Inductance converter for position sensor - Page 1
Forum section: electronics / projects

author: rvalente
author_rank: Frequent Contributor
timestamp: October 19, 2022, 11:09:49 am
content: Hello Mates,I need to use and test an special Wabco position sensor, it is a variable inductor varying from 18 to 50mH accordingly to the plunger position.Also, I need to detect sensor short and sensor open. The signal will be read by a industrial PLC so I need to have a frequency, voltage or duty cycle output.The environment is industrial, a little noisy with a 20KW AC motor running really close to everything.1st strategy - Wien oscillator at 1KHZ + 180 series resistor, inductor (DUT) - Rectifier - integrator - final amplifier (attached picture, my design, I'm definitely not a analog pro)Pros: not a relaxation oscilatorCons: 6 opamps, need symmetrical power, big circuit2st strategy:Relaxation circuit, read the inductance as frequency (e.g. 1 to 10Khz)Pros: not sure what circuit to pickCons: the cable inductance and environment noisy seams to play a bigger hole here.Any suggestions?Greetings,Renan

author: RoGeorge
author_rank: Super Contributor
timestamp: October 19, 2022, 11:22:30 am
content: TI has some chips dedicated to inductive sensing, inductive to digital converters. See if they might be useful for your sensor:https://e2e.ti.com/blogs_/b/analogwire/posts/inductive-sensing-meet-ldc-the-industry-s-first-inductance-to-digital-converterhttps://www.ti.com/tool/LDC1000EVMhttps://www.mouser.com/datasheet/2/405/ldc1000-556645.pdf

author: rvalente
author_rank: Frequent Contributor
timestamp: October 19, 2022, 11:39:35 am
content: HelloI need something simpler, analog based, easier to develop, no micro controller dependent

author: moffy
author_rank: Super Contributor
timestamp: October 19, 2022, 12:06:33 pm
content: I would go for the external oscillator, in your case Wein bridge, to make it more noise immune. You can then use a bandpass filter for reception or even synchronous demodulation. Problem with that approach is that you don't get a direct read out of inductance i.e. |Vo| = (|Vin|*wL)/(w*L*w*L + R*R)^0.5Amplitude stability is important unless it is a ratiometric measurement.

author: rvalente
author_rank: Frequent Contributor
timestamp: October 19, 2022, 12:36:11 pm
content: Actually I do not need to read inductance directly, I need to read the sensor position which is inductance based.Any circuit suggestions? App notes or books?

author: moffy
author_rank: Super Contributor
timestamp: October 19, 2022, 12:54:24 pm
content: Quote from: rvalente on October 19, 2022, 12:36:11 pmWhat is the formula for inductance vs position? that is important to know in giving advice. If you want to measure inductance directly, you could use the formula V = L*di/dt or excite the inductor with a constant voltage pulse of fixed duration so that L*Ie = Vp*Dt, so that the value of Ie at the end of the pulse is inversely related to inductance. Not a great method for your application as it would be susceptible to noise. If instead you excite the inductor with a constant voltage then measure how long it takes to get to a fixed current value (below saturation, use a comparator) the period will be linearly related to inductance, because both Vp and Ie will be constants. But it is still susceptible to noise. I am also assuming that the inductor has 0 current at the beginning of the pulse.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: October 19, 2022, 04:24:48 pm
content: I'd think any L-C oscillator circuit would be simple, followed by a simple slope converter to do the F-V conversion. If the oscillator stops oscillating if the coil is open or short, you'll know by the output voltage.

author: jonpaul
author_rank: Super Contributor
timestamp: October 19, 2022, 08:22:25 pm
content: use optical encoder

author: rvalente
author_rank: Frequent Contributor
timestamp: October 21, 2022, 08:49:48 pm
content: Quote from: Conrad Hoffman on October 19, 2022, 04:24:48 pmI'm not choosing the free oscillator, all the cable parasitic inductances and capacitances would take a bigger whole in the circuit behavior.Quote from: moffy on October 19, 2022, 12:54:24 pm