OPTICAL PROXIMITY SWITCH AND METHOD FOR DETECTING THE LOSS OF A MINIMUM DISTANCE

Abstract

To achieve the smallest possible yet stable switching hysteresis in a proximity sensor, and thus ensure reliable switching with both highly and weakly reflective objects, the present invention provides a signal processing method that differs from the prior art. Previously, a distance signal was determined from the position of a light spot on a photodiode array, but the intensity of the reflection was not taken into account in the calculation. To account for the varying signal strength of the received reflections, a correction value is alternately added to and subtracted from the tapped photostreams, with the order being reversed for the two tapped photostreams. The resulting offset is proportional to the signal intensity, allowing for small yet reliable switching hysteresis values ​​to be selected for different signal intensities.

Description

[0001] The present invention relates to an optical proximity switch for detecting when a minimum distance to an object within a monitoring area has been undershot, comprising a light source for emitting light into the monitoring area and a receiving unit for detecting a reflection of the emitted light, wherein the receiving unit comprises a photodiode arrangement with a sensor surface which is arranged between a first tap and a second tap, the reflection causing a first photocurrent at the first tap and a second photocurrent at the second tap, and the receiving unit further comprises a, preferably analog, computing circuit for determining a ratio between the difference between the first photocurrent and the second photocurrent and the sum of the first photocurrent and the second photocurrent, as well as an evaluation circuit for generating a switching signal from this ratio, and a method therefor.

[0002] Such a solution is already known from DE 43 11 691 C2. This involves a light distance sensor in which the two photocurrents are connected to adjustable amplifiers / attenuators. In contrast to the present application, the two photocurrents are subject to correction values ​​with different magnitudes. EP 1 209 487 A1 discloses a light sensor in which one of the two photocurrents can be grounded via a controllable switch. DE 198 52 173 C2 and DE 197 53 836 B4 disclose further light sensors without correction of the photocurrents. Furthermore, such a solution is also previously known from DE 36 40 159 A1. Optical sensors with background suppression determine the distance to an object using triangulation. A switching signal is output when a certain distance is exceeded. With this measuring method, even dark objects against a light background can be reliably detected.Important for the user is the easy possibility of adjusting this switching distance. In this solution, this is achieved via a position-sensitive diode, which allows the switching distance to be adjusted purely electronically.

[0003] Generally, a light source is used that emits a light signal into a monitoring area monitored by the proximity sensor. The light signal is reflected by an object and bounced back toward the proximity sensor. To detect the reflected light, the proximity sensor has photodiodes onto which the reflected light is bounced back. Depending on the angle of the bounced light, different diodes or different areas of a diode are hit. Since the angle of incidence of the reflected light allows conclusions to be drawn about the distance of the reflecting object from the proximity sensor, such a solution can distinguish between background and foreground.

[0004] DE 10 2015 009 619 A1 uses a mechanically adjustable double diode for this purpose. The switching distance—that is, the moment at which the reflected light no longer hits a diode intended for the background but instead hits a diode intended for the foreground—is mechanically adjusted. This is achieved using a planetary gear system and is therefore a mechanically complex solution.

[0005] When using a connectable array of multiple photodiodes, a large number of individual diodes can be combined with analog switches to form two diodes using an electronic circuit, depending on the desired switching distance. The disadvantage of this is the complex and expensive circuitry. However, mechanical adjustment is eliminated.

[0006] In the position-sensitive diode mentioned above, there is an active area between two taps X1 and X2 onto which the reflected light falls.

[0007] The light spot on the position-sensitive diode generates a photocurrent that is distributed between the two terminals X1 and X2. The position x on the diode is then calculated using the following formula: x=k⋅X1−X2X1+X2 where k is a constant factor. Using this formula, the position of the light spot can be converted into a proportional analog voltage that is zero on a center line between the taps. A shift toward tap X1 makes the signal positive; a shift toward tap X2 makes it negative.

[0008] If the voltage exceeds an adjustable reference voltage, the object is detected and the output is switched. Division in the calculation formula cancels out the signal strength of the photocurrents; the result is independent of the brightness of the object being scanned, which is also desired in optical triangulation.

[0009] However, the signal level of the transmitted signal can be made available as a constant voltage value for further processing.

[0010] In the calculation block, the stored values ​​X1 and X2 are evaluated according to the formula above. Adding an offset yields a distance value y, which is output as the result. This value y corresponds to the path of the light spot on the active area of ​​the position-sensitive diode. However, the circuit's output value y only contains information about the distance of the object from the sensor, not about the amount of light emitted by the object onto the position-sensitive photodiode.

[0011] Each photodiode also generates a constant noise current superimposed on the photocurrent. The signals X1 and X2 are thus additionally modulated by the noise signal dx.

[0012] The higher the wanted signals X1 and X2 are relative to the noise signal dx, the smaller the effect on the distance signal y. Since the distance signal y no longer contains any information about the original value of the wanted signal, the output signal y has only a small noise component for high signals X1 and X2, thus measuring the distance very precisely. For weak signals X1 and X2, the distance signal y has a very high noise component, resulting in a very imprecise distance value.

[0013] If the distance signal y is queried using a comparator threshold with switching hysteresis to generate a switching point, the constant hysteresis of the switching threshold is never properly adjusted. For dark materials, it is too small, causing the switching output to toggle uncontrollably. For light objects, however, the switching hysteresis is much too large.

[0014] For sensors with only one switching point, it is still possible to generate a hysteresis that is inversely proportional to the signal level by applying a small current directly to the terminals of the position-sensitive diode, which is switched on and off with the switching signal. This allows a signal-independent distance hysteresis to be generated for a switching output.

[0015] However, a sensor with a switching output and an analog output for the distance measurement cannot work with this method, as the switched current at one or both terminals of the position-sensitive diode would also cause the distance value y to jump back and forth, which is unacceptable for the analog output. A sensor with two independent switching outputs for the same distance signal cannot be implemented using this method either, or only with great effort.

[0016] For correct functioning, the amount of signal noise in the distance signal y should be determinable at any time so that the necessary switching hysteresis for the comparator(s) can be set in such a way that the smallest possible but stable switching hysteresis is always guaranteed.

[0017] Determining the useful signal level by measuring directly on the position-sensitive diode is very difficult and expensive, since the minimum photocurrents are in the nanoampere range and a measurement must function over the full dynamic range of up to 100 dB.

[0018] Based on this, the present invention is based on the object of creating an optical proximity switch and a method for detecting when a minimum distance to an object is exceeded, which enable the smallest possible but stable switching hysteresis and thus ensure reliable switching both for highly and weakly reflective objects.

[0019] This is achieved by an optical proximity switch according to the features of independent claim 1 and by a method for detecting the undershoot of a minimum distance to an object according to the features of independent claim 8. Useful embodiments of both the proximity switch and the method can be found in the respective subsequent dependent claims.

[0020] Accordingly, an optical proximity switch is provided for detecting when a minimum distance to an object within a monitoring area has been undershot, comprising a light source for emitting light into the monitoring area and a receiving unit for detecting a reflection of the emitted light, wherein the receiving unit comprises a photodiode arrangement with a sensor surface which is arranged between a first tap and a second tap, the reflection causing a first photocurrent at the first tap and a second photocurrent at the second tap, and the receiving unit further comprises a, preferably analog, computing circuit for determining a ratio between the difference between the first photocurrent and the second photocurrent and the sum of the first photocurrent and the second photocurrent, and an evaluation circuit for generating a switching signal from this ratio.Such a proximity sensor is characterized in that a first adder for adding a first correction value to the first photocurrent is assigned to the first tap and a second adder for adding a second correction value to the second photocurrent is assigned to the second tap, wherein the first correction value corresponds to the inverted second correction value and the first correction value and the second correction value are invertible by means of a changeover switch, wherein within a measuring cycle the evaluation circuit detects a first ratio in a first position of the changeover switch and a second ratio in a second position of the changeover switch and determines the switching signal on the basis of an average value and a difference between the first ratio and the second ratio.

[0021] A measurement cycle therefore consists of sending the transmitted light and simultaneously storing the two measured values ​​X1 and X2. The calculation block then calculates the value according to the formula y=k⋅X1−X2X1+X2 the distance value y is determined. In this respect, the present solution still complies with the state of the art. However, according to the invention, the distance value is now determined twice using the same measured values ​​X1 and X2, with the correction value Δx or -Δx being added once each time before the above calculation is performed. The result is that the distance value y is slightly offset each time. Due to the calculation formula, this offset becomes greater the smaller the measured values ​​X1 and X2 are.

[0022] If this constant value Δx or -Δx is chosen to be approximately equal to or slightly larger than the value dx caused by the photodiode noise, the difference between the successively determined distance values ​​y1 and y2 yields exactly the value that must be set as hysteresis on the comparator to achieve a stable switching process. The mean value y = (y1 + y2) / 2 corresponds to the original, unadulterated distance value.

[0023] The result for a first switch position S=0 is: y1=kx2−x1+2Δxx2+x1+b and for a second switch position S=1: y2=kx2−x1−2Δxx2+x1+b

[0024] A difference y1 - y2 results in y1−y2=4kΔxx2+x1y1+y2=4kx2−x1x2+x1+2b

[0025] For simplification, if we assume 4kΔx = C as a constant and set 4k = K, we get y1−y2=C1x2+x1, therefore (y1−y2)∼1x2+x1

[0026] The value y1+y2=2kx2−x1x2+x1+2b corresponds to the initial formula with an offset of b = 0. At the end of the measurement cycle, the required hysteresis value and the mean distance value are calculated, and the corresponding switching signals or distance measurements are generated and output.

[0027] The advantage of this measuring method is that with only one measuring cycle, not only a distance value is determined, but also the exactly matching hysteresis value for the switching point comparators, so that at the maximum operating frequency of the sensor, a switching point with an optimally small distance hysteresis can always be determined.

[0028] Only with this method is it possible to generate two independent switching points, each with an optimal distance hysteresis, from a determined distance value and optionally output the analog distance value at the same time.

[0029] A further advantage can be the use of an analog arithmetic circuit for the arithmetic block. With an upstream, automatically adjustable amplifier circuit within the arithmetic block, it is possible to achieve a high dynamic range of more than 100 dB at low cost, thus reliably detecting matte black parts at maximum switching distance and high-gloss parts at close range. However, with such a circuit, it is no longer possible to detect the absolute value of the signal intensity. However, with the described inventive method, it is still possible to determine the necessary hysteresis value without the signal intensity information.

[0030] In a first specific embodiment, it can be provided that the first tap and the second tap are each assigned a sample-and-hold element, each with a capacitor, in which the signal levels of the photocurrents are stored as voltage values ​​during a measurement cycle. To separate the ambient light from the useful light, the light source can be operated with a pulsed transmission current. Along with the transmission signal, the signal level of a preamplifier is also stored in a capacitor by means of the sample-and-hold element. The signal level is then available as a voltage value in the capacitor for the entire period as a constant value for further processing.

[0031] Preferably, the evaluation circuit can further comprise a comparator circuit with switching hysteresis for monitoring the mean value and generating the switching signal therefrom when an adjustable reference voltage is exceeded. In this comparator circuit, the determined output values ​​y are compared with reference values, and based on the reference values, it is determined whether a predetermined output value is exceeded or not reached. The determined distance, and thus the switching signal output by the proximity sensor, is correspondingly large or small.

[0032] In a specific development, the magnitude of the correction values ​​can be selected to be greater than or equal to a noise signal from the photodiode array. This results in the difference between y1 and y2 yielding exactly the value that must be set as hysteresis on the comparator to achieve a stable switching process. The mean value (y1 + y2) / 2 then corresponds to the original, unadulterated distance value.

[0033] A first alternative for the design of the photodiode array can be if the photodiode array comprises a row with several photodiodes, wherein the sensor area extends over several photodiodes. This can be a double diode in particular, but also a plurality of photodiodes. The active regions of the photodiodes together form the sensor area, whereby localization can be achieved simply by the response of a specific diode at a specific position in the row. Preferably, however, the area of ​​the diodes is regarded as a common sensor area and the diodes are connected together to form a common photodiode array. Taps located between the diodes are connected, and only the outer taps are treated as such, where the photocurrents are taken.

[0034] Accordingly, a second alternative is for the photodiode array to comprise an optical position sensor with a continuous sensor surface. In this case, the photocurrents are generated at the taps on both sides. If taps are present on four sides, they can be limited to two sides to obtain a purely two-dimensional evaluation.

[0035] Finally, with particular advantage, the evaluation circuit can linearize the mean value using a linearization circuit and determine an absolute distance signal from the linearized mean value.

[0036] The present invention is explained in more detail below using an embodiment. It shows Fig. 1 a position-sensitive photodiode in a schematic plan view, Fig. 2 a photodiode arrangement comprising two photodiodes in a schematic plan view, Fig. 3 a monitoring area with an object approaching a proximity sensor in a schematic representation, Fig. 4 an evaluation circuit for evaluating the photocurrents of a position-sensitive photodiode in a basic circuit diagram, as well as Fig. 5 a supplementary circuit for adding a correction value in a block diagram.

[0037] Fig. 1 shows a photodiode array 1 for use in a proximity sensor. In a first alternative, the photodiode array consists of a single position-sensitive photodiode 2, which is bounded on both sides by electrical taps 4 and 5. If a light spot 6, in particular as a reflection of a light beam emitted by a light source of the proximity sensor, falls on an active area of ​​the photodiode 2, this triggers a first photocurrent X1 at a first tap 4 and a second photocurrent X2 at a second tap 5 of the photodiode 2. By evaluating these photocurrents X1 and X2, it can be determined how far from a center line 3 of the photodiode 2 the light spot occurred. From this, in turn, the angle of incidence and thus the distance of the object 7 from which the light was reflected can be deduced. The light spot 6 in Fig. 1 is closer to the first tap 4, so it is a background reflection. The closer the object 7 comes, the further the light spot 6 moves toward the second tap 5.

[0038] Fig. Figure 2 shows an alternative to this, which is, for example, a double diode composed of a first photodiode 17 and a second photodiode 18. This arrangement does have a wider area between the components, which, like the center line 3 in Fig. 1 works, but otherwise the diodes 17 and 18 are more or less identical to the position-sensitive photodiode 2. However, the function is not with the position of the light spot 6, but with the portion of light of the in reality more diffuse light spot 6, which is incident on the first photodiode 17 and the photodiode 18. With a position of the light spot 6 as in Fig. 2, the largest portion of the light is directed onto the first photodiode 17, with only a small portion falling onto the photodiode 18.

[0039] Here, a distinction can be made between the photodiodes. A background diode can be activated when the reflection is returned by a distant object 7, while a foreground diode is activated when the reflection is returned by a nearby object 7, i.e., in the foreground. Here, too, this is determined by the angle at which the reflection returns to the proximity sensor.

[0040] This is evident from the illustration in Fig. 3. Object 7 moves within the monitored area 8 and, for example, approaches. The light emitted by light source 9 is initially radiated into the monitored area 8 and strikes object 7. As long as the object is far away from light source 9, light reflected from object 7 also falls back onto photodiode 2 at a steep angle, initially in the area of ​​the first tap 4 in the limiting case. The closer object 7 approaches, the further the light spot 6 moves toward the second tap 5.

[0041] In the Fig. This is shown as a schematic diagram in Figure 4. The upper part of the schematic diagram initially includes a controller 21 and a driver 20, with the aid of which a pulsed light beam is emitted from a light source 9, in particular a light-emitting diode. By using a pulsed light beam, it can be particularly well distinguished from the ambient light. In particular, the emitted patterns are also stored in the system to be taken into account for comparison when evaluating the reflected light.

[0042] As indicated in the previous figures, the photocurrents X1 and X2 are taken from taps 4 and 5 on both sides of the position-sensitive diode 2 (although a photodiode array 1 in all embodiments described in this document would also be possible here), and are each fed to a preamplifier 19. This is followed by a sample-and-hold element 15, which samples the signal of the respective photocurrents X1 and X2 and stores it in a capacitor 16 for a complete measurement cycle.

[0043] For the complete measuring cycle, the same value of the photocurrent X1 and the same value of the photocurrent X2 are applied to the subsequent analog computing circuit 10.

[0044] In the analog computing circuit 10, the photocurrents X1 and X2 are then processed in a manner already known per se. This results in y=k⋅X1−X2X1−X2+b where y represents the output signal of the analog arithmetic circuit, which results from a constant factor k, the photocurrents X1 and X2, and an offset b, which can also be zero. The output signal y thus determined is then evaluated in an evaluation circuit 11, and a switching signal is generated from it.

[0045] However, this alone is not considered sufficient because this output signal y does not take into account the signal intensity of the reflected light. As a result, for weakly reflective objects 7, the switching hysteresis may be insufficient, resulting in a constant switching between an on and off level. For highly reflective objects 7, the hysteresis may be too large, and possibly no switching signal may be output at all, even though the object is sufficiently close.

[0046] To remedy this, as in Fig.5, a first correction value Δx is applied. This first correction value is selected so that it is only slightly higher than a value dx caused by photodiode noise. The first correction value Δx is first applied to the photocurrent X1, and in parallel with reverse polarity as a second correction value -Δx to the photocurrent X2. After determining a first output signal y1 with y1=kx2−x1+2Δxx2+x1+b The switch 14 is then activated and the polarity of the correction values ​​Δx or -Δx is reversed. Using the same calculation procedure, a second output signal y2 with y2=kx2−x1−2Δxx2+x1+b Since the measured signal remains present during the measurement cycle due to the function of the sample-and-hold element, the values ​​X1 and X2 do not change during the measurement cycle. After the two successive calculations, an average value y is obtained as y=(y1+y2) / 2=kx2−x1x2+x1+b where the hysteresis is now set to a value y1−y2=4kΔxx2+x1y1+y2=4kx2−x1x2+x1+2b can be set, whereby this value is optimal to obtain a stable switching process.

[0047] The above describes an optical proximity switch and a method for detecting when a minimum distance to an object is exceeded, which enable the smallest possible but stable switching hysteresis and thus ensure reliable switching for both highly and weakly reflective objects. LIST OF REFERENCE SYMBOLS 1 photodiode array 2 position-sensitive photodiodes 3 Center line 4 first tap 5 second tap 6 Light spot 7 Subject 8 Surveillance area 9 Light source 10 analog arithmetic circuit 11 Evaluation circuit 12 first adder 13 second adder 14 switches 15 Sample and hold element 16 Capacitor 17 first photodiode 18 second photodiode 19 preamplifiers 20 drivers 21 Control 22 inverters 23 Linearization circuit X1 first photocurrent X2 second photocurrent Δx first correction value -Δx second correction value y output signal z Distance signal k constant factor b Offset

Claims

[1] Optical proximity switch for detecting when a minimum distance to an object (7) within a monitoring area (8) is exceeded, comprising a light source (9) for emitting light into the monitoring area (8) and a receiving unit for detecting a reflection of the emitted light, wherein the receiving unit comprises a photodiode arrangement (1) with a sensor surface which is arranged between a first tap (4) and a second tap (5), the reflection causing a first photocurrent (X1) at the first tap (4) and a second photocurrent (X2) at the second tap (5), and the receiving unit further comprises a, preferably analog, computing circuit (10) for determining a ratio between the difference between the first photocurrent (X1) and the second photocurrent (X2) and the sum of the first photocurrent (X1) and the second photocurrent (X2),and an evaluation circuit (11) for generating a switching signal from this ratio, , characterized by that a first adder (12) for adding a first correction value (Δx) to the first photocurrent (X1) is assigned to the first tap (4), and a second adder (13) for adding a second correction value (-Δx) to the second photocurrent (X2) is assigned to the second tap (5), wherein the first correction value (Δx) corresponds to the inverted second correction value (-Δx) and the first correction value (Δx) and the second correction value (-Δx) are invertible by means of a changeover switch (14), wherein, within a measuring cycle, the evaluation circuit (11) detects a first ratio in a first position of the changeover switch (14) and a second ratio in a second position of the changeover switch, and determines the switching signal on the basis of an average value and a difference between the first ratio and the second ratio. [2] Proximity switch according to claim 1, characterized by that the first tap and the second tap are each assigned a sample-and-hold element (15) each having a capacitor (16) in which the signal levels of the photocurrents (X1, X2) are stored as voltage values ​​during a measuring cycle. [3] Proximity switch according to one of claims 1 or 2, characterized by that the evaluation circuit (11) comprises a comparator circuit with switching hysteresis for monitoring the mean value and for generating the switching signal therefrom when an adjustable reference voltage is exceeded. [4] Proximity switch according to one of the preceding claims, characterized by that the amount of the correction values ​​(Δx, -Δx) is selected to be greater than or equal to a noise signal of the photodiode arrangement (1). [5] Proximity switch according to one of the preceding claims, characterized bythat the photodiode arrangement (1) comprises a row with a plurality of photodiodes (17, 18), wherein the sensor surface extends over a plurality of photodiodes (17, 18). [6] Proximity switch according to one of claims 1 to 4, characterized by that the photodiode arrangement (1) comprises a position-sensitive photodiode (2) with a continuous sensor surface. [7] Proximity switch according to one of the preceding claims, characterized by that the evaluation circuit (11) is assigned a linearization circuit (23) for determining a distance value from the mean value. [8] Method for detecting the undershoot of a minimum distance of an optical proximity switch to an object (7) within a monitoring area (8), in which a light source (9) emits light into the monitoring area (8) and a receiving unit detects a reflection of the emitted light, wherein the receiving unit comprises a photodiode arrangement (1) with a sensor surface which is arranged between a first tap (4) and a second tap (5), the reflection at the first tap (4) causes a first photocurrent (X1) and at the second tap (5) a second photocurrent (X2), and the receiving unit further determines a ratio between the difference between the first photocurrent (X1) and the second photocurrent (X2) and the sum of the first photocurrent (X1) and the second photocurrent (X2), preferably with the aid of an analogue computing circuit (10),and generating a switching signal from this ratio with the aid of an evaluation circuit (11), , characterized by that in each measuring cycle, a first correction value (Δx) is added to the first photocurrent (X1) at the first tap (4) and a second correction value (-Δx) is added to the second photocurrent (X2) at the second tap (5), wherein the first correction value (Δx) corresponds to the inverted second correction value (-Δx) and within the same measuring cycle, the first correction value (Δx) and the second correction value (-Δx) are inverted with the aid of a changeover switch (14) and the measurement is repeated, wherein in a first position of the changeover switch (14) a first ratio is detected and in a second position of the changeover switch (14) a second ratio is detected and with the aid of the evaluation circuit (11) an average value and a difference between the first ratio and the second ratio is formed and the switching signal is determined therefrom. [9] Method according to claim 8, characterized by that the light source (9) emits a pulsed light signal, wherein the light signal is stored by means of a sample-and-hold element (15) and compared with a reflected light signal. [10] Method according to one of claims 8 or 9, characterized by that the evaluation circuit (11) linearizes the mean value with the aid of a linearization circuit (23) and determines an absolute distance signal (z) from the linearized mean value.

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