The invention relates to a rotary motor that is relatively small and lightweight.

One known type of motor is a strain wave motor (which may also be referred to as a harmonic motor or a harmonic drive). Strain wave motors or harmonic drives are commonly used in applications such as robotics, where high resolution, excellent repeatability and no backlash are required. A strain wave motor typically comprises three principle components, a circular spline, a flexible spline and means to distort the flexible spline so that the geared surfaces mesh at angular locations that move progressively around the cylindrical spline, thereby driving relative rotation of the splines if the two geared surfaces have a different number of teeth. The output can then be taken from either the flexible spline or the circular spline depending on the specific configuration used.

In a common configuration, an elliptical drive shaft is used to distort the flexible spline. In this configuration the harmonic drive acts as a reducer to the rotation provided by the drive shaft. US2002/0135241 discloses an example of this kind of drive.

Alternative methods have been suggested to induce the distortion in the flexible spline including electromagnetic forces (as disclosed in US-2,906,143 and US-2005/253675), electrostatic forces (as disclosed in JP- 1990-0230014) and pneumatic forces (as disclosed in US-7,086,309). More complex arrangements have also been suggested where the flexible spline has internal and external gears (as disclosed in WO-2009/071604) and also uses an electrostrictive polymer to distort the flexible spline.

Each of these designs have issues that make them undesirable for small light weight motors.

In the case of using electromagnetic forces, the required coils and metal are relatively complex and heavy for motors smaller than 20mm in size.

In the case of using electrostatic forces, in order to achieve the required displacement, the voltages required to achieve significant electrostatic forces or motion of an electrostrictive polymer are very high. This is not generally compatible with the low cost requirement for consumer electronics.

In the case of using pneumatic forces, the required plumbing and pumps are not generally compatible with motors smaller than 20mm in size.

It would be desirable to provide a strain wave motor that alleviates at least some of these issues.

According to the present invention, there is provided a strain wave motor comprising: a static portion; a flexible spline having an annular geared surface, the flexible spline being prevented from rotating relative to the static portion; a cylindrical spline that is rotatable relative to the static portion, the cylindrical spline having an annular geared surface, wherein one of the splines has its geared surface on an inner face and the other of the splines has its geared surface on an outer face, the two splines being placed coaxially so that the geared surfaces mesh on distortion of the flexible spine, the two geared surfaces comprising a number of teeth that is different; plural shape memory alloy wires configured to distort the flexible spline at different angular positions so as to cause the geared surfaces to mesh; and a drive circuit configured to apply drive currents to each shape memory alloy wire with waveforms selected to distort the flexible spline so that the geared surfaces mesh at one or more angular locations that move progressively around the cylindrical spline.

The strain wave motor comprises a flexible spline and a cylindrical spline which have geared surfaces that mesh on distortion of the flexible spine. It has been appreciated that such distortion may be driven by plural shape memory alloy (SMA) wires configured to distort the flexible spline at different angular positions. This may be achieved by the flexible spline being prevented from rotating relative to the static portion, so that the plural shape memory alloy wires configured to distort the flexible spline without needing to accommodate rotation of the flexible spline.

Accordingly, by applying drive currents to each shape memory alloy wire with selected waveforms, the flexible spline may be distorted to cause the geared surfaces mesh at one or more angular locations that move progressively around the cylindrical spline. As the two geared surfaces have a different number of teeth, this causes rotation of the cylindrical spline relative to the flexible spline. As successive teeth on each geared surface mesh, it follows that the cylindrical spline shifts relative to the flexible spline by the difference in the number of teeth, after each complete rotation of the angular location where meshing occurs.

The motor can be configured to be relatively small and lightweight, because the distortion of the flexible spline is driven by SMA wires which are compact with respect to the force they apply compared to other forms of actuator. For example, coils and other metallic components associated with electromagnetic drive are avoided, the high voltages associated with using electrostatic forces are avoided, and there the plumbing and pumps associated with pneumatics are avoided.

Similarly, the motor can provide relatively high torque and low speed characteristics. The torque and gearing can be chosen by selecting the configuration of the splines, including the difference in the number of teeth.

The speed of response of SMA wire increases with smaller diameter wires. To ensure a fast speed of response, the diameter of the SMA wire should preferably be ΙΟΟμιη or less, more preferably 50μιη or less, or even more preferably 25μηι or less.

Advantageously, the SMA wires lie across the cylindrical spline as viewed along the axis of the cylindrical spline. In this manner, the overall dimensions of the motor may be constrained for a given length of SMA wire designed to provide the necessary amount of deformation of the flexible spline.

The diameter of the circular spline is preferably 50mm or less, more preferably 30mm or less, or even more preferably 20mm or less.

Embodiments of the present invention will now be described by way of non-limitative example, with reference to the accompanying drawings, in which:

Fig. 1 is a plan view of a strain wave motor;

Fig. 2 is a diagram of the drive currents applied to SMA wires in the strain wave motor;

Figs. 3 to 5 are schematic plan views of modified versions of the strain wave motor;

Fig. 6 is a plan view of an alternative means for preventing rotation of the flexible spline in the strain wave motor;

Figs. 7 and 8 are circuit diagrams of alternatives for the drive circuit of the strain wave motor and its connection arrangement.

A strain wave motor 1 is shown in Fig. 1. The strain wave motor 1 comprises a flexible spline 2 and a cylindrical spline 3. The diameter of the cylindrical spline 3 is 50mm or less, more preferably 30mm or less, or even more preferably 20mm or less.

The strain wave motor 1 also has a static portion 4 which remains static in use. The flexible spline 2 comprises bosses 5 that fit in recesses 6 in the static portion 4. The mechanical interference between the bosses 5 and the recesses 6 in the static portion 4 prevent the flexible spline 2 from rotating relative to the static portion 4.

The cylindrical spline 3 is rotatable, being fixed to an axle 7 mounted in the static portion 4 on which rotation about a rotational axis relative to the static portion 4 is supported. The cylindrical spline 3 acts as a rotor that is rotated in use. Rotational drive may be taken from the cylindrical spline 3, for example by connecting an output shaft thereto.

The flexible spline 2 has an annular geared surface 8 on its inner face and the cylindrical spline 3 has an annular geared surface 9 on its outer face, i.e. the flexible spline 2 is outside the cylindrical spline 3. The cylindrical spline 3 and the flexible spline 2 are placed coaxially so that the geared surfaces 8 and 9 face each other. The flexible spline 2 may be distorted, in this example by flexing inwardly. The annular geared surfaces 8 and 9 mesh with each other on such distortion of the flexible spine 2.

The two geared surfaces 8 and 9 have a number of teeth that is different, preferably by 2 or more. Either one of the geared surfaces 8 and 9 may have more teeth, but in one example the flexible spline 2 which has its geared surface 9 on an inner face has more teeth than the cylindrical spline 3 which has its geared surface 8 on an outer face.

The strain wave motor 1 further comprises plural SMA wires 10 that are configured as follows to distort the flexible spline 2. The SMA wires 10 are each connected at one end to the flexible spline 2 and at the other end to the static portion 4. The connections at each end of the SMA wires 10 are made by the SMA wires 10 being crimped by crimp elements 11 mounted on the flexible spline 2 and crimp elements 12 mounted on the static portion 4. The crimp elements 11 and 12 provide electrical and mechanical connections.

The SMA wires 10 are connected to the flexible spline 2 at different angular positions, which are equally angularly spaced. The SMA wires 10 are arranged symmetrically around the rotational axis of the cylindrical spline 2. The SMA wires 10 lie across the cylindrical spline 3 as viewed along the axis of the cylindrical spline 3, so that the overall dimensions of the strain wave motor 1 are constrained for the given length of the SMA wires 10 which provide the necessary amount of deformation of the flexible spline 2. The SMA wires 10 span from one side of the flexible spline 2 to a point on the static portion 4 approximately on the opposite side of the flexible spline 2 in a radial direction.

Advantageously, the SMA wires 10 do not pass through the rotational axis of the cylindrical spline 3 so that space is left between the SMA wires 10 for attachment of the axle 7 to the cylindrical spline 3.

The SMA wires 10, on contraction, distort the flexible spline 2 so as to cause the geared surfaces 8 and 9 to mesh at different angular locations which correspond to the angular positions at which the SMA wires 10 are connected to the flexible spline 2 and so are equally angularly spaced.

In this example, there are six SMA wires 10 arranged in three pairs, wherein the SMA wires 10 of each pair are connected to the flexible spline 2 at opposed angular positions. More generally, there may be higher even numbers of SMA wires 10 arranged in similar pairs connected to the flexible spline 2 at opposed angular positions. Even more generally, there may be any number of at least three SMA wires 10 that are angularly spaced but not necessarily in pairs.

A drive circuit 13 is electrically connected to each SMA wire 10 so as to apply drive currents thereto. In the example shown in Fig. 1, the electrical connection is as follows. The ends of the SMA wires 10 at the flexible spline 2 are electrically connected together by providing an electrical connection between the crimp elements 11 (for example by forming the crimp elements 11 from a common component), and the drive circuit 13 is electrically connected in common thereto by a flexible electrical connector 14. The drive circuit 13 is electrically connected in parallel to the crimps 12 and hence to the ends of the SMA wires 10 at the static portion.

The drive signals applied to each SMA wire 10 have waveforms selected to distort the flexible spline so that the geared surfaces 8 and 9 mesh at one or more angular locations that move progressively around the cylindrical spline 2.

Fig. 2 shows an example of the drive currents applied to SMA wires 10. In particular, Fig. 2 shows the six waveforms applied to the six SMA wires 10 in order around the rotational axis. Thus, the drive currents are phased pulses. Each waveform is the same shape, but offset in phase to progress the force applied to the flexible spline 2 around the rotational axis. The pulses are square waves in this example, but other shaped waveforms may be used. Similarly, the length of the pulses may be varied depending on the mechanical configuration. Also, in this example, the SMA wires 10 in each pair are driven by the same waveform, so that they contract together causing the geared surfaces 8 and 9 to mesh at opposed angular locations. Thus, in this example, the deformed shape of the flexible spline 3 is a cylinder with an elliptical cross section.

As successive SMA wires 10 around the cylindrical spline 3 are caused to contract, the angular locations at which the geared surfaces 8 and 9 mesh correspondingly move progressively around the cylindrical spline 3. As the two geared surfaces 8 and 9 have a different number of teeth, this causes rotation of the cylindrical spline 3 relative to the flexible spline 2. As successive teeth on the two geared surfaces 8 and 9 mesh, after each complete rotation of the angular location where meshing occurs, the cylindrical spline 3 shifts relative to the flexible spline 2 by the difference in the number of teeth. The direction of rotation depends on which of the geared surfaces 8 and 9 has the larger number of teeth.

The torque and gearing can be chosen by selecting the configuration of the splines, including the difference in the number of teeth. In one example, the cylindrical spline 3 may have 100 teeth and a diameter of 20mm, and the flexible spline may have two additional teeth (so 102 teeth). The teeth on both gearing surfaces 8 and 9 have a suitable height to permit meshing and release, for example 0.5mm.

Although in this example, the geared surfaces 8 and 9 mesh at two angular locations that move progressively around the cylindrical spline 3, more generally the geared surfaces 8 and 9 may mesh at a single angular location or at more than two angular locations that move progressively around the cylindrical spline 3. This depends on the configuration of the SMA wires 10 and the waveforms applied thereto.

The configuration of the strain wave motor 1 shown in Fig. 1 is not limitative and various modifications may be made. Figs. 3 to 5 illustrate examples of modifications which will now be described, the strain wave motor 1 being otherwise as described above.

Fig. 3 illustrates a modification wherein the SMA wires 10 are connected to the flexible spline 2 at both ends, instead of being connected to the static portion 4 at one of their ends. In this example, three SMA wires 10 are present but in general larger numbers of SMA wires 10 could be provided. In this case, as contraction of the SMA wires 10 distorts the flexible spline 2 at each end, the geared surfaces 8 and 9 to mesh at opposed angular locations. In a similar manner to that described above, the drive signals applied to the SMA wires 10 have waveforms selected so that the angular locations at which the geared surfaces 8 and 9 mesh move progressively around the cylindrical spline 2.

Fig. 4 illustrates a modification wherein the SMA wires 10 have a similar configuration to Fig. 1, but the drive signals applied to the SMA wires 10 have waveforms selected so that the geared surfaces 8 and 9 mesh at a single angular location that moves progressively around the cylindrical spline 2.

Fig. 5 illustrates a modification wherein the flexible spline 2 has an annular geared surface 8 on its outer face and the cylindrical spline 3 has an annular geared surface 9 on its inner face, i.e. the cylindrical spline 3 is outside the flexible spline 2. In addition, the SMA wires 10 are connected to the flexible spline 2 at both ends, although they could alternatively be connected to the static portion 4 at one of their ends in a similar manner to that described above. The SMA wires 10 distort the flexible spline 10 at different angular positions so as to cause the geared surfaces 8 and 9 to mesh, but whereas the above examples achieve this by the SMA wires 10 pulling the geared surfaces 8 and 9 together to achieve meshing, here the SMA wires 10 pull the geared surfaces 8 and 9 apart so as to mesh at other angular locations separated from the SMA wires 10.

Fig. 6 illustrates a modification that may be applied to any other of the above examples, where the bosses 5 and recesses 6 are replaced by the flexible spline 2 and the static portion 4 each having respective further geared surfaces 15 and 16 which mesh with each other. The further geared surfaces 15 and 16 have the same number of teeth and so prevent the flexible spline 2 from rotating relative to the static portion 4.

Figs. 7 and 8 illustrate two alternatives for the drive circuit 13 and its connection arrangement.

In the alternative of Fig. 7, the drive circuit 13 is connected to the SMA wires 10 as described above. Optionally, the drive circuit may also include a resistance measurement circuit 17 and/or an ambient temperature sensor 18.

Where provided, the resistance measurement circuit 17 measures the resistances of the SMA wires 10. The measured resistances may be used to adjust the power of the drive currents supplied to the SMA wires 10, for example using resistance feedback control.

Where provided, the ambient temperature sensor 18 measures the ambient temperature. The measured ambient temperature may be used to adjust the power of the drive currents supplied to the SMA wires 10, for example by increasing the power as the ambient temperature decreases and thereby increases heat loss from the SMA wires 10.