Q: Simulating an Old Vacuum Sensor I have an old vacuum sensor. This sensor consists of two windings on a coil and a metal shaft that moves according to the ambient pressure and the vacuum level.
The sensor is calibrated at the factory so that a known atmospheric pressure gives a known inductance on the two coils.
Other people have reverse engineered the operation to produce charts that show the relationship between pressure and inductance of the coils.
Over the vacuum operating range the inductance varies from 0.3H to 1.4H.
How can I simulate this sensor using a combination of software and hardware? For example suppose I have an MCU running firmware that accepts a user input for the current vacuum level. In the firmware I can calculate the inductance of both coils. How can I then produce that result in hardware so that a device which thinks it is attached to one of these sensors is actually attached to my hardware instead?
I know that programmable pots exist for changing resistance programmatically, but what about inductance?
Thanks, Andy

A: It seems unclear what the configuration is and how it works. Presumably some kind of differential arrangement. Perhaps an LVDT or RVDT as suggested above.
An LVDT arrangement typically has at least 4 wires and three coils. The inductance is not directly used in an LVDT arrangment. There is one pair of wires that excites the LVDT and the other pair picks off the output.
If it is an LVDT, the task is to measure the excitation voltage and produce an output voltage that is linearly proportional to the desired output (usually signed, so 4-quadrants). That's pretty much just a 4-quadrant analog multiplier. Typically LVDTs are excited by signals in the kHz range, which is not difficult for a multiplier chip to handle. You would need a bipolar DAC to produce the control voltage.
It could be done all digitally, of course, but you'd need an ADC then and a reasonably fast processor.
Edit:
Possibly could be done with a relatively high voltage digital pot and bipolar power supplies. Some measurements of the voltages and currents would be required. At least an oscilloscope! And a working unit.

A: Supposing you have a LVDT sensor. It does need a conditioner circuit which consists of an excitation voltage that feeds the primary and then measures the voltage at secondary coil. The most important parameter is the coupling ratio, not the inductance. You have to determine this coupling ratio by measuring the Vrms value of the excitation coil and then meesuring the Vrms on secondary coil at max. displacement.
$$k=\dfrac{V_{sec}}{V_{prim}}$$
Then you can fake the secondary voltage with your circuit - still needs the excitation from the conditioner circuit.

simulate this circuit – Schematic created using CircuitLab
I have drawn a basic opamp circuit, just as an idea. You do buffer the excitation voltage from conditioner, then you pass it to the difference amplifier. The Rdig is a digital potentiometer controlled from MCU. The diff. amp shall output zero when poti is mid. position, it shall invert and amplify signal when poti is displaced at both end positions.

simulate this circuit
$$V_{sec}=V_{prim}\cdot (2x-1)\cdot k$$
Where x is the position of digital potentiometer, at 1/2 the output is zero. K is the divider factor R3/(R3+R4) that has to match with LVDT k.
https://www.rdpe.com/ex/hiw-lvdtdc.htm
