DE102012200153B4 - Optical proximity switch with a correlation receiver - Google Patents

Abstract

Optical proximity switch, preferably energetic light sensor, with a transmitter (10) for emitting a light signal modulated with a pseudo-random number code, preferably with a 32-bit code, into a monitoring area (20), a correlation receiver (30) for receiving the modulated light signal and an evaluation unit (40) for generating a binary switching signal, characterized in that the correlation receiver (30) has a PMD pixel whose outputs are connected to the evaluation unit (40) which over the length of the pseudo-random number Codes can compare summed difference signal of the outputs with a switching threshold, the bit length within the pseudo-random number code is at least 10 times the regular light propagation time from the transmitter (10) to the receiver (30).

Description

The invention relates to an optical proximity switch, in particular an energetic light scanner with a correlation receiver according to the preamble of patent claim 1. A method for operating the optical proximity switch according to the invention is also claimed.

Optical proximity switches are widely used in automation technology. You will e.g. also manufactured and sold by the applicant. A particular problem is their immunity to interference. In addition to the other electromagnetic interference, there is extraneous light, but also the mutual influence of the sensors, especially in light grids.

An improvement in the immunity to interference can be achieved with correlation receivers. the DE 41 01 206 A1 shows a light barrier that emits light modulated by a noise generator. A synchronous demodulator is connected downstream of the photoreceiver, which correlates the received signal with the signal from the noise generator. By using statistically independent noise sources, even identically constructed light barriers do not influence each other. However, separating the signals requires a certain integration time, which is further increased by electromagnetic interference.

the DE 198 21 974 B4 discloses a light curtain in which the individual light barriers are encoded with binary signal sequences. Each transmitter contains a pseudo-random number generator, with which the transmitted light beams are encoded. These individual signal sequences are identified by an evaluation unit. The monitoring area is only released if these are received in full. The disadvantage is that faults are interpreted incorrectly, ie the system has to be shut down as a precaution for safety reasons.

Furthermore, the DE 199 26 214 A1 a method for suppressing interference signals in optical sensors and a circuit arrangement for carrying out this method. Chirp sequences of light pulses are emitted by a transmission unit and an autocorrelation function is formed between the transmitted and received signals, with an obstacle being inferred if the autocorrelation function exceeds a threshold value.

From the WO 02/33817 A1 a method and a device for detecting and processing signal waves is known, in which a distance is determined on the basis of the phase difference between an emitted and received modulated radiation.

the EP 1 318 606 A1 discloses a photoelectric proximity switch for detecting an object in a surveillance area. The proximity switch contains a pulsed light transmitter which, triggered by an output signal of a random signal generator, emits transmitted light pulses at a time interval from one another into the monitored area. When an object is present, the light is detected by a receiver and an object detection signal is emitted.

From the DE 102 11 387 A1 a sensor for detecting transmitted pulses of radiation is known. By comparing the detected radiation pulses with a standard bit pattern, real and false electrical pulses can be distinguished.

For these reasons, the optical correlation receivers described above should be further improved. Additional shielding is expensive and not always successful. While increasing the integration time is an effective tool, it also increases the response time of the sensors. Therefore, another efficient way to improve the noise immunity is sought.

The object of the invention is to improve the immunity to interference in optical proximity switches. In particular, the mutual influencing of adjacently arranged sensors with overlapping reception areas should also be reduced without increasing their reaction time.
This problem is solved according to the features of patent claim 1 . The further claims relate to the advantageous embodiment and an evaluation method according to the invention.

The essential idea of the invention is the correlation with the transmission pulse sequence at the earliest possible point in time, i. H. to be carried out in the optical receiver. In this way, the bandwidth of the subsequent electronic circuit can be reduced. In this way, stray high-frequency interference signals are effectively suppressed. However, this requires a component that allows mixing with the transmission signal in its receiving area.

A suitable component is from DE19821974B4 famous. The photomixing detectors described there have two light-sensitive modulation photogates and two non-light-sensitive accumulation gates. The modulation pho togates are each connected to a modulating device and the accumulation gates are connected to a readout device. These mixing elements are referred to as PMD pixels. They allow the evaluation of phase differences resulting from the transit time of light and were actually developed to generate three-dimensional camera images. With the circuit according to the invention, however, they are also suitable as a correlation receiver for an energetic light sensor.

The invention is explained in more detail with reference to the drawing. the 1 shows an energetic light scanner with a PMD pixel used according to the invention as a correlation receiver 30 . A generator 11 controlled by a microcontroller 40 generates a transmission signal Q in the form of a pseudo-random number code. According to the invention, a “truly orthogonal code set” is stored either in the generator 11 or in the μC 40. These codes are suitable for multiple access methods (CDMA=code division multiple access). An asynchronous orthogonal code with a length of 32 bits is preferably generated. The advantage is that the multiplication of two codes over a certain period of time results in zero even with a bit shift. The use of multiple orthogonal codes allows the sensors to operate in a common surveillance area. The codes can be broken down at the factory, manually or by an algorithm stored in the evaluation unit. In addition, the codes should be balanced, ie the number of low and high bits is approximately the same.

The light emitter 10 and the first modulation photogate K+ are modulated with the code Q. The second modulation photogate K- is modulated with the inverted code Q', which is also generated by the generator 11. The second modulation photogate K- is therefore only activated in the transmission pauses, i. H. the correlation takes place on the photosensitive surface. It should be noted here that the generator 11 can in principle be simulated as software in the microcontroller 40 .

The photocurrents are summed up in the accumulation gates of the PMD pixel 30, or integrated, and output as voltage values u _a and u _b . They are then fed to the differential amplifier 31 shown without external wiring. The voltage difference occurring at the output of the differential amplifier 31 is compared in the trigger 32 with a switching threshold. These operations can also take place in the microcontroller 40 if analog/digital converters are present and the processing speed is sufficient. The evaluation, ie the comparison with the switching threshold, of course, only takes place after the transmission pulse sequence has elapsed. The PMD pixel is then reset by a reset command and the generator is restarted. Of course, the assessment can only be made after several transmission pulse sequences have elapsed.

Without a reflex signal from the monitoring area 20, both modulation photogates are equally exposed to extraneous light or stray light, so that their difference signal results in zero. A useful signal only becomes effective in the first modulation photogate synchronized with the transmission signal Q.

Neither the transmission pulse sequence described above nor the type of evaluation are suitable for the time-of-flight measurement. The integration of the received signal and the one-off measurement at the end of the transmitted pulse sequence do not allow any statement to be made about the phase shift and thus about the light propagation time. This requires a symmetrical transmission signal and at least 2, generally even 4 measurements in different phase positions (0°, 90°, 180°, 270°). The light propagation time is only important if the clock frequency is too high. The clock, i.e. the bit length of the transmission signal, is therefore adapted to the monitoring area 20 to be expected according to the invention. For a reflector at a distance of 30 cm, the light propagation time is approx. 2 nanoseconds. With a clock frequency of 0.5 MHz, the bit length is 1 microsecond. The light propagation time is negligible because the phase of the received signal is only shifted by 1 per mil compared to the transmitted signal. However, distant reflectors are weakened or can be hidden by adjusting the clock frequency. With a maximum monitoring distance of 30 cm, the light propagation time (time of flight) for the regular light pulse is 2 nanoseconds. For a background object at a distance of 180 cm, the running time is 12 nanoseconds. If a bit length of 24 nanoseconds is selected, which results in a clock frequency f = 1/(ton+toff) of approx. 21 MHz, the photocurrents of the two modulation photogates cancel each other out. The object is masked out, but the useful signal is only weakened by around 16 percent. Interference signals statistically distributed to both modulation photogates are completely canceled out by the synchronous rectification with subsequent difference formation. Thus, by generating a random sequence and correlating the received signal with the transmitted signal, the immunity to interference can be improved almost at will. All that is required for this is the lengthening of the random sequence and thus also the lengthening of the integration time if the transmission clock is to be retained.

As indicated by the low-pass filter, the bandwidth of the electronic circuit can be limited to that used to process the mechanical movement in excess wachungsbereich 20 be limited to the extent necessary.

The microcontroller 40 controls the generator 11 and is used for further signal processing.

With the two switching outputs or ports 41 and 42 shown, the switching status can be displayed or reported to a higher-level control unit. Furthermore, learning processes, parameterization or other control functions can be triggered. The connection to the generator 11 allows the selection of the pseudo-random number code Q or the clock frequency that is most suitable for the current measurement task.

The present invention relates to an optical proximity switch, preferably an energetic light sensor, but it is also suitable for one-way or reflex light barriers. It contains a transmitter 10 which receives a light signal modulated with a pseudo-random number code Q, preferably with a 32-bit code, from a generator 11 or from a control unit 40 and emits it into a monitoring area 20 .

A correlation receiver 30, according to the invention a PMD pixel, is fed synchronously with the code Q and the inverse code Q' for receiving the modulated light signal.

The outputs of the PMD pixel acting as a correlator are connected to an evaluation unit 40 . Here, the difference signal of the two outputs is summed over at least one length of the pseudo-random number code and compared with a switching threshold, and a binary switching signal is generated as a result.

In contrast to the time-of-flight measurement, only the amplitude of the differential signal at the end of the pseudo-random sequence, which preferably consists of 32 bits, is evaluated.

Advantageously, the bit length of the pseudo-random code is at least 10 times the regular light propagation time from the transmitter 10 to the receiver 30. Regular light propagation time means the direct path without mirroring around or further reflections.

Furthermore, a method for operating an optical proximity switch is claimed, with a light signal emitted into a monitoring area 20 being correlated in a PMD pixel with the received signal and a binary switching signal being generated. In a first method step, a transmitter 10 emits a light signal modulated with a pseudo-random number code, preferably with a 32-bit code, into a surveillance area 20 .

In a second method step, the light signal is received with a synchronously modulated PMD pixel 30 and added up over the length of the pseudo-random number code.

In a third method step, a difference signal is preferably formed in an evaluation unit 40, the difference signal is compared with a switching threshold and a binary switching signal is generated therefrom.

Reference List

10

optical transmitter, LED, laser diode

11

Pseudo-random generator, function generator

20

Surveillance area, reflector, object, target

30

Correlation Receiver, PMD Receiver, PMD Pixel

31

differential amplifier

32

triggers

40

Control unit, evaluation unit, microcontroller, µC

41

first switching output, port 1

42

second switching output, port 2

Claims (2)

Optical proximity switch, preferably energetic light sensor, with a transmitter (10) for emitting a light signal modulated with a pseudo-random number code, preferably with a 32-bit code, into a monitoring area (20), a correlation receiver (30) for receiving the modulated light signal and an evaluation unit (40) for generating a binary switching signal, characterized in that the correlation receiver (30) has a PMD pixel whose outputs are connected to the evaluation unit (40) which over the length of the pseudo-random number Codes can compare summed difference signal of the outputs with a switching threshold, the bit length within the pseudo-random number code is at least 10 times the regular light propagation time from the transmitter (10) to the receiver (30). Method for operating an optical proximity switch claim 1 , characterized in that a light signal modulated with a pseudo-random number code, preferably with a 32-bit code, is emitted, the A light signal is received with a synchronously modulated PMD pixel and summed up over the length of the pseudo-random number code, a difference signal is formed, compared with a switching threshold and a binary switching signal is generated from this, the bit length within the pseudo-random number code being at least 10 times the regular light propagation time from the transmitter (10) to the receiver (30).

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