Testing and Troubleshooting Capacitive Sensor Probes

In summary, the capacitive probes are measuring blade tip health. Test equipment is being built to help with system troubleshooting. There is not a lot of information available on how to troubleshoot these systems. A transmission line test may be a good option.
  • #1
daniel1211
28
1
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?
 
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  • #2
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.
 
  • #3
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?
 
  • #4
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.
 
  • #5
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.
 
  • #6
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
 
  • #7
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?
 
  • #8
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.
 

1. How do you test the accuracy of a capacitive sensor probe?

To test the accuracy of a capacitive sensor probe, you can use a known reference object and compare the readings from the probe to the actual measurements of the object. You can also perform multiple tests with varying distances and orientations to ensure consistent and accurate readings.

2. What are the common troubleshooting techniques for capacitive sensor probes?

Some common troubleshooting techniques for capacitive sensor probes include checking the wiring and connections for any damage or loose connections, adjusting the sensitivity and threshold settings, and cleaning the probe surface to remove any dirt or debris that may interfere with the measurements.

3. How do you calibrate a capacitive sensor probe?

To calibrate a capacitive sensor probe, you will need a known reference object and a calibration tool. The calibration tool can be a special calibration kit or a simple metal plate. You will need to measure the capacitance of the reference object using the tool and then adjust the probe's settings to match the measured capacitance.

4. What are some factors that can affect the accuracy of a capacitive sensor probe?

Some factors that can affect the accuracy of a capacitive sensor probe include changes in temperature and humidity, electromagnetic interference, and surface conditions of the object being measured. It is important to consider these factors when setting up and using the probe to ensure accurate measurements.

5. Can capacitive sensor probes be used in harsh environments?

Yes, some capacitive sensor probes are designed to withstand harsh environments such as extreme temperatures, moisture, and vibrations. However, it is important to check the specifications of the specific probe to ensure it is suitable for the intended environment and application.

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