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Thread: Measing distance with capacitive touch

  1. #1
    Junior Member
    Join Date
    Aug 2014

    Measing distance with capacitive touch

    I have been using a custom but traditional load cell design with strain gauges to measure force but i recently realised that this can also be done with a load cell designed to use capacitive measurment. I have designed a load cell that will have around 0.3 mm movment between the capacitive sensor and target. From basic experimentation it seems that this will work quite well with the touchRead function with it having great responce and accuracey at very small distances (1mm).
    In my use i dont really need the accuracey the strain gauges offer and the main motivation is that i really hate the slow assembly time of strain gauges and im really facinated by the capacitive measurment.

    My question is I have seen in another thread people are adding a capacitor to the plate used to transmit, what effect does this have ? In experimenting it seems to definitley make a difference but im unsure how this works or how to calculate its effect. Is there a better method to do this task than the touchRead library?
    If any one has any tips on tuning this set up for a smooth signal and good accuracey i would like to hear them.

  2. #2
    Junior Member
    Join Date
    Aug 2014
    Still after any info any one might have but thought i would post that i made a basic test unit with 2 old pcbs cut into a very stiff bending beam load cell and got a stable output and enough variation in capcitance to show it working as expecte.i need to get more range change of capacitance yet but its suprising what can be done with next to nothing.

  3. #3
    Senior Member Epyon's Avatar
    Join Date
    Apr 2013
    You've built a basic a capacitive proximity sensor. This works because your finger (or other object you used) acts as a a dielectric for the PCB capacitor. When inserted into the electric field the PCB cap projects, your finger disturbs this field resulting in a change of capacitance that can be measured (usually by an oscillator circuit). As long as the object being measured has the same dielectric constant, changes in capacitance and therefore distance measurements will be reliable. If the conductance changes, e.g. you want to measure the distance of something metallic in stead of your finger, the distance/capacitance ratio will be different as well.

    If you use two seperate plates, the effect is the same but usable range is higher. If the distance between the two plates changes, so will the amount of dielectric (air) and the resulting capacitance. Basically you've then made a variabele air capacitor.

    Capacitive distance sensing can be extremely precise when operating conditions do not change, but because that only happens in specific cases (or in MEMS) it is generally more used as a relative proximity or touch sensor. Usable range is not that high, with a few inches being the max most sensors can handle.

    You can make your own measurement circuit by building an oscillator that uses the variable capacitor to generate a frequency. The frequency will change with the capacitance. You can then use FreqMeasure to measure the frequency and calculate the distance, of use a frequency-to-voltage converter to read a varying voltage into an ADC and do the same. Lots of examples on the internet.
    Last edited by Epyon; 02-27-2017 at 11:45 AM.

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