Proximity sensor with nonlinear filter and method
A sensor for a portable connected device comprising a filter 30 is arranged to reduce a noise component on a sampled input signal, wherein the filter is arranged to consider only input measurements that change systematically in a same direction, updating an output value when all the input samples in a predetermined time window are above or below a current output value and, repeating the current output value when the input samples in the time window are below and above the current output value.
1. A portable connected device comprising a sensor arranged to determine whether a user is in proximity to the portable connected device, the sensor comprising a noise-reducing filter receiving a series of digital input values and generating a series of digital output values, wherein the filter is arranged to process input values in a window having a predetermined length time and to:
update the output value shifting the output value towards the maximum of the input samples in the window when the current output value is above all the input samples in the time window, or
update the output value shifting the output value towards the minimum of the input samples in the window when the current output value is below all the input samples in the time window, or
repeat the current output value in all other cases.
2. The portable connected device of claim 1 , wherein updating the output value includes shifting the output value by a predetermined fractional amount between 0 and 1.
3. The portable connected device of claim 1 , wherein the output value is updated according to the formula
U ( k )= a ·min( k )+(1− a )· U ( k− 1) if U ( k− 1)<min( k )
U ( k )= a ·max( k )+(1− a )· U ( k− 1) if U ( k− 1)>max( k )
where ‘U(k)’ denotes the output value, ‘U(k−1)’ the current output value, ‘max (k)’ and ‘min(k)’ the maximum, respectively minimum of the input values in the window, and ‘a’ a predetermined fractional amount between 0 and 1.
4. The portable connected device of claim 1 , arranged to adapt a RF power of a radio transmitter based on the proximity of a user to the portable connected device.
5. The portable connected device of claim 1 , wherein the noise-reducing filter is adapted to compare an average change of the input signal with an expected spread value and to update the output value only if the average change is larger than a predetermined fraction of the expected spread.
6. The portable connected device of claim 1 , wherein the noise-reducing filter is arranged to compute a minimum value of the input values in the window and a maximum value of the input values in the window, to compare the minimum value and the maximum value with a current output value, and to:
update the output value to the minimum value if the current output value is less than the minimum value, or
repeat the previously-generated output value if the current output value is comprised between the minimum value and the maximum value, or
update the output value to the maximum value if the current output value is more than the maximum value.
7. The portable connected device of claim 1 , comprising a linear low-pass filter.
8. The portable connected device of claim 1 , comprising a baseline estimator arranged to read the output values and generate a series of estimated baseline values, and a subtraction unit arranged to subtract the estimated baseline values from the output values.
9. The portable connected device of claim 1 , wherein the sensor determines a capacity seen by a sense electrode and decides whether a user is in proximity based on the capacity.
10. The portable connected device of claim 6 , wherein the sense electrode is also an emitter of radio waves.