Self-Mixing Interferometer vs Capacitive Sensor

[Isaiah Gray]

I'm trying to build a non-contact displacement sensor, sensitive to motion in the micrometer to millimeter range, for a research project. My two best choices are a self-mixing laser interferometer and a capacitive displacement sensor. Has anyone had enough experience with these types of sensors to know if they are doable?

 

[berkeman]

I'm trying to build a non-contact displacement sensor, sensitive to motion in the micrometer to millimeter range, for a research project. My two best choices are a self-mixing laser interferometer and a capacitive displacement sensor. Has anyone had enough experience with these types of sensors to know if they are doable?

I'm not able to say that I have a lot of experience, but the laser interferometer would seem to the the better choice, IMO.

Capacitive sensing involves lots of problematic issues, in my limited experience. It may be that your measurement distances are short enough that capacitive sensing may be practical, though. Can you provide more complete specs on the physical space available for the sensor (capacitor area) and accuracy requirements? What are related EMI issues (are there any things nearby the space that would be interfered with by the capacitive sensing voltage waveform)? Can you control the humidity of the gas in the gap of the variable capacitive sensor?

 

[berkeman]

BTW, you probably already know that the laser interferometer measurement gives you relative displacement information. It does not give you any absolute displacement information. You need a "limit stop" type of input to give you a starting position, before you can use the interferometer's information to give you the displacement from that limit stop.

 

[lionprecision]

You must determine your desired resolution/accuracy to make a good selection. All technologies have their weaknesses and you can only choose when you know when the those weaknesses are beyond your tolerance.

Regarding Capacitive Sensors:
I would say capacitive sensors are also relative measurement devices (again, depending on tolerance). Capacitive sensors usually exhibit offset shifts over long periods of time (months/years) making them poor absolute position devices at the sub-micron/nm level.

The humidity issue for capacitive sensors mentioned by berkeman is real but not usually a problem. Humidity-related errors are offsets, not sensitivity errors, so they remain accurate over a wide range of humidity provided humidity doesn't change significantly during measurement. These errors are usually in the submicron range.

Larger ranges require larger sensing areas. Millimeters is a large range for cap sensors. Up to 2mm can be achieved with a 3/8" diameter probe. If you want 5mm you're now using an 18mm diameter probe. The target diameter should be 130% of the probe diameter, or special calibration will be required.

Capacitive probes must be very near the target compared to interferometers.

Cap sensors cost much less than interferometers.

I've never heard of a cap probe causing EMI because of the sensing field. Our probes have 1MHz, 100Vp-p drive voltage present on the sensing surface and extremely low currents. They are used in electron microscopy positioning stages in the presence of high DC voltages (2-3kV) without any problem.

 

[Mike_In_Plano]

It's always best to get what you can off the shelf.
I haven't worked with interferometry systems, but according to people I worked with, they purchased the required materials from HP.
I used to design capacitance based sensors for a living. They could give you amazing resolution, without the fuss of lasers (i.e. 10nm), but we never attempted to use them more than about 1/20 of a mm away. Also, you either needed to be able to design exceptional capacitance measurement or buy it from another instrument house (Andeen-Hagerlan).