GB2184996A - Robot finger - Google Patents

Abstract

A robot gripper finger 7B is formed by a single piece of metal in which a number of holes 12-16 are formed to provide dual guided cantilevers arranged so that they bend and twist in response to X, Y and Z forces and XY, YZ and XZ torques. Suitable strain sensors 17-22 are positioned on the arms of the dual guided cantilevers to detect these forces and torques. The outputs from the strain sensors are fed to six Wheatstone bridges, one for each of forces and torques measured. The outputs from the bridges are multiplexed, converted to digital form and processed to perform force transformations. These calculate the forces and torques in the coordinate system, X, Y, Z which are used to control the robot and gripper. <IMAGE>

Description

SPECIFICATION Robot finger This invention relates to a manipulating mechanism such as a robot finger which alone or together with other fingers is used to manipulate an object.

It is often desirable to obtain measurements of the forces and torques applied to the manipulating mechanism of a robot during manipulation of the object and these measurements can be used to control the robot eg to enable it to operate in a compliant fashion.

Techniques have been proposed in the past where a gripper comprising a number of fingers is attached to a wrist designed to sense torques and forces. Such a system has limitations because it does not produce individual measurements for each finger, which information is useful for grasping operations.

This invention arose in an endeavour to design a robot finger which can measure force in X, Y and Z directions and torque in XY, YZ and XZ directions in an cartesian co-ordinate system.

The aforementioned member can be formed as a single body having holes, each of which defines the two individual arms of a dual guided cantilever on opposite sides of it. A construction made like this can be sufficiently compact and robust to serve the purpose of a robot finger.

It would be possible to use six dual guided cantilevers, three for forces and three for torques; but this makes inefficient use of space.

It is thus preferred that at least one of the dual guided cantilevers should serve a dual function of measuring force and torque. Preferably two of the dual guided cantilevers each serves such a dual function. It would be possible to construct an arrangement having just three dual guided cantilevers each of which measures torque and force but this is not preferred because one of them would, it is believed, have a very low sensitivity to torque.

Thus an arrangement having four dual guided cantilevers is preferred two of which measure both force and torque, one of which measures just torque and one of which measures just force.

One of the dual guided cantilevers arranged to measure a force component (eg FZ) can conveniently be arranged to measure a torque component (eg XZ) about a central axis parallel to plates forming its dual cantilever arms.

This torque twists the plates, putting them under shear, and this shear can be measured using one or more strain gauges on at least one of the plates.

Another of the dual guided cantilevers arranged to measure a force component (eg FX) can also conveniently be arranged to measure a torque component (eg YZ) about an axis perpendicular to and passing through its dual cantilever arms. This torque bends the plates in such a way that they act as two parallel simple cantilevers bending under the influence of the torque. The extent of such bending can of course be measured using a suitably arranged strain sensor or sensors.

A third torque component (eg XY) may be measured by generating a force acting at a fixed distance from the axis of the torque whose magnitude is therefore proportional to the torque. This force can then be measured using a third dual guided cantilever. The force can be generated by two lines of weakness formed opposite sides of a hole. One of these lines of weakness is co-incident with the torque axis and is connected to one end of the dual guided cantilever. The other line of weakness is connected to the other end of the dual guided cantilever. In such an arrangement a strain sensor or sensors are arranged to measure bending of the dual guided cantilever.

The dual guided cantilever is preferably positioned at a corner of an L-shaped finger one arm of which defines the previously mentioned first and second dual guided cantilevers and the other arm of which defines a further dual guided cantilever as will now be described.

A third force component (eg FY) may be measured by a fourth dual guided cantilever arranged so that the force bends it in the normal manner, with a strain induced by such bending.

One way in which the invention may be performed will now be described by way of example with reference to the accompanying drawings in which: Fig 1 illustrates a robot gripper employing the features of the invention; Fig 2 is a perspective view of one finger of the gripper shown in fig. 1; Fig 3 is similar to fig. 2 but shows the finger with its outer sheath removed; Fig 4 is a block diagram showing how outputs from various strain gauges shown on fig 3 are processed; and Figs 5 to 10 show respectively the deformations of the finger of figs 2 and 3 when subjected to forces in directions X, Y & Z of fig 3 and torques YZ, XZ and XY also as indicated in fig 3.

Referring to fig 1 the illustrated gripper comprises a supporting plate 1 which, in use, is bolted to the end point or wrist of a robot.

The plate 1 supports a housing 2 containing a motor (not shown) connected to operate a dual rack and pinion mechanism 3, 4A and 4B, the racks 4A and 4B of which are fixed to brackets 5A, and 5B. The brackets 5A and 5B are secured, by screws (not shown) passing through holes 6, to respective fingers 7A and 7B. In operation the motor drives the fingers 7A and 78 together or apart to grip an object to be manipulated. During such manipulation signals appear on multi-core cables 8A and 8B indicating various forces and torques supplied to each finger. These signals are generated in a manner to be described later and are used to control the robot. The cables 8A and 8B also include cores for the application of supply voltages to sensors within each finger.

Referring now to figs 2 and 3 the illustrated finger comprises a machined metal element 9 which in this particular example is made of aluminium. It has the shape of an inverted letter L and is covered by sheaths 10A and 10B made of a rigid material, in this example steel. Sheath 10A is secured to the element 9 by the same screws, passing through holes 6, and serve to fix the finger 7B to the bracket 5B. Sheath 10B is secured to the element 9 by screws passing through holes 11. Each sheath is spaced from the associated arm of the L shaped element 9 except of course in the region where it is secured to the latter.

The sheaths are also spaced from each other, as shown at L2, to allow some slight relative movement between them due to flexing of the element 9.

Referring now particularly to fig. 3 it will be seen that the element 9 has machined in it, five holes 12, 13 14, 15 and 16. Secured to the surface of this element 9 are a number of strain gauges 17A, 17B, 17C, 17D, 18A, 18B, 18C, 18D, 19A, 19B, 19C, 19D, 20A, 20B, 20C, 20D, 21A 21B 21C, 21D, 22A, 22B, 22C, and 22D. AtI these gauges are simple strain gauges (micromeasurements EA-13- 062TV-350), except for 21A, 21 B, 21C and 21D which are shear strain gauges (micromeasurements EA-13-062AQ-350), and are denoted on Fig 3 by a rectangular shape containing a straight line indicating the strain direction to which they give maximum response.

Gauges 21C and 21A on the one hand and 21B and 21D on the other hand are included in a single unit commercially available for the purpose of measuring shear.

Referring now to fig. 4 the individual gauges are joined together to form six Wheatstone bridges. This is done by means of wires (not shown) lying on the surface of the element 9 (Fig 3). The outputs and voltage supplies to the six bridges lead from and to a connecting device 23 (fig 3) by which connection is made to the cable 8B, which passes through a hole 24. The cable 8B connects the finger 7B to a processing circuit 25 (fig 4). In this circuit the output from each bridge passes through a low pass filter 26 to remove noise picked up in the cable and in the strain gauges themselves.

The resulting filtered signal is amplified at 27 and filtered at 28 to remove noise generated in the amplifier. Six identical separate channels are provided and produce outputs representing the three force and three torque components to be measured. The six output signals are multiplexed at 29, converted to digital form at 30 and processed in a computer 3 1 to perform force transformations. The effect of these transformations is to calculate the forces and torques applied in the particular coordinate system x, y, z shown on Fig. 3, centred at an origin 0. The information thus obtained is used to control the robot and the gripper.

Fig 5, illustrates the effect of a force FX applied in the X direction to the sheath 10B.

The sheath 10B is secured by screws 1 1A fitting in holes 11. These screws and holes connect the sheath 10B to the element 9 at a point which is notably below the hole 13 and, in this particular example, is on a bottom face of the element 9.

The hole 13 defines, with adjacent vertical sides of the member 9, a dual-guided cantilever consisting of two simple cantilevers 13A, 13B in parallel. Under the application of a force in the X direction such as FX as shown in Fig 5 this dual-guided cantilever deforms, as shown in fig. 5 with a characteristic s-shape. The shape arises because the sheath 10B and also the parts of the member 9 above and below the hole 13 are both much stiffer than the cantilevers 13A, 13B. For the particular case shown in Fig 5, strain gauges 17A, 17D are in compression and strain gauges 17B, 17C are in tension. The force FX can be applied at any point on the sheath and cause the same distortion of the dual guided cantilever 13A, 13B.Hence, by measuring the strains at the four points where strain gauges 17A, 17B, 17C and 17D are located the magnitude of the force (independent of its point on application) can be obtained. It is not necessary to measure individually the strains at each of these points. It is sufficient to provide a comparison between the strains at 17A and 17B or at 17C and 17D. It is better however if the comparison is made between both of these pairs of points and this can conveniently be done using the Wheatstone bridge arrangement shown in Fig. 4.

In Fig. 5 the points of application of the force FX is chosen such that a measurement of XY torque, to be described later, is zero.

This however is a special case and, in general, a force in the direction X will produce a non-zero value for such a torque.

Referring to Fig 6 there is shown the effect of a force FY the -Y direction. This acts on a dual guided cantilever defined on either side of the hole 16 so as to put strain gauges 18A, 18B, 18C and 18D into tension and compression in a similar way as described with reference to Fig. 5. The point of application of the force FY does not affect the measurement obtained from the bridge formed by strain gauges 18A to 18D but, as in the case of Fig. 5, the point of application of the force FY illustrated is selected so as to illiminate XZ torque above the Y axis.

Fig. 7 shows the affect of a force FZ applied in the Z direction. This acts on a dualguided cantilever formed by the hole 12. The resulting strain values at the output of strain gauges 1 9A to 1 9D give a measurement in the same way as previously described for the X and Y forces. Again the position of application of the force FZ does not affect the measurement but is illustrated as being at a point where the YZ torque measurement will be zero.

Figure 8 illustrates the effect of a pure YZ torque about a point 0, also shown on fig 3.

Such a torque would occur if equal YZ forces were applied in opposite directions to the sheath at equal distances above and below the point 0. There is thus no net effect on the strain gauges 19A to 19D. The effect of such a torque is to bend the cantilever formed by the whole of the upright leg of the element 9 as shown in fig. 8 putting the strain gauges 20A and 20D in compression and strain gauges 20B and 20C in tension. The strain gauges 20A and 20B are separated as far as possible to produce the greatest output; as are the strain gauges 20C and 20D. It will be noted that, with the strain gauges 20A to 20D in the position shown no output is generated by a force in the X direction. This is because the strain gauges are in the middle of the dual cantilevers formed either side of the hole 13.This however is not considered to be of critical importance and, in alternative arrangements they could be provided at different positions along the height of the member 9. It is however important that 20A and 20B on the one hand and 20C and 20D on the other hand should be at the same height.

Figure 9 shows the effect of a XZ torque about a central axis P passing through the vertical leg of the element 9. It twists the dual cantilever formed by the hole 12 to produce shear forces which are detected by gauges 21A to 21D.

XY torque about axis through point S is measured by an arrangement comprising holes 14 and 15 as shown in fig. 10. These define a dual-guided cantilever composed of simple cantilevers 15A and 15B. The hole 14 has the effect of producing two lines of weakness passing through S and T so that the torque is converted into a force acting at T on the dualguided cantilever 15A, 15B. The deformation of the cantilever produces an output signal at the bridge composed of strain gauges 22A to 22D. These produce an output which is proportional to the aforementioned force which in turn is proportional to the XY torque about point S.

In the illustrated embodiment the X, Y and Z force directions are orthogonal and the XY, YZ, XZ torque directions are the same as the X, Y and Z directions. However these are not essential requirements and a useful result can be obtained with other directions.

Claims (8)

1. A manipulating mechanism including a member which in use contacts an object to be manipulated and is adapted to produce output signals representing X, Y and Z forces and XY YZ and XZ torques applied to it during manipulations of the object, the member comprising a set of dual guided cantilevers three of which are arranged to distort in first fashions in response to the respective X, Y and Z forces and three of which are arranged to distort in second fashions in response to the respective torques; and strain sensors for sensing the six distortions to produce the said output signals.

2. A manipulating mechanism according to claim 1 in which the aforementioned member is a single body having holes each of which defines a dual guided cantilever the arms of which are on opposite sides of it.

3. A manipulating mechanism according to claim 1 or 2 in which each of two of the dual guided cantilevers is arranged to distort in a first fashion and in a second fashion.

4. A manipulating mechanism according to any preceding claim in which one of the dual guided cantilevers, arranged to measure a force component, is also arranged to measure a torque component about a central axis parallel to plates forming its dual cantilever arms.

5. A manipulating mechanism according to any preceding claim in which one of the dual guided cantilevers is arranged to measure a force component and to measure a torque component about an axis perpendicular to and passing through its dual cantilever arms.

6. A manipulating mechanism according to any preceding claim comprising a dual guided cantilever and means for converting a torque component into a force acting at a fixed distance from the axis of the torque on the dual guided cantilever.

7. A manipulating mechanism according to any preceding claim including a dual guided cantilever arranged so that a force component bends it.

8. A robot finger substantially as described with reference to the accompanying drawings and substantially as illustrated therein.