Methods and apparatus for limiting the excursion of a transducer

1. A method of providing excursion protection for a transducer comprising:

receiving a transducer signal; and

limiting the transducer signal, or a signal derived therefrom, to generate a limited transducer signal for driving the transducer such that an electrical response caused by the limited transducer signal in an electrical model of the transducer would be less than a threshold electrical response, wherein the threshold electrical response has been determined by:

inputting a stimulus input signal into the electrical model of the transducer, wherein the stimulus input signal is designed to cause the transducer to reach a maximum excursion; and

determining the threshold electrical response as a maximum of the electrical response caused by the stimulus input signal in the electrical model of the transducer.

2. The method of claim 1 further comprising:

determining an electrical response caused by the transducer signal in the electrical model of the transducer; and

limiting a delayed version of the transducer signal to generate the limited transducer signal based on a comparison of the electrical response caused by the transducer signal with the threshold electrical response.

3. The method of claim 1 further comprising:

determining an electrical response caused by the limited transducer signal in the electrical model of the transducer;

comparing the electrical response of the limited transducer signal with the threshold electrical response; and

adjusting the limitation of the transducer signal based on the comparison.

4. The method of claim 1 wherein the electrical response comprises a representation of the back electromotive force, EMF, voltage in the electrical model.

5. The method of claim 4 wherein the step of limiting comprises:

attenuating the transducer signal or the signal derived therefrom to generate the limited transducer signal, such that when the limited transducer signal is input into the electrical model, the representation of the back EMF voltage in the electrical model remains below a maximum of the representation of the back EMF voltage in the electrical model caused by the stimulus input signal.

6. The method of claim 5 wherein the step of limiting comprises:

setting the maximum of the representation of the back EMF voltage equal to 1.

7. The method of claim 5 wherein the transducer comprises a Linear Resonant Actuator, LRA, and wherein the electrical model comprises an electrical model of a moving mass of the transducer, and wherein the step of determining the maximum back EMF voltage comprises:

measuring the voltage across the electrical model of the moving mass of the transducer as the stimulus input signal is input into the electrical model of the transducer; and

setting the maximum voltage reached in the step of measuring as the maximum back EMF voltage caused by the stimulus input signal.

8. The method as claimed in claim 1 wherein the electrical response comprises a total energy across the electrical model.

9. The method of claim 8 wherein the step of limiting comprises:

attenuating the transducer signal or the signal derived therefrom to generate the limited transducer signal such that when the limited transducer signal is input into the electrical model, the total energy across the electrical model remains below a maximum of the total energy across the electrical model caused by the stimulus input signal.

10. The method of claim 1 wherein the electrical response comprises an inductor current in the electrical model.

11. The method of claim 10 wherein the step of limiting comprises attenuating the transducer signal or the signal derived therefrom to generate the limited transducer signal such that when the limited transducer signal is input into the electrical model, an inductor current in the electrical model remains below the maximum inductor current in the electrical model caused by the stimulus input signal.

12. The method of claim 1 wherein the stimulus input signal comprises a nominal resonance frequency associated with the transducer.

13. The method of claim 12 wherein the stimulus input signal comprises a signal in which the frequency is varied across a range of frequencies comprising the nominal resonance frequency.

14. A controller for providing excursion protection for a transducer comprising:

an input configured to receive a transducer signal;

excursion limiting circuitry configured to limit the transducer signal or a signal derived therefrom to generate a limited transducer signal for driving the transducer such that an electrical response caused by the limited transducer signal in an electrical model of the transducer would be less than a threshold electrical response, wherein the threshold electrical response has been determined by:

inputting a stimulus input signal into the electrical model of the transducer, wherein the stimulus input signal is designed to cause the transducer to reach a maximum excursion; and

determining the threshold electrical response as a maximum of the electrical response caused by the stimulus input signal in the electrical model of the transducer.

15. The controller of claim 14 further comprising:

an electrical modelling block configured to determine an electrical response caused by the transducer signal in the electrical model of the transducer; wherein the excursion limiting circuitry is configured to limit a delayed version of the transducer signal to generate the limited transducer signal based on a comparison of the electrical response caused by the transducer signal with the threshold electrical response.

16. The controller of claim 14 further comprising:

an electrical modelling block configured to determine an electrical response caused by the limited transducer signal in the electrical model of the transducer;

a comparison block configured to compare the electrical response of the limited transducer signal to the threshold electrical response; wherein the excursion limiting circuitry is configured to adjust the limitation of the transducer signal based on the comparison.

17. The controller of claim 14 wherein the electrical response comprises a representation of the back electromotive force, EMF, voltage in the electrical model.

18. The controller of claim 17 wherein the excursion limiting circuitry is configured to:

attenuate the transducer signal or the signal derived therefrom to generate the limited transducer signal, such that when the limited transducer signal is input into the electrical model, the representation of the back EMF voltage in the electrical model remains below a maximum of the representation of the back EMF voltage in the electrical model caused by the stimulus input signal.

19. The controller of claim 18 wherein the excursion limiting circuitry is configured to:

set the maximum of the representation of the back EMF voltage equal to 1.

20. The controller of claim 14 wherein the electrical response comprises a total energy across the electrical model.

21. The controller of claim 20 wherein excursion limiting circuitry is configured to:

attenuate the transducer signal or the signal derived therefrom to generate the limited transducer signal such that when the limited transducer signal is input into the electrical model, the total energy across the electrical model remains below a maximum of the total energy across the electrical model caused by the stimulus input signal.

22. The controller of claim 14 wherein the electrical response comprises an inductor current in the electrical model.

23. The controller of claim 22 wherein the excursion limiting circuitry is configured to: attenuate the transducer signal or the signal derived therefrom to generate the limited transducer signal such that when the limited transducer signal is input into the electrical model, an inductor current in the electrical model remains below the maximum inductor current in the electrical model caused by the stimulus input signal.

24. The controller of claim 14 wherein the stimulus input signal comprises a nominal resonance frequency associated with the transducer.

25. The controller of claim 24 wherein the stimulus input signal comprises a signal in which the frequency is varied across a range of frequencies comprising the nominal resonance frequency.