WO1981002781A1

WO1981002781A1 - Optical position sensor

Info

Publication number: WO1981002781A1
Application number: PCT/SE1981/000086
Authority: WO
Inventors: L Stenmark
Original assignee: L Stenmark
Priority date: 1980-01-21
Filing date: 1981-03-16
Publication date: 1981-10-01
Also published as: SE8000460L; GB2083620A; SE420352B; JPS57500350A

Abstract

Novel optical position sensor which, when illuminated by a lightline produces an electrical signal containing information about where the lightline strikes the surface of the sensor in relation to its origin (10). The sensor is provided with two or more detectors (5, 6, 7, 8) with wedge-shaped light sensitive area. Said detectors are located with point symmetry in relation to the origin. Two of said detectors are inner detectors (5, 6) accurately measuring the position of the lightline and located on both sides of a symmetry line (9) being parallel to the measuring direction (x) and substantially at right angles to the lightline. When the sensor is used to sense a position of an object, the lightline strikes the two inner detectors (5, 6) which are so orientated that the signal from one increases and the signal from the other one decreases if the position sensor is laterally displaced in the measurement direction (x).

Description

Optical Position Sensor

The present invention relates to an optical position sensor which, when illuminated with light projected along a line, i.e. a lightline, produces an electrical signal, being a measure of the position of the illuminating lightline. The sensor includes two or more photodetectors and compensates for ambient light.

It is often useful to be able to continuously measure latent displacements of an object, or to set the direction of an instrument etc. where it, for some reason, is not possible or wanted to use mechanical methodes, such as theodolites. In this connection a laser is often used as a light source, the beam of which is fanlike expanded in a plane. When this beam hits an object, a lightline occurs across the surface of the object. By directing such a light beam against an optical position sensor that produces an electric signal containing information about where on the surface the pro¬ jected lightline hits, it is either possible to measure changes in the position of the sensor or .in the direction of the light beam. The requirements of the position sensor are that it firstly must have a good resolution in the position indicated, se¬ condly must have a large enough working span, i.e. extension in the measuring direction, thirdly that it must have a high sensitivity to the laser light, fourthly that it must be independent of variations in the ambient light and fifthly that it, if possible is simple in construction and in¬ expensive.

Most of the existing position sensors use either a diode matrix with a large number of aligned descrete small photo diodes in combination with CCD-technigue. Both types give good resolution but a relatively small working span. The former one requires a complex electronic system for the sig¬ nal handling and the later type is still rather expensive and has a small working span; in the order of one or two centimeters.

Another position sensor is described in the US patent appln no 3,819,273. It contains 2 or 3 detector element located in a plan (fig. 1). A lightline can hit one or two of thes elements which alltogether gives five different positions t be sensed. This is in certain applications not enough accurate.

Another known variant is disclosed in the Swedish patent no 360170 (corresponding to the German patent appln no 1913399 In this device a photodetector with an hourglas-shaped sur- face (fig. 2) is used. Parallel to this, a linear shaped photodetector is positioned. The main advantage of this de¬ vice is that it gives a continuous position indication, i.e has a good resolution. It has however the disadvantage of a large detector surface if the working span is large. Becaus of this the portion of the sensitive area that is hit by th lightline becomes very small in relation to the total area which is exposed to the ambient light. This means that eith a powerful laser has to be used which easily brings you in conflict with current worker protektion regulations, or giv a low signal to background ratio. The present invention will solve the problems mentioned by positioning two or more light sensitive detectors behind an optical transmission filter of band-pass type. Said detecto are electrically connected to an electronic analysis circui which produces signals to an indication unit. According to the invention, said sensors, by masking or cutting, are arranged to have a wedge shape light sensitive surface and positioned in pairs on both sides of a symmetry line which also passes through the origin of the sensor. The advantage of this is that instead of one large detector several small ones are used which gives a good signal to background ratio and hence requires a low laser power. The wedge shape detector area gives good resolution in the posi tion indicated in comparison to existing sensors with rec¬ tangular shaped detector elements. According to the* invention it is further advantageous to locate the wedge shaped detectors around the symmetry line with a certain degree of overlapping in relation to the lightline. Thereby the lightline always illuminates at least one detector element, which assures that no dead poij occurs. It has further proved to be an advantage to arrange the wedge shaped detectors with symmetry around thr origin (point symmetry) . As a result of this good position indica¬ tion is achieved even if the lightline not crosses the symmetry line at right angles and the zero point will still coincide with the origin. This is particularly advantageous if changes in the position also gives rise to turnings which is not un¬ usual. Another quality of the invention is that it provides , a particularly high degree of accuracy when centering. This is due to the fact that with a so called window compensator, it is possible to compare the signals from the two central detectors and indicate when they are equal. By using this methode one is completely independent of possible unlineari- ties in the compensation of the ambient light. In a preferred embodiment of the invention at least four detector elements are used. As a result of this at least one of the outer two detectors is not stricken by the light- line, and this detector is preferably used as a reference detector for sensing the ambient light level. Naturally it is possible to use more than one detector for the background compensation, and to use more than four detectors in the position sensor according to the invention.

In order to achieve best possible signal to background ratio it can be advantageous to let the lightline consist of puls- ing light, either by using a scanning beam to create the fanlike expansion or by straight forward modulation of the expanded beam.

If the position sensor is situated in a rotatable casing, it is easy to turn it to the right orientation inrelation to the lightline. By furnishing the casing with two legs pro¬ vided with magnets it is easy to let the origin coincide with the iddlepoint between two plane surfaces, i.e. the saw blades of a frame-saw. In order to transfer the information of the position, it is often useful to let a light emitting diode work as a trans¬ mitter in or in relation to the sensor unit and to use a photodiode as reciever in the indication unit. Alternatively an ultrasonic transmission may be used. By way of this either on of or both the sensor unit and the indication unit can be free moving.

In enviroments with much noise and with large variations in the ambient light, it may be useful to present the position information by using a series of lamps where position and/o blink frequency indicates the distance between the position of the lightline and the origin of the sensor. An example of the embodiment of the position sensor accor¬ ding to the invention is described in more detail herein¬ after with reference to the drawings. Figures 1 - 4, where in Fig. 1 shows an example of a prior art sensor. Fig. 2 shows a preferred embodiment of the present invention. Fig. 3 shows how a position sensor. according to the inven¬ tion can be used to detect the middlepoint between the saw- blades of a frame-saw, and wherein Fig. 4 shows an embodi— ment of the electronic system of the present invention.

Referring to Fig. 1, a prior art position sensor is provi¬ ded with three detector elements 1, 2 and 3. A lightline 4 can strike one of the elements 1, 2 or 3, or it can strike element 1 together with element 2 or, at last, it can strik element 2 together with element 3. These five steps give a too low resolution in many cases.

Referring to Fig. 2, an optical position sensor according to the present invention contains four wedge-shaped detec¬ tors 5, 6, 7 and 8, located in pairs on both sides of a symmetry line 9, going through the origin 10 of the sensor. The detectors 5, 6, 7 and 8 are employed in a casing 11, behind an optical bandpass-filter (not shown) . The position sensor provides a signal corresponding to the position and movement in X-direction of a lightline with an elongation in direction Y. As a result of the symmetry around the orig 10, the direction of the lightline is not very critical. Sa direction can be within plus and minus 20° from the directi Y with the retained accuracy at centering. The working span D of the position sensor can be divided in zones A,^"B and C, depending on which one of the four detec¬ tors are struck by the lightline. Zone A of the three men¬ tioned zones, gives the highest resolution. The detectors 5 and 6 gives the high resolution while the detectors 7 and 8 gives the large working span. _/" ^•""BTJ

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^■ The position sensor may be located on a vehicle or any moving object. Another application is shown in Fig. 3. The casing 11 is furnished with two legs 12 and 13 of equal length with magnets provided in the ends 14, 15. The sensor can then be used for centering a frame-saw and is then to be placed be¬ tween two saw blades 16, 17 in the frame. The origin is then accurately located half way between the blades. If instead one leg is made twice the length of the other, measured from J the origin, the origin will then be situated one third from one of the saw blades independent of the distance between them. Naturally the distance between the blades must not exceed the distance between the feet 12, 13. Further the distance has to exceed the diameter of the casing 11. Fig. 4 shows a functional block diagram of an embodiment of the electronic system utilized in the system of the inven¬ tion. Each detector 5, 6, 7 and 8 is connected to one pre¬ amplifier 18, 19, 20 and 21. These pre-amplifiers are in turn connected to a background compensation circuit which compares the different signals. Said units are all prefer- ably located in the casing 11. The resulting signal, that is compensated for the ambient light, is via a cable 23 or by means of some other type of communication, transferred to an electronic unit 28, containing a signal handling cir¬ cuit 24, display logics 25 and a display unit 26. In said electronic unit may as well a power supply unit be located.

Claims

1. Optical position sensor for the determination of latera displacements in a predetermined direction (x) provided with at least two detectors (5, 6, 7, 8) located behind a bandpass type optical transmission filter and, when illuminated by a lightline produces electrical signals to an electronic ana¬ lyses circuit (22, 24) , whereby said lightline preferable is generated by a fan-like expanded laser beam and substan¬ tially orientated at right angles to the predetermined measurement direction (x) , c h a r a c t e r i z e d in that two of said detectors (5, 6, 7, 8) are inner detectors (5, 6 arranged to be illuminated simultaneously by the lightline in the main part of their working area, and that further detectors (7, 8) are outer detectors, whereby said inner detectors (5, 6) by masking or cutting has a wedge-shaped light sensitive area and located beside each other on both sides of a symmetry line (9) substantially parallel to the measurement direction (x) and orientated with the apexes in opposite directions in such a way that the signal from one o them decreases and the signal from the other one increases if the position sensor is displaced in relation to the light line in said measurement direction (x) .

2. Optical position sensor according to claim 1, c h a- r a c t e r i z e d in that said inner detectors (5, 6) are located between said outer detectors (7, 8) and that the inner detectors are used for measurements with great accurac measurement area (A) , and that said outer detectors are used for a more coarse measurement, area (C) .

3. Optical position sensor according to claim 1 or 2, c h a r a c t e r i z e d in that said detectors (5, 6, 7, 8 are located in pairs on both sides of said symmetry line (9) and with point symmetry in relation to the origin (10) .

4. Optical position sensor according to any of the precedi claims, c h a r a c t e r i z e d in that at least four de¬ tectors (5, 6, 7, 8) are used and that the one outer detecto that is not illuminated by the lightline is provided to sense the ambient light, and if no one of the outer detectors are illuminated, that at least one of them is provided to sense the ambient light.

5. Optical position sensor according to any of the preceding claims, c h a r a c t e r i z e d in that the electronic analysis circuit (22) provides electrical signals to a light . emitting diode, emitting visible or infrared radiation, or provides signals to a transmitter for ultrasonics, whereby information about the position of the sensor currently is transferred to a display means. (26)_. by said light emitting diode or ultrasonic transmitter.

6. Optical position sensor according to any of the preceding claims, c h a r a c t e r i z e in that a display means

(25, 26) is provided with at least two lamps or light-emitting diodes indicating the distance between said lightline and the origin (10) by means of blinking frequency and/or position.

7. Optical position sensor according to any of the preceding claims, c h a r a c t e r i z e d in that said lightline con¬ sists of pulsed light, either in the form of scanning light beam or in the form of plain modulation.