JP2006322932A - Signal detection circuit - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a signal detection circuit having a simple constitution capable of detecting surely, for example, decline of a signal level of an interference component or the like of laser light, and preventing wrong recognition thereof. <P>SOLUTION: This circuit for inputting a signal whose frequency is changed corresponding to the change of a physical quantity, and detecting the physical quantity by analyzing the frequency is equipped with the first and second hysteresis comparators wherein mutually different hysteresis levels are set, and a processing circuit for comparing outputs from each hysteresis comparator and determining the decline of the signal level. For example, the output from each comparator is counted respectively by using the first and second counters, and the ratio between measured values is compared with a threshold set beforehand, to thereby determine the decline of the signal level. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a signal detection circuit with a simple configuration suitable for signal detection in an interferometer using laser light, for example.

  The laser interferometer measures various physical quantities using, for example, interference of laser light, and is configured to obtain the physical quantities by frequency analysis of interference components of the laser light. For example, as shown in FIG. 1, the distance information from the interference component between the laser light applied to the measurement object and the laser light reflected from the measurement object (return light) to the detection object is obtained, or not particularly illustrated. Is configured to cause two laser beams having different wavelengths to interfere with each other and detect the physical state of the measurement object from the interference state. Incidentally, the frequency analysis of the interference component described above can be performed by using, for example, a fast Fourier transform (FFT).

However, if complex signal processing such as FFT is used, the system cost cannot be denied. Therefore, for example, the above-described interference component is discriminated using a comparator such as a hysteresis comparator or a Schmitt inverter, and the frequency component is detected by counting the discrimination output using a counter. At this time, two hysteresis comparators (comparators) are used in parallel, and the outputs of the two hysteresis comparators are logically processed while changing the threshold value of one of the hysteresis comparators according to the signal frequency, thereby being included in the signal. It has also been proposed to remove a noise component (glitch) (see, for example, Patent Document 1).
JP 2001-94401 A

  By the way, the frequency of the interference component described above varies greatly depending on the physical quantity such as the distance L to the detection target. For example, when the frequency of the interference component when the distance L is 10 mm is about 10 kHz as shown in FIG. 2A, the frequency of the interference component is 1 MHz as shown in FIG. 2B when the distance L is 1 m. It will be about. Therefore, when detecting such a frequency component of an interference signal having a wide change width, the operation band of the hysteresis comparator described above needs to be sufficiently wide.

  Further, when the level of the interference component described above becomes low, there is a possibility that the interference component cannot be reliably detected by the hysteresis comparator described above. By the way, when the interference signal component is detected intermittently by the hysteresis comparator, the count value of the counter that counts the output pulse signal of the comparator decreases, so that the frequency component is erroneously detected lower than the actual value. There is a possibility. Furthermore, since the above-mentioned signal has a wide band, it often includes a sub-frequency (sub-harmonic) in addition to the fundamental frequency component, and there is a possibility that such a sub-frequency is erroneously recognized.

  The present invention has been made in view of such circumstances, and its object is to detect a signal level of an interference component or the like reliably and to prevent erroneous recognition of the signal detection circuit. Is to provide.

In order to achieve the above-described object, the signal detection circuit according to the present invention inputs a signal whose frequency changes in accordance with a change in physical quantity, analyzes the frequency and detects the physical quantity,
A first comparator that sets a first hysteresis level and discriminates the signal to generate a first pulse signal (hysteresis comparator);
A second comparator that is set to a second hysteresis level different from the first hysteresis level and discriminates the signal to generate a second pulse signal;
And a processing circuit for comparing the first and second pulse signals respectively obtained by the first and second comparators to determine the level drop of the signal.

  Incidentally, the signal whose frequency changes in accordance with the change in the physical quantity is a self-coupling effect between the output of the laser interferometer, for example, the laser light irradiated toward the measurement object and the laser light reflected by the measurement object. It consists of what shows the interference component by. Preferably, the signal is supplied to the first and second comparators through a bandpass filter (BPF) having a preset pass bandwidth.

  For the processing circuit, for example, a first counter that counts the first pulse signal output from the first comparator and a second counter that counts the second pulse signal output from the second comparator. And a counter for calculating the ratio of the count values of the first and second counters, and a determination unit for comparing the ratio of the count values determined by the calculator with a threshold value. Just do it. Alternatively, the first and second pulse signals may be logically processed, and a so-called tooth missing state of the pulse signal may be detected from the logical processing result.

  For example, the first hysteresis level is set as a discrimination threshold for determining that the signal is equal to or higher than a predetermined level, and the second hysteresis level is set for discrimination for counting the signal. It may be set as a threshold value.

  According to the signal detection circuit having the above-described configuration, the decrease in the signal level is caused by the output pulse signal of the first comparator (hysteresis comparator) in which the first hysteresis level (discrimination threshold) is set, and the second hysteresis level. Since it can be easily detected by comparing with the output pulse signal of the second comparator (hysteresis comparator) for which the (discrimination threshold) is set, the detection object due to the decrease in the signal count value due to the glitch It becomes possible to prevent erroneous recognition.

  In particular, in the present invention, for example, the count value of the output pulse of each comparator (hysteresis comparator) counted by the first and second counters over a preset period is compared and determined, and the signal is Since it is determined that the level has dropped to an inappropriate level for signal detection, the detection range can be sufficiently widened to the limit. Furthermore, even if the signal to be detected includes a sub-frequency (sub-harmonic) other than the fundamental frequency component, it is determined that the signal has dropped to an inappropriate level for the signal detection. Therefore, the erroneous detection can be prevented beforehand.

  In addition, the above-described first and second comparators (hysteresis comparators) are used to detect the signal with a simple circuit configuration that determines the ratio of the count values of the output pulses by the first and second counters, for example. Reliability can be sufficiently increased.

Hereinafter, with respect to a signal detection circuit according to an embodiment of the present invention with reference to the drawings, the state of a measurement object irradiated with the laser light is detected from an interference component generated by the self-coupling effect of the laser light in the laser element A case where the present invention is applied to a measuring system will be described as an example.
The signal detection circuit according to this embodiment inputs an output (light reception signal) from a light receiver (PD) 3 that receives an interference component (laser light) generated in the laser element 1 and obtains its frequency component. is there. For example, as shown in FIG. 4, this signal detection circuit passes through a bandpass filter (BPF) 10 having a passband characteristic of about 10 kHz to about several tens of MHz, which is a frequency component of the interference laser light to be detected. The light receiving signal is input.

  In particular, this signal detection circuit includes first and second hysteresis comparators (comparators) 11 and 12 in which different hysteresis levels are set in parallel, and the received light signal is received by each of these hysteresis comparators 11 and 12. First and second counters 13 and 14 are provided for counting first and second pulse signals generated by discrimination over a preset period, respectively. Incidentally, the first hysteresis level for the first hysteresis comparator 11 is set as a discrimination threshold [Th1] for determining the presence or absence of a signal exceeding a predetermined level. In contrast, the second hysteresis level for the second first hysteresis comparator 12 is slightly lower than the discrimination threshold [Th1], and the discrimination threshold [Th2 ( <Th1)]. Further, the counting cycle of the pulse signal by each of the counters 13 and 14 is set, for example, as synchronized with the wavelength modulation cycle of the laser beam described above.

  Normally, the count value [N] of the pulse signal discriminated by the first hysteresis comparator 11 and counted by the first counter 13 is output via the output unit 15 as the frequency (measurement value) of the interference component. Is done. Further, as will be described later, the pulse signal counted by the first counter 13 is compared with the count value [M] of the pulse signal discriminated by the first hysteresis comparator 12 and counted by the second counter 14. When the ratio of the count value [N] is equal to or greater than a preset ratio [P], for example, the count value [M] of the pulse signal counted by the second counter 14 is the frequency of the interference component ( (Measured value) is output via the output unit 15. When the ratio of the count value [M] by the second counter 14 and the count value [N] by the first counter 13 is less than the ratio [P], the reliability of the input signal itself is poor. The output of the measurement value from the output unit 15 is blocked and only information indicating that an interference component exists is output as (there is a possibility of erroneous recognition due to a decrease in signal level).

  Specifically, the count values [N] and [M] of the pulse signal counted at predetermined intervals by the first and second counters 13 and 14 are ratio calculation unit 16 in synchronization with the counting cycle. And the ratio [N / M] is calculated. When the ratio [N / M] obtained by the ratio calculation unit 16 is [1], the determination unit 17 indicates that the count value [N] by the first counter 13 and the count value [N] by the second counter 14 [ When M] is equal, the level of the input signal (interference component) is sufficiently high, and it is determined that the discrimination of the input signal by the second hysteresis comparator 12 is normally performed. Then, the count value [M] of the second pulse signal output from the second hysteresis comparator 12 by the second counter 14 is adopted as the frequency (measurement value) of the interference component, and this is output to the output unit 15. Output through.

  The determination unit 17 determines whether the ratio [N / M] exceeds the above-described ratio [P] when the ratio [N / M] obtained by the ratio calculation unit 16 is less than [1]. Is judged. That is, when the signal level of the input signal itself is lowered and fluctuations occur as shown in FIG. 5 due to the influence of noise, the signal level is set to the first hysteresis comparator 11. In the vicinity of the hysteresis level [Th1], the output pulse signal may be interrupted. Then, the count value [N] by the first counter 13 that counts the output pulse signal becomes smaller than the frequency component that the input signal originally has.

  In this respect, since the second hysteresis comparator 12 is set with the second hysteresis level [Th2] smaller than the first hysteresis level [Th1] described above, the output pulse signal is the first hysteresis comparator. 11 is less disruptive due to noise components (level fluctuations) contained in the input signal than the output pulse signal. As a result, the value [N / M] of the count values obtained by the ratio calculator 16 decreases.

  Therefore, as described above, the ratio [N / M] of the respective count values [N] and [M] by the first and second counters 13 and 14 is obtained, and this ratio [N / M] is set to a preset ratio [P ], If the count value [N] by the first counter 13 is somewhat less than the count value [M] by the second counter 14, that is, When the ratio [N / M] exceeds the preset ratio [P], it can be determined that the reliability of the count value is low, but the influence of the signal level and noise (signal level fluctuation) is small. .

  Further, when the count value [N] by the first counter 13 is significantly lower than the count value [M] by the second counter 14, that is, the ratio [N / M] is a preset ratio. When it is less than [P], it can be determined that the difference between the count values of both counters 13 and 14 is large and the reliability of the count value [M] of the second counter 14 is poor. Then, it can be determined that the cause is caused by, for example, a decrease in signal level.

  Thus, according to the signal detection circuit configured as described above, the levels of the input signals are discriminated using the first and second hysteresis comparators (comparators) 11 and 12 having different hysteresis levels, respectively. It is determined whether the ratio [N / M] of the count values [N] and [M] by the first and second counters 13 and 14 that respectively count the output pulse signal exceeds a preset ratio [P]. Therefore, even when the count value [M] by the second counter 14 becomes small due to fluctuations in the signal level due to noise, it is not erroneously detected that the frequency component has become low. Specifically, when applied to a laser interferometer, for example, the measurement object 2 that has been measured at a long distance position is gradually moved away, but is not erroneously detected as being suddenly present at a short distance position. Moreover, there is no possibility of erroneous detection that the measurement object 2 does not exist even though the measurement object 2 exists at a long distance position as in the past.

  In particular, the measurement object 2 is located at a very far position in the distance range to be measured, the signal level of the laser reflected light from the measurement object 2 becomes low, and the frequency of the laser interference wave accompanying this is the above-described band pass. When it exists in the vicinity of the high-frequency cutoff frequency of the filter 10, signal detection becomes difficult in the detection circuit described in Patent Document 3 described above. That is, since the frequency of the laser interference wave is wide, the frequency of the sub-frequency (sub-harmonic) and noise (signal level fluctuation) other than the fundamental frequency component overlaps the band of the laser interference wave, making it difficult to detect the signal. It becomes.

In this respect, in the signal detection circuit according to the present invention, since the effectiveness of the signal is determined from the ratio of the number of pulses of the signal discriminated at different hysteresis levels as described above, the frequency of the signal overlapping the band of the interference wave is determined. It is possible to detect a decrease in the reliability of the signal itself due to noise or the like, and the measurement reliability can be sufficiently enhanced.
The present invention is not limited to the embodiment described above. For example, in the above-described embodiment, the output pulse signals from the comparators 11 and 12 are counted using the first and second counters 13 and 14, respectively. It is also possible to execute the calculation of the ratio [N / M] of the count value, the predetermined ratio [P] and the comparison determination process, etc. collectively as described above. Further, a so-called missing state of the pulse signal is detected from the logical processing result of the output pulse signal, such as obtaining an exclusive OR of the output pulse signals from the first and second hysteresis comparators 11 and 12, and so on. It is also possible to detect a decrease in signal level by determining the degree of.

  Furthermore, if the hysteresis levels set in the first and second hysteresis comparators 11 and 12 and the counting periods of the first and second counters 13 and 14 are determined according to the specifications of the signals to be detected, It ’s good. In addition, the present invention can be variously modified and implemented without departing from the scope of the invention.

The figure which shows schematic structure of a laser interferometer. The figure which shows the example of the interference wave which changes with physical quantities. The figure which shows the example of the level change of the signal with respect to interference wave frequency (detection distance). 1 is a schematic configuration diagram of a signal detection circuit according to an embodiment of the present invention. The figure which shows the relationship between the input signal in the signal detection circuit shown in FIG. 4, and the output pulse signal of the 1st and 2nd hysteresis comparator.

Explanation of symbols

1 Laser element (LD)
2 Measurement object 3 Light receiver (PD)
10 Bandpass Filter 11 First Hysteresis Comparator (Comparator)
12 Second hysteresis comparator (comparator)
13 First Counter 14 Second Counter 15 Output Unit 16 Ratio Calculation Unit 17 Determination Unit

Claims (6)

A signal detection circuit that inputs a signal whose frequency changes in accordance with a change in physical quantity, analyzes the frequency and detects the physical quantity,
A first comparator that is set with a first hysteresis level to discriminate the signal and generate a first pulse signal;
A second comparator that sets a second hysteresis level different from the first hysteresis level and discriminates the signal to generate a second pulse signal;
A signal detection circuit comprising: a processing circuit for comparing the first and second pulse signals obtained by the first and second comparators to determine a decrease in the level of the signal.   The signal detection circuit according to claim 1, wherein the signal whose frequency changes according to a change in the physical quantity is an output of a laser interferometer.   The signal whose frequency changes according to the change of the physical quantity is an interference component of a self-coupling effect generated by the laser light irradiated toward the measurement object and the laser light reflected by the measurement object. Item 2. The signal detection circuit according to Item 1.   4. The signal detection circuit according to claim 2, wherein the signal is supplied to the first and second comparators through a bandpass filter having a preset pass bandwidth. 5.   The processing circuit includes: a first counter that counts a first pulse signal output from the first comparator; a second counter that counts a second pulse signal output from the second comparator; 2. An arithmetic unit for obtaining a ratio of count values obtained by the first and second counters, and a determination unit for comparing the ratio of the count values obtained by the arithmetic unit with a threshold value. The signal detection circuit described in 1. The first hysteresis level is set as a discrimination threshold for determining that the signal is equal to or higher than a predetermined level,
The signal detection circuit according to claim 1, wherein the second hysteresis level is set as a discrimination threshold for counting the signal.

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