DIY LVDT Readout  - Page 1
Forum section: electronics / general

author: Noobster
author_rank: Contributor
timestamp: November 14, 2022, 08:21:26 pm
content: Hi,i hope you are not sick of more amateur questions from me, but you guys have been to helpful in the past. Sadly i should not need a precision voltage source for this application. My plan is to build a LVDT readout for precision equipment (indicators ,gages etc.). It should be able to be used with the large amount of probes i have aquired over the years (two i even wound myself). Because of different manufacturers they of course differ in resistance and inductance. I made a readout based on a ADC (peak detection circuit to get dc signal from ac). As AC source i used a 40kHz pwm signal. Due to my inexperience this is not very precise (0.5µm fluctuations which is too much for some measurents). The digital readout is very impractical in practice so now my new readout shall be analog. I plan to use this voltmeter, hopefully i can change the internal resistors to get lower voltage maximums. The AC signal i plan to create again with a 555 timer, the peak voltage adjustable with a adjustable regulator (in order to be able to "calibrate" gages with different sensitivities) .The reference signal is provided with a potentiometer, maybe two for rough and fine adjust (to be able to zero the readout). the signal from the lvdt and the reference signal go into an instrumantation amplifier AD8221 (maybe ?) with different gains selectable by jumpers. Circuit idea here.https://youtu.be/UG6LV8v8W-0?t=1515here is a video from Robin Renzetti showing an application of such a measurement. This commercial solution is great but i would be limited to the OEM probes. Concerning my plans:- is this a decent plan or am i delusional?- is it stupid to use a pwm/offset square wave signal (is there a reason for a better solution like amplified resonator etc?)- what do i need to look for concerning the negative voltage for the amplifier (negative voltage generator ? does this influence accuracy ?)- concerning cables: i have used usb cables (shielded) for the probes in the past, are there better solutions? what to look for?

author: Kleinstein
author_rank: Super Contributor
timestamp: November 14, 2022, 09:31:06 pm
content: The plan is rather strange. Normally the LVDT will have separte drive an detect coils and already kind of a bridge build in. Normally somewhere in the middle there will be zero output and than to both sides more output with a change in phase a round zero.Peak detection is not a good idea. The suitable way is phase sensitive rectification. So the sense signal is mixed (e.g. switch between gain + 1 and -1) with the drive signal. In phase this gives a positive out, out of phase this gives a negative output. A digital way to implement the phase senstive recitification is from multiple ADC readings over the signal period and than taking the difference between the readings for the times where the drive is positive / negative, maybe skiping the edge samples.A square wave drive is OK and it also provides a good reference signal for the rectifier. One usually needs to make sure to have zero DC part, e.g. quite accurate 50:50 PWM ratio.A resonant drive is problematic, as it gets more tricky to get a stable amplitude.A universal unit can be somewhat tricky with different drive levels needed. The sense part is resonable universal at least.The cable should not be that critical - it is about gettting the ciruit right.

author: Noobster
author_rank: Contributor
timestamp: November 14, 2022, 09:59:22 pm
content: QuoteThis is how i thought of lvdts too, but a lot of my probes (oem) have only two windings, driving the coils as voltage dividers seemed logical to me when i built my first unit (and it worked okish).Quoteyes, in hindsight i am actually astonished by the mediocre accuracy i was able to pull of. I guess it relied heavily on the averaging of all the ADC samples. I hope the analog readout will achieve a similar effect.

author: Henrik_V
author_rank: Regular Contributor
timestamp: November 15, 2022, 09:08:15 am
content: I would give a modern soundcard a try. Generator and two input channels. Capture excitation and LVDT output.Use sine-fitting on both inputs , calc the ratio.

author: Noobster
author_rank: Contributor
timestamp: November 16, 2022, 01:54:45 pm
content: Quote. Good idea, but you would have to boost the signal, the windings have lower impedance compared to earphones. However i want to try the analog solution, probably battery powered. Seems like there is are not too many opinions on the topic so i will procede to test it out.Still one question: Is a standard negative voltage generator good enough for an instrumentation amplifier or does it significantly impact the readings ?

author: MarginallyStable
author_rank: Regular Contributor
timestamp: November 16, 2022, 06:10:53 pm
content: QuoteThis --^^^^

author: Noobster
author_rank: Contributor
timestamp: November 16, 2022, 09:15:09 pm
content: QuoteI might need some help understanding, i studied this linkhttps://www.eeeguide.com/what-is-phase-sensitive-detector-or-phase-meter-circuit/and as far as i understand this particular circuit is "DC biasing" (best i can think of), so lets say for supply voltage = reference voltage = 3v AC the output is 6v DC increasing for more voltage over coil1 and decreasing for more voltage over coil2 ?. You would then still (reading directly on the voltmeter won´t be precise enough) use an intrumentation amplifier to compare this voltage to a "zeroing voltage" from a dc voltage divider, giving you basically the same output my circuit would give BUT now you can feed the LVDT with pure AC ? What about just half wave rectifiying the voltage between center tab coils to center tab resistive divider ? Then you can also use pure AC and amplifier.

author: Noobster
author_rank: Contributor
timestamp: November 16, 2022, 09:23:33 pm
content: Quotewhoopsie voltage drop over diodeAnd i guess supplying the instrumentation amplifier with only ground on vs- while giving ac input is a big no no ?