Baselining techniques in force-based touch panel systems

1. A method for determining a touch location of a touch on a touch screen, comprising:

developing a first reference level for a force responsive touch signal;

developing a second reference level for the force responsive touch signal; and

determining the touch location by calculating a difference between the touch signal and at least one of the first and second reference levels;

wherein developing the second reference level comprises using an extrapolated value as the second reference level, and wherein using an extrapolated value as the second reference level comprises:

acquiring a first touch signal sample during a quiescent period;

acquiring a second touch signal sample during the quiescent period, the second touch signal sample acquired a predetermined number of sample times after the first touch signal sample;

determining a rate of change of the touch signal during the quiescent period using the first and the second touch signal samples;

extrapolating a value of the touch signal based on the first touch signal sample, the second touch signal sample and the rate of change of the touch signal; and

using the extrapolated value as the second reference level.

2. A method for determining a touch location of a touch on a touch screen, comprising:

developing the first reference level for a force responsive touch signal;

developing a second reference level for the force responsive touch signal; and

determining the touch location by calculating a difference between the touch signal and at least one of the first and second reference levels;

wherein developing the second reference level comprises using an interpolated value as the second reference level, wherein using an interpolated value as the second reference level comprises:

acquiring a first touch signal sample during a quiescent period;

acquiring a second touch signal sample when the touch signal returns to a quiescent state following application of a touch;

determining a rate of change of the touch signal between the first touch signal sample and the second touch signal sample;

interpolating a value of the touch signal based on the first touch signal sample, the second touch signal sample and the rate of change of the touch signal; and

using the interpolated value as the second reference level.

3. A method for determining a touch location of a touch on a touch screen, comprising:

developing a first reference level for a force responsive touch signal;

developing a second reference level for a force responsive touch signal; and

determining the touch location by calculating a difference between the touch signal and at least one of the first and second reference levels;

wherein determining the touch location includes transitioning from using the first reference level to using the second reference level, wherein transitioning from using the first reference level to using the second reference level comprises calculating X and Y touch location coordinates as:

X=WX ref2 +(1 −W ) X ref1 ,

Y=WY ref2 +(1 −W ) Y ref1 ,

where:

W is a weighting factor,

X ref1 and Y ref2 are touch location coordinates calculated using the second reference level, and

X ref1 and Y ref1 are touch location coordinates calculated using the first reference level.

4. The method of claim 3 , wherein transitioning from using the first reference level to using the second reference level comprises:

calculating touch signal location values using each of the first and second reference levels;

assigning weights to each touch signal location value; and

determining the touch location based upon the weighted touch signal location values.

5. The method of claim 4 , wherein the weights assigned to each touch signal location value vary with distance as the touch is moved across the touch screen.

6. The method of claim 3 , wherein transitioning from using the first reference level to using the second reference level comprises eliminating an effect associated with either the first or the second reference level from the touch location determination when the touch is moved a predetermined distance.

7. The method of claim 3 , wherein transitioning from using the first reference level to using the second reference level comprises eliminating an effect associated with either the first or the second reference level from the touch location determination after a predetermined time interval.

8. The method of claim 3 , wherein the weighting factor, W, is calculated as:

W

=

Max

⁡

[

1

-

D

0.2

⁢

S

,

0

]

where D is a greatest distance traveled in either an X direction of a Y direction, and S is a size of the touch screen.

9. The method of claim 3 , wherein the weighting factor, W, declines linearly from 1 to 0 over a period of one second.

10. A method of establishing a reference level for a touch signal, comprising:

sensing a quiescent touch signal prior to an application of a touch force;

detecting a touch signal responsive to the application of the touch force; and

establishing a reference level for the touch signal based on one or more values of the touch signal acquired contemporaneously with the detection of the touch force, the reference level representing a zero applied force reference condition, wherein a difference calculated between the reference level and the touch signal is used to determine a location of the touch force;

wherein establishing the reference level comprises establishing the reference level based on a particular time at which touch location information is obtained, wherein establishing the reference level comprises:

detecting a first quiescent touch signal, the first quiescent touch signal representing the force responsive touch signal before the touch is applied to the touch screen;

detecting a second quiescent touch signal, the second quiescent touch signal representing the force responsive touch signal after the touch is removed from the touch screen;

determining an interpolated touch signal between the first and the second quiescent touch signals; and

using a value of the interpolated touch signal at the particular time at which the touch location is determined as the reference level.

11. The method of claim 10 , wherein establishing the reference level comprises:

detecting a quiescent touch signal, the quiescent touch signal representing the force responsive touch signal during a period in which no touch force is applied to the touch screen;

extrapolating the quiescent touch signal to the particular time of the touch location measurement; and

using a value of the extrapolated quiescent touch signal at the particular time at which the touch location is determined as the reference level.

12. The method of claim 10 , wherein the second quiescent touch signal is detected if a touch signal sample is below a predetermined threshold and at least one subsequent touch signal sample has a value less than the touch signal sample.

13. The method of claim 10 , wherein the second quiescent touch signal is detected if a predetermined number of successive samples remain within a predetermined range of values.