RU2551700C1 - Laser pulse distance meter - Google Patents

Abstract

FIELD: measurement equipment.

SUBSTANCE: distance meter includes a pulse semiconductor laser with an optical system, a laser pumping circuit, an avalanche photodiode with an optical system, which is in-series connected to an amplifier of photodetected signals, a controllable power supply (CPS) for the avalanche photodiode, a multiplier, a low-pass filter (LPF), an analogue-to-digital converter (ADC), a microcontroller (MC), a digital-to-analogue converter (DAC), an inverting amplifier, two comparators, a three-input multiplexer, an adder, two two-input multiplexers, random access memory (RAM), a clock pulse generator, an address counter, a counter of the summing number, three comparison circuits, a trigger and a range indicator.

EFFECT: reducing measurement error and improvement of mass-and-dimensional parameters of a distance meter.

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Description

The present invention relates to measuring technique for measuring the distance to various objects on the ground using laser radiation.

Known laser pulsed range finder (RF patent No. 2323245 C1, G01C 3/00, declared. 09.11.2006). The laser range finder contains two pulsed lasers (PCG) with an optical system, a PCG pump pulse amplifier, a controlled PCG pump pulse generator, a photodetector with an optical system coupled to a PCG optical system field, a photodetected signal amplifier, two comparators, a time interval meter, an object detection indicator , synchronizer, integrators, adder, photodetector power regulator, inhibit pulse generator, controllable counter, strobe generator.

The disadvantage of this laser rangefinder is the low detection range. Reliable range determination requires that the reflected signal exceed the noise level. In Fig. 1, the useful signal is allocated by the comparator 11 and the controllable counter 14 and the threshold on the comparator is set above the noise level, otherwise there will be a false response from the noise.

In addition, a laser pulse range finder is known (RF patent No. 2288449 C2, G01C 3/08, application form 28.06.2004), which is a prototype of the invention and consisting of a master pulse generator, a pump generator and a laser, as well as forming optics, interconnected the output of which is optically connected to the photodetector path, consisting of a series-connected photodetector, a differentiating stage, a preamplifier and a video amplifier, the output of which, which is the output of the photodetector path, is connected in series with the start-stop a range meter and a range indicator, and the laser is optically connected to the gate signal driver, the output of which is connected to the control input of the video amplifier and to the controlled input of the start-stop range meter, between the output of the photodetector path and the input of the start-stop range meter, a series-connected unit of weighted bipolar summation signals received from individual locations of successive ranging locations, as well as a maximum search device that determines Astok of locating, corresponding to the maximum accumulated in the summation signal amplitude, and a counter n of m to validate the selection of locating portion corresponding to the maximum accumulated in the summation signal amplitude, which in a control input coupled to the output of the strobe signal.

The disadvantage of the prototype is too large a measurement error (± 5 m), because the weighted summation unit as part of the UVX, a multi-polar ADC, multiplier and RAM does not allow to achieve high performance due to the high bit depth of the multiplier, adder and RAM.

The task of the invention is to reduce the measurement error and improve the overall dimensions of the range finder.

The basis of the invention on the device is "Method of incoherent accumulation of radar signals" (RF patent No. 2359226 C1, application. 10.10.2007, publ. 20.06.2009). As shown in this invention, two-level storage devices are optimal for detection and require a minimum of equipment volume, less device capacity, while the storage efficiency approaches the theoretical limit. The storage device sums up or subtracts one, therefore it requires less bit depth and works with higher clock frequencies, which means that the discreteness of the range and, accordingly, the measurement error will be less than that of a multi-threshold storage device. In the prototype, a multi-threshold drive based on ADC is used, it requires more binary bits during accumulation and therefore has lower speed and, accordingly, a larger measurement error.

Figure 1 shows the functional diagram of the laser rangefinder, figure 2 - plot signals. The proposed device (figure 1) contains:

1 - controlled power source (UIP);

2 - microcontroller (MK);

3 - optical system;

4 - avalanche photodiode (APD);

5 - amplifier photodetected signals;

6 - multiplier;

7 - low-pass filter (low-pass filter);

8 - analog-to-digital Converter (ADC);

9 - digital-to-analog converter (DAC);

10 - a comparator;

11 - inverting amplifier;

12 - a comparator;

13 - multiplexer;

14 - adder;

15 - optical system of the receiver;

16 - pulsed semiconductor laser;

17 is a diagram of a laser pump;

18 is a comparison diagram;

19 - multiplexer;

20 - random access memory (RAM);

21 - clock generator (GTI);

22 - multiplexer;

23 - counter;

24 - counter;

25 is a comparison diagram;

26 - range indicator;

27 is a comparison diagram;

28 - trigger.

The start of the measurement is initiated by the MC, which first gives the “Reset” signal, which resets the counters 23 and 24 to zero. The zero code of the counter 24 is fed to the first input of the comparison circuit 25, which outputs an equal signal at a high level to the input of the data selection of multiplexer 19. The multiplexer selects the second input, i.e. zero code. The multiplexer output is connected to the RAM data input, so in the first period the RAM memory cells will be zeroed, and in the second and subsequent periods the multiplexer will switch to the output of the adder 14 and accumulation will be carried out.

After issuing the “Reset” signal, the MK generates a “Start” signal with a high-level pulse supplied to the input of the trigger setup 28. The trigger output is set to logical unit “1”, switches the multiplexer 22 to broadcast the GTI 21 and sets the data recording mode of RAM 20. Counter 23 is incremented with the frequency of the GTI, the output code of which is supplied to the comparison circuit 18 and the address input of RAM. At the second input of the comparison circuit 18, a binary code “M” is supplied, which sets the duration of the emitted laser light pulse. The output of the comparison circuit 18 is set to “1” when the counter code 23 is less than the “M” code. The high-level signal of the comparison circuit is supplied to the pump circuit 17, which generates the required pump current for the laser 16. When the counter code 23 exceeds the code "M", the comparison circuit 18 will generate a logic zero signal, completing the formation of a pulse of the emitted laser light signal.

The light signal reflected from the obstacle enters the optical system 3 and then focuses on the receiving platform of the APD. The photodetected LFD signal, which is a mixture of the useful signal with background radiation and noise, is amplified by an amplifier 5 and fed to an analog multiplier 6 and comparators 10 and 12. The multiplier 6 performs the mathematical operation of squaring and then using the low-pass filter is filtered, which is equivalent to the integration operation. With the help of the ADC, the signal is converted into a digital binary code received by the MK. MK performs the square root extraction operation on this signal, obtaining the root mean square (effective) value of the noise with the signal. At long ranges, the signal level is much less than the rms noise value, so the noise will be calculated with great accuracy.

One of the tasks of the MC is to maintain an effective noise level over the entire temperature range, which is performed by regulating the power supply voltage of the APD. For this, the calculated effective noise value is compared with a predetermined threshold, if the noise value is greater than the threshold, then the power supply voltage of the APD decreases, and if less, it increases. Management is carried out by the code issued at UIP1. A change in the voltage on the APD leads to a change in the multiplication coefficient of the photodiode and, accordingly, to a change in the signal level with background radiation and inherent noise level.

In addition, the MC sets the optimal voltage level - the threshold at the second inputs of the comparators 10 and 12. As shown in the above-mentioned incoherent accumulation method, the optimal detection threshold is + 0.5σ and -0.5σ, where σ is the rms noise value. MK issues a code on the DAC 9, the output voltage of which will be equal to 0.5 Ueff. The output of the inverting amplifier 11 produces a negative voltage of -0.5 Ueff. The comparator 10 gives a logical (log.) "1" if the input voltage exceeds 0.5 Ueff, and a log. "0" if not exceeding. Comparator 12 issues a log. “1” if the input voltage is less (lower) -0.5 Ueff and logical “0” if it is more than a threshold.

The output signals of the comparators are fed to the inputs of the data selection of the multiplexer 13. If the comparator 10 and the comparator 12 give a level log. “0”, then in the multiplexer 13 the first input with the given binary code “0” is selected and a zero code is fed to the input of the adder 14 - there is no accumulation. If comparator 10 issues a log. "1", and the comparator 12 log. “0”, then in the multiplexer 13 the second input with the binary code “1” is selected, and in the adder the output code of the current RAM cell with “1” is added. The output code of the adder is transmitted through the multiplexer 19 and is fed to the input and recorded in RAM with the same address with which it was read. Reading and writing to RAM is performed in a single clock cycle of the GTI frequency. If comparator 10 issues a log. "0", and the comparator 12 log. “1”, then in the multiplexer 13 the third input is selected with a binary additional code “-1”, and in the adder the output code of the current RAM cell is added to “-1”, i.e. subtraction unit. With each new GTI cycle, the delay from the moment of laser radiation increases and, accordingly, each RAM address is uniquely “tied” to a certain discrete distance. In each RAM cell, signal accumulation occurs.

After recording the last RAM cell, the next clock counter 23 is reset and generates a transfer pulse at the second output, which is received at the clock input of the counter 24, the state of which increases by "1". A new period of laser pulse emission and accumulation begins. As soon as the state of the counter 24 reaches the code "N", the comparison circuit 27 will output a log at the output of the equality. "1" and reset the trigger 28 to zero, signaling the end of accumulation - the signal "Ready", received at the input of the MC, the input of the data selection of the multiplexer 22 and the write input of the RAM, translating it into a read state.

After receiving the “Ready” signal, processing of the accumulated MK information begins. First, the MK overwrites the contents of the RAM into its internal RAM, for this the MK gives out a “Reset” pulse, setting the counter 23 to zero, then it reads the RAM memory cell through the input port and writes the code to the internal memory array. Then it gives an increment pulse, which through the multiplexer 22 is fed to the clock input of the counter 23, increasing the state of the counter by "1". At the output of RAM, data from the first memory cell appears, which are read by the MK and written to the first cell of the internal memory array. So the process of reading and writing continues to the end of the entire memory.

After overwriting the information, the MK normalizes the accumulated values by range by multiplying the data array by weight coefficients, the values of which depend on the range. At shorter ranges, the coefficients are smaller, the dependence in the initial portion of the range is quadratic, then the coefficient is constant. The multiplication operation equalizes the sensitivity of the system in range and reduces the amplitude of the backscatter signal from aerosol interference.

After normalizing the information, MK searches for a maximum in the array of internal memory. The maximum is taken not for one memory cell, but for the sum of the "M" consecutive memory cells, equal in number of clock cycles of the pulse radiated by the laser and, accordingly, the received pulse. The search is conducted not from the zero address, but from a certain, corresponding minimum range, excluding the transient process of the receiving path from the action of the emitted laser pulse. The found maximum is compared with the threshold, and if it exceeds the threshold, then the result is considered reliable.

The rms value of the noise in the drive after the accumulation of N periods will be equal to:

(стр.20 в книге под редакцией В.Е. Карасика «Лазерные приборы и методы измерения дальности». М.: 2012, изд. МГТУ им. Н.Э. Баумана).At the same time, it is known that to set the probability of detection of a reflected signal to be 0.99, it is required that the signal-to-noise ratio be at least 6, therefore, the detection threshold should be equal to $$6\sqrt{N}$$

(p. 20 in a book edited by VE Karasik “Laser devices and methods for measuring range.” M .: 2012, ed. MSTU named after NE Bauman).

, например, если N=10000, то сигнал/шум вырастет в 100 раз, при этом максимальная дальность увеличится в 10 раз.The increase in signal-to-noise ratio as a result of accumulation will be $$\sqrt{N}$$

For example, if N = 10000, then the signal-to-noise ratio will increase 100 times, while the maximum range will increase 10 times.

To further improve the measurement accuracy and reduce the error, the MC finds the energy center in the found maximum sum of M elements, for example, for M = 6, the delay time of the reflected signal will be equal to

Where

Here, j is the number of the time discrete in which the accumulated sum is maximum, K _(a) is the accumulated sum in the (a) mth disc, p is the correction number characterizing the temporary fixation point, and ΔT is the discrete duration (GTI period).

Formulas are given on p. 122 V. Vilner, A. Laryushin, E. Rud "Methods for improving the accuracy of pulsed laser rangefinders", g. Electronics: Science, Technology, Business 3/2008.

At a GTI clock frequency of 150 MHz, the sampling duration is 1 m; finding the delay using the above formulas increases the measurement accuracy to 0.2 m.

Range is determined by the formula

D = C · Tz / 2,

where C = 3 · 10 ⁸ m / s is the speed of light.

The implementation of the digital part, including the microcontroller, can be performed on a modern element base - on a programmable logic integrated circuit (FPGA), which allows to reduce the weight and size parameters of the range finder.

Claims (1)

A laser pulse range finder, consisting of a pulsed semiconductor laser connected in series with an optical system and a laser pump circuit, as well as of an avalanche photodiode (APD) connected in series with an optical system, a photodetected signal amplifier, and also consisting of a controlled power source (SPS) ), a clock generator, random access memory (RAM), a microcontroller (MK) and a range indicator, with the first output of the MK connected to the input of the UIP One of which is connected to the LFD power input, the RAM output is connected to the second input of the MK, and the output of the clock generator is connected to the clock input of the RAM, the fourth output of the MK is connected to the input of the range indicator, characterized in that a multiplier, a low-pass filter (LPF) are introduced, analog-to-digital converter (ADC), first and second comparator, digital-to-analog converter (DAC), inverting amplifier, first, second and third multiplexer, adder, first, second and third comparison circuit, first and second counter and RS trigger, moreover the output of the photodetector amplifier is connected to the first and second input of the multiplier, as well as to the first input of the first and first input of the second comparator, the output of the multiplier is connected to the input of the low-pass filter, the output of which is connected to the input of the ADC, the output of the ADC is connected to the first input of the MK, the second output of the MK is connected with the input of the DAC, the output of which is connected to the second input of the comparator and the input of the inverting amplifier, the output of which is connected to the second input of the second comparator, the output of the first and second comparator are connected respectively to the first and second the first input of the data selection of the first multiplexer, the first input of which is supplied with a binary zero code, the binary input of the unit code is supplied to the second input, and the binary code minus one in the additional code is supplied to the third input, the output of the first multiplexer is connected to the first input of the adder, the second input of which is connected to RAM output, the adder output is connected to the first input of the second multiplexer, to the second input of which a binary zero code is supplied, the output of the second multiplexer is connected to the RAM data input, the first input of the third multiplexer is sub It is connected to the output of the clock generator, and the second input is connected to the third output of the MK, the output of the third multiplexer is connected to the clock input of the first counter, the output of which is connected to the first input of the first comparison circuit and the RAM address input, the transfer output of the first counter is connected to the clock input of the second counter whose output is connected to the first input of the second and third comparison circuits, the binary code of the radiation pulse duration "M" is supplied to the second input of the first comparison circuit, the output of the first comparison circuit is connected to laser pump, the binary zero code is supplied to the second input of the second comparison circuit, and the binary code of the number of accumulation pulses “N” is supplied to the second input of the third comparison circuit, the equality output of the second comparison circuit is connected to the data selection input of the second multiplexer, and the equality output of the third circuit the comparison is connected to the reset input of the trigger, the output of which is connected to the third input of the MK, the data selection input of the third multiplexer and the RAM write input, the fifth output of the MK is connected to the reset input of the first and second counter, and MC oops output connected to the trigger input of the installation.

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