Capacitance Sensor Guarding Principles

Let’s talk a moment about guarding principles. Guarding is a very important aspect of probe construction when you’re making capacitive-based displacement sensors.

So electricity, and in particular, the principle of capacity relies on creating electric fields, and these electric fields, when visualized, look very similar to magnetic fields. So if you think of the traditional view of a magnet you have these curling lines that go from the north pole to the south pole, or from one north pole to another south pole, and that’s very similar to what electric lines look like.

On this right side view you have a cross-section view of what the probe looks like. Here you have these electric field lines that are generated between the sensing surface of a probe, as well as the ground and target, and they go straight down, but as you get closer to these edges here you can see that they kind of curl out a little bit. Whenever you have these lines curling out, that actually will change the linearity of your result. So if your target moves 10 microns you’d actually get different results if you don’t have a linear response.

In this case what guarding does, is you’ll see a sensing area which is that center ring in the center, you have a guarding area which is that concentric ring that goes around it, as well as the body which is also grounded.

What the guarding does is by forming itself to the same electrical potential as the sensing area, but being actively driven, it’s able to kind of nullify these curling electric potential lines. So these electric potential lines because they’re being driven by that guard ring no longer are relevant in the actual measurement of capacitance between sensing and target and this helps to change the resulting capacitance graph from something that looks very curved to a more linear response as one would expect when you’re moving in a linear fashion from target to sensing phase.

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