FDC1004: FDC1004 suitable for capacitive force transducer design ?

Hello Ralph,

For sensing metal targets, I recommend inductive sensing instead of capacitive.   You can observe a greater delta in the data and higher resolution.  

Take a look at the LDC1612(28 bit) and LDC1312(12 bit) devices, which may work in your application.    

https://www.ti.com/product/LDC1312 

https://www.ti.com/product/LDC1612 

well the mechanic constrains are the easiest to solve using capacitive principle, so ma question remains open

Can you explain the use case further?  My understanding is that you will have 2 metal washers(parallel to each other?) which you would like to determine the distance between.  Where would you place the sensing electrode?  If you have a simple drawing, that may help as well.  

The two washers will be around 10mm apart, but what is the expected min/max distance that needs to be measured and at what resolution?

well ideally the washer are the electrodes and they are parallel. The application is not interested in absolute values, it is about if changes are occurring over up to 50 years and more. the resolution should be ideally 500nm or 0.5 µm  

I thought maybe the washer would be used as the electrode.  Thanks for clarifying!  I'm looking into this and will provide feedback shortly.

Ralph,

This should be possible to use the washer as an electrode and measure distance based on capacitance change.  In a 50 year period, I would expect that the composition of the metal washer will change due to environment, so it would be useful to also have a reference sensor that can be used to calibrate over time.  

Whether your required resolution is feasible would depend on what the total range of movement will be.  

Well we would use stainless steel of course. However I studied the inductive alternative, while the capacitive principle should not give any issue with stainless how would that look with your inductive alternative, lets say using a 316 or 302 stainless steel alloy? these are not magnetic What SS alloys would you recommend for using inductive principle?

And will the FDC1004 be suitable? it looks that the capacitance must be VERY small.

Hi Ralph,

For inductive sensing, stainless steel alloys can be used.  The important specification will be the resistivity of the material, which will have an effect on the skin depth.  Section 2.2 of the app note below discusses this.  It is recommended for target thickness of 2 to 3 skin depths.  

https://www.ti.com/lit/an/snoa957b/snoa957b.pdf   

You can also use the LDC calculator tool to estimate the skin depth for a given material, thickness, and frequency.  You may find that your target thickness or frequency needs to be adjusted for your proposed material.  

https://www.ti.com/tool/download/LDC-CALCULATOR-TOOLS 

In regards to the FDC, I am running some testing with FDC1004EVM and ring lug.  The initial testing shows that the ring lug does work as a sensor with ~1.1pF capacitance.  With my finger, I am able to easily saturate it to 16pF.  With a second ring lug as the target, I am seeing less range.  I plan to test this more and will provide the results later today.