WO1994001788A1

WO1994001788A1 - Device and method for distance or velocity measurements

Info

Publication number: WO1994001788A1
Application number: PCT/NL1993/000144
Authority: WO
Inventors: Jacob Cornelis Arend Van Loon
Original assignee: B.V. Optische Industrie 'de Oude Delft'
Priority date: 1992-07-09
Filing date: 1993-07-08
Publication date: 1994-01-20
Also published as: EP0671016A1; NL9201233A

Abstract

The invention provides a system which functions with laser radiation on the basis of the FM-CW principle and with which it is possible to measure the velocities of, or the distances from, all the objects in a 3-dimensional target region simultaneously without the laser beam transmitted having to perform any scanning movement over the said region. For this purpose, the system transmits a conical beam of laser radiation having a periodically varying intensity, laser radiation reflected from the target region being imaged by an optical system on the cathode of an image intensifier tube. The image formed on the anode of said tube after synchronous demodulation is converted image point by image point into separate electrical signals, the frequency spectrum of each of which is determined to obtain the required distance or velocity information.

Description

Title: Device and method for distance or velocity measurements

The invention relates to a device for distance or velocity measurements, comprising means for generating a periodically varying signal; means for radiating a beam of radiation; means for modulating the intensity of the beam of radiation with the periodically varying signal; means for receiving reflected radiation; means for detecting the received radiation and converting it into an electrical signal corresponding thereto; means for synchronously demodulating the electrical signal and means for determining the frequency spectrum of the signal obtained by demodulation.

The use of a periodically varying signal for measuring distances and velocities is known as FM-CW and is described, for example, in the paper entitled "Mehrzielfahiges FM-C -Radar zur eindeutigen Messung von Entfernung und Geschwindigkeit" by U. Raudonat et al. in ntz, vol. 30 (1977), no. 3. If the frequency of the measuring signal is constant, the velocity of objects in a target area can be measured, and if the frequency of the measuring signal is periodically varied, for example periodically increasing linearly, the distance with respect to objects in the target area can be determined. This article describes the use of an FM-CW radar system.

A system for distance measurement with the aid of laser radiation has been described in US-A-3,649,123. In that case, however, the modulated optical signal does not have a periodically varying frequency but is controlled with the aid of the received reflected signal in order that a predetermined phase relationship is obtained between the transmitted and reflected signal. The use of laser radiation has the advantage that smaller details can be detected. A drawback of the known measuring instruments employing radar or laser radiation is that, in principle, only a point measurement is performed and that, to obtain distance or velocity information from a 3-dimensional region, for example the field of view of an observer, the radiation beam must be caused to perform a scanning movement along successive lines by means of a mechanical system in order to be able to cover the entire area. If the measuring system is intended, for example, for use in a low-flying aircraft, such a mechanically scanning system is often too slow to be able to provide the desired information about any obstacles present in good time and if the system were already to have the desired high scanning speed, it would be extremely expensive because of the required high speed and accuracy.

The object of the invention is to provide a system which functions with laser radiation on the basis of the FM-CW principle and with which it is possible to measure the velocities of, or the distances from, all the objects in a 3-dimensional target area simultaneously without the laser beam transmitted having to perform any scanning movement over the said region.

The object of the invention is also to provide a measuring system for distances or velocities which can have an appreciably lower cost price and is less suscep¬ tible to wear than a mechanically scanning system which can provide the desired information from a target region with a comparable speed, assuming that such a mechanical system can be constructed.

For this purpose, the invention provides a device of the type described above, which is characterized in that the radiation is laser radiation, in that the means for transmitting the laser radiation are designed to radiate a conical beam having substantial transverse dimensions, in that the means for detecting and demodu¬ lating the received reflected radiation comprise an image intensifier tube having a cathode and an anode, in that an imaging system is present for imaging the reflected radiation on the cathode, in that an image intensifier tube component which influences the gain thereof is connected to the means for generating a periodically varying signal, in that means are present for converting brightness information on the anode image point by image

SUBSTITUTE SHEET point into separate electrical signals, and in that means are present for determining the frequency spectrum separately for each of the separate electrical signals. The invention is based on the insight that it is possible to simplify a known optical rangefinder con¬ siderably and at the same time to make it suitable for simultaneously determining the velocities of, or the distances from, objects in a target region, such as the field of view of an observer, by replacing the detector and demodulator by an image intensifier tube, more particularly, an image intensifier tube of the first, second or third generation.

It is pointed out that such image intensifier tubes are well known per se, for example for use in night viewing equipment. For a discussion of a second-gener¬ ation image intensifier tube, reference may be made to the paper entitled "Bringing the dark into light with gen II" by H.R. Alting-Mees in Electro-optical Systems Design, July 1972, pages 20 - 24 inclusive. In contrast to the two other types, a first-generation image inten¬ sifier tube has no so-called "multi-channel plate" (MCP) and a third-generation image intensifier tube differs from the second-generation type in that the cathode material is sensitive to other wavelengths. For the purpose of demodulation, the periodically varying signal can be fed to the cathode or to a grid of an image intensifier tube of the first, second or third generation or to the MCP of an image intensifier tube of the second or third generation. The invention also provides a method for distance or velocity measurement, comprising radiating a beam of laser radiation whose intensity is periodically varied, receiving and detecting reflected laser radiation, demodulating the signal generated by the detected radi- ation synchronously with the periodic variation in the intensity of the beam of laser radiation and determining the frequency spectrum of the signal obtained by demodulation, characterized in that a conical beam having substantial transverse dimensions is radiated, in that the detection and demodulation takes place with the aid of an image intensifier tube on which the reflected radiation is imaged, and in that the frequency spectrum is determined for each point of the image on the anode of the image intensifier tube.

The invention will be explained in greater detail below on the basis of an exemplary embodiment with reference to the drawing, in which :

Figure 1 shows a block diagram of a known FM-CW laser rangefinder for performing a point measurement.

Figure 2 shows a block diagram of an FM-CW laser rangefinder according to the invention with which dis¬ tances from any objects in a 3-dimensional target region can be simultaneously determined; and Figure 3 shows a more detailed block diagram of the device shown in Figure 2.

For the sake of clarity, identical components in the figures will be indicated by identical reference numerals, preceded, however, by the figure number. Also for the sake of clarity, the invention will be explained hereinafter solely as elaborated for use in distance measurements. The invention can, however, be used with the same effect for velocity measurements, for which purpose, as is known, the frequency of the signal which modulates the intensity of the laser source is not periodically varied as in the case of distance measure¬ ment, but is kept constant.

Figure 1 shows a generator 101 for generating a periodically linearly increasing output voltage u, where u = bt, where t = time [sec] and b = the so-called chirp velocity [rad/sec²]. The output voltage of the generator 101 is fed to a voltage-controlled oscillator (VCO) 102. Said VCO generates a signal sin ωt, so that, because ω = bt, a signal sin bt² is emitted at the output of the VCO 102, which signal is fed to a laser source 103, more particularly a laser diode in order to modulate, in a manner known per se, the intensity of the laser radiation transmitted with the amplitude of the signal sin bt².

The modulated laser radiation is fed to an

SUBSTITUTE SHEET EP optical system 104 in order to be able to direct the radiation at a desired target point 105. The radiation reflected by a target point is picked up by means of an optical system 106 and fed to a detector 107, which converts the optical signal into an electrical signal. The reflected radiation is received again with a delay τ with respect to the instant of transmission, τ being given by τ = 2xl/c (1), where 1 is the distance from the target point [m] and c is the speed of light [m/sec] . The electrical signal obtained at the output of the detector 107 is amplified by an amplifier 108 and the signal amplified in this manner is fed to a first input of a synchronous demodulator 109, to the other input of which the output signal of the VCO is applied.

If it is assumed that the instantaneous trans¬ mission power P(z) [W] of the signal radiated by the laser source 103 is given by: P(z) = P(g)-sin bt² (2), where P(g) = mean transmission power, and also that, of the transmitted power, a propor¬ tion A is received again by the detector 107 as a result of reflection, the received power P(o) [W] is given by: P(o) = A-P(g)-sin b(t + τ)² (3).

In the synchronous demodulator, the product of P(z) and P(o) is formed and, using the formulae (2) and (3) it can readily be calculated that:

P(z) -P(o) = A-P(g)²- [cos(2btτ+bτ²) - cos(2bt² + 2btτ+bτ²) ] (4) Of these product terms, the second cosine term, which contains only high frequencies, is blocked with the aid of a low-pass filter 110 and the first cosine term, which is in fact allowed through by said filter, contains the desired distance information as will be explained below.

Because T = 2-1/c, the first cosine term in (4) can be rewritten as:

A-P(g)²-cos[(4blt/c) + (4bl²/c²) ] = A-P(g)²-cos(ωt + φ) (5), with ω = 4bl/c (6) and φ = 4bl²/c².

From (6) it follows that f = 2bl/πc² (7) , so that the demodulated signal is found to contain a frequency which

SUBSTITUTE SHEET is determined by the distance from the target point. The phase component is not utilized further because it contains no relevant information.

The demodulated signal is spectrally analysed in a Fast Fourier Transformation (FFT) processor 111 and the distance from the target point(s) can be determined in a manner known per se from the spectral component(s) obtained. Even in the case of multiple reflections from various target points struck by the same laser beam, the various distances from said target points can be deter¬ mined because the demodulated signal then has the form a-cos(ω₁t + φ-_j + b-cos(ω₂t + φ₂ ) + (8) wherein the diverse frequency components represent the various distances. Figures 2 and 3 show the device according to the invention, with which it is possible to determine dis¬ tances from all the target points in a region, such as the field of view of an observer, simultaneously. The components which may be completely identical to the components shown in the rangefinder according to Figure 1 and have corresponding reference numerals will not be discussed further.

In view of the desired high transmission power, use is preferably made, for the laser source 203, 303, of an array of, for example, 64 x 64 laser diodes, the intensity of each of which is varied simultaneously by influencing the output signal of the VCO 202, 302. In principle, it is, however, also possible to make use of a single laser diode if it is capable of radiating sufficient power. The laser diodes are preferably GaAε diodes, which emit radiation having a wavelength of approximately 800 nm, which wavelength has been found beneficial from the point of view of the sensitivity of the detector and also because these laser diodes are commercially available on an ample scale. To obtain an adequate transmission power (in practice approximately

100 W) , a high-frequency amplifier 318 has been provided.

The radiation reflected by target points 205 in the irradiated 3-dimensional region, the object space, is picked up by the receiving optics 206, 306, which is provided with an objective 306a capable of imaging the entire object space on the detector 213, 313 and with an optical band-pass filter 306b whose purpose is to allow through only the wavelength of the radiation transmitted by the laser source and to reject the other radiation (light) , which results in additional noise. Such a band¬ pass filter having a pass-band width of approximately 10 nm is commercially available in the form of an inter- ference filter.

According to one embodiment of the invention, the radiation allowed through by the filter 306b is fed to an image intensifier tube 213, 313, preferably an image intensifier tube of the second or third generation, which comprises a cathode 214, 314, and MCP 215, 315 capable of intensifying the electrons generated by the cathode under the influence of the incident optical radiation and feeding them to an anode 216, 316. The output signal of the VCO 202, 302 is also fed to the cathode 314 or to the MCP in order to modulate the current of the electrons generated, respectively, by the cathode or by the MCP to obtain a signal in the form of formula (4) . In the case where a first generation image intensifier tube is used, the output signal of the VCO 202, 302 can also be fed to an additional grid provided in such a tube in order to modulate the grid current thereof. If desired, however, such an additional grid can also be provided in an image intensifier tube of the second or third generation.

The MCP, which may be regarded as a collection of photomultiplier tubes, is supplied from, and the gain thereof can be controlled by, a high-voltage unit 319. The anode 216, 316 comprises a phosphor layer which converts the energy of the incident electrons into visible light. As a consequence of the inherent slowness of the phosphorescence process, only the low-frequency components in the electron current are converted into visible light and those are also precisely the desired signal components containing the required distance information. As a result of the measures according to the invention, the functions of the detector 107, the demodulator 109, the amplifier 108 and the low-pass filter 110 in the known device according to Figure 1 are therefore fulfilled by a single image intensifier tube in which said functions are performed, respectively, by the cathode 214, 314, by the modulation of the cathode or MCP voltage, by the MCP 215, 315 and by the phosphor on the anode 216, 316. The visible light generated by the anode is fed via a bundle 320 of optical fibres to an array 317 of photosensitive elements, such as CCD elements or photodiodes, in such a way that the entire object space is imaged on said array. The array may comprise 64 x 64 = 4096 elements (image points) . Depending on the mutual ratio of the dimensions of the anode 316 and the array 317, the bundle 320 is a straight bundle or a tapering bundle.

The output signal of each of the photosensitive elements is therefore representative of the amount of light, formed during a short time, namely the time between two consecutive read-outs, on a corresponding position, image point, of the anode. The photosensitive elements are read out under the control of a timing circuit 326. The signal read-out from each photosensitive element is converted in an analog/digital converter 322 into a corresponding digital signal and fed to an FFT processor 211, 311, which is capable of performing a very fast FFT on all the digital signals which represent the output signals of the respective photosensitive elements. Such an FFT processor is obtainable commercially, for example the PDSP 16510 stand-alone processor supplied by Plessey which can perform a 256-point real FFT in 12.5 μsec. An N-point FFT means that N/2 usable complex output values which can each be converted into N/2 moduli representing the amplitudes of N/2 frequency components can be obtained from 256 real input values. Instead of an FFT, use can also be made of other techniques known per se for obtaining the frequency components from a signal

SUBSTITUTE SHEET supplied .

The spectrally analysed output signals of the FFT processor are fed to a further signal processing circuit 212, 312 in order to derive the desired distance informa- tion therefrom. This can be done by selecting a number, for example six, of spectral components having the largest amplitude from each of the 4096 frequency spectra obtained. In this connection, it is pointed out that the frequency of a spectral component is a measure of the distance from a target point in the object space and the amplitude of a spectral component is a measure of the intensity of the reflection by a target point. Obviously, a certain minimum amplitude is adopted in said selection of spectral components in order to prevent only noise components being selected if a certain spectrum does not comprise any distance information at all because no target point is present in the object space in the corresponding image point.

The distance information obtained after conver- ting the frequencies found for relevant spectral components can be reproduced on a display screen 323, for example in the form of a 3-dimensional image or an image containing "false" colours, a particular colour repre¬ senting a particular distance, or by means of symbols on a display screen. The method of reproduction of the information obtained does not, however, form part of the present invention, so that this will not be dealt with further here.

A few general properties of the rangefinder system according to the invention will also be explained below.

If it is assumed that the image repetition frequency of the image intensifier tube is Fb, measure¬ ment can be carried out for a time 1/Fb, which means that the spectral resolution in the signal obtained is also Fb. If use is made of an N-point FFT, the maximum measur¬ able frequency Fmax is given by Fmax = Fb-N/2 (9) and the sampling frequency Fs should at least be equal to 2 ^•Fmax = Fb-N = Fs (10) . Using formula (7) , the maximum distance measuring range Lmax can now be determined as L ax = π-Fmaχ-c/2 -b = π-N-Fb*c/4 ^■b (11). In a system in which the image frequency and the size (N) of the FFT are fixed, the maximum "chirp speed" b, i.e. the speed with which the frequency of the VCO 202 is varied, is deter¬ mined by the desired Lmax in accordance with: b = π-N-Fb-c/4-Lmax [rad/sec²] (12) or B = N-Fb-c/8-Lmax [Hz/sec] (13).

For an image repetition frequency Fb, the dis- tance resolution of the system is given by: ΔL = π-Fb-c/2-b, so that using (12), it follows that: ΔL = π-Fb-c-4 -Lmax/2 -π-N-Fb-c = 2-Lmax/N (14).

A calculated example will illustrate the above. If Fb = 12Hz; N = 256 and Lmax = 1000m, it follows for B from (13) that B = 115.2 [MHz/sec] and from (9) that Fmax = 1536 Hz.

Finally, it can be further calculated that, using the formulae (9) , (10) and (14) , it can be derived for the minimum sampling frequency Fs that Fs = 2-Fb-Lmax/ΔL (15) . If it is desired that ΔL = 8 m, it follows from this that Fs = 3000 Hz. This means that an array 217, 317 containing photodiodes must be read out 3000 times per second. With an array size of 64 x 64 diodes, this results in an information flow of 64 x 64 x 3000 = 12,288,000 signals per second. Semiconductor sensors having such a sampling speed are commercially available.

After the above explanation of the operation as rangefinder, the explanation of the operation as velo¬ city-measuring instrument can be kept simple. The only difference is that for use as velocity-measuring instru¬ ment, the intensity of the laser radiation is modulated with a signal whose frequency is constant. As a conse¬ quence of the movement of an object in the target region, the frequency of the modulation of the laser radiation reflected by said moving object will differ from the constant frequency at which the transmitted laser radia¬ tion is modulated as a consequence of the Doppler effect. In this case, the frequency of the received signal in the mixer 214, 314 also differs from the frequency of the

SUBSTITUTE SHEET transmitted signal but only for those parts of the image formed by the optics 216 on the cathode 214 which are the imaging of a moving object. In a manner which is com¬ pletely identical to that described above in relation to distance measurement, it can now be determined, for each point, with the aid of the array of photosensitive elements and the FFT processors whether a particular image point represents a (part of a) moving object. As stated above, in relation to distance measurements, the method of reproduction on, for example, a display screen falls outside the scope of this invention.

The Doppler effect will also have an influence on the distance measurement, in particular the distance measurement of moving objects, during a distance measure- ment. This effect is, however, in practice often so small that it can be ignored. If, however, an extremely accu¬ rate measurement is desired or it is nevertheless desired to make a correction for the Doppler effect for other reasons, this can be achieved as follows. The effect of different velocities of various objects in the target region can be allowed for in the result of the distance measurement by processing the result of said distance measurement with the result of a velocity measurement prior thereto. In this way, the processed result of the distance measurement reproduces only distances. The said processing takes place as follows.

First a velocity measurement takes place. It is then known for each point of the image of the target region whether it is moving (along the line of sight) and, if so, at what velocity. From this, it can readily be calculated for each point of the image of the target region what the effect (positive or negative frequency shift and magnitude thereof) will be in a distance measurement. The result of the calculation for each image point is stored in a memory. The distance measure¬ ment immediately follows the velocity measurement and, for each point of the image of the target region, the uncorrected "measured" distance, i.e. the distance which can be derived directly from the frequency difference

SUBSTITUTE SHEET ISA/EP between the instantaneously transmitted signal and the received signal and which is dependent on the speed of the object because both distance and velocity determine the instantaneous frequency difference, is corrected using the "apparent distance" which is the consequence of the movement of the object along the line of sight in that image point, so that the true distance finally results. The sequence of distance measurement and velocity measurement can, of course, also be reversed. In an environment in which no rapidly altering velocities are expected, one velocity measurement may also be alternated with a number of distance measurements, each of the distance measurements being corrected by the result of one velocity measurement.

Claims

1. Device for distance or velocity measurements comprising means for generating a periodically varying signal; means for radiating a beam of radiation, means for modulating the intensity of the beam of radiation with the periodically varying signal; means for receiving reflected radiation; means for detecting the received radiation and converting it into an electrical signal corresponding thereto; means for synchronously demodulating the electrical signal and means for deter¬ mining the frequency spectrum of the signal obtained by demodulation, characterized in that the radiation is laser radiation, in that the means for transmitting the laser radiation are designed to radiate a conical beam having substantial transverse dimensions, in that the means for detecting and demodulating the received reflected radiation comprise an image intensifier tube having a cathode and an anode, in that an imaging system is present for imaging the reflected radiation on the cathode, in that an image intensifier tube component which influences the gain thereof is connected to the means for generating a periodically varying signal, in that means are present for converting brightness informa¬ tion on the anode image point by image point into separate electrical signals, and in that means are present for determining the frequency spectrum separately for each of the separate electrical signals.

2. Device according to Claim 1, characterized in that the component influencing the gain of the image intensifier tube is the cathode.

3. Device according to Claim 1, in which the image intensifier tube comprises a grid, characterized in that the component influencing the gain of the image inten¬ sifier tube is the grid.

4. Device according to Claim 1, in which the image intensifier tube comprises a multichannel plate, charac¬ terized in that the component influencing the gain of the image intensifier tube is the multichannel plate.

5. Device according to at least one of Claims 1 to 4 inclusive, characterized in that the means for trans¬ mitting the beam of laser radiation comprise a 2-dimen¬ sional array of laser diodes.

6. Device according to Claim 1 or 5, characterized in that the transmitted laser radiation has a wavelength of approximately 800 nm.

7. Device according to at least one of Claims 1 to 4 inclusive, characterized in that the imaging system comprises a band-pass filter having a pass band for the laser radiation transmitted.

8. Device according to at least one of Claims 1 to 4 inclusive, characterized in that the means for converting the brightness information on the anode image point by image point into separate electrical signals comprise a 2-dimensional array of photosensitive elements, which array is optically linked to the anode of the image intensifier tube.

9. Device according to at least one of Claims 1 to 4 inclusive or 8, characterized in that the means for determining the frequency spectrum for each separate electrical signal comprise a separate FFT processor for each separate electrical signal.

10. Device according to Claim 1, 2 or 3, character¬ ized in that the image intensifier tube is a third generation image intensifier tube.

11. Method for distance or velocity measurements comprising radiating a beam of laser radiation whose intensity is periodically varied, receiving and detecting reflected laser radiation, demodulating the signal generated by the detected radiation synchronously with the periodic variation in the intensity of the beam of laser radiation and determining the frequency spectrum of the signal obtained by demodulation, characterized in that a conical beam having substantial transverse dimensions is radiated, in that the detection and demodulation takes place with the aid of an image intensifier tube on which the reflected radiation is imaged, and in that the frequency spectrum is determined for each point of the image on the anode of the image intensifier tube.

12. Method according to Claim 11, characterized in that the frequency spectrum is determined simultaneously for at least a number of points in the image on the anode of the image intensifier tube.

13. Method according to Claim 11 or 12, characterized in that the periodically varying signal has a constant frequency.

14. Method according to Claim 11 or 12, characterized in that the periodically varying signal has a period¬ ically varying frequency.

15. Method according to Claim 11 or 12, in which the image intensifier tube comprises a cathode and a multichannel plate, characterized in that an alternating electrical voltage is applied to the cathode or to the multichannel plate synchronously with the periodical variation of the radiated laser radiation.