JP2005221330A - Distance calibration method of range sensor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a distance calibration method of an AM modulation system range sensor by solving the problem wherein a response delay is generated by operating a reference value of 0m by one step at one scanning time and calibrating the distance, and thereby distance measurement in all the scanning directions becomes difficult. <P>SOLUTION: A position having the lowest reflected light level is detected and stored by the first time scanning, and distance calibration operation is performed by operating the reference value of 0m at the detection position stored at the second time scanning. Distance measurement of the one-step detection position wherein the distance calibration operation is performed is performed by calculating the mean value of measured values by both two steps adjacent to one step. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a distance calibration method for a scanner-type range sensor that scans laser light or LED light with a rotating mirror and measures the distance to a projection target.

  The basic structure of a scanner-type range sensor (distance measurement sensor) mounted on an automated guided vehicle (AGV) or a cleaning robot that runs on a floor in a factory or the like is shown in FIG. 5 and applied to the distance measurement of this range sensor. A typical AM modulation type light wave distance measuring method will be described with reference to FIG.

  The range sensor 11 of FIG. 5 illustrates the projection light A1 of laser light or LED light from the light projecting optical system 1 by rotating the light projecting optical system 1 and the light receiving optical system 2 arranged coaxially in the vertical direction with a motor 3 at high speed. Scanning is performed 360 degrees in the horizontal direction using the rotating mirror, and the reflected light A2 from the projection object 10 in the peripheral two-dimensional area is received by the light receiving optical system 2 and output to the light receiving circuit 4 to be output to the distance calculating circuit 5 The distance L is measured by calculation. The projection light A1 is modulated at a certain frequency by the projection circuit 6 of the projection optical system 1, and the distance L is measured from the difference between the phase of the modulated signal and the phase of the reflected light A2 from the projection target 10. . That is, when the light modulated at a specific frequency hits the projection object 10 and returns, the phase difference φ is caused by the speed and distance of the light as shown in FIG. Since the value of the phase difference φ depends on the speed of light and the distance L, the distance L is measured by detecting the phase difference φ. Accordingly, the distance of a two-dimensional area can be measured by horizontally rotating the frequency-modulated projection light A1 by a rotating mirror and scanning 360 degrees. Further, the distance in the three-dimensional area can be measured by continuously increasing and decreasing the vertical angle while scanning the projection light A1 by 360 degrees.

  As the detection distance of the 2D range sensor that measures the distance of the two-dimensional area, the maximum distance is determined in advance on the arithmetic circuit side. For example, in a 2D range sensor having a maximum detection distance of 1.5 m, as shown by a broken line in FIG. 7, the entire circumference of the range sensor is 1.5 m from the detection area E1, up to the projection object in the detection area E1. Measure and display the distance. This type of range sensor has a light intensity margin that can detect a projection object in a two-dimensional detectable area E2 outside the detection area E1 that is about twice the maximum detection distance, and guarantees the stability of the distance detection operation. I have to do it. In addition, one step angle θ at the time of one scan for scanning 360 degrees of the 2D range sensor shown in FIG. 7 is 0.5 °, for example, and one scan time is 25 ms.

  In the case of an AM modulation type scanner type range sensor, the distance calculation characteristic is greatly affected by the reflectance of the projection target. In particular, reflection from a high-reflectance projection object such as a mirror or stainless steel returns strong reflected light from a long distance exceeding the maximum detection distance for detecting a phase difference of 360 degrees or more, and the phase difference is 360 degrees. When the distance becomes the above, it cannot be distinguished from 0 degrees (short distance), and the distance calculation is often wrong when the frequency of the projection light is one. Therefore, the projection light is modulated at a plurality of different frequencies, and the final measurement distance is output when the distances calculated at the respective frequencies coincide. Normally, the distance is calculated at each step by changing the frequency for each scan in which the projected light scans 360 degrees, and it is checked whether the distances at the same mirror angle detection position at each scan match. ing. In this case, although the probability of malfunctioning decreases as the number of types of frequencies used increases, the distance measurement time increases and the response speed decreases, so it is normal to use two types of frequencies in a 2D scanner type range sensor. It is.

  The scanner-type 2D range sensor that uses two types of frequencies alternately changes the frequency for each scan, and the distance calculated by the frequency at the first scan coincides with the distance calculated by the frequency at the second scan. Sometimes the measurement distance data is output. Therefore, a total of two scans, the first and second for the coincidence calculation, and one scan corresponding to the timing difference between the projection object and the scan, the response time required for measurement in one step is 3 scans, The response time of a 2D range sensor with one scan time of 25 ms is 75 ms.

  In addition, an AM modulation type scanner range sensor corrects constants of various circuit components such as a light projecting circuit, a light projecting optical system, a light receiving optical system, and a light receiving circuit, and particularly variations in temperature characteristics of these components. The distance calibration is performed in real time during one scan. For example, there are variations in various characteristics such as a delay in emission with respect to the emission current when a laser diode is caused to emit light, and a response delay in a light receiving amplifier and phase comparison circuit that receives reflected light and performs phase calculation. Affects the measurement distance. If measurement is continued without calibrating the measurement distance in real time during each scan, errors in the measurement value accumulate due to variations in the temperature characteristics of the circuit components, and the reliability of the measurement value decreases. Therefore, distance calibration is performed at the detection position in a predetermined step for each scan using, for example, the optical path changing means 7 shown in FIG.

FIG. 8 shows an outline of a range sensor that projects the projection light (laser light) A1 from the laser diode D1 onto the projection target 10 through the lens B1, and receives the reflected light A2 at the photodiode D2 through the lens B2. Indicated. In this case, optical path changing means 7 such as a reflection mirror, a polarizing element, and an optical fiber (for example, refer to Patent Document 1) is arranged in the optical axis of the projection light A1. In cases other than distance calibration, the reflected light A2 from the projection light A1 is received by the photodiode D2, the phase of the light reception signal of the photodiode D2 and the emission signal of the laser diode D1 is compared, and the phase difference φ1 is calculated. Output the measured distance. This distance measurement is continuously performed in each step except one step of one scan, the optical path is changed at one step for calibration, and a reference value calculation for distance calibration is performed. This reference value calculation is performed using a closed loop of light that causes the optical path changing means 7 to directly enter the photodiode D2 without passing the laser light A1 from the laser diode D1 through the lens B1. That is, the phase of the light receiving signal for measuring the reference value of substantially 0 m by the closed loop of the light that is changed by the optical path changing means 7 and the phase of the light emitting signal of the laser diode D1 are compared, and the phase difference φ2 Is calculated. Then, the phase difference φ used for the actual measurement value is defined as a difference [φ = φ1-φ2] between the phase difference φ1 at the time of the former measurement and the phase difference φ2 at the time of the latter reference value measurement of 0 m. Distance calibration at each step of time is performed.
JP-A-6-230111 (FIG. 1)

  As in the above distance calibration method, a closed loop of light is formed using the optical path changing means inside the range sensor, the distance in the closed loop is used as a reference value corresponding to 0 m, and the actually measured distance is compared with the reference value. When calculating distances with calibration, the circuit components used for phase difference calculation including laser diodes and photodiodes use the same components together with distance calculation and distance calibration. Correction can be performed, distance calibration can be performed with high performance, and distance calculation performance can be improved.

  However, in the case of a 360-degree scanner type range sensor using a rotating mirror or the like, the optical path changing means is temporarily inserted into the optical axis at the detection position of a specific step during one scan, so that the 0 m phase difference detection is performed. The detection area of the step becomes a missing detection area where the distance cannot be measured, which makes it difficult to detect substantially 360 degrees in all directions.

  In addition, if the distance measurement without the detection area is performed during the next one scan, the response is delayed by one scan and the response time is required for four scans, which may affect the measurement characteristics of the range sensor. For example, when the projection target exists outside the detection area in front of the range sensor moving in parallel at high speed on the floor, and the projection target enters the detection area by the movement of the range sensor, the above-described one scan is performed. The response delay may have a non-negligible effect.

  The present invention has been made in view of the present situation, and an object of the present invention is to provide a distance sensor distance calibration method that facilitates distance measurement in all scanning directions without causing a response delay substantially.

  The method of the present invention that achieves the above object is a distance calibration method of a range sensor that measures the distance to a projection object from the phase difference between the projection light scanned at a predetermined step angle and the reflected light from the projection object. The position where the reflected light level is the lowest at one scan is detected and the detected position is memorized, and the projection light is projected to the outside at one step at the detection position memorized at the previous one scan at the next one scan. A reference value for calibrating the measurement distance in another step is calculated using a closed loop of light provided inside the range sensor.

  Here, the range sensor is an AM modulation method in which the projection light is sequentially modulated with a plurality of different frequencies for each scan or for each step, or one step of the projection light is modulated with two types of frequencies. The AM modulation method is applied in which two frequencies are separated by a filter (digital filter or the like) in an optical system that receives the reflected light by the projection light, and that the frequencies are mixed and simultaneously emitted. In this case, the smaller the frequency type, the better the response of distance detection, so the minimum two frequencies are effective. The detection position with the lowest reflected light level during one scan is an area where there is no projection object in the detection area or the detectable area, or there is a projection object with extremely low reflectivity, and it is measured even if the distance is measured. This corresponds to an area where the value is unstable and lacks reliability, and a substantial distance calibration operation is performed to calculate a distance calibration reference value at one step at the detected position where the level is detected. The reference value for distance calibration is typically 0 m that facilitates distance calibration. In addition, the detection position of one step for performing the distance calibration operation is a position where the detection area is not actually measured, but since this area is originally an area that does not need to be detected with high reliability, it is substantially the same. No problems arise. Other than the detection position with the lowest reflected light level, that is, the other detection positions with the higher reflected light level, there is a projection object in the detection area or detectable area, and there is a high need for distance measurement with high reliability. Therefore, this is an area where a response delay occurs, and since there is no distance calibration operation in this area, there is no possibility of a response time delay occurring.

  The closed loop of light provided inside the range sensor for calculating the distance calibration reference value is a closed loop formed by temporarily changing the optical path of the projected light projected to the detection area, or emits the projected light. A closed loop formed by using a second light source dedicated for calibration that emits light of the same quality as the projection light can be applied.

  In the present invention, it is desirable that the average value of the distance measurement values of the two steps adjacent to the one-step detection position for calculating the distance calibration reference value is the measurement value at the one-step detection position for calculating the reference value. . By measuring the distance of the detection position in one step for calculating the reference value for distance calibration in this way, the distance measurement of the detection position can be easily performed without a response delay, and the response delay is substantially achieved. It is possible to provide a range sensor that can easily measure the distance in all scanning directions. Here, the scanning all directions include all directions within 360 degrees in a scanning angle set smaller than 360 degrees in addition to all directions of 360 degrees.

  Further, another method of the present invention for achieving the above object is that the phase difference between the projection light scanned at a predetermined step angle and the reflected light from the projection object while moving relative to the projection object to the projection object. A distance sensor distance calibration method for measuring the distance of the light, which is provided inside the range sensor by interrupting projection of the projection light to the outside during one step in the direction opposite to the relative movement direction of the range sensor with respect to the projection target. This is a method of calculating a reference value for distance calibration in another step in the closed loop.

  The range sensor here is a mobile range sensor installed in a moving body such as an automated guided vehicle or a cleaning robot, or a fixed range sensor that measures the distance from a moving body that is installed at a fixed position and moves close to and away from it. It is. In the case of the former movable range sensor, the reference value calculation for distance calibration of 0 m is performed at one step in the backward direction opposite to the forward direction of the moving direction of the moving body moving on the floor in the forward / backward / left / right arbitrary direction. Response delay in the detection area in the forward direction of the moving body of a moving body such as an automated guided vehicle is a problem, but the response delay in the opposite backward direction is not a problem, so distance calibration in the backward direction If the operation is performed, no matter what direction the moving body moves, there is no fear of a substantial response delay in the detection area, and there is no problem. In the case of the latter fixed range sensor, the moving direction of the moving body with respect to the range sensor may be detected, and the distance calibration operation may be performed in the opposite direction.

  According to the method of the present invention, the reference value measurement for the distance calibration of the scan-type range sensor is performed, the detection position where the response level with the lowest reflected light level is the lowest in the detection area, or the movement of the moving moving body Because it is performed at a position where the response delay behind the opposite direction does not matter, it is easy to measure the distance in all scanning directions without causing a response delay, and movement that moves at high speed like an automated guided vehicle or a cleaning robot There is an effect suitable for the body.

  Hereinafter, embodiments of the method of the present invention will be described with reference to FIGS.

  The first distance calibration method of the present invention will be described with reference to FIG. The 360 degree scan type 2D range sensor 11 has, for example, a detection area E1 having a radius of 1.5 m and a detectable area E2 having a radius slightly more than twice that of the detection area E1. The range sensor 11 scans a laser beam frequency-modulated at a step angle of 0.5 ° in all 360 ° horizontal directions, calculates a phase difference from the reflected light at each step position at intervals of 0.5 °, and The distance from the projecting body 10 is measured. There are two types of laser beam frequencies. When the measurement values at each step in two consecutive scans are switched for each scan, the measurement values are judged to be correct and output. Since such a measurement method is the same as the conventional method, detailed description thereof is omitted.

  A curve LL in FIG. 1 schematically shows the reflected light level in one scan, and appears as continuous levels of various sizes. Assuming that the lowest level of the reflected light level LL at the time of this one scan is La, the detection position (rotation mirror angle) of this lowest reflected light level La is stored and stored at the first time during the second first scan. A distance calibration operation is performed to obtain a reference value for distance calibration at one step S1 of the detection position. That is, the projection of the projection light from the range sensor 11 to the outside is interrupted, and the reference value calculation of 0 m is performed using the light closed loop provided inside the range sensor. As the closed loop at this time, for example, the circuit configuration shown in FIG. 3 or 4 is effective.

  FIG. 3 shows the range sensor 11 in which the closed loop 28 is formed of an optical fiber or a light guide. The laser drive circuit 20 causes the laser diode 21 to emit light at a predetermined frequency, the projection light A 1 is projected onto the projection target 10 through the lens 22, and the reflected light A 2 is received by the distance measurement photodiode 24 through the lens 23. The light reception signal is input to the light reception amplifier circuit 26 via the switching circuit 25, the phase of the light reception signal and the phase of the projection light A1 are compared by the phase comparison circuit 27, the phase difference φ1 is calculated, and the distance is measured.

  On the other hand, a 0 m measurement photodiode 29 and a laser diode 21 having the same characteristics as the distance measurement photodiode 24 are optically coupled by a closed loop 28. The length of the closed loop 28 inside the range sensor is extremely shorter than the detection area and can be defined as 0 m. In the first scan of the second time in FIG. 1, the light of the laser diode 21 is directly sent to the 0-meter measurement photodiode 29 through the closed loop 28 only in one step S1 in which the distance calibration operation is performed. The switching circuit 25 is switched to send the light reception signal of the photodiode 29 to the light reception amplifier 26. The phase of the light reception signal and the phase of the light of the laser diode 21 are compared by the phase comparison circuit 27, and become the reference value for distance calibration. The phase difference φ2 is calculated. Then, the phase difference used for the actual measurement value is defined as φ = φ1-φ2, and distance calibration is performed at each step during one scan. In this case, the above-described distance calibration can always be performed with high accuracy by sufficiently approximating the characteristics of the measurement photodiode 24 and the 0-meter measurement photodiode 29, particularly the temperature characteristics.

  In FIG. 4, a 0 m measurement laser diode 31 provided separately from the measurement laser diode 21 is directly optically coupled to the photodiode 24 to form a closed loop. In the case of the range sensor 11, a switching circuit (not shown) that switches the operations of the two laser diodes 21 and 31 is provided. In the first scan for the second time in FIG. 1, the measurement laser diode 21 is stopped emitting light only during 1 step S1 of performing the distance calibration operation, and only the 0 m measurement laser diode 31 is emitted. The phase difference φ2 is obtained by comparing and calculating the phase of the received light signal and the phase of the light of the laser diode 31. In this case, distance calibration can be performed with high accuracy by sufficiently approximating the characteristics of the pair of laser diodes 21 and 31.

  In FIG. 1, the detection position with the lowest reflected light level at the time of the first scan is the distance measurement because there is no projection object in the detection area E1 or the detectable area E2, or there is a projection object with extremely low reflectance. Even in this area, the measured value is unstable and lacks reliability. Even if the distance calibration operation is performed in the second scan in this area, the delay in the response time hardly causes a problem. In addition, the detection positions other than the detection position with the lowest reflected light level at the time of the first scan have the projection object 10 in the detection area E1 and the detectable area E2, and it is necessary to perform distance measurement with high reliability. It is an area with high characteristics. Since this area is a normal distance measurement area in which the distance calibration operation is not performed at the time of the second scan, there is no fear of a response delay, and distance measurement with high reliability is executed. For example, when the projection object 10 enters the detection area E1 at a high speed from the detectable area E2, there is no response delay, and therefore the distance can be reliably measured in the detection area E1.

  In the second scan of FIG. 1, the distance measurement in the first step S1 with the lowest reflected light level can be performed in the third and subsequent scans, but 2 adjacent to the detection position in the first step S1. Since the reflected light level in steps S2 and S3 approximates the reflected light level in step 1 S1, the average value of the distances measured in steps 2 and S3 is the measured value at the detection position in step 1 S1. It is desirable to do so. Thus, by measuring the distance of the detection position in one step S1 for the distance calibration operation, the distance detection at the same position can be performed with a simple circuit configuration without a response delay. Enables 360 degree omnidirectional distance measurement.

  Next, a second embodiment of the method of the present invention will be described with reference to FIG. When the 360-degree scan type 2D range sensor 11 is mounted on an automated guided vehicle and moves at high speed in an arbitrary direction on the floor, the movement direction T is detected one by one and is in the direction X opposite to the movement direction T by 180 °. A distance calibration operation similar to that in the first embodiment is performed at the detection position in step S10. For example, the movement direction of the range sensor 11 is detected at the time of the first scan, the data in the direction X opposite to the movement direction is calculated, and one step S10 that matches the data in the opposite direction X at the second time of the first scan The reference value calculation of 0 m is performed using the closed loop of FIG. 3 or FIG.

  When the range sensor 11 of FIG. 2 moves at a high speed in the desired moving direction T, a response delay in distance measurement becomes a problem in the area ahead of the moving direction, but there is substantially no rearward in the direction X opposite to the moving direction. In most cases it does not matter. Therefore, even when the projection object 10 in the moving direction T enters the moving detection area E1, this is detected without a response delay and the distance is measured. Moreover, even if the projection object exists in the direction X opposite to the moving direction T, since it moves away, there is no problem even if there is a response delay in the distance measurement.

  Further, when it is necessary to reduce the response delay in the opposite direction X, the average value of the measurement values at the detection positions of the two steps S20 and S30 adjacent to one step S10 may be set as the measurement value of one step S10. It is possible and desirable to do so. In this way, distance detection at the rear position can be performed with a simple circuit configuration without a delay in response, and 360-degree omnidirectional distance measurement with substantially no response delay is possible.

It is a top view for demonstrating the 1st distance calibration method of this invention. It is a top view for demonstrating the 2nd distance calibration method of this invention. FIG. 3 is a block diagram of a distance calibration circuit for implementing the method of the present invention. FIG. 5 is a block diagram of another distance calibration circuit for implementing the method of the present invention. It is a side view which shows the outline | summary of a range sensor. It is a wave form diagram for demonstrating AM modulation system. It is a top view which shows the area of a range sensor. It is a calibration circuit block diagram for demonstrating the conventional calibration method.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Projection object 11 Range sensor 20 Laser drive circuit 21 Laser diode 24 Photodiode 25 Switching circuit 26 Light reception amplifier circuit 27 Phase comparison circuit 28 Closed loop 29 Photodiode 31 Laser diode A1 Projection light A2 Reflected light E1 Detection area E2 Detectable area L Distance LL Reflected light level La Lowest reflected light level S1 to S3 Steps S10 to S30 Step T Movement direction X Opposite direction θ Step angle φ Phase difference

Claims (4)

A range sensor distance calibration method for measuring a distance from a phase difference between a projection light scanned at a predetermined step angle and a reflected light from the projection object to the projection object,
The detection position with the lowest reflected light level is stored during one scan, the projection of the projection light is interrupted during one step of the stored detection position during the next one scan, and the light is provided in a closed loop inside the range sensor. A distance calibration method for a range sensor, comprising calculating a reference value for distance calibration in another step.   2. The range according to claim 1, wherein an average value of distance measurement values of two steps adjacent to one step for calculating the reference value is used as a measurement value at a detection position of one step for calculating the reference value. Sensor distance calibration method. A range sensor distance calibration method for measuring a distance to a projection object from a phase difference between a projection light scanned at a predetermined step angle while moving relative to the projection object and a reflection light from the projection object. ,
Projection of projection light to the outside is interrupted at one step in the direction opposite to the relative movement direction of the range sensor with respect to the projection target, and a reference value for distance calibration in another step is calculated in a closed loop of light provided inside the range sensor. A distance calibration method for a range sensor.   5. The range according to claim 4, wherein an average value of distance measurement values of two steps adjacent to one step for calculating the reference value is used as a measurement value at a detection position of one step for calculating the reference value. Sensor distance calibration method.