impedance measurement, research for an accurate solution

Thread Starter

LittleZ

Joined Mar 5, 2021
3
Hi everyone,
I would like to create a sensor that will measure the impedance of a system with 2 rings. In between these rings is a thin layer of isolation but this one is not perfect, or at least I suppose. On the "long" run the system can be simulated as a capacitor.
For now I thought of doing an simple voltage divider but am not sure this is the more accurate option because of the possible parasitic capa coming from the cables and the structure. I read about the Auto-balancing bridge but was wondering if some of you have a better option? I am concerned of the space as well a bit... I kind of would like it to be not too big

Does anyone has some knowledge in this field? or could recommend some resources?
Thank you a lot in advance for your help :)
LittleZ
 

wayneh

Joined Sep 9, 2010
17,028
I would like to create a sensor that will measure the impedance of a system with 2 rings.
Two rings? What is the nature of the "system" that you want to measure? I mean, is it biological tissue, a circuit, pond water, what?

What frequency and range of frequencies are we talking about?
 

Mickey57

Joined Mar 5, 2021
2
Hi everyone,
I would like to create a sensor that will measure the impedance of a system with 2 rings. In between these rings is a thin layer of isolation but this one is not perfect, or at least I suppose. On the "long" run the system can be simulated as a capacitor.
For now I thought of doing an simple voltage divider but am not sure this is the more accurate option because of the possible parasitic capa coming from the cables and the structure. I read about the Auto-balancing bridge but was wondering if some of you have a better option? I am concerned of the space as well a bit... I kind of would like it to be not too big

Does anyone has some knowledge in this field? or could recommend some resources?
Thank you a lot in advance for your help :)
LittleZ
There is a nice article in circuit cellar from March 2019 #344
“Impedance Spectroscopy using the AD5933”.
I used the 5933 to measure brine oil mixtures to estimate Oil content.

Check out both the data sheets and application sheets at ADI. They will give you an idea on how to handle both low and high impedance measurements. Wires to dielectric constants of ceramics. Best of luck.
 

Thread Starter

LittleZ

Joined Mar 5, 2021
3
Hi everyone,
first all thank you a lot for your answer, I reply a bit late because it wanted to read a bit of the doc you advised me and made also some more research beside.

There is a nice article in circuit cellar from March 2019 #344
“Impedance Spectroscopy using the AD5933”.
I used the 5933 to measure brine oil mixtures to estimate Oil content.

Check out both the data sheets and application sheets at ADI. They will give you an idea on how to handle both low and high impedance measurements. Wires to dielectric constants of ceramics. Best of luck.
thanks for the advise Mickey57, I went to read the article and looked a bit to the datasheet. One point where I am not so sure, the impedance that I will measure will change at a time. How big is the change, I don't really know right now but I had the impression by reading the blog that it is mostly made to measure a "fix" value of impedance? or I misunderstood it ^^'

Two rings? What is the nature of the "system" that you want to measure? I mean, is it biological tissue, a circuit, pond water, what?

What frequency and range of frequencies are we talking about?
to answer you Wayneh, I want to measure the impedance generated by a tapered bearing. The value may varied at the beginning and once the system is well on running. And the frequency of the signal should already start around 1kHz, it could get higher than 1MHz but from what I read this could be critical because of the skin effect.

@Papabravo, yeah exactly however in the best of case i would like to make my own LCR meter ^^ I was wondering if you already have some tips/ways on what I should watch for or circuit/design I could use at the beginning :)
Thanks a lot for your help
LittleZ
 

Papabravo

Joined Feb 24, 2006
15,469
Hi everyone,
first all thank you a lot for your answer, I reply a bit late because it wanted to read a bit of the doc you advised me and made also some more research beside.


thanks for the advise Mickey57, I went to read the article and looked a bit to the datasheet. One point where I am not so sure, the impedance that I will measure will change at a time. How big is the change, I don't really know right now but I had the impression by reading the blog that it is mostly made to measure a "fix" value of impedance? or I misunderstood it ^^'


to answer you Wayneh, I want to measure the impedance generated by a tapered bearing. The value may varied at the beginning and once the system is well on running. And the frequency of the signal should already start around 1kHz, it could get higher than 1MHz but from what I read this could be critical because of the skin effect.

@Papabravo, yeah exactly however in the best of case i would like to make my own LCR meter ^^ I was wondering if you already have some tips/ways on what I should watch for or circuit/design I could use at the beginning :)
Thanks a lot for your help
LittleZ
Start here:
https://www.element14.com/community...ardware/blog/2019/07/23/building-an-lcr-meter
 

Mickey57

Joined Mar 5, 2021
2
We are all bound by a time frame and assumptions. Assumptions;1) a dielectric bering, air or oil film bearing (Not a contact type roller etc). 2) electrical contact to both stator and rotor. 3) The system is concentric (eccentricity, wobble gets hairy but measurable) The real portion of the impedance is very high 10K to meg ohms (essentially fixed, till the rotor touches the journal) the Reactance(X) is what will change as the two rings change gap or as fluid film gets thinner.

Try a capacitive voltage divider, an AC signal generator running 10 to 50 times sampling rate, and an Oscilloscope to measure the output. This setup can measure very fast phenomena. "Slow" changes can be interpreted as changing gaps, periodic fluctuations in output as eccentricity wobble or precession. Once you've got C measured, games can be played to follow the bering gap, dielectric constant of the fluid in the gap.

Lots of fun pushing the wave form through a spectrum analyzer (software) should highlight eccentricity and precession.
Best of luck
 
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