CA1287149C - Range finder and method for range finding by using light pulses - Google Patents

Abstract

ABSTRACT
A method for range determination between a transmitter receiver and a target using optical pulse signals is described, which only requires extremely low transmission pulse energy, but still has a much higher sensitivity or interference suppression than conventional methods. For this purpose, optical pulse groups with a pulse rate in the range approximately 10 to approximately 150 kHz are directed onto the target. The reflected and received signal sequence is scanned with a scanning frequency dependent on the transmission pulse rate and digitized. The scan values obtained are then continuously added to the corresponding value for each individual transmission pulse with the clock of the scanning frequency. The ranging information is derived from the resulting signal. Preferably the scanning of the incoming signal sequence is carried out with a scanning frequency in the nanosecond range. For example, signal processing takes place in a parallel adder and for each transmission pulse there is a continuous parallel addition of the digitized received signals to the addend signal of the adder. In particular, the output of adder can be connected across a shift register to the input for the second addend. Alternatively the processor can be constituted by a microprocessor operating in the nanosecond range and in which is contained the function of the transmission pulse-related parallel adder

Description

~ ~ ~7 1~ 9 The present invention relates to a range determination or ranging method using optical pulse signals transmitted by à
transmitter in the direction of a target and which are received after reflection, converted into electrical signals and converted in a signal processing means to a distance or range information.
The invention also relates to an apparatus for performing this method.

Ranging methods are known which, in accordance with the radar principle, use as aids pulsed electromagnetic signal so that, with the knowledge of certain boundary conditions, it is possible to determine the range by measuring the signal behaviour between the target and the transmitter-receiver.

More sensitive ranging methods operating in the optical frequency spectrum use solid state lasers (e.g. YAG, ruby or the like) as transmitters. These lasers are optically pumped, the range being determined by measuring the behaviour of an individ-ual laser pulse with a correspondingly high energy. The electri-cal efficiency of an optically pumped solid state laser is gener-ally very poor due to the discharge lamps used for pumping. The frequently necessary battery change is also disadvantageous in operation. In order that an individual backscattered pulse has sufficient energy to permit its detection, the energy of the individual transmitted pulses must be very high. However, pulses, whose energy exceeds a certain threshold value are pre~u-dicial to the eyes, unless special safety measures are taken.
Semi-conductor lasers which, although allowing higher pulse rates, e.g. lO to 100 kHz for GaAs have not hitherto been consid-ered for ranging methods ln open terrain, i.e. at least over sev-eral hundred metres, due to their relatively low peak output power, which may not be exceeded for thermal reasons.

The present invention improves a ranging method and apparatus of the aforementioned type, so that on the one hand higher pulse rates than hitherto can be used for ranging purposes ,.....
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~ ~71q~3 and that on the other hand the signal strength of the received signals is sufficient in order to permit completely satisfactory signal processing and therefore ranging over the deslred range with an adequate reliability level.

According to the invention there is provided a ranging method between a transmitter-receiver for optical pulse signals and a target, by the transmission of optical pulse signal groups, the reception of the signals reflected by the target, converting the received optical signals into electrical signals and then processing the signals for deriving a measuring criterion, where-in the transmitter directs pulse groups with a pulse rate in the range between approximately 10 and approximately 150 kHz onto the target, the reflected and received signal sequence is scanned with a scanning frequency dependent on the transmission pulse rate and digitized, the scan values obtained are continuously added to the corresponding value for each individual transmission pulse in the clock of the scanning frequency and the range infor-mation is derived from the resulting signal. Suitably scanning of the incoming signal sequence is performed with a scanning fre-quency in the n~nosoaond range. Desirably the scanning of the incoming signals is per~form~ed with a scanning frequency of approximately ~ nun~eee*dG.

The present invention also provides an apparatus for effecting said method which comprlses an analog-digital converter supplied with the incoming signal and whose scanning frequency can be controlled by a processor as a function of the pulse rate of the transmission signal and downstream of said converter are connected means for the parallel addition of the data supplied on a parallel line from the analog-digital converter and the addend signals of adder means processed in parallel form. Suitably downstream of the analog-digital converter is connected a paral-lel adder, whose output is connected across a shift register to the input for the second addend of the parallel adder. Desirably in the output line of parallel adder is provided a sensor for 7 ~4 indicating an overflow bit MSB to processor and in the parallel connection between shift register and the input for the second addend on parallel adder is provided a switch controlled by pro-cessor and which, when an overflow bit MSB is lndicated by sen-sor, downwardly displaces by one position the parallel signallines from the shift register to the parallel adder. Suitably the processor is a microprocessor operating in the nanosecond range and which contains the function of the transmission pulse-related parallel adder.

The advantage of this solution is that, contrary to original expectations, despite their low peak output powers, the relatively inexpensive laser diodes which can be very adequately controlled from the circuitry standpoint can be used for range measurements over at least several hundred metres. This has sur-prisingly led to considerable improvement in the sensitivity of the measuring method by at least a power of ten, by typically by several powers of ten, e.g. by a factor of 100. In addition, equipment operating according to this method can be made very small and have a light weight. The energy supply and control of the laser diodes, as well as the subsequent signal processing can be realized particularly simply, largely using standard compo-nents. The higher electrical efficiency of a semi-conductor laser compared with the hitherto used solid state lasers, as well as the possibility of being able to operate with higher pulse rates also constitute advantages.

Despite the lower peak power the inventive measure per-mits larger ranges to be covered when measuring in a manner safe for the eyes than when using individual pulsed lasers. Due to the marked focusing of the laser beam, this method permits the measurement of target distances with extremely high precision and even without reflectors, i.e. without the prior fitting of reflecting elements at the target.

Whereas hitherto scanning or sampling methods have been ~ 2~7~49 used in signal processing procedures for improving the resolution of the signals received, i.e. for the better dlrect identifica-tion thereof, the present method and/or apparatus aims at the use of the sampling method for ~mproving the sensitivity of the receiver and therefore for improving the wanted~spurious signal ratio S/N.

The invention is described in greater detail here-inafter relative to non-limitative embodiments and the accompany-ing drawings, whereln:-Fig. 1 is the block circuit diagram of a preferredembodiment for illustrating the method;

Fig. 2 is a time diagram for illustrating the method;
and Fig. 3 is the block circuit diagram of a simplified embodiment.

The principle of the inventive method essentially com-prlses the use of the knowledge that the sensitivity of the mea-suring method can be improved through the use of N pulses by a factor of V N in accordance with information theory rules. It has been found that through an optimum utilization of high pulse rates, in accordance with such information theory rules it ls not only posslble to overcome the disadvantages of the relatively low admissible peak output powers for laser diodes, but in fact that the measuring sensitivlty can be significantly improved, e.g. by a factor of 100 compared with conventional methods.

Through the use of th0 scanning or sampling method on the received pulse signal groups, it is possible to derive an extremely precise decision criterion for the arriving again of the pulses reflected by the target and therefore for the time delay of the pulses between the time of transmission and the - 3a -~37'149 rearrival. Despite a low transmission energy relatlvely large distances can be accurately measured in the case of a very good S/N ratio.

As is diagrammatically shown by Fig. 1, the pulse sequence transmitted by a laser diode arrangement 1 is reflected by a target 2 and then received by a light-sensitive cell, e.g.
an - 3b -,, ~ 2~37149 avalanche diode ~ preferably arranged in the transmitter receiver. The~selected repetition frequency is e.g. in the range 10 to ~g~ kHz. Pulsing is controlled and preferably program-controlled by a micro processor 5.

The signals detected by the avalanche diode 3 are amplified in an amplifier 4 to the extent necessary for the following processing. In a following analog - digital converter 6, the pulses received are digitized with a scanning frequency given by the microprocessor 5. The clock of the scanning operation is in the example 100 ns (nanoseconds). The digitized data are e.g. transferred in the form of 4 or 6 bit parallel signals to a parallel adder 7 and directly in the clock of the a~forementioned scanning operation are added to the corresponcling value for each individual pulse within a scanning interval. This adding of the scan values of the periodically t:ransmitted pulse sequences related in each case to correspondLng scanning times leads to an increase in the evaluated incoming signals and therefore to the indicated rise in the s~ensitivity for the overall arrangement.

In order to obtain this action, in the represented example a 4 bit line from the analog - digital converter 6 is supplied as a first adclend input to the parallel adder 7. The second addend input of parallel adder 7 is in the form of a 5 bit input and the adder output also has a 5 bit line. The parallel line corresponding to the lowest point of the parallel transmitted signal is designated LSB and that associated with the highest point is designated MSB. A
sensor 11 is provided for establishing a bit signal appearing on line MSB at the output of parallel adder 7 and is connected across a MSB indicator line 10 to an input of microprocessor 5. Microprocessor 5 establishes in program-controlled manner whether a bit appearing on line MSB
is present during a complete scanning cycle between two pulses transmitted by the laser diode arrangement 1.

The output of parallel adder 7 is connected to the input of a 7~49 shift reg:ister 9, in which there is a continuous intermediate storage o:E the scanning cycle value supplied by parallel adder 7.

A switch 8 controlled by microprocessor 5 is preferably provided at the output of shift register 9 via a 4 bit parallel lin.e. At the output side, switch 8 is connected via a 5 bit parallbl line to the second addend input of parallel adder 7. Thusr compared with its input, switch 8 has an additional bit line on its output leading to the adder.

In the conn,ection between shift register 9 and switch 8, according t:o fig 1 the lines for the lowest and highest positions a,re once again designated LSB and MSB. The output bit lines of switch 8 are switched up or down by one bit position in. accordance with a criterion described hereinafter and supplled by microprocessor 5, so that the association of the incoming and outgoing bit lines is in each case displaced by one position.

If an overflow signal from parallel adder 7 is detected on the MSB indlcator line by microprocessor 5 and is maintained over a complete scanning period, then microprocessor 5 supplies a switching signal to switch 8. The switch then switched all its input lines to in each case an output line lower by one place and remains throughout the next scanning period in said position. Thus, during this time, the previous MSB is now supplied as the second hiqhest bit to parallel adder 7, the second highest as the third highest etc and the information of the lowest position is not taken into account during this time. All incoming bits during this scanning period are therefore displaced downwards by one pOSitiOIl by this measure.

Fig 2 diagrammatically illustrates the action of the described signal processing on the received pulse signals I.
Whereàs signal line A shows the actual course of the signal sequence received, line B represents the result of the signal 1~37~49 processing with the clearly raised scanning pulses. Such a signal permits target acquisition and therefore reliable ranging with a roughly 100 times better sensitivity when using 10,000 pulses compared with a known method using a threshold value detection within a given pulse window.

As a result of the selected, completely parallel siqnal processing, there is a very high processing rate for the pulses received from the avalanche diode. There is a correspondingly high resolution or sensitivity of the means for relatively weak pulse signals of the laser diodes received over a greater distance. As a variant of the previously described embodiment, the parallel adder 7 represented in fig 1 as a discrete component can be integrated into microprocessor 5. When using a correspondingly fast microprocessor 5, it is even possible to obviate the use of a discrete shift register 9 and its function can then be performed by the processor.

Fig 3 shows a simplified exemplified variant, in which a microprocessor 20 with parallel adder integrated therein is used for direct signal processing. Preferably this function is fulfilled by very fast signal processors, whose operating frequency is in the nanosecond range. The functions described for the first embodiment according to fig 1 are realized by corresponding programming of processor 20. As the processing principle has already been described, details of a corresponding program are not explained here. As in the embodiment according to fig 1, here again a gain control signal AGC for amplifier 4 can be derived from microprocessor 20.

Apart from the indicated embodiments, other solutions realised by programming or circuitry are possible, which make use of the same, previously described method features, in order to obtain a usable criterion for ranging from relatively weak incoming signals.

. .

Claims (7)

1. A ranging method between a transmitter-receiver for optical pulse signals and a target, by the transmission of opti-cal pulse signal groups, the reception of the signals reflected by the target, converting the received optical signals into electrical signals and then processing the signals for deriving a measuring criterion, wherein the transmitter directs pulse groups with a pulse rate in the range between approximately 10 and approximately 150 kHz onto the target, the reflected and received signal sequence is scanned with a scanning frequency dependent on the transmission pulse rate and digitized, the scan values obtai-ned are continuously added to the corresponding value for each individual transmission pulse in the clock of the scanning fre-quency and the range information is derived from the resulting signal.

2. A method according to claim 1, wherein scanning of the incoming signal sequence is performed with a scanning fre-quency in the Megahertz range.

3. A method according to claim 2, wherein the scanning of the incoming signals is performed with a scanning frequency of approximately 10 Megahertz.

4. An apparatus for effecting said method which com-prises an analog-digital converter supplied with the incoming signal and whose scanning frequency can be controlled by a pro-cessor as a function of the pulse rate of the transmission signal and downstream of said converter are connected means for the par-allel addition of the data supplied on a parallel line from the analog-digital converter and the addend signals of adder means processed in parallel form.

5. An apparatus according to claim 4, wherein down-stream of the analog-digital converter is connected a parallel adder, whose output is connected across a shift register to the input for the second addend of the parallel adder.

6. An apparatus according to claim 5, wherein in the output line of parallel adder is provided a sensor for indicating an overflow bit MSB to processor and in the parallel connection between shift register and the input for the second addend on parallel adder is provided a switch controlled by processor and which, when an overflow bit MSB is indicated by sensor, down-wardly displaces by one position the parallel signal lines from the shift register to the parallel adder.

7. An apparatus according to claim 4, wherein the pro-cessor is a microprocessor operating in the nanosecond range and which contains the function of the transmission pulse-related parallel adder.