JP2001050711A - Optical displacement meter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical displacement meter capable of shortening measuring time and correctly measuring a dark object to be measured. SOLUTION: A laser beam emitted from a laser diode 31 irradiates an object to be measured 27 through a projection lens 35. A part of the laser beam reflected by the object to be measured 27 enters into a CCD linear sensor 33 through a light receiving lens 36. The electric charge accumulated in the CCD linear sensor 3 in accordance with the light receiving amount and a relative position of the object to be measured 27 is read out by a CCD driving circuit 34, and converted into a voltage signal in a time series. A reset circuit 48 sends a reset signal to the CCD driving circuit 34 every period selected from a plurality of periods in accordance with a command from a microprocessor 44. Whereby a sampling period including a dummy reading-out period is changed, and a charge accumulation time of the CCD linear sensor 33 is changed.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of irradiating light from a light source to a measuring object, receiving reflected light from the measuring object with an image sensor, and receiving a light receiving signal accumulated in the image sensor at a predetermined sampling period. The present invention relates to an optical displacement meter that reads out and converts digital data, and calculates a distance to an object to be measured based on the digital data.

[0002]

2. Description of the Related Art An optical displacement meter of this type is also called a laser displacement meter and is used to measure the displacement of an object (measurement object) in a non-contact manner using the principle of triangulation. The principle will be described with reference to FIG.

FIG. 1 is a schematic diagram showing an example of a conventional optical displacement meter. The laser light emitted from the laser diode 12 under the control of the LD driver 11 passes through the light projecting lens 13 and irradiates the measurement object 10. A part of the laser light reflected by the measuring object 10 passes through the light receiving lens 14 and is received by the PSD (light position detecting element) 15.

When the measuring object 10 is displaced away from the laser diode 12, as indicated by a broken line in FIG. 1, the optical path of the laser light reflected by the measuring object 10 and reaching the PSD 15 changes as indicated by the broken line. . as a result,
The position of the light receiving spot on the light receiving surface of the PSD 15 moves. The PSD 15 outputs two differential signals according to the position of the light receiving spot on the light receiving surface. One of the differential signals has a current value proportional to the distance from one end of the light receiving surface to the light spot, and the other signal has a current value proportional to the distance from the other end of the light receiving surface to the light spot.

[0005] These differential signals (currents) are converted into differential voltages by respective current-voltage conversion circuits (IV conversion circuits) 16 and 17. Then, the processing circuit 18 detects the displacement of the measuring object 10 based on these two differential voltages. In addition, among the above components, at least the laser diode 12, the light projecting lens 13, and the light receiving lens 1
4 and the PSD 15 are integrated as a sensor head.

Recently, an optical displacement meter using a CCD (solid-state image sensor) image sensor instead of a PSD (optical position detector) has been put to practical use. Usually, the position of the light receiving spot on the light receiving surface is digitally detected using a linear (one-dimensional) image sensor. In this case, a CCD drive circuit is provided instead of the current-voltage conversion circuits 16 and 17,
The CCD drive circuit reads out a charge proportional to the amount of received light accumulated in each pixel of the linear image sensor at a constant cycle (that is, a sampling cycle).

The sampling period is substantially equal to the product of the number of pixels of the linear image sensor and the transfer rate for each pixel. For example, when the transfer rate is 1 microsecond with a 256 pixel linear image sensor, the sampling period is 256
Microseconds. Conventionally, this sampling period is determined by an optical displacement meter and is not variable.

[0008] Usually, the measurement object 1 as described above is used.
Measurement of zero displacement (relative distance) multiple times (for example, 10 times)
The processing circuit 18 performs an average value of the obtained plurality of data, and uses the average value as a final measurement result. Such an average value calculation process is performed to increase the measurement accuracy. On the other hand, when it is necessary to reduce the measurement time, the number of times of measurement of the data for obtaining the average value is reduced (for example, three times). There is a trade-off between the measurement accuracy and the measurement time, and there is an optical displacement meter in which the number of measurements can be set according to which of the measurement accuracy and the measurement time is prioritized.

[0009]

However, there is a limit to a method for reducing the measurement time by reducing the number of times of data measurement for obtaining the average value. Even if the number of measurements is made one and the obtained data is used as the final measurement result, the measurement time does not become shorter than the sampling period. As described above, the sampling period is substantially equal to the product of the number of pixels of the linear image sensor and the transfer rate for each pixel.

When the number of pixels is reduced, the sampling period is shortened, but the position detection accuracy of the light receiving spot on the light receiving surface is deteriorated. The transfer rate is determined by the performance of a linear image sensor (CCD), and is usually used at its maximum rate. If the transfer rate is set too high, the time (accumulation time) for the linear image sensor to receive light from the object to be measured and accumulate charges corresponding to the amount of received light in each pixel becomes shorter. As a result, when the object to be measured is dark, sufficient charge is not accumulated, and proper measurement cannot be performed.

SUMMARY OF THE INVENTION It is an object of the present invention to solve the above-mentioned conventional problems and to provide an optical displacement meter capable of shortening the measurement time and appropriately measuring a dark object to be measured. And

[0012]

An optical displacement meter according to the present invention irradiates light from a light source to an object to be measured, receives reflected light from the object to be measured by an image sensor, and accumulates the light in the image sensor. An optical displacement meter that reads out the received light signal at a predetermined sampling period, converts it into digital data, and calculates the distance to the object to be measured based on the digital data. The optical displacement meter accumulates in the image sensor by making the sampling period variable. The storage time of the received light signal is variable.

According to the above configuration, when it is necessary to measure a bright measurement object in a short measurement time, the sampling period is set short, and when a dark measurement object is measured, the sampling period is set long. In any case, the displacement of the measuring object can be accurately measured.

[0014] Preferably, a dummy reading period is added following the reading period of the image sensor.
The sampling period is made variable without changing the transfer speed for each pixel of the image sensor. This makes it easy to adapt to high-speed measurement (short measurement time) or to measure a dark object by changing the sampling period without changing the frequency characteristics of the received light signal. In this case, since the frequency characteristic of the light receiving signal does not change, it is not necessary to change, for example, the time constant (cutoff frequency) of the filter of the signal processing circuit.

[0015]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 2 is an external view of an optical displacement meter according to an embodiment of the present invention. 2, (a) is a plan view, (b)
Is a side view. This optical displacement meter includes a sensor head section 21 and a controller section 22. The sensor head unit 21 has a built-in laser diode that emits laser light to irradiate the measurement target 27 and a CCD linear sensor that receives light reflected from the measurement target 27. Controller 2
2 controls the operation of the sensor head unit 21 and
A signal obtained from the CCD linear sensor is processed to obtain a distance (displacement) to the measurement target 27 and output it.

The sensor head 21 and the controller 2
2 is a cable 2 for exchanging electric signals with each other
3 is connected. Also, the sensor head unit 21
It is fixed to a predetermined mounting table 25 using two bolts 24. Each of the two mounting holes through which the bolt 24 is inserted is provided along the reference surface 26 of the sensor head 21. The reference surface 26 is a surface from which the laser light for measurement is emitted and the reflected light from the measurement object 27 is incident.

That is, the emission surface of the laser diode and C
The light receiving surface of the CD linear sensor is disposed near the reference surface 26, and the sensor head 2 extends along the reference surface 26.
1 will be fixed at a predetermined position. Therefore, even if the housing of the sensor head 21 expands and contracts due to, for example, a change in the ambient temperature, the influence on the position of the reference surface 26 (the emission surface of the laser diode and the light receiving surface of the CCD linear sensor) is small. If the position of the reference plane 26 fluctuates, the relative distance between the measurement object 27 and the reference plane 26 fluctuates, leading to a decrease in measurement accuracy.

FIG. 3 is a block diagram showing a main configuration of the optical displacement meter of the present embodiment. Sensor head 21
Includes a laser diode 31 and its drive circuit (LD driver) 32, a CCD linear sensor 33 and its drive circuit (CCD drive circuit) 34, a light projecting lens 35 and a light receiving lens 36. The controller section 22 includes a low-pass filter (LPF) 41, a peak hold circuit 42, AD converters 43 and 47, a microprocessor 44, a DA converter 45, an AGC (automatic gain control) amplifier 46, and a reset circuit 48.

The laser light emitted from the laser diode 31 passes through the light projecting lens 35 and irradiates the object 27 to be measured. Part of the laser light reflected by the measurement target 27 passes through the light receiving lens 36 and enters the CCD linear sensor 33. CCD according to the amount of received light and the relative position of the measuring object 27
The electric charges accumulated in the linear sensor 33 are read out by the CCD driving circuit 34 and converted into time-series voltage signals.

When the CCD drive circuit 34 supplies a predetermined transfer clock to the CCD linear sensor 33,
The D linear sensor 33 sequentially transfers the charge stored in each pixel one pixel at a time. For example, CCD linear sensor 3
In the case where 3 is composed of 256 pixels and the transfer rate for each pixel is 1 microsecond, the accumulated charges of all the pixels are read out in 256 microseconds and output as time-series voltage signals from the CCD drive circuit 34. The time required to read out the accumulated charges of all the pixels is a sampling cycle.

The output signal of the CCD drive circuit 34 is passed to the controller 22, where a high-frequency component is first removed by a low-pass filter 41. This high frequency component includes CC
The transfer rate component of the D linear sensor 33 is included.

FIG. 4 is a diagram schematically illustrating how the voltage signal output from the CCD drive circuit 34 changes through the low-pass filter 41 and the peak hold circuit 42. The output signal 51 of the CCD drive circuit 34 is a continuous rectangular wave as shown in FIG.
This corresponds to the transfer rate of the CD linear sensor 33. When the output signal 51 passes through the low-pass filter 41, the high-frequency component corresponding to the transfer rate is removed, and the voltage signal 52 includes only the low-frequency component corresponding to the envelope. This voltage signal 52 is transmitted to the one-dimensional position (2
Information on the distribution of the amount of received light with respect to which one of the 56 pixels). The higher the voltage value, the greater the amount of light received at that position.

The voltage signal 52 output from the low-pass filter 41 is applied to a peak hold circuit 42. The peak hold circuit 42 holds the peak value Vp of the voltage signal 52 and outputs it to the first AD converter 43. The AD converter 43 converts the peak value Vp into a digital value and provides the digital value to the microprocessor 44.

The voltage signal 52 is amplified by an AGC amplifier 46 and converted into a digital value by a second AD converter 47. The digital value is converted by the microprocessor 44
Are sequentially given to The gain of the AGC amplifier 46 is automatically controlled so that the amplified voltage value falls within a predetermined range.

The microprocessor 44 receives the peak data input through the first AD converter 43 and the second A
The position of the barycentric value of the received light amount is determined based on the sequential data input through the D converter 47. Once the position of the center of gravity of the received light amount is determined, as described in the description of the related art,
From the principle of triangulation, the distance (relative distance or displacement) to the measurement target 27 is obtained.

In practice, the output (digital data) of the second AD converter 47 is compared with a predetermined threshold value in order to eliminate spurious peaks due to noise, and only data larger than this threshold value is valid. Data. The distance (relative distance or displacement) to the measurement target 27 determined in this way is provided from the microprocessor 44 to the DA converter 45, converted into an analog voltage and output.

In FIG. 3, the intensity of the laser beam emitted from the laser diode 31 is controlled by the microprocessor 44 via the LD driver 32. If the intensity of the laser beam changes, the laser beam is reflected by the object to be measured 27 and the CCD
The amount of light (the amount of received light) incident on the linear sensor 33 also changes. Therefore, by adjusting the intensity of the laser light emitted from the laser diode 31 in accordance with the light reflectance (brightness) of the measurement object 27, the variation width of the amount of light received by the CCD linear sensor 33 is suppressed as much as possible. Specifically, the intensity of the laser light is adjusted by changing the duty ratio of the pulse for driving the laser diode 31.

Further, the optical displacement meter according to the present embodiment can change the sampling period, and
The accumulation time of the light reception signal accumulated in each pixel of the linear sensor 33 can be changed. For example, when it is necessary to measure a relatively bright object at high speed, the sampling period is shortened, while when a dark object to be measured is measured, the sampling period is lengthened.

Referring to FIG. 3, the reset circuit 48
The sampling cycle is changed by the microprocessor 44 controlling the cycle of the reset signal applied to the D drive circuit. As described above, the product (256 microseconds) of the number of pixels (for example, 256 pixels) of the CCD linear sensor 33 and the transfer rate (for example, 1 microsecond) of each pixel is usually a sampling period, and reset every period. The signal is provided to the CCD drive circuit. Upon receiving the reset signal, the CCD drive circuit 34 starts reading the CCD linear sensor 33.

However, in the optical displacement meter of this embodiment,
The reset circuit 48 does not necessarily output a reset signal for each of the above-described sampling periods.
, A reset signal is output at intervals of an integral multiple of the sampling period. In particular,
1 time, 2 times, 4 times, 8 times, 16 times of the above sampling period
A reset signal is supplied to the CCD drive circuit 34 at each cycle selected from the double (hereinafter, variable sampling cycle). Then, the transfer rate for each pixel is set to 0.5 microsecond. Therefore, the number of pixels is 256 CCs
When the D linear sensor 33 is used, 128 microseconds,
The variable sampling period will be selected from among five types of 256 microseconds, 512 microseconds, 1024 microseconds, and 2048 microseconds.

If the shortest 128 microsecond variable sampling period is selected by microprocessor 44, reset circuit 48 provides a reset signal to CCD drive circuit 34 every 128 microseconds. The CCD drive circuit 34 receives the next reset signal immediately after reading the accumulated charges of all the pixels of the CCD linear sensor 33, and immediately starts a new CCD read cycle. This is the fastest measurement.

On the other hand, if the microprocessor 44 selects a variable sampling period of, for example, 256 microseconds, the reset circuit 48 supplies a reset signal to the CCD drive circuit 34 every 256 microseconds. When the CCD drive circuit 34 finishes reading the accumulated charges of all the pixels of the CCD linear sensor 33 in 128 microseconds, it does nothing for the remaining 128 microseconds and waits for the next reset signal. That is, a period (so-called dummy reading period) having the same length as the reading period of 128 microseconds is added, and even during this period, charges corresponding to the amount of received light are accumulated in each pixel of the CCD linear sensor 33.

The amount of charge stored in each pixel of the CCD linear sensor 33 increases by the added dummy readout period, so that even dark objects can be accurately measured. Selecting a variable sampling period of 512 microseconds, 1024 microseconds, or even 2048 microseconds further increases the accumulation time (the amount of accumulated electric charge), which is advantageous for measuring a dark object.

As described above, the optical displacement meter according to the present embodiment (first embodiment) sets the transfer rate of each pixel of the CCD linear sensor 33 to a constant value (0.5 microsecond) while maintaining the transfer rate for the dummy reading period. The sampling period is made variable by addition. However, as another embodiment (second embodiment), the sampling period may be made variable by changing the transfer rate for each pixel. Of course, it is also possible to make the sampling period variable by combining both the addition of the dummy read period and the variable transfer rate.

FIG. 5 is a signal waveform diagram for explaining the difference between the variable sampling period according to the first embodiment and the variable sampling period according to the second embodiment. In FIG.
(A) shows waveforms in the first embodiment, and (b) shows waveforms in the second embodiment, and these waveforms correspond to the output signal 52 of the low-pass filter 41 shown in FIG.

As can be seen from FIG. 5A, in the first embodiment, the transfer rate does not change even if the sampling period is changed, and thus corresponds to the one-dimensional distribution of the amount of light received on the light receiving surface of the CCD linear sensor 33. The voltage waveform does not change. On the other hand, in the second embodiment shown in FIG. 5B, if the sampling period is doubled, the transfer rate is also doubled, and accordingly, the voltage waveform is also doubled in the time axis direction. It is.

Therefore, in the case of the second embodiment, it is necessary to change the time constant (that is, the cutoff frequency) of the low-pass filter 41 in FIG. 4 as the sampling period is changed. As a result, it becomes difficult to design the low-pass filter 41. The first embodiment is superior to the second embodiment in that it is not necessary to change the time constant (cutoff frequency) of the low-pass filter 41 even if the sampling period is changed.

The embodiment of the present invention is not limited to the above embodiment, and various design changes can be made. For example, CC
D image sensors are not limited to one-dimensional linear sensors.
A dimensional area sensor may be used. Further, the values of the transfer rate and the sampling period in the above embodiment are only specific examples, and these values can be arbitrarily changed. The number of variable sampling periods that can be selected is not limited to four.

[0040]

As described above, according to the optical displacement meter of the present invention, since the sampling period is variable,
To measure the displacement of the measurement object accurately in any case, set the sampling period short when measuring a bright measurement object at high speed, and set the sampling period long when measuring a dark measurement object. Can be.

According to the embodiment in which the sampling period is made variable without changing the transfer rate for each pixel of the CCD linear sensor by adding the dummy reading period,
There is no need to change the constant of the low-pass filter for removing the high-frequency component of the signal read from the CCD linear sensor according to the sampling cycle.

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram showing an example of a conventional optical displacement meter.

FIG. 2 is an external view of an optical displacement meter according to an embodiment of the present invention.

FIG. 3 is a block diagram illustrating a main configuration of an optical displacement meter according to the present embodiment.

FIG. 4 is a diagram schematically illustrating how a voltage signal output from a CCD driving circuit changes through a low-pass filter and a peak hold circuit.

FIG. 5 is a signal waveform diagram for explaining a difference between a variable sampling period according to the first embodiment and a variable sampling period according to the second embodiment.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 21 Sensor head part 22 Controller part 23 Cable 24 Mounting bolt 25 Mounting stand 27 Object to be measured 31 Laser diode 33 CCD linear sensor (image sensor) 34 CCD drive circuit 35 Light emitting lens 36 Light receiving lens 41 Low pass filter 44 Microprocessor 45 DA Converter 48 reset circuit

 ──────────────────────────────────────────────────続 き Continued on the front page F-term (reference) 2F065 AA06 AA09 DD06 FF09 GG06 GG08 HH13 JJ02 JJ08 JJ25 LL04 NN02 NN12 NN13 NN18 PP22 QQ00 QQ01 QQ02 QQ03 QQ25 QQ28 QQ33 2F112 AA08 BA05 CA12 DA05

Claims (2)

[Claims] An object is illuminated with light from a light source, reflected light from the object is received by an image sensor, and a light receiving signal stored in the image sensor is read out at a predetermined sampling period to obtain a digital signal. An optical displacement meter that converts the data into data and calculates a distance to the object to be measured based on the digital data, wherein an accumulation time of a light reception signal accumulated in the image sensor by changing the sampling period. An optical displacement meter characterized by having a variable length. 2. An optical system according to claim 1, wherein a sampling period is variable without changing a transfer speed of each pixel of said image sensor by adding a dummy reading period following said reading period of said image sensor. Displacement gauge.