GB2223315A

GB2223315A - Tactile sensor for robot system

Info

Publication number: GB2223315A
Application number: GB8920150A
Authority: GB
Inventors: Ryszard Tadeusz Rakowski; Payman Adl
Original assignee: Brunel University
Current assignee: Brunel University
Priority date: 1988-09-06
Filing date: 1989-09-06
Publication date: 1990-04-04
Anticipated expiration: 2009-09-06
Also published as: GB8920150D0; GB2223315B; GB8820889D0

Abstract

A tactile sensing assembly for detecting relative movement in a slip force sensing apparatus of a robot comprises first and second adjacent layers (20, 10), the first layer (20) comprising a plurality of magnetoresistive elements (21) and the second layer (10) comprising one or more conductive members (12) having an alternating current passing therethrough. Relative movement of the layers e.g. by application of force on the second layer (10) causes a resistance change in the magnetoresistive elements indicative of the magnitude and direction of the movement. The layers may be separated by a layer (30) of flexible material. The magnetoresistive elements may comprise an elongate strip folded to form a non-inductive, serpentine shaped structure. The elements (21) may be provided in units of four with the elements being connected in bridge circuits. A constant bias field may be applied to the magnetoresistors by passing a direct current therethrough to linearise the response. <IMAGE>

Description

TACTILE SENSOR FOR ROBOT SYSTEMS This invention relates to a tactile sensor for robot systems.

In order to be able to automatically manipulate a delicate article, e.g. to pick it up in a gripper, the latter must be able to sense when it has applied a certain gripping force. It should also be able to sense when an article is 'slipping' through its grip. Various forms of 'skin' have been proposed, e.g. Canadian Patent 1223936.

It is the aim of the invention to provide a tactile sensor which is improved compared with known forms, and which in particular is more sensitive to 'slip'.

According to invention there is provided a tactile sensing apparatus comprising an alternating current power source, resistance measurement means, and a sensing assembly comprising first and second adjacent layers, the first layer comprising an electric circuit incorporating a plurality of magnetoresistive elements connected to said resistance measurement means and the second layer comprising one or more conductive elements that are supplied with an alternating current by said power source, thereby producing an alternating magnet field, wherein relative movement of the layers causes a resistance change in the magnetoresistive elements that is indicative of the magnitude and direction of the movement.

Preferably, the sensing assembly comprises one or more units of four magnetoresistors arranged with their easy axes of magnetisation at right angles to those of their adjacent neighbours in the unit. The conductive elements comprise elongate members passing over the magnetoresistors.

In a first embodiment of the invention each of the magnetoresistors comprises an elongate magnetoresistive strip, having the easy axis of magnetisation along its own major axis, folded back and forth to form a serpentine shaped structure.

In a further embodiment of the invention each unit comprises four magnetoresistors arranged such that their easy axes of magnetisation form a square.

In order that the invention and its various other features may be understood more easily, embodiments thereof will now be described, by way of example only, with reference to the drawings wherein: Figure 1 is a plan view of a tactile sensing device in accordance with a first embodiment of the present invention; Figure 2 is a cross sectional view on II-II in Fig.l; Figure 3 shows the static characteristic behaviour of a magnetoresistor of the type incorporated in a sensor in accordance with the first embodiment of the invention; Figure 4 is a plan view of a tactile sensing device in accordance with a second embodiment of the present invention; and Figure 5 shows the static characteristic behaviour of magnetoresistors of the types incorporated in a sensor in accordance with the second embodiment of the invention.

The Tactile sensing device in Figures 1 and 2 comprises a conductive skin 10 which overlays a bridge sensor 20.

The bridge sensor 20 comprises four identical sensing elements 21 connected in a resistance bridge configuration. Each sensing element comprises a strip 22 of a magnetoresistive material and two metallic connectors 23 (Figure 2). The easy axis of magnetisation of the strips 22 is coincident with their major axis.

A magnetoresistive material has the property that its electrical resistance changes when an external magnetic field is applied to it.

The magnetoresistive strip 22 is folded back and forth upon itself to produce a magnetoresistor 24 that is square in shape. Metallic connectors 23 are joined to the ends of the magnetoresistive strip 22 and the magnetoresistor 24 is configured so that the metallic connectors 23 are at opposite corners of the magnetoresistor 24 when viewed from above. (Fig.l) To form the bridge sensor 20 four sensing elements 21 are joined so that the four magnetoresistors 24 form a square when viewed from above. The magnetoresistive strips 22 in adjacent sensing elements 21 are mutually at right angles to each other. The sensing elements 21 in the bridge sensor 20 are separated from each other by a thin layer of an insulating material 25.The metallic connectors 23 in the bridge sensor 20 are connected to a measurement system which measures the resistance of the sensing elements 21 in the bridge sensor 20. The bridge sensor 20 is overlaid by a layer of flexible material 30 that separates the bridge sensor 20 from the conductive skin 10 (Fig.2).

The conductive skin 10 is made in the form of a flexible printed circuit board 11. The electrical circuitry on the conductive skin 10 carries an alternating electric current to the overlay conductor 12 which is embedded in the conductive skin 10. On the surface of the conductive skin 10 is a circular, ribbed rubber pad 13.

In operation of the device, the bridge sensor is fixed to a rigid surface 40. When a normal force is applied to the conductive skin 10 the overlay conductor 12 will be displaced towards the bridge sensor 20.

Associated with the alternating current in the overlay conductor 12 is an alternating magnetic field. The intensity of this field decreases as the radial distance from the overlay conductor 12 increases. Hence, when a normal force is applied to the conductive skin 10 the magnetic field intensity at the bridge sensor 20 will increase and this will change the resistance of the four magnetoresistors 24. This change in resistance can be measured by passing a direct current through the magnetoresistors 24. A pressure normal to the skin will tproduce an equal change in resistance in each magnetoresistor 24. The larger the normal force applied the greater will be the change in resistance of the magnetoresistors 24, but the bridge balance will be unaffected.

If a force is applied to the conductive skin 10 at right angles to the overlay conductor 12 and parallel to the plane of the skin (a 'slip' force) the overlay conductor 12 will be displaced so that its major axis is no longer above the centre 25 of the bridge sensor 20.

This will result in a decrease in magnetic field intensity at the two magnetoresistors 24 that the overlay conductor 12 has been displaced away from and an increase in magnetic field intensity at the two magnetoresistors 24 that the overlay conductor 12 has been displaced towards. This will produce a measurable change in the resistance balance of the bridge sensor 20, the magnitude of this change being proportional to the magnitude of the applied slip force.

Forces applied in directions between right angles to the overlay conductor. 12 and the direction of the overlay conductor 12 will also produce resistance changes in the magnetoresistors 24. Measurement of the magnitude of these resistance changes and the difference in resistance changes between different magnetoresistors 24 in the sensing bridge 20 permits computation of the magnitude and direction of the applied force. The ribbed rubber pad 13 on the surface of the conductive skin 10 allows objects to grip the tactile sensing device and apply a slip force to it.

During the operation of the device the alternating current in the overlay conductor 12 produces a magnetic field whose magnetic field intensity varies sinusoidally. This means that the resistance of the magnetoresistors 24 changes with time in a cyclic manner. The resistance values measured in the bridge sensor 20 are the peak values which correspond to the peak magnetic field intensity values. Displacement of the overlay conductor 12, relative to the bridge sensor, causes a change in the peak resistance values of the magnetic resistors 24 that is indicative of the magnitude and direction of the force applied to the conductive skin 10.

If the resistance of a single magnetoresistor 24 is measured for a number of different applied magnetic field intensities a graph similar to that in Fig.3 is formed. A region of the graph is linear and it is desirable to operate the magnetoresistor 24 in this region. Then, the change in resistance of the magnetoresistors is directly proportional to the change in intensity of the applied magnetic field. Because of this, operating in this region makes computation of the applied forces simpler than if operation takes place on a non-linear part of the graph.

To do this it is necessary to apply a residual fixed intensity magnetic field to the magnetoresistor 24.

This will make the mean intensity of the oscillating magnetic field at the magnetoresistor 24 equal to that at the bias point in Fig.3. This residual magnetic field is formed by the direct current that is passed through the magnetoresistor 24 to measure its resistance.

The serpentine configuration of the strip 22 is adopted so that the alternating magnetic field produced by the overlay conductor 12 does not induce an alternating current in the magnetoresistors 24 that would make measurement of their resistances difficult.

To form a robot skin, a number of tactile sensors, e.g. four, are positioned in an array in an area approximately lcm x 1cam. The sensors themselves have dimensions of about 400 m x 400 m.

An alternative form of tactile sensor is shown in Fig.4. In this case the connectors 50 are shown crosshatched.

The four magnetoresistors 51-54 are very short and wide; they do not employ a meandering path as in Fig.l. The magnetoresistors are fabricated with their axes of easy magnetisation at +45 or -45 degrees to the overlay conductor (not shown in drawing). Each magnetoresistor 51-54 has dimensions of about 90 m x 30 m, and a thickness of about 50-80nm.

The magnetoresistors 51-54 may be excited using a sinusoidal or switched AC supply and this determines the type of signal processing that may be employed. In either case the sense field H5 is provided in the form of an AC current Is flowing through an overlay conductor, as in Fig.l.

The static characteristics of both types of magnetoresistor are shown in figure 5. The response curves +45, -45 show the saturation limit of the magnetoresistors to be approximately 2 Oersteds, much lower than that of the magnetoresistors 24 in the first embodiment of the invention. Over an applied field range between +2 and -2 Oersteds the response of the sensor is approximately linear. The response curves pass through the origin, and this eliminates the need for an applied bias field.

The advantages of this design are as follows; Magnetoresistors with an easy axis of magnetisation of 45 degrees and those with an easy axis of -45 degrees are complementary. A differential pair of these used in a bridge circuit eliminates the steady component of the magnetic field and is immune to the temperature changes of thin film resistance. The double magnetoresistors 51-54 also offer a linearity of +1%, far superior to the linearity of a single element.

The thickness to Width ratio of 5 x 10-4 to 9 x 10-4 makes the magnetoresistors less sensitive than normal read heads and provides further immunity to surrounding magnetic fields.

The magnetoresistors are fabricated in clean room conditions using RF sputtering technics to deposit thin films onto Borosilicate substances. These are then etched into sensor patterns using relevant mask aligning, developing and etching methods.

Claims

1. A tactile sensing apparatus comprising an alternating current power source, resistance measurement means, and a sensing assembly comprising first and second adjacent layers, the first layer comprising an electric circuit incorporating a plurality of magnetoresistive elements connected to said resistance measurement means and the second layer comprising one or more conductive elements that are supplied with an alternating current by said power source, thereby producing an alternating magnet field, wherein relative movement of the layers causes a resistance change in the magnetoresistive elements that is indicative of the magnitude and direction of the movement.

2. An apparatus according to claim 1, wherein the first layer of the sensing assembly comprises one or more units of four magnetoresistors, wherein the easy axis of magnetisation of each magnetoresistor in said unit is at right angles to the easy axis of magnetisation of the adjacent magnetoresistors in said unit.

3. An apparatus according to claim 1 or 2, wherein said conductive elements comprise elongate members passing over the magnetoresistors.

4. An apparatus according to any preceding claim wherein a constant value bias magnetic field is applied to the magnetoresistors during operation, such that the resistance of the magnetoresistors varies linearly with the intensity of the magnetic field produced by the conductive elements.

5. An apparatus according to claim 4, wherein said constant value bias magnetic field is produced by passing a direct current through the magnetoresistors.

6. An apparatus according to claim 5, wherein said direct current is utilised by the resistance measurement means.

7. An apparatus according to any preceding claim, wherein each magnetoresistive element comprises an elongate magnetoresistive strip, having the easy axis of magnetisation along its own major axis, folded back and forth to form a serpentine shaped structure having substantially zero inductance.

8. An apparatus according to claim 7 when appended to any of claims 2 to 6 wherein the magnetoresistors in said unit(s) are separated by a layer of insulating material.

9. An apparatus according to any of claims 2 to 6, wherein said unit(s) comprises four magnetoresistors arranged such that their easy axes of magnetisation form a square.

10. An apparatus according to any of claims 2 to 9, wherein the four magnetoresistors in said unit(s) are connected in a bridge measurement circuit.

11. An apparatus according to any of claims 3 to 10, wherein the elongate member(s) are at 45 degrees to the easy axes of magnetisation of the magnetoresistors.

12. An apparatus according to any of claims 3 to 11, wherein the elongate member(s) passes over the geometrical centre of a unit.

13. An apparatus according to any of claims 3 to 12, wherein the second layer of the sensing assembly comprises a flexible printed circuit board incorporating said elongate member(s).

14. An apparatus according to claim 13, wherein the second layer of the sensing assembly further comprises one or more flexible pads attached to the surface of the flexible printed circuit board that is remote from the first layer.

15. An apparatus according to any preceding claim, wherein the first and second layers of the sensing assembly are separated by a layer of flexible material interposed therebetween.

16. A tactile sensing apparatus substantially as described herein, with reference to the drawings attached hereto.