WO1986001589A1

WO1986001589A1 - Position sensing apparatus

Info

Publication number: WO1986001589A1
Application number: PCT/GB1985/000401
Authority: WO
Inventors: Frank Ernest Richard Cannings
Original assignee: Renishaw Plc
Priority date: 1984-09-06
Filing date: 1985-09-06
Publication date: 1986-03-13
Also published as: GB8422535D0; EP0192721A1

Abstract

A sensor (10) for sensing the position of an object (13) has a source (16) of incident light, a single lens (14) having a first zone (18) for imaging the incident light to a waist (20A) at an object plane (19). The single lens (14) further has a second zone (21) for imaging light, reflected from a surface (12) of the object (13) situated at or near the object plane (19), on to a sensing plane (24A) of a transducer (24) there to form a reflected image (25) whose position on the sensing plane (24A) is indicative of the relative position of the sensor (10) and the object (13).

Description

POSITION SENSING APPARATUS

Background to the invention

This invention relates to position-sensing apparatus wherein a sensor and an object are moved one relative to the other to sense a surface of the object with a view to determining the position thereof.

More specifically, this invention relates to a sensor for sensing the position of an object during relative movement therebetween, the sensor being of the kind comprising a a source of incident light, first imaging means adapted to direct the incident light onto a surface of the object there to define an incident image, an opto-electronic transducer having a sensing plane, second imaging means adapted to direct light reflected from said surface onto said sensing plane to define a reflected image thereon, said relative movement resulting in displacement of the incident image relative to the second imaging means and in corresponding displacement of the reflected image across the sensing plane thereby to provide an indication of the relative position of the sensor and the object in terms of an output of the transducer.

Such sensors are often referred to as triangulation sensors because, as is well-known the position of the reflected image depends on the geometry of a triangle two sides of which are formed by the incident and the reflected light between the respective imaging means and the object, and the third side of which- is formed by the spacing of the two imaging means. The distance between the two imaging means on the one hand and the object on the other hand is referred at the "stand-off" of the sensor. The angle between the incident and reflected light, the "displacement angle", determines the displacement rate of the reflected image i.e. the greater the displacement angle the greater the displacement of the reflected image for a given relative displacement of the sensor and the object, and the better the resolution of the sensor. However, the greater the displacement angle the smaller is the stand-off of the sensor for a given spacing of the two imaging means, or the greater is the spacing of the two imaging means for a given stand-off. Further, the greater the displacement angle the smaller is the sensing range of the sensor. It is also a feature of sensors of the kind described that the intensity of the reflected light depends on the nature of the reflecting surface and can in some circumstances be relatively weak because of scatter.

It is an object of this invention to provide a sensor in which the stand-off and the displacement angle are optimized in relation to the spacing of the two imaging means. It is optionally an object of this invention to provide a sensor of simple mechanical construction. It is optionally an object of this invention to provide a sensor in which the two imaging means can be changed relatively easily with a view to varying the displacement angle and thus the sensing range of the sensor.

It is optionally an object of the invention to provide a sensor in which the aperture of the second imaging means can be large relative to that of the first imaging means, without unduly increasing the spacing of the two imaging means, so as to improve the light gathering capacity of the second imaging means.

According to this invention there is provided a sensor for sensing the position of an object during relative movement between the sensor and the object; the sensor comprising a source of incident light, first imaging Beans adapted to direct the incident light onto a surface of the object there to define an incident image, an opto-electronic transducer having a sensing plane, second imaging means adapted direct light reflected from said surface onto said sensing plane to define a reflected image thereon, said relative movement resulting in displacement of the incident image relative to the second imaging means and in corresponding displacement of the reflected image across the sensing plane, characterised in that said first and second imaging means comprise respective first and second zones of a single imaging system.

The single imaging system may be an axisym etric optical element or combination of elements adapted to both focus the incident light to a point or a narrow waist and of gathering the light reflected from a said surface when the surface is situated in the region of said point or waist.

By using different zones of the single imaging system for the two imaging means a rigid mechanical relationship is established in a simple way between the two imaging means, and no difficulty arises in establishing and maintaining a selected displacement angle between them. The single imaging system may be constituted by a member arranged to be readily movable relative to a support for exchange with another such system defining a different displacement angle thereby to provide a different stand-off or different resolution. The second zone βay be arranged to be larger than the first zone so that the second zone has a larger effective aperture with corresponding benefit in case of relatively weak reflected light.

Brief Description of Drawings

A preferred embodiment of this invention will now be described with reference to the accompanying drawing wherein:

Fig.1 is a sectional eleveation of a sensor according to this invention.

Fig.2 is a section on the line II-II in Fig.1

Fig.3 is a view on the line III-III in Fig.1

Description of Preferred Embodiment

Referring to Figs.l and 2, there is shown a sensor 10 secured to the head 11 of a co-ordinate measuring machine for the purpose of sensing surfaces e.g. a surface 12, of an object or workpiece 13. The sensing is required for measuring the dimensions of the workpiece. The machine and the use of a sensor for such measuring are known per se.

The sensor 10 has an imaging system defined by a lens 14 which is axisymmetric, i.e. rotationally symmetrical, and has an optical axis 14A. The lens is secured to one end of a housing 15. A laser 16 secured to the housing 15 at the other end thereof is positioned to direct an incident beam 17 of light onto a first or incident zone 18 of the lens 14. The zone 18 defines a first imaging means and occupies a portion of the lens 14 to one side, the "incident side", of a plane 14B through the axis 14A. The beam 17 has an axis 17A being the axis of the laser 16. The zone 18 directs or images the incident light to the side of the lens opposite to that at which the laser is situated so that the incident light emerges from the housing 15. The laser 16 is so positioned that, after refraction through the lens .14, the incident light passes along an axis 17B having an acute angle 17C, the "displacement angle", with the axis 14A and intersecting the latter axis in a position outside the housing 15. The lens 14 is designed, and corrected for abberation, to substantially focus the incident light to a point or narrow waist 20A at an object plane 19. If the surface 12 is situated at or near the plane 19 the incident light falls onto the surface 12 at a small spot where the incident light defines a first or incident image 20. The image 20 is reflected by the surface 12 generally back toward the lens 14. At the side of the plane 14B opposite to that αf the zone 18, referred to as the "reflection side", the lens 14 has a second or reflection zone 21 defining a second imaging means. That portion of the reflected light which falls onto the zone 21 defines a reflected beam 22 having axes 22A,22B corresponding to the axes 17A.17B. The housing 15 also supports a mirror 23 positioned substanitally at the reflection side of the plane 14B and so as to direct the reflected light away from the axis 14A. Also at the reflection side of the plane 14B the housing 15 has secured thereto a planar opto-electronic transducer 24 defining a sensing plane 24A. The mirror 23 and the transducer 24 are so positioned that the reflected light is directed or imaged onto the sensing plane 24A there to form a reflected image 25 corresponding to the incident image 20.

The axes 17B and 22B define light paths which converge onto the object plane 19. The arrangement is such that an angle 14C is defined between the axes 17B.22B and the arrangement is such that the angle 14C is divided by the axis 14A.

It will be seen that if the head 11 is moved relative to the workpiece 13, preferrably in the direction of the axis 17B, the reflected image 25 travels across the plane 24A and the position of the reflected image 25 in the plane 24A to either side of a centre position 25A (Fig.3) is a measure of the displacement of the surface 12 along the axis 17A to either side of a centre position 12A. The transducer which is known per se, has electrodes 24B,24C spaced apart in the direction of the displacement of the image 25. The arrangement is such that there is a flow of two photo-currents from the image 25 toward the respective terminals 24B.24C and the ratio of the two currents varies according to the position of the image 25 between the terminals 24B,24C i.e. the position of the image 20 relative to the sensor 10 is given in terms of said ratio.

In the present example, the lens 14 is of the convergent-convergent type so that not only the axes 17B,22B are convergent onto the. object plane 19, but also the axes 17A,22A are convergent from the lens 14 onto the axis 14A at the output source point, 16A, of the laser 16. Since the transducer 24 cannot physically be placed at the said source point 16A , a mirror 23 is provided to deflect the reflected light in a direction away from the axis 14A i.e. clear of the laserlβ.

The zones 18,21 themselves are not symmetrical about the axis 14A but are defined by a mask 26 arranged to give the best overall optical efficiency. For example, as shown in Fig.2, the mask 26 is such that the zone 21 is of larger area than the zone 18 so that best use is made of the reflected light which, because of scatter, is usually of lesser intensity than the incident light. As will be seen the zone 18 is defined by a circular or oval perimeter 18A defining the optical aperture of the zone 18. On the other hand the zone 21 is defined by a crescent-shaped perimeter 21A defining the optical aperture of the zone 21. As shown, the perimeter 21A extends to the incident side of the plane 14B and at least partially embraces the perimeter 18A. In this way the areas of the zones 18,21 are optimised as regards light intensity. Also, inasmuch as the perimeter 18A is of oval shape, it corresponds to the shape often found in the cross-section of a laser beam and it will be clear that this shape favours the relative geometry of the perimeters 18A,21A. In other words, the. oval shape of the zone 18 its reasonably well into a half-circle portion of the lens 14 to one side of the plane 14A. The mirror 23 extends correspondinly across the plane 14B and has portions 23A corresponding to end portions 21B of the crescent shape of the zone 21. The zone 18 is as remote as possible from the axis 14A so as to maximize the spacing between them within the confines of the lens 14.

The housing 15 comprises a portion 15A to which the lens 14 is secured and which is connected to^' the remainder of the housing 15 by a screw thread 15B so that the lens 14 can be removed for replacement of another such lens of a different focal length with a view to varying the stand-off or the resolution of the sensor.

Claims

CLAIMS :

1. A sensor for sensing the position of an object during relative movement between the sensor and the object; the sensor (10) comprising a source (16) of incident light, first imaging means (18) adapted to direct the incident light onto a surface (12) of the object (13) there to define an incident image (20), an opto-electronic transducer (24) having a sensing plane (24A) , second imaging means (21) adapted to direct light reflected from said surface (12) onto said sensing plane (24A) to define a reflected image (25) thereon, said relative movement resulting in displacement of the incident image (20) relative to the second imaging means (21) and in corresponding displacement of the reflected image (25) across the sensing plane (24A), characterised in that said first and second imaging means (18,21) comprise respective first and second zones (18,21) of a single imaging system (14).

2. A sensor according to claim 1 wherein said single imaging system (14) comprises at least one axisymmetric optical element adapted to both focus the incident light to a point or a narrow waist (20A) at an object plane (19) and of gathering the light reflected from a said surface (12) situated in the region of said object plane (19).

3. A sensor according to claim 1 wherein said single imaging means (14) comprises a member (14) and the sensor (10) includes a support (15) relative to which the member (14) is arranged to be movable.

4. A sensor according to claim 1 wherein said second zone (21) is of larger optical aperture (21A) than the correspondingly smaller optical aperture (18) of said first zone (18) .

5. A sensor according to claim 4 wherein said larger aperture (21A) is shaped to at least partially embrace the smaller aperture (18A).

6. A sensor according to claim 5 wherein said smaller aperture (18A) is of circular or oval shape and the larger aperture (21A) is crescent-shaped and situated to partially embrace the smaller aperture (18A).

7. Sensor according to claim 2 wherein said first and second zones (18,21) lie substantially at diametrally opposite portions of said at lesast one element (14).

8. Sensor according to claim 2 wherein said light source (16) and said transducer (24) are situated at the side of said at least one element (14) away from said object plane (19) in positions such that at least one of the light source (16) and transducer (24) are situated remote from the axis (14A) of the element (14).

9. Sensor according to claim 8 wherein said at least one element (14) is of the convergent-convergent type, and at least one of said light source (16) and transducer (24) are situated in a position remote from the adjacent focus of the element(s) (14) .

10. Sensor according to claim 8 or 9 including means (23) arranged between said at least one element (14) and said at least one of the light source (16) and transducer (24) for diverting light away from said axis (14A).