A time discrimination system for an improved pulsed lidar

Abstract

The present invention relates to the technical field of lidar mapping, and discloses a time discrimination system for an improved pulsed lidar, which includes a signal amplification module, an automatic gain control module, a constant fraction timing discrimination module, and a time measurement module; the signal amplification module is used to convert the optical signal of the received echo pulse signal of the lidar system into a voltage signal and amplify it; the automatic gain control module is used to construct an automatic gain control circuit by using the chip AD603 to process the input signal so that the constant fraction timing circuit receives a signal with a smaller dynamic range; the improved constant fraction timing discrimination module includes a leading edge discrimination unit, a constant fraction timing discrimination unit, and a logic gate unit; the time measurement module includes a time measurement unit and an MCU unit. The present invention can compress the signal dynamic range through the automatic gain control unit to reduce the timing point error caused by the amplitude change.

Description

Technical Field

[0001] The present invention relates to the technical field of lidar mapping, and particularly to a time discrimination system for an improved pulsed lidar mainly based on circuit design. Background Art

[0002] As a core sensor device in autonomous driving, lidar has gradually come into the public eye with the continuous popularity of autonomous driving. To achieve high-precision measurement of lidar, improving the time discrimination accuracy in the receiving circuit is an important topic. A pulsed lidar system includes a laser emission controller, a transmission driving circuit, a photoelectric conversion circuit (APD), an amplifier circuit, an automatic gain control circuit (AGC), a constant fraction timing discrimination circuit, and a high-precision time measurement circuit (TDC). The time measurement method uses the time of flight for distance measurement.

[0003] According to existing research results, there are mainly three typical signal time discrimination methods: The first is the leading edge time discrimination method, which uses the input pulse signal to compare with a fixed threshold, generates a trigger signal at the moment when the input pulse signal is greater than the reference voltage, and sends it to the TDC to record the receiving pulse time, taking this time as the timing point. Because the amplitude change of the echo pulse will generate a large walk error, affecting the ranging accuracy; The second is the high-pass RC method, which uses a high-pass filter to convert the unipolar echo signal into a bipolar signal, that is, converts the peak value of the pulse signal into the zero point of a sine-like signal, and then records the time point of the receiving pulse through a zero-crossing comparator. The zero-crossing time discrimination method is insensitive to the amplitude of the input signal and can reduce the walk error caused by the pulse amplitude change. However, this method is affected by the slope of the pulse amplitude point, and the pulse width size will also bring measurement errors; The third is the constant fraction timing method. The input signal is divided into two paths, one path is attenuated, and the other path is delayed. A high-speed comparator is used to compare the two paths of signals, and the signal jump is captured through the positive and negative poles of the comparator to reduce the time discrimination error caused by the amplitude change of the echo signal.

[0004] The signal from the automatic gain passes through the attenuated and delayed signals to obtain the timing point through a comparator. However, before the delayed signal arrives, the equivalent amplitudes of the attenuated signal and the delayed signal will make the comparator output unstable or even oscillate, so that the error caused by the amplitude change cannot be eliminated. Therefore, a bias level is generally added, that is, a fixed level is added to the delayed signal at the intersection of the attenuated signal and the delayed signal, and the amplitude is the minimum flip level of the comparator. This bias voltage involves the problem of the superposed time, and at the same time increases the complexity of the circuit structure. Commonly used delay circuits include RLC delay and transmission line delay. RLC delay often causes waveform distortion and complex circuit debugging. Summary of the Invention

[0005] In view of the deficiencies of the above-mentioned traditional time discrimination methods in the background art, the present invention provides a time discrimination system for an improved pulsed lidar, a more perfect time discrimination circuit system, to achieve high-precision time discrimination, so as to solve the technical problem of inability to eliminate errors proposed in the above background art.

[0006] The present invention provides the following technical solutions:

[0007] A time discrimination system for an improved pulsed lidar, comprising a signal amplification module, an automatic gain control module, an improved constant fraction timing discrimination module, and a time measurement module;

[0008] The signal amplification module is used to convert the optical signal of the echo pulse signal received by the lidar system into a voltage signal and amplify it;

[0009] The automatic gain control module is used to construct an automatic gain control circuit by using the chip AD603, process the input signal, and enable the constant fraction timing circuit to receive a signal with a smaller dynamic range;

[0010] The improved constant fraction timing discrimination module includes a leading edge discrimination unit, a constant fraction timing discrimination unit, and a logic gate unit; the leading edge time discrimination unit is composed of a TLV3501 high-speed comparator and an ADA4665 operational amplifier as a voltage follower; the constant fraction timing discrimination unit is composed of a signal amplification circuit, a signal delay circuit, and a high-speed comparator. The difference after the intersection of the attenuated signal and the delayed signal is smaller than the difference after the intersection of the amplified signal and the delayed signal, which results in a delay of several nanoseconds between the actual timing point and the ideal timing point generated by the attenuated signal and the delayed signal. However, when using the amplified signal to intersect with the delayed signal, the delay time between the actual timing point and the ideal timing point is greatly shortened. This reduces the timing error and improves the timing accuracy; the operational amplifier circuit has no problem of resistance value consistency compared with the traditional attenuation circuit, and the amplification factor is convenient to adjust, which is convenient for later debugging;

[0011] The time measurement module includes a time measurement unit and an MCU unit.

[0012] Preferably, the signal amplification module includes a transimpedance amplifier and an inverting amplifier. The transimpedance amplifier is composed of the chip OPA657. This operational amplifier is high-speed and high-bandwidth, and is usually used for pulsed light conversion. The minimum current resolution of the module is 0.1 nA, and it also has an analog signal bandwidth better than 100 M. The extremely low bias current makes the output error extremely small. The inverting amplifier is composed of the chip OPA695. The transimpedance amplifier amplifies the optical signal and converts it into a voltage signal, and the inverting amplifier flips the voltage signal and sends it to the automatic gain control circuit.

[0013] Preferably, the automatic gain control module includes a first-stage amplifier circuit, a second-stage amplifier circuit, and a detector. While amplifying the previous-stage signal, the two-stage amplifier circuit will also amplify the noise. To improve the signal-to-noise ratio of the two-stage amplifier circuit, the two-stage circuit adopts a sequential control connection method. Also, to reduce the bandwidth loss after cascading the two-stage amplifiers, a direct capacitor coupling method is selected in the design. The triodes 2N3904 and 2N3906 are cascaded to form a detector. The input signal is output after passing through the first-stage amplifier circuit and the second-stage amplifier circuit in sequence. The detector composed of two triodes detects the amplitude of the output signal at the output end of the second-stage amplifier circuit, and controls the gain by comparing and quantifying the voltage amplitude to achieve automatic gain control.

[0014] Preferably, through actual tests, when the signal is within a certain range, the constant ratio timing discrimination unit has a good timing point. Therefore, the output signal of the automatic gain control circuit needs to be amplified appropriately. The first pulse signal and the set reference voltage are input into the leading-edge discrimination unit together. The TLV3501 comparator is the core of the leading-edge discrimination unit; the second pulse signal enters the TLV3501 comparator after being amplified again and the third delayed signal. The moment when the amplitude of the third delayed signal is equal to the amplitude of the amplified second pulse signal is taken as the timing point. The constant ratio timing discrimination unit obtains the timing point of the pulse signal regardless of the input pulse amplitude; finally, the output signals of the two comparators pass through a logic gate, and the moment corresponding to the rising edge of the output signal of the logic gate is the moment point of the discriminated pulse signal.

[0015] Preferably, in the comparator circuit, the echo pulse signal and the threshold signal of the reference voltage are sent to the two input terminals of the comparator for differential comparison. Appropriately setting the threshold value of the reference voltage can reduce the misjudgment of the timing discrimination point caused by noise.

[0016] Preferably, the time measurement module uses the time-to-digital conversion method to achieve high-precision time interval measurement. When the timing discrimination system is powered on, the emission pulse of the programmable logic controller enters the time measurement module as a trigger signal as the start signal. The echo pulse signal passes through the signal amplification module and the automatic gain control module and enters the improved constant ratio timing discrimination module. The timing moment generated by the logic gate is used as the stop signal. The time difference between the start signal and the stop signal is calculated to obtain the measured distance. The measured distance and the true distance are numerically fitted to obtain a functional relationship, and the MCU unit is programmed.

[0017] Preferably, the automatic gain control module needs to control the output pulse amplitude range within 1.362V - 2.487V.

[0018] Preferably, the amplification factor of the time discrimination circuit and the delay time of the delay circuit are selected as follows: Since the intersection of the falling edge slope of the amplified pulse signal at its maximum and the delay signal is required, the amplification factor and the delay time need to be controlled. In the simulation circuit, the range of the delay time of the delay circuit is measured to be 5 - 7 ns, and the range of the amplification factor of the time discrimination circuit is 1.3 - 1.5 times, which can make the timing point occur at the steeper part of the falling edge of the amplified signal. During actual measurement, amplified signals and delay signals with different input amplitudes are sent to an oscilloscope to observe the timing point. The delay time is continuously adjusted while keeping the amplification factor unchanged, or the amplification factor is adjusted while keeping the delay time unchanged to obtain the optimal timing point, and the parameters at this time are recorded.

[0019] The present invention has the following beneficial effects:

[0020] 1. The dynamic range of the signal is compressed by the automatic gain control unit, reducing the timing point error caused by amplitude changes.

[0021] 2. Pre - time discrimination is performed through the leading - edge time discrimination circuit. The comparator is set with a relatively high threshold to reduce the probability of misjudgment. Then, the high - level signals obtained from the fixed comparator and the high - level signals obtained from the constant - fraction timing comparator are passed through an AND gate to obtain the final required signal, which can improve the time discrimination accuracy.

[0022] 3. In the constant - fraction timing discrimination system of the improved pulsed lidar, an operational amplifier is used to amplify the signal, and the moment when the amplitude is equal to that of the delay signal is taken as the timing point, avoiding the error caused by subtracting the delayed signal from the attenuated signal due to small signals, and improving the timing accuracy. The delay circuit is composed of two operational amplifiers to form a single - pole active all - pass filter. By utilizing the characteristic that the gain of the all - pass filter is 1 and the phase changes for time delay, the problem of changing the waveform while delaying the signal by the traditional RLC circuit is avoided. In addition, the circuit is convenient for debugging, and the delay time can be changed by adjusting the RC network of the first - stage filter. The time measurement unit obtains the time - difference information, and the time - difference is processed in the controller through a software algorithm, while eliminating the hardware error caused by the analog circuit. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 It is a schematic diagram of the module structure and process of the time discrimination system of the improved pulsed lidar according to the embodiment of the present invention;

[0024] Figure 2 It is a schematic diagram of the module structure and process of the automatic gain control system according to the embodiment of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0025] The present invention will be further described in detail below with reference to the drawings and specific embodiments.

[0026] Embodiment

[0027] The time discrimination system of the improved pulsed lidar provided by the embodiment of the present invention, its specific system module structure and process are as Figure 1 shown. The time discrimination system of the improved pulsed lidar provided by this embodiment includes: a signal amplification module, an automatic gain control module, an improved constant fraction timing discrimination module, and a time measurement module;

[0028] (1) Signal amplification module: As the front-end amplification unit, it is used to convert the optical signal of the received echo pulse signal of the lidar system into a voltage signal and amplify it; the signal amplification module is mainly composed of a transimpedance amplifier and an inverting amplifier. The transimpedance amplifier is composed of the chip OPA657. This operational amplifier is high-speed and high-bandwidth, and is usually used for pulsed light conversion. The minimum current resolution of the module is 0.1 nA, and it has an analog signal bandwidth of better than 100 M. The extremely low bias current makes the output error extremely small. The inverting amplifier is composed of the chip OPA695. The transimpedance amplifier amplifies the optical signal and converts it into a voltage signal, and the inverting amplifier flips the voltage signal and sends it to the automatic gain control circuit.

[0029] (2) Automatic gain control module: It is used to construct an automatic gain control circuit using the chip AD603 to process the input signal so that the constant fraction timing circuit receives a signal with a smaller dynamic range. As Figure 2 shown, the automatic gain control module is composed of a first-stage amplification circuit, a second-stage amplification circuit, and a detector. The two-stage amplification circuit will amplify the noise while amplifying the pre-stage signal. In order to improve the signal-to-noise ratio of the two-stage amplification circuit, the two-stage circuit adopts a sequential control connection method. Also, in order to reduce the bandwidth loss after the cascade of the two-stage amplifiers, a direct capacitance coupling method is selected in the design. The triodes 2N3904 and 2N3906 are cascaded to form a detector. The input signal is output through the two-stage amplification circuit. The detector composed of two triodes detects the amplitude of the output signal at the output end of the amplification circuit, and controls the size of the gain by comparing and quantifying the voltage amplitude to achieve automatic gain control.

[0030] (3)Improved constant fraction timing discrimination module: It includes a leading edge discrimination unit, a constant fraction timing discrimination unit, and a logic gate unit; the leading edge timing discrimination unit is composed of a TLV3501 high-speed comparator and an ADA4665 operational amplifier as a voltage follower; the constant fraction timing discrimination unit is composed of a signal amplification circuit, a signal delay circuit, and a high-speed comparator. The difference after the intersection of the attenuated signal and the delayed signal is smaller than the difference after the intersection of the amplified signal and the delayed signal. This causes a delay of several nanoseconds between the actual timing point and the ideal timing point generated by the attenuated signal and the delayed signal. However, when using the intersection of the amplified signal and the delayed signal, the delay time between the actual timing point and the ideal timing point is greatly shortened. This reduces the timing error and improves the timing accuracy; the operational amplifier circuit has no problem with the consistency of resistor values compared with the traditional attenuation circuit, and the amplification factor is convenient to adjust, which is convenient for later debugging; after actual testing, when the signal is within a certain range, the constant fraction timing discrimination unit has a good timing point. Therefore, the output signal of the automatic gain control circuit needs to be amplified appropriately. The first pulse signal and the set reference voltage are input into the leading edge discrimination unit together, and the TLV3501 comparator is the core of the leading edge discrimination unit; the second pulse signal is amplified again and then enters the TLV3501 comparator together with the third delayed signal. The moment when the amplitude of the third delayed signal is equal to the amplitude of the signal after being amplified again is taken as the timing point. The constant fraction timing discrimination unit obtains the timing point of the pulse signal independently of the input pulse amplitude; finally, the output signals of the two comparators pass through the logic gate, and the moment corresponding to the rising edge of the output signal of the logic gate is the timing point of the discriminated pulse signal.

[0031] In the comparator circuit, the echo pulse signal and the threshold signal of the reference voltage are sent to the two input terminals of the comparator for difference comparison together. Appropriately setting the threshold value of the reference voltage can reduce the misjudgment of the timing discrimination point caused by noise.

[0032] (4)Time measurement module: It includes a time measurement unit and an MCU unit. The time measurement module uses the time-to-digital conversion method to achieve high-precision time interval measurement. When the timing discrimination system is powered on, the emission pulse of the programmable logic controller enters the time measurement module as a trigger signal as the start signal. The echo pulse signal passes through the signal amplification module, the automatic gain control module, and enters the improved constant fraction timing discrimination module. The timing moment generated by the logic gate is used as the stop signal. The time difference between the start signal and the stop signal is calculated to obtain the measured distance. The measured distance and the actual distance are numerically fitted to obtain a functional relationship, which is burned into the MCU unit.

[0033] Optimal input amplitude range of the improved constant fraction timing discrimination module: The high-precision improved constant fraction timing discrimination module needs to determine the optimal input pulse amplitude range through experiments, record the time difference between different input pulse amplitudes and the timing point. When the input signal is between 1.362V and 2.487V, the pulse amplitude has nothing to do with the timing discrimination point. The automatic gain control module needs to control the output pulse amplitude range to improve the discrimination accuracy of the received pulse timing point.

[0034] Selection of amplification factor of the timing discrimination circuit and delay time of the delay circuit: Since the falling edge slope of the amplified pulse signal at its maximum and the delayed signal need to intersect, the amplification factor and delay time need to be controlled. In the simulation circuit, it is measured that when the delay time is 5 - 7ns and the amplification factor is 1.3 - 1.5 times, the timing point can occur at the steeper part of the falling edge of the amplified signal. During actual measurement, the amplified signals and delayed signals with different input amplitudes are sent to the oscilloscope to observe the timing point. Continuously adjust the delay time while keeping the amplification factor unchanged or adjust the amplification factor while keeping the delay time unchanged to obtain the optimal timing point, and record the parameters at this time.

[0035] In summary, the transmitted pulse of the programmable logic controller in the above embodiments of the present invention enters the time measurement module as a trigger signal and serves as the start signal. The echo pulse signal enters the improved constant fraction timing discrimination module after passing through the signal amplification module and the automatic gain control module. The timing moment generated by the logic gate is used as the stop signal, and the time difference between the start signal and the stop signal is calculated to obtain the measured distance.

[0036] The above is only the basic solution of the specific implementation method of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be thought of by those skilled in the art within the technical scope disclosed by the present invention should be covered by the protection scope of the present invention. Therefore, the protection scope of the present invention should be subject to the protection scope of the claims. All changes falling within the equivalent meaning and scope of the claims will be included within the scope of the claims.

Claims

1. An improved time discrimination system for a pulsed lidar, characterized in that, It includes a signal amplification module, an automatic gain control module, an improved constant fraction timing discrimination module, and a time measurement module; The signal amplification module is used to convert the optical signal of the received echo pulse signal of the lidar system into a voltage signal and amplify it; The automatic gain control module is used to construct an automatic gain control circuit using the chip AD603, process the input signal, and enable the improved constant fraction timing discrimination module to receive a signal with a smaller dynamic range; The improved constant fraction timing discrimination module includes a leading edge discrimination unit, a constant fraction timing discrimination unit, and a logic gate unit; The time measurement module includes a time measurement unit and an MCU unit; The automatic gain control module includes a first-stage amplifier circuit, a second-stage amplifier circuit, and a detector. The input signal is output after passing through the first-stage amplifier circuit and the second-stage amplifier circuit in sequence. The detector composed of two triodes detects the amplitude of the output signal at the output end of the second-stage amplifier circuit, and controls the size of the gain by comparing and quantifying the voltage amplitude to achieve automatic gain control; The output signal of the automatic gain control circuit is processed as follows: The first pulse signal and the set reference voltage are input into the leading edge discrimination unit together. The TLV3501 comparator is the core of the leading edge discrimination unit; The second pulse signal enters the high-speed comparator of the constant fraction timing discrimination unit after being amplified again and together with the third delayed signal. The moment when the amplitudes of the third delayed signal and the amplified second pulse signal are equal is taken as the timing point; finally, the output signals of the two comparators pass through the logic gate, and the moment corresponding to the rising edge of the output signal of the logic gate is the moment point of the discriminated pulse signal.

2. The time discrimination system of the improved pulsed lidar according to claim 1, characterized in that, The signal amplification module includes a transimpedance amplifier and an inverting amplifier. The transimpedance amplifier is used to amplify the optical signal and convert it into a voltage signal, and the inverting amplifier is used to invert the voltage signal and send it into the automatic gain control circuit.

3. The time discrimination system of the improved pulsed lidar according to claim 1, characterized in that, In the comparator circuit, the echo pulse signal and the threshold signal of the reference voltage are sent into the two input terminals of the comparator together for differential comparison.

4. The time discrimination system of the improved pulsed lidar according to claim 1, characterized in that, The time measurement module uses the time-to-digital conversion method to achieve high-precision time interval measurement. When the time discrimination system is powered on, the emission pulse of the programmable logic controller enters the time measurement module as a trigger signal as the start signal. The echo pulse signal enters the improved constant fraction timing discrimination module after passing through the signal amplification module and the automatic gain control module. The timing moment generated by the logic gate is used as the stop signal, and the time difference between the start signal and the stop signal is calculated to obtain the measured distance. The measured distance and the true distance are numerically fitted to obtain a functional relationship, and the MCU unit is programmed.

5. The time discrimination system of the improved pulsed lidar according to claim 3, characterized in that, The automatic gain control module needs to control the output pulse amplitude range within 1.362V to 2.487V.

6. The time discrimination system of the improved pulsed lidar according to claim 1, characterized in that, The improved constant fraction timing discrimination module includes a timing circuit and a delay circuit. The amplification factor range of the timing circuit is: 1.3 to 1.5 times, and the delay time range of the delay circuit is: 5 to 7 ns.

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