GB2118741A - Electrically variable linkage joint - Google Patents

Abstract

A cylindrical housing (1) divided into two chambers (2 and 3) filled with electro-rheological fluid, contains an angularly deflectable shaft (6) having two axial vanes (9 and 10) which extend into the two chambers respectively. The fluid at the two sides of each vane is in parallel connection with the two ends of a respective one of two electrically variable flow restrictors (14 and 15) arranged in series flow connection in an electro- rheological fluid circuit. Electrical variation of each flow restrictor varies the pressure drop across each respective vane and hence the torque applied to the shaft. The two torques are arranged to act in opposition and control voltages are applied to the flow restrictors by a servo-control circuit so as to angularly deflect the shaft in response to the difference between an external demand signal and an angular position signal. <IMAGE>

Description

SPECIFICATION Electrically variable linkage joint This invention relates to an electrically variable linkage joint which is fluid powered and suitable for use, for example, in an articulated arm of an automated manipulating device, i.e. industrial robot, or in a prosthetic apparatus such as an artificial limb.

Numerous electro-mechanical actuators are known for use in automated manipulative processes, which are readily adaptable to computer control, but these actuators normally demand a fairly heavy current supply. Hydraulic and pneumatic actuators are also known for such processes but these are relatively slow acting and require additional electro-hydraulic/pneumatic valve systems for interfacing with a computer.

Some faster responding electro-hydraulic control devices are known which employ the non Newtonian flow properties imparted to an electrorheological fluid, i.e. a slurry of finely divided hydrophilic solids in a hydrophobic liquid, when it is subjected to an electric field, typically of the order of 2 to 3 kV/mm. Under the influence of such electric field the fluid behaves approximately as a Bingham plastic resisting all shear stresses below a given yield-point determined by the value of the field. Shear stresses in excess of the yieldpoint cause shearing to occur at a rate roughly proportional to the difference between the applied stress and the yield-point.This behaviour therefore permits rapid adjustment of flow rate by relatively low power electrical means, a typical control device being an electro-rheological valve or flow restrictor, either of which comprises an array of laminar electrodes between which the electro-rheological fluid flows and across which variable voltages are applied so as to vary the shear resistance of the fluid and hence to vary the pressure drop engendered by the device.

It is an object of the present invention to use the properties of an electro-rheological fluid to provide a fast response linkage joint which can be readily interfaced with an electronic control system.

In accordance with the present invention, an electrically variable linkage joint includes: a housing containing at least one axial partcylindrical chamber; a rotatable shaftjournalled in the housing coaxially with the chamber, which shaft carries an axial vane extending throughout the chamber thereby to sectorially divide the chamber into a pair of sub-chambers of reciprocally variable cross-section; an electrorheological fluid circuit including an electrorheological flow restrictor each end of which is arranged in fluid communication with a respective one of the two sub-chambers; an angular position sensor disposed in association with the rotatable shaft and providing an actual position voltage signal indicative of the angular position of the shaft; and an electronic control circuit so connected and arranged as to continuously measure the difference between the actual position voltage signal and an externally provided demand position voltage signal and to apply, in response to that difference, control voltages to each flow restrictor so as to reduce the difference to zero.

.Conveniently, the housing contains two axial part-cylindrical chambers mutually disposed about a single rotatable shaft which carries two axial vanes each extending into a respective one of the two chambers, thereby to define two pairs of sub-chambers, each pair being connected in opposing sequence across a respective one of a pair of electrorheological flow restrictors connected in series in the fluid circuit thereby to provide that the torques imposed upon the shaft by the differential pressure of each restrictor via the two vanes respectively, are in opposition.

In operation, the net torque acting upon the rotatable shaft of this arrangement is determined by the magnitude of the difference existing between pressure drops engendered in the fluid circuit by the two flow restrictors when subject to electric fields, the direction of vane displacement being dependent upon the sign of that difference.

Preferably the control voltages applied to the two flow restrictors by the control circuit are varied reciprocally, thereby to ensure that no variation of overall flow rate in the fluid circuit occurs and hence to permit a plurality of separate joints to be used in series flow connection in a single fluid circuit.

Embodiments of the invention will now be described by way of example only, with reference to the accompanying drawings of which Figures 1 and 2 are diagrammatic and partcutaway axial sections respectively of a first embodiment of the linkage joint having static fluid chambers, Figure 3 is a flow diagram representing the operation of the linkage joint illustrated in Figures 1 and 2, Figure 4 is an exploded view of an exemplary practical arrangement of the embodiment of Figures 1 and 2, indicating a method of manufacture, and Figure 5 is a diagrammatic representation of a second embodiment having continuous fluid flow through the chambers.

The linkage joint illustrated in Figures 1 and 2 includes a cylindrical housing 1 which is subdivided into two half cylindrical chambers 2 and 3 by opposed diametral webs 4 and 5.

A rotatable shaft 6 coaxially journalled in the housing 1 via sealed bearings 7 and 8 is in slideable contact with the inner radial extremities of the webs 4 and 5. The shaft 6 carries two axial vanes 9 and 10, the outer radial extremities of which are in slideable sealing contact with the housing 1 , thereby dividing the chambers 2 and 3 into sub-chambers 2a, 2b, and 3a, 3b, respectively.

The vanes 9 and 10 are provided with fluid transfer ports 11 and 1 2 respectively which inter communicate via a set of ports 13 transversely through the shaft 6.

A conjoint pair of U-shaped electro-rheological flow restrictors 14 and 1 5 each having an inlet end a and an outlet end b, and each containing a stack of parallel electrode plates 16, are attached to the housing 1, each symmetrically bridging the diametral web 5.

An inlet port 17 is provided at the inlet end 1 4a of the restrictor 14, which end also communicates with the chamber 2b via a fluid transfer port 1 8 in the cylindrical wall of housing 1.

The outlet end 1 4b of the restrictor 14 communicates with the adjoining inlet end 15a of the restrictor 1 5 and also with the chamber 3a via an extended fluid transfer port 1 9 in the cylindrical wall of the housing 1. Chamber 3a is of course also in fluid communication with the chamber 2a via the ports 11,12 and 13.

An outlet port 20 is provided at the outlet end 1 sub of the restrictor 15, which end also communicates via a fluid transfer port 21 with an annular gallery 22 surrounding the housing 1 and providing fluid communication with the chamber 3b via a fluid transfer port 23.

An electrical control circuit 24 is connected and arranged to receive an input voltage signal indicative of the angular position of the shaft 6 from a position sensor 25 disposed in association with the shaft, and to apply control voltages derived from a low current, 3 kV source 26, to alternate electrodes 1 6a of the two flow restrictors 14 and 15, via connectors 27 and 28 respectively, in response to a position demand signal D, so as to rotate the shaft to the demanded position. The remaining alternate electrodes 16b are connected, with the housing, to a common earth.

Operation of the linkage joint will now be described with reference to the symbolic flow diagram depicted at Figure 3. Electro-rheological fluid continuously pumped through the two restrictors 14 and 1 5 in series enters the flow restrictor 14 at pressure p1 and leaves the restrictors 14 and 1 5 at pressures p2 and p3 respectively. The chambers 2a, 2b, 3a and 3b are all primed with electro-rheological fluid and consequently the pressures of the flow line are communicated to them, p, to chamber 2b, p2 to chambers 2b and 3a and p3 to chamber 3b.A force proportional to p1-p2, i.e. the pressure differential engendered by the restrictor 14, therefore tends to rotate the vane 9 in an anticlockwise direction (as drawn), and a force proportional to p2-p3, i.e. the pressure differential engendered by the restrictor 15, tends to rotate the vane 10 in a clockwise direction (as drawn). As these two rotating forces act upon the shaft 6 in opposing directions no net rotation of the shaft will occur when the pressure drop is the same in both restrictors.

In operation, the flow resistance of the two restrictors are varied reciprocally by the voltages applied to their electrodes by the control circuit 24 so as to maintain the total pressure drop p1- p3 constant, thereby ensuring a constant flow rate in the fluid circuit. The direction and amplitude of the resulting rotation imparted to the shaft 6 is of course dependent upon the sign and value of (p1-p2)-(p2-p3).

The maximum angular deflection of this embodiment is approximately + or 60o. If greater deflection is required however the two part-cylindrical chambers 2 and 3 and the vanes 9 and 10 can be disposed in axial alignment within the housing so that the contained angle of each chamber can approach 3600, thus permitting an angular deflection of up to about +1500. This end-to-end arrangement inevitably has considerably higher frictional losses than the twin half cylinder arrangement.

The embodiment of Figures 1 to 3 has the advantage of being robust, compact and lightweight unit which can be housed within an articulated manipulative arm, thus permitting a series of such joints to be mounted in the arm so as to provide multi-directional movement at the manipulative tip. The joints may be interconnected for series fluid flow thus minimising pipe work and requiring connection to a single fluid pump only.

The linkage joint is also applicable to an artificial limb, in which application it is envisaged that the necessary control voltages for application to the flow restrictors could be picked up from available human body potentials.

A simple method of manufacturing the linkage joint of Figures 1 and 2 will now be described with reference to Figure 4. The U-shaped flow restrictor pair 14 and 1 5 is assembled from a length of extruded or milled channelling 30 having four symmetrically disposed rectangular grooves 31, 32,33 and 34 defined by two side walls 35, a cross-web 36 and inner walls 37.

Earth and live electrode plates 38 and 39 respectively are alternately stacked in each groove parallel with the cross web 36, the earth plates 38 being the full width of the groove and engageable at their edges with the side wall 35 and the inner wall 37, and the live plates 39 being slightly narrower and having their edges covered with a channelled strip 40 of electrically insulating material, e.g. PTFE, which engages with the side and inner walls 35 and 37 respectively and also with the two neighbouring earth plates 38, thereby to define the inter-electrode spacing.

Each earth and live plate is provided with an end tab 41 and 42 respectively, disposed in staggered relationship so as to permit separate electrical interconnection between each earth plate and between each live plate of a stack.

The four stacks of plates are held in place by top and bottom cover plates 43 and 44, and by end plates 45 and 46. Insulating material blocks 47 to 54 are inserted at the ends of the stacks, which blocks are appropriately perforated to provide the necessary fluid transfer ports and flow paths already described with reference to Figures 1 and 2.

The end plate 46 is perforated to co-operate with the ports in the blocks 47 to 50 and mates directly with a correspondingly perforated face 55 provided on the cylindrical housing 1.

This method of manufacture is simple and has the advantage that the whole assembly can be easily dismantled for cleaning if and when any silting occurs.

If settlement of solid particles from the specific electro-rheological fluid to be used is likely to cause excessive silting however, it is preferable to interconnect the chambers and flow restrictors of the linkage joint so as to ensure a continuous flow of fluid throughout. A second embodiment of the linkage joint adapted for continuous flow is illustrated in Figure 5.

In this embodiment, the diametral webs 4 and 5 and the rotatable axial vanes 9 and 10 of the first embodiment define the chambers 2a, 2b, 3a and 3b within a cylindrical housing 60. The housing 60 is provided with an inlet port 61 to the chamber 2b and an outlet port 62 from the chamber 3b.

Two U-shaped electrorheological flow restrictors 63 and 64 are interconnected with the housing 60, the restrictor 63 being in fluid communication with the chambers 2b and 3a via ports 65 and 66 respectively, and the restrictor 64 being in fluid communication with the chambers 2a and 3b via ports 67 and 68 respectively.

Operation of this embodiment is identical with that of the first embodiment, but static fluid is eliminated from the chambers by fluid flushing through each chamber in turn, as indicated by the flow line drawn in Figure 5.

Claims (6)

Claims

1. An electrically variable linkage joint including a housing containing at least one axial partcylindrical chamber, a rotatable shaftjournalled in the housing coaxially with the chamber, which shaft carries an axial vane extending throughout the chamber thereby to sectorially divide the chamber into a pair of sub-chambers of reciprocally variable cross-section, an electro-rheological fluid circuit including an electro-rheological flow restrictor each end of which is arranged in fluid communication with a respective one of the two sub-chambers, an angular position sensor disposed in association with the rotatable shaft and providing an actual position voltage signal indicative if the angular position of the shaft, and an electronic control circuit so connected and arranged as to continuously measure the difference between the actual position voltage signal and an externally provided demand position voltage signal and to apply, in response to that difference, control voltages to the flow restrictor so as to reduce the difference to zero.

2. A linkage joint as claimed in Claim 1 wherein the housing contains two axial partcylindrical chambers mutually disposed about a single rotatable shaft which carries two axial vanes each extending into a respective one of the two chambers, thereby to define two pairs of subchambers, each pair being connected in opposing sequence across a respective one of a pair of electrorheological flow restrictors connected in series in the fluid circuit, thereby to provide that the torques imposed upon the shaft by the differential pressure of each restrictor via the two vanes respectively, are in opposition.

3. A linkage joint as claimed in Claim 2 wherein variable control voltages are applied recipirocally to the two flow restrictors by the control circuit.

4. An electrically variable linkage joint substantially as hereinbefore described with reference to the accompanying Figures 1 to 3.

5. An electrically variable linkage joint substantially as hereinbefore described with reference to the accompanying Figure 4.

6. An electrically variable linkage joint substantially as hereinbefore described with reference to the accompanying Figure 5.