There are a few intrinsic advantages of capacitive sensing:
There are varous applications:
A capacitive sensor is a sensor, mounted on a moving object, which transfers a displacement of the object (on which is it mounted) into a change of it’s capacitor value. The change is capacitor value is measured.
In an ideal measurement set-up the change in capacitance is transformed, in a linear way, to a change in voltage. The block diagram represents this transformation. Speed op operation: we aim for a system which can measure mechanical vibrations, this means that besides the measurement of static distances we also want to measurement fast movements of mechanical structures.
A common sensitivity setting is $100 \mu m$ displacement results in an DC output of $1V$. This means that for every $100 \mu m$ change in distance, the output voltage changes exactly $1.0V$.
Before explaining the principle of a capacitive sensors let’s explain why an AC voltage is used to bias the capacitive sensor.
When a DC voltage is applied to a capacitor the plates of the capacitor will be charged and that’s it. When an AC voltage is applied to a capacitor the current which flow’s ‘through’ this capacitor depends on the value of the capacitor.
The larger the distance between the plates, the smaller the current will be (assuming a constant AC voltage applied to this capacitor), this according to the capacitor impedance equation $Z=1/j \omega C$ with $\omega$ the frequency [in radians] of the AC voltage applied to the capacitor, and C the value of the capacitor.
As shown in the following slides, any change in the capacitor can be used to sense something with a capacitive set-up:
When using the (approximated) expression an error is made due to stray fields (fringes) at the edges of the plates. The practical implication of this error is a non-linear relationship between displacement and capacitance.
Generally, electric fields are better manageable than magnetic fields: by (active) guarding it is easy to create electric fields that are homogeneous over a wide area. This is the major reason why displacement sensors based on capacitive principles have excellent linearity
The effect of stray fields can be reduced by the application of guarding. One electrode is grounded; the other, active electrode of the capacitor is completely surrounded by an additional conducting electrode in the same plane, and isolated from the active electrode. The potential of the guard electrode is made equal to that of the active electrode (active guarding). The result is that the electric field is homogeneous over the total area of the active electrode, assuming infinite guard electrodes and a zero gap width between the two electrodes. However, since the guard electrode has finite dimensions and the gap width is not zero, a residual error occurs. This error depends on the dimensions of the guard electrode and the gap. As a rule of thumb, for x/d and d/s equal to 5, these errors are less than 1 ppm.
There are four commonly used basic methods to read-out a capacitive sensor:
These are the chapters for the Sensor Technology course: