Proximity sensor with nonlinear filter and method
A sensor for a portable connected device. The sensor has a filter arranged to reduce a noise component on a sampled input signal. The 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:
compute an upper limit and a lower limit based on the input samples in the window, and a central value of the input samples in the window, the central value being comprised between the upper limit and the lower limit,
shifting the output value towards the upper limit when the current input value is above the upper limit, or
shifting the output values towards the lower limit when the current input value is below the lower limit, or
shifting the output value towards the central value in all other cases.
2. The portable connected device of claim 1 , wherein the central value is computed as equal to an average of the minimum value and the maximum value, or wherein the central value is computed as equal to a median value of the input samples in the window.
3. The portable connected device of claim 1 , wherein the operation of shifting the output value towards the central value comprises a linear combination of the previous output value and the central value, wherein the previous value is given a weight b and the central value is given a weight (1-b), b being a number between 0 and 1.
4. The portable connected device of claim 1 , wherein the operation of shifting the output value towards the upper limit or the lower limit value comprises a linear combination of the previous output value and the corresponding limit, wherein the previous value is given a weight a and the central value is given a weight (1-a), a being a number between 0 and 1.
5. The portable connected device of claim 1 , comprising a linear low-pass filter.
6. The portable connected device of claim 1 , comprising a baseline correction unit and a discriminator.
7. The portable connected device of claim 1 , the sensor being a capacitive proximity sensor arranged to determine whether a user is in proximity to the portable connected device based on a capacity seen by a sense electrode.
8. The portable connected device of claim 1 , wherein the sense electrode is also an emitter of radio waves.