RU2554279C2 - Laser distance meter - Google Patents

Abstract

FIELD: measurement equipment.

SUBSTANCE: invention relates to optic devices for contactless measurement of distance and can be used at production of laser distance meters or tacheometers. A distance meter includes a transmitting channel including a setting generator connected to the input of a laser transmitter with an output optic system, a receiving channel, as well as an optic pilot channel. The receiving channel includes an input optic system, in the focus of which an avalanche photodiode is installed which is connected to a signal input of the measurement unit, the reference input of which is connected to the setting generator. The pilot channel is made in the form of an external optic line closing input and output channels. Housings of the avalanche photodiode and the laser diode are provided with a temperature sensor connected to the measurement input of the measurement unit, and the measurement unit itself is provided with non-volatile memory.

EFFECT: enlarging functional capabilities.

6 cl, 4 dwg

Description

The claimed invention relates to optical measuring devices, in particular to devices for non-contact range measurement, and can be used in the manufacture of laser rangefinders or total stations, which can accurately measure the distance to the object or its individual parts, and can also be used for topographic surveying of the area.

State of the art

At present, in the field of geodesy, construction, engineering surveys, land surveying and cadastral works, laser range finders are widely used for highly accurate distance control, which allows determining distances with an accuracy of 1 mm at distances up to 3 km. The main requirements for these range meters are:

- simplicity and speed of work with the consumer, which can significantly simplify operating costs and reduce the level of personnel servicing the device;

- the cheapness of the device during its mass production, which will significantly reduce the costs of enterprises that use dozens and hundreds of devices at the same time;

- ensuring guaranteed reliability of devices in the operating temperature range from - 40 ° C in winter and up to + 50 ° C in summer, because work can be carried out year-round in hard-to-reach places, for example, when laying pipelines in the northern latitudes, which excludes the possibility of frequent calibration of devices in the metrology laboratory.

A laser range finder device with two photodetector devices (FPUs) is known, which makes it possible to measure range using the phase method with high accuracy (see US Patent No. 7023531, class G01C 3/08, 2006). The device contains a transmitting channel, including a master oscillator connected to the input of the laser transmitter, and two receiving channels (signal and reference), each of which is an FPU based on an avalanche photodiode, the output of which is through a signal converter (demodulator, bandpass filter, amplifier, and ADC) ) connected to the microcontroller. Both receiving channels operate simultaneously: the signal channel receives radiation reflected from the measured object, and a small part of the radiation reflected from the translucent mirror at the output of the optical transmitter is supplied to the reference channel.

The main principle of operation of this circuit is based on the assumption that the temperature drift of phases in these channels should be the same, and, accordingly, should not affect the phase difference.

The main disadvantages of the known device are, firstly, the presence of two APDs, each of which is the most expensive component of the circuit.

Secondly, these APDs must be complementary, that is, they are made in one batch and matched to each other so that at the output of each FPU they have the same temperature phase drift. The use of one model in this scheme, but from different batches, will not fully compensate for the temperature drift, which will lead to an additional error.

Closest to the claimed laser rangefinder is taken as a prototype two-channel device that works on the principle of a phase rangefinder (see US patent No. 7221435, CL G01C 3/08, 2007). The device comprises: a transmitting channel; receiving channel; optical control channel and electronic unit. The transmitting channel includes a master oscillator connected to the input of the laser transmitter with the output optical system. The receiving channel includes an input optical system, in the focus of which is installed an APD connected to the signal input of the electronic unit. The reference input of the electronic unit is connected to the master oscillator. The optical control channel is made in the form of a mechanical switch with two optically coupled mirrors, one of which is installed in front of the outlet of the transmitting channel, and the other in front of the inlet of the receiving channel. The presence of an optical control channel with two switchable optically coupled mirrors makes it possible to increase the accuracy of range measurement due to the fact that before each measurement, a calibration measurement of the length of the control optical channel is performed, in which due to two rotary mirrors the output radiation of the transmitting channel without leaving the device immediately goes to input channel input. The presence of the reference channel makes it possible to eliminate errors arising during distance measurement that arise due to time delays introduced by the avalanche photodiode, laser transmitter, and some other electronic elements of the device circuit. These errors have a value of the order of several tens of millimeters, which is not acceptable for solving the tasks.

The reference channel immediately before each measurement with the help of rotary mirrors "closes" the radiation of the optical transmitter directly to the FPU, and makes a control measurement of the length of the reference channel. The result of the control measurement is subtracted from the result of the subsequent range measurement, which is carried out immediately after the control measurement, when the turning mirrors overlap the reference channel and open the optical path to the measured object.

The main disadvantages of the known device are, firstly, the complexity of the design containing the movable elements. It is known that mechanically movable parts of a device are usually the least reliable parts of devices operating at very low temperatures (up to -40 ° C).

Secondly, the need for a control measurement before each range measurement increases the total measurement time, which also has a particularly negative effect in very cold or very hot climatic conditions.

Disclosure of invention

The basis of the invention is the task of expanding the functionality of the laser rangefinder by eliminating the above disadvantages while reducing the cost of the product.

This task is in a laser rangefinder containing a transmitting channel, including a master oscillator connected to the input of the laser transmitter with the output optical system, and a receiving channel, including the input optical system, in the focus of which an avalanche photodiode is connected, connected to the signal input of the electronic unit, the reference input of which connected to the master oscillator, as well as the optical control channel, it is decided that the optical control channel is made in the form of an external optical line that closes the input and output channel Ala. In addition, the avalanche photodiode and laser diode cases are equipped with a thermal sensor connected to the measuring input of the electronic unit, and the electronic unit is equipped with non-volatile memory.

An external optical control channel allows one to measure temperature-dependent phase shifts that occur inside the rangefinder, and the presence of a temperature sensor allows one to construct a calibration dependence (SC) of phase shifts on temperature, which is recorded in non-volatile memory. Thus, once measured short circuit allows you to eliminate the need to use the data of the reference channel before each measurement, which accordingly increases the measurement speed, ease of use, operating temperature range and at the same time reduces the cost of the device.

To control the temperature of the avalanche photodiode body and laser diode, they are mounted on a common metal heat pipe, the temperature of which is monitored by a temperature sensor.

For short-circuit measurements, each device equipped with an optical control channel, made, for example, in the form of two fixed mirrors mounted to each other at a right angle, or in the form of a two- or trihedral prism, the edges of which form a right angle, or in the form of a fiber section, installed in a climate chamber. At the same time, an optical radiation attenuator, for example, darkened glass, is installed inside the optical control channel, which makes it possible to direct the desired level of optical power to the LFD working platform.

Short circuit measurement for each rangefinder is carried out only once at the factory and is recorded in non-volatile memory, such as flash memory.

Brief Description of the Drawings

Figure 1 shows a block diagram of the inventive laser rangefinder in operating mode.

Figure 2 shows a block diagram of the inventive laser rangefinder in the measurement mode short circuit.

Figure 3 shows the block diagram of the measuring unit of the laser rangefinder.

Figure 4 shows the structural embodiment of the optical control channel using a two-sided prism.

The best embodiment of the invention

The inventive device (figure 1) includes: a transmitting channel 1, consisting of a master oscillator 2 and a laser transmitter consisting of a driver 3 and a laser diode 4 with an output optical system 5 that converts a diverging light beam into a parallel light beam 6; a receiving channel 7, consisting of an input optical system 8, converting a parallel beam of light 9 into a converging beam, in the focus of which an avalanche photodiode 10 is installed; metal heat conduit 11 on which the cases of the laser diode 4 and the avalanche photodiode 10 are installed, as well as the temperature sensor 12; a power source 13 supplying a high voltage to the avalanche photodiode 10; an electronic unit 14 comprising a microcontroller 15, a high-frequency signal converter 16, a control and indication interface 17, a data transmission interface 18, a battery pack 19 and a non-volatile memory 20.

Presented in figure 2, the device further includes: an optical control channel 21, consisting of two stationary mirrors 22 and 23, located at right angles to each other, as well as a laser radiation attenuator 24.

The block diagram shown in FIG. 3 includes: a digital-to-analog converter (DAC) 25; high-frequency signal converter 16, consisting of two analog-to-digital converters (ADCs) 26 and 27, two mixers (Down converter) 28 and 29, a local oscillator 30 (Local oscillator) and an amplifier 31.

Figure 4 presents the design of the pairing of the claimed device with an optical control channel, made in the form of a dihedral prism 25, the edges of which form a right angle. The heat conduit 11, in which the laser diode 4 and the avalanche photodiode 10 are pressed, is mounted directly on the printed circuit board 26.

After the range finder is assembled, its calibration by temperature is the final stage of manufacture, and it is performed directly in the factory.

To do this, one or more rangefinders with attached optical control channels 21 (Fig. 4) are simultaneously placed in a climate chamber.

The calibration of each device is as follows. A device or devices placed in a climate chamber is heated to a temperature above 50 ° C. After that, power is supplied to the device and the microcontroller 15 starts a calibration program, which, as the device cools to a temperature of minus 40 ° С, measures the length of the optical control channel with a given step, for example, 0.5 or 1.0 ° С. These data, hereinafter referred to as calibration values (SC), using the microcontroller 15 are recorded in non-volatile memory 20 and are used in the operation of the device during the entire life of the device.

The method of measuring the length of the control channel is similar to the method of measuring range and is carried out as follows.

The master oscillator 2 generates an electrical signal having the form:

, $$A\left(2\pi \times f_0\times t\right)\left(\mathrm{one}\right)$$

,

where A is the amplitude of the electric signal, f ₀ is the frequency of the master oscillator 2, t is time.

The laser driver 3 converts this signal into power-modulated laser radiation 6 of the laser diode 4. Using the output optical system 5, the laser radiation is directed to the fixed mirrors 22 and 23, attenuating in the attenuator 24. After that, the laser radiation 9 is directed through the input optical system 8 to an avalanche photodiode 10, which converts the optical signal into an electrical one, having the form:

, $$B\left(2\pi \times f_0\times t+\Delta \varphi \left(L\right)\right)\left(2\right)$$

,

where B is the amplitude of the received electric signal, L is the measured distance, Δφ (L) is the phase difference between the electric signal from the output of the master oscillator 2 and the electric signal from the output of the APD.

The phase difference Δφ (L) is described by the following formula:

; $$\Delta \varphi \left(L\right)=\mathrm{four}\pi \times L\times f_0/C\left(3\right)$$

;

where C is the speed of light, or

; $$L=\Delta \varphi \left(L\right)\times C/\mathrm{four}\pi f_0\left(\mathrm{four}\right)$$

;

Since the frequency f ₀ can have values from several hundred megahertz to gigahertz, and the distance L in the general case can reach several kilometers, the value Δφ (L) in the general case has the form:

, $$\Delta \varphi \left(L\right)=2\pi \times N+\delta ,0\le \delta <2\pi ,\left(5\right)$$

,

where δ is the fractional part of the phase, and N is the integer, or the so-called "phase uncertainty".

The fractional part of the phase δ can be determined by standard methods for measuring electrical signals (for example, using a phase meter).

To determine the uncertainty of the N phases, it is necessary to use measurements at several f ₀ values using the well-known phase range finder algorithm (V.V. Grigorin-Ryabov, Radar devices. - M.: Soviet Radio, 1970, pp. 19-24,).

In the measuring unit 14 of the device by means of a high-frequency signal converter 16, comprising a local oscillator 30 with a frequency f1 and two mixers 28 and 29, signals B (2π × f ₀ × t + Δφ (L)) and A (2π × f ₀ × t) transferred to the low-frequency region (demodulation method):

$$B\left(2\pi \times f_0\times t+\Delta \varphi \left(L\right)\right)\to b\left(2\pi \times \Omega \times t+\Delta \varphi \left(L\right)\right),\left(6\right)$$

$$A\left(2\pi \times f_0\times t\right)\to a\left(2\pi \times \Omega \times t\right).\left(7\right)$$

Ω = f ₀ −f ₁ , where f ₁ is the frequency of the reference oscillator.

The value of Ω is usually selected in the range from 1 kHz to 100 kHz.

The phase difference Δφ (L) is thus retained.

The signals b (2π × Ω × t + Δφ (L) and a (2π × Ω × t) using two ADCs 26 and 27 are recorded in the RAM of the microcontroller 15, in which the length of the optical control channel 21 is calculated by the formula (4) .

After removing the short circuit from the device, the optical control channel 21 is removed, the working range measurement program is recorded in the microcontroller 15, after which the device (Fig. 1) is ready for operation.

The range measurement technique in the work program differs from the calibration one, from the obtained range value the short circuit corresponding to the temperature closest to the temperature value measured by the temperature sensor 12 is subtracted.

Thus, in the inventive device eliminates the need for a control measurement before each range measurement.

Technical applicability

An experimental sample of the claimed device was installed instead of the standard rangefinder unit on a 3Ta5D tacheometer manufactured by the Ural Optical and Mechanical Plant with a receiving aperture diameter of 45 mm. The overall dimensions of the rangefinder unit are 40 × 80 × 20 mm., The overall dimensions of the external optical control channel are 40 × 20 × 0 mm.

As a control and display interface, the standard interface of the total station was used.

Brief characteristics of the laser range finder as part of the 3Ta5D tacheometer.

Operating modes:

- automatic;

- prism;

- reflectionless.

Distance measurement mode - automatic (automatic detection of the reflectivity type of the target):

The minimum measurement time is 0.4 sec;

In terms of range and accuracy, all other parameters correspond to the parameters of the PRISM and REFLECTIVE modes for the corresponding types of reflectors (target).

Distance Measurement Mode - PRISMA

Coaxial visible red laser 658 nm.

Laser safety class 2 according to IEC 60825-1 (Laser safety Class 2)

Range:

Reflector GPR1 - 3000 m;

The minimum distance is 0.1 m;

Accuracy / measurement time - 1 mm + 1.5 ppm / 2.4 s;

Screen resolution - 1 mm.

Distance Measurement Mode - REFLECTIVE

Coaxial visible red laser 658 nm.

Laser safety class 2 according to IEC 60825-1 (Laser safety Class 2)

Screen resolution - 0.1 mm.

Range and time of measurement:

The measurement distance of 1 km is 50 seconds (reflective surface Kodak gray 90%).

300 m. - 0.4 sec. (Kodak gray reflective surface 90%).

The minimum distance is 0.1 m.

Accuracy / Measurement Time:

Up to 100 m (Kodak gray 90%) - 1 mm + 2 ppm / 0.4 sec .. (automatic mode);

0.1 sec (reflectionless mode);

More than 300 m - 3 mm + 2 ppm / 1-6 sec, maximum 12 sec.

The size of the laser spot per 100 m is 20 × 10 mm.

Claims (6)

1. A laser range finder, comprising a transmitting channel including a master oscillator connected to the input of the laser transmitter with an output optical system, and a receiving channel including an input optical system, in the focus of which there is an avalanche photodiode connected to the signal input of the electronic unit, the reference input of which is connected with a master oscillator, as well as an optical control channel, characterized in that the optical control channel is made in the form of an external optical line that closes the input and output channels s and housings avalanche photodiode and a laser diode provided with a temperature sensor connected to the measuring input of the electronic unit, and the electronic control unit equipped with nonvolatile memory. 2. The laser rangefinder according to claim 1, characterized in that the avalanche photodiode, laser diode and thermal sensor cases are mounted on a common metal heat conduit. 3. The laser rangefinder according to claim 1, characterized in that the external optical line is made in the form of two fixed mirrors mounted to each other at right angles, and is equipped with a laser radiation attenuator. 4. The laser rangefinder according to claim 1, characterized in that the external optical line is made in the form of a two- or trihedral prism, the edges of which form a right angle, and is equipped with a laser radiation attenuator. 5. The laser rangefinder according to claim 1, characterized in that the external optical line is made in the form of a segment of a fiber and is equipped with a laser radiation attenuator. 6. The laser rangefinder according to claim 1, characterized in that flash memory is used as non-volatile memory.

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