LVDT Signal Processor Circuit - Page 1
Forum section: electronics / beginners

author: Aliasmk
author_rank: Newbie
timestamp: November 06, 2015, 06:26:36 pm
content: I'm building aLinear Variable Differential Transformerin my spare time and am trying to come up with an signal processing circuit that I can later feed into ADC and then into a Microcontroller. An LVDT works by exciting a primary coil with an AC voltage, magnetizing a core, and in turn exciting two secondary coils on the outside of the primary with a differential voltage depending on where the core is located. I basically want to take two AC signals, rectify them to a constant DC signal, then subtract the two.I've come up with two circuit designs (after a few iterations), shown as attachments and I've provided links to the circuit simulator to visualize them. Before I build it though I have a couple of questions about it.Am I correct in generalizing an energized transformer coil as an AC voltage source?I'm confused about the behavior of the bridge rectifier... as far as I know I /should/ have to use a cap tied to group to smooth the rectified AC into ~Vrms DC,but it doesn't seem to need it in the simulator for the second revision. Why could this be? Just a software error maybe?Notice the two op-amp buffers - I implemented them because when I did have caps tied to ground the response time if I changed the voltage mid-simulator was slowed significantly. Is this too much of a hack way of isolating the rectifier circuit from the differential op amp?Try changing the voltages of AC sources on the left by double clicking them, in the second revision the output changes right away, whereas in the first revision it doesn't in any sort of time I saw. I think this may be because of those caps. The output is top voltage - bottom voltage, range of +/- 5V. I've removed the schematics for the time being but will repost them after the 25th of November 2015Thank you! Let me know if you need more info.

author: Aliasmk
author_rank: Newbie
timestamp: November 12, 2015, 02:32:39 am
content: Okay so since posting this question I've changed up and built the design and have answered my own questions. In case anyone stumbles upon this thread, here is what I have found. The answers in order:Yes, energized coil will have the same wave type (sine) and frequency as the exciting coilSoftware error. Tying a 100G ohm resistor to ground establishes the ground for that part of the circuit and it behaves as expected - full wave rectification with no DC smoothingDidn't end up needing them. Over-complicated the circuit.After building the real circuit I discovered the latency when changing voltages is only around a half second when using a 100u smoothing cap, definitely within my design constraints. In the end the circuit ended up pretty close to the first revision.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 12, 2015, 01:50:14 pm
content: The secondaries will behave as voltage sources BUT not ideal. The Thevenin equivalent impedance depends on the winding construction, and probably, core position as well. This will more significantly impact the reading of a peak voltage detector. An average voltage detector will have less error.There are also other methods to consider (but probably not worthwhile), like synchronous detection. This would be more important for the rotating, phased type, or synchros.Tim