GB2317055A

GB2317055A - Contact sensor

Info

Publication number: GB2317055A
Application number: GB9718672A
Authority: GB
Inventors: Donald Irwin-Houston
Original assignee: IRWIN HOUSTON DONALD
Current assignee: IRWIN HOUSTON DONALD
Priority date: 1996-09-06
Filing date: 1997-09-04
Publication date: 1998-03-11
Also published as: DE19738736A1; GB9618599D0; GB9718672D0

Abstract

A contact sensor for mapping the surface of a workpiece comprises a probe element 64, 70 moveable in relation to electrical contacts 52, 54, 56, 58, 60, 62 within the sensor, a biasing means 80 and first 52, 54, 56, 58, 60, 62 and second 66, 68 electrical contact means. The biasing means 80 biases the probe element to a rest position. The first electrical contact means 52, 54, 56, 58, 60, 62 are mounted on an element 50 of a rigid insulating material. The second electrical contact means 66, 68 are mounted in association with the probe element 64, 70 for movement therewith. When the probe element is in the rest position, the first and second electrical contact means make electrical contact and when the probe element is moved from the rest position the electrical contact is altered.

Description

2317055 AN IMPROVED SENSOR This invention relates to providing an improved

structure for a contact sensor.

Contact sensors have been known for many years. Such sensors comprise a probe element which is taken into contact with a work piece by a movement means (perhaps a robotic arm or XY table, etc.). As the probe element contacts the work piece displacement of the probe element is detected.

This detection of the contact between the probe element and the work piece can be used to map the surface of the work piece. For instance if the co-ordinates of an end portion of the probe element are known (there may simply be a translation from the co-ordinates of the moving means to the co-ordinates of the end portion of the probe element) then the co-ordinate of the point of contact is known. By repeatedly contacting the work piece a map of its surface can be constructed.

Sensors capable of performing such tasks have been available for a number of years. However, prior art sensors are somewhat complex internally having many discrete components and are therefore more expensive to manufacture, and difficult to repair.

It is an object of this invention to provide a sensor capable of providing the function of prior art sensors but which is less complex.

According to a first aspect of the invention there is provided a contact sensor comprising a probe element moveable in relation to electrical contacts within the sensor, a biasing means, and first and second 2 electrical contact means, in which the biasing means is adapted to bias the probe element to a rest position, the first electrical contact means is mounted on a rigid insulating material element in proximity to the probe element, the second electrical contact means is mounted in association with the probe element for movement therewith, the said first and second electrical contact means making electrical contact when the probe is in the rest position, the probe element on making a contact when in use moving from the rest position thereby altering the electrical contact.

The rigid insulating material is preferably ceramic.

The expression "altering the electrical contacC includes breaking the contact between the first and second contact means and also varying a parameter of the electrical contact, such as the conductance between the 15 first and second electrical contact.

An advantage of the contact sensor of the present invention is that it is mechanically simpler than prior art sensors, and therefore cheaper to manufacture.

The ceramic element may be in the form of an annulus. This provides a convenient shape as will be appreciated from the remaining structure of the contact sensor.

The first electrical contact is preferably provided by a plurality of electrically conducting balls positioned on the surface of the ceramic element. Such balls offer a convenient surface on which the second electrical contact means may locate.

3 Six balls may be provided, preferably in pairs. There may be three pairs of balls. Such a configuration provides a convenient way of operating the sensor.

The two balls in each pair should be positioned so that there is a gap between them. The gap is bridged by the second electrical contact means. The arrangement is such that when the probe element is in the rest position an electrical conduction path (perhaps of a known conductance) is provided from one ball in the pair to the other ball in the pair via the second electrical contact means. Movement of the probe means may vary the conductance of the conduction path. This provides a simple yet effective structure for the contact sensor.

The pairs of balls may be provided at between 80' to 160' spacings around the annular ceramic element. Preferably the pairs of balls are provided at between 110' to 130 spacings around the annular ceramic element. Most conveniently the pairs of balls are located at substantially 120 spacings around the ceramic annular element.

Most preferably a ceramic element electrical conduction path is provided which connects adjacent the pairs of balls when the probe element is in the rest condition (that is with the gap between each ball in a pair bridged by the second electrical contact means).

The conductance of the ceramic element electrical conduction path may be adapted to vary as the probe element moves. This provides a parameter which can be monitored, simply, to detect a contact of the probe means.

4 Preferably the ceramic element conduction path is provided by electrical conductors which are positioned on a surface of the ceramic element.

Advantages of using a ceramics material are that it is thermally very stable, non-absorptive, relatively inexpensive to manufacture in quantity and is an electrical insulator. Additionally some ceramic materials can be machined easily The electrical conductors may be applied by known techniques, e.g.printing, on the surface of the ceramic element.

The probe element may comprise an upper, preferably frusto conical, portion and a lower elongate portion. The elongate portion, in use, contacts the surface of a work piece, the upper portion may be provided with means to interact with the ceramic element.

The means to interact with the annular element preferably comprises a number of cylindrical elements extending outwardly from the upper portion of the probe element. Three such elements may be provided which interact with the pairs of balls. Most preferably there are an equal number of cylindrical elements and pairs of balls provided.

Conveniently the cylindrical elements extend so that they overlie an upper surface of the annular ceramic element. This provides a structure in which the cylindrical elements can contact the balls relatively easily.

Preferably the probe element is supported by the cylindrical elements resting on and between each of the balls in pairs thereby ensuring good electrical contact is maintained between the first electrical contact means (the balls) and the second electrical contact means (the cylindrical elements).

Preferably a housing contains all or substantially all of the components of the contact sensor. This provides a sensor which is robust and self contained.

The biasing means may be a biconical coil spring, acting between the probe elements upper surface and the housing in a direction normal to the plane of the ceramic annular to bias the probe element to the rest position.

Most preferably electrical connection means are provided on the housing. This provides a simple way of connecting the conduction path of the sensor to an outside device. The electrical connection means may comprise two electrical contacts.

According to a second aspect of the invention there is provided a sensor system comprising a movement means (for example a robot arm), a processing means and a contact sensor according to the first aspect of the invention.

An embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

Figure 1 shows an exploded diagrammatic illustration of the essential components of a prior art contact sensor;

6 Figure 2 shows an exploded diagrammatic illustration of the essential components of a contact sensor according to the invention; and Figure 3 shows a plan view of an annular ceramic element used in the contact sensor according to the invention.

Referring to Figure 1, at the centre of the contact sensor is an anodised aluminium annulus 2. On an upper surface of the annulus 2 there are provided three pairs of conducting balls (six balls 16,18,20,22,24,26 in total) spaced at 120' intervals around the annulus 2.

The pairs of balls are arranged so that there is a gap between each of the balls in the pair.

A frusto conical portion 30 of a probe extends through the centre of the annulus 2. Near to an upper edge portion of the frusto conical portion 30 are provided three cylindrical elements (two of which 32,34 can be seen in Figure 1). These cylindrical elements are arranged at 120 spacing around the perimeter of the frusto conical portion 30 and contact the balls 16,18,20,22,24,26 on the annulus 2.

An elongate portion 28 forming the probe contact element extends from the apex of the frusto conical portion 30.

Six coil springs 4,6,8,10,12,14 are provided, one in association with each of the balls 16,18,20,22,24,26. The coil springs 4,6,8,10,12,14 are formed of a conductive material. In the assembled contact sensor a lower end portion of each of the springs 4,6,8,10,12,14 7 rests on one of the balls 16,18,20,22,24,26. An upper end portion of each of the springs 4,6,8,10,12,14 contacts a conducting annulus 36 having surface parallel to the surface of the annulus 2.

Two wires 38,40, or other conductors, are provided for electrical connection to the conduction annulus 36 which provide an output from the sensor.

A biconical spring 42 is provided as a biasing means and extends from a housing member 43 to an upper surface of the frusto conical portion 30. The spring 42 ensures that the frusto conical portion 30 is biased to a rest portion.

In use, an electrical path is formed which starts at wire 38 passes round the conduction annulus 36, through spring 8, ball 20, the corresponding cylindrical element (not shown), ball 22, spring 10, conduction annulus 36, spring 12, ball 24, cylindrical element 34, ball 26, spring 14, conduction annulus 36, spring 4, ball 16, cylindrical element 32, ball 18, spring 6, conduction annulus 36 and finishes at wire 40.

The necessary conductors to provide this path are formed on the conduction annulus 36.

In a rest position the three cylindrical elements are in equal contact with their respective pairs of balls. This provides an electrical conductance around the above circuit.

As the elongate portion 28 of the probe means is moved (by contacting an object) the frusto conical portion of the probe means moves 8 and so varies the contact force between at least one of the pairs of balls and respective cylindrical elements. This varies the electrical conductance around the circuit and so contact can be detected.

At the heart of the prior art contact sensor is the anodised aluminium annulus 2. It is vital for the operation of the prior art sensor that the anodised coating provides a good insulating coating to the aluminium. This is both expensive and difficult to provide.

Referring now to Figure 2, an annulus 50 formed from a machinable ceramic sold under the trade mark BIACORS by Corning, Inc., (of course, other suitable ceramics may be used) is provided with three pairs of conducting balls 52,54,56,58,60,62 (first electrical contact means) spaced at 120' intervals on the upper surface thereof.

A frusto conical portion 64 of a probe element extends through the centre of the annulus 50. As with prior art sensor there are three cylindrical elements provided at an upper edge portion of the frusto conical portion 64 (only two of these cylindrical elements 66,68 are shown in Figure 2). The cylindrical elements are provided at 120 intervals around the perimeter of the frusto conical portion 64.

An elongate portion 70 of the probe element extends downwardly from the lower portion of the frusto conical portion 64.

The working parts are enclosed in a housing (not shown) with the elongate portion 70 protruding from a bottom face of the housing.

On the upper surface of the ceramic element 50 there are provided conductors 72,74,76,78 (Figure 3). These conductors are arranged so that 9 the pairs of balls can be electrically connected to each other. Conductors 72,74,76,78 are formed on the upper surface of the ceramic annulus 50 to provide a conductive path between adjacent pairs of balls, a nonconductive zone 90,91,92 remaining between the balls in each pair.

A biconical coil spring 80 (biasing means) is provided which extends from a housing member 81 to an upper surface of the frusto conical portion 64 and biases the frusto conical portion to a rest position. In other embodiments a spring other than a biconical spring may be used.

Two conductive elements, e.g.wires 82,84, (other conductors, such as springs may be used) are provided in contact with the conductors 72, 78 and are attached to the conductors using a conductive adhesive, e.g. silver loaded epoxy adhesive (other conducting adhesive means for example solder, are equally applicable as will be appreciated by the skilled person).

The balls 52,54,56,58,60,62 are adhered to indentations in the surface of the ceramic annulus 50 using epoxy loaded adhesive. Whilst the adhesive is still pliable, the frusto conical portion 64 together with the cylindrical elements may be supported by the pairs of balls. This allows the balls to seat so that they are in a position to ensure good electrical contact between the balls and the cylindrical elements. It will be appreciated that the balls 52,54,56,58,69,62 must be mechanically attached to the ceramic whilst electrically attached to the conductors 72, 74,76,78. The silver loaded epoxy allows this to be achieved using a single operation, although other means of achieving the desired result are equally possible.

The cylindrical elements each rest between and in electrical contact with a pair of balls. This interaction supports the frusto conical and the elongate portions of the probe.

The contact between the three cylindrical elements and the three pairs of balls completes an electrical path starting at wire 84, passing through conductor 72, ball 54, cylindrical element (not shown), ball 52, conductor 74, ball 62, cylindrical element 68, ball 60, conductor 76, ball 58, cylindrical element 66, ball 56, conductor 78 to end at wire 82.

Problems arose in prior art sensors because the annulus was metallic, e.g. aluminium, which needed to be covered with an insulating coating. By providing an annulus from an insulating material it is not necessary to coat the annulus with an insulating material and, indeed, the surface may be partially coated with a conductor.

It will be appreciated that good electrical contact must be made between each of the balls and the conductor associated with it.

The probe is used in a similar way to the prior art sensor. As elongate portion 70 is contacted, it displaces fractionally, causing the frusto conical portion 64 to move. Movement of the frusto conical portion causes at least one of the cylindrical elements to move with respect to its pair of balls. This movement of the cylindrical element changes the conductance of the circuit described above, and so contact can be detected.

It should be noted that the cylindrical elements do not necessarily loose contact with the balls, but generally the conductance of the 11 electrical path through the balllcylindrical element system is varied. However, the cylindrical element may loose contact with the balls if displacement of the elongate portion is sufficient.

At a top portion of the housing a single externally threaded member is provided which is designed to be received by a complementary internally threaded portion. Of course, this housing arrangement is not the only possible technique. Electrical contacts are provided on top of the externally threaded member which are connected to the wires 82,84.

Complementary contacts are also provided at a top of the internally threaded portion.

The internally threaded portion will generally be provided on the end of a robot arm, XY table, etc. To use the probe a user simply screws the externally threaded member mounted on the probe into the internally threaded portion.

This not only mechanically locates the probe but also completes the electrical connection to the probe.

Claims

12 CLAIMS

1. A contact sensor comprising a probe element moveable in relation to electrical contacts within the sensor, a biasing means, and first and second electrical contact means, in which the biasing means is adapted to bias the probe element to a rest position, the first electrical contact means is mounted on a rigid insulating material element in proximity to the probe element, the second electrical contact means is mounted in association with the probe element for movement therewith, the first and second electrical contact means making electrical contact when the probe is in the rest position, the probe element on making contact when in use moving from the rest position thereby altering the electrical contact.

2. A contact sensor according to claim 1 in which the rigid insulating material is ceramic.

3. A contact sensor according to claim 1 or claim 2 in which the rigid insulating material element is in the form of an annulus.

4. A contact sensor according to any preceding claim in which the first electrical contact is provided by a plurality of electrically conducting balls positioned on the surface of the rigid insulating material element.

5. A contact sensor according to claim 4 in which six balls are provided.

6. A contact sensor according to claim 4 or 5 in which three pairs of balls are provided.

13 7. A contact sensor according to claim 6 in which the pairs of balls are provided at between 80 and 160 spacings around the rigid insulating material element.

8. A contact sensor according to claim 6 or 7 in which the pairs of balls are provided at between 110 to 130' spacings around the rigid insulating material element.

9. A contact sensor according to any of claims 6 to 8 in which the pairs of balls are provided at substantially 120' spacings around the rigid insulating material element.

10. A contact sensor according to any of claims 6 to 9 in which a pair of balls is positioned so that a gap is provided between the balls of the pair.

11. A contact sensor according to claim 10 in which the gap between the balls of the pair are bridged by the second electrical contact means.

12. A contact sensor according to any of claims 6 to 11 in which a rigid insulating material element conduction path is provided which connects adjacent balls when the probe element is in the rest condition. 13. A contact sensor according to claim 11 in which the conductance of 25 the rigid insulating material element conduction path is adapted, in use, to vary as the probe element moves. 14. A contact sensor according to claim 12 or claim 13 in which the rigid insulating material element conduction path is provided by electrical 14 conductors which are provided on a surface of the rigid insulating material element.

15. A contact sensor according to any preceding claim in which the probe element comprises an upper, frusto conical portion and a lower elongate portion.

16. A contact sensor according to claim 15 in which the upper frusto conical portion is provided with means to interact with the rigid insulating 10 material element.

17. A contact sensor according to claim 16 in which the means to interact with the rigid insulating material element comprises a number of cylindrical elements extending outwardly from the upper portion of the probe element.

18. A contact sensor according to claim 17 in which the cylindrical elements extend so that they overlie an upper surface of the rigid insulating material element.

19. A contact sensor according to claim 17 or 18 in which the probe element is supported by the cylindrical elements as they rest on the first electrical contact means.

20. A contact sensor according to any preceding claim in which a housing is provided to contain all or substantially all of the components of the contact sensor.

21. A contact sensor according to any preceding claim in which the 30 biasing means is a biconical coil spring.

22. A contact sensor substantially as described herein with reference to the accompanying drawings.

23. A sensor system comprising a movement means, a processing means and a contact sensor according to any of claims 1 to 22.

24. A contact sensor according to claim 1 substantially as herein described with reference to Figures 2 and 3 of the drawings.