The best way to measure low capacitance

[Keiv]

Hi! I want to build a volumetric sensor to determine the volumen of fruits while they are still in the tree, i mean a "non-intrusive" sensor, by measuring the capacitance variation of a simple (and big) parallel plates capacitor when I put the fruit between the plates. Anybody knows a simple way to measure this kind of capacitance? I mean, to turn the capacitance into a voltage signal that can be read by a A/D converter. I was thinking in a square wave generator feeding the RC circuit an collecting the time the capacitor takes in discharging to a certain level using the A/D and doing the calculations with a PIC micro (I will show the volumen in a digital display).

Any suggestions? Thanks in advance, and sorry about my english

 

[ambertec]

Hello, Kiev.

It would be interesting to know the pF to fruit ratio involved!

One problem of measuring a small capacitance is doing so in the presence of the capacitance of the cable that feeds your parallel plates. Please see if you can find a copy of Electronic Design for December 5, 1991. Turn to page 113 and you will find an item I wrote entitled "Vary Capacitance to Positive or Negative". It shows a way to negate the bulk of your cable's capacitance so that the small capacitance of your plates will be easier to discern with your sensing circuitry.

If you can't find the item (ten years is a long time), e-mail me and I'll send you a scanned copy of it.

Good luck.

John Dunn - Consultant
ambertec@ieee.org

 

[Keiv]

Thanks for the advice! I'll try to find the article. When the sensor is ready, I'll post some conclusions here.

Anyone can give me advice in how to measure the capacitance by RF means?, or there is any IC capable to interpret RF amplitudes (RMS) to DC volts?

Thanks!

 

[jbartos]

Suggestion: The capacitance between plates C is:
C=Ke x Eo x A / d in Farads
where
Ke is relative permitivity dependent on the volume of fruit in the tree, frequency in Hz, etc.
Eo is permittivity of free space = 8.854188 x 10**(-12) ~ 1/(36 x pi x 10**9) in Farad/meter
d is a distance between the plates in meters
A is the area of conductive plates in meters**2