Testing and Troubleshooting Capacitive Sensor Probes

[daniel1211]

I am working on a system that uses capacitance sensor probes, the system components are the probe, triaxial cable with LEMO connectors, BNC cable, oscillator, and a demodulator. I have had some trouble finding ways to troubleshoot the system other then basically testing continuity. Are there any proven methods that provide more in-depth ways to test this type of system?

 

[Baluncore]

What are the capacitive probes measuring?

Build a test jig that can place a probe in the situations where it's operation is most critical. That will make it possible to test the functionality of the complete system. Keep a certified good system handy so that when you find a system that does not work, you can substitute known good modules into the faulty system. That will identify the faulty module. Always keep the certified good modules with the test jig.

 

[daniel1211]

Hi Baluncore,
Thank you for your reply, we are testing blade tip health. I actually do have a test rig being built but it is more for the lab and would not be portable, I would also like to avoid removing the probe once it is in place unless we absolutely have to. I suppose we could remove the components from the suspect probe and test those with a known good probe.

Currently we view the data using an oscilloscope with a voltage vs time scale, do you know if this is the best way to view the data that we are receiving?

 

[Baluncore]

You are making the assumption that I know which field you are working in.
Are the blade tips part of a compressor or turbine?

To avoid sensor replacement for testing, one possibility would be to install two sensors and then verify that they are in agreement.

Are you testing as part of commissioning new equipment, or are you performing routine service testing through a long blade life.

 

[daniel1211]

Sorry, I am testing both compressor and turbine blades and it is for both new equipment and routine service. I am new to this field, but I have noticed that when it comes to methods for troubleshooting these systems that there is not much information that I can find. Some of the basic methods that I've seen employed are testing the coaxial cable, changing out the triaxial cable (we have no way of testing the triaxial cable right now), or pulling and recalibrating the probe. I believe that there should be a more methodical approach to troubleshooting these systems rather then shooting from this hip and hoping something works.

 

[Baluncore]

You might consider some form of transmission line testing. If you measure the capacitance of the sensors on the end of the cable at low frequency you will get a consistent result if the cable is intact.

Now consider using a very fast step time domain reflectometer. That will reveal an open circuit cable, but the end of the cable trace on the screen will show only that it has a short circuit or a capacitor to environment.
http://en.wikipedia.org/wiki/Transmission_line
http://en.wikipedia.org/wiki/Time-domain_reflectometer
http://en.wikipedia.org/wiki/Time-domain_reflectometer#TDR_in_aviation_wiring_maintenance

Now consider the situation where you use a sine wave to test the cable and sensor as a single unit. By selecting the optimum frequencies you can make accurate tests. The impedance of the transmission line can be known. When the frequency is such that the transmission line is multiples of 1/8 wavelength long, the capacitive sensor can be made to look like a resistor or an inductor.
http://en.wikipedia.org/wiki/Smith_chart

 

[daniel1211]

Awesome and thank you! I actually was wondering if you could use a Smith Chart but when I asked about it, either nobody knew what a Smith Chart was or they didn't know how to use it.

Can I ask when I was first learning about these probes I made a simplified assumption that the probe was basically a capacitor + wave guide + transmission line, would that be accurate?

 

[Baluncore]

There are a great many different types of capacitive sensors. I have no idea of the particular topology of your capacitive blade tip sensors. All I can do is consider a capacitive termination at the far end of a transmission line. I have no wave guide in my mind/model. Maybe a part number or a link to a manufacturers specification would help us identify a rational test procedure.