I have a capacitive distance sensor that is used for THC (torch height control) for plasma or fuel/oxygen cutting. It consists mainly of a metal ring that is placed around the torch tip and moves a few millimeters above the sheet metal to be cut. The original sensor uses an analogue circuit and is sold for lots of money (~$400). I have the feeling that this could be done cheaper and better theese days, digitally and of course with a propeller.
The metal ring (see picture) has around 50mm inner and 80mm outer diameter. I measured the capacitance against the metal surface with an LCR meter. It's between 30pF when nearly touching the plate and 20pF when held 100mm above it. Of course the relation is highly non-linear but that's not the problem. It could be corrected easily with a lookup table.
I have a few ideas for the measuring principle:
1) RC decay. Because of the low capacitance this would need a very high resistance of >1Mohm to achieve reasonable resolution. I think near a source of lots of ionized particles (smoke from the plasma arc) this is not a good idea.
2a) build an LC oscillator and measure the resonant frequency. This can be done very easily with a counter. However, I don't know what influence the parasitic effects have. The ground plate can't be connected with a short cable as the sensor is moving long distances horizontally.
2b) do the same with an RC oscillator.
3) apply a high frequency signal with fixed amplitude and frequency and measure the AC current. If the frequency is high enough and the signal current is well above the leakage current this should work relativey independent of the ground impedance (length of ground return path). However, small HF AC currents are not easily measured accurately.
I know that capacitive sensors are used in scales with amazingly high precision. This is achieved by measuring differentially. The sensor plate moves between two reference plates, both capacitances are measured and the ratio is calculated. This way all unwanted effects like thermal expansion and drift of component values are cancelled out. In my situation this is not possible. But on the other side I don't need high precision. 0.1mm resolution and 1mm accuracy in the range 1..20mm and 5mm accuracy to 100mm distance would be sufficient.
Anyone who has a different idea or knows a schematic trick that could be helpful?
Of course, protection is important. In the case of a crash the arc (up to 200V and 300A) could hit the ring. The ring can be replaced quickly but the circuit should survive this. The arc current is DC so a T-filter with two high-voltage caps and a gas discharge tube (used for lightning protection) should do the job. The caps are effectively in series with the capacitance to be measured. If they are 1nF or greater their influence can be neglected.