Compliant actuator

Abstract

An actuator (12) includes a moving member (18) pivotally connected to a base unit (14) for rotation about an axis. The driving force for rotation of the moving member (18) relative to the base unit (14) is provided by a pair of antagonistically operating shape memory alloy (SMA) wires (48, 50) and transmitted via a torsional spring (56). An endoscope, or a snake-like robot (66), may include one or more of the actuators (12).

Claims

1. An actuator comprising: a moving member pivotally connected to a base unit for rotation about an axis; wherein the driving force for rotation of the moving member relative to the base unit is provided by a pair of antagonistically operating shape memory alloy (SMA) wires and transmitted via a torsional spring.

2. The actuator of claim 1 wherein the pair of antagonistically operating SMA wires are mounted within the body of the actuator.

3. The actuator of claim 2 wherein the pair of antagonistically operating SMA wires or the bulk of the pair of antagonistically operating SMA wires are mounted within the moving member.

4. The actuator of claim 2 wherein the pair of antagonistically operating SMA wires or the bulk of the pair of antagonistically operating SMA wires are mounted within the base unit.

5. The actuator of claim 1 wherein the pair of antagonistically operating SMA wires are coiled or wound round within the body of the actuator.

6. The actuator of claim 5 wherein the pair of antagonistically operating SMA wires run around a plurality of pulleys within the body of the actuator.

7. The actuator of claim 6 wherein the pair of antagonistically operating SMA wires run around a plurality of pulleys mounted within the body of the moving member.

8. The actuator of claim 1 wherein the moving member and base unit are hollow or comprise frame structures having open space within.

9. The actuator of claim 1 wherein the pair of SMA wires drives a wheel or shaft mounted in the actuator body and rotating on the rotation axis; and wherein the rotation of the wheel or shaft is transmitted to the joint between the base unit and the moving member via the torsional spring.

10. The actuator of claim 9 wherein the torsional spring is in the form of a wire engaging at one end with the wheel or shaft and the other with the base unit.

11. The actuator of claim 1 wherein the torsional spring is coiled or looped around the rotation axis.

12. The actuator of claim 1 wherein the torsional spring is a rod or bar of a resiliently flexible material that twists as torque is applied.

13. The actuator of claim 1 wherein the torsional spring is a magnetic torsional spring.

14. The actuator of claim 13 wherein the magnetic torsional spring comprises a circular array of permanent magnets is mounted on the wheel or shaft driven by the SMA wires and a corresponding circular array of permanent magnets is mounted about the rotation axis on the base unit.

15. The actuator of claim 1 wherein the actuator body is cylindrical or generally cylindrical.

16. The actuator of claim 1 wherein the base unit comprises a circular or generally circular ring.

17. The actuator of claim 16 wherein the moving member comprises a frame having a circular or generally circular end, distal to the ring and connecting to the ring by means of two longitudinally extending connector arms which attach to diametrically opposite pivot positions on the ring.

18. The actuator of claim 1 wherein the SMA wires comprise an NiTi alloy.

19. The actuator of claim 1 wherein a distributed power and control system is employed.

20. The actuator of claim 1 comprising two moving members.

21. The actuator of claim 20 wherein the two moving members are disposed to either side of the base unit, each of the moving members being independently pivotally connected to the base unit for rotation about an axis transverse to the longitudinal axis of the actuator; and wherein the driving force for rotation of each moving member relative to the base unit is provided by a pair of antagonistically operating shape memory alloy (SMA) wires and transmitted via a torsional spring, for each moving member.

22. The actuator of claim 21 wherein the rotation axes of the two moving members are at right angles to each other, providing two degrees of freedom to the motion of the actuator.

23. The actuator of claim 21 wherein the rotation axes of the moving members are at right angles to each other and intersect.

24. The actuator of claim 21 wherein one moving member is pivotally connected to the base unit for rotation about the longitudinal axis and the rotation relative to the base unit is driven by means of a further pair of antagonistically operating shape memory alloy (SMA) wires and transmitted via a torsional spring.

25. The actuator of claim 24 wherein both moving members are pivotally connected to the base unit for rotation about the longitudinal axis and for each moving member the rotation relative to the base unit is driven by means of a further pair of antagonistically operating shape memory alloy (SMA) wires and transmitted via a torsional spring.

26. The actuator of claim 1 wherein an end of a moving member distal to the base unit has connector arms that project away from the base unit, for pivotal connection to another base unit.

27. An endoscope or a snake-like robot comprising at least one actuators; the actuator comprising a moving member pivotally connected to a base unit for rotation about an axis; wherein the driving force for rotation of the moving member relative to the base unit is provided by a pair of antagonistically operating shape memory alloy (SMA) wires and transmitted via a torsional spring.

28. An endoscope or a snake-like robot comprising a plurality of actuators, connected together; each actuator of the plurality of actuators comprising a moving member pivotally connected to a base unit for rotation about an axis; wherein the driving force for rotation of the moving member relative to the base unit is provided by a pair of antagonistically operating shape memory alloy (SMA) wires and transmitted via a torsional spring.

29. An endoscope or a snake-like robot according to claim 28 wherein the plurality of actuators are joined one to the next by link pieces.

30. An endoscope or a snake-like robot according to claim 28 wherein the plurality of actuators are joined one to the next by attaching ends of moving members, distal to their respective base units, directly together.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 illustrates schematically the operation of an actuator;

(2) FIGS. 2A to 2C illustrate actuators;

(3) FIGS. 3A to 3E illustrate aspects of actuators;

(4) FIGS. 4A to 4D illustrate alternative moving members for actuators;

(5) FIG. 5 illustrates schematically a snake-like robot; and

(6) FIG. 6 illustrates schematically a power and control system.

(7) FIGS. 7A and 7B illustrate alternative aspects of actuators.

DETAILED DESCRIPTION OF THE INVENTION WITH REFERENCE TO SOME PREFERRED EMBODIMENTS

(8) FIG. 1 shows schematically the principle of the drive mechanism 1 of an actuator of the invention. A pair of shape memory alloy (SMA) wires 2,4 act antagonistically to cause drive either clockwise or anti-clockwise to a drive wheel 6. (Power supply and control system to heat the SMA wires 2,4 not shown in this figure, see FIG. 6). As shown on the drawing a torque T.sub.S is being applied as the resultant of the forces F.sub.S1 and F.sub.S2. The torque is transmitted via torsional spring 8 to provide output torque T.sub.O, and hence motion, to a joint represented by wheel 10 in the schematic of the figure.

(9) The working principle is showed in FIG. 1 and equation 1 below describes the statics, Where it is assumed that T.sub.O=T.sub.S:

(10) $\begin{array}{cc}\left(F_{S2}-F_{S1}\right)R_S={}_S=K=-{}_O& \left(1\right)\\ K=\frac{-{}_O}{{}_{\mathrm{MAX}}}=\frac{{}_S}{{}_{\mathrm{MAX}}}& \left(2\right)\end{array}$
where F.sub.S1 and F.sub.S2 are the forces provided by each SMA, T.sub.S is the resultant torque, K the torsional spring elastic constant, R.sub.S the radius of the SMA pulley, R.sub.O is half of the joint length (radius of the wheel representing the joint), the angle of the SMA pulley, the angle of the torsional spring, .sub.MAX the max compliance angle. Eq. 2 provides a definition of K used to design the torsional spring.

(11) An SMA provides a residual force of about F.sub.MAX/3, where F.sub.MAX is the maximal contracting force, which implies a residual torque of T.sub.S/3, when the power supply is off. The resultant force provided by SMA wires in antagonistic configuration is as follows:

(12) $F_S=F_{\mathrm{MAX}}-\frac{1}{3}{F}_{\mathrm{MAX}}=\frac{2}{3}{F}_{\mathrm{MAX}}$

(13) This allows the joint to have a residual compliance of .sub.MAX/3. Each joint can provide an output torque T.sub.O as described by the following equation:

(14) $\left(\frac{2}{3}{F}_{\mathrm{MAX}}\right)R_S={}_{\mathrm{MAX}}$
which is reduced by about due to the residual strain of the opposite antagonistic SMA wire when it is not powered. The residual torque can be adjusted by controlling the temperature of the SMA, resulting in increased joint stiffness.

(15) By decoupling the input torque T.sub.S produced by the SMA wires 2,4 from the output torque T.sub.O, the joint has increased efficiency, and hence reduced heat generation. At the same time the joint is compliant, the motion to a particular position is not rigid but flexible to the extent dictated by the torsional spring 8. Furthermore if an actuator containing the drive 1 is acted upon by an external force, distortion of the torsional spring 8 will allow compliance of the joint direction in response to the amount of force encountered. For example, if the actuator is a component of an endoscope encountering a bend in the wall of a passage.

(16) FIG. 2A shows an actuator 12 of cylindrical form comprising three main body parts. A base unit 14 in the general form of a ring has two moving members 16,18 connected to it at respective pairs of pivot points 20,22 and 24,26. The moving members comprise frames for carrying mechanical and electrical components. Each moving member 16,18 has distal circular ends 28,30 that connect to the base unit 14 by means of respective pairs longitudinally extending connector arms (32,34 and 36,38) which attach to respective and diametrically opposite pivot points on the base unit 14 ring.

(17) The arrangement depicted in FIG. 2A has rotation axes 40,42 at right angles and intersecting at the centre of the base unit, providing a compact form of universal joint with two degrees of freedom allowing both pitch and roll motion as illustrated by FIG. 2B, which shows the actuator 12 of FIG. 2A tilted about both rotation axes.

(18) FIG. 2C shows an actuator 12 of cylindrical form and similar to that of FIGS. 2A and 2B but comprising only two main body parts, moving member 18 and base unit 14. Like parts are numbered the same. This actuator 12 provides only one degree of freedom (one axis of rotation). However, joining together with another of the same type base unit 14 to base unit 14, can provide a two degree of freedom actuator if the rotation axes are arranged to be e.g. at right angles as in the arrangement of FIGS. 2A and 2B.

(19) The drive mechanism employed to cause motion between the moving member 18 (FIG. 2C) or moving members 16,18 (FIGS. 2A, 2B) and base unit 14 is more easily with reference to FIG. 3.

(20) FIG. 3A shows a moving member 18. A drive wheel 44 is mounted between a connector arm 38 and a drive wheel support arm 46 for rotation about rotation axis 40. Rotation of the drive wheel 44 is driven by a pair of shape memory alloy (SMA) wires 48,50 acting antagonistically on the wheel 44 as indicated by arrows F.sub.S1 and F.sub.S2 as depicted in schematic FIG. 3B showing drive wheel 44 viewed looking along axis 40. The arrows suggest the applied force generated by activation of one or both wires by their power supply (not shown). Power supply may conveniently be provided by a single common power connection to drive wheel 44 for the ends of both SMA wires 48, 50 and a further connector wire for each SMA wire to complete the circuits.

(21) Long lengths of SMA wire are accommodated in the moving member 18 by running each wire 48, 50 around a series of pulleys. In this example each wire is guided from the drive wheel 44 by a respective pulley 52 in the distal end 28 of the moving member, to then run around two further pairs of pulleys 54. The pulleys 52 can be more easily seen in the view from below of the actuator 12 of FIG. 2A, as shown in FIG. 3C.

(22) FIG. 3C also shows one of the ends of each SMA wire 48 and 50 on a drive wheel 44. After wrapping round pulleys 52 and 54 an SMA wire may have its other end terminating at one of the pulleys 54 or by attaching to the moving member carrying the pulleys.

(23) Pulleys 54 are conveniently located in the end 30 of the moving member. Coiling longer lengths of wire within the actuator body in this fashion allows greater motion to be achieved on activation of the SMA wires. Typically only up to 5% contraction of the wire length is achievable.

(24) The drive from wheel 44 is transmitted to the base unit 14 via torsional spring 56 of a suitable material which may be metal such as steel or of a plastic or polymer composition. Spring 56 acts along the axis 40. The spring 56 in this example wraps around a bushing 58 of wheel 44. As shown in FIG. 3D the spring 56 has two ends 60, 62. In use one end 60 fits into an aperture on wheel 44, the other 62 into an aperture in base unit 14 (see FIG. 3E). Thus action of the SMA wires 48,50 driving the wheel 44 is transmitted to move the moving body 28 relative to the base unit 14. FIG. 3E also shows mounts 63 for ball bearings (not shown) at the pivot points of base unit 14. These allow smooth and reduced friction pivoting of a moving member 18 when attached by its connector arms 36,38.

(25) The arrangement shown is advantageous. The drive wheel 44 is controlled by the two SMA wires in antagonist configuration and it fixes the equilibrium point of the system, which is a stable configuration of the system if no external force is applied. The equilibrium point can be controlled even if an external torque is applied because the two SMA wires are directly connected to the wheel 44. If an external torque is supplied, the drive wheel 44 will tend to stay in the chosen equilibrium point, even though the moving member may movethe torsion spring allows motion of the moving member without disturbing the drive wheel. In the prior art systems using springs in tension, each SMA wire has a serialised spring that does not allow a fixed equilibrium position. If an external torque is applied, one spring is compressed and the other stretched. In contrast, with actuators as described herein, the equilibrium position of the drive wheel can be accurately controlled. Control of the drive wheel position can be for example by embedding a sensor in the wheel 44 to determine the position and adjustments made by powering the SMA wires. The position of the drive wheel 44 can also be determined for control purposes by measuring the length of the SMA wires. The length of an SMA wire can be determined by measuring the resistance of the wire, which varies with the length.

(26) FIG. 4A shows two moving members 16, 16a bonded end 28 to end 28a. Only the frame part of each moving member and not components carried within are shown in this illustration for clarity. This arrangement allows actuators such as shown in FIG. 2 to be connected directly one to another to provide two or more actuators linked together in series. The angle between the rotation axis 40 of one moving member and that of the next (40a) may be varied about the longitudinal axis as shown in the examples of FIGS. 4B and 4C. In FIG. 4C the support arms 46, 46a are opposed so that drive wheels 44 (not shown here, see FIG. 3A), when fitted to their respective moving members, are radially opposite, Such arrangements can be more convenient for routing of power and control cables and the placing of electronic components within the actuator bodies, and/or to balance the forces exerted on operating the series of actuators.

(27) As an alternative to bonding together two separate components 16,16a as shown in FIG. 4 a single component 64 may be manufactured having the same general form of the bonded together pair shown in FIG. 4A. See FIG. 4D. This can be convenient in manufacture, where a series of actuators are required, e.g. in building a snake-like robot. One moving body 64 connects between two base units in the series.

(28) FIG. 5 shows a schematic view of a snake-like robot (SNLR) 66 comprising a series of actuators of the invention mounted end to end resulting in an articulated snake. The SNLR is capable of adopting relatively complex shapes and thus can be suitable for exploring convoluted narrow channels such as a human colon when used as an endoscope. In this example pairs of moving members 16,16a bonded together as in FIG. 4A or a single component 64 having two pairs of connector arms as in FIG. 4D may be employed to link base units 14 together in making up the SNLR.

(29) FIG. 6 shows schematically an arrangement for powering and controlling a series of connected actuators of the invention. Such an arrangement may be found in an endoscope employing several actuators for improved range of motion, or more generally in a snake-like robot. In the figure three segments 68 of a distributed power and control system are shown. Each segment is located in an actuator such as those shown in the preceding figures. Each independent segment embeds all the required hardware to control the two degrees of freedom motion of its corresponding actuator.

(30) In order to achieve this requirement, the electronic system relies on a distributed system using a parallel bus to exchange control data. Four wires are needed for the actuators: two providing power supply 70, 72, and two 74,76 for the serial communication bus. A digital signal processor (DSP) 78, processes and implements a low level control, setting several parameters, such as torque, position or force for each of the four SMA wires. Four mosfets and current sensors will regulate the power for each SMA wire.

(31) Further control, a mid-level control (not shown), can be located in a main control board at the end of the robot or endoscope, whereas high level control can be provided by an external PC connected to the control system shown, for more advanced computational work.

(32) FIG. 7A shows in partial schematic view part of an actuator 12 generally similar to that of FIG. 2A but employing a magnetic torsional spring. Only one moving member 18 is shown fitted to the base unit 14 to allow better viewing of the drive details and other details are omitted. In this embodiment the drive wheels 44 are powered by pairs of SMA wires (not shown, see FIG. 3A) that wind round pulleys 52, 54. The drive wheels 44 have an array of permanent magnets 78 mounted to the outward facing face. See schematic elevation detail of a drive wheel 44 in FIG. 7B. A corresponding array of permanent magnets 80 is mounted about the rotation axis on the base unit 14, for each drive wheel 44. The drive wheel 44 is rotated by the SMA wires, in one direction or the other, as suggested by the double headed arrow in FIG. 7B. The magnetic field between the permanent magnets 78 and 80 acts to produce movement of the moving member 18 relative to the base unit 14. In this example the drive wheels 44 are fitted on shafts 82 on the rotation axes and mounted on the support arms 36 of the moving member. This example does not make use a drive wheel support arm such as shown (part 46) in FIG. 3A.