JP7568907B2 - Laser radar - Google Patents

Description

The present invention relates to a laser radar that calculates the distance to an object based on the time between emitting a pulsed laser beam and receiving the reflected beam.

Conventionally, this type of laser radar calculates the count value of the clock signal from the rising edge of the clock signal immediately after the peak level of the search wave transmitted toward the target to the rising edge of the clock signal immediately after the peak level of the reflected wave is detected, and the fractional time of the clock signal from the detection of the peak level of the reflected wave to the rising edge of the clock signal immediately after (see Patent Document 1).

Patent No. 3961428

The inventors of this application have noticed that in the laser radar described in Patent Document 1, when the detection of the peak level of the reflected wave coincides with the rising edge of the clock signal, the count value of the clock signal becomes unstable and does not know whether it will take the value before the rising edge of the clock signal or the value after the rising edge. In this case, even if an attempt is made to accurately calculate the time from the transmission of the search wave to the reception of the reflected wave by calculating the fractional time of the clock signal, a time error equivalent to the count value of 1 (1 count) of the clock signal may occur.

Even in laser radars that detect the rising edge of a reflected wave instead of detecting the peak level of the reflected wave, a similar problem can occur if the detection of the rising edge of the reflected wave coincides with the rising edge of the clock signal.

The present invention has been made to solve the above problems, and its main objective is to accurately calculate the round-trip time of pulsed laser light in a laser radar that calculates the round-trip time of pulsed laser light based on the time calculated from the count value of a clock signal and the fractional time of the clock signal.

A first means for solving the above problem is a laser radar,
a clock generator for generating a clock signal having a constant period, the clock signal being composed of a signal of a first level which continues from a start timing and a signal of a second level which is different from the first level and continues;
a light projection unit that projects a pulsed laser beam in synchronization with the clock signal;
a light receiving unit that receives light reflected by an object from the pulsed laser light;
a first counter that counts a first counter value that is the number of starts of the first level signal corresponding to a period from a light projection timing at which the pulsed laser light is projected by the light projection unit to a light reception timing at which the reflected light is received by the light reception unit;
a second counter that counts a second counter value that is the number of starts of the second level signal corresponding to the period from the light projection timing to the light reception timing;
a selector that selects, in each period of the clock signal, the second counter value as the third counter value at a timing earlier than a predetermined timing that is later than a first start timing that is a start timing of the signal of the first level and earlier than a second start timing that is a start timing of the signal of the second level, and selects the first counter value as the third counter value at a timing later than the predetermined timing;
a fraction calculation unit that calculates a fractional time, which is a time from a beginning of the clock signal to the light receiving timing or a time from the light receiving timing to an end of the clock signal, in a cycle of the clock signal that includes the light receiving timing;
a time calculation unit that calculates a round-trip time of the pulse laser light based on the third counter value and the fractional time calculated by the fraction calculation unit;
a distance calculation unit that calculates a distance to the object based on the speed of the pulsed laser light and the round-trip time calculated by the time calculation unit;
Equipped with.

According to the above configuration, the clock generator generates a clock signal with a constant period, which is composed of a first level signal that continues from a start timing and a second level signal that is different from the first level and continues. The light projecting unit projects a pulsed laser beam in synchronization with the clock signal. Therefore, the generation timing of the clock signal can be made to correspond to the projection timing of the laser beam. The light receiving unit receives the reflected light of the pulsed laser beam reflected by an object. The first counter counts a first counter value, which is the number of starts of the first level signal corresponding to the timing from the projection timing at which the pulsed laser beam is projected by the light projecting unit to the reception timing at which the reflected light is received by the light receiving unit. The second counter counts a second counter value, which is the number of starts of the second level signal corresponding to the timing from the projection timing to the reception timing. Therefore, the first counter value and the second counter value counted by the first counter and the second counter, respectively, can be used to calculate an approximate time from the projection timing to the reception timing.

Here, for example, if the light receiving timing overlaps with the first start timing, which is the start timing of the first level signal, the first counter value becomes unstable as to whether it is a value before the first start timing or a value after the first start timing. On the other hand, in this case, since the light receiving timing does not overlap with the second start timing, which is the start timing of the second level signal, the second counter value becomes a stable value. In this regard, the selection unit selects the second counter value as the third counter value at a timing before a predetermined timing that is a timing after the first start timing and before the second start timing, in each period of the clock signal, and selects the first counter value as the third counter value at a timing after the predetermined timing. Therefore, when the light receiving timing overlaps with the first start timing, the second counter value is selected as the third counter value, and the third counter value can be a stable value. Similarly, when the light receiving timing overlaps with the second start timing, the first counter value is selected as the third counter value, and the third counter value can be a stable value.

The fraction calculation unit calculates a fractional time, which is the time from the beginning of the clock signal to the light reception timing or the time from the light reception timing to the end of the clock signal, in the cycle of the clock signal including the light reception timing. The time calculation unit calculates a round-trip time of the pulsed laser light based on the third counter value and the fractional time calculated by the fraction calculation unit. Therefore, in a laser radar that calculates the round-trip time of the pulsed laser light based on the time calculated from the third counter value, which is the count value of the clock signal, and the fractional time of the clock signal, it is possible to accurately calculate the round-trip time of the pulsed laser light. And, the distance calculation unit can accurately calculate the distance to the object based on the speed of the pulsed laser light and the round-trip time calculated by the time calculation unit.

A second means may be adopted in which only the selection unit of the first means is modified as follows. In the second means, the selection unit selects, in each period of the clock signal, the second counter value as the third counter value at a timing earlier than a first predetermined timing that is later than a first start timing that is the start timing of the signal of the first level and earlier than a second start timing that is the start timing of the signal of the second level, selects the second counter value as the third counter value at a timing later than a second predetermined timing that is later than the second start timing that is the start timing of the signal of the second level and earlier than an end timing of the signal of the second level, and selects the first counter value as the third counter value at a timing later than the first predetermined timing and earlier than the second predetermined timing.

Even with the above configuration, when the light receiving timing overlaps with the first start timing, the second counter value is selected as the third counter value, and the third counter value can be a stable value. Similarly, when the light receiving timing overlaps with the second start timing, the first counter value is selected as the third counter value, and the third counter value can be a stable value. Therefore, in a laser radar that calculates the round trip time of the pulsed laser light based on the time calculated using the third counter value, which is the count value of the clock signal, and the fractional time of the clock signal, the round trip time of the pulsed laser light can be accurately calculated.

The third means includes a delay circuit configured by connecting delay units in multiple stages, each of which transmits an input signal while delaying it, and into which the clock signal is continuously input, and the fraction calculation unit calculates the fractional time based on the number of transmission stages, which is the number of stages of the delay units through which the head of the clock signal has been transmitted from when the head of the clock signal is input to the first delay unit until the light reception timing, in a period of the clock signal that includes the light reception timing.

According to the above configuration, the delay circuit is configured by connecting delay units in multiple stages, which transmit the input signal while delaying it, and the clock signal is input continuously. Therefore, the clock signal is input from the beginning to the first delay unit, and is transmitted sequentially to the subsequent delay units. Then, when the clock signal is input to the first delay unit up to the end, the clock signal is input again from the beginning to the first delay unit, and this process is repeated. Therefore, the number of delay units to which the beginning of the clock signal is transmitted from the first delay unit in the delay circuit represents the fractional time, which is the time from the beginning of the clock signal to the light reception timing. Note that if the time from the light projection timing to the light reception timing is to be measured using only a delay circuit, a large number of delay units would be required.

The fraction calculation unit calculates the fractional time based on the number of transmission stages, which is the number of delay units through which the head of the clock signal has been transmitted from when the head of the clock signal was input to the first delay unit until the light receiving timing, in a cycle of the clock signal that includes the light receiving timing. The time calculation unit then calculates the round trip time of the pulsed laser light based on the third counter value and the fractional time calculated by the fraction calculation unit. This makes it possible to accurately calculate the round trip time of the pulsed laser light while preventing a large number of delay units from being required.

In a laser radar that inputs a start pulse to a pulse delay circuit and measures fractional time based on the outputs of all delay units that are latched when a stop pulse is input, after measuring a fractional time using the delay circuit, it is necessary to reset the outputs of all delay units in order to measure a fractional time using the delay circuit again.

In this regard, the multiple stages of delay units repeatedly transmit clock signals at a constant cycle. Therefore, after measuring a minute time using a delay circuit, it is not necessary to reset the outputs of all delay units before measuring a minute time using the delay circuit again. Therefore, minute time measurements are performed by receiving reflected light from smoke, dust, etc. that exists between the laser radar and the target object, and even if reflected light from the target is continuously received, minute time measurements can be performed continuously. Then, the distance calculation unit calculates the distance to the object based on the speed of the pulsed laser light and the round trip time calculated by the time calculation unit. Therefore, even if reflected light is continuously received, the distance to the target can be calculated.

Specifically, as in the fourth means, the time calculation unit can be configured to calculate the round-trip time of the pulsed laser light based on the sum of the time obtained by multiplying the third counter value by the constant period and the time obtained by multiplying the number of transmission stages by the delay time by which each delay unit delays the signal.

In the fifth means, the delay unit outputs a signal of the first level when a signal of the first level is input, and outputs a signal of the second level when a signal of a level different from the first level is input. With this configuration, a signal identical to a clock signal of a constant period can be repeatedly transmitted by a plurality of delay units, and the head of the clock signal can be transmitted sequentially by the delay units. Therefore, the fraction calculation unit can calculate the fractional time based on the number of transmission stages in the period of the clock signal that includes the light receiving timing.

In the sixth means, the fraction calculation unit obtains a measurement output including an output from a first delay unit to a delay unit capable of transmitting the head of the clock signal in the constant period, and the number of stages from the first delay unit to the delay unit corresponding to an edge between the first level signal and the second level signal in the measurement output is set as the number of transmission stages. The laser radar described in claim 5.

According to the above configuration, the fraction calculation unit acquires a measurement output including an output from the first delay unit to the delay unit capable of transmitting the head of the clock signal in the fixed period. Therefore, it is possible to acquire information necessary for calculating the number of stages of the delay unit to which the head of the clock signal is transmitted in the fixed period of the clock signal. The fraction calculation unit can easily calculate the number of transmission stages by setting the number of stages from the first delay unit to the delay unit corresponding to the edge between the first level signal and the second level signal in the measurement output as the number of transmission stages.

FIG. 1 is a schematic diagram showing an overview of a laser radar. FIG. 2 is a block diagram showing the configuration of an FPGA according to the first embodiment. FIG. 13 is a diagram showing an image of transmission of a carry signal. 6 is a time chart showing a manner in which a round-trip time of a pulsed laser beam is calculated. FIG. 11 is a block diagram showing a configuration of an FPGA according to a second embodiment. 6 is a time chart showing a manner in which a round-trip time of a pulsed laser beam is calculated. 11 is a graph showing the relationship between the distance to the object and the measurement time. 10 is a time chart showing a modified example of the manner of calculating the round-trip time of the pulsed laser beam. 10 is a time chart showing another modified example of the manner in which the round-trip time of the pulsed laser beam is calculated.

First Embodiment
Hereinafter, a first embodiment embodied in a laser radar that calculates a distance to an object will be described with reference to the drawings.

As shown in FIG. 1, the laser radar 10 includes a light-projecting unit 20 and a light-receiving unit 30. The light-projecting unit 20 projects a pulsed laser beam in response to a START signal synchronized with a clock signal. The light-receiving unit 30 receives the reflected light of the pulsed laser beam reflected by an object T (body), and outputs a STOP signal when the reflected light is detected. The laser radar 10 calculates the distance from the laser radar 10 to the object T by multiplying the speed of light, which is the speed of the pulsed laser beam, by the time obtained by dividing the time from the START signal to the STOP signal (the round-trip time T1 of the pulsed laser beam) by 2.

Figure 2 is a block diagram showing the configuration of an FPGA (Field Programmable Gate Array) 40 that calculates the round trip time T1. The FPGA 40 is mounted on the laser radar 10. The FPGA 40 includes a clock generator 41, a counter 42, a delay circuit 44, a sampling unit 46, an encoder 47, an integration unit 48, etc.

The clock generator 41 generates a clock signal of, for example, 400 MHz (several hundred MHz), i.e., a period (constant period) of 2.5 ns. The clock signal is composed of a 1.25 ns "1" (high level, first level) signal followed by a 1.25 ns "0" (low level, second level) signal. In other words, the clock signal is composed of a first level signal that continues from the start timing, and a second level signal that is different from the first level and continues.

The counter 42 counts a counter value, which is the number of clock signals generated from the timing when a START signal synchronized with the generation timing (head) of the clock signal is input to the timing when a STOP signal is input. That is, the counter 42 counts the number of clock signals generated from the timing when the light projecting unit 20 projects the pulsed laser light to the timing when the light receiving unit 30 receives the reflected light. The light receiving unit 30 detects the reflected light when the intensity of the reflected light exceeds the judgment value Ir, and this time becomes the light receiving timing. The counter value starts from 0 and is incremented by 1 when a rising edge (rising) from "0" to "1" of the clock signal is detected until the light receiving timing. The counter 42 calculates a counter time T2, which is the time obtained by multiplying the counter value by the period of the clock signal. The counter 42 outputs the calculated counter time T2 to the integration unit 48.

The delay circuit 44 includes a number of delay units 44a, 44b, 44c, ... connected in series. Each delay unit transmits the input signal while delaying it. The delay circuit 44 includes a number of delay units that can delay the input signal for a time longer than one period of the clock signal. In other words, when one period of the clock signal has elapsed since the beginning of the signal was input to the first delay unit 44a, the beginning of the signal has not yet reached the last delay unit.

Each delay unit is composed of, for example, a carry circuit of an adder, adds the input signals, and outputs a carry signal (carry signal) to the next delay unit and the sampling section 46. The carry signal from the previous adder and "1" are input to each adder. The first delay unit 44a is input with a clock signal and "1". Therefore, each delay unit outputs "1" as the carry signal while the clock signal is "1", and outputs "0" as the carry signal while the clock signal is "0". In other words, each delay unit outputs a signal of the first level when a signal of the first level is input, and outputs a signal of the second level (a level different from the first level) when a signal of a level different from the first level is input.

When the STOP signal is input, the sampling unit 46 samples the output of each delay unit. The sampling unit 46 connects the sampled outputs of each delay unit to obtain a binary measurement output. For example, if the outputs of delay units 44a, 44b, 44c, etc. are "1", "1", "0", etc. respectively, the measurement output will be 110.... The sampling unit 46 outputs the obtained measurement output to the encoder 47.

The encoder 47 receives the measurement output and calculates, based on the measurement output, a fractional time T3, which is the time from the beginning of the clock signal to the light reception timing in the period of the clock signal that includes the light reception timing.

Specifically, as shown in FIG. 3, the carry signal is transmitted sequentially by the delay units, causing the measurement signal to change over time. For example, when the sampling unit 46 receives a STOP1 signal, it samples the output of each delay unit at that time to obtain the measurement output. In the figure, "11000001111100000" is obtained as the measurement output. In the measurement output, "10" represents the rising edge of the clock signal from "0" to "1," i.e., the beginning of the clock signal.

Therefore, the encoder 47 searches for "10" from the left in the acquired measurement output and calculates the number of stages by which "10" has advanced. The number of stages by which "10" has advanced in the measurement output represents the number of transmission stages, which is the number of delay units through which the head of the clock signal has been transmitted from the input of the head of the clock signal to the first delay unit 44a to the light receiving timing in a clock signal period including the light receiving timing. The encoder 47 acquires a measurement output including the output from the first delay unit 44a to a delay unit capable of transmitting the head of the clock signal in one clock signal period, and sets the number of transmission stages to the number of stages from the first delay unit 44a to the delay unit corresponding to the edge of the first level signal "1" and the second level signal "2" in the measurement output. The encoder 47 calculates the fractional time T3 by multiplying the number of transmission stages by the delay time by which each delay unit delays the signal. That is, the encoder 47 encodes the measurement output to calculate the fractional time T3. The encoder 47 outputs the calculated fractional time T3 to the integration unit 48. The sampling unit 46 and the encoder 47 constitute the fraction calculation unit.

The integrating unit 48 adds the counter time T2 input from the counter 42 and the fractional time T3 input from the encoder 47 to calculate the round trip time T1 of the pulsed laser light (T1 = T2 + T3). That is, the integrating unit 48 calculates the round trip time T1 of the pulsed laser light based on the sum of the counter time T2 and the fractional time T3. The integrating unit 48 outputs the calculated round trip time T1 of the pulsed laser light to the distance calculating unit 50. The distance calculating unit 50 is mounted on the laser radar 10. The sampling unit 46, the encoder 47, and the integrating unit 48 constitute a time calculating unit.

The distance calculation unit 50 calculates the distance from the laser radar 10 to the target object T by multiplying the speed of light, which is the speed of the pulsed laser light, by the time obtained by dividing the round-trip time T1 of the pulsed laser light by 2. In other words, the distance calculation unit 50 calculates the distance to the object based on the speed of the pulsed laser light and the round-trip time T1 of the pulsed laser light calculated by the time calculation unit.

Figure 4 is a time chart showing how the round-trip time of pulsed laser light is calculated.

At time t1, a pulsed laser beam is emitted in synchronization with the generation of a clock signal, and the intensity of the pulsed laser beam increases. The counter 42 starts counting the counter value from "0". The number of transmission stages (carry transmission stage number), which is the number of delay units through which the head of the clock signal is transmitted in one cycle of the clock signal, increases from 0 over one cycle of the clock signal. Note that here, an example is shown in which the number of transmission stages increases from 0 to 99 in one cycle of the clock signal.

At time t2, when the beginning of the next cycle of the clock signal is input to the delay circuit 44, the counter 42 increments the counter value from "0" to "1." The number of transmission stages increments again from 0 to 99.

At time t4, for example, when the pulsed laser light is reflected by smoke and the intensity of the reflected light exceeds the judgment value Ir, the reflected light is detected and the first light receiving timing is reached. At this time, the counter time T21 is the counter value = n multiplied by the period of the clock signal. The fractional time T31 is the number of transmission stages from time t3 to time t4 multiplied by the delay time by which each delay unit delays the signal. The integration unit 48 then adds the counter time T21 and the fractional time T31 to calculate the round trip time T11 of the pulsed laser light.

At time t6, for example, when the pulsed laser light is reflected by the object T and the intensity of the reflected light exceeds the judgment value Ir, the reflected light is detected and the second light receiving timing is reached. At this time, the counter time T22 is the counter value = n + 1 multiplied by the period of the clock signal. The fractional time T32 is the number of transmission stages from time t5 to time t46 multiplied by the delay time by which each delay unit delays the signal. The integration unit 48 then adds the counter time T22 and the fractional time T32 to calculate the round trip time T12 of the pulsed laser light.

Then, the distance calculation unit 50 multiplies the speed of light, which is the speed of the pulsed laser light, by the round-trip times T11 and T12 of the pulsed laser light divided by 2 to calculate the distance from the laser radar 10 to the smoke and the target T, respectively.

The present embodiment described above has the following advantages:

The clock generator 41 generates a clock signal with a fixed period (2.5 ns period). The light projecting unit 20 projects a pulsed laser beam in synchronization with the clock signal. This allows the generation timing of the clock signal to correspond to the projection timing of the laser beam. The light receiving unit 30 receives the reflected light of the pulsed laser beam reflected by an object. The counter 42 counts a counter value which is the number of clock signals generated from the projection timing at which the light projecting unit 20 projects the pulsed laser beam to the reception timing at which the reflected light is received by the light receiving unit 30. This allows the counter value counted by the counter 42 to calculate an approximate time from the projection timing to the reception timing.

The delay circuit 44 is configured by connecting multiple stages of delay units 44a, 44b, 44c, ... that transmit the input signal while delaying it, and the clock signal is input continuously. Therefore, the clock signal is input from the beginning to the first delay unit 44a, and is transmitted sequentially to the subsequent delay units 44b, 44c, .... Then, when the clock signal is input to the first delay unit 44a up to the end, the clock signal is input again from the beginning to the first delay unit 44a, and this process is repeated. Therefore, the number of delay units to which the beginning of the clock signal is transmitted from the first delay unit 44a in the delay circuit 44 can represent the minute time since the beginning of the clock signal is input to the first delay unit 44a. Note that if the time from the light projection timing to the light reception timing is to be measured using only the delay circuit 44, a large number of delay units 44a, 44b, 44c, ... are required.

The time calculation unit calculates the round trip time of the pulsed laser light based on the counter value counted by the counter 42 and the number of transmission stages, which is the number of delay units through which the head of the clock signal is transmitted from when the head of the clock signal is input to the first delay unit 44a until the light receiving timing in the period of the clock signal that includes the light receiving timing. This makes it possible to accurately calculate the round trip time of the pulsed laser light while preventing the need for a large number of delay units.

・Multiple stages of delay units 44a, 44b, 44c, ... continue to repeatedly transmit a clock signal with a constant period. Then, the time calculation unit calculates the round-trip time of the pulsed laser light based on the counter value and the number of transmission stages in the period of the clock signal including the light receiving timing. Therefore, after measuring a minute time using the delay circuit 44, it is not necessary to reset the outputs of all delay units 44a, 44b, 44c, ... until the next time measuring a minute time using the delay circuit 44. Therefore, even if the minute time is measured by receiving reflected light from smoke, dust, etc. existing between the laser radar 10 and the object T, and the reflected light from the object T is continuously received, the minute time measurement can be continuously performed. Then, the distance calculation unit 50 calculates the distance to the object based on the speed of the pulsed laser light and the round-trip time calculated by the time calculation unit. Therefore, even if the reflected light is continuously received, the distance to the object T can be calculated. In addition, after measuring the minute time using the delay circuit 44, one to four cycles (several cycles) of the clock signal are required before the outputs of all delay units 44a, 44b, 44c, ... are reset, and during that time the laser radar described in Patent Document 1 is unable to measure minute time using the delay circuit 44.

The constant-period clock signal is composed of successive first-level signals "1" and successive second-level signals "0" different from the first level, and the delay units 44a, 44b, 44c, ... output the first-level signal "1" when the first-level signal "1" is input, and output the second-level signal "0" when a signal different from the first level is input. With this configuration, a signal identical to the constant-period clock signal can be repeatedly transmitted by multiple stages of delay units 44a, 44b, 44c, ..., and the head of the clock signal can be transmitted sequentially by delay units 44a, 44b, 44c, .... Therefore, the time calculation unit can calculate the round-trip time of the pulse laser light based on the number of transmission stages in the period of the clock signal including the light receiving timing.

The time calculation unit acquires a measurement output including the output from the first delay unit 44a to the delay unit capable of transmitting the beginning of the clock signal in a fixed period (2.5 ns). Therefore, it is possible to acquire information necessary for calculating the number of stages of the delay unit through which the beginning of the clock signal has been transmitted in a fixed period of the clock signal. The time calculation unit can easily calculate the number of transmission stages by determining the number of stages from the first delay unit 44a to the delay unit corresponding to the edge of the first level signal "1" and the second level signal "0" in the measurement output as the number of transmission stages.

Second Embodiment
The second embodiment will be described below with reference to the drawings, focusing on the differences from the first embodiment. The laser radar of this embodiment includes a first counter that counts a first counter value, a second counter that counts a second counter value, and a selector that selects one of the first and second counter values as a third counter value. The time calculator calculates a round-trip time T1 of the pulsed laser light based on the third counter value and the fractional time T3 calculated by the fraction calculator.

As shown in FIG. 5, FPGA 140 includes a counter 142 and a selection unit 43 instead of the counter 42 of FPGA 40 of the first embodiment. The rest of the configuration of FPGA 140 is the same as FPGA 40.

For example, in the time t13 to t14 in FIG. 6, if the light receiving timing (STOP) overlaps with the first start timing, which is the rising edge of the clock signal (the start timing of the first level signal "1"), the first counter value (the same as the counter value in the first embodiment) becomes unstable as to whether it takes the value "0" before the first start timing or the value "1" after the first start timing. In this case, as shown in FIG. 7, depending on the distance to the target T, there is a risk of a jump occurring in the measurement time of the round trip time of the pulsed laser light. On the other hand, as shown in FIG. 6, since the light receiving timing in this case does not overlap with the second start timing, which is the falling edge of the clock signal (the start timing of the second level signal "0"), the second counter value (the counter value shifted by a half cycle of the clock signal from the first counter value) becomes a stable value.

The counter 142 includes a first counter 142a and a second counter 142b (see FIG. 5). The first counter 142a counts the number of rising edges of the clock signal from "0" to "1" from the light projection timing to the light reception timing, i.e., the number of starts of a first level signal "1" corresponding to the light projection timing to the light reception timing, as the first counter value. The second counter 142b counts the number of falling edges of the clock signal from "1" to "0" from the light projection timing to the light reception timing, i.e., the number of starts of a second level signal "0" corresponding to the light projection timing to the light reception timing, as the second counter value.

The selection unit 43 sets a threshold value that divides the carry transmission stage into the front and rear at a position that does not overlap with the rising and falling of the clock signal. Since the clock signal rises and falls around the carry transmission stage number of "0" and "50", respectively, the threshold value is set to "30", for example. The selection unit 43 selects the second counter value as the third counter value when the carry transmission stage number is "30" or less, and selects the first counter value as the third counter value when the carry transmission stage number exceeds "30". That is, the selection unit 43 selects the second counter value as the third counter value at a timing before the predetermined timing t12, t15 (corresponding to the threshold value = 30) that is a timing after the first start timing that is the start timing of the first level signal "1" and before the second start timing that is the start timing of the second level signal "0" in each cycle of the clock signal, and selects the first counter value as the third counter value at a timing after the predetermined timing t12, t15. Then, the selection unit 43 outputs the counter time T2, which is the third counter value multiplied by the period of the clock signal, to the integration unit 48 (see FIG. 5).

The integration unit 48 adds the counter time T2 and the fractional time T3 input from the encoder 47 to calculate the round trip time T1 of the pulsed laser light (T1 = T2 + T3).

The present embodiment described above has the following advantages. Here, only the advantages that differ from the first embodiment will be described.

- The first counter 142a counts a first counter value, which is the number of starts of a first level signal "1" corresponding to the time from the light projection timing when the pulsed laser light is projected by the light projection unit 20 to the light reception timing when the reflected light is received by the light reception unit 30. The second counter 142b counts a second counter value, which is the number of starts of a second level signal "0" corresponding to the time from the light projection timing to the light reception timing. Therefore, the approximate time from the light projection timing to the light reception timing can be calculated from the first counter value and the second counter value counted by the first counter 142a and the second counter 142b, respectively.

- In each period of the clock signal, the selection unit 43 selects the second counter value as the third counter value at a timing earlier than the predetermined timings t12 and t15, which are timings later than the first start timing and earlier than the second start timing, and selects the first counter value as the third counter value at a timing later than the predetermined timings t12 and t15. Therefore, when the light reception timing overlaps with the first start timing, the second counter value is selected as the third counter value, and the third counter value can be a stable value. Similarly, when the light reception timing overlaps with the second start timing, the first counter value is selected as the third counter value, and the third counter value can be a stable value.

The fraction calculation unit calculates a fractional time T3, which is the time from the beginning of the clock signal to the light reception timing in the cycle of the clock signal that includes the light reception timing. The time calculation unit calculates a round-trip time T1 of the pulsed laser light based on the third counter value and the fractional time T3 calculated by the fraction calculation unit. Therefore, the laser radar 10 that calculates the round-trip time T1 of the pulsed laser light based on the counter time T2 calculated from the third counter value, which is the count value of the clock signal, and the fractional time T3 of the clock signal, can accurately calculate the round-trip time T1 of the pulsed laser light. Then, the distance calculation unit 50 can accurately calculate the distance to the object based on the speed of the pulsed laser light and the round-trip time T1 calculated by the time calculation unit.

The second embodiment can also be modified as follows. The same parts as those in the second embodiment are given the same reference numerals and will not be described.

- Only the selection unit 43 of the second embodiment can be modified as follows. As shown in FIG. 8, in each period of the clock signal, the selection unit 43 selects the second counter value as the third counter value at a timing earlier than a first predetermined timing t12 that is later than a first start timing that is a start timing of a first level signal "1" and earlier than a second start timing that is a start timing of a second level signal "0", selects the second counter value as the third counter value at a timing later than second predetermined timings t22, t25 that are later than a second start timing that is a start timing of a second level signal "0" and earlier than an end timing of the second level signal "0", and selects the first counter value as the third counter value at a timing later than the first predetermined timing and earlier than the second predetermined timing.

Even with the above configuration, when the light receiving timing overlaps with the first start timing, the second counter value is selected as the third counter value, and the third counter value can be a stable value. Similarly, when the light receiving timing overlaps with the second start timing, the first counter value is selected as the third counter value, and the third counter value can be a stable value. Therefore, in the laser radar 10 that calculates the round trip time T1 of the pulsed laser light based on the counter time T2 calculated from the third counter value, which is the count value of the clock signal, and the fractional time T3 of the clock signal, the round trip time T1 of the pulsed laser light can be accurately calculated.

- As shown in FIG. 9, in the cycle of the clock signal including the light receiving timing t32 (STOP), the time from the light receiving timing t32 to the rising timing t33 of the clock signal changing from "0" to "1" can be set as the fractional time T3, and the time from the light projection timing t31 to the rising timing t33 can be set as the counter time T2. In this case, the round trip time T1 of the pulsed laser light can be calculated by subtracting the fractional time T3 from the counter time T2 (T1=T2-T3).

Even with this configuration, by applying the first counter 142a, the second counter 142b, and the selection unit 43 of the second embodiment, when the light receiving timing overlaps with the first start timing, the second counter value is selected as the third counter value, and the third counter value can be a stable value. Similarly, when the light receiving timing overlaps with the second start timing, the first counter value is selected as the third counter value, and the third counter value can be a stable value. Therefore, in the laser radar 10 that calculates the round-trip time of the pulsed laser light based on the counter time T2 calculated from the third counter value, which is the count value of the clock signal, and the fractional time T3 of the clock signal, the round-trip time T1 of the pulsed laser light can be accurately calculated. Then, the distance calculation unit 50 can accurately calculate the distance to the object based on the speed of the pulsed laser light and the round-trip time T1 calculated by the time calculation unit.

- In the clock signal, a "1" (high level) signal can be considered as a second level signal, and a "0" (low level) signal can be considered as a first level signal. In this case, in FIG. 6, the first start timing and the second start timing are swapped, the position of the predetermined timing changes accordingly, and the first counter value and the second counter value are swapped. With this configuration, it is possible to achieve the same effect as the second embodiment.

The first and second embodiments can be modified as follows. The same parts as those in the first and second embodiments are denoted by the same reference numerals and will not be described.

- In a clock signal, the ratio between the length of a "1" (high level) signal and the length of a "0" (low level) signal is arbitrary.

The round-trip time T1 of the pulsed laser light can also be corrected taking into account at least one of the following: the time from generation of the START signal to projection of the pulsed laser light, the processing time for calculating the counter times T2, T21, and T22 by the counter 42, the first counter 142a, and the second counter 142b, the processing time of the encoder 47, the processing time of the integration unit 48, and the change in the light reception timing due to the intensity of the reflected light.

The delay units 44a, 44b, 44c, etc. are not limited to adders that add input signals and output a carry signal, but can also be buffer circuits that output the input signals as is.

- The counter value does not have to start from 0, but may start from 1. In that case, when calculating the counter time T2, the counter value minus 1 is multiplied by the period of the clock signal.

- The timing for receiving reflected light is not limited to when the intensity of the reflected light exceeds the judgment value Ir, but can also be when the intensity of the reflected light reaches its peak value (maximum value).

10...laser radar, 20...light projecting unit, 30...light receiving unit, 40...FPGA, 41...clock generator, 42...counter, 43...selecting unit, 44...delay circuit, 44a...delay unit, 44b...delay unit, 44c...delay unit, 46...sampling unit, 47...encoder, 48...integrating unit, 50...distance calculation unit, 140...FPGA, 142...counter, 142a...first counter, 142b...second counter.

Claims (10)

a clock generator for generating a clock signal having a constant period, the clock signal being composed of a signal of a first level which continues from a start timing and a signal of a second level which is different from the first level and continues;
a light projection unit that projects a pulsed laser beam in synchronization with the clock signal;
a light receiving unit that receives light reflected by an object from the pulsed laser light;
a first counter that counts a first counter value that is the number of starts of the first level signal corresponding to a time from a light projection timing at which the pulsed laser light is projected by the light projection unit to a light reception timing at which the reflected light is received by the light reception unit;
a second counter that counts a second counter value that is the number of starts of the second level signal corresponding to the period from the light projection timing to the light reception timing;
a selector that selects, in each period of the clock signal, the second counter value as a third counter value at a timing before a predetermined timing that is later than a first start timing that is a start timing of the signal of the first level and earlier than a second start timing that is a start timing of the signal of the second level, and selects the first counter value as the third counter value at a timing after the predetermined timing;
a fraction calculation unit that calculates a fractional time, which is a time from a beginning of the clock signal to the light receiving timing or a time from the light receiving timing to an end of the clock signal, in a cycle of the clock signal that includes the light receiving timing;
a time calculation unit that calculates a round-trip time of the pulsed laser light based on the third counter value and the fractional time calculated by the fraction calculation unit;
a distance calculation unit that calculates a distance to the object based on the speed of the pulsed laser light and the round-trip time calculated by the time calculation unit;
A laser radar comprising: a clock generator for generating a clock signal having a constant period, the clock signal being composed of a signal of a first level which continues from a start timing and a signal of a second level which is different from the first level and continues;
a light projection unit that projects a pulsed laser beam in synchronization with the clock signal;
a light receiving unit that receives light reflected by an object from the pulsed laser light;
a first counter that counts a first counter value that is the number of starts of the first level signal corresponding to a period from a light projection timing at which the pulsed laser light is projected by the light projection unit to a light reception timing at which the reflected light is received by the light reception unit;
a second counter that counts a second counter value that is the number of starts of the second level signal corresponding to the period from the light projection timing to the light reception timing;
a selector that selects, in each period of the clock signal, the second counter value as the third counter value at a timing earlier than a predetermined timing that is later than a first start timing that is a start timing of the signal of the first level and earlier than a second start timing that is a start timing of the signal of the second level, and selects the first counter value as the third counter value at a timing after the predetermined timing;
a fraction calculation unit that calculates a fractional time, which is a time from a beginning of the clock signal to the light receiving timing or a time from the light receiving timing to an end of the clock signal, in a cycle of the clock signal that includes the light receiving timing;
a time calculation unit that calculates a round-trip time of the pulsed laser light based on the third counter value and the fractional time calculated by the fraction calculation unit;
a distance calculation unit that calculates a distance to the object based on the speed of the pulsed laser light and the round-trip time calculated by the time calculation unit;
A laser radar comprising: a clock generator for generating a clock signal having a constant period, the clock signal being composed of a signal of a first level which continues from a start timing and a signal of a second level which is different from the first level and continues;
a light projection unit that projects a pulsed laser beam in synchronization with the clock signal;
a light receiving unit that receives light reflected by an object from the pulsed laser light;
a first counter that counts a first counter value that is the number of starts of the first level signal corresponding to a period from a light projection timing at which the pulsed laser light is projected by the light projection unit to a light reception timing at which the reflected light is received by the light reception unit;
a second counter that counts a second counter value that is the number of starts of the second level signal corresponding to the period from the light projection timing to the light reception timing;
a selector that selects, in each period of the clock signal, the second counter value as a third counter value at a timing before a first predetermined timing that is later than a first start timing that is a start timing of the signal of the first level and earlier than a second start timing that is a start timing of the signal of the second level, selects the second counter value as a third counter value at a timing after a second predetermined timing that is later than the second start timing that is a start timing of the signal of the second level and earlier than an end timing of the signal of the second level, and selects the first counter value as the third counter value at a timing after the first predetermined timing and earlier than the second predetermined timing;
a fraction calculation unit that calculates a fractional time, which is a time from a beginning of the clock signal to the light receiving timing or a time from the light receiving timing to an end of the clock signal, in a cycle of the clock signal that includes the light receiving timing;
a time calculation unit that calculates a round-trip time of the pulsed laser light based on the third counter value and the fractional time calculated by the fraction calculation unit;
a distance calculation unit that calculates a distance to the object based on the speed of the pulsed laser light and the round-trip time calculated by the time calculation unit;
A laser radar comprising: a clock generator for generating a clock signal having a constant period, the clock signal being composed of a signal of a first level which continues from a start timing and a signal of a second level which is different from the first level and continues;
a light projection unit that projects a pulsed laser beam in synchronization with the clock signal;
a light receiving unit that receives light reflected by an object from the pulsed laser light;
a first counter that counts a first counter value that is the number of starts of the first level signal corresponding to a period from a light projection timing at which the pulsed laser light is projected by the light projection unit to a light reception timing at which the reflected light is received by the light reception unit;
a second counter that counts a second counter value that is the number of starts of the second level signal corresponding to the period from the light projection timing to the light reception timing;
a selector that selects, in each period of the clock signal, the second counter value as a third counter value at a timing before a first predetermined timing that is later than a first start timing that is a start timing of the signal of the first level and earlier than a second start timing that is a start timing of the signal of the second level, selects the second counter value as the third counter value at a timing after a second predetermined timing that is later than the second start timing that is the start timing of the signal of the second level and earlier than an end timing of the signal of the second level, and selects the first counter value as the third counter value at a timing after the first predetermined timing and earlier than the second predetermined timing;
a fraction calculation unit that calculates a fractional time, which is a time from a beginning of the clock signal to the light receiving timing or a time from the light receiving timing to an end of the clock signal, in a cycle of the clock signal that includes the light receiving timing;
a time calculation unit that calculates a round-trip time of the pulse laser light based on the third counter value and the fractional time calculated by the fraction calculation unit;
a distance calculation unit that calculates a distance to the object based on the speed of the pulsed laser light and the round-trip time calculated by the time calculation unit;
A laser radar comprising: a clock generator for generating a clock signal having a constant period, the clock signal being composed of a signal of a first level which continues from a start timing and a signal of a second level which is different from the first level and continues;
a light projection unit that projects a pulsed laser beam in synchronization with the clock signal;
a light receiving unit that receives light reflected by an object from the pulsed laser light;
a first counter that counts a first counter value that is the number of starts of the first level signal corresponding to a period from a light projection timing at which the pulsed laser light is projected by the light projection unit to a light reception timing at which the reflected light is received by the light reception unit;
a second counter that counts a second counter value that is the number of starts of the second level signal corresponding to the period from the light projection timing to the light reception timing;
a selector that selects, in each period of the clock signal, the second counter value as a third counter value at a timing before a first predetermined timing that is later than a first start timing that is a start timing of the signal of the first level and earlier than a second start timing that is a start timing of the signal of the second level, selects the second counter value as the third counter value at a timing after a second predetermined timing that is later than the second start timing that is a start timing of the signal of the second level and earlier than an end timing of the signal of the second level, and selects the first counter value as the third counter value at a timing after the first predetermined timing and earlier than the second predetermined timing;
a fraction calculation unit that calculates a fractional time, which is a time from a beginning of the clock signal to the light receiving timing or a time from the light receiving timing to an end of the clock signal, in a cycle of the clock signal that includes the light receiving timing;
a time calculation unit that calculates a round-trip time of the pulsed laser light based on the third counter value and the fractional time calculated by the fraction calculation unit;
a distance calculation unit that calculates a distance to the object based on the speed of the pulsed laser light and the round-trip time calculated by the time calculation unit;
A laser radar comprising: a clock generator for generating a clock signal having a constant period, the clock signal being composed of a signal of a first level which continues from a start timing and a signal of a second level which is different from the first level and continues;
a light projection unit that projects a pulsed laser beam in synchronization with the clock signal;
a light receiving unit that receives light reflected by an object from the pulsed laser light;
a first counter that counts a first counter value that is the number of starts of the first level signal corresponding to a period from a light projection timing at which the pulsed laser light is projected by the light projection unit to a light reception timing at which the reflected light is received by the light reception unit;
a second counter that counts a second counter value that is the number of starts of the second level signal corresponding to the period from the light projection timing to the light reception timing;
a selector that selects, in each period of the clock signal, the second counter value as a third counter value at a timing earlier than a first predetermined timing that is later than a first start timing that is a start timing of the signal of the first level and earlier than a second start timing that is a start timing of the signal of the second level, selects the second counter value as a third counter value at a timing later than a second predetermined timing that is later than the second start timing that is a start timing of the signal of the second level and earlier than an end timing of the signal of the second level, and selects the first counter value as the third counter value at a timing after the first predetermined timing and earlier than the second predetermined timing;
a fraction calculation unit that calculates a fractional time, which is a time from a beginning of the clock signal to the light receiving timing or a time from the light receiving timing to an end of the clock signal, in a cycle of the clock signal that includes the light receiving timing;
a time calculation unit that calculates a round-trip time of the pulsed laser light based on the third counter value and the fractional time calculated by the fraction calculation unit;
a distance calculation unit that calculates a distance to the object based on the speed of the pulsed laser light and the round-trip time calculated by the time calculation unit;
A laser radar comprising: a delay circuit configured by connecting delay units in a plurality of stages, each delaying and transmitting an input signal, to which the clock signal is continuously input;
The laser radar according to any one of claims 1 to 6, wherein the fraction calculation unit calculates the fraction time based on a transmission stage number, which is the number of stages of delay units through which the head of the clock signal is transmitted from when the head of the clock signal is input to a first delay unit until the light reception timing, in a period of the clock signal that includes the light reception timing. 8. The laser radar according to claim 7, wherein the time calculation unit calculates a round-trip time of the pulsed laser light based on a sum of a time obtained by multiplying the third counter value by the constant period and a time obtained by multiplying the number of transmission stages by a delay time by which each delay unit delays a signal. 9. The laser radar according to claim 7 or 8, wherein the delay unit outputs a signal of the first level when a signal of the first level is input, and outputs a signal of the second level when a signal of a level different from the first level is input. 10. The laser radar according to claim 9, wherein the fraction calculation unit obtains a measurement output including outputs from a first delay unit to the delay unit capable of transmitting a head of the clock signal in the constant period, and the number of transmission stages is the number of stages from the first delay unit to the delay unit corresponding to an edge between the first level signal and the second level signal in the measurement output.

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