Wireless battery-powered daylight sensor

Abstract

A wireless battery-powered daylight sensor for measuring a total light intensity in a space is operable to transmit wireless signals using a variable transmission rate that is dependent upon the total light intensity in the space. The sensor comprises a photosensitive circuit, a wireless transmitter for transmitting the wireless signals, a controller coupled to the photosensitive circuit and the wireless transmitter, and a battery for powering the photosensitive circuit, the wireless transmitter, and the controller. The photosensitive circuit is operable to generate a light intensity control signal in response to the total light intensity in the space. The controller transmits the wireless signals in response to the light intensity control signal using the variable transmission rate that is dependent upon the total light intensity in the space. The variable transmission rate may be dependent upon an amount of change of the total light intensity in the space. In addition, the variable transmission rate may be further dependent upon a rate of change of the total light intensity in the space.

Claims

1. A wireless sensor comprising: a photosensitive circuit configured to generate a light intensity control signal in response to a light intensity; a wireless transmitter configured to transmit wireless signals; and a controller coupled to the photosensitive circuit and the wireless transmitter, the controller configured to transmit wireless signals in response to the light intensity control signal; wherein the controller is configured to: sample the light intensity control signal to collect a first plurality of samples during a first time interval, and a second plurality of samples during a second, subsequent time interval, the first and second time intervals being non-overlapping; determine a minimum sample among the first plurality of samples; determine a minimum sample among the second plurality of samples; determine a maximum sample among the second plurality of samples; calculate a first adjustment percentage based on a difference between the minimum sample among the first plurality of samples and the minimum sample among the second plurality of samples; calculate a second adjustment percentage based on a difference between the minimum sample among the second plurality of samples and the maximum sample among the second plurality of samples; and transmit a digital message if the first adjustment percentage is greater than a first predetermined amount and the second adjustment percentage is less than a second predetermined amount.

2. The sensor of claim 1, further comprising: a battery configured to power the photosensitive circuit, the wireless transmitter, and the controller.

3. The sensor of claim 2, wherein the controller is configured to enable the photosensitive circuit and subsequently sample the light intensity control signal at a sampling period, the controller further configured to disable the photosensitive circuit after the light intensity control signal has been sampled, such that the photosensitive circuit only draws current from the battery during each sampling period.

4. The sensor of claim 3, wherein the photosensitive circuit comprises a photosensitive diode for conducting a photosensitive diode current having a magnitude responsive to the light intensity, the magnitude of the light intensity control signal responsive to the magnitude of the photosensitive diode current.

5. The sensor of claim 4, wherein the photosensitive circuit further comprises a controllable switch coupled in series with the photosensitive diode, the photosensitive diode configured to conduct the photosensitive diode current when the controllable switch is closed, the controller coupled to the controllable switch for opening the switch, such that the photosensitive diode does not conduct the photosensitive diode current and the photosensitive circuit is disabled.

6. The sensor of claim 5, wherein the photosensitive circuit further comprises a transimpedance amplifier coupled between the photosensitive diode and the controller, the transimpedance amplifier configured to generate the light intensity control signal in response to the photosensitive diode current.

7. The sensor of claim 2, further comprising: a wireless receiver coupled to the controller and configured to receive a wireless signal, the wireless receiver powered by the battery; and a laser pointer circuit adapted to be exposed to light from a laser pointer, the laser pointer circuit coupled to the controller; wherein the controller is configured to enable the wireless receiver in response to light from a laser pointer shining on the laser pointer circuit, and to subsequently receive a wireless signal.

8. The sensor of claim 1, further comprising: a memory coupled to the controller for storing samples of the sampled light intensity control signal, the controller configured to analyze the sampled light intensity control signal stored in the memory to determine the amount of change of the light intensity.

9. The sensor of claim 1, wherein the controller is configured to determine when it is nighttime in response to the light intensity control signal, and to increase a length of the sampling period during the nighttime.

10. The sensor of claim 1, wherein the digital message includes a value representative of the light intensity.

11. The sensor of claim 1, wherein the first adjustment percentage is representative of an amount of change of the light intensity.

12. The sensor of claim 1, wherein the controller configured to transmit the wireless signals using a variable transmission rate that is dependent upon the first adjustment percentage.

13. The sensor of claim 1, wherein the digital message includes a command to control a lighting load in response to the light intensity.

14. A method of transmitting a digital message in response to a light intensity, the method comprising: measuring the light intensity by a daylight sensor which comprises a memory coupled to a controller; storing in the memory light intensity values periodically sampled in a first time interval and a second time interval by the controller in response to the step of measuring; determining a minimum sample among the light intensity values sampled in the first time interval; determining a minimum sample among the light intensity values sampled in the second time interval; determining a maximum sample among the light intensity values sampled in the second time interval; calculating a first adjustment percentage based on a difference between the minimum sample among the light intensity values sampled in the second time interval and the minimum sample among the light intensity values sampled in the second time interval; calculate a second adjustment percentage based on a difference between the minimum sample among the light intensity values sampled in the second time interval and the maximum sample among the light intensity values sampled in the second time interval; and transmitting a digital message if the first adjustment percentage is greater than a first predetermined amount and the second adjustment percentage is less than a second predetermined amount.

15. The method of claim 14, wherein the first adjustment percentage is representative of an amount of change of the light intensity.

16. The method of claim 14, wherein transmitting the digital message comprises transmitting wireless signals using a variable transmission rate that is dependent upon the first adjustment percentage.

17. The method of claim 14, wherein the digital message includes a command to control a lighting load in response to the light intensity.

18. The method of claim 14, further comprising: receiving a light intensity control signal from a photosensitive circuit; enabling the photosensitive circuit; subsequently sampling the light intensity control signal at a sampling period; and disabling the photosensitive circuit after the light intensity control signal has been sampled, such that the photosensitive circuit only draws current from the battery during each sampling period.

19. The method of claim 18, further comprising: determining when it is nighttime in response to the light intensity control signal; and increasing a length of the sampling period during the nighttime.

20. The method of claim 14, further comprising: enabling a wireless receiver in response to light from a laser pointer shining on a laser pointer circuit; and subsequently receiving a wireless signal via the wireless receiver.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention will now be described in greater detail in the following detailed description with reference to the drawings in which:

(2) FIG. 1 is a simple diagram of a radio-frequency (RF) lighting control system comprising a dimmer switch and a daylight sensor according to the present invention;

(3) FIG. 2 is a simplified diagram of a room in which the daylight sensor of FIG. 1 may be mounted;

(4) FIG. 3 shows a few example plots of total light intensities at the daylight sensor mounted in the room of FIG. 2 with respect to time during a sunny day, a cloudy day, and an intermittent-cloudy day;

(5) FIG. 4 is an enlarged perspective view of the daylight sensor of FIG. 1;

(6) FIG. 5 is a simplified block diagram of the dimmer switch of FIG. 1;

(7) FIG. 6A is a simplified block diagram of the daylight sensor of FIG. 1;

(8) FIG. 6B is a simplified schematic diagram of the daylight sensor of FIG. 6A;

(9) FIG. 7 is a simplified flowchart of a transmission algorithm executed by a controller of the daylight sensor of FIG. 1 according to a first embodiment of the present invention, such that the daylight sensor transmits digital messages using a variable transmission rate;

(10) FIG. 8 is a simplified flowchart of a variable transmission control procedure executed by the controller of the daylight sensor of FIG. 1 according to the first embodiment of the present invention;

(11) FIG. 9 is a simplified flowchart of a receive procedure executed by a controller of the dimmer switch of FIG. 1 according to the first embodiment of the present invention;

(12) FIG. 10A is a simplified flowchart of a laser pointer interrupt procedure executed by the controller of the daylight sensor of FIG. 1;

(13) FIG. 10B is a diagram of an example test setup for the daylight sensor of FIG. 1 according to the first embodiment of the present invention;

(14) FIG. 10C is a plot of an example test waveform for the daylight sensor of FIG. 1 according to the first embodiment to be used in the test setup shown in FIG. 10B;

(15) FIG. 11 is a simplified flowchart of a variable transmission control procedure executed by the controller of the daylight sensor of FIG. 1 according to a second embodiment of the present invention;

(16) FIG. 12 is a simplified flowchart of a receive procedure executed by the controller of the dimmer switch of FIG. 1 according to the second embodiment of the present invention;

(17) FIG. 13 is a simplified flowchart of a load control procedure executed periodically by the controller of the dimmer switch of FIG. 1 according to the second embodiment of the present invention;

(18) FIG. 14 is a simplified flowchart of a variable transmission control procedure executed by the controller of the daylight sensor of FIG. 1 according to a third embodiment of the present invention;

(19) FIG. 15 is a simplified flowchart of a transmission algorithm executed by the controller of the daylight sensor of FIG. 1 according to a fourth embodiment of the present invention;

(20) FIG. 16A is a simplified flowchart of a variable transmission control procedure executed by the controller of the daylight sensor of FIG. 1 according to the fourth embodiment of the present invention;

(21) FIG. 16B is a plot of an example test waveform for the daylight sensor of FIG. 1 according to the fourth embodiment to be used in the test setup shown in FIG. 10B; and

(22) FIG. 17 is a simplified flowchart of a control procedure executed periodically by the controller of the daylight sensor of FIG. 1 according to a fifth embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

(23) The foregoing summary, as well as the following detailed description of the preferred embodiments, is better understood when read in conjunction with the appended drawings. For the purposes of illustrating the invention, there is shown in the drawings an embodiment that is presently preferred, in which like numerals represent similar parts throughout the several views of the drawings, it being understood, however, that the invention is not limited to the specific methods and instrumentalities disclosed.

(24) FIG. 1 is a simple diagram of a radio-frequency (RF) lighting control system 100 comprising a dimmer switch 110 and a daylight sensor 120 according to a first embodiment of the present invention. The dimmer switch 110 is adapted to be coupled in series electrical connection between an alternating-current (AC) power source 102 and a lighting load 104 for controlling the amount of power delivered to the lighting load. The dimmer switch 110 may be wall-mounted in a standard electrical wallbox. Alternatively, the dimmer switch 110 could be implemented as a table-top load control device. The dimmer switch 110 comprises a faceplate 112 and a bezel 113 received in an opening of the faceplate. The dimmer switch 110 further comprises a control actuator 114 (i.e., a button) and an intensity adjustment actuator 116. Successive actuations of the toggle actuator 114 toggle, i.e., turn off and on, the lighting load 104. Actuations of an upper portion 116A or a lower portion 116B of the intensity adjustment actuator 116 respectively increase or decrease the amount of power delivered to the lighting load 104 and thus increase or decrease a present light intensity L.sub.PRES of the lighting load 104 from a minimum intensity (e.g., 1%) to a maximum intensity (e.g., 100%). A plurality of visual indicators 118, e.g., light-emitting diodes (LEDs), are arranged in a linear array on the left side of the bezel 113. The visual indicators 118 are illuminated to provide feedback of the intensity of the lighting load 104. An example of a dimmer switch having a toggle actuator 114 and an intensity adjustment actuator 116 is described in greater detail in U.S. Pat. No. 5,248,919, issued Sep. 29, 1993, entitled LIGHTING CONTROL DEVICE, the entire disclosure of which is hereby incorporated by reference.

(25) The daylight sensor 120 is mounted so as to measure a total light intensity L.sub.T-SNSR in the space around the daylight sensor (i.e., in the vicinity of the lighting load 104 controlled by the dimmer switch 110). The daylight sensor 120 includes an internal photosensitive circuit, e.g., a photosensitive diode 232 (FIG. 6A), which is housed in an enclosure 122 having a lens 124 for conducting light from outside the daylight sensor towards the internal photosensitive diode 232. The daylight sensor 120 is responsive to the total light intensity L.sub.T-SNSR measured by the internal photosensitive circuit. Specifically, the daylight sensor 120 is operable to wirelessly transmit digital messages (i.e., wireless signals) to the dimmer switch 110 via RF signals 106, such that the dimmer switch 110 controls the present light intensity L.sub.PRES of the lighting load 104 in response to the total light intensity L.sub.T-SNSR measured by the daylight sensor 120.

(26) During a setup procedure of the RF lighting control system 100, the daylight sensor 120 may be assigned to (i.e., associated with) the dimmer switch 110. As mentioned above, the daylight sensor 120 transmits digital messages wirelessly via the RF signals 106 to the dimmer switch 110 in response to the total light intensity L.sub.T-SNSR measured by the daylight sensor. A digital message transmitted by the daylight sensor 120 includes, for example, identifying information, such as, a serial number (i.e., a unique identifier) associated with the daylight sensor. The dimmer switch 110 is responsive to messages containing the serial numbers of the daylight sensor 120 to which the dimmer switch is assigned. Each digital message may further comprise a value representative of the measured total light intensity L.sub.T-SNSR measured by the daylight sensor 120 (e.g., in foot-candles). Accordingly, the dimmer switch 110 controls the present light intensity L.sub.PRES of the lighting load 104 in response to receiving a digital message with the total light intensity L.sub.T-SNSR as measured by the daylight sensor 120. According to the present invention, the daylight sensor 120 is operable to transmit digital messages to the dimmer switch 110 using a variable transmission rate f.sub.TX that is dependent upon the measured total light intensity L.sub.T-SNSR, such that the daylight sensor 120 only transmits digital messages when needed (as will be described in greater detail below).

(27) Examples of RF lighting control systems are described in greater detail in U.S. patent application Ser. No. 12/033,223, filed Feb. 19, 2008, entitled COMMUNICATION PROTOCOL FOR A RADIO-FREQUENCY LOAD CONTROL SYSTEM; U.S. patent application Ser. No. 12/203,518, filed Sep. 3, 2008, entitled RADIO-FREQUENCY LIGHTING CONTROL SYSTEM WITH OCCUPANCY SENSING; U.S. patent application Ser. No. 12/203,500, filed Sep. 3, 2008, entitled BATTERY-POWERED OCCUPANCY SENSOR; and U.S. patent application Ser. No. 12/371,027, filed Feb. 13, 2009, entitled METHOD AND APPARATUS FOR CONFIGURING A WIRELESS SENSOR, the entire disclosures of which are hereby incorporated by reference.

(28) Alternatively, the dimmer switch 110 could be replaced with an electronic switch comprising, for example, a relay, for simply toggling the lighting load 104 on and off. The electronic switch could be adapted to simply turn the lighting load 104 on when the measured total light intensity L.sub.T-SNSR drops below a predetermined threshold and turn the lighting load off when the measured total light intensity L.sub.T-SNSR rises above approximately the predetermined threshold (e.g., using some hysteresis).

(29) The lighting control system 100 could additionally comprise one or more motorized window treatments, such as roller shades, draperies, Roman shades, or blinds, for controlling the amount of daylight entering the space around the daylight sensor 120. Examples of load control systems having motorized window treatments are described in greater detail in U.S. Pat. No. 7,111,952, issued Sep. 26, 2006, entitled SYSTEM TO CONTROL DAYLIGHT AND ARTIFICIAL ILLUMINATION AND SUN GLARE IN A SPACE, the entire disclosure of which is hereby incorporated by reference.

(30) FIG. 2 is a simplified diagram of a room 130 in which the daylight sensor 120 may be mounted. The daylight sensor 120 is mounted to a ceiling 132 of the room 130 at a distance from a window 134 through which natural light (i.e., daylight) shines. The lighting load 104 is also mounted to the ceiling 132 of the room. The room 130 contains a task surface 136 (e.g., a table) that is illuminated by the natural light shining through the window 134 and the electric light (i.e., artificial light) generated by the lighting load 104. Thus, a total light intensity L.sub.T-TASK produced on the task surface 136 is the sum of a light intensity L.sub.D-TASK on the task surface from only daylight entering the room 130 through the window 134 and a light intensity L.sub.E-TASK on the task surface from only the lighting load 104 (i.e., L.sub.T-TASK=L.sub.D-TASK+L.sub.E-TASK). The daylight sensor 120 is operable to measure the total light intensity L.sub.T-SNSR at the daylight sensor, which is also a combination of the natural light and the electric light in the room 130. The natural and electric light that shine onto the task surface 136 may be reflected to the daylight sensor 120, while the natural light from the window 134 may shine directly onto the daylight sensor. Thus, the total light intensity L.sub.T-SNSR measured by the daylight sensor 120 is the sum of a light intensity L.sub.D-SNSR at the daylight sensor from only daylight entering the room 130 through the window 134 and a light intensity L.sub.E-SNSR at the daylight sensor from only the lighting load 104 (i.e., L.sub.T-SNSR=L.sub.D-SNSR+L.sub.E-SNSR).

(31) The dimmer switch 110 adjusts the present light intensity L.sub.PRES of the lighting load 104 so as to control the total light intensity L.sub.T-TASK on the task surface 136 towards a target total task surface light intensity L.sub.TRGT-TASK. For example, the target total task surface light intensity L.sub.TRGT-TASK may be preset to be approximately fifty foot-candles. In addition, the target total task surface light intensity L.sub.TRGT-TASK may be decreased by actuating the intensity adjustment actuator 116. Alternatively, the dimmer switch 110 could be operable to receive one or more digital messages from an advanced programming device, such as a personal digital assistant (PDA) or a personal computer (PC), such that the target total task surface light intensity L.sub.TRGT-TASK may be entered using a graphical user interface (GUI) and transmitted to the dimmer switch 110. Further, the target total task surface light intensity L.sub.TRGT-TASK could alternatively be adjusted using an advanced programming mode of the dimmer switch 110. An example of an advanced programming mode for a dimmer switch is described in greater detail in U.S. Pat. No. 7,190,125, issued Mar. 13, 2007, entitled PROGRAMMABLE WALLBOX DIMMER, the entire disclosure of which is hereby incorporated by reference.

(32) Since the total light intensity L.sub.T-SNSR measured by the daylight sensor 120 (e.g., as reflected on the daylight sensor) is less than the total light intensity L.sub.T-TASK shining directly on the task surface 136, the lighting control system 100 is characterized by one or more gains. Specifically, the dimmer switch 110 uses a daylight gain G.sub.D and an electrical light gain G.sub.E to control the present intensity L.sub.PRES of the lighting load 104. The daylight gain GD is representative of the ratio between the light intensity L.sub.D-TASK on the task surface 136 from only daylight and the light intensity L.sub.D-SNSR measured by the daylight sensor 120 from only daylight (i.e., G.sub.D=L.sub.D-TASK/L.sub.D-SNSR). The electric light gain G.sub.E is representative of the ratio between the light intensity L.sub.E-TASK on the task surface 136 from only the lighting load 104 and the light intensity L.sub.E-SNSR measured by the daylight sensor 120 from only the lighting load (i.e., G.sub.E=L.sub.E-TASK/L.sub.E-SNSR). The daylight gain G.sub.D and the electrical light gain G.sub.E of the lighting control system 100 are set during a gain calibration procedure. An example of a gain calibration procedures are described in greater detail in commonly-assigned, co-pending U.S. patent application Ser. No. 12/727,923, filed Mar. 19, 2010, entitled METHOD OF CALIBRATING A DAYLIGHT SENSOR, the entire disclosure of which is hereby incorporated by reference.

(33) During days when there are intermittent clouds passing the building in which the room 130 is located, the total light intensity L.sub.T-SNSR at the daylight sensor 120 may fluctuate between high values when the clouds are not blocking the sunlight and low values when the clouds are blocking the sunlight. FIG. 3 shows a few example plots of the total light intensity L.sub.T-SNSR as measured by the daylight sensor 120 with respect to time during a sunny day, a cloudy day, and an intermittent-cloudy day. The total light intensity L.sub.T-SNSR during a day typically takes the shape of a parabola. On a sunny day, a total sunny-day light intensity L.sub.T-SUNNY may increase from sunrise (at time t.sub.SUNRISE) to a maximum sunny-day light intensity L.sub.MAX-SUNNY around midday (at time t.sub.MIDDAY), and then decrease until sunset (at time t.sub.SUNSET). On a cloudy day, a total cloudy-day light intensity L.sub.T-CLOUDY may increase from sunrise to a maximum cloudy-day light intensity L.sub.MAX-SUNNY around midday, and then decreases until sunset. The maximum sunny-day light intensity L.sub.MAX-SUNNY is typically greater than the maximum cloudy-day light intensity L.sub.MAX-CLOUDY. On a day having intermittent clouds, a total light intensity L.sub.T-IC may fluctuate between the total cloudy-day light intensity L.sub.T-CLOUDY and the total sunny-day light intensity L.sub.T-SUNNY as shown in FIG. 3.

(34) FIG. 4 is an enlarged perspective view of the daylight sensor 120. The lens 124 is transparent such that the light from the room 130 is able to shine onto the internal photosensitive diode 232 of the daylight sensor 120. The daylight sensor 120 is positioned on the ceiling 132 such that an arrow 140 points towards the window 134, such that lens 124 is directed towards the window 134. As a result, more natural light than artificial light will shine through the lens 124 and onto the internal photosensitive diode 232. A plurality of actuators (e.g., a calibration button 150, a test button 152, and a link button 154) are used during the setup and calibration procedures of the daylight sensor 120. The daylight sensor 120 further comprises a laser-pointer receiving opening 156, which is adapted to receive energy from a laser pointer (not shown). The daylight sensor 120 is responsive to the energy of the laser pointer shining through the laser-pointer receiving opening 156. When the daylight sensor 120 is mounted to the ceiling 132, a user may shine the laser pointer through the opening 156 rather than actuating the calibration button 150 during the gain calibration procedure.

(35) FIG. 5 is a simplified block diagram of the dimmer switch 110. The dimmer switch 110 comprises a controllably conductive device 210 coupled in series electrical connection between the AC power source 102 and the lighting load 104 for control of the power delivered to the lighting load. The controllably conductive device 210 may comprise any suitable type of bidirectional semiconductor switch, such as, for example, a triac, a field-effect transistor (FET) in a rectifier bridge, or two FETs in anti-series connection. The controllably conductive device 210 includes a control input coupled to a drive circuit 212. The controllably conductive device 210 is rendered conductive or non-conductive in response to the control input, which in turn controls the power supplied to the lighting load 104.

(36) The drive circuit 212 provides control inputs to the controllably conductive device 210 in response to control signals from a controller 214. The controller 214 is, for example, a microcontroller, but may alternatively be any suitable processing device, such as a programmable logic device (PLD), a microprocessor, or an application specific integrated circuit (ASIC). The controller 214 receives inputs from the control actuator 114 and the intensity adjustment actuator 116 and controls the visual indicators 118. The controller 214 is also coupled to a memory 216 for storage of the preset intensity of lighting load 104, the serial number of the daylight sensor 120 to which the dimmer switch 110 is assigned, the daylight gain G.sub.D, the electrical light gain G.sub.E, and other operational characteristics of the dimmer switch 110. The controller 230 may recall the daylight gain G.sub.D and the electrical light gain G.sub.E from the memory 216 at startup. The memory 216 may be implemented as an external integrated circuit (IC) or as an internal circuit of the controller 214. A power supply 218 generates a direct-current (DC) voltage Vcc for powering the controller 214, the memory 216, and other low-voltage circuitry of the dimmer switch 110.

(37) A zero-crossing detector 220 determines the zero-crossings of the input AC waveform from the AC power supply 102. A zero-crossing is defined as the time at which the AC supply voltage transitions from positive to negative polarity, or from negative to positive polarity, at the beginning of each half-cycle. The zero-crossing information is provided as an input to controller 214. The controller 214 provides the control signals to the drive circuit 212 to operate the controllably conductive device 210 (i.e., to provide voltage from the AC power supply 102 to the lighting load 104) at predetermined times relative to the zero-crossing points of the AC waveform using a phase-control dimming technique.

(38) The dimmer switch 110 further comprises an RF transceiver 222 and an antenna 224 for receiving the RF signals 106 from the daylight sensor 120. The controller 214 is operable to control the controllably conductive device 210 in response to the messages received via the RF signals 106. Examples of the antenna 224 for wall-mounted dimmer switches, such as the dimmer switch 110, are described in greater detail in U.S. Pat. No. 5,982,103, issued Nov. 9, 1999, and U.S. Pat. No. 7,362,285, issued Apr. 22, 2008, both entitled COMPACT RADIO FREQUENCY TRANSMITTING AND RECEIVING ANTENNA AND CONTROL DEVICE EMPLOYING SAME. The entire disclosures of both patents are hereby incorporated by reference.

(39) FIG. 6A is a simplified block diagram of the daylight sensor 120. The daylight sensor 120 comprises a controller 230 that is responsive to a photosensitive circuit 231, which includes the photosensitive diode 232. The cathode of the photosensitive diode 232 is coupled to the controller 230 via a transimpedance amplifier 234, which operates as a current-to-voltage converter. The anode of the photosensitive diode 232 is coupled to circuit common through a controllable switch 235, which allows the controller 230 to enable and disable the photosensitive circuit 231 (using a photosensitive circuit enable control signal V.sub.PS_ENABLE) as will be described in greater detail below.

(40) The photosensitive diode 232 conducts a photosensitive diode current I.sub.PD having a magnitude dependent upon the magnitude of the light that shines on the photosensitive diode (i.e., the total light intensity L.sub.T-SNSR as measured by the daylight sensor 120). The transimpedance amplifier 234 provides the controller 230 with a total light intensity control signal V.sub.TOT representative of the total light intensity L.sub.T-SNSR. Specifically, the magnitude of the total light intensity control signal V.sub.TOT generated by the transimpedance amplifier 234 is dependent upon the magnitude of the current I.sub.PD conducted by the photosensitive diode 232, and thus the total light intensity L.sub.T-SNSR. The controller 230 comprises an analog-to-digital converter (ADC), such that the controller is operable to sample the total light intensity control signal V.sub.TOT to generate a total light intensity sample S.sub.TOT. The controller 230 uses a sampling period T.sub.SMPL of, for example, approximately one second, such that the controller samples the total light intensity control signal V.sub.TOT approximately once every second during normal operation of the daylight sensor 120.

(41) The daylight sensor 120 further comprises an RF transceiver 236, which is coupled to the controller 230 and an antenna 238. The controller 230 is operable to cause the RF transceiver 236 to transmit digital messages to the dimmer switch 110 via the RF signals 106 in response to the magnitude of the total light intensity control signal V.sub.TOT. The controller 230 may also be operable to receive a digital message from the dimmer switch 110 or another remote control device, such as a personal digital assistant (PDA), for configuring the operation of the daylight sensor 120. The controller 230 provides the digital message to be transmitted by the RF transceiver 236 and obtains received digital messages from the RF transmitter via an RF data control signal V.sub.RF_DATA. The controller 230 also is operable to enable and disable the RF transceiver via an RF enable control signal V.sub.RF_ENABLE. Alternatively, the RF transceiver 236 of the daylight sensor 120 could comprise an RF transmitter and the RF transceiver 222 of the dimmer switch 110 could comprise an RF receiver to allow for one-way communication between the daylight sensor and the dimmer switch. The RF transmitter may comprise, for example, part number CC1150 manufactured by Texas Instruments Inc.

(42) The controller 230 of the daylight sensor 120 is also responsive to a plurality of actuators 240 (i.e., the calibration button 150, the test button 152, and the link button 154), which provide user inputs to the daylight sensor for use during calibration of the daylight sensor. The controller 230 is operable to control one or more LEDs 242 to illuminate the lens 124 to thus provide feedback during calibration of the daylight sensor 120. A laser pointer circuit 244 is coupled to the controller 230 and is responsive to light that shines through the laser-pointer receiving opening 156 from a laser pointer. Specifically, the controller 230 responds to an input from the laser pointer circuit 244 in the same manner as an actuation of the calibration button 150. The controller 230 is further coupled to a memory 246 for storing the operational characteristics of the daylight sensor 120. The daylight sensor 120 also comprises a battery V1 that provides a battery voltage V.sub.BATT (e.g., approximately three volts) for powering the controller 230, the photosensitive circuit 231, the RF transceiver 236, and the other circuitry of the daylight sensor 120.

(43) The controller 230 is operable to control the photosensitive circuit 231 and the RF transceiver 236 in order to conserve battery power. Specifically, the controller 230 is operable to enable the photosensitive circuit 231 (by closing the switch 235 via the photosensitive circuit enable control signal V.sub.PS_ENABLE) for a small time period T.sub.PD (e.g., 50 msec) during each sampling period T.sub.SMPL, such that that the photosensitive diode 232 only conducts current for a portion of the time during normal operation (e.g., 5% of the time). In addition, the controller 230 only enables the RF transceiver 236 (via the RF enable control signal V.sub.RF_ENABLE) when required. As previously mentioned, the controller 230 only enables the RF transceiver 236 to transmit digital messages when needed, i.e., using the variable transmission rate (as will be described in greater detail below with reference to FIG. 8). The controller 230 only enables the RF transceiver 236 to receive digital messages in response to the laser pointer circuit 244 receiving light from a laser pointer through the laser-pointer receiving opening 156. When the photosensitive circuit 231 and the RF transceiver 236 are disabled, the controller 230 is operable to enter a sleep mode in which the controller consumes less power.

(44) FIG. 6B is a simplified schematic diagram of the daylight sensor 120 showing the transimpedance amplifier 234 and the laser pointer circuit 244 in greater detail. The transimpedance amplifier 234 comprises an operational amplifier (“op-amp”) U250 having a non-inverting input terminal coupled to circuit common. A feedback resistor R252 is coupled between an inverting input terminal and an output terminal of the op-amp U250. The output terminal of the op-amp U250 provides to the controller 230 the total light intensity control signal V.sub.TOT, which has a magnitude that varies in response to the magnitude of the photosensitive diode current I.sub.PD. The cathode of the photosensitive diode 232 is coupled to the inverting input terminal of the op-amp U250, such that the photosensitive diode current I.sub.PD is conducted through the feedback resistor R252. Thus, the magnitude of the total light intensity control signal V.sub.TOT is dependent upon the magnitude of the photosensitive diode current I.sub.PD and the resistance of the feedback resistor R252. For example, the resistor R252 may have a resistance of approximately 300 kΩ, such that the magnitude of the total light intensity control signal V.sub.TOT ranges from approximately zero volts to three volts as the light intensity shining directly on the photosensitive diode 232 ranges from approximately zero lux to 1000 lux.

(45) The transimpedance amplifier 234 further comprises a feedback capacitor C254 (e.g., having a capacitance of approximately 0.022 μF) for providing some low-pass filtering, such that the magnitude of the total light intensity control signal V.sub.TOT is not responsive to high-frequency noise in the photosensitive diode current I.sub.PD. In addition, the op-amp U250 is powered from the battery V1 through a low-pass filter comprising a resistor R256 (e.g., having a resistance of approximately 22Ω and a capacitor C258 (e.g., having a capacitance of approximately 0.1 μF). The low-pass filter prevents high-frequency noise that may be coupled to the battery V1 from the RF transceiver 236 from affecting the operation of the photosensitive circuit 231.

(46) The laser pointer circuit 244 comprises a laser-responsive element, e.g., a light-emitting diode (LED) D260. The LED D260 is positioned inside the daylight sensor 120 such that light from a laser pointer may shine through the laser-pointer receiving opening 156 and onto the LED. The LED D260 may be a green LED, such that a laser current I.sub.LASER conducted through the LED increases in magnitude when a green laser pointer is shined onto the LED. A resistor R262 is coupled between the anode of the LED D260 and circuit common and has, for example, a resistance of approximately 1 MΩ. A capacitor C264 is coupled in parallel with the resistor R262 and has, for example, a capacitance of approximately 0.01 μF. The junction of the LED D260 and the resistor R262 is coupled to the controller 230 through a capacitor C265 (e.g., having a capacitance of approximately 0.22 μF) and a resistor R266 (e.g., having a resistance of approximately 10 kΩ). The junction of the capacitor C265 and the resistor R266 is coupled to circuit common through a resistor (e.g., having a resistance of approximately 1 MΩ). When a laser pointer is shined onto the LED D260 and the laser current I.sub.LASER increases in magnitude, the voltage across the parallel combination of the resistor R262 and the capacitor C264 also increases in magnitude. Accordingly, the capacitor C265 conducts a pulse of current and the laser pointer control signal V.sub.LASER also increases in magnitude. The input of the controller 230 that receives the laser pointer control signal V.sub.LASER is an interrupt pin, such that the controller 230 is operable to come out of sleep mode in response to the laser pointer. The controller 230 may then be operable to enable the RF transceiver 236 to receive a digital message as will be described in greater detail below with reference to FIG. 10A.

(47) According to the present invention, the daylight sensor 120 is operable to transmit digital messages to the dimmer switch 110 using the variable transmission rate that is dependent upon the present change in the total light intensity L.sub.T-SNSR as measured by the daylight sensor 120. The daylight sensor 120 is operable to determine the total light intensity L.sub.T-SNSR from the magnitude of the total light intensity control signal V.sub.TOT, and to only transmit one or more values representative of the total light intensity L.sub.T-SNSR (e.g., in foot-candles) to the dimmer switch 110 when the total light intensity L.sub.T-SNSR has changed by at least a first predetermined percentage ΔS.sub.MAX1. Since the total light intensity L.sub.T-SNSR as measured by the daylight sensor 120 changes throughout a typical day, the variable transmission rate also changes throughout the day (as shown in FIG. 3). The variable transmission rate ensures that the daylight sensor 120 only transmits digital messages when needed (i.e., when the total light intensity L.sub.T-SNSR is changing quickly, but not too quickly). Because the controller 230 is able to disable the photosensitive circuit 231 (by opening the switch 235 via the photosensitive circuit enable control signal V.sub.PS_ENABLE), the daylight sensor 120 is able to conserve battery power by not transmitting digital messages to the dimmer switch 110 as often when the total light intensity L.sub.T-SNSR is relatively constant with respect to time.

(48) FIG. 7 is a simplified flowchart of a transmission algorithm 300 executed by the controller 230 of the daylight sensor 120 according to the first embodiment of the present invention, such that the daylight sensor 120 transmits digital messages using the variable transmission rate. The transmission algorithm 300 as shown in FIG. 7 is generalized and specific embodiments are detailed below. According to the first embodiment, the controller 230 collects a predetermined number N.sub.SMPL of measurements of the total light intensity L.sub.T-SNSR (e.g., ten) during consecutive non-overlapping time intervals (i.e., windows) that each have a length equal to a predetermined time period T.sub.WIN (i.e., T.sub.WIN=N.sub.SMPL.Math.T.sub.SMPL). The controller 230 determines one or more estimators from a previous time interval and uses the estimator to estimate one or more predicted light intensity values in the present time interval. At end of the present time interval, the controller 230 determines whether a digital message including one or more values representative of the total light intensity L.sub.T-SNSR should be transmitted to the dimmer switch 110 in response to the error between the measured light intensity values and the predicted light intensity values. The transmission algorithm 300 is executed with at a period equal to the predetermined time period T.sub.WIN. Accordingly, the minimum time period between transmissions by the daylight sensor 120 according to the first embodiment is equal to the predetermined time period T.sub.WIN. For example, the predetermined time period T.sub.WIN may be approximately ten seconds, but may alternatively range from approximately five seconds to thirty seconds.

(49) Referring to FIG. 7, the controller 230 first measures the predetermined number N.sub.SMPL of new total light intensity values at step 310, and stores the measured total light intensity values at step 312. Next, the controller 230 determines the predicted light intensity value(s) at step 314 using the estimator(s) determined during one of the previous time intervals, and calculates an error between the measured total light intensity values and the predicted total light intensity values at step 316. If the error is outside of predetermined limits (i.e., is too great) at step 318, the controller 230 calculates the new estimator(s) for use during the subsequent time interval at step 320 and transmits a digital message including one or more values representative of the total light intensity L.sub.T-SNSR as measured by the daylight sensor 120 to the dimmer switch 110 at step 322. For example, the controller 230 may transmit one or more of the measured total light intensity values to the dimmer switch 110. Alternatively, the controller 230 may transmit the new estimator(s) determined at step 320 to the dimmer switch 110. After transmitting a digital message to the dimmer switch 110, the transmission algorithm 300 loops around, such that the controller 230 may collect the predetermined number N.sub.SMPL of measurements of the total light intensity L.sub.T-SNSR during the subsequent non-overlapping time interval. If the error is within the predetermined limits at step 318, the controller 230 does not calculate the new estimator(s) at step 320 and does not transmit the value representative of the total light intensity L.sub.T-SNSR at step 324, but simply analyzes the next non-overlapping time interval.

(50) According to the first embodiment of the present invention, the controller 230 of the daylight sensor 120 uses a single data point as the estimator. For example, the controller 230 may use the minimum value of the measured light intensity values from the previous time interval as the estimator. Alternatively, the controller 230 may use the average or median value of the measured light intensity values from the previous time interval as the estimator. Since the estimator is a single data point, the controller 230 only uses one predicted light intensity value at step 314 of the transmission algorithm 300. For example, the predicted light intensity value may be equal to the estimator. The controller 230 then calculates the error using the minimum value of the measured light intensity values from the present time interval and the predicted light intensity value (i.e., the estimator).

(51) FIG. 8 is a simplified flowchart of a variable transmission control procedure 400 executed by the controller 230 of the daylight sensor 120 according to the first embodiment of the present invention. The controller 230 executes the variable transmission control procedure 400 periodically (e.g., approximately once every second) during normal operation in order to sample the total light intensity control signal V.sub.TOT, to thus collect the predetermined number N.sub.SMPL of samples (e.g., approximately ten samples) during each of the consecutive non-overlapping time intervals. Specifically, the controller 230 first enables the photosensitive circuit 231 at step 410 by closing the controllable switch 235 using the photosensitive circuit enable control signal V.sub.PS_ENABLE. The controller 230 waits for the time period T.sub.PD (i.e., 50 msec) at step 412 to allow the photosensitive diode current I.sub.PD to become representative of the total light intensity L.sub.T-SNSR at the daylight sensor 120. The controller 230 samples the total light intensity control signal V.sub.TOT to generate a new total light intensity sample S.sub.TOT at step 414, and disables the photosensitive circuit 231 by opening the switch 235 using the photosensitive circuit enable control signal V.sub.PS_ENABLE at step 416. The controller 230 then increments a variable n by one at step 418 and stores the new total light intensity sample S.sub.TOT as sample S[n] in the memory 246 at step 420. If the variable n is less than the predetermined number N.sub.SMPL of samples at step 422, the variable transmission control procedure 400 simply exits without processing the samples S[n] stored in the memory 246. The controller 230 will execute the variable transmission control procedure 400 once again to collect a new sample of the total light intensity control signal V.sub.TOT.

(52) If the variable n is greater than or equal to the predetermined number N.sub.SMPL of samples at step 422, the controller 230 processes the samples S[n] stored in the memory 246 in order to determine if a digital message should be transmitted to the dimmer switch 110. First, the controller 230 resets the variable n to zero at step 424. The controller 230 then determines if the total light intensity L.sub.T-SNSR has changed by at least the first predetermined percentage ΔS.sub.MAX1. Specifically, the controller 230 determines a present minimum sample S.sub.MIN-PRS of the samples S[n] stored in the memory 246 (i.e., samples S[0] through S[N.sub.SMPL]) at step 426. The controller 230 then calculates a minimum sample adjustment percentage ΔS.sub.MIN that is representative of the amount of change of the total light intensity L.sub.T-SNSR at step 428 using the equation:

(53) $\begin{array}{cc}\Delta S_{\mathrm{MIN}}=\frac{.Math.S_{\mathrm{MIN}-\mathrm{PRS}}-S_{\mathrm{MIN}-\mathrm{PRV}}.Math.}{S_{\mathrm{MIN}-\mathrm{PRV}}},& \left(\mathrm{Equation}1\right)\end{array}$
where the sample S.sub.MIN-PRV is the previous minimum sample determined during the previous time period T.sub.WIN that is stored in the memory 246. If the minimum sample adjustment percentage ΔS.sub.MIN is less than the first predetermined percentage ΔS.sub.MAX1 at step 430, the variable transmission control procedure 400 exits without the controller 230 transmitting a digital message to the dimmer switch 110. In other words, the controller 230 has determined that the total light intensity L.sub.T-SNSR has not changed significantly enough to merit a transmission of a digital message. For example, the first predetermined percentage ΔS.sub.MAX1 may be approximately 15%, but may alternatively range from approximately 1% to 20%.

(54) If the minimum sample adjustment percentage ΔS.sub.MIN is greater than or equal to the first predetermined percentage ΔS.sub.MAX1 at step 430, the controller 230 sets the previous minimum sample S.sub.MIN-PRV equal to the present minimum sample S.sub.MIN-PRS at step 432. The controller 230 then loads a digital message including a value representative of the total light intensity L.sub.T-SNSR as measured by the daylight sensor 120 (e.g., in foot-candles) in a transmit (TX) buffer at step 434, before the variable transmission control procedure 400 exits. For example, the controller 230 may include the minimum present minimum sample S.sub.MIN-PRS in the digital message loaded into the TX buffer. The controller 230 will transmit the digital message to the dimmer switch 110 via the RF signals 106 using a transmit procedure (not shown). An example of a transmit procedure is described in previously-referenced U.S. patent application Ser. No. 12/203,518.

(55) FIG. 9 is a simplified flowchart of a receive procedure 500 executed by the controller 214 of the dimmer switch 110 when a digital message is received from the daylight sensor 120 at step 510. As previously mentioned, the dimmer switch 110 adjusts the present light intensity L.sub.PRES of the lighting load 104 so as to control the total light intensity L.sub.T-TASK on the task surface towards the target total task surface light intensity L.sub.TRGT-TASK. Specifically, the dimmer switch 110 uses a present dimming percentage d.sub.PRES to control the present light intensity L.sub.PRES of the lighting load 104. The present dimming percentage d.sub.PRES is calculated in response to the received digital messages and a target task surface light intensity value L.sub.TRGT-TASK during the receive procedure 500. For example, the present dimming percentage d.sub.PRES may be a number between zero and one. The controller 214 may apply the present dimming percentage d.sub.PRES to different dimming curves depending upon the load type of the lighting load 104 (i.e., incandescent, fluorescent, etc.) to determine the actual new present light intensity L.sub.PRES of the lighting load.

(56) Referring to FIG. 9, if the received digital message does not include a light intensity value received from the daylight sensor 120 at step 512, the controller 214 processes the digital message appropriately at step 514 and the receive procedure 500 exits. For example, the digital message may comprise a calibration message transmitted during a calibration procedure of the daylight sensor 120. However, if the received digital message includes a light intensity value at step 512, the controller 214 stores the total light intensity L.sub.T-SNSR as measured by the daylight sensor 120 (and as received in the digital message) in the memory 248 at step 516. As noted above, the value representative of the total light intensity L.sub.T-SNSR in the received digital message may be equal to the minimum present minimum sample S.sub.MIN-PRS from the variable transmission control procedure 400 executed by the controller 230 of the daylight sensor 120 (i.e., L.sub.T-SNSR=S.sub.MIN-PRS).

(57) At step 518, the controller 214 calculates the light intensity L.sub.E-SNSR measured by the daylight sensor 120 from only the lighting load 104 using the electric light gain G.sub.E, i.e.,

(58) $\begin{array}{cc}{L}_{E-\mathrm{SNSR}}=\frac{d_{\mathrm{PRES}}.Math.L_{\mathrm{EM}-\mathrm{TASK}}}{G_E},& \left(\mathrm{Equation}2\right)\end{array}$
where L.sub.EM-TASK represents the light intensity on the task surface 136 from only the lighting load 104 when the lighting load is at the maximum light intensity. For example, the controller 214 may set the light intensity L.sub.EM-TASK from Equation 2 equal to the light intensity L.sub.E-TASK on the task surface from only the lighting load 104 (from the gain calibration procedure), or to a predetermined value, such as, fifty foot-candles. At step 520, the controller 214 calculates the light intensity L.sub.D-SNSR at the daylight sensor 120 from only natural light by subtracting the light intensity L.sub.E-SNSR at the daylight sensor from only the lighting load 104 (as calculated at step 518) from the total light intensity L.sub.T-SNSR measured by the daylight sensor (as received in the digital message), i.e.,
L.sub.D-SNSR=L.sub.T-SNSR−L.sub.E-SNSR.  (Equation 3)
At step 522, the controller 214 calculates the light intensity L.sub.D-TASK on the task surface from only daylight by multiplying the light intensity L.sub.D-SNSR at the daylight sensor 120 from only daylight by the daylight gain G.sub.D, i.e.,
L.sub.D-TASK=G.sub.D.Math.L.sub.D-SNSR.  (Equation 4)
At step 524, the controller 214 calculates the new present dimming percentage d.sub.PRES as a function of the target total task surface light intensity L.sub.TRGT-TASK, the light intensity L.sub.D-TASK on the task surface from only daylight, and the light intensity L.sub.EM-TASK on the task surface 136 from only the lighting load 104 when the lighting load is at the maximum light intensity, i.e.,

(59) $\begin{array}{cc}{d}_{\mathrm{PRES}}=\frac{L_{\mathrm{TRGT}-\mathrm{TASK}}-L_{D-\mathrm{TASK}}}{L_{\mathrm{EM}-\mathrm{TASK}}}.& \left(\mathrm{Equation}5\right)\end{array}$
Finally, the controller 214 controls the lighting load 104 according to the new present dimming percentage d.sub.PRES, before the receive procedure 500 exits.

(60) FIG. 10A is a simplified flowchart of a laser pointer interrupt procedure 600 executed by the controller 230 in response to the laser pointer circuit 244 detecting light from a laser pointer when the controller is in the sleep mode. Specifically, when the laser pointer control signal V.sub.LASER is pulled high towards the battery voltage V.sub.BATT at step 610, the controller 230 first enables the RF transceiver 236 via the RF enable control signal V.sub.RF_ENABLE at step 612. The controller 230 then waits until a digital message is received at step 614 or a timeout expires at step 616. If a digital message is received at step 614 (e.g., from the dimmer switch 110), the controller 230 stores the received digital message in a receive (RX) buffer at step 618, such that the controller 230 may process the received digital message at a later time. The controller 230 then disables the RF transceiver 236 via the RF enable control signal V.sub.RF_ENABLE at step 620, and the laser pointer interrupt procedure 600 exits. If the timeout expires at step 616 before a digital message is received at step 614, the controller 230 simply disables the RF transceiver 236 at step 620, before the laser pointer interrupt procedure 600 exits.

(61) FIG. 10B is a diagram of an example test setup 650 and FIG. 10C is a plot of an example test waveform 670 for the daylight sensor 120 of the first embodiment. The test setup 650 comprises test box 652 having a first compartment in which a test lighting load 604 is located and a second compartment in which the daylight sensor 120 and a light meter 654 are located. The first and second compartments of the test box 652 are separated by a light diffuser 656. The test setup 650 further comprises a personal computer (PC) 658, which coupled to the daylight sensor 120 and the light meter 654 via serial connections 662. The PC is operable to adjust the intensity of the lighting load 604 using a dimmer circuit 660 according to the test waveform 670 and to observe the transmission rate of the daylight sensor 120 as well as the actual light intensity as measured by the light meter 654. The test waveform 670 controls the intensity of the test lighting load 604 linearly from a minimum test intensity L.sub.T-MIN to a maximum test intensity L.sub.T-MAX and has a length T.sub.TEST, such that the test waveform 670 has a slope m.sub.TEST, i.e., m.sub.TEST=(L.sub.T-MAX−L.sub.T-MIN)/T.sub.TEST. If the test waveform 670 has a first slope, the rate of transmission of the daylight sensor 120 of the first embodiment will remain constant at a first rate. For example, the minimum test intensity L.sub.T-MIN may be zero foot-candles, the maximum test intensity L.sub.T-MAX may be 50 foot-candles, and the length T.sub.TEST may be two hours. If the test waveform 670 is altered to have a second slope less than the first slope (e.g., if the length T.sub.TEST is increased to approximately three hours), the rate of transmission of the daylight sensor 120 will change to a second rate less than the first rate.

(62) According to a second embodiment of the present invention, the controller 230 uses a linear least-squares prediction model to determine the predicted light intensity values. Specifically, the controller 230 is operable to perform a linear least-squares fit on the measured light intensity values from a present time interval to determine a slope m and an offset b of a line (i.e., y=mx+b) that best represents the change in the measured light intensity values with respect to time. The controller 230 uses these estimators (i.e., the slope m and the offset b) to determine the predicted light intensity values for one or more of the subsequent time intervals. The controller 230 then determines a mean-square error e between the measured light intensity values and the predicted light intensity values.

(63) FIG. 11 is a simplified flowchart of a variable transmission control procedure 700 executed by the controller 230 of the daylight sensor 120 according to the second embodiment of the present invention. As in the first embodiment, the controller 230 executes the variable transmission control procedure 700 of the second embodiment periodically (e.g., approximately once every second) during normal operation to sample the total light intensity control signal V.sub.TOT and to collect the predetermined number N.sub.SMPL of samples during each of the consecutive non-overlapping time intervals. The controller 230 closes the controllable switch 235 to enable the photosensitive circuit 231 at step 710, and waits for the time period T.sub.PD (i.e., 50 msec) at step 712 to allow the photosensitive diode current I.sub.PD to become representative of the total light intensity L.sub.T-SNSR. The controller 230 then samples the total light intensity control signal V.sub.TOT (to generate a new total light intensity sample S.sub.TOT) at step 714, and opens the controllable switch 235 to disable the photosensitive circuit 231 at step 716. The controller 230 increments a variable n by one at step 718 and stores the new total light intensity sample S.sub.T-SNSR as sample S[n] in the memory 246 at step 720. If the controller 230 has not yet collected the predetermined number N.sub.SMPL of samples during the present time interval at step 722, the variable transmission control procedure 700 simply exits without processing the samples S[n] stored in the memory 246.

(64) When the controller 230 has collected the predetermined number N.sub.SMPL of samples during the present time interval at step 722, the controller 230 processes the samples S[n] stored in the memory 246 to determine if a digital message should be transmitted to the dimmer switch 110. The controller 230 first increments a variable q at step 724. The controller 230 uses the variable q to keep track of how many time intervals have occurred after the time interval in which the estimators were last calculated. The controller 230 then calculates the predicted light intensity values at step 726 using the estimators (i.e., the slope m and the offset b) from a previous time interval, i.e.,
P[i]=m.Math.i+b,  (Equation 6)

(65) for i=q.Math.T.sub.WIN+1 to 2q.Math.T.sub.WIN.

(66) At step 728, the controller 230 determines the mean-square error e between the measured light intensity values and the predicted light intensity values, i.e.,
e=(1/N.sub.MAX).Math.Σ(S[i]−P[i]).sup.2,  (Equation 7)

(67) for i=q*T.sub.WIN+1 to 2q*T.sub.WIN.

(68) If the mean-square error e is less than a predetermined maximum error e.sub.MAX (e.g., approximately 15%) at step 730, the variable transmission control procedure 700 exits without transmitting a digital message to the dimmer switch 110.

(69) However, if the mean-square error e is greater than or equal to the predetermined maximum error e.sub.MAX at step 730, the controller 230 then determines the new estimators at step 732 by performing a linear least-squares fit on the measured light intensities from the present time interval to thus determine the slope m and the offset b of the line that best represents the measured light intensities from the present time interval. The controller 230 loads a digital message including one or more values representative of the total light intensity L.sub.T-SNSR in the TX buffer at step 734. For example, the controller 230 may include the estimators (i.e., the slope m and the offset b) determined at step 732 in the digital message. Since the slope m and the offset b determined at step 732 represent the measured intensity values from the present time interval, the predicted intensity values determined in the next subsequent time interval will begin at time T.sub.WIN, which is equal to the predetermined number N.sub.SMPL of samples per interval. Therefore, the controller 230 resets the variable n to N.sub.SMPL and the variable q to one at step 736, before the variable transmission control procedure 700 exits.

(70) Since both the slope m and the offset b as determined by the daylight sensor 120 are transmitted to the dimmer switch 110, the dimmer switch is operable to continuously re-calculate (i.e., estimate) the total light intensity L.sub.T-SNSR as a function of time, and to adjust the present light intensity L.sub.PRES of the lighting load 104 in response to the estimated total light intensity L.sub.T-SNSR. FIG. 12 is a simplified flowchart of a receive procedure 800 executed by the controller 214 of the dimmer switch 110 when a digital message is received from the daylight sensor 120 at step 810 according to the second embodiment of the present invention. If the received digital message includes light intensity values received from the daylight sensor 120 at step 812, the controller 214 stores the slope m and the offset b from the received digital message in the memory 216 at step 814. The controller 214 calculates the total light intensity L.sub.T-SNSR as measured the daylight sensor 120 at step 816 using the slope m and the offset b from the received digital message, as well as the predetermined period T.sub.WIN of each interval, i.e.,
L.sub.T-SNSR=m.Math.T.sub.WIN+b.  (Equation 8)
The controller 214 then stores the calculated total light intensity L.sub.T-SNSR in the memory 216 at step 818, before the receive procedure 800 exits. If the received digital message does not include light intensity values received from the daylight sensor 120 at step 812, the controller 214 processes the digital message appropriately at step 820 and the receive procedure 800 exits.

(71) FIG. 13 is a simplified flowchart of a load control procedure 900 executed by the controller 214 of the dimmer switch 110 periodically according to an adjustment period T.sub.ADJ (e.g., one second), such that the load control procedure 900 is executed once per second. The controller 214 first updates the total light intensity L.sub.T-SNSR (with respect to time) at step 910 using the slope m stored in the memory 216, i.e.,
L.sub.T-SNSR=L.sub.T-SNSR+m.Math.T.sub.ADJ.  (Equation 9)
The controller 214 then determines the new present dimming percentage d.sub.PRES for the lighting load 104 in a similar manner as in the receive procedure 500 of the first embodiment. Specifically, the controller 214 calculates the light intensity L.sub.E-SNSR measured by the daylight sensor 120 from only the lighting load 104 at step 912, calculates the light intensity L.sub.D-SNSR at the daylight sensor 120 from only natural light at step 914, calculates the light intensity L.sub.D-TASK on the task surface from only daylight at step 916, and calculates the new present dimming percentage d.sub.PRES at step 918. The controller 214 then finally controls the lighting load 104 according to the new present dimming percentage d.sub.PRES at step 920, before the load control procedure 900 exits.

(72) According to a third embodiment of the present invention, the controller 230 uses a parabolic model to determine the predicted light intensity values. In other words, the controller 230 is operable to perform a parabolic least-squares fit on the measured light intensity values from a present time interval to fit measured light intensity values to a parabola (i.e., y=ax.sup.2+bx+c) that best represents the change in the measured light intensity values with respect to time. The controller 230 uses these estimators (i.e., the coefficients a, b, c of the parabola) to determine the predicted light intensity values for one or more of the subsequent time intervals. The controller 230 then determines a mean-square error e between the measured light intensity values and the predicted light intensity values.

(73) FIG. 14 is a simplified flowchart of a variable transmission control procedure 1000 executed by the controller 230 of the daylight sensor 120 periodically (e.g., approximately once every second) according to the third embodiment of the present invention. The variable transmission control procedure 1000 is very similar to the variable transmission control procedure 700 of the second embodiment. However, the controller 230 calculates the predicted light intensity values at step 1026 using the coefficients a, b, c (i.e., the estimators) and the parabola equation, i.e.,
P[i]=ai.sup.2+bi+c,  (Equation 10)

(74) for i=q.Math.T.sub.WIN+1 to 2q.Math.T.sub.WIN.

(75) At step 1028, the controller 230 determines the mean-square error e between the measured light intensity values and the predicted light intensity values. If the mean-square error e is greater than or equal to the predetermined maximum error e.sub.MAX at step 1030, the controller 230 determines the new estimators at step 1032 by performing a parabolic least-squares fit on the measured light intensities from the present time interval to thus determine the coefficients a, b, c of the parabola that best represent the measured light intensities from the present time interval. The controller 230 then loads a digital message including one or more values representative of the total light intensity L.sub.T-SNSR in the TX buffer at step 1034, e.g., the estimators (i.e., the coefficients a, b, c of the parabola) determined at step 1032. Accordingly, the dimmer switch 110 will execute a receive procedure (not shown) similar to the receive procedure 800 of the second embodiment in order to calculate the total light intensity L.sub.T-SNSR as measured by the daylight sensor 120 using the coefficients a, b, c. In addition, the dimmer switch 110 will periodically adjust the present light intensity L.sub.PRES of the lighting load 104 using a load control procedure (not shown) similar to the load control procedure 900 of the second embodiment.

(76) According to another alternative embodiment of the present invention, the controller 230 of the daylight sensor 120 could use a linear predictor to determine the predicted light intensity values. For example, the predicted light intensity values may be calculated using the equation:
P[i]=Σ(α.sub.i.Math.x[n-i])  (Equation 11)

(77) for i=1 to K,

(78) where x[n-i] are the previous measured light intensity values, α.sub.i are the predictor coefficients, and K is the maximum number of values used to calculate the predicted light intensity.

(79) According to a fourth embodiment of the present invention, the daylight sensor 120 does not transmit digital messages in response to the measured total light intensity L.sub.T-SNSR if the measured data is “misbehaving” so as to reduce the transmission rate and further conserve battery life. For example, the daylight sensor 120 may ignore fluctuations in the measured total light intensity L.sub.T-SNSR that are large in magnitude and short in time duration (i.e., during intermittent-cloudy days as shown in FIG. 3), such that the variable transmission rate of the daylight sensor is also dependent upon the rate of change of the total light intensity L.sub.T-SNSR measured by the daylight sensor (i.e., the “dynamic” change in the total light intensity). Specifically, the daylight sensor 120 does not transmit digital messages to the dimmer switch 110 if the total light intensity L.sub.T-SNSR has changed by more than a second predetermined percentage ΔS.sub.MAX2 during the predetermined time period T.sub.WIN. Accordingly, the variable transmission rate of the daylight sensor 120 of the fourth embodiment of the present invention results in the average time between transmissions by the daylight sensor during the course of a day being greater than approximately 420 seconds (as determined by experimental study).

(80) FIG. 15 is a simplified flowchart of a transmission algorithm 1100 executed by the controller 230 of the daylight sensor 120 according to the fourth embodiment of the present invention, such that the daylight sensor 120 transmits digital messages using the variable transmission rate. The transmission algorithm 1100 of the fourth embodiment is similar to the transmission algorithm 300 of the first, second, and third embodiments (as shown in FIG. 7). The controller 230 first measures and stores the predetermined number N.sub.SMPL of new total light intensity values at steps 310 and 312. Next, the controller 230 determines the predicted light intensity value(s) at step 314 using, for example, any of the estimators described with reference to the first through third embodiments, and calculates the error between the measured total light intensity values and the predicted total light intensity values at step 316.

(81) However, according to the fourth embodiment, the controller 230 further analyzes the measured total light intensity values if the error calculated at step 316 is outside of the predetermined limits (i.e., is too great) at step 318. Specifically, the controller 230 using the measured total light intensity values to calculate a data behavior metric at step 1124, compares the calculated data behavior metric to predetermined data behavior metric limit(s) at step 1126, and determines if the data is misbehaving at step 1128, i.e., is outside of the data behavior metric limit(s). For example, the controller 230 may analyze the total light intensity values to determine if the rate of change of the total light intensity L.sub.T-SNSR measured by the daylight sensor 120 is too great. If the data is not misbehaving at step 1128, the controller 230 calculates the new estimator(s) for use during the subsequent time interval at step 320 and transmits a digital message including one or more values representative of the total light intensity L.sub.T-SNSR as measured by the daylight sensor 120 to the dimmer switch 110 at step 322, before the transmission algorithm 1100 loops around. If the data is misbehaving at step 1128, the controller 230 does not calculate the new estimator(s) at step 320 and does not transmit the values representative of the total light intensity L.sub.T-SNSR at step 324, but simply analyzes the next non-overlapping time interval.

(82) FIG. 16A is a simplified flowchart of a variable transmission control procedure 1200 executed by the controller 230 of the daylight sensor 120 periodically (e.g., approximately once every second) according to the fourth embodiment of the present invention. The variable transmission control procedure 1200 of the fourth embodiment is very similar to the variable transmission control procedure 400 of the first embodiment (as shown in FIG. 8). According to the fourth embodiment of the present invention, the controller 230 uses a single data point as the estimator (as in the first embodiment). However, the controller 230 could alternatively use a linear prediction model or a parabolic prediction model to determine the estimators as described above with reference to the second and third embodiments respectively.

(83) Referring to FIG. 16A, if the minimum sample adjustment percentage ΔS.sub.MIN is greater than or equal to the first predetermined percentage ΔS.sub.MAX1 at step 430, the controller 230 determines if the data (i.e., the samples S[n] stored in the memory 246) is misbehaving by determining if the total light intensity L.sub.T-SNSR has changed by more than the second predetermined percentage ΔS.sub.MAX2 during the present time period T.sub.WIN. Specifically, the controller 230 determines a present maximum sample S.sub.MAX-PRS of the samples S[n] stored in the memory 246 (i.e., samples S[0] through S[N.sub.SMPL]) at step 1236. The controller 230 then calculates a present sample adjustment amount ΔS.sub.PRS, which is representative of the rate of change of the total light intensity L.sub.T-SNSR, at step 1238 using the equation:

(84) $\begin{array}{cc}\Delta S_{\mathrm{PRS}}=\frac{S_{\mathrm{MAX}-\mathrm{PRS}}-S_{\mathrm{MIN}-\mathrm{PRS}}}{S_{\mathrm{MAX}-\mathrm{PRS}}}.& \left(\mathrm{Equation}12\right)\end{array}$
If the present sample adjustment amount ΔS.sub.PRS is greater than or equal to the second predetermined percentage ΔS.sub.MAX2 at step 1240, the variable transmission control procedure 1200 exits without transmitting a digital message to the dimmer switch 110. For example, the second predetermined percentage ΔS.sub.MAX2 may be approximately 10%, but may alternatively range from approximately 5% to 25%.

(85) However, if the present sample adjustment amount ΔS.sub.PRS is less than the second predetermined percentage ΔS.sub.MAX2 at step 1240, the controller 230 sets the previous minimum sample S.sub.MIN-PRV equal to the present minimum sample S.sub.MIN-PRS at step 432. The controller 230 then loads a digital message including a value representative of the total light intensity L.sub.T-SNSR as measured by the daylight sensor 120 (i.e., the minimum present minimum sample S.sub.MIN-PRS) in a transmit (TX) buffer at step 434, before the variable transmission control procedure 1200 exits.

(86) FIG. 16B is a plot of an example test waveform 1250 for the daylight sensor 120 of the fourth embodiment to be used in the test setup 650 shown in FIG. 10B. The test waveform 1250 comprises a pulsed waveform added on top of a linear ramp waveform and has peaks and valleys, such that the text waveform models the total light intensity L.sub.T-SNSR as measured by the daylight sensor 120 on an intermittent-cloudy day. The test waveform 1250 has a minimum light intensity (during the valleys) that increases with respect to time at a first slope m.sub.1, and a maximum light intensity (during the peaks) that increases with respect to time at a second slope m.sub.2. Each of the peaks (during which the text waveform 1250 is at the maximum light intensity) have a length T.sub.PULSE, which may be approximately five seconds. The magnitude of the test waveform 1250 during the valleys is approximately 15% of the magnitude of the test waveform during the peaks. When the test waveform 1250 is used in the test setup 650 to control the test lighting load 604, the daylight sensor 120 of the fourth embodiment will not transmit digital messages in response to the temporary excursions of the light intensity during the peaks. Accordingly, the rate of transmission of the daylight sensor 120 of the fourth embodiment will remain constant at a rate determined by the slope m.sub.1 of the valleys.

(87) As described above, the controller 230 of the daylight sensor 120 of the first, second, third, and fourth embodiments collects the predetermined number N.sub.SMPL of measurements of the total light intensity L.sub.T-SNSR during consecutive non-overlapping time intervals, and only analyzes the measurements at the end of each time interval (i.e., as determined by the predetermined time period T.sub.WIN). Alternatively, the controller 230 could analyze the measurements of the total light intensity L.sub.T-SNSR in a sliding window time interval. Specifically, the controller 230 could store each new measurement of the total light intensity L.sub.T-SNSR in a first-in, first-out (FIFO) register (e.g., having a size equal to the predetermined number N.sub.SMPL of measurements). The controller 230 could then analyze the data stored in the FIFO registered each time that the controller samples the total light intensity control signal V.sub.TOT.

(88) In addition, the controller 230 of the daylight sensor 120 transmits digital messages including one or more values representative of the measured total light intensity L.sub.T-SNSR according to the first, second, third, and fourth embodiments. According to a fifth embodiment of the present invention, each digital message transmitted by the daylight sensor 120 to the dimmer switch 110 may alternatively comprise a command, such as a specific new light intensity L.sub.NEW for the lighting load 104. The controller 230 of the daylight sensor 120 determines the new intensity levels L.sub.NEW in response to the measured total light intensity L.sub.T-SNSR. The dimmer switch 110 controls the present light intensity L.sub.PRES of the lighting load 104 to the new light intensity L.sub.NEW in response to receiving a digital message with a command from the daylight sensor 120.

(89) According to the fifth embodiment, each time the controller 230 of the daylight sensor 120 samples the total light intensity control signal V.sub.TOT, the controller 230 calculates a new dimming percentage d.sub.NEW, which may be transmitted to the dimmer switch 110. As in the previous embodiments, the new dimming percentage d.sub.NEW may be a number between zero and one, which is representative of the new light intensity L.sub.NEW for the lighting load 104. The controller 214 of the dimmer switch 110 is operable to determine the light intensity L.sub.NEW from the new dimming percentage d.sub.NEW received from the daylight sensor 120, for example, by applying the new dimming percentage d.sub.NEW to different dimming curves depending upon the load type of the lighting load. The controller 230 of the daylight sensor 120 only transmits digital messages to the dimmer switch 110 when the new dimming percentage d.sub.NEW is outside a deadband, i.e., only when a change to the present light intensity L.sub.PRES of the lighting load 104 is required. Accordingly, the daylight sensor 120 only transmits digital messages to the dimmer switch 110 using a variable transmission rate that is dependent upon the measured total light intensity L.sub.T-SNSR.

(90) In addition, the controller 230 may also store a historical record of the total light intensity L.sub.T-SNSR as measured by the daylight sensor 120 each time the controller samples the total light intensity control signal V.sub.TOT. The controller 230 is operable to determine when it is daytime and nighttime in response to the total light intensity control signal V.sub.TOT and the historical record stored in the memory 246. The controller 230 may increase the length of the sampling period T.sub.SMPL (e.g., to approximately three seconds) during the nighttime, such that the controller samples the total light intensity control signal V.sub.TOT less frequently and consumes even less power.

(91) FIG. 17 is a simplified flowchart of a control procedure 1300 executed periodically (e.g., every one to three seconds) by the controller 230 of the daylight sensor 120 according to the fifth embodiment of the present invention. At step 1310, the controller 230 enables the photosensitive circuit 231 by closing the switch 235 using the photosensitive circuit enable control signal V.sub.PS_ENABLE. The controller 230 waits for the time period T.sub.PD (i.e., 50 msec) at step 1312 to allow the photosensitive diode current I.sub.PD to become representative of the total light intensity L.sub.T-SNSR at the daylight sensor 120. The controller 230 then samples the total light intensity control signal V.sub.TOT (using the ADC) to generate a new total light intensity sample S.sub.TOT at step 1314, and disables the photosensitive circuit 231 by opening the switch 235 using the photosensitive circuit enable control signal V.sub.PS_ENABLE at step 1316. At step 1318, the total light intensity sample S.sub.TOT is applied to a digital filter (such as a linear predictor) to generate a filtered total light intensity sample FS.sub.TOT.

(92) The controller 230 is operable to periodically store the filtered total light intensity samples FS.sub.TOT (e.g., every 30 minutes) to create the historical record in the memory 246 of the total light intensity L.sub.T-SNSR at the daylight sensor 120. Specifically, if the controller 230 should store the present filtered total light intensity sample FS.sub.TOT at step 1320, the controller stores the present filtered total light intensity sample FS.sub.TOT in the memory 246 at step 1322.

(93) Next, the controller 230 uses the filtered total light intensity sample FS.sub.TOT and a present dimming percentage d.sub.PRES to determine the new dimming percentage d.sub.NEW for the lighting load 104 using similar calculations as the receive procedure 500 of the first embodiment. Specifically, the controller 230 calculates the light intensity L.sub.E-SNSR measured by the daylight sensor 120 from only the lighting load 104 at step 1324, calculates the light intensity L.sub.D-SNSR at the daylight sensor 120 from only natural light at step 1326, calculates the light intensity L.sub.D-TASK on the task surface from only daylight at step 1328, and calculates the new dimming percentage d.sub.NEW at step 1330.

(94) At step 1332, the controller 230 determines if the new dimming percentage d.sub.NEW is outside of a deadband, e.g.,
d.sub.PRESΔ<d.sub.NEW<d.sub.PRES+Δ,  (Equation 13)
where Δ represents a predetermined increment by which the new dimmer percentage d.sub.NEW must differ from the present dimming percentage d.sub.PRES before the daylight sensor 120 will transmit a digital message to the dimmer switch 110 causing the dimmer switch to adjust the intensity of the lighting load 104 to the new intensity L.sub.NEW. For example, the predetermined increment Δ may be approximately 1%. If the new dimming percentage d.sub.NEW is within the deadband at step 1332, the control procedure 1300 simply exits. However, if the new dimming percentage d.sub.NEW is outside the deadband at step 1332, the controller 230 stores the new dimming percentage d.sub.NEW as the present dimming percentage d.sub.PRES at step 1334. The controller 230 loads a digital message (including a command to control the intensity of the lighting load 104 according to the new dimming percentage d.sub.NEW) into a transmit (TX) buffer at step 1336, before the control procedure 1300 exits.

(95) A lighting control systems including wired daylight sensors (i.e., wired photosensors) is described in greater detail in U.S. Pat. No. 7,369,060, issued May 6, 2008, entitled DISTRIBUTED INTELLIGENCE BALLAST SYSTEM AND EXTENDED LIGHTING CONTROL PROTOCOL, the entire disclosures of which is hereby incorporated by reference.

(96) While the present invention has been described with reference to the dimmer switch 110 for controlling the intensity of the lighting load 104, the concepts of the present invention could be applied to load control systems comprising other types of load control devices, such as, for example, fan-speed controls for fan motors, electronic dimming ballasts for fluorescent loads, and drivers for light-emitting diodes (LEDs). Further, the concepts of the present invention could be used to control other types of electrical loads, such as, for example, fan motors or motorized window treatments.

(97) Although the present invention has been described in relation to particular embodiments thereof, many other variations and modifications and other uses will become apparent to those skilled in the art. It is preferred, therefore, that the present invention be limited not by the specific disclosure herein, but only by the appended claims.