Patient Monitoring Using Capacitive Sensors

Capacitive sensors can be used to measure many different physical parameters that are important when monitoring the health of patients. In particular, the technical advances made by sensor manufacturers in using micro electro mechanical systems (MEMS), fabricated using silicon microchip manufacturing techniques, have opened up new possibilities for integration which improve the ease and adaptability of their use.

Capacitive sensors offer advantages over resistive or piezo-electric sensors because of their well-understood characteristics. In addition, they are free from the intrinsic noise found in all resistance (Johnson noise). The small changes in capacitance, however, place exacting requirements on the interface circuit.

Accelerometers

MEMS accelerometers were among the first devices to be used extensively. They are manufactured using silicon lithography techniques of deposition, etching, and lithography. In principle, a capacitive sensor consists of two parallel electrodes. The following formula determines the total capacitance:

C=ε ₀ε _rA/d, where ε₀ is the permittivity of free space, ε_r is the relative permittivity, A is the area of the electrodes, and d is the distance between them.

As a result, the capacitance changes with the distance between electrodes.

A common problem in the single electrode technique is that temperature will affect the result, along with other common mode effects. A much better approach is to use a differential input where a third electrode is used. Thus, any temperature effects will be cancelled out since both electrodes are affected in a similar way.

Accelerometer structures often consist of an array of movable fingers and a fixed mass. As the sensor accelerates, the cantilevers move and the capacitance between the fingers changes.

Typically, a change in capacitance of such a structure will be on the order of between 0.2pF/g to 1pF/g.

Gyroscopes

MEMS-based gyroscopes use the Coriolis Effect to determine angular velocity. The effect produces a coupling of energy between two vibration modes of a structure when it is rotated. The signal produced by the Coriolis Effect can be on the order of 4aF change in capacitance at the drive frequency, which is on the order of 10KHz. This type of readout requires advanced lownoise techniques such as synchronous demodulation.