Where to improve this transducer amplifier? - Page 1
Forum section: electronics / projects

author: rvalente
author_rank: Frequent Contributor
timestamp: March 31, 2024, 03:52:57 pm
content: Hello Mates,I need to read this position transducer, its a rod variable inductor. There are two models, one ranges from 36 to 70mH and the other 10 to 50mH, both the DC resistance is something around 70ish ohms. I need an 0-10V with some diagnostic (short to ground and open)I came up with this circuit.Wien Oscillator at 5Vpp 1Khz -> Voltage divider > Rectifier > Filter > Offset + Gain > Final gain-1.2V for short to gnd | 10.5V for open | 0,9V for 10mH | 8,3V for 70mHBefore prototyping, where would you suggest improvements?Better wien oscillator voltage stabilization?Better filtering?Improved amplifier for less offset problems?Greetings

author: Zero999
author_rank: Super Contributor
timestamp: March 31, 2024, 08:23:06 pm
content: That will work, but why make it so complicated?Just make an oscillator with the inductor. An LR oscillator should give a fairly linear voltage/inductance characteristic. A microcontroller with PWM or DAC and an amplifier could be used to give 0 to 10V out.Another thing to consider is changing to a linear potentiometer, for the transducer so there's no longer any need for this.

author: rvalente
author_rank: Frequent Contributor
timestamp: April 01, 2024, 11:15:54 am
content: Quote from: Zero999 on March 31, 2024, 08:23:06 pmI intentionally avoided making a LR/LC oscillator, my experience tank oscillators tell me that many external variables plays, they're are not that reliable :'(

author: Zero999
author_rank: Super Contributor
timestamp: April 01, 2024, 12:51:12 pm
content: Quote from: rvalente on April 01, 2024, 11:15:54 amAn RL oscillator is only a reliable or unreliable as an RL filter which is what you have there.

author: rvalente
author_rank: Frequent Contributor
timestamp: April 02, 2024, 04:12:23 pm
content: Agreed, I juts feel more confidence in the RL filter, would you suggest anything else?

author: moffy
author_rank: Super Contributor
timestamp: April 03, 2024, 12:26:08 am
content: If you don't mind a digital solution you could use an 8 pin micro with an ADC, switch a pulse through a resistor to the inductor and measure the voltage after a predetermined period, knowing R you can derive L. Some simple component measurers use this technique quite effectively. The micro could then via filtered PWM output your 0-10V signal.

author: MathWizard
author_rank: Super Contributor
timestamp: April 03, 2024, 02:52:30 am
content: So what distances can such a sensor measure/differentiate between ? From making a few RF circuits, I can imagine some frequency changing as inductance changes. But yeah on a simple variable inductor like off an old TV, I wonder how much distance a slug moves, vs the measured frequency, and then how repeatable it would be?It wouldn't be very good with my little protoboard through-hole oscillator circuits, they aren't very steady or stable, compared to any modern IC clock.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: April 03, 2024, 05:21:06 am
content: Quote from: rvalente on March 31, 2024, 03:52:57 pmAre you worried about this not being quite inductance linear ? ie you already have a lookup table somewhere ?You do not need a good sine wave or full wave rectification for inductance-ratio capture. Even a triangle wave is ok.

author: Kleinstein
author_rank: Super Contributor
timestamp: April 03, 2024, 07:14:39 am
content: If the signal will finally go to a µC anyway, one could skip the rectifier and let the ADC in the µC directly sample the AC signal and do the rectification and filtering numerically. Than one could also generate the drive signal from there, with some filtering to get a bit closer to a sine (a triangle could already be good enough). The µC could than also use phase sensitive rectification.The weak point in the circuit may be amplitude stabilitzation. A square wave and filter may give a more stable amplitude.How much effort is worth it depends on the needs (stability, noise, linearity, interference, costs).