GB2140156A - Position and/or attitude sensing system and methods - Google Patents

Abstract

The position and/or attitude of a member (1) is determined by means of an optical sensor or sensors (2) mounted on the member detecting a scanning optical beam produced by scanning unit (4). The position of the scanning optical beam as a function of time is known and thus the position and/or attitude of the member can be determined by determining the time(s) of arrival of the optical beam at the optical sensor(s). The scanning optical beam is preferably generated by an acousto-optic deflector. Applications include automatic milling and cutting machines, robot arms, laser disc readers and writers, and an electronic notepad; in the latter the position of a pen bearing two photodiodes is sensed in two dimensions and a corresponding dot written on a CRT or liquid crystal display. Also mentioned is a system where the beam is switched between any two positions and the sensor is caused to move to the new position by closed loop control. <IMAGE>

Description

SPECIFICATION Position and/or attitude sensing systems and methods This invention relates to position and/or attitude sensing systems and methods and in particular, but not exclusively, to a solid state position and/or attitude sensing system.

According to one aspect of the present invention there is provided a system for determinig the position and/or attitude of a member, comprising one or more optical sensors which in use ofthe system are mounted to the member, means for generating a scanning optical beam whose position as a function of time is known, means for determining the time of arrival ofthe scanning optical beam atthe or each sensor, and means for determining the position and/or attitude ofthe member from said time or times ofarrival.

According to a further aspect ofthe present invention there is provided a method for determining the position and/orattitude of member, comprising the steps of mounting one or more optical sensors to the member; scanning a field including the member with a scanning optical beam whose position as a function of time is known; determining the time of arrival of the scanning optical beam atthe or each sensor, and determining the position and/or attitude ofthe memberfrom said time ortimes of arrival.

According to another aspect ofthe present inven tion there is provided a method for controlling the position and/or attitude of a member provided with an optical sensorthereon, including the steps of driving the member to a required position in which the optical sensor detects an optical beam whose position corresponds to the required position; monitoring the member in the required position by closed loop control until a new required position is selected, when the optical beam position is changed and driving the memberto redetectthe optical beam and thus assume the new required position.

Embodiments ofthe present invention will now be described, byway of example, with reference to the accompanying drawings, in which: Fig. 1 illustratesschematicallya basic arrangement of a position sensing system; Fig. 2 illustrates schematically an example of the scanning unit of Fig. 1; Fig. 3 illustrates schematically a scanning optical beam and a photodiode; Fig. 4 illustrates the electrical output of the photo diode of Fig. 3; Fig. 5 illustrates schematically an arrangementfor attitude sensing; Fig. 6 illustrates the electrical outputs of the photodiodes of Fig. 5; Fig. 7 illustrates schematically an enhanced resolu tion variant of the position sensing system of Fig. 1;; Fig. 8 illustrates schematically a scanning optical beam and a plurality of diodes employed to provide enhanced positional resolution; Fig. 9 illustrates schematically a basic arrangement of multiphotodiode position sensing system; Figs. 1 ova, band cshowthe electrical outputs of a photodiode, an associated monostable and a bandpass filter, respectively; Fig. 11 a and b illustrate schematically two types of optical beam scanners; Fig. 12 illustrates schematically a scanner for use as an x-y position indicator; Fig. 13 illustrates a telephoto lens system; Figs. 14a and b illustrate two practical forms of the scannertype of Fig. 11 a, and Fig. 15 illustrates schematically generation of a scanning light barfrom a scanning beam.

Referringfirstlyto Figs. 1 to4a basic one dimensional position sensing system will be described. Let us assume that it is required to know the position of a device 1 in they-direction of a field. To the device 1 is attached a photodetector, for example a photodiode, 2. Ascanning optical beam 3 (Figs. 2 and 3) is generated by a scanning unit 4(Figs. 1), an embodiment of which is shown in Fig. 2, and directed generally towards device 1. At a particular time relative to the start of the scan, t5 in Fig. 3, the optical beam 3 will strikethe photodiode 2 and this will result in an electrical output from the photodiode as indicated in Fig. 4.

The position sensing system illustrated in Fig. 1 comprises the photodiode 2 togetherwith associated electronics 5for signal processing purposes, for example, the scanning unit4, control and synchronisation means 6, a timing unit 7 and a data converter 8. The scanning unit4 illustrated in Fig. 2 comprises an optical beam source, for example a laser 9, control electronics 10 forthe laser 9, an acousto-optic deflector 11, an input optical system 12 for deflector 11, an output optical system 13, and an r.f. drive means 14forthe deflector 11.

The acousto-optic deflector 11 comprises a body of an acousto-optic material, for example lithium niobate (LiNbO3), into which an acoustic wave can be launchedfrom appropriatetransducersto produce a frequency controllable diffraction grating. Light incident at the appropriate angle onto this grating is then coupled into the first grating orderwhich can be scanned in proportion to the grating period. The device may be realised in either bulk or surface wave forms. For random access operation the response of the device is determined by the transittime of the acoustic wave across the optical beam which, typical- ly, will be less than tewnty microseconds. Raster scanning is also possiblewithoutsignificant penalty to resolution at speeds approaching one hundred microseconds per scan.

Since the direction of the optical scanning beam is a known function oftime, because the time reference of timing unit 7 and the acousto-optic deflector 11 ofthe scanning unit 4are synchronised by the control and synchronisation unit 6, then the position of the photodiode 2, and hence ofthe device 1, may be derived from the time of appearance of the electrical output ofthe photodiode 2. The time information obtained attiming unit may be converted to distance (position) information in they direction by, for example, a data converter circuit 8 based on a preprogrammed ROM. Thus the device postion information is derived from the time arrival of a scanning optical began on a photodiode attached to the device.

Referring to Figs. 5 and 6, a basic one dimensional attitude sensing system, a plan view ofwhich is shown in Fig. 5, will now be described. To a device 15 whose attitude isto be sensed are attached two photodetec tors, for example, photodiodes 76 and 17.The photodiodes 16 and 17 are separated buy a known distance r. An optical scanning beam 18 is employed which has a bar cross-section ratherthan a spot cross-section as in the Figs. 1 to 4 embodiment. The lightpropagates in the z-direction and the beam scans in the x-direction. The outputs ofthe photodiodes are ofthe form shown in Fig. 6.From the time difference At between the two photodiode outputs and knowing the beam position as a function of time, the difference in the x position, Ax, ofthetwo photodiodes 16 and 17 may be derived.The attitude angle e may then be derived from the relationship, 6 = cos~t (Axtrl.

The determination of Ax and thus 9may be readily carried out by a data converter circuit based on a preprogrammed ROM. The attitude information is thus derived from the times ofarrival of a scanning optical beam with a bar cross-section on the photodiodes.

By combining the position and attitude sensing systems described above, completethree-dimension- al attitude and position information of a device can be obtained. The position photodiode, or, as described hereinafter, position photodiodes, should preferably be placed in an appropriate position on the device such asto be unaffected byangularchanges ofthe device. Alternatively, appropriate post-processing of the position data will be required.

As mentioned above, acousto-optic deflectors can be used in a random access mode, that is where the beam position is changed from one to any other position without scanning intervening positions, and they may thus be used to control the position of a sensor (photodiode) and thus the position of a device carrying the sensor. Such a position controller may operate as follows. A photodetector (sensor) on a device senses a deflected beam when the device is in a required position and then by means of closed loop control the device is maintained in this position.When a new position is required, that is programmed, the sensor will automatically be moved towards the new position and once in the newpositionthe sensorwill detectthe corresponding deflected beam and hold the device in that position.

The basic position sensing system described above detectsthe time of arrival ofthe scanning optical beam. Since the beam has a finitewidth, a finite width electrical pulse will result from the photodiode.

Positional resolution may be increased by, for example, the use of athreshold comparator after the photodiode such that an output is generated when the photodiode output reaches, for example, 80% of maximum. The time to distance data converter ROM must in this case be appropriately programmed such thatthe corresponding time measurementgivesthe correct distance (position). An arrangement employing a threshold comparator 18 is shown in Fig. 7.

The photodiode 2, photodiode electronics 5, scanning unit 4 and the control and synchronisation unit 6 correspond to those of Fig. 1. In addition the arrange ment includes a counter and oscillator 19 and a data converter 20. In this arrangement the output pulse of the comparator 18 is used to stop the count of the counter, of counter and oscillator 19, which started counting, and hence measuring time, when the optical beam started scanning. Thusthere is provided an electronic method of improving the resolution ofthe system by sensing the leading edge of the electrical pulse generated in the photodiode.

Improvementofthe signal to noise ratio may be achieved in various ways. For example, an optical filter may be placed overthe photodetector,thefilter having a wavelength bandpass at the wavelength used bythe scanning unit. Thus other optical wavelengths will suffer greater attenuation before reaching the photodetector. Alternatively, an electrical high pass filter may be used afterthe photodetector.

The lower frequency unit of the filter should be just below the reciprocal of the transit period ofthe scanning beam acrossthe photodetector.Athird possibility is that of intensity modulating the scanning beam such thatthe photodetector picks up a number of cycles of the intensity modulation frequency during a scan. In this case a bandpass filter atthe intensity modulation frequency is employed afterthe photodetector.

Atechniqueforthe enhancement of positional indication using a number of photodetectors (photodiodes) will now be described with reference to Figs. 8 to 1 0.Aschematic plan ofthe layout is shown in Fig. 8.

The scanning optical beam 3 scans along a line of photodiodes 21,22,23 and 24. The more photodiodes used, the betterthe discrimination against noise and the betterthe positional resolution. A schematic block diagram ofthe overall multi-photodiode position sensing system is shown in Fig. 9 and comprises a scanning unit4,synchronisation and control means 6, photodiode electronics 25, a respective rising edge triggered monostable 26,27,28 or 29 for each photodiode 21 to 24, a bandpass amplifier 30, a counter and oscillator 31, a reference oscillator32, a phase detector 33, an analogue to digital converter 34, and a data converter 35.

The output pulses ofthe photodiodes 21 to 24 are indicated figuratively in Fig. 1 Oa. These output pulses are each fed into a respective monostable, of monostables 26to 29, which delivers a pulse that is short compared with the scanning time of the optical beam across a photodiode. The pulses obtained, which are indicated in Fig. 1 Ob, are then fed through bandpass amplifier 30 whose centre frequency correspondsto the period between pulses. The output of bandpass amplifier 30 is a sinusoidal waveform, as indicated in Fig. 1 0c. Since the position in time ofthe pulses in Fig.

lOb depends on the position of the photodiodes, the phase of the sinusoidal signal in Fig. will depend on the position ofthe photodiodes. Hence by measuring the phase ofthesinusoidal signal the fine position of the photodiodes may be determined. Forthis purpose the phase of the output of bandpass amplifier 30 is detected by comparing the sinusoidal signal with the output of reference oscillator32 by means of phase detector33. The analogue output of detector33 is converted to digital form by means of converter34 and to the required output format by the data converter 35. Thus the output corresponds to the fine position of the photodiodes.The coarse position ofthe photodiodes is determined as described previously by measuring the time of arrival ofthe scanning beam at a photodiode.! In the above two optical scanners have been referred to, a scanning light spot and a scanning light bar. The optics systems for producing a scanning spot or barwill depend, in detail, on the requirements of the sensing system. In general, for producing a scanning lightspot, optics may be required to shape andcollimatethe input beam, and optics following thescannerwill be required to generatethe desired scan geometry from the scanner output. The scanner output beam should have desired cross-sectional dimensions and, normally, be collimated. The position ofthe beam as a function oftime should be known.Two types of scanners which fulfill these requirements are shown in Fig. 11. In Fig. a, only the position of the beam changes, not its angle, and the position across the scan will be known as a function of time, ifforexample the scan rate is constant. Fig. 11 bshows a scanner where the output beam pivots about a point and the angular position thereof may be known as a function oftime. For this type of scan the distance ofthe beam, along an axis being scanned,from its central position can be calculated from the angle of the beam relative to the central position, ifthe distance between the pivot and the axis at the cental position is known.

Two scanners ofthe pivottype as illustrated in Fig.

11 and operating at right angles as indicated in Fig. 12 may be used to determine the x andy position of a sensor (photodetector) relative to an origin. The pivot point of one scanner is arranged at a point 36 a distance H, measured in the direction of they-axis, from an origin 37 with coordinates (0, O). The pivot point ofanotherscanner is arranged at a point 38 a distance I, measured in the direction of the x-axis, from the origin 37. The beam of the one scanner is pivotable between extremes of -q) and + relative to its central position, whereas the beam of the other scanner is pivotable between extremes of -6' and +0' relative to its central position.For a sensor at point39thecoordinates (x,y)thereof can be calculated from the expressions

The (x, y) position of a sensor can thus be calculated forall positionswithinthe region (field) of overlapor field40which is indicated by dashed lines.

An inherentfeature of acousto-optic beam deflectors is the relatively small angular deflection of their scan. The output normally travelsthrough approx imately + 1.5 about its centre point and in order to make the device "high resolution" the optical aperture has to be large. The optics at the output ofthe scannershould reduce the beam size, recollimate and possibly increase the angular scan. These requirements may be met by using a telephoto lens systems, such as illustrated in Fig. 13. Lenses 41 and 42 have positive focal lengths FA andfs, respectively, and are positioned sothattheirfocal planes coincide. The collimated input beam isfocussed by lens 41 and recollimated by lens 42.The angular magnification of the system is given approximately by fplfs, and the cross-sectional size ofthe beam is changed by approximately fBIfA. In practice the system may use several lenses to correct aberrations and the operation of each element may not be clear cut. Most acoustic-optic deflectors have rectangular apertures and the scanned beam may not be circular, therefore the system may also include cylindrical lens, prisms and other anamorphic elements.

The scan produced by most acousto-optic deflectors is of the type shown in Fig. 1 ib. Ascan ofthetype illustrated in Fig. 11 a can be generated by adding extra optical elements atthe outputofan acoustooptic deflector. A simple positive focal length lens 43 positioned with its focal pointatthe position ofthe scanner may be suitable (Fig. .14a), alternatively a holographic element 44 may be used (Fig. 14b).

A scanning light bar may be generated from a scanning beam (light spot-type) by using cylindrical optics 45, as illustrated in Fig. 15.

The solid state position and/or attitude sensing systems and methods described above may be applied to any system which has many parts whose position and orientations must be known, for example automatic milling and cutting machines, preprogrammed "robot" arms, laser disc readers and writers.

The inventive approach is particularlysuitablefor such applications because ofthe high speed with which position and orientation information can be obtained, and the high resolution obtainable. The scanning optical beams or bars are generated by solid state acousto-optic deflectors, which may be either of the surface acoustic wave or bulk acoustic wave variety. The use of scanning optical beams or bars also means that no mechanical contactwith a position is required.

Afurther possible application comprises an "electronic notepad". Atwo-dimensional position sensing system is combined with a cathode ray tube or liquid crystal display and appropriate electronics to form a "notepad". The "pen" has two photodiodes mounted orthogonally on it. The dimensional x andy position of the"pen" is sensed using the scanner ofthe "notepad". A dot is then electronically written on the screen or display at the position ofthe "pen". The information may also be stored in a RAM. As the "pen" is moved across the screen or display a trace may thus be described thereon. Such an "electronic notepad" may be employed as an alternative to the cathode ray tube "light pen", or for electronically recording hand written information, replacing pen and paper.

Claims (20)

1. A system for determining the position and/or attitude of a member, comprising one or more optical sensors which in use ofthe system are mounted to the member, meansfor generating a scanning optical beam whose position as afunction of time is known, meansfor determining the time of arrival of the scanning optical beam atthe or each sensor, and meansfordetermining the position and/orattitudeof the memberfrom said time or times of arrival.

2. Asystem as claimed in claim 1, wherein the scanning optical beam generating means comprises a solid-state acousto-optic deflector.

3. Asystem as claimed in claim 1 or claim 2, wherein the or each optical sensor comprises a photodetector, wherein in use ofthe system the photodetectors produce electrical pulses upon arrival of the scanning optical beam thereat.

4. A system as claimed in claim 3, wherein the meansfordeterminingthetimeofarrival ofthe scanning optical beam serve to sense the leading edge of the electrical pulses.

5. Asystem as claimed in any one of claims 1 to4 andfordeterminingthe attitude angle (9)ofthe member relative to a first direction, comprising two sensors which in use are mounted to the member and spaced apart bya first distance (r), wherein the scanning optical beam comprises a scanning optical bar, andwherein the determining meansservesto determine the difference in position (vex) in the first direction ofthetwosensorsandtodetermine9from the relationship 6 = cos~1 (Ax/r).

6. A system as claimed in claim 1 or claim 2, wherein each sensor comprises a plurality of photodiodes, wherein the time ofarrival of the scanning optical beam at a first ofthe plurality of photodiodes serves two determine the approximate position ofthe member and wherein the times of arrival ofthe scanning optical beam at all of the photodiodes serves to finely determine the position of the member

7. Asystem as claimed in claim 6, including a plurality of rising-edge triggered monostables, each photodiode output being applied to a respective monostable; a bandpass amplifier to which the monostable outputs are applied; and means for determining the phase ofthe bandpass amplifier output signal, which phase is dependent onthe photodiodes positions.

8. Asystem as claimed in any one of claims 3to 7, including an optical filter disposed between the photodetector and the scanning optical beam gener atop, the optical filter having a wavelength bandpass at the wavelength employed by the scanning unit, whereby to improve the signal to noise ratio of the system.

9. A system as claimed in any one of claims 3to 7, including an electrical high-passfilterforfiltering the photodetector output prior to application to the determination means,thefrequency unitofthefilter being just below the reciprocal of the transit period of the scanning beam across the photodetector, where byto improve the signal to noise ratio of the system.

10. Asystem as claimed in any one of claims 3to 7, including means whereby the scanning beam is intensity modulated such that the photodetector picks up a number of cycles of the intensity modula tion frequency during a scan, and including a bandpass filterforfiltering the detector output at the intensity modulation frequency whereby to improve the signal to noise ratio ofthe system.

11. A method for determining the position and/or attitude of member, comprising the steps of mounting one or more optical sensors to the member; scanning a field including the member with a scanning optical beam whose position as a function oftime is known; determining the time ofarrival of the scanning optical beam atthe or each sensor, and determining the position and/or attitude ofthe memberfrom saidtimeortimesofarrival.

12. Amethodasclaimea in claim 11,wherein the scanning optical beam is generated by means of a solid state acousto-optic deflector.-

13. A method as claimed in claim 11 or claim 12, wherein the or each optical sensorcomprises a photodetector, the or each photodetector producing electrical pulses upon arrival ofthe scanning optical beam thereat.

14. A method as claimed in claim 13 including the step of sensing the leading edges of the electrical pulses.

15. A method as claimed in any one of claims 11 to 14 andfor determining the attitude angle (6) of the member relative to a first direction, comprising mounting two sensors, spaced apart by a first distance (r),tothe member; scanningthefieldwith an optical beam intheform of a bar; determining the difference in position (Ax) in the first direction of the two sensors and determining 8 from the relationship 6 = cos' (Ax/r).

16. A method as claimed in claim 11 or claim 12, wherein each sensorcomprises a plurality of photo diodes, including the steps of determining the approximate position ofthe memberfrom thetime of arrival ofthescanning optical beam atafirstofthe pluralitgof photodiodes, and finely determining the position ofthe member from the times of arrival of the scanning optical beam at all ofthe photodiodes.

17. A method for controlling the position and/or attitude of a member provided with an optical sensor thereon, including the steps of driving the memberto a required position in which the optical sensor detects an optical beam whose position corresponds to the required position; monitoring the member in the required position by closed loop control until a new required position is selected, when the optical beam position is changed, and driving the member to redetectthe optical beam and thus assumethe new required position.

18. A method as claimed in claim 17,whereinthe optical beam is generated by an acousto-optic deflector operating in the random access mode.

19. A system for determining the position and/or attitude of a member substantially as herein described with reference to Figs. 1 to 4, Figsr 5 and 6, Fig.

7 or Figs. 8,9 and 10, with our without reference to any oneofFigs.11to15.

20. A method of determining the position and/or attitude of a member substantially as herein described with reference to Figs. 1 to 4, Figs. and 6, or Figs. 8,9 and 10, with orwithout referenceto any one of Figs. 11 to 15.