Extension sensing actuator

1. A method for measuring the extension of a solid state actuator comprising:

inputting an acoustic signal to the actuator;

measuring the acoustic signal that has passed along the actuator; and

using the measured signal to determine the actuator length or a change to that length.

2. A method as claimed in claim 1 comprising monitoring changes in the measured signal, thereby to determine changes in the actuator length.

3. A method as claimed in claim 1 wherein inputting comprises transmitting the input signal from one end of the actuator where the signal is reflected from an opposing end of the actuator and said measuring comprises measuring the signal at the transmit end.

4. A method as claimed in claim 3 wherein inputting comprises transmitting the input signal from one end of the actuator where the signal is reflected from an opposing end of the actuator and makes multiple passes along the actuator and said measuring comprises measuring the signal at the transmit end.

5. A method as claimed in claim 1 wherein inputting comprises transmitting the input signal from a point along the actuator and reflected from one or more ends.

6. A method as claimed in claim 1 wherein inputting comprises transmitting two input signals simultaneously from opposing ends of the actuator.

7. A method as claimed in claim 1 wherein inputting comprises transmitting the input signal from one end of the actuator and measuring comprises measuring at the other end of the actuator.

8. A method as claimed in claim 1 , wherein using comprises using the input and measured signals to determine an actuator transit time, and the method further involves using the transit time to determine the actuator length or changes in the actuator length.

9. A method as claimed in claim 1 comprising determining the resonant frequency of the actuator and using the determined resonant frequency of the actuator to determine its length or a change in length.

10. A method as claimed in claim 1 comprising:

using a transducer to input and measure the acoustic signal; and

determining the resonant frequency of the transducer and using the determined resonant frequency of the transducer to determine length or a change in length of the actuator.

11. A method as claimed in claim 1 comprising applying a DC voltage to the actuator to cause a change in its length.

12. A method as claimed in claim 1 wherein the solid-state actuator is one of: a piezo ceramic actuator; a piezo electric actuator; an electrostrictive actuator.

13. A system comprising means for implementing the method of claim 1 .

14. A device comprising:

a solid-state actuator;

means for inputting an acoustic signal to the actuator; and

means for measuring a resultant signal to determine the actuator length or a change to that length.

15. A device as claimed in claim 14 wherein the input means and/or the measuring means are at an end of the actuator.

16. A device as claimed in claim 15 wherein input means and measuring means are provided at both ends of the actuator configured to input acoustic signals at both ends of the actuator and measure acoustic signals at both ends.

17. A device as claimed in claim 14 wherein the actuator comprises layers of material, and the input means and/or the measuring means are integrated with those layers.

18. A device as claimed in claim 17 wherein the input means and/or the measuring means are defined by layers of the same material as the actuator.

19. A device as claimed in claim 18 wherein the input means and/or the measuring means are defined in the same laminar structure as the actuator.

20. A device as claimed in claim 14 wherein input means and/or the measuring means are separate from but attached to the actuator.

21. A device as claimed in claim 20 wherein input means and/or the measuring means are attached using epoxy or by a covalent bonding process.

22. A device as claimed in claim 20 wherein input means and/or the measuring means are formed on the actuator by a thin film deposition process.

23. A device as claimed in claim 14 wherein input means and the measuring means are implemented by the same single element.

24. A device as claimed in claim 23 wherein an input and measurement element is provided at both ends of the actuator.

25. A device as claimed in claim 14 wherein input means and the measuring means are separate elements.

26. A device as claimed in claim 25 wherein the input means and the measuring means are located at opposite ends of the actuator.

27. A device as claimed in claim 14 wherein the solid-state actuator is one of: a piezo ceramic actuator; a piezo electric actuator; an electrostrictive actuator.

28. A system comprising:

a solid-state actuator;

means inputting an acoustic signal to the actuator;

means measuring the acoustic signal that has passed along the actuator; and

means using the measured signal to determine the actuator length or a change to that length.

29. A device comprising:

a solid-state actuator;

an acoustic or ultrasonic transducer or resonator incorporated with said solid-state actuator; and

means for measuring a frequency of the transducer or resonator to determine the solid-state actuator length or a change to that length.

30. A device as claimed in claim 29 wherein the solid-state actuator is one of: a piezo ceramic actuator; a piezo electric actuator; an electrostrictive actuator.