CN202182717U - Laser ranging device based on TDC technology - Google Patents

Abstract

The utility model discloses a laser distance measuring device based on TDC technology, which includes a single-chip microcomputer FPGA, a time digital converter TDC-GP2, a laser emitting device, a photoelectric detector and an optical element, wherein the single-chip microcomputer FPGA is connected to the time digital converter TDC-GP2, the time digital converter TDC-GP2 is connected to the photoelectric detector, and the photoelectric detector and the laser emitting device are connected to the optical element. The utility model can be used as a multidisciplinary integrated monitoring and measurement solution, and has good application prospects in distance measurement, positioning, two-dimensional profile measurement, speed measurement, regional monitoring, three-dimensional spatial positioning, etc.

Description

A Laser Distance Measuring Device Based on TDC Technology

technical field

The utility model belongs to the field of laser ranging, in particular to a laser ranging device based on TDC technology.

Background technique

In today's technologically advanced society, the application of laser ranging is becoming more and more common. In many fields, electric power, water conservancy, communication, environment, construction, geology, police, fire protection, blasting, navigation, railway, anti-terrorism/military, agriculture, forestry, real estate, leisure/outdoor sports, etc. can use laser range finder . Laser range finders generally use two methods to measure distances: pulse method and phase method. Pulse laser range finders calculate the distance by measuring the time between laser emission and return. Therefore, time measurement is a very important link for the pulsed laser range finder. Because lasers are so fast, the time between the emitted and received laser pulses is very small. For example, to measure a distance of 1 kilometer, the resolution is required to be 1 cm, and the resolution of time interval measurement is required to be as high as 67 ps. The time-to-digital converter TDC-GP2 of German acam company has a typical single measurement resolution of 65ps, ultra-low power consumption, high integration, and high measurement flexibility. It is an ideal choice for time-of-flight (TOF) measurement of pulsed laser rangefinders. .

Laser rangefinder is an instrument that uses laser to accurately measure the distance of the target (also known as laser rangefinder). The laser rangefinder emits a laser beam to the target when it is working, and the photoelectric element receives the laser beam reflected by the target. The timer measures the time from the launch to the reception of the laser beam, and calculates the distance from the observer to the target. Because the laser has the advantages of high directivity, high monochromaticity and high power, these are very critical for measuring long distances, determining target orientation, improving the signal-to-noise ratio of the receiving system, and ensuring measurement accuracy. Therefore, laser rangefinders increasingly receiving attention. The laser radar developed on the basis of the laser range finder can not only measure the distance, but also measure the target orientation, transportation speed and acceleration. The laser rangefinder has the advantages of high accuracy and resolution, strong anti-interference ability, small size, light weight, etc. It has a wide range of applications, numerous industry needs, and a large market demand space.

There are three main development directions of laser ranging technology: one is the expansion of the application field, such as combining the ranging system with the scanning mechanism to form a laser three-dimensional topography mapping system or laser radar; the other is to study how to improve the laser ranging system in ensuring The distance measurement accuracy in the case of range measurement; the third is the laser range finder that is safe for human eyes in use.

Laser range finder is a comprehensive application of multidisciplinary technologies such as laser, precision machinery, embedded technology and optoelectronics. With the development of laser technology, embedded technology and integrated optics, the laser rangefinder is developing in the direction of digitalization, automation, low cost, small and light, and its application range is becoming wider and wider. Usually, the key technology of the pulsed laser ranging system is mainly the strong pulse transmission and reception technology and the high-precision time interval measurement technology.

1. Optical pulse transmission and reception technology

The basic principle of pulsed laser ranging is to calculate the distance traveled by the laser within this time by measuring the time difference between the received laser pulse and the transmitted pulse. In order to accurately generate the emission and reception laser pulse reference, the laser emission pulse is required to be as narrow as possible. Due to the limitation of the resistance and capacitance parameters of the laser and the modulation device, the narrow pulse width of the laser emission can reach 10-20ns at present. The receiving system of semiconductor laser ranging is a typical direct detection system. The photodetector in the system directly converts the received optical signal into an electrical signal, and then its demodulation circuit can detect the information carried by the optical signal. The choice of detectors in this system generally uses avalanche diodes. Usually, the surface of the target to be measured is diffuse reflection, and the power of the light signal returned to the photodetector is very small, especially for long-distance measurement, the reflected signal has to be fully amplified, shaped and denoised Only after processing can it be recorded, calculated and displayed. Therefore, the signal amplification and extraction circuit in the photoelectric detection system is an important part of it. Its main purpose is to suppress noise to the greatest extent, amplify the signal, and improve the signal-to-noise ratio as much as possible, so as to obtain useful information carried in the returned signal. . All functions of the entire receiving system are to amplify, limit bandwidth, and separate information from the low-level narrow pulse signal received by the detector, and then send it to the next-level signal processing unit. The laser emits and receives pulses into a Gaussian shape, and the waveform forms a reference signal through a comparator. With the change of the electrical pulse, the position of the leading edge of the reference signal will change slightly, especially when the measurement distance is farther and the amplitude of the received signal changes greatly, causing The greater the ranging error.

2. High-precision time interval measurement technology

In the time-of-flight ranging method and in the research of many transient processes, the precision time measurement technology is the most core technology. In view of the importance of this technology, many companies, research institutes and universities at home and abroad have carried out research in this area. At present, some precision time measurement devices with different precision, different volume quality and different cost have been formed. In summary, there are three main time measurement methods:

(1) The analog method has the advantage of high measurement accuracy and can reach an order of magnitude. However, since the charging and discharging of the capacitor is not absolute, that is, there is a nonlinear effect, the measurement error is about one ten thousandth of its measurement range. This method is also greatly affected by temperature.

(2) Digital method, the digital method uses synchronous clock pulses to time the time interval. The digital method has good linearity and is not affected by the measuring range. The measurement accuracy is mainly related to the clock frequency, and its measurement accuracy is plus or minus one clock cycle. Clocks of a few hundred megahertz are generally used, with an accuracy of the order of magnitude. Even if the clock frequency is as high as it is, its measurement accuracy is only mere, and the distance corresponding to it is on the order of decimeters. From this point of view, the ranging accuracy is obviously not high. However, it is also possible to improve the measurement accuracy by adopting the method of taking the average of multiple measurements.

(3) Digital interpolation method. The digital interpolation method uses digital methods combined with various interpolation methods to achieve accurate measurement. It can improve the accuracy and linearity of single pulse measurement at the same time. It has a wider application range and field, and can adapt to high-speed, large-scale Measuring range and high-precision measurement.

Utility model content

In order to solve the above problems, the utility model provides a laser ranging device based on TDC technology. The device can be used as a multi-disciplinary comprehensive monitoring and measurement scheme, and has good application prospects in distance measurement, positioning, two-dimensional contour measurement, speed measurement, area monitoring, and three-dimensional space positioning.

In order to achieve the above object, the utility model adopts the following technical solutions:

A laser distance measuring device based on TDC technology, the laser distance measuring device includes a single-chip FPGA, a time-to-digital converter TDC-GP2, a laser emitting device, a photodetector and an optical element, and the single-chip FPGA is connected to the time-to-digital converter TDC-GP2, the time-to-digital converter TDC-GP2 is connected to the photodetector, and the photodetector and the laser emitting device are connected to the optical element.

In a preferred solution of the present utility model, the single-chip FPGA and the time-to-digital converter TDC-GP2 are connected through an SPI serial port.

Further, the trigger pulse width of the time-to-digital converter TDC-GP2 is greater than 2.5 ns.

Further, the photodetector is a receiving circuit based on CFD technology.

Still further, the laser distance measuring device also includes an on-site real-time display screen.

Still further, the laser distance measuring device is provided with a metal shell shielding electromagnetic interference.

Compared with the prior art, the utility model obtained according to the foregoing scheme has the following advantages:

(1) The utility model adopts TDC technology, TDC (Time-to-Digital Converter) is a kind of measurement method that carries out high-precision time interval measurement by the propagation delay of the signal through the internal gate circuit;

(2) Adopt a data processing method based on mean square error, so that the time measurement accuracy reaches the picosecond level, and the distance measurement accuracy reaches the centimeter level. In addition, the electromagnetic anti-interference of the system is also shielded;

(3) The design of the transceiver circuit, the device adopts the design of the receiving circuit based on CFD technology, so that the frequency of the distance measurement reaches one kilohertz;

(4) Use FPGA control and data processing to solve the problem of high-precision time slot measurement, and realize chip-level control and algorithm flow control;

(5) Using high-speed serial port and USB joint transmission mode, the device provides two different data transmission interfaces for different application requirements;

(6) The device is a portable device equipped with a local display function.

Description of drawings

The utility model will be further described below in conjunction with the accompanying drawings and specific embodiments.

Fig. 1 is a schematic diagram of the utility model;

Fig. 2 is the block flow diagram that the utility model implements;

Fig. 3 is the measurement schematic diagram of the utility model;

Fig. 4 is the measurement data before unprocessing;

Figure 5 shows the processed measurement data.

Detailed ways

In order to make the technical means, creative features, goals and effects achieved by the utility model easy to understand, the utility model will be further elaborated below in conjunction with specific illustrations.

Referring to FIG. 1 , the TDC technology-based laser ranging device 100 provided by the present invention includes a single-chip FPGA101, a time-to-digital converter TDC-GP2102, a laser emitting device 103, a photodetector 104 and an optical element 105.

Wherein, the microcontroller FPGA101 is connected to the time-to-digital converter TDC-GP2102, the time-to-digital converter TDC-GP2102 is connected to the photodetector 104, and the photodetector 104 and the laser emitting device 103 are connected to the optical element 105.

In the present utility model, the laser emitting device 103 is an emitting circuit mainly composed of a laser driver and a pulsed laser diode.

For the photodetector 104 it is a receiving circuit based on CFD technology.

On the basis of the above scheme, the utility model is provided with an on-site real-time display screen in the laser distance measuring device for realizing local display.

In order to further improve the measurement accuracy of the utility model, the utility model is provided with a metal shell shielding electromagnetic interference on the laser distance measuring device.

The utility model that obtains according to above-mentioned scheme, its course of work is as follows:

Referring to Figure 1, the laser emitting device emits light pulses and simultaneously inputs the emission pulses to the start port of TDC-GP2 to trigger time difference measurement. Once the reflected pulse transmitted back from the object 200 reaches the photodetector (receiving circuit), a Stop signal is generated to the TDC, and the time difference measurement is completed at this time. Then the time difference between Start and Stop pulses is accurately recorded by TDC-GP2, which is used to calculate the distance between the measured object and the transmitter.

In this principle, FPGA performs register configuration and time measurement control for TDCGP2, and the time measurement result is sent back to FPGA to calculate the distance accurately through the algorithm, and at the same time, if there is a display device, the distance will be displayed.

In this principle, the distance measurement is not only related to the time difference measurement accuracy of TDC-GP2, but also related to many other factors:

1. Laser peak power;

2. Laser beam divergence;

3. Optical components;

4. Media for optical transmission (air, rain, fog, etc.);

5. The light reflection ability of the object;

6. The sensitivity of the light receiving part and so on.

The characteristics of the measured object and the characteristics of the transmission medium are generally given by the conditions of the application, so the laser transmitter (wavelength, driving conditions, characteristics of the beam, etc.) and receiver (type, sensitivity, bandwidth, etc.). The measurement range increases with higher peak laser powers and higher signal-to-noise ratios. The accuracy of the time difference measurement is not only related to the measurement accuracy of the TDC-GP2 chip itself, but also related to the pulse characteristics of the laser, such as the pulse shape (width, rising and falling edge time), and the detector bandwidth and signal processing circuit. For tdc-gp2, the faster the speed of the pulse signal and the wider the bandwidth, the higher the measurement accuracy will be.

The measurement work of the TDC-GP2 chip is all completed by the TDC high-speed measurement unit. The start channel, stop1, and stop2 channels of GP2 are all available. Each stop channel has a measurement capability of 4 pulses. Under this measurement range, the measurement result can choose the calibration result (32 bits) or the non-calibration result 16 bits. It is recommended to use 32-bit calibration results, that is, to calibrate the TDC measurement unit once for each measurement. Issues requiring attention:

1. For TDC-GP2, the trigger pulse width must be greater than 2.5ns.

2. The time interval between the trigger edge of the start channel and the pulse edge of the first stop channel should be greater than 3.5ns.

3. Recommend automatic calibration results, and choose to perform automatic calibration after each measurement is completed. This feature is enabled by setting the autocalibration bit in register 0 to 0.

4. If the pulse time difference of stop1 and stop2 channels is calculated, the range of pulse time difference can be reduced to 0. The distance from Start to the last stop pulse cannot exceed 1.8us, which is limited by the hardware itself. The measurement process and typical register settings in this measurement mode are shown in Figure 2:

The communication between FPGA and tdc-gp2 is completed through the spi serial port, then a typical measurement process for measurement range 1 is:

Then FPGA can process the data after reading the data from gp2 to calculate the distance of the pulse back and forth. In the above measurement process, if gp2 does not receive any start signal after being initialized, the measurement will not take place. No interrupt is generated either. The measurement is triggered only after the start signal is accepted, so whether the measurement is normal or the stop pulse is not received within the specified time, an interrupt signal will be generated on the INTN pin of gp2, and the measurement is judged by judging the content of the status register. normal.

Note: Before accepting the start and stop pulses, the pins en_start and en_stop of gp2 must be set high, otherwise the start and stop channels will not be strobed, and the measurement will not be triggered!

Apply the method of averaging to improve the accuracy: the situation mentioned above is that the laser start pulse is given to the start channel of tdc-gp2, and the laser return pulse is given to the stop channel of tdcgp2. In this case, gp2 has a single measurement accuracy of 65ps. When the output frequency of the measurement is not very important, such as outputting results 1 to 2 times per second, then in order to improve the measurement accuracy at this time, the system error can be eliminated by averaging multiple measurements. In order to enable gp2 to greatly reduce the error by averaging, the measurement design recommended below is very effective, and can reduce the peak value of the system error to less than 10 ps.

As shown in Figure 3, in this case, the utility model uses the measurement range 1, the laser emission and reception pulse signals are given to stop1 and stop2, and in the start channel of tdc-gp2, the start signal is given by FPGA A start signal that does not participate in the measurement. The measurement process is as follows:

First, the FPGA sends a dummy start that does not participate in the measurement but is used to trigger the measurement. This signal is needed because the signal of the start channel tells gp2 to enter the measurement state now. Then after at least 50ns, the FPGA triggers the laser to generate a launch signal and simultaneously inputs this signal to the stop1 channel. Then the received laser pulse signal is input to the stop2 channel. That is to say, stop1 and stop2 are used to measure the time difference between laser emission and reception, and the start signal is given by FPGA to trigger gp2.

The reason for this is that there is a noise unit inside the tdc-gp2, which can be triggered by register settings. The noise unit will add random distribution noise to the start channel pulse of gp2, so the purpose of this is to greatly eliminate the quantization error and system error during averaging. Then the setting of this bit is the setting of EN_STARTNOISE in register 5.

In the absence of averaging, the measurement results are shown in Figure 4, and the measurement results after averaging 500 times are shown in Figure 5. The optimized features of the system are:

1. The time interval measurement of stop1 and stop2 can be as low as 0.

2. After this measurement, if the measurement results of gp2 are averaged, the system error can be greatly eliminated. Unlike the number of averages, the accuracy of gp2 can be improved to less than 6ps at most.

3. For the temperature change is quite stable, it should be noted that the time between the start signal given by the FPGA and the start signal of the laser (that is, the stop1 signal) should be more than 50ns. This time is to add noise to the start signal. In the measurement process in this case, the configuration of the above register 1 needs to be slightly modified: SPIwrite(0x81194900);//stop1 and stop2 channels receive a pulse respectively, define the calculation method, subtract the first pulse of stop2 The first pulse of stop1. In the application of gp2 to evaluate the measurement system test situation, the noise curve is as follows after measuring the 1us time interval on average 1000 times: in the case of averaging 1000 times, the peak-to-peak noise of the output is reduced to within 10ps, which is equivalent to resolving the distance of 1mm. Then, improving the measurement accuracy through this average method is very helpful for laser ranging applications with low measurement frequency.

The basic principles, main features and advantages of the present utility model have been shown and described above. Those skilled in the art should understand that the utility model is not limited by the above-mentioned embodiments. The above-mentioned embodiments and descriptions only illustrate the principle of the utility model. Without departing from the spirit and scope of the utility model, the utility model The new model also has various changes and improvements, and these changes and improvements all fall within the scope of the claimed utility model. The scope of protection required by the utility model is defined by the appended claims and their equivalents.

Claims (6)

1. laser ranging system based on TDC technology; It is characterized in that; Said laser ranging system comprises single-chip microcomputer FPGA, time-to-digit converter TDC-GP2, laser beam emitting device, photodetector and optical element; Said single-chip microcomputer FPGA is connected in time-to-digit converter TDC-GP2, and said time-to-digit converter TDC-GP2 and photodetector join, and said photodetector and laser beam emitting device are connected in optical element.

2. a kind of laser ranging system based on the TDC technology according to claim 1 is characterized in that, joins through the SPI serial ports between said single-chip microcomputer FPGA and the time-to-digit converter TDC-GP2.

3. a kind of laser ranging system based on the TDC technology according to claim 1 is characterized in that the trigger pulse width of said time-to-digit converter TDC-GP2 is greater than 2.5ns.

4. a kind of laser ranging system based on the TDC technology according to claim 1 is characterized in that said photodetector is the receiving circuit based on the CFD technology.

5. a kind of laser ranging system based on the TDC technology according to claim 1 is characterized in that said laser ranging system also comprises an on-site real-time display screen.

6. a kind of laser ranging system based on the TDC technology according to claim 1 is characterized in that said laser ranging system is provided with the metal shell of shield electromagnetic interference.

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