Capacitive Measuring System

Abstract

A capacitive measuring system includes capacitive sensors and an evaluation circuit having a multiplexer, a synchronous rectifier, a sinusoidal signal generator, and a reference voltage divider. The capacitive sensors are acted on by a mono-frequency voltage signal generated by the sinusoidal signal generator, output signals of the capacitive sensors are transmitted in alternation to the synchronous rectifier via the multiplexer, and signal amplification of output signals of the synchronous rectifier are calibrated as a function of an activatable reference impedance. The synchronous rectifier is formed by a MOS semiconductor switch. A source-drain section of the MOS semiconductor switch forms a shunt that is controlled by the mono-frequency voltage signal. A channel of the multiplexer is provided for transmitting a calibration signal, generated by the reference voltage divider, to the synchronous rectifier alternately with the output signals of the capacitive sensors.

Claims

1. A capacitive measuring system comprising: a plurality of capacitive sensors; an evaluation circuit having a multiplexer, a synchronous rectifier, a sinusoidal signal generator, and a reference voltage divider; wherein the capacitive sensors are acted on by a mono-frequency voltage signal generated by the sinusoidal signal generator, output signals of the capacitive sensors are transmitted in alternation to the synchronous rectifier via the multiplexer, and signal amplification of output signals of the synchronous rectifier are calibrated as a function of an activatable reference impedance; and the synchronous rectifier is formed by a MOS semiconductor switch, a source-drain section of the MOS semiconductor switch forms a shunt that is controlled by the mono-frequency voltage signal, and a channel of the multiplexer is provided for transmitting a calibration signal, generated by the reference voltage divider, to the synchronous rectifier alternately with the output signals of the capacitive sensors.

2. The capacitive measuring system of claim 1 wherein: the evaluation circuit includes another reference voltage divider; another channel of the multiplexer is provided for transmitting a different calibration signal, generated by the another reference voltage divider, to the synchronous rectifier alternately with the output signals of the capacitive sensors; wherein one of the calibration signals corresponds to an upper limit and the other one of the calibration signals corresponds to a lower limit.

3. The capacitive measuring system of claim 1 wherein: the evaluation circuit further includes a microcontroller; and the sinusoidal signal generator generates the mono-frequency voltage signal using a Chebyshev filter and three centered pulse width-modulated voltages generated by the microcontroller.

4. The capacitive measuring system of claim 1 wherein: the capacitive measuring system is provided for hands on/off detection on a steering wheel of a vehicle.

5. The capacitive measuring system of claim 1 wherein: the evaluation circuit further includes a microcontroller; and the microcontroller evaluates the output signals of the synchronous rectifier based on a comparison voltage that depends on the calibration signal.

6. The capacitive measuring system of claim 1 wherein: a gate of the MOS semiconductor switch is controlled by the mono-frequency voltage signal for the shunt to be controlled by the mono-frequency voltage signal.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Embodiments of the present invention are explained in greater detail below and illustrated with reference to the drawings, which show the following:

[0019] FIG. 1 illustrates a first block diagram of a capacitive measuring system having a steering wheel sensor system;

[0020] FIG. 2 illustrates a second block diagram of the capacitive measuring system; and

[0021] FIG. 3 illustrates a diagram of the structure of the steering wheel sensor system.

DETAILED DESCRIPTION

[0022] Detailed embodiments of the present invention are disclosed herein; however, it is to be understood that the disclosed embodiments are merely exemplary of the invention that may be embodied in various and alternative forms. The figures are not necessarily to scale; some features may be exaggerated or minimized to show details of particular components. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a representative basis for teaching one skilled in the art to variously employ the present invention.

[0023] FIG. 1 illustrates a block diagram of the functional components of a capacitive measuring system. The capacitive measuring system includes multiple capacitive sensors. The capacitive sensors are situated on a steering wheel LR of a motor vehicle. The capacitive sensors together form a steering wheel sensor system LS for recognizing contact of steering wheel LR by a human hand, particularly, a finger F.

[0024] FIG. 3 illustrates a schematic of the structural principle of steering wheel sensor system LS. Steering wheel sensor system LS is made up of an arrangement of conductors L and insulators I layered in alternation. Conductors L and insulators I are situated on or around the steering wheel rim, and optionally also on or around the spokes of steering wheel LR. Conductors L are preferably made of copper foils or metal mesh. Insulators I are preferably made of plastic films. The conductive steering wheel core is labeled Core in FIG. 3. Situated above the steering wheel core, separated by an insulator I, is a shield conductor. The shield conductor is labeled Shield in FIG. 3.

[0025] The uppermost layer of conductors L is formed by multiple adjacently situated sensor foils SF1, SF2. Sensor foils SF1, SF2, in contrast to the other conductors L, are arranged not continuously, but, rather, in multiple separate adjacent sections on steering wheel LR. Sensor foils SF1, SF2 may have different shapes and sizes. The diagram illustrated in FIG. 3 shows in particular an arrangement of two capacitive sensors S1, S2. The number of capacitive sensors may be easily increased to an intended value by adding additional sensor foils.

[0026] Capacitive sensors S1, S2 divide the shield conductor. The shield conductor is intended to shield sensor foils SF1, SF2 from the grounded metal core of steering wheel LR. For this purpose, sensor foils SF1, SF2 overlap the shield conductor.

[0027] As shown in FIG. 1, the capacitive measuring system further includes an evaluation circuit AS. Evaluation circuit AS includes a micro-controller MC, a multi-channel analog multiplexer MUX, a synchronous rectifier SG, a sinusoidal signal generation device SIN, first and second reference voltage dividers REF, and a differential amplifier DV.

[0028] Strictly by way of example, steering wheel sensor system LS illustrated in FIG. 1 is designed with six capacitive sensors (not illustrated in detail in FIG. 1) situated on steering wheel LR. A total of six connecting lines L.sub.Sensor lead individually from the capacitive sensors on steering wheel LR to evaluation circuit AS. Connecting lines L.sub.Sensor are connected to sensor foils SF1, SF2. In addition, in each case there is a connecting line L.sub.Shield to the shield conductor and a connecting line L.sub.Core to the metal core of steering wheel LR. The number of connecting lines provided in each case is indicated in FIG. 1 by the information in parentheses (6, 1, 1, for example).

[0029] In evaluation circuit AS, connecting line L.sub.Core to the metal core of steering wheel LR is connected to the vehicle ground GND via a ground resistor. Connecting line L.sub.Shield to the shield conductor is coupled to the output of sinusoidal signal generation device SIN of evaluation circuit AS. Connecting line L.sub.Shield to the shield conductor is used for coupling a mono-frequency sinusoidal signal U.sub.sin from sinusoidal signal generation device SIN into steering wheel sensor system LS.

[0030] The six sensor lines L.sub.Sensor are led in evaluation circuit AS to respective inputs of multiplexer MUX of evaluation circuit AS. Two further inputs of multiplexer MUX (illustrated here with eight channels) are occupied by the respective center connection of the reference voltage dividers REF of evaluation circuit AS.

[0031] Multiplexer MUX is controlled by microcontroller MC of evaluation circuit AS. A particularly simple and cost-efficient design of multiplexer MUX may be achieved in that multiplexer MUX is made up of a number of controllable analog switches that correspond to the intended number of channels. The analog switches of multiplexer MUX are in each case controlled in alternation in a clocked manner via a programmable universal connection GPIO (general purpose input-output) of microcontroller MC.

[0032] For EMC reasons, mono-frequency sinusoidal signal U.sub.sin having a frequency in the range of 100 kHz is used as a measuring signal for steering wheel sensor system LS situated on steering wheel LR. Three centered pulse width-modulated signals PWM, generated entirely by the hardware of microcontroller MC, are used to generate sinusoidal signal U.sub.sin. Sinusoidal signal U.sub.sin results from forming these pulse width-modulated signals PWM using a Chebyshev filter CF. The signal generation is functionally illustrated here by a circuit block, referred to as sinusoidal signal generation device SIN.

[0033] Use is made of the attenuation of mono-frequency sinusoidal signal U.sub.sin, when generating a detectable measuring signal by steering wheel sensor system LS. For this purpose, via connecting line L.sub.Shield, sinusoidal signal U.sub.sin is applied to the shield conductor, which in each case forms a capacitive voltage divider with sensor foils SF1, SF2.

[0034] An attenuated sinusoidal signal having the same frequency and phase, but with a smaller amplitude, falls at the output of the respective capacitive voltage divider, i.e., at sensor foils SF1, SF2 (i.e., at the output of the respective capacitive voltage divider, i.e., at the sensor foils SF1, SF2, each have a damped sinusoidal signal with the same frequency and phase, but with a smaller amplitude). The capacitance values of capacitive sensors S1, S2 are influenced by the presence of a hand or a finger, as the result of which the amplitudes of sensor signals S.sub.out present at sensor foils SF1, SF2 change.

[0035] At the same time, sensor signals S.sub.out outputted from capacitance sensors S1, S2 (i.e., the output signals of capacitance sensors S1, S2) are present at the inputs of multiplexer MUX and, controlled by microcontroller MC, are transmitted in succession to the output of multiplexer MUX in alternation.

[0036] Each signal MUX.sub.OUT present at the output of multiplexer MUX is relayed to the input of synchronous rectifier SG. Synchronous rectifier SG is supplied with the mono-frequency sinusoidal signal U.sub.sin as a calibration signal. Synchronous rectifier SG rectifies, on a frequency-selective basis, output signals MUX.sub.OUT of multiplexer MUX having the same frequency as the mono-frequency sinusoidal signal U.sub.sin, which is coupled into shield line L.sub.Shield. This effectively suppresses interference signals having different frequencies.

[0037] The output signal SG.sub.out of synchronous rectifier SG is amplified by a differential amplifier DV of evaluation circuit AS. Differential amplifier DV is supplied with a comparison (or comparative) voltage V.sub.Bias for adapting the sensitivity. The output signal OUT of differential amplifier DV is filtered via a further low pass circuit TP2 and digitized by an analog-digital converter AD that is part of microcontroller MC, then evaluated and optionally used by microcontroller MC for controlling functions associated with the recognized signals.

[0038] Microcontroller MC computes the comparison voltage V.sub.Bias that is optimal in each case for adapting the sensitivity. To specify this comparison voltage for the differential amplifier DV, microcontroller MC influences the duty cycle of a pulse width-modulated signal PWM. Pulse width-modulated signal PWM is smoothed with respect to comparison voltage V.sub.Bias by a low pass circuit TP.

[0039] Comparison voltage V.sub.Bias is a function of at least one calibration or reference voltage signal that is generated by at least one reference voltage divider REF. The calibration voltage signal is affected by environmental influences of which the at least one reference voltage divider REF is exposed. If multiple reference voltage dividers REF, such as two in the present exemplary embodiment, are provided, then they may also specify multiple parameters such as a maximum value and a minimum value.

[0040] The calibration voltage signal, present at each center tap of a reference voltage divider REF, is supplied in each case to an input of multiplexer MUX. The calibration voltage signal thus passes through the same signal path as sensor signals S.sub.out, and at the end is likewise digitized by analog-digital converter AD and evaluated by microcontroller MC. By adapting the output pulse width-modulated signal PWM, microcontroller MC may thus adapt comparison voltage V.sub.Bias to the instantaneously present calibration voltage values without having to interrupt the detection of the sensor signals at steering wheel LR.

[0041] Advantageous details of evaluation circuit AS are illustrated in FIG. 2, which shows several individual circuit details with regard to the arrangement of electronic components. Evaluation circuit AS is shown here also in a simplified, schematic illustration. In particular, for reasons of better clarity in FIG. 2 the illustration of microcontroller MC that carries out the control is omitted. Various function blocks are designated in accordance with the corresponding function blocks in FIG. 1. Since the basic operating principle has already been explained with reference to FIG. 1, the explanation of FIG. 2 is limited to a few special features.

[0042] The circuit block depicts capacitive steering wheel sensor system LS illustrated in FIG. 3 based on an equivalent circuit diagram, which in particular shows that steering wheel sensor system LS forms one or multiple capacitive voltage dividers (in the present case, only one of multiple sensor lines L.sub.Sensor is illustrated). In addition, human finger F is illustrated as an equivalent circuit diagram made up of capacitive and ohmic components that act electrically between one of sensor foils SF1, SF2 and the vehicle ground.

[0043] Sensor line L.sub.Sensor leads to the input of an analog switch AN of multiplexer MUX. Corresponding to the eight-channel design of multiplexer MUX by way of example here, multiplexer MUX contains eight individual analog switches AN, only one of which is depicted here. The inputs of six of these eight analog switches AN are occupied by sensor lines L.sub.Sensor, and two further inputs are connected to the center connections of a respective reference voltage divider REF (illustrated in FIG. 1).

[0044] To allow grounded elements, such as hands or fingers on steering wheel LR, to be detected with good sensitivity in the vicinity of a capacitive sensor, the circuit is calibrated with regard to the environmental conditions (temperature, moisture, aging, etc.). For this purpose, reference voltage dividers REF, illustrated in FIG. 1, are used with known impedances. As indicated in FIG. 1, reference voltage dividers REF may have a simple design as ohmic voltage dividers, whose external connections are each connected to a fixed potential, for example to the poles of the supply voltage of evaluation circuit AS.

[0045] Analog switches AN of multiplexer MUX are switched on in succession by microcontroller MC in alternation so that its respective input signal reaches the output of multiplexer MUX, and thus, also the input of synchronous rectifier SG.

[0046] To obtain a direct voltage signal, output signal MUX.sub.OUT of multiplexer MUX is half-wave rectified with phase sensitivity via a switched shunt and is subsequently filtered by a low pass circuit TP1. The switched shunt is in the form of the source-drain section SD of a MOSFET semiconductor switch FET, which forms synchronous rectifier SG. The gate G of the MOSFET semiconductor switch FET is controlled by mono-frequency sinusoidal signal U.sub.sin. Operational amplifier OP connected upstream from the gate G is used for level adjustment.

[0047] The difference between the filtered measuring signal and the comparison voltage V.sub.Bias is amplified in differential amplifier DV. The sensitivity of the circuit is calibrated using the difference between comparison voltage V.sub.Bias and the measuring signal. Adapting the sensitivity of differential amplifier DV allows use to be made of the entire input voltage range of analog-digital converter AD for determining a useful signal.

REFERENCE SYMBOLS

[0048] AD analog-digital converter [0049] AN analog switch [0050] AS evaluation circuit [0051] Core core electrode [0052] CF Chebyshev filter [0053] DV differential amplifier [0054] FET MOSFET semiconductor switch [0055] G gate [0056] GND vehicle ground [0057] GPIOs universal connections (general purpose input-output) [0058] I insulators, insulation (foil) [0059] L conductor [0060] LR steering wheel [0061] LS steering wheel sensor system (capacitive sensors) [0062] L.sub.Core connecting line or cable (core line or cable) [0063] L.sub.Sensor connecting lines or cables (sensor line(s) or cable(s)) [0064] L.sub.Shield connecting line or cable (shield line or cable) [0065] MC microcontroller [0066] MUX analog multiplexer [0067] MUX.sub.OUT output signals of the multiplexer [0068] OP operational amplifier [0069] PWM, PWM pulse width-modulated signal [0070] REF reference voltage divider [0071] S1, S2 capacitive sensors [0072] SD source-drain section [0073] SF1, SF2 sensor foils [0074] SG synchronous rectifier [0075] SG.sub.out output signals of the synchronous rectifier [0076] SIN sinusoidal signal generation device [0077] Shield shield conductor or electrode [0078] S.sub.out output signals of the capacitive sensors (sensor signals) [0079] TP, TP1, TP2 low pass circuit [0080] U.sub.sin mono-frequency sinusoidal signal [0081] V.sub.Bias comparison or comparative voltage [0082] OUT output signal of the differential amplifier

[0083] While exemplary embodiments are described above, it is not intended that these embodiments describe all possible forms of the present invention. Rather, the words used in the specification are words of description rather than limitation, and it is understood that various changes may be made without departing from the spirit and scope of the present invention. Additionally, the features of various implementing embodiments may be combined to form further embodiments of the present invention.