This inductor is in motion and any self induced emf must be ignored, only the change in inductance is to be sensed.
This inductor is in motion and any self induced emf must be ignored, only the change in inductance is to be sensed.
Hi,
emf voltages are cancelled out by using AC test signals, as you suggest anyways.
The test frequency has to be > 2x emf / induced voltage / pink noise (in amplifier!) frequency.
The "Smart Tweezers" (Smart Tweezers ST5S  Professional Handheld  Smart Tweezers LCR Meter )
use a brilliant method to measure also inductance. Unfortunately the exact method is secret.
But what I have found out is that they use two different frequencies after onother (960Hz > 96Hz) to calculate R, C & L with a TMS mikrocontroller.
I am pretty sure they have the sine generator (DAC) and in series is a resistor they use as a shunt for current measurement.
Together with a voltage measurement at the test object, they can calculate the ratio of these two and the phase shift between them.
Out of this it is possible to calculate the inductance value. I don't know exactly why they use two different frequencies, but they can calculate not only inductance but its parasitic resistance simultaneously. For precise calculation of L this is necessary anyways.
Or
you may supply a triangle shaped ACcurrent and calculate L & C out of the voltage signal, that will give a rectangle+triangle shaped low level voltage (=> zerodriftOPAs). The amplitude of the pure rectangle signal in it is a measure of inductance, the triangle shaped part of it gives the series resistance. But I have no experience about achievable accuracy and errors caused by hysteresis of inductor.
Best regards
Axel
Hi AxelI really like your second suggestion. It obeys the KISS principle. The hysteresis would be absent and the dc resistance is only between 3 to 10 ohms. The triangular current drive can be done at a frequency above any self induced signal frequencies. The result should be reasonably linear, if not I can design a banana bender circuit to make it linear. I know there are several proven designs for current pumps, so I can proceed.Thanks again for your elegantly simple solution.Gary
From: Alexxx <analog@analogvm.hosted.jivesoftware.com>
To: Gerhard Spangenberg <garyspang@att.net>
Sent: Wednesday, April 25, 2018 3:41 AM
Subject: Re:  Re: Need a linear inductance (35mHY) to DC voltage converter (10Hz to 1KHz). Any nonbridge suggestions? Thanks
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Re: Need a linear inductance (35mHY) to DC voltage converter (10Hz to 1KHz). Any nonbridge suggestions? Thanks
reply from Alexxx in Instrumentation Amplifiers  View the full discussionHi, The "Smart Tweezers" (Smart Tweezers ST5S  Professional Handheld  Smart Tweezers LCR Meter )
use a brilliant method to measure also inductance. Unfortunately the exact method is secret.
But what I have found out is that they use two different frequencies after onother (960Hz > 96Hz) to calculate R, C & L with a TMS mikrocontroller.
I am pretty sure they have the sine generator (DAC) and in series is a resistor they use as a shunt for current measurement.
Together with a voltage measurement at the test object, they can calculate the ratio of these two and the phase shift between them.
Out of this it is possible to calculate the inductance value. I don't know exactly why they use two different frequencies, but they can calculate not only inductance but its parasitic resistance simultaneously. For precise calculation of L this is necessary anyways.
Oryou may supply a triangle shaped ACcurrent and calculate L & C out of the voltage signal, that will give a rectangle+triangle shaped low level voltage (=> zerodriftOPAs). The amplitude of the pure rectangle signal in it is a measure of inductance, the triangle shaped part of it gives the series resistance. But I have no experience about achievable accuracy and errors caused by hysteresis of inductor. Best regards Axel
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Hi garysapng,
I am looking in to your question will have to get back to you ASAP once information is available.
Regards,
Joven