GB2247129A

GB2247129A - Laser sensor systems

Info

Publication number: GB2247129A
Application number: GB9113880A
Authority: GB
Inventors: Gunther Sepp; Walter Hermann
Original assignee: Messerschmitt Bolkow Blohm AG
Current assignee: Airbus Defence and Space GmbH
Priority date: 1990-06-29
Filing date: 1991-06-27
Publication date: 1992-02-19
Anticipated expiration: 2011-06-27
Also published as: GB9113880D0; GB2247129B; DE4020833C2; FR2664041A1; FR2664041B1; DE4020833A1

Description

1 A 1 1 LASER SENSOR SYSTEMS The invention relates to a laser sensor.

Laser sensors are generally well known in various forms. Where two objects are to be simultaneously detected and measured, it is customary to use either a single laser sensor with reflecting mirror provisions relative to two spatial directions, or two complete laser sensors with fixed spatial directions. Furthermore, it has been known to use am-cw diode-laser sensors with two-channel phase-sensitive rectification for rangefinding.

These known systems tend to be elaborate, constructionally and operationally, and with respect to material and assembly.

Embodiments of the invention provide a laser sensor having two measuring beams and two corresponding lasers with associated transmission optics and receiving means that reduces impact of the aforementioned disadvantages. Effectively, such embodiments generally involve reduced costs by dispensing with use of reflecting mirrors or two complete sensors, but still enable measuring and control for two received signals representing two different objects, essentially by combining them in order to process single signals.

According to this invention, there is provided laser sensor comprising two lasers for producing measuring beams directed in different spatial directions, a common receiver with receiving optics and detector having a field of view for receiving reflections for both spatial directions of the two laser measuring beams, modulation means for differently modulating the lasers periodically, and a single-channel evaluation circuit responsive to output from the detector to provide a measuring signal that has been averaged over several modulation periods, the sign and size of which will depend on intensity differences in signals reaching the receiver from objects detected by the two laser measuring beams.

Suitable lasers are amplitude-modulated, continuous-beam diode lasers controlled by two 180' phase-opposed outputs of a common oscillator of the modulation means. A suitable evaluation circuit comprises an alternating current amplifier to receive signals from the detector (16), a following phasesensitive rectifier controlled by one of the outputs of the oscillator, and further following low pass circuitry for producing the measuring signal with averaging over several modulation periods.

In a preferred embodiment, the receiving signal represents deviation of the laser sensor from a predetermined nominal position relative to targets for the two laser beams.

Both lasers can be repeatedly operated for short periods using in-phase outputs of the oscillator, and output of the lasers controlled during those short periods according to the sum of laser radiation received by the detector in order to hold that sum constant.

Preferably, switching between measuring and output control modes of operation is by means of a switch for one of the lasers controlling pushpull or in-phase operation relative to the other of the lasers. Further preferably, output from the low pass circuitry goes to sample-and-hold means operative to hold the measuring signal as at its last value prior to switching to inphase operation of the lasers, and a comparator or discriminator serves in controlling said switching relative to approaching a set value, and in controlling the sample-and-hold means.

An exemplary embodiment is now described with reference to the accompanying drawings, in which:

Fig. 1 is a block schematic circuit diagram of the exemplary embodiment having two measuring beams and a common receiver; Fig. 2 is a block circuit diagram of one main circuit; Fig. 3 is a block circuit diagram of another circuit for output control; and Fig. 4 is a schematic illustration for possible application of the laser sensor to controlling a driver less transport system.

The embodiment to be described is based on the concept that a common control signal is produced from two laser measuring beams by way of differencing in a common control signal, whereby the cost of the required sensor is substantially that of a single sensor, i.e. a sensor which copes with only one receiver, one receive optics, one detector, one modulation means and a single- channel evaluation circuit.

Fig. 1 shows two laser measuring beams 13b, 14b from two lasers 13 and 14 both associated with modulation means 15 and going to two objects 13a, 14a in different spatial directions. The two measuring beams as returned from those objects go to a common receiver 30 having receive optics 16a which passes the received beams 13c, 14c to a detector 16. The latter has a receive field of view that includes the two objects 13a, 14a targeted by the spatial directions of the measuring beams 13b, 14b. Lasers 13, 14 are periodically modulated diiferently by the modulation means 15.

Lasers 13, 14 can be of amplitude-modulated continuous-beam (am/cw) diode type controlled from a common oscillator using two 180' phase-offset outputs, in the modulation means 15.

Detector 16 of receiver 30 sends its output to a single-channel evaluation circuit 32, output of which supplies a measuring signal 19 which is averaged over several modulation periods. In Fig. 2, the evaluation circuit 32 is shown with an alternating-current amplifier 16a feeding a phase-sensitive rectifier 17 controlled by one of the two out- of-phase outputs of the modulator 15. Low pass circuitry 18 serves in averaging over several modulation periods in order to produce the measuring signal 19.

In many measuring operations of control systems, the relative position of the two objects 13a, 14a and other apparatus associated with the laser sensor 12 is z 1 measured and serves in control functions. Deviations from a nominal position are mean values over several modulation periods (from low pass circuitry 18), and the measuring signal 19 indicates by its sign the direction of deviation or can be a control signal representing required correction. Where control takes into account absolute values of the received signals 13c, 14c, say for sharpness of correction, other factors can be significant, such as reflection properties of targets 13a, 14a and their distances from the sensor 12. To compensate these effects, it is proposed repeatedly to operate both laser diodes 13, 14 in phase, i.e. without phase-offset, for short periods. The signal then output from the rectifier 18 represents the sum of received intensities for both of the objects 13a, 14a. Clearly, that sum signal depends on the laser beams and the targets, and it is advantageous for the sum to be virtually constant for controlled deviations from the nominal position. This sum signal, shown taken at 19a, is held constant by control of the two laser outputs. A switch 23, controlled by a comparator or discriminator 22, switches at a condition to be explained later from receiving the measuring signal 19 to receiving the sum signal 19a. For the latter, laser 14 is switched from the 180 phase-output of oscillator 15 to its 00-phase output, so that both lasers 13, 14 now produce beams inphase rather than in phase- opposition, preferably pushpull, fashion. A switch-over signal 23a indicates completion of switching of output control, and the sum signal 19a for the received signals 13c, 14c is fed to the output-control unit 24 of the two lasers 13, 14. For the two lasers, an output offset can be set during a calibration phase so that the deviation measuring signal 19 provides a nil value for no deviation from the nominal position. Thus, inherent variations for the two laser beams 13a, 14a and their positions can be rendered ineffective, i.e. compensated by output offset between the two lasers 13, 14. Control signal 19 is shown fed to a sample-and- hold circuit 21 which serves to keep the two signals 19, 19a separately available. Prior to switching to sum signal (19a) insertion, sample-andhold output 21a is caused to be held at its last prevailing value by the switch-over signal 23a at its "hold" input 21b.

It is advantageous to activate the sum-signal mode of operation only when virtually no deviation is detection from the nominal position, i.e. when the signal to be stored approaches zero. Different intensity distributions for the two laser beams 13b, 14b then have no influence. For this purpose, switch-over to output control is made when the signal 19 goes below a predetermined value at output 21a. This condition is detected by the comparator or discriminator 22, the output of which actuates switch 23 if appropriate. After setting the output at the predetermined value, the output-control unit 24 will send a switch-back signal 23b to switch 23, which then switches back to the other mode of operation, i.e. storing current measuring signals. The signal 19 is thus virtually independent of the distance of the objects 13a, 14a, their reflection properties, and anything else differentially affecting transmission/reflection.

The exemplary application to a driver-less transport system (FTS), is described relative to Fig. 4.

2 71 A reflective band 11 is attached above, for example to the ceiling of a factory or warehouse. This retroreflector band 11 constitutes a guide track, the edges of which correspond to the objects 13a, 14a. The FTS 60 has a central control unit 50 with microprocessor 51 and program memory 52 which receives the signals from the laser sensor 12, a steering sensor 53 and a wheelrotation sensor 54. Using the signals 19, 53a, 54a, and as defined by contents of the program memory 52, the processor 51 will determine signals 55a, 56a for steering device 55 and wheel-drive 56 of the FTS 60.

It will be appreciated that, as described, a laser sensor of the initially mentioned type is provided with functions effectively providing measurement signals and control capabilities substantially as for two complete individual sensors. However, structural conten and costs are substantially as for an individual sensor, save only for a second laser with related optics.

Claims

1. Laser sensor comprising two lasers for producing measuring beams directed in different spatial directions, a common receiver with receiving optics and detector having a field of view for receiving reflections for both spatial directions of the two laser measuring beams, modulation means for differently modulating the lasers periodically, and a single-channel evaluation circuit responsive to output from the detector to provide a measuring signal that has been averaged over several modulation periods, the sign and size of which will depend on intensity differences in signals reaching the receiver from objects detected by the two laser measuring beams.

2. Laser sensor according to claim 1, wherein the lasers are amplitudemodulated, continuous-beam diode lasers controlled preferably in pushpull fashion by two 1800 phase-opposed outputs of a common oscillator of the modulation means.

3. Laser sensor according to claim 2, wherein the evaluation circuit comprises an alternating current amplifier to receive signals from the detector (16), a following phase-sensitive rectifier controlled by one of the output of the oscillator, and further following low pass circuitry for producing the measuring signal with averaging over several modulation periods.

4. Laser sensor according to any preceding claim, wherein the receiving signal represents deviation of the laser sensor from a predetermined nominal position relative to targets for the two laser beams.

Q 1 C.

9 -

5. Laser sensor according to claim 4 with claim 2, wherein both lasers repeatedly operate for short periods using in-phase outputs of the oscillator, and output of the lasers is controlled during those short periods according to the sum of laser radiation received by the detector and to keep that sum constant.

6. Laser sensor according to claim 5, wherein switching between measuring and output control modes of operation is by means of a switch for one of the lasers controlling push-pull or in-phase operation relative to the other of the lasers.

7. Laser sensor according to claim 6, wherein output from the low pass circuitry goes to sample-and-hold means operative to hold the measuring signal as at its last value prior to switch to in-phase operation of the lasers, and a comparator or discriminator serves in controlling said switching relative to approaching a set value, and in controlling the sample-and-hold means.

8. Laser senor system arranged and adapted to operate substantially as herein described with reference to and as shown in the accompanying drawings.

Published 1992 at The Patent Office. Concept House. Cardifr Road. Newport. Gwent NP9 1 RH. Further copies njay be obtained froni Sales Branch. Unit 6. Nine Mile Point. Cwrnfelinfach. Cross Keys. New- port. NP1 7HZ. Printed by Multiplex techniques lid. Si Man.

Cra. Kent-