RU2058525C1 - Laser distance meter - Google Patents

Abstract

FIELD: measurement technology; optical systems for meters of light conduit characteristics and for check of quality of optical oscillations, meters for determination of distance to point of break of light conduit, laser altimeters and range finders. SUBSTANCE: device is provided with scaling unit 11, adaptation unit 9, correction unit 10 and tuneable filter 8. Distance to object (or nonuniformity of propagation channel) is measured in wide dynamic range at high accuracy irrespective of level of Rayleigh scattering signal. Besides that, device provides for simplification of measurements in automated mode since it excludes manual adjustment. EFFECT: enhanced accuracy of measurement of distance to object in wide dynamic range at presence of jamming scattering signal. 7 dwg

Description

 The invention relates to measuring equipment and can be used in optical systems: measuring instruments for the characteristics of optical fibers and the quality of joints of optical cables, measuring instruments for distance to the point of fiber breakage, laser altimeters and rangefinders.

 A device is known for determining the location of damage to optical cables, comprising a master oscillator with a synchronizing output, an optical radiation generator connected to it, an optical coupler with two output arms sequentially arranged along the radiation, a mode selector and a photodetector connected to a recording unit, the synchronizing output of the master oscillator connected to the registration unit, and the input of the mode selector through the matching unit is connected to the synchronizing output of the master oscillator. The disadvantage of this device is the low reliability of distance measurement in the presence of interfering signals.

 A fiber optic reflectometer is known, consisting of a fiber optic coupler connected in series, a photodetector, a gating unit, an amplifier and a recording unit, as well as an emitter connected to the second arm of the coupler, the third arm of which is connected to the signal propagation medium (with the fiber under study).

 The disadvantage of this device is the low reliability of distance measurement. This is due, firstly, to the possibility of saturating the photodetector (FPU) with a powerful optical scattering signal from the near zone of an imperfectly transparent propagation medium. In this case, a weak pulse reflected from the medium-object boundary (from the place where the fiber under study is cut off) cannot be detected due to the “blindness” of the FPU, and accordingly, the delay time of the pulse and the scattering to the object cannot be reliably measured. Secondly, when the amplifier is gated, an impulse with a steep edge will be formed ("cut out") from an exponentially decaying scattering signal, along which the recording unit will make a false measurement.

где τ_ф.и длительность фронта излучаемого (зондирующего импульса);
Т₁ постоянная времени эквивалентной схемы фотодетектора T₁=

Т₂ постоянная времени эквивалентной схемы усилителя T₂=

При полосе пропускания фотодетектора w_гр.1= 4π•10⁷

усилителя w_гр.2= 2π•10⁺⁷

и при τ_ф.и. 30 нс случайные изменения амплитуды отраженного сигнала приводят к погрешности. Δ_дин 50,74 нс.Thirdly, even when the amplitude of the signal scattered from the near zone is less than the saturation level of the FPU, the scattering measurement accuracy is not ensured, since the additive mixture "scattering signal + pulse from the object" is sent to the registration unit. With a constant detection threshold, this leads to an error in measuring the temporary position of the pulse reflected from the object. Indeed, the dynamic range of the "scattering signal + pulse from the object" mixture is at least 40 dB. Under these conditions, the error in fixing the position of the pulse along the front is determined by the duration of the pulse front at the output of the FPU:
Δ _din = τ _ffpu =

where τ _{f.i and the} duration of the front of the emitted (probing pulse);
T ₁ time constant of the equivalent photodetector circuit T ₁ =

T ₂ is the time constant of the equivalent amplifier circuit T ₂ =

With the passband of the photodetector w _gr. 1 = 4π • 10 ⁷

amplifier w _gr . ₂ = 2π • 10 ⁺⁷

and at τ _f.i. 30 ns random changes in the amplitude of the reflected signal lead to an error. Δ _din 50.74 ns.

Thus, the error in measuring the distance to the object, due to this destabilizing factor, is
ΔR c · Δ _din / 2 7.61 m, which is unacceptable in most fiber-optic measuring systems and location devices. In addition, the significant amplitude of the signal reflected from the place of imperfect docking with the studied fiber makes it difficult to measure small distances (to a close object or to a cliff). Indeed, the cutoff duration of the pulse reflected from the docking point is τ _for 20–2000 ns for various types of semiconductor laser emitters. At this time, the photodetector should be closed by introducing gating. Thus, the range of measured distances is limited from below to
R _min = c τ _s / 2 = 3 ÷ 300 m
Due to the scatter in the parameters of the fiber optic connectors, it is not possible to predict and take into account the effect of the pulse reflected by them.

In known devices for high-precision measurement of the time interval between the components of the radiation and the arrival of the pulse reflected from the object, a high repetition rate of clock pulses is required. So at a group pulse velocity of 2 · 10 ⁸ m / s in the fiber, the accuracy of measuring a distance of 0.1 m can be achieved due to a clock frequency of more than 2 GHz. However, the corresponding elemental base (counters, registers) is missing.

 The aim of the invention is to improve the accuracy of measuring the distance to the object in a wide dynamic range in the presence of an interfering signal.

 A wide dynamic range refers to the range of an additive mixture of an interfering scattering signal and a pulse from an object of at least 40 dB.

 This is achieved by the fact that a device containing a receiving optical system in series, a photodetector, an amplifier and a recording unit, as well as an emitter with a transmitting optical system and a gating unit, a tunable filter, an adaptation unit and a correction unit are introduced, while the input of the adaptation unit is connected to the photodetector output, and the output to the control input of the amplifier, the first input of the correction unit is connected to the output of the amplifier, and the output with the regulatory input of the registration unit, a tunable filter is connected n for entry yield photodetector, and an output with the input of the adaptation unit. The optical output of the emitter is connected to the input of the transmitting optical system, and the clock output is connected to the input of the strobing unit, the output of which is connected to the gate input of the amplifier and to the second input of the correction unit, the output of the scaling unit is connected to the second input of the recording unit, the third input of which is connected to the sync output of the emitter.

 In this solution, all the features indicated in the characterizing part of the claims show the inherent known properties in the process of interaction, each individually giving a known positive effect.

 At the same time, an over-summarized positive effect is provided, due to the combination of these signs, namely that the destabilizing effect of the interfering signals and the dependence of the measurement results on the amplitude of the useful signal are eliminated.

 In FIG. 1 shows a structural diagram of a device; in FIG. 2 diagram explaining the operation of a tunable filter, adaptation unit and amplifier; in FIG. 3 frequency response of the tunable filter (1 and 2) for the corresponding spectra of the scattering signal (3 and 4); in FIG. 4 shows a schematic diagram of a tunable filter; in FIG. 5 schematic diagram of the register and the gating unit; in FIG. 6 schematic diagram of the emitter; in FIG. 7 shows a schematic diagram of a photodetector amplifier, an adaptation unit, and a correction unit.

 The proposed device contains a receiving optical system 1, a photodetector 2, an amplifier 3, a recording unit 4, a transmitting optical system 5, an emitter 6, a gating unit 7, a tunable filter 8, an adaptation unit 9, a correction unit 10, a scaling unit 11.

 The device operates as follows.

 The emitter 6 forms an optical pulse and a sync pulse coinciding in time with its front. The optical pulse through the transmitting optical system 5 enters the propagation medium (measured signal). The sync pulse arrives at the gating block 7, where a strobe pulse with the necessary time characteristics (delay, duration) is formed. The optical signal reflected from the object and the propagation medium is supplied to the photodetector 2 by means of the receiving optical system 1, where it is converted into an electrical signal. The registration unit 4, made in the form of a time-amplitude-code converter, generates an electrical signal (voltage), the amplitude of which is proportional to the time interval between the clock pulse and the received signal, and then the generated analog signal is converted into binary code. This allows accurate measurement of time intervals in the range of a few thousand nanoseconds without the use of ultra-wideband digital devices.

 The linearity of the conversion, and, accordingly, the high reliability of measuring the distance to the object, the registration unit 4 provides for a limited time interval, to expand the limits of the high-frequency measurement in the scaling unit 11, a coarse distance measurement is made, the discrete intervals of which are measured using the registration unit 4 with a given accuracy.

). Соответственно меньше потери энергии импульса от объекта, обусловленные подавлением низкочастотной части его спектра. В блоке адаптации 9 из неотфильтрованного электрического сигнала рассеяния формируется управляющее напряжение, компенсирующее медленный неотфильтрованный сигнал рассеяния, за счет чего рабочая точка усилителя 3 не смещается и не обеспечивается передача от объекта с максимальным коэффициентом. На стробирующий вход усилителя 3 с блока стробирования 7 подается строб-импульс и, если он совпадает во времени с импульсом до объекта, то последний после усиления поступает на сигнальный вход блока регистрации 4 и на первый вход блока коррекции 10. Чтобы напряжение переходных процессов при стробировании усилителя 3 и напряжение его шума не накапливались блоком коррекции 10, на его второй вход также подается строб-импульс. Таким образом по амплитуде импульса от объекта блок коррекции 10 вырабатывает регулирующее напряжение, поступающее на регулирующий вход блока регистрации 4.The tunable filter 8 passes an impulse from an object to the input of amplifier 3, suppressing an electrical signal caused by the scattering of radiation from the near zone of an imperfectly transparent propagation medium. Moreover, the boundary frequency of the tunable filter changes according to information about the amplitude and steepness of the decay of the scattering signal: the slower the level of the scattering signal decreases (according to the law U _p U _o e ^{-t / τ} p), the lower the boundary frequency of the tunable filter (w

) Accordingly, there is less loss of pulse energy from the object due to the suppression of the low-frequency part of its spectrum. In the adaptation unit 9, a control voltage is generated from the unfiltered electrical scattering signal, which compensates for the slow unfiltered scattering signal, due to which the operating point of the amplifier 3 is not shifted and transmission from the object with the maximum coefficient is not ensured. A strobe pulse is supplied to the gate input of amplifier 3 from gate block 7 and, if it coincides with the pulse to the object, the latter, after amplification, is fed to the signal input of registration block 4 and to the first input of correction block 10. So that the transient voltage during gating amplifier 3 and the voltage of its noise was not accumulated by the correction unit 10, a strobe pulse is also supplied to its second input. Thus, according to the amplitude of the pulse from the object, the correction unit 10 generates a regulatory voltage supplied to the regulatory input of the registration unit 4.

 As a result of this, the moment of arrival of the pulse from the object, and hence the distance to the object, are measured with high accuracy regardless of the amplitude of the pulse.

обусловленную фронтом полезного импульса в 5-10 раз. Значение коэффициента к определяется точность воспроизведения формы фронта импульса блоком коррекции 10.From a consideration of the stress diagrams illustrating the operation of the proposed device (see Fig. 2), it can be seen that eliminating the influence of the scattering signal due to the tunable filter 8 and adaptation unit 9 prevents false corrections 10, makes it possible to reduce the error in measuring the distance ΔR

due to the front of the useful pulse by 5-10 times. The value of the coefficient k is determined by the accuracy of reproducing the shape of the pulse front by the correction unit 10.

 A specific implementation of the proposed device is as follows.

 The tunable filter 8 (see Fig. 4) attenuates the extended exponentially scattered signal with a slight attenuation of the short pulse reflected from the object. In order to minimize the indicated pulse power loss from the object, the filter boundary frequency is tuned in accordance with the spectral width of the scattering signal (see Fig. 3). With a significant amplitude of the scattering pulse, the boundary frequency of the tunable filter also increases, which improves the ratio of the object signal / scattering signal.

 A diagram of the registration unit 4 and the scaling unit 11 is shown in FIG. 5. A time interval meter used as a recording unit consists of threshold formers, a time-amplitude converter, and an ADC. With the arrival of the "Start" clock from the emitter 6, the comparator DA1 generates a pulse of negative polarity. At the same time, the DD1, DD2 trigger is tipped, allowing the operation of the DD2.1, DD2.2 generator, and the counters at the parallel load input are also reset. For each pulse of the generator, the capacitance C is discharged through the transistor VT1, then its discharge from the stable current generator begins. The signal "Stop" returns the trigger DD1.1, DD1.2 to its original state. At the same time, the transistor VT2 opens, bypassing the GTS and the ADC is launched through the waiting multivibrator DD1.3, DD1.4. As a result, an eighteen-digit binary code is formed on the digital output of the meter, the lower ten digits of which are the result of an accurate count, the older 8 are the result of a coarse count.

при грубом отсчете расстояние измеряется с точностью ΔR₁ 100 м в пределах R₁ R_o 25600 м, где V скорость распространения излучения; R_o "мертвая зона".Thus, with the switching period of the scaling unit 11 T _m =

with a rough reading, the distance is measured with an accuracy of ΔR ₁ 100 m within R ₁ R _o 25600 m, where V is the speed of propagation of radiation; R _o "dead zone".

≃ 0,2 м

в пределах R R₁ + 100 м, что позволяет получить высокую точность измерения в широком диапазоне измерения расстояния до объекта.With the beginning of each switching period of the scaling unit through the DD2.3 element, the registration unit is started and the time interval meter measures the distance with an accuracy ΔR

≃ 0.2 m

within RR ₁ + 100 m, which allows to obtain high measurement accuracy in a wide range of measuring the distance to the object.

 A schematic diagram of the emitter 6 is shown in FIG. 6. The blocking generator on the transistor VT1 generates power pulses for the avalanche diode (S-diode) VD1. An avalanche diode VD1 generates current pulses with a duration of 10-100 ns for pumping a semiconductor laser VD2.

 At the time of the breakdown of the avalanche diode VD1, using the differentiating circuit R1C1, a clock pulse is obtained, which starts the gate former 7 (see Fig. 6) on the DD1 chip. The gate pulse is fed to the second (control) input of the correction unit 10, allowing the useful signal to pass to the output of the peak detector (diode VD2 in Fig. 7), the output voltage of which is supplied to the control input of the recording unit 4. As a result, the threshold level of the comparator DA2 (see Fig. 5) varies in proportion to the amplitude of the useful signal.

 The photodetector 3 (see Fig. 7) is assembled on the basis of an avalanche photodiode LFD-6 and a preliminary video amplifier on transistors VT1, VT2. A cascade video amplifier circuit with a common frequency-dependent negative feedback with a gain of 30-60 increases the input impedance, extends the passband (up to 20-100 MHz) and improves the stability of the detector. The amplifier 3 (see Fig. 7) is made in the form of cascades of OE-OE (VT3, VT4), inverter (VT5) and the cascade of OE-OK (VT6-VT7), while achieving a stable gain of up to 25000 and matching with the load (inputs registration units 4 and correction unit 10).

 The adaptation unit 9 (see Fig. 7) is based on two emitter repeaters (VT8, VT9), which decouple the low-pass filter (R4, C2) from the output of the tunable filter 8 and from the control input of the amplifier 3. The low-pass filter suppresses the useful pulse forming an adaptation voltage proportional to the scattering signal (see Fig. 2). The adaptation voltage applied to the control input of the amplifier 3 (emitter of the transistor VT3) finally eliminates the scattering signal (see Fig. 2, e).

 As the receiving (1) and transmitting (5) optical systems, serial OLP-M type lenses (for laser rangefinders) or a fiber-optic splitter with optical connectors on the output arms can be used. In the second case, the APD of the photodetector 2 and the laser of the emitter 6 should have a fiber input and output, respectively.

 The circuit diagram of the correction unit 10 is shown in FIG. 7. It consists of an emitter follower VT10, a peak detector VD2, C3 and a source follower VT1. The output voltage of the correction unit 10, proportional to the amplitude of the useful signal, is fed to the regulatory input of the registration unit 4.

Claims (3)

 1. LASER DISTANCE METER, containing optically coupled emitter and transmitting optical system, receiving optical system and photodetector, serially connected amplifier and recording unit, as well as a gating unit, the input of which is connected to the second output of the emitter, and the output with the first input of the amplifier, the second input the registration unit is connected to the second output of the emitter, characterized in that, in order to improve the accuracy of measuring distances in a wide dynamic range in the presence of an interfering signal Ia, an adaptation unit, a correction unit, and a scaling unit are introduced into it, the output of which is connected to the third input of the registration unit, the output of which is connected to the second input of the scaling unit, the input of the adaptation unit is connected to the output of the photodetector, the first input of the correction unit is connected to the output of the gating unit, the second input with the output of the amplifier, the third input of which is connected to the output of the adaptation unit, the output of the correction unit is connected to the fourth input of the registration unit.  2. The distance meter according to claim 1, characterized in that, in order to improve the accuracy of measuring distances to objects with low reflectivity, a tunable filter is introduced into it, the input of which is connected to the output of the photodetector, and the output to the second input of the amplifier and to the input of the unit adaptation.  3. The distance meter according to claim 1, characterized in that the registration unit is made in the form of a time amplitude converter code.

Images