DE102015221178B4 - Capacitive sensor - Google Patents

Abstract

Capacitive sensor with a first transmission electrode (1) and a second transmission electrode (2), a code generator (3) and a modulator (4) for generating two square-wave transmission signals (E1, E2), as well as an evaluation unit (5) with a synchronous demodulator ( 6), the transmission electrodes (1, 2) being driven with the square-wave transmission signals (E1, E2), and the reception signals being demodulated synchronously with one and the same square-wave signal (E1, E2, E3), characterized in that the transmission signals (E1 , E2) are sent modulated with two different, non-orthogonal codes (C1, C2), the transmission signals (E1, E2) having the full voltage swing, and the reception signals with one of the transmission signals (E1, E2) or a signal (E3) be demodulated, the signal (E3) being differently correlated with the transmission signals (E1, E2) and the sensitivity of the electrode system can be set by varying the code signals (C1, C2, C3), the transmission elect clearing (1, 2) can be switched with a chopper (4) in the chopper cycle (NF).

Description

The invention relates to non-contact capacitive sensors and thus also to capacitive proximity switches and operating elements according to the preamble of claim 1.

Non-contact proximity switches are used as electronic switching devices, especially in automation technology. They have long been known and are u. a. also manufactured and sold by the applicant.

They essentially consist of a sensor for detecting preferably electrical, but also optical or other physical properties of moving objects, the change in the relevant physical variable being used as a measure for the approach of an object, and a control unit which, when a threshold value is reached, is used preferably binary switching signal generated.

Capacitive sensors have a displacement circuit with at least one sensor electrode, the capacitance of the measuring electrodes to the environment or their impedance being evaluated as a rule.

To external factors such as B. to hide humidity, is often measured with reference or compensation electrodes, and the difference between the two capacities is evaluated. It is also known to influence the spatial distribution of the electrical measuring field by means of shielding and / or auxiliary electrodes.

The DE 10 2014 216 246 A1 shows a circuit for evaluating a capacitive sensor, whereby a counter signal with the same frequency as the control signal but with a variable phase position is generated and superimposed on the evaluation signal to adjust the system.

The DE 697 19 321 T2 shows a three-dimensional capacitive position sensor with a number of transmitting and receiving electrodes which are excited with several frequencies, the signals of which are demodulated synchronously and evaluated by a switching logic.

The DE 198 13 013 A1 shows a method and a circuit arrangement for operating a capacitive proximity switch in which a frequency-spread noise or binary random signal is emitted by a transmitting electrode and the signal received by a measuring electrode is evaluated with a correlator. In this way, the spectral energy density can be reduced without changing the transmission voltage.

The DE 103 23 030 A1 shows a capacitive sensor with a plurality of transmitting electrodes and at least one receiving electrode, the transmitting electrodes being acted upon one after the other with a square-wave signal, and the displacement current at the receiving electrode being measured. The evaluation is carried out with a clocked rectifier (synchronous rectifier).

The transmission electrodes are coded or driven one after the other and differentiated from one another by a selective detection circuit.

The DE 10 2012 015 423 A1 and B4 disclose a capacitive sensor with a transmitting electrode, a compensation electrode, one or two receiving electrodes, and an evaluation method suitable for this. It is proposed here to transmit a specified code sequence (code signal) with a first transmission electrode and an inverse code sequence with a second transmission electrode, the compensation electrode, the compensation electrode acting in a known manner on the receiving electrode and the transmission path of the first transmitting electrode being influenced by a measurement object .

A processing unit correlates the codes, determines the most likely code and its probability. The transmission power of the compensation electrode is regulated in such a way that the detection probability of the inverse code sequence is zero, with the transmission signal from the compensation electrode serving as a measured value for the transmission path.

The control of the transmission power is viewed as disadvantageous because the logic modules used for fixed voltages, e.g. B the TTL level of 5V are designed. A further disadvantage is seen in the fact that to drive the compensation electrode another, i. additional feed signal must be generated and demodulated.

The object of the invention is to provide a cost-effective capacitive sensor and an evaluation method that does not have these disadvantages.

This object is achieved with the characterizing features of claim 1. Advantageous embodiments of the invention are specified in the subclaims.

The essential idea of the invention is to transmit with two different codes, but to demodulate the two received signals with the same code. The more a transmission code differs from the demodulation code, the more the contribution of the respective transmitted signal to the received signal is lower. This enables the signal components of the respective transmitting electrode to be controlled in the receiving signal without separating them in time or evaluating them with the aid of two receivers with different demodulation codes. Taking advantage of this fact, the control of the transmission power can be replaced by a code change.

In an advantageous embodiment, the demodulation code corresponds to one of the transmission codes. In a further embodiment, the second transmission code is selected such that the spectral components are the same as in the first transmission code, only the number of clock edges transmitted per unit of time (transmission energy) decreases. The transmission signals E1 and E2 have the same spectral components, but with a different distribution.

With capacitive sensors, the transmitting electrode is often fed with a square-wave signal. In contrast to sinusoidal signals, this enables simple generation of the feed signal with digital components and thus simple application of the in the DE 10 2007 041 646 A1 described digital coding method.

The electrode is recharged with each clock edge, with the potential increasing by the full voltage swing, e.g. changes by the amount of the operating voltage. A positive clock edge generates a positive current pulse and a negative clock edge a negative current pulse. The amount of charge Q = C * U is moved per current pulse. After appropriate synchronous rectification and low-pass filtering, the result is a constant sensor current, which depends on the voltage swing at the transmitter electrode, the number of clock edges per unit of time and, of course, on the sensor capacitance. In contrast to this, the control of the transmission power through amplitude or voltage control is relatively complex. Control via the number of edges, on the other hand, is very simple and can still be implemented using digital modules. Since the sensor characteristics and especially the electromagnetic compatibility (EMC) of the system depend on the spectrum used, simple control of the transmission power by adjusting the feed frequency is only conditionally effective.

Coding methods with deterministic spreading sequences are already in use today. This allows the spectral distribution of the signal energy to be controlled and the sensor and EMC properties of the system to be optimally adjusted.

When controlling the transmission power of individual electrodes, it is therefore advantageous to retain the spectrum, or at least the proportions used.

By reducing the number of transmission edges, a signal with reduced transmission energy can be generated, which nevertheless has the full voltage swing on the transmission electrode and does not contain any signal components that are not demodulated and would represent potential interference signal components.

The modulator 4 and above all the transmitter can be equipped with inexpensive logic components such as the 74HCT86 containing four XOR gates.

Capacitive proximity switches are typically set application-specific. The switching point is adjusted to a reference state at which the sensor should switch at the output.

Since the sensor behavior is particularly relevant in connection with a binary switching output at the switching point, the ratio of the proportions of the two transmitting electrodes or the transmitting power of the second transmitting electrode can also be compared to the reference state by the second code signal.

In a special embodiment, the portion of the second transmitting electrode is derived directly from the switching point setting. The proportion of the second transmitting electrode can increase or decrease linearly with the switching point setting.

This opens up the possibility of controlling different electrodes in such a way that an optimal detection area for the target is created.

Another advantage is that the sensitivity of the electrode system to the different effects can be adjusted for the customer.

In addition, the effects of environmental influences such as temperature fluctuations or electrode contamination can be reduced.

The invention is explained in more detail below on the basis of exemplary embodiments with reference to the drawings.

• 1 zeigt eine erste erfindungsgemäße Messanordnung mit XOR-Gattern,
• 2 zeigt eine zweite erfindungsgemäße Messanordnung mit einem AND-Gatter,
• 3 zeigt ein Impulsdiagramm mit den Codes und den Signalen an den Elektroden,
• 4 zeigt eine dritte erfindungsgemäße Messanordnung mit Chopper-Umtastung,
• 5 zeigt eine erste Elektrodenanordnung zur Durchführung des Messverfahrens,
• 6 zeigt eine zweite Elektrodenanordnung zur Durchführung des Messverfahrens.

They show schematically:

• 1 shows a first measuring arrangement according to the invention with XOR gates,
• 2 shows a second measuring arrangement according to the invention with an AND gate,
• 3 shows a pulse diagram with the codes and the signals at the electrodes,
• 4th shows a third measuring arrangement according to the invention with chopper shift keying,
• 5 shows a first electrode arrangement for carrying out the measuring method,
• 6th shows a second electrode arrangement for carrying out the measuring method.

In the following description of the preferred embodiments, the same reference symbols designate the same or comparable components.

1 shows a measuring arrangement according to the invention with a first transmitting electrode (main electrode) 1 and a second transmitter electrode (additional electrode) 2 , both of a modulator 4th driven by a code generator 3 a clock signal CLK and two code signals C1 and C2 receives.

The modulator 4th contains four exclusive OR gates (XOR) linked to one another, or also exclusive OR elements, where XOR means that a high level must be present at exactly one input so that the output also has the high level. In other words, the clock signal is inverted when the code signal is high and output is not inverted when the code signal is low. In the case of edge-synchronous signals, a send edge is suppressed with each code edge. From a signal theory perspective, this corresponds to a multiplication or modulation of the send clock with the code signal.

Since the modulator 4th represents a pure logic function, the signals can also be wholly or partially generated in a microcontroller or a programmable logic module, the modulator 4th to relieve the microcontroller and to improve the signal routing. The modulator 4th can therefore be implemented discretely, or completely or only partially as a digital logic module, for example as a complex programmable logic device (CLPD).

Like in the 3 is shown, the modulator converts 4th the codes C1 and C2 into two clock-synchronized square-wave transmit signals E1 and E2 around. These generate a high-frequency displacement current with their environment, which here acts as functional earth. The current then flows from the functional earth via a coupling capacitor and a synchronous demodulator 6th into a current-voltage converter 7th , the one with the code generator 3 forms a circuit via the device ground. The signal is then filtered in a known manner and finally digitized.

The transmission power of the two electrodes 1 and 2 is according to the invention with the help of the codes 1 and 2 regulated by sending with two similar codes, but both receiving signals with the same signal, preferably with that of the main electrode 1 be demodulated.

Alternatively, as in the 4th shown with a third code C3 , or with one C3 based demodulation signal E3 be demodulated.

In this way, the amplitude regulation, which is complex in terms of circuitry, is replaced by the different correlation coefficients, that is, by software, and a second demodulator is avoided due to the properties of the codes.

In addition to the code signals, the transmission signal is further modulated with a low-frequency chopper signal ( NF ) of 500 Hz, for example. Due to this relatively small frequency, the effect on the spectral distribution is insignificant in practice. However, the advantages known from chopper amplifiers are used. In particular, the received signal in the two LF phases can be viewed independently of one another, with a constant component or offset being canceled out in a known manner by forming the difference. This is advantageous because parasitic signal components can only affect the signal evaluation from the line between the modulator and the electrode. Therefore, in an advantageous embodiment, the XOR gates are close to the transmission electrodes 1 and 2 to place.

In addition to the assemblies mentioned above, the evaluation unit contains 5 a low pass filter 8th for generating an analog output signal, and an analog-digital converter 9 for generating a digitally processable output signal, with assemblies familiar to those skilled in the art having been omitted for a better overview.

It should also be noted that the evaluation unit 5 can also have a receiving electrode instead of the coupling capacitor connected to functional earth.

2 shows a similar measuring arrangement as 1 . Only an XOR gate has been replaced by an AND (AND) gate, which simplifies the circuit, and also a masking of the transmission signal E2 of the second transmitting electrode 2 made possible by applying another signal to the second input of the AND gate.

3 shows the pulse diagram of a measuring arrangement according to the invention. In the top row is the square-wave clock signal CLK shown. Below you can see the first code C1 and in the third row the code C2 . The two codes are different, but not orthogonal. The two lower rows show the transmission signals E1 and E2 on the electrodes 1 and 2 . The signals are created by XORing the clock signal CLK with one of the codes C1 or C2 which corresponds mathematically to a multiplication or in signal theory to a modulation.

4th shows a measuring arrangement with a chopper 4th , with the transmitting electrodes 1 and 2 in chopper cycle ( NF ) can be switched. The demodulation takes place with a third code signal C3 based demodulation signal E3 that is different with the two transmit signals E1 and E2 correlated, so that the sensitivity can also be adjusted here by varying the code signals.

5 shows a circular main electrode arranged on a double-sided printed circuit board 1 with a ring-shaped additional electrode 2 and a back electrode R. on the back side. In this way, a directional characteristic that is rotationally symmetrical and largely insensitive to rear interferers is achieved

6th shows an electrode arrangement with spatially resolving properties, the main electrode being wedge-shaped into a rectangular additional electrode 2 protrudes.

List of reference symbols

1

First transmitter electrode (main electrode)

2

Second transmitter electrode (compensation electrode, additional electrode)

3

Code generator

4th

Modulator or chopper

5

Evaluation unit

6th

Synchronous demodulator

7th

Current-to-voltage converter

8th

filter

9

Analog-to-digital converter

C1

First code signal (first code)

C2

Second code signal (second code) for the compensation electrode

C3

Third code signal (demodulation code)

CLK

Clock signal

E1

Transmission signal of the first electrode (first square wave signal)

E2

Transmission signal of the second electrode (second square wave signal)

E3

Third signal, demodulation signal (third square wave signal)

NF

Low-frequency chopper signal, chopper clock

R.

Back electrode

Claims (3)

Capacitive sensor with a first transmission electrode (1) and a second transmission electrode (2), a code generator (3) and a modulator (4) for generating two square-wave transmission signals (E1, E2), as well as an evaluation unit (5) with a synchronous demodulator ( 6), the transmission electrodes (1, 2) being driven with the square-wave transmission signals (E1, E2), and the reception signals being demodulated synchronously with one and the same square-wave signal (E1, E2, E3), characterized in that the transmission signals (E1 , E2) are sent modulated with two different, non-orthogonal codes (C1, C2), the transmission signals (E1, E2) having the full voltage swing, and the reception signals with one of the transmission signals (E1, E2) or a signal (E3) be demodulated, the signal (E3) being differently correlated with the transmission signals (E1, E2) and the sensitivity of the electrode system can be adjusted by varying the code signals (C1, C2, C3), the transmission electronics trodes (1, 2) can be switched with a chopper (4) in the chopper cycle (NF). Capacitive sensor after Claim 1 , characterized in that the rectangular transmission signals (E1, E2) have the same spectral components, the ratio of the transmission power at the transmission electrodes (1, 2) being set by the ratio of the transmission edges of the transmission signals (E1, E2), the number of sent clock edges per time unit is different. Capacitive sensor according to one of the preceding claims, characterized in that, in addition to the one code signals (C1, C2, C3), the transmission signals (E1, E2) are further modulated with a low-frequency chopper signal (NF), with a constant component or offset being formed by forming the difference is extinguished.

Images