GB2329780A

GB2329780A - Distance measuring device

Info

Publication number: GB2329780A
Application number: GB9900625A
Authority: GB
Inventors: Masahira Akasu
Original assignee: Mitsubishi Electric Corp
Current assignee: Mitsubishi Electric Corp
Priority date: 1994-06-28
Filing date: 1995-06-27
Publication date: 1999-03-31
Anticipated expiration: 2015-06-27
Also published as: GB2318240A; GB2318240B; GB2329779A; GB9727074D0; GB2318239A; GB2318242B; GB9727066D0; GB2318241A; GB2318242A; GB2318241B; GB2329780B; GB9727070D0; GB2318238A; GB2329779B; GB9727076D0; GB9727072D0; GB2318239B; GB2318238B

Abstract

A distance measurement device comprising a light-sending unit 2 for sending pulsed light towards an object whose distance is to be measured a light-receiving unit 3 for receiving incident light including pulsed light reflected by the object a distance measurement unit 4 for measuring the time from the sending of the pulsed tight to the receiving of the incident light and calculating the distance to the object and an interference detection unit 5 for determining whether interference light exists in the incident light, based on whether the intensity of the incident light fluctuates with distance (as would be expected of reflected light) or not (indicating interference).

Description

2329780

DISTANCE MEASUREMENT DEVICE BACKGROUND OF THE INVENTION Field of-theInvention

The present invention relates to a distance measurement device which sends pulsed light toward an object of measurement where a distance thereto is measured, receives light reflected by the object, and measures the time required from the sending of light to the receiving of light to obtain the distance to the object. Description of the Related Art

A distance measurement device of the above kind is shown in FIG.4. In the figure, the distance measurement device comprises clock pulse generation means 1, light-sending means 2, a plus drive unit 21, a laser diode 22 that is driven by the pulse drive unit 21, and a light-sending lens 23. Lightreceiving means 3 consists of a light- receiving lens 31 and a light-rezeiving element 32. And, distance measurement means 4 obtains a distance to an object 10 which is an object of measurement.

Next, the operation of the conventional device thus constructed will be described. The clock pulse generation means 1 generates a clock pulse CP which is a reference. The lightsending means 2 drives the pulse drive unit 21 to generate a drive pulse DP in synchronization with the clock pulse CP that the clock pulse generation means 1 generates, and then drives the laser diode 22 to generate pulsed light A. The pulsed light A generated by the laser diode 22 is irradiated forward as a 12 pulsed light beam B that is condensed by the light-sending lens 23. This irradiated light beam B will be reflected Iby the object 10 if the object is within a range of the irradiation. This reflection light E reflected by the object 10 is incident on the light-receiving lens 31 of the light- receiving means 3 as incident light G and is condensed at the light- receiving surface of a light-receiving element 32 as focused light H. The lightreceiving element (a photoelectron converter) 32 converts the focused light H into a ligh t reception signal J.

The above-described distance measurement means 4 compares the light reception signal i from the light-receiving element 32 with a predetermined threshold value and detects the significan't light reception signal J based on the reflection light E from the object 10. Further, the distance measurement means 4 measures, from the clock pulse CP, i.e., the time of occurrence (ta) of the drive pulse DP of the laser diode of the lightsending means. 2 and the time of detection (tb) of the light reception signal i based on the reflection light E from the abovedescribed object 10, the turnaround time to the object 10 (t = tb - ta) by using, for example, a high frequency oscillator and a high speed counter, and obtains the distance between the distance measurement device and the object, d, by the following Equation (l):

d = t x c/2 (Equation 1) where c represents the speed of light.

in summary, instead of the drive pulse and the light reception signal which are electric signals, the above-described time, t (= tb - ta), is modified to the time from the sending of 3 the pulsed light A to the incidence of the focused light H and is substituted into Equation (1).

is Also, the distance measurement means 4 is constructed so that it calculates a distance based the first significant light reception signal J and would not calculate a distance even if other significant light reception signals were between the first distance calculation and the next distance calculation.

A distance measurement device such as described above detects the distance to the object 10 by sending the light beam B to the object 10 and receiving the light E reflected from the object 10. However, if interference light S, such as pulsed light from other light sources or pulsed reflection light therefrom, is incident on the light-receiving means 3 as incident light G before the reflection light E from the object 10 is incident, the light-receiving means 3 cannot determine whether the light incident thereon is the reflection light E from the object 10 based on the light beam B sent by the lightreceiving means 3 or the interference light S from other light sources, so the interference light S from other light sources is recognized as reflection light E by mistake. And, the distance measurement means 4 performs a distance measurement calculation with the light reception signal J based on the interference light S and will calculate a mistaken distance d (Equation 1).

The above-described problem of the incidence of the interference light 5 from other light sources occurs when a plurality of distance measurement devices of the above kind are used. For example, this kind of distance measurement device is mounted in a vehicle and utilized as a device which measures the 4_ distance between vehicles and alarms to maintain a safe distance between vehicles. And, if two opposite vehicles traveling on two opposite lanes are provided with similar devices, the pulsed light of the distance measurement device of the opposite vehicle will surely be incident on the distance measurement device of a self-vehicle as interference light S.

Since the interference light S from the opposite vehicle is direct light, even if the opposite vehicle were far away, the level (illuminance) of the incident light to the light-receiving means 3 would be far stronger than that of the reflection light E from a normal preceding vehicle. At this time, if the reflection light E from the distance measurement device of a self-vehicle and the pulsed interference light 5 from the distance measurement device of the opposite vehicle occur at the substantially same time, the interference light S from the opposite vehicle will be detected by mistake, so an alarm is to be given even when there is no preceding vehicle on the same lane as a self-vehicle.

Thus, the mistaken operation caused by receiving the interference light S other than the regular light E reflected from the object 10 becomes an important problem associated with the safety and reliability of a system, when this kind of device is used in a sensor of a system for controlling an alarm occurrence or equipment.

Also, when the interference light S of the distance measurement device of an opposite vehicle is received, conversely the pulsed light A. of the distance measurement device of a self-vehicle is also irradiated to the opposite vehicle.

Therefore, since the driver of the opposite vehicle is also subjected to the irradiation of the pulsed light A, the conventional device is undesirable from the standpoint of safety. SUMMARY OF THE INVENTION

This invention has been made to solve problems such as described above. Accordingly, it is an object of the present invention to provide a distance measurement device which is capable of judging whether interference light exists in the light incident on light-receiving means.

According to this invention, there is provided a distance measurement device comprising:

light-receiving means for receiving incident light including pulsed reflection light reflected by the object; 6 distance measurement means for measuring the time from the sending'of the pulsed light to the receiving of the incident light and calculating the distance to the object; and interference detection means for determining whether interference light exists in the incident light, based on the intensity of the incident light, the 7 interference detection means stores a plurality of distance data measured by the distance measurement means and a plurality of intensity data of incident light corresponding to the plurality of distance data, and determines whether interference light exists in the incident light, based on fluctuations in the distance data and the intensity data of the incident light.

Pulsed light which becomes interference light is not sent in synchronization with the light-sending means. Therefore, when the incident light includes interference light, a fluctuation in the distance data calculated based on that 8 incident light becomes greater. On the other hand,,a fluctuation in the intensity data of incident light is small. As a result, when the fluctuation in the distance data is grea and the fluctuation in the intensity data of incident light is great, it can be determined that interference light exists in the incident light.

Further, when various objects of measurement are measured, the fluctuations in the distance data and the intensity of incident light are both great. Therefore, in such case, it may be determined that interference light does not exist in the incident light.

/11 9 The present application has been divided from UK patent application no. 9513205.6 which relates to a light-sending distance measurement device which identifies interference light from its intensity.

UK patent application no. 972707.6 has been divided from UK patent application no. 9513205.6. That relates to a light-sending distance measurement device which identifies interference light from the time at which it is received (which implies a distance).

UK patent application no. 9727072.2 has also been divided from UK patent application no. 9513205.6. That relates to a light-sending distance measurement device which determines that interference is present.from light detected while light is not being sent.

UK patent application no. 9727076.3 has also been divided from UK patent application no. 9513205.6. That relates to a light-sending distance measurement device which alters the timing of light-sending in order to avoid interference light.

UK patent application no. 9727074.8 has also been divided from UK patent application no. 95 13205.6. That relates to a light-sending distance measurement device which detects interference light on the basis of direction of incident light.

UK patent application no. 9727066.4 has also been divided from UK patent application no. 951-31205.6. That relates to a light-sending distance measurement device which reduces or stops output of pulsed light in response to detection of interference light.

UK patent application no. has also been divided from UK patent application no. 9513205.6. That relates to a light-sending distance measurement device which alters the intensity of outgoing light and determines the existence of interfering light from that intensity and the intensity of the incident light.

The above and other objects and advantages of the present )o invention will become apparent from the following detailed description of the preferred embodiments of the invention when the same is read in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS.

FIG. 1 is a block diagram showing a distance measurement device as background to this invention; his invention;

FIG. 2 is a diagram showing the determination level as background to this invention;

FIG. 3 is a flowchart showing the operation of a first embodiment of this invention; 11 FIG. 4 is a block diagram showing a conventional distance measurement device.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of the present invention will hereinafter be described in detail with reference to the accompanying drawings.

FIG. 1 shows a distance measurement device constructed in accordance with background to this invention. In the figure, reference numerals 1, 2, 3, 4 and 10 are the same as the above-described conventional device and therefore a description thereof is omitted by applying the same reference numerals. Interference detection means 5 has a display unit 5a, determines that interference light exists, when the level of a light reception signal J from light-receiving means 3 exceeds a predetermined value, and displays that effect on the display unit 5a and also generates interference detection signal AL.

In the distance measurement device thus constructed, light- 1.

sending means 2 sends a light beam B at the timings T1, T2, T3, and TN of a drive pulse DP synchronized with the clock pulse CP generated by clock pulse generation means 1, and the light beam is reflected by an object 10. The light-receiving means 3 receives incident light G including reflection light E reflected by the object 10 and converts it into a light reception signal J, which is input to distance measurement means 4 and interference detection means 5.

From the time difference between the timings T1, T2, T3, and TN of the drive pulse DP synchronized with the clock pulse CP input by clock pulse generation means 1 and the timings T1, T2, T3,..., and TN of the first light reception signal i output by the light-receiving means 3 after each light beam is sent, the distance measurement means 4 calculates and outputs distances D1, D2, D3,..., and DN based on Equation (1).

At the same time, the interference detection means 5 compares with a predetermined value aO (see FIG. 2) the level L of the light reception signal i based on the incident light G that is first incident on the light-receiving means 3 after each light beam is sent. When the level L of the light reception signal J is aO or more, the interference detection means 5 determines that interference light S exists in incident light G and displays that effect on the display unit 5a and also generates interference detection signal AL.

That is, each time the distance measurement means 4 calculates a distance based on the light reception signal J first incident on the lightreceiving means 3, the level L of the light reception signal i is compared with a predetermined [5 value. And, when the level L of the light reception signal J is a predetermined value or more, that effect is displayed on the display unit Sa, so it can be understood that the distance calculated at that time is based on interference light. incidentally, if the light beam B sent by the light-sending means 2 is irradiated on object 10, reflection light E with an intensity corresponding to the reflectivity of the object 10 will be reflected. when the light-sending means 2 sends light beam B with a power po and a radiant solid angle DO, the illuminance Hr on the light-receiving means 3 by the reflection light E reflected by the object 10 having an area St at a point spaced by a distance R, a reflectivity K, and a radiant solid angle Qt is given by the following laser equation Hr = PO x St x K/R4 x DO x Qt (Equation 2) is The level L of the light reception signal J of the lightreceiving means 3 is proportional to the illuminance Hr. Therefore, the level L of the light reception signal J by the reflection light E of the object 10 is inversely proportional to the distance R.

On the other hand, the illuminance Hd on the lightreceiving means 3 on which interference light is directly incident from a similar distance measurement which is spaced by a distance R and sends pulsed light having the same power and radiant solid angle (PO, 90) as the device of the present invention is given by Hd = PO/R 2 X no..... (Equation 3) That is, since interference light (pulsed light from an opposite device) is directly incident, very strong light is 14- incident on the light-receiving means 3, and accordingly, the level of the light reception signal J that is output by the light-receiving means 3 becomes high.

A ratio of light reception levels on light-receiving means. 3 between interference light S and reflection light E is given by Hd/Hr = R 2 x Pt/St x K (Equation 4) Assume now that the object 10 is a reflex reflector that is mounted on the rear reflecting mirror of a vehicle. If the radiant solid angle, reflection area, and reflectivity of the reflector are Rt = 10-3 (sr), St = 2 X 10-3 (M2), and K = 0.3, the ratio of light reception levels on the light-receiving means 3 will be given by Hd/Hr = R2 x 1.67 (Equation 5) According to this ratio, the level of the incident light S from another distance measurement device 50 m (R = 50) ahead is 4000 times as much as that of the reflection light E from the reflex reflector.

Therefore, when in this kind of distance measurement device an object of measurement 10 is assumed to be the above-described reflex reflector of a vehicle, a value which is, for example, 10 times as much as the reflection light E from the reflex reflector is set as a reference value aO for determining whether interference light exists. In this arrangement, the level of interference light S is very high like 4000 times, so the interference light S and the normal reflection light E from the object 10 can be distinguished with reliability.

Thus, interference light S is far stronger than reflection light E. Therefore, even if reference value czO for determining interference light 5 were set to a value sufficiently greater than the maximum level that light-receiving means 3 receives (in this embodiment, 10 times), it could reliably be determi ned whether interference light exists.

In the above arranaement, it has been described that the interference detection means Sa compares the level L of the light reception signal i output by the light-receiving means 3 with a predetermined value aO to detect interference light.

However, in this arranp-ement, according to the distance data D calculated by distance measurement means 4, a determination level av for the level L of the light reception signal of light-receiving means 3 is set to a greater value for a short distance and a smaller value for a long distance so that the incidence of interference light S can be detected more reliably.

As described above, the reflection light E from object 10 is scattered, the illuminance Hr of the reflection light E on light-receiving means 3 is reduced in inverse proportion to distance R4, as shown in Equation 2, and the reflection light E becomes weaker if the object 10 is far away, so the level L of the light reception signal J output by the light- receiving means 3 becomes smaller. on the other hand, the illuminance Hd of interference light S that is incident from other distance measurement devices directly on the light-receiving means 3, is attenuated in inverse proportion to distance R 2, as shown in Equation 3.

1 Therefore, the determination level av, as shown in FIG. 2, is set to a value which is greater than the level Hr of reflection light E, smaller than the level Hd of interference light 5, and reduced in inverse proportion to distance R2. With this, there can be detected interference light S which is stronger than the reflection light E from the object 10 and is incident from other distance detection means directly on the lightreceiving means 3, and other interference light which is weaker than this interference light S and greater than av. As a result, interference light can be detected more reliably.

First Embodiment Also, interference detection means 5 stores the levels Ll, L2, L3,..., and LN of the light reception signals of light receiving means 3 and the distance data D1, D2, D3,..., and DN output by distance measurement means 4, over a plurality of times, i.e., at the timings T1, T2, T3,..., and TN of a drive pulse DP. And, for the respective values, the interference detection means 5 calculates from the stored values a statistical fluctuation value such as a standard deviation or sum of absolute values of differences between continuous data.

And, when a fluctuation in the distance data D1 to DN is greater than a predetermined value and a fluctuation in the levels L1 to LN of the light reception signals is smaller than a predetermined value, it can also be determined that interference light exists.

Generally speaking, when a distance measurement device continuously detects the sam e object 10, the fluctuation in the distance data D1 to DN is small and the intensity GK of incident U light G is substantially constant, so the fluctuation in the levels Ll to LN of the light reception signals is small. When, on the other hand, the distance measurement device detects a plurality of objects in various positions, the fluctuation in the distance data D1 to DN becomes greater and also the fluctuation in the levels L1 to LN of the light reception signals becomes greater.

However, since pulsed light which is generated by an opposite distance measurement device and becomes interference light S is not emitted in synchronization with light-sending means 2, the timing at which distance measurement means 4 measures and the timing at which interference pulsed light S is incident on light-receiving means 3 are not synchronized.

Therefore, the distances D1 to DN calculated by the distance measurement means 4 are not constant and become random values, so the fluctuation becomes very greater.

Conversely, since the position of a source of interference light is not greatly changed within a plurality of distance measurement times, the levels Ll to LN of the light reception signals of light-receiving means 3 become substantially constant and-the fluctuation is small. Interference detection means 5 calculates a fluctuation aD in distance data D1 to DN and a fluctuation al, in light reception signal levels Ll to LN and, when the distance fluctuation aD is great and the level fluctuation aL is small, determines that interference light 5 has been incident. Therefore, whether interference light 5 exists in incident light S can be determined with reliability.

The above-described operation of the interference detection 19 means 5 will be described further in detail with reference to a flowchart of FIG. 3. Nine distance data D1 to D9 were measured with a resolving power of 0.1 m, and the light reception signal levels L1 to L9 were normalized with a maximum value of 1.

The following process operation is performed once for a single measurement, and the results of measurement for the past nine measurements have been stored in distance data storage registers RD1 to RD9 and signal level storage registers RL1 to RL9, respectively.

First, in step 1 an i-nterference detection flag F for storing a detection result of the incidence of incident light S is cleared (F = 1). In step 2, distance data DO measured this time is stored in the register RDO, and a light reception signal level LO is stored in the register RLO..

In step 3, an average value Dm of ten consecutive distance data including a current measurement result and the past nine data is obtai-ned. In step 4, a standard deviation aD is obtained by an equation, E(Dm - Dn)2/9} 112 where Dm represents the average value of distance data DO to D9 obtained in step 3 and Dn represents the measured value of each data.

Next, in step 5 the standard deviation aD of distance data is compared with a predetermined value KD which is, for example, a value (1 m) ten times greater than a resolving power 0. 1 m of distance measurement. If aD is less than KD, a f luctuation in distance data DO to D9 will be small and stable, and since this is not the incident of interference light S, step 5 will advance to step 11.

If, on the other hand, aD is greater than KD, distance data 1 will have a fluctuation and step 5 will advance to step 6. In step 6, an average value Lm of light reception signal levels LO to L9 is obtained.

In step 7, a standard deviation aL of light reception signal levels LO to L9 is obtained in the same way as step 4. Further, in step 8 the degree of fluctuation, VL, of the light reception signal levels L is obtained by dividing the standard deviation aL of the light reception signal levels by the average value Lm of the light reception signal levels.

When in step 9 the degree of fluctuation VI, of the light reception signal levels is greater than or equal to a predetermined value KI, (for example, 0.01), the light reception signal level L has a fluctuation and is not constant. Since this is not the incidence of interference light 5, step 9 is advances to step 11.

On the other hand, when in step 9, VI, is less than KI, (0.01), there is no fluctuation in the light reception signal levels L1 to L9 and step 9 advances to step 10. In step 10, the incidence of interference light 5 is assumed to exist and the interference detection flag F is set to a 1, because it has been determined in step 5 that distance data DO to D9 has a fluctuation and it has been determined in step 8 that light reception signal levels LO to L9 have no fluctuation.

In step 11, current distance data and the stored contents of registers RDO to RD8 in which the past eight distance data DO to D8 have been stored are transferred to registers RD1 to RD9, respectively. That is, stored data are transferred so that the content of the register RDS is transferred to the register RD9 2-0 and then the content of the register RD7 is transferred to the register RD 8. In step 12, the contents of registers RLO to RL8 in which the data LO to LS of the light reception signal levels have been stored are likewise transferred to registers RL1 to RL9, respectively, and prepared for the interference detection process in the next distance measurement.

With the above-described operation, the interference detection flag F is set to a 1 when there is a fluctuation in distance data DO to D9 and there is no fluctuation in light reception signal levels L1 to L9. Therefore, when distance data is used, whether interference light exists can be detected by monitoring this interference detection flag F. Also, depending on the state of this interference detection flag F, that effect can be displayed on the display unit Sa and also the is interference detection signal A1 can be generated.

21 While the subject invention has been described with reference to the preferred embodiments thereof, it will be appreciated by those skilled in the art that numerous variations, modifications, and embodiments are possible, and accordingly, all such variations, modifications, and embodiments are to be regarded as being within the spirit and scope of the invention.

Claims

2. A distance measurement device substantially as herein described with reference to figure 3 of the accompanyin,cy drawings.

2- 1. A distance measurement device comprising: light-sending means far. sending pulsed light toward an object of measurement where a distance thereto is measured. light-receiving means for receiving incident light including pulsed reflection light reflected by said object; distance measurement means for measuring the time from the sending of said pulsed light to the receiving of said incident li ght and calculating the distance to said object; and interference detection means for determining'whether interference light exists in said incident light, based or! the intensity of said incident light, wherein said interference detection means stores a plurality of distance data measured by said distance measurement means and a plurality of intensity data of incident light corresponding to said plurality of distance data, and determines whether interference light exists in said incident light, based on fluctuations in said distance data and said intensity data of 2. A distance measurement device substantially as herein described with reference to figure 3 of the accompanying drawings.

C. 3 Amendments to the claims have been filed as follows 1. A distance measurement device comprising: light-sending means for. sending pulsed light toward an object of measurement where a distance thereto is measured; light-receiving means for receiving incident light including pulsed reflection light reflected by said object; distance measurement means for measuring the time from the sending of said pulsed light to the receiving of said incident light and calculating the distance to said object; and interference detection means for determining whether interference light exists in said incident light, based on the intensity Of said incident light, wherein said interference detection means stores a plurality of distance data measured by said distance measurement means and a plurality of intensity data of incident light corresponding to said plurality of distance data, and determines whether interference light exists in said incident light, based on fluctuations in said distance data and said intensity data of said incident light.