JPWO2009031528A1 - Signal detection device - Google Patents

Abstract

For example, a signal detection device for amplifying a sensor signal in a reflective photoelectric sensor through an amplifier and detecting the level of the sensor signal from an output obtained by digitally converting the output signal of the amplifier using an A / D converter, A predetermined offset is given to the output of the amplifier, and the average value of the output of the A / D converter when there is no signal is stored in a memory in advance, and the output of the A / D converter and the average value during normal operation Is detected as the level of the sensor signal.

Description

  The present invention relates to a signal detection device that detects a signal output from a sensor.

  For example, a reflective proximity switch, which is a typical photoelectric sensor, is configured to detect the presence of an object in the monitoring region by detecting light reflected by the object irradiated to a predetermined monitoring region. Specifically, for example, as described in detail in Japan; Japanese Patent Application Laid-Open No. 2007-184589 and Japanese Patent Application Laid-Open No. 2006-80896, light is emitted from a light-emitting element (for example, a light-emitting diode; LED) toward a monitoring region. Then, the reflected light from the monitoring area is received by a light receiving element (for example, a photodiode; PD). The output current from the light receiving element is I / V converted, amplified to a predetermined voltage level using an amplifier, and this is taken into an arithmetic unit (for example, CPU) via an A / D converter, and the light receiving element The amount of received light is detected and the presence of an object in the monitoring area is detected.

  The amplifier includes, for example, an inverting input terminal (− terminal) for inputting an I / V converted sensor signal via a capacitor, and a non-inverting input terminal (+ terminal) to which a reference voltage Vref is applied, and an output terminal. It comprises an operational amplifier provided with a feedback circuit between the inverting input terminal. The sensor signal is amplified by applying negative feedback so that the level (voltage) Vin of the sensor signal applied to the inverting input terminal becomes equal to the reference voltage Vref.

  By the way, in this kind of proximity switch, it is desirable to be able to detect the presence of an object in a wide distance range from a short distance to a long distance. By the way, the light reception level (sensor signal) of reflected light from a short distance is somewhat high, but the reflected light from a long distance is attenuated by light propagation, so its light reception level compared to the reflected light from a short distance. I cannot deny that it becomes low. Therefore, conventionally, the operating condition of the amplifier is set so that the A / D conversion output (digital value) is zero [0] when there is no signal (when light is shielded) without receiving reflected light. An A / D conversion output (digital value) corresponding to the light reception level is obtained. Specifically, the drive voltage Vdriv of the A / D converter that defines the dynamic range (full scale) of the A / D converter is given as the reference voltage Vref given to the amplifier (the non-inverting terminal of the operational amplifier). ing.

  By the way, when the light receiving element is shielded from light, it can be generally said that the current output from the light receiving element is [0]. However, in reality, a circuit constituting the light receiving system includes thermal noise caused by a bias resistance of the light receiving element and a voltage converting resistance in the I / V converter. The appearance frequency of this thermal noise is generally a normal distribution, and on average, it can be considered that the positive and negative distributions cancel each other and are zero [0]. For this reason, a weak noise component due to the thermal noise is also superimposed on the amplified output of the amplifier. Therefore, even if the light receiving element is shielded from light and becomes no signal, the noise component is directly added to the A / D converter.

  However, the A / D converter is configured to perform an A / D conversion operation only on an input signal having a single polarity (for example, positive polarity) based on a normal [0] level. For this reason, only a unipolar (positive polarity) noise component of the thermal noise input to the A / D converter is digitally converted. Then, as shown by hatching in FIG. 4, the noise component of reverse polarity (negative polarity) is ignored. As a result, a level shift depending on the noise distribution characteristic shown in the right half of FIG. 4 occurs in the output of the A / D converter, particularly in the digital conversion value when there is no signal. In other words, even when there is no signal, the output of the A / D converter has a minute value deviated from the [0] level. For this reason, it becomes difficult to distinguish (discriminate) between when the light receiving level of the reflected light is low and the no-signal state formed by the light shielding. Specifically, the long-distance detection characteristics of the reflective photoelectric sensor are deteriorated.

  An object of the present invention is to provide a signal detection device having a simple configuration capable of reducing the influence of thermal noise and capable of setting a sufficiently long object detection distance in a proximity switch using a reflective photoelectric sensor, for example. It is in.

The present invention is suitable for application to, for example, a proximity switch using a reflective photoelectric sensor, and amplifies a sensor signal through an amplifier, and digitally converts an output signal of the amplifier using an A / D converter. The present invention relates to a signal detection device that detects the level of the sensor signal from the converted output.
In particular, the signal detection device according to the present invention is made paying attention to the output of the A / D converter when there is no signal, and gives a predetermined offset to the output of the amplifier, and also the A / D when there is no signal. The average value of the output of the converter is stored in a memory in advance, and the difference between the output of the A / D converter and the average value during normal operation is detected as the level of the sensor signal.

Incidentally, the sensor signal is, for example, a voltage signal obtained by I / V converting the output current of the light receiving element, and is set by shielding the light receiving element when there is no signal.
The amplifier includes, for example, an inverting input terminal for inputting a sensor signal and a non-inverting input terminal to which a reference voltage is applied, and includes an operational amplifier provided with a feedback circuit between the output terminal and the inverting input terminal, The sensor signal is amplified by applying negative feedback so that the voltage of the inverting input terminal becomes equal to the reference voltage. Then, by setting the reference voltage applied to the non-inverting input terminal of the amplifier as a voltage obtained by applying a predetermined offset voltage to the power supply voltage of the A / D converter, an offset corresponding to a noise level is output to the output. Is given. Note that it is desirable that the reference voltage after applying a predetermined offset voltage does not fall below [0].

  According to the signal detection device having the above-described configuration, a predetermined offset is given to the output of the amplifier, and the average value of the output of the A / D converter at the time of no signal is stored in advance in the memory. Since the difference between the output of the D converter and the average value stored in the memory is handled as the detection value of the sensor signal, the level of the input signal can be accurately detected regardless of the offset. In particular, it is possible to accurately detect the level of the input signal without being affected by noise.

The figure which shows schematic structure of the photoelectric sensor which employ | adopted the signal detection apparatus which concerns on one Embodiment of this invention. The figure which contrasts and shows the case where the sensor signal given to an A / D converter from an amplifier and this sensor signal are given an offset. The figure which shows the output distribution characteristic of the A / D converter at the time of no signal when giving an offset to a sensor signal. The figure which shows the output distribution characteristic of the A / D converter at the time of no signal resulting from the thermal noise superimposed on the sensor signal.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Light receiving element 10 I / V converter 20 Amplifier 30 A / D converter 40 Calculator (CPU)
50 memory

Hereinafter, a signal detection apparatus according to an embodiment of the present invention will be described with reference to the drawings, taking as an example an apparatus for detecting the level of reflected light detected by a light receiving element of a reflective photoelectric sensor.
FIG. 1 is a schematic configuration diagram of a main part of a photoelectric switch constructed by incorporating a signal detection device according to the present invention. For example, the photoelectric switch irradiates a predetermined monitoring area with pulse light having a period T from a light projecting element (not shown), and receives light reflected by an object existing in the monitoring area by a light receiving element (photodiode) 1. And it is comprised so that presence of the object in the said monitoring area | region may be detected from the light reception amount (light reception level of reflected light) by the light receiving element 1. FIG. In the case of a transmissive photoelectric switch, a light projecting element and a light receiving element are arranged to face each other with a monitoring area interposed therebetween, and light from the light projecting element is blocked by an object entering the monitoring area. The object is detected from a change in the amount of light received by the light receiving element.

  Specifically, the light receiving system of the photoelectric switch includes an I / V converter 10 that performs I / V conversion on the current output from the light receiving element 1 according to the amount of received light, and the I / V converter 10. And an amplifier 20 that amplifies the voltage signal (sensor signal) output from the signal to a predetermined signal level. Then, the sensor signal amplified through the amplifier 20 is digitally converted through the A / D converter 30 and then taken into the arithmetic unit (for example, CPU) 40, and the signal level is detected. Is detected. In the case of the reflective type, when the received light amount exceeds a predetermined threshold value, and in the case of the transmissive type, it is detected that the object is present in the monitoring area when the received light amount is less than the predetermined threshold value. Configured.

  Incidentally, the I / V converter 10 is mainly composed of an operational amplifier 12 in which a feedback circuit 11 is provided between an output terminal and an inverting input terminal (−). The light receiving element 1 is connected to the power supply VDD through a resistor. The voltage at the connection point between the resistor and the light receiving element 1 decreases when the output current of the light receiving element 1 increases due to light reception. The I / V converter 10 inputs the voltage at the connection point (the output current of the light receiving element 1) to the inverting input terminal (−) of the operational amplifier 12 via the capacitor 13, and the operating point of the operational amplifier 12. Is input to the non-inverting input terminal (+) of the operational amplifier 12 to obtain a sensor signal V obtained by converting the output current I of the light receiving element 1 into a voltage at the output terminal of the operational amplifier 12. . This sensor signal V increases as the amount of light received by the light receiving element 1 increases.

  The amplifier 20 basically has a non-inverting input terminal (−) for inputting a sensor signal V output from the I / V converter 10 and applied via a capacitor 21 and an input resistor, and a reference voltage VR2. The operational amplifier 23 includes an inverting input terminal (+) and a feedback circuit 22 provided between the output terminal and the inverting input terminal (−). The operational amplifier 23 is configured to amplify the sensor signal V by applying negative feedback via the feedback circuit 22 so that the voltage V of the inverting input terminal (−) becomes equal to the reference voltage VR2. .

  In some cases, the amplification gain of the amplifier 20 can be variably set by changing the circuit constant of the feedback circuit 22 as indicated by a broken line in FIG. The output signal voltage of the amplifier 20 is equal to the reference voltage VR2 when the light receiving amount of the light receiving element 1 is zero [0], and decreases from the reference voltage VR2 as the light receiving amount of the light receiving element 1 increases to zero [0]. ]. The output signal of the amplifier 20 is converted into a digital value by the A / D converter 30 and then input to the arithmetic unit (CPU) 40. This computing unit (CPU) 40 obtains the difference between the reference voltage VR2 stored in advance in the memory 50 and the output signal of the A / D converter 30 as the amount of light received by the light receiving element 1 as will be described later. .

  Incidentally, in the prior art, the voltage VR2 applied to the non-inverting input terminal (+) of the operational amplifier 23 is set to the A / D converter 30 provided at the subsequent stage of the amplifier 20 in order to ensure the maximum dynamic range. It is set equal to the drive voltage Vdriv of the / D converter 30. Incidentally, the range of the A / D converter 30 is [0 to Vdriv]. That is, the output signal voltage of the amplifier 20 is set to [VR2 = Vdriv] when the amount of light received by the light receiving element 1 is zero [0].

  However, in the signal detection apparatus according to the present invention, the output of the A / D converter 30 when the light receiving element 1 described above is shielded from light so that there is no signal, that is, when the level of the sensor signal is zero [0]. The voltage VR2 applied to the non-inverting input terminal (+) of the operational amplifier 23 is set as [Vdriv−Voff] so that has an offset Voff corresponding to the thermal noise level of the light receiving element 1. Then, the average value of the output of the A / D converter 30 in the no-signal state is stored in the memory 50 in advance. The arithmetic unit (CPU) 40 detects the level of the sensor signal using the output average value at the time of no signal stored in the memory 50 as a reference value. In other words, the arithmetic unit (CPU) 40 detects the difference as the level of the sensor signal by subtracting the average value stored in the memory 50 from the output of the A / D converter 30 obtained during normal operation. To do.

  More specifically, the reference voltage VR2 (voltage applied to the non-inverting input terminal of the operational amplifier 23) applied to the amplifier 20 is set to a value equal to the drive voltage Vdriv of the A / D converter 30 as in the prior art. In this case, the amplifier 20 responds to the pulsed sensor signal, and the pulsed amplified output A based on the [Vdriv] level is supplied to the A / D converter 30 as shown by the solid line in FIG. Output. As a result, the A / D converter 30 sets the level of the amplified output A when the level of the sensor signal is [0] as the digital conversion value Vdriv, and the pulse of the amplified output A based on this level [Vdriv]. A digital conversion value corresponding to high WA (pulse amplitude) is output.

  On the other hand, when the light receiving element 1 is shielded from light, ideally, the current output from the light receiving element 1 is [0] and the output from the amplifier 20 is [0]. However, in reality, thermal noise is generated in the bias resistance of the light receiving element 1 and the voltage conversion resistance of the I / V converter 10 in the circuit constituting the light receiving system, so that the light receiving element 1 is not receiving light. However, the amplifier 20 outputs a weak signal due to thermal noise. As shown in FIG. 3, this thermal noise generally has a normal distribution of positive and negative frequencies around zero [0], and can be regarded as zero [0] on average. For this reason, a weak noise component due to the thermal noise is also superimposed on the amplified output A of the amplifier 20, and the noise component remains as it is even if the light receiving element 1 is shielded from light and is in a no-signal state. Added to. That is, a noise component normally distributed in the positive and negative directions around [Vdriv] is input to the A / D converter 30 even when the amount of light received by the light receiving element 1 is [0].

  However, since the A / D conversion 30 is configured to perform A / D conversion only for one polarity (for example, negative polarity) on the basis of the Vdriv level, as shown in FIG. Only the part) is digitally converted, and the noise component with no reverse polarity (positive polarity) (the part without diagonal lines) is ignored. As a result, a level shift depending on the noise distribution characteristic occurs in the output of the A / D converter 30, particularly in the digital conversion value when there is no signal. In other words, even when there is no signal, the output of the A / D converter 30 has a minute value deviated from the Vdriv level to the negative side. For this reason, if the received light signal WA of the reflected light is small, it is difficult to distinguish (discriminate) from the no-signal state formed by the light shielding, and the long-distance detection characteristics of the reflective photoelectric sensor are deteriorated.

  In this regard, in the present invention, by giving a predetermined offset Voff to the amplifier 20 as described above, an amplified output B having a predetermined offset from the Vdriv level is obtained as shown in FIG. / D converter 30 is input. As a result, the A / D converter 30 amplifies all the thermal noise components that change around the offset level [Vdriv−Voff] as shown in FIG. Since the minute output change of the A / D converter 30 caused by the thermal noise component of the light receiving element 1 when the light receiving element 1 is shielded is [0] on average, the A / D conversion is performed when there is no signal. The output level is substantially only [Vdriv−Voff].

  On the other hand, the light receiving element 1 is set in a light shielding state in advance, and the output of the A / D converter 30 in this light shielding state (no signal) is sampled a plurality of times. The average value of the output is stored in the memory 50 as the average value Voff of the magnitude of thermal noise of the light receiving element 1. In the subsequent normal operation, the arithmetic unit 40 subtracts the average value Voff stored in the memory 50 from the output B of the A / D converter 30, and calculates the difference as the level WB of the sensor signal. According to the present apparatus that detects the level of the sensor signal in this way, it is possible to obtain the level of the sensor signal (the amount of received light) with high accuracy without being affected by thermal noise. Therefore, problems such as deterioration of the long-distance detection characteristics of the reflective photoelectric sensor are not caused.

  In particular, the level (light receiving amount) of the sensor signal obtained as described above is the level of the driving voltage Vdriv of the amplifier 20 to which the offset Voff is given, that is, the [Vdriv−Voff] level which is a level when there is no signal. Similarly, since the offset Voff is a difference from the pulse height of the sensor signal B to which the offset Voff is applied, the above-described offset Voff does not cause trouble in detecting the level of the sensor signal. Therefore, when this signal detection device is applied to a reflective photoelectric sensor (proximity sensor), it is possible to accurately detect an object in a monitoring target region in a wide distance range from a short distance to a long distance.

  That is, according to the present invention, for example, even when determining the output of a reflective photoelectric sensor, reflected light from a long distance with a small input signal level can be reliably detected, and the detection dynamic range can be increased. It can be set wide enough. Specifically, a predetermined offset is given to the output of the amplifier, and the zero level itself is shifted. Therefore, as the output of the A / D converter that does not take a negative value, generally, thermal noise that is normally distributed is used. All of the included signals can be A / D converted. As a result, when no signal is input (when light is blocked), all of the thermal noise generated in the light receiving system can be A / D converted. Therefore, by calculating the average value of the signals including the thermal noise described above, The output level of the light receiving system can be obtained accurately.

  The average value of the output signal at the time of light shielding including the thermal noise is stored in the memory 50 as a reference value, and the value obtained by subtracting the reference value from the output of the A / D converter 30 in the normal operation is received by the light receiving element 1. In general, it is possible to cancel out the normally distributed thermal noise by averaging the output of the A / D converter. Therefore, it is possible to accurately detect the amount of received light (light reception level) without being affected by thermal noise. In other words, due to the fact that the output of the A / D converter does not take a negative value, the problem that the output shifts due to the influence of thermal noise is solved, and the amount of light received by the light receiving element is reduced. It can be detected accurately. As a result, for example, an object can be detected over a wide range without sacrificing the long-distance detection characteristic of the reflective photoelectric sensor.

  In addition, this invention is not limited to embodiment mentioned above. For example, the detection of the average value of the output of the A / D converter 30 when there is no signal may be performed at the time of calibration at the time of shipment of the photoelectric sensor, and the detection value may be preset. However, it may be detected by forming a light shielding state in the usage environment of the photoelectric sensor and operating a preset switch incorporated in the photoelectric sensor. At this time, the above-described offset can be appropriately adjusted according to the average value of the output of the A / D converter 30 in a light-shielded state.

  Although the reflective proximity switch has been described here as an example, the present invention can be similarly applied to the case where the output of the light receiving element in the transmissive photoelectric sensor is detected. Furthermore, although the case where the output of the light receiving element 1 is detected has been described as an example, the present invention can be similarly applied to the case where the output level of other sensing elements is detected. In the embodiment, an example in which an A / D conversion is performed on a signal that swings in the minus (−) direction from the Vdriv level by using an inverting amplifier. However, a non-inverting amplifier is used in the plus (+) direction from the Vdriv level. Needless to say, the present invention can also be implemented in the same manner when a signal that fluctuates is A / D converted and taken into the arithmetic unit (CPU) 40. The memory 50 may have a memory function built in the arithmetic unit (CPU) 40. In addition, the present invention can be variously modified and implemented without departing from the scope of the invention.

Claims (5)

A sensor that converts a physical quantity into an electric signal, an amplifier that amplifies the electric signal output from the sensor, an A / D converter that converts the electric signal output from the amplifier into a digital signal, and the A / D conversion In a signal detection device comprising an arithmetic unit that calculates a physical quantity from a digital signal output from a device,
An offset unit that gives a predetermined offset to the output of the amplifier; and a memory that stores in advance an average value of the output of the A / D converter when there is no signal, and the arithmetic unit includes the A during normal operation. A signal detection apparatus that calculates a difference between an output of a / D converter and the average value stored in the memory as a physical quantity. The signal detection device according to claim 1, wherein the sensor signal is a voltage signal obtained by performing I / V conversion on an output current of the light receiving element, and is set by shielding the light receiving element when there is no signal. The amplifier includes an inverting input terminal for inputting a sensor signal and a non-inverting input terminal to which a reference voltage is applied, and includes an operational amplifier provided with a feedback circuit between the output terminal and the inverting input terminal. Amplifying the sensor signal by applying a negative feedback so that the terminal voltage is equal to the reference voltage,
The offset means gives an offset to the output of the amplifier by setting the reference voltage given to the non-inverting input terminal as a voltage obtained by giving a predetermined offset voltage to the power supply voltage of the A / D converter. The signal detection device according to claim 1. The signal detection apparatus according to claim 1, wherein the offset is set as an average value of the output of the A / D converter when there is no signal. The signal detection apparatus according to claim 1, wherein detection of an average value of the output of the A / D converter at the time of no signal is performed at the time of calibration.

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