Compensation of motion artifacts in capacitive measurement of electrophysiological signals
The invention relates to a system and a method in which an electrophysiological signal is sensed capacitively with at least two closely spaced electrodes such that the electrodes experience strongly correlated skin-electrode distance variations. To be able to derive a motion artifact signal, the capacitive coupling between the electrodes and skin is made intentionally different. With a signal processing means the motion artifact signal can be removed from the measured signal to leave only the desired electrophysiological signal. Since the measured quantity is dependant on the electrode-skin distance itself, the system and method do not need to rely on the constancy of a transfer function. Hereby, they give reliable motion artifact free output signals.
1. A system for providing motion compensated capacitive measurement of electrophysiological signals, the system comprising:
a sensor probe comprising a first capacitive plate and a second capacitive plate arranged to be placed in a proximity of a skin of a human or animal body, wherein a first capacitance between the first capacitive plate and the skin is different from a second capacitance between the second capacitive plate and the skin, and wherein the first capacitive plate is connected to an input of a first amplifier for providing a first measurement signal and the second capacitive plate is connected to an input of a second amplifier for providing a second measurement signal as a measure of an electrophysiological signal of the human or animal body; and
a processor connected to the sensor probe and configured for performing the acts of:
receiving said first and second measurement signals from the first and second capacitive plates;
determining a motion modulated signal from at least one signal of the first measurement signal and the second measurement signal; and
providing a motion compensated output signal from the motion modulated signal and the at least one signal.
2. The system according to claim 1 , wherein the first and second capacitive plates of the sensor probe have mutually different areas.
3. The system according to claim 1 , wherein the sensor probe moreover comprises an active shield.
4. The system according to claim 1 , wherein the determining act comprises calculating a difference between the first measurement signal and a factor applied to the second measurement signal, and wherein the factor is a ratio between the first and second measurement signals integrated by respective integrators.
5. The system according to claim 1 , wherein the determination act comprises averaging or low pass filtering of the first and second measurement signals over a predetermined time interval.
6. The system according to claim 1 , wherein the providing act comprises using a least-mean-square algorithm to adaptively remove the effect of the motion modulated signal from the at least one signal.
7. The system according to claim 1 , wherein the first and second capacitive plates are placed so close that they effectively measure the same signal of the user.
8. The system according to claim 1 , wherein the first and second capacitive plates of the sensor probe have different distances to a measurement surface of the sensor probe.
9. The system according to claim 1 , wherein the first and second capacitive plates of the sensor probe have different distances to a measurement surface of the sensor probe and the first and second capacitive plates have mutually different areas.
10. A signal processing unit for compensating for motion, said signal processing unit comprising:
a first amplifier configured to receive a first measurement signal from a first capacitive plate of a sensor probe arranged to be placed in a proximity of a skin of a human or animal body; and
a second amplifier configured to receive a second measurement signal from a second capacitive plate of the sensor probe, wherein a first capacitance between the first capacitive plate and the skin is different from a second capacitance between the second capacitive plate and the skin, wherein the first and second measurement signals are measures of an electrophysiological signal of the human or animal body, said signal processing unit being configured for performing the acts of:
determining a motion modulated signal from the first and/or second measurement signals; and
providing a motion compensated output signal at least one signal of the first measurement signal and the second measurement signal.
11. A non-transitory computer readable medium embodying computer instructions executable by a processor to enable a computer system comprising at least one computer having a data storage associated therewith to control a signal processing unit according to claim 10 .
12. A method for providing motion compensated capacitive measurement of electrophysiological signals, the method comprising the acts of:
placing a sensor probe in a proximity of a skin of a human or animal body, wherein said sensor probe comprises a first capacitive plate and a second capacitive plate, wherein a first capacitance between the first capacitive plate and the skin is different from a second capacitance between the second capacitive plate and the skin;
providing a first measurement signal from the first capacitive plate to an input of a first amplifier;
providing a second measurement signal from the second capacitive plate to an input of a second amplifier, the first and second measurement signals being a measure of an electrophysiological signal from the first and second capacitive plates;
determining a motion modulated signal from at least one signal of the first measurement signal and the second measurement signal; and
providing a motion compensated output signal from the motion modulated signal and the at least one signal.
13. A sensor probe for providing motion compensated capacitive measurement of electrophysiological signals, the probe comprising:
a first capacitive plate connectable to an input of a first amplifier;
a second capacitive plate connectable to an input of a second amplifier, wherein the first capacitive plate and the second capacitive plate are arranged to be placed in a proximity of the a of a human or animal body, wherein a first capacitance between the first capacitive plate and the skin is different from the second capacitance between the second capacitive plate and the skin, and wherein the first and second capacitive plates are arranged for providing a first measurement signal and a second measurement signal as a measure of an electrophysiological signal of the human or animal body.