Measurement of inductor's reactance - Page 1
Forum section: electronics / projects

author: ricko_uk
author_rank: Super Contributor
timestamp: January 27, 2020, 04:19:13 am
content: Hi,I need to measure the complex impedance of an inductor for Eddy current testing so I can display the complex impedance on a complex impedance plane exactly like this:https://www.nde-ed.org/EducationResources/CommunityCollege/EddyCurrents/Instrumentation/impedanceplane.htmThat is exactly the application I am trying to replicate.What is the best circuit to interface to a micro so I can read two signals giving both the resistance and the inductive reactance?Can anybody suggest any reference circuit? I googled quite a bit but couldn't find any with a proper explanation. Any feedback/info/ref-schematics are much appreciated!Many thanks

author: MasterT
author_rank: Frequent Contributor
timestamp: January 27, 2020, 05:29:32 am
content: Study AD5933.What kind of microCPU?

author: ricko_uk
author_rank: Super Contributor
timestamp: January 27, 2020, 09:37:54 pm
content: Thank you MasterT,that is an interesting IC but too expensive. Every device we are building has up to 64 channels and using that IC just for the impedance would mean more than 700 USD just for those ICs and there is plenty more ICs/electronics for each channel. So we are looking for a much lower cost solution. The frequency can vary from 10KHz to 350KHz and the coils impedance can vary between 1R to 1K so outside what that IC can do. Any other suggestion?Ref the micro we are planning to use any mid-range Atmel micros which are the ones we usually use (no DSP nor ultra high speed micros).Many thanks

author: DaJMasta
author_rank: Super Contributor
timestamp: January 27, 2020, 09:59:40 pm
content: What you're making is basically an LCR meter. So you need to be able to generate a signal to send into the DUT, then be able to measure the current and voltage waveform that results. From there it's just math, which is fairly well documented, but the whole procedure gets harder and harder to execute as test frequency increases.The actual hardware depends on what your requirements are. It's going to be much better to use an analog mux than to use a complete system on every channel unless you need synchronized simultaneous sampling. 10kHz shouldn't be too bad in terms of performance requirements, but a 350kHz capable LCR meter is generally a pretty expensive thing (even good benchtop ones can get real pricey beyond 100kHz). Measuring this accurately depends on fine detail simultaneous current and voltage measurements (or ones that are correlated to the same phase of the input waveform), and fine discrimination of the phase of the signal. You could think of the requirements as a signal generator, a high resolution scope with two channels, and a current probe.I'd suggest reading up a bit on impedance and what factors into it as a starting point, then very clearly define how you need to make these measurements? Do they need to be time correlated? Do they need to be continuous? How much time can be spent for each measurement if switching between multiple inputs? Then it would be a deep-dive into LCR meter architecture, and taking a look at budget solutions and whether they could work at all for you.

author: MasterT
author_rank: Frequent Contributor
timestamp: January 27, 2020, 10:36:35 pm
content: Quote from: ricko_uk on January 27, 2020, 09:37:54 pmI did a project in the past that replicated analog device AD5933, using arduino DUE (sam3x8 cortex-M3). It has dual DAC & ADC with sampl. rate up to 1 MHz. For smaller micro, like atmega it still would be possible to get 350 kHz with help ad9833 and running adc in "undersampling" mode 10-76 ksps or so.Math is quite simple, Goertzel or FFT provides phases on two channels than complex impedance easy to obtain out of equation.Here they talk about capacitance, but there is no difference for inductance,https://meettechniek.info/passive/capacitance.htmledit:inductance also discussed:https://meettechniek.info/passive/inductance.html

author: graybeard
author_rank: Frequent Contributor
timestamp: January 28, 2020, 01:42:16 am
content: I give examples of maesuring impedance from 10 Hz to 10 MHz in the video above. It works for any passive device including inductors. I beleiv e I even have some inductive examples.