Light detector employing trapezoidal chips

Abstract

A position sensor employing silicon photodiodes formed from trapezoidal chips mounted on a printed circuit board detects angular positions of a rotor shaft within a galvanometer-based optical scanner.

Claims

1. A light detector comprising a trapezoidal chip having an arcuate light sensor positioned thereon.

2. A light detector comprising segmented light sensors disposed in pairs, each pair comprising one A detector element and one B detector element, at least one of the segmented light sensors positioned on a trapezoidal chip.

3. The light detector according to claim 2, wherein the pairs are disposed so that each A detector element is positioned between two B detector elements and each B detector element is positioned between two A detector elements.

4. The light detector according to claim 2, further comprising a circuit operable with a signal connection for measuring signals from the A detector elements and the B detector elements relating to an amount of light impinging thereon.

5. The light detector according to claim 2, wherein the segmented light sensors are generally defined within the same plane.

6. The light detector according to claim 2, wherein each of the segmented light sensors comprises an arcuate sector shape.

7. The light detector according to claim 2, wherein each of the segmented light sensors comprises a substantially toroidal shape.

8. The light detector according to claim 2, wherein the segmented light sensors comprises four light sensors.

9. The light detector according to claim 2, wherein the segmented light sensors comprise a light sensor material responsive to light, and wherein a linear increase in light per unit area impinging thereon causes a substantially linear increase in output signal.

10. The light detector according to claim 2, wherein the trapezoidal chip defines a first edge and a second edge, wherein an angle between the first edge and the second edge is about 120 degrees.

11. The light detector according to claim 2, wherein the trapezoidal chip defines a first edge and a second edge, wherein an angle between the first edge and the second edge is about 117 degrees.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Embodiments of the invention are described by way of example with reference to the accompanying drawings in which:

(2) FIG. 1 is a perspective view of a rotary position detector operable with silicon photodiodes on a PCB according to the teachings of the present disclosure;

(3) FIG. 2 is a diagrammatical plan view of one embodiment of a light detector comprising a PCB-mounted trapezoidal chip having a photodiode light sensor according to the teachings of the present disclosure;

(4) FIG. 3 is a perspective view of the embodiment of FIG. 2;

(5) FIGS. 4A, 4B, and 4C are end, plan and perspective views, respectively, of a trapezoidal chip having a photodiode light sensor according to the teachings of the present disclosure;

(6) FIG. 5 is a schematic circuit diagram according to the teachings of the present invention including a parallel connection of photodiodes; and

(7) FIG. 6 is an illustration of a wafer including photodiode light sensors arranged according to the teachings of the present disclosure.

DETAILED DESCRIPTION OF EMBODIMENTS

(8) Embodiments of the present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which embodiments of the invention are shown by way of illustration and example. The invention may, however, be embodied in many forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art.

(9) With reference initially to FIG. 1, an embodiment includes a rotary position detector 10 comprising a shaft 12 rotatable about an axis thereof and a light source 14. The light source 14 may comprise a unitary light source generally aligned with the axis. Certain embodiments of the rotary position detector 10 may include a housing 13, although other suitable configurations are within the scope of the present disclosure. Light rays emanating from the light source 14 may optionally be modified by a lens 15 or other light modifying or directing element. A light detector 16 is positioned in a spaced relation to the light source 14 for receiving the light rays. Embodiments may also include a motor (not shown) operable with the shaft 12.

(10) A light blocker 22 is positioned between the light detector 16 and the light source 14, the light blocker 22 having at least one element 24 rotatable with the shaft 12. The light blocker 22 may be attached to the shaft 12, and hovers over the surface of the light sensors 20, such as, for example, silicon photodiodes. The light blocker 22 may be butterfly shaped, having two opaque wings, which partially cover or uncover the photosensitive areas.

(11) As the shaft 12 rotates, the light blocker 22 will expose a greater amount or lesser amount of photosensitive area on the light detector's light sensor 20, thereby permitting a determination of the relative position of the shaft 12. One embodiment, described by way of example, includes four photosensitive areas that produce output as the light blocker 22 uncovers or covers the active areas. A signal connection 26 to the light detector 16 may also be provided for measuring the output corresponding to an amount of light impinging on the at least one light sensor 20 to thus measure a rotary position of the shaft 12.

(12) As depicted in FIGS. 2 and 3, one embodiment of the light detector 16 comprises at least one trapezoidal chip 18 mounted on a printed circuit board (PCB) 21, the at least one trapezoidal chip having at least one light sensor 20 thereon for receiving light rays emitted from the light source 14. As used herein, the term trapezoidal excludes a rectangle or square. The trapezoidal chip 18 defines at least a first edge 19a and a second edge 19b. As depicted in FIGS. 4A-4C, an angle between the first edge 19a and the second edge 19b is approximately 120 degrees. In one embodiment, the angle between the first edge 19a and the second edge 19b is approximately 117 degrees. Other suitable angles resulting in trapezoidal chips are considered to be within the scope of the present disclosure.

(13) The PCB 21 is used both to mount the silicon sensors thereon, and to facilitate motor and LED connection. The PCB 21 defines an aperture 17 through which the shaft 12 extends. The PCB 21 may be a four-layer circuit board, to permit the connections that are needed for the silicon sensor chips and a motor. By way of non-limiting example, the circuit board thickness can be between 0.050 inches and 0.065 inches, and made from standard PCB material such as FR4. In one manufactured unit, 0.040 inches was used. To facilitate desirable quality connections, any exposed pads on the circuit board are preferably gold (whether immersion gold or nickel-gold) or of a material having equivalent beneficial features. Again by way of non-limiting example, a solder mask used on the circuit board is a Black Matte finish, so that any stray light produced by the LED is effectively absorbed by the solder mask. A legend or silk screen layer for the PCB is optional.

(14) With continued reference to FIGS. 2 and 3, an embodiment of a light detector 16 includes a first number of segmented light sensors 20 mounted to a printed circuit board (PCB) 21 and disposed in pairs about an axis, each pair comprising one A detector element 28 and one B detector element 30, the pairs disposed so that each A detector element 28 is circumferentially positioned between two B detector elements 30 and each B detector element 30 is positioned between two A detector elements 28. Counterclockwise sensors are herein referred to as Sensor A sensors and clockwise sensors are referred to as Sensor B sensors. In one embodiment, the first number of segmented light sensors 20 comprises four light sensors. The PCB 21 may electrically connect both of the A sensors 28 together (in parallel) and separately, both of the B sensors 30 together (in parallel), ultimately delivering only an A output and a B output along with a common output.

(15) The overall position sensor and photodiodes are used in a Photo Voltaic mode. Essentially the A sensors 28 are amplified with a simple inverting op-amp, and B sensors 30 are amplified with a separate op-amp. The optical sensor thus provides a differential output, which works well for measuring the rotational position of the shaft 12. It is of interest to note that Photoconductive mode may also be used with proper biasing, so although the implementation herein described by way of example favors use of the photovoltaic mode, this is not intended to be a limitation.

(16) In one embodiment (not shown), the light blocker 22 includes a second number of opaque, substantially equal-surface-area elements 24 rotatable with the shaft 12, the second number equal to one-half of the first number, wherein a radial extent of the light blocker elements 24 is at least equal to a radial extent of the light sensors 20. An example of this type of rotary position detector configuration is depicted and described in U.S. Pat. No. 8,508,726, the entirety of which is incorporated by reference. Another example of a rotary position detector is depicted and described in U.S. Pat. No. 7,688,432, the contents of which are also incorporated by reference. The light blocker 22 may comprise a plurality of openings, each opening positioned between adjacent light blocker elements 24. In certain exemplary embodiments utilizing the trapezoidal chips as herein described, the angular subtense of each of the light blocker openings is at least as great as the angular subtense of the light sensors 20.

(17) Embodiments may further comprise a circuit operable with the signal connection 26 for measuring signals from the A detectors 28 and the B detectors 30 relating to an amount of light impinging thereon, wherein a difference between the A detector signal and the B detector signal is related to an angular position of the shaft 12. Although the embodiments disclosed herein all show a single active area on each chip, such is not intended to be a limitation, and it is possible that each chip may have more than one active area. For example, it is possible that each chip may have both Sensor A and Sensor B on a single chip. This would reduce the amount of rotation that could be sensed, but could be desirable for certain applications.

(18) In the embodiments depicted in FIGS. 1-3, the segmented light sensors 20 are generally defined within a plane perpendicular to the axis. In other embodiments, the light sensors may be parallel to the axis. The segmented light sensors 20 may have an arcuate sector shape. In another embodiment, the arcuate sector shape comprises a substantially toroidal shape.

(19) With continued reference to FIGS. 1-3, the light sensors 20 include a light sensor material responsive to light, such as a photodiode, wherein a linear increase in light per unit area impinging thereon causes a substantially linear increase in an output signal. Over the years photodiodes have been tested whose capacitance was quite lowon the order of a few tens of picofarrad each, as well as photodiodes whose capacitance is much higheron the order of hundreds of picofarrad each. The photodiodes having low capacitance are said to be based on high ohm-centimeter silicon material such as around 2000 ohm-centimeters. On datasheets, these silicon photodiodes are claimed by manufacturers to produce higher noise in photovoltaic applications. However, in a computer analysis, it was found that capacitance really plays a dominant role in overall position sensor noise because of how the capacitance manifests itself with the signal amplifier. Therefore, a sensor material that offers low device capacitance may be desirable.

(20) Certain silicon photodiodes used in photovoltaic applications are amplified using a simple op-amp, and have a direct connection from the photodiode to the op-amp. However, according to the teachings of the present disclosure, a small resistor may be connected to the photodiode and the op-amp, as illustrated with reference to FIG. 5. The circuit diagram of FIG. 5 shows the paralleled connection between individual photodiodes (i.e. two Sensor A in parallel) as well as typical amplifier component values.

(21) The resistor between the photodiode and op-amp is placed there to isolate the photodiode capacitance (and also any cable capacitance between the photodiode and amplifier board) from the op-amp. This tends to reduce noise that would otherwise be produced by the op-amp. This works desirably well to reduce noise. However, this resistor does effectively allow the photodiode to develop a small and varying voltage across it during sensor operation. Component values are selected that limit this voltage to around 0.1 to 0.2 volts. It has been determined that if this voltage gets too high (for example around 0.5 volts or so), the linearity of the sensor is negatively impacted because eventually the diode becomes forward biased. Experiments have shown that some photodiodes will allow this voltage to rise to around 1 volt before any noticeable linearity affects come into play.

(22) With reference now to FIG. 6, embodiments of the present disclosure also include a method for manufacturing a light detector 16 operable with a rotary position detector 10, the method comprising:

(23) providing a wafer having a plurality of light sensors;

(24) cutting the wafer by performing at least one cut at a first angle counterclockwise relative to horizontal (dashed lines);

(25) cutting the wafer by performing at least one cut at a second angle clockwise relative to horizontal (dotted lines); and

(26) cutting the wafer by performing at least one horizontal cut (solid lines), wherein the cutting results in at least one trapezoidal chip having at least one light sensor;

(27) mounting the at least one trapezoidal chip on a printed circuit board; and

(28) positioning the light detector in spaced relation to a light source for assisting with a measuring of a rotary position of a shaft.

(29) In one embodiment, the first and second angles are greater than or less than about 120 degrees. In another embodiment, the first and second angles are about 117 degrees. In some embodiments, there are twice as many horizontal cuts as there are diagonal cuts.

(30) By rotating a dicing saw so that it operates at either about 117 degree or about 120 degree angles, instead of the usual 90-degree angles, it is possible to make chips that are trapezoidal instead of rectangular. During the dicing process, the chips take on a triangular shape. But after the final cut, small triangular waste pieces are eventually removed, leaving the trapezoidal chip itself. The use of trapezoidal chips permits a larger hole, or aperture 17, in the PCB through which the shaft 12 may extend.

(31) While herein describing how about 117-degree or about 120-degree cuts may be performed, this is not intended to be a limitation. Other angles may also be used to optimize wafer utilization, the amount of active area of the photosensitive material, and the size of the hole in the center of the PCB where the motor shaft comes through. As long as a trapezoidal shape results, the embodiments may be used.

(32) Moreover, when comparing the trapezoidal approach of the present invention to the prior-art approach described in U.S. Pat. No. 7,688,432 that uses five-sided chips, the prior-art approach requires the total silicon area to be much larger than would be required for the new trapezoidal approach as herein described for embodiments of the invention. The trapezoidal approach allows for silicon usage very similar to the standard prior art square-chip approach, such as that described in U.S. Pat. No. 5,844,673, but has the benefit of being able to have a larger size hole in the middle of the circuit board for a shaft to come through.

(33) To evaluate the performance of the present invention with prior art approaches discussed here, one should compare the size of the motor shaft that attaches to light blocker 22, and the hole in the circuit board through which the motor shaft must pass. The most popular United States-based manufacturer of prior-art galvanometers uses a shaft size of 0.060 inches passing through a hole in the circuit board having a diameter of 0.078 inches. A China-based galvanometer manufacturer uses a shaft size of 0.070 inches passing through a hole in the circuit board having a diameter of 0.080 inches. By comparison, galvanometers being manufactured by the inventor and using the teachings of the present disclosure use a shaft size of 0.0935 inches passing through a hole in the circuit board having a diameter of 0.112 inches. In fact the shaft size can safely be increased to around 0.100 inches.

(34) Since the stiffness of a shaft is proportional to the fourth power of its diameter, galvanometers using a shaft diameter of 0.0935 inches are 3.18 times as stiff as those having a diameter of 0.070 inches, and almost 6 times as stiff as those having a diameter of 0.060 inches (as long as the length of the shaft is the same). The stiffer shaft is highly desirable in galvanometer applications since it pushes torsional resonant frequency higher and reduces positional uncertainty.

(35) Of course it is possible to increase the diameter of the hole in the circuit board of prior art position sensors, allowing a larger shaft to pass through. However, doing so would necessitate that the dimensions of the silicon sensor chips change as well. Changing the inner dimensions of these chips (those dimensions closest to the aperture 17 in the printed circuit board 21) demands that either the sensor angle be reduced, or the active area of the sensor to be reducedlowering signal-to-noise ratio. Changing the outer dimension of the chips demands that the diameter of the light blocker 22 be increased, which increases inertia and lowers torsional and bending-mode resonances of the position sensor. Any of these changes results in a reduction in overall position sensor performance.

(36) Manufacturing a position sensor using trapezoidal chips according to the teachings of the present disclosure allows for the hole size in the circuit board to be increased dramatically, which ultimately dramatically pushes bending- and torsional-mode resonant frequencies higher, while simultaneously reducing positional uncertainty. These benefits are accrued while not sacrificing position sensing angle, signal-to-noise ratio, or increased inertia. Moreover, wafer utilization (in terms of the number of sensor chips that can be made from a single wafer) for the trapezoidal chip approach of the present invention is similar to that of the commonplace and prior-art rectangular-chip approach, thus the monetary cost of implementing the new sensor of the present invention is also similar.

(37) Although the invention has been described relative to various selected embodiments herein presented by way of example, there are numerous variations and modifications that will be readily apparent to those skilled in the art in light of the above teachings. It is to be understood that, within the scope of claims supported by this specification, the invention may be practiced other than as specifically described.