Control methods for a force sensor system

1. A method of controlling a force sensor system to define at least one button implemented by at least one force sensor, the method comprising:

receiving a force sensor input;

determining first derivatives and second derivatives of the force sensor input to provide corresponding first-derivative values and second-derivative values; and

controlling the force sensor system based on the first-derivative values and the second-derivative values;

wherein:

each said derivative is a derivative with respect to time; and

each said second derivative is a rate of change of a corresponding first derivative with respect to time.

2. The method according to claim 1 , comprising:

comparing the first-derivative values and the second-derivative values to threshold values to control the force sensor system.

3. The method according to claim 1 , comprising:

comparing the first-derivative values and the second-derivative values to threshold values to determine if a user press event has occurred and/or to determine a re-calibration requirement of the force sensor system.

4. The method according to claim 1 , wherein the first-derivative values and the second-derivative values are determined as successive pairs of derivative values, each pair comprising a first-derivative value and a second-derivative value corresponding to a given value of the force sensor input or to a given point in time.

5. The method according to claim 4 , comprising:

mapping the pairs of derivative values to a 2D representation; and

performing quadrant tracking or state space tracking with respect to the 2D representation of the successive pairs of derivative values to determine if a user press event has occurred.

6. The method according to claim 5 , wherein the method comprises defining a noise box as part of the 2D representation,

and wherein:

a force sensor input having first-derivative values and second-derivative values falling outside of the noise box is subjected to the quadrant tracking or state space tracking to determine if the user press event has occurred; and/or

pairs of derivative values falling inside the noise box are excluded from the quadrant tracking or state space tracking,

optionally wherein the noise box is dynamically calculated.

7. The method according to claim 5 , wherein the 2D representation comprises a plot of first-derivative values against second-derivative values, or vice versa.

8. The method according to claim 5 , wherein the method comprises determining a user press event has occurred if the quadrant tracking or state space tracking follows a defined sequence.

9. The method according to claim 4 , wherein the method comprises determining that a user press event has occurred if the successive pairs of first-derivative and second-derivative values sequentially progress through first, second, third and fourth stages, wherein the stages are defined as follows:

first stage: positive first-derivative value, positive second-derivative value;

second stage: positive first-derivative value, negative second-derivative value;

third stage: negative first-derivative value, negative second-derivative value; and

fourth stage: negative first-derivative value, positive second-derivative value.

10. The method according to claim 4 , wherein the method comprises defining a first-derivative noise range as a range of first-derivative values attributable to noise, and excluding a pair of first-derivative and second-derivative values from determining a user press event has occurred if the first-derivative value of that pair is within the first-derivative noise range.

11. The method according to claim 4 , wherein the method comprises defining a second-derivative noise range as a range of second-derivative values attributable to noise, and excluding a pair of first-derivative and second-derivative values from determining a user press event has occurred if the second-derivative value of that pair is within the second-derivative noise range.

12. The method according to claim 4 , wherein the method comprises defining a noise space as a space defined by pairs of first-derivative and second-derivative values attributable to noise, and excluding a pair of first-derivative and second-derivative values from determining a user press event has occurred if that pair of first-derivative and second-derivative values falls within the noise space,

optionally wherein the method comprises updating the noise space dynamically, optionally based on preceding first-derivative values, second-derivative values and/or pairs of first-derivative and second-derivative values.

13. The method according to claim 1 , comprising:

receiving a temperature level of the force sensor system; and

performing the method when the temperature level is above a temperature threshold,

optionally comprising determining whether the temperature level is above the temperature threshold.

14. The method according to claim 13 , comprising:

performing said determining and controlling when the temperature level is above the temperature threshold; and

controlling the force sensor system without using a gradient of the force sensor input, or without performing said determining, when the temperature level is not above the temperature threshold.

15. A force sensor system such as a host device, comprising:

at least one force sensor; and

a controller connected to the at least one force sensor and configured to carry out the method of claim 1 .

16. A control method for a force sensor system used to define at least one virtual button, the control method comprising the steps of:

receiving a force sensor input;

determining first and second derivatives of the force sensor input to provide velocity and acceleration values;

mapping the velocity and acceleration values to a 2D representation; and

performing quadrant tracking or state space tracking of the 2D representation of the velocity and acceleration values to determine if a user press event has occurred;

wherein:

each said derivative is a derivative with respect to time; and

each said second derivative is a rate of change of a corresponding first derivative with respect to time.