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Reading Voltage off a Capacitive Force Sensor

wtwm94

Hi everyone, this is my first time visiting the element 14 community as I am struggling with a project at the moment and I hear that this was a great place to get help with electronics. As I am new, I apologise in advance if this pops up within the wrong community. As the title suggests, my project involves me building a Capacitive Force Sensor, which I have done and have done some initial testing on it using an LCR Bridge and I managed to get the Capacitance to vary when I applied a Force to it.

 

The main issue i'm having at the moment is with building a circuit in which I can obtain a voltage reading off the sensor which I can then use to find the force by using the equation V=kF^2. So far the circuit designed is as follows:

 

image

 

As can be seen from the picture, the circuit consists of a 5V input from a bench power supply and a constant current source, which is represented by a PSSI2021SAY IC (http://assets.nexperia.com/documents/data-sheet/PSSI2021SAY.pdf ) with a 10K ohm resistor connected across pins 4 and 5 to set the current to 0.1mA. The sensor is represented by the Capacitor with the output being measured on an oscilloscope where a Sawtooth wave should be observed. An IN4148 Diode and Pulse Generator are also used.

 

The pulse generator generates a square wave where the frequency is set by using the following equations:

 

image

i/C is inverted to find the frequency. For example, if I were to have a current of 0.1mA and a 1pF capacitor, i/C =1x10^8 V/s which would correspond to 100Mhz which would what the pulse generator would be set to.

 

For the purposes of testing and calibrating the concept, I have opted for testing fixed capacitors to try and obtain a base reading as my sensors seem to fluctuate a bit.

 

Here is the circuit created up close:

 

image

 

And here is the circuit hooked up to the pulse generator, oscilloscope and bench power supply:

 

image

 

The issue is that I am not getting the desired sawtooth wave output, even if I fiddle around with the frequency. I have used various capacitors to no avail and have checked the circuitry multiple times to try and see if it was built wrong. I have tried grounding the circuit as well using the ground of the power supply and without which doesn't seem to make a difference. The PSSI2021SAY was tested and was found to be working.

 

Perhaps there is a missing step in my maths? Or the circuit is built wrong? or my power supply and/or pulse generator are configured wrongly? Any help would be greatly appreciated as the project is due to be finished in a few days. Thanks for any help in advance!

  • 0

    To get a sawtooth, the pulse generator must periodically discharge the capacitor through the diode. (briefly short the capacitor to ground)

    In your circuit it is not clear what the pulse generator circuit is, so we can't analyze why it is not working.

    Typically the pulse generator would not be powered by this circuit but would drive a transistor to short out the capacitor.

    An SCR might work to periodically short the capacitor without a pulse generator or possibly an avalanche diode would work as well.

  • 0

    Hi,

    You've placed a formula in your text, but have you used it with the values you're testing with. The capacitor in the photo looks of the order of microfarads, and the photo also shows the function generator set to 10MHz. What voltage do you expect across the capacitor? (not going to give the answer because this looks like college work).

    Secondly, what volts/division setting will the tool you're using to observe this (oscilloscope) need to be set to, in order to see this?

    By the way, unless there is a specific well thought out reason, it is best practice to connect the ground clip of the oscilloscope to the 0V point on your circuit (not suggesting there is an issue with this causing the problem you observe, but it could cause accidental damage in other future circuits if your 0V point ends up connected to ground). It isn't good to assume that the oscilloscope is like a multimeter set to volts, where you can use the two probes (red and black) at any points.

     

    Also you may have other issues when you scale things to the settings for your real sensor (guessing that will have a capacitance of the order of pF) but that's a different story. For that, you cannot assume that all parts are designed to function at very high frequencies in the same manner that they worked at low frequencies, because the internal circuitry and structures of parts have capacitance too, and (for some parts and prototype construction methods) it could be of the same order as the capacitance you're trying to measure.