measuring the inductance of a solenoid, whilst driving it? - Page 1
Forum section: electronics / projects

author: max_torque
author_rank: Super Contributor
timestamp: February 19, 2016, 09:35:52 pm
content: The inductance of a mechanical solenoid is proportional to the position of the core within the winding, so it must be possible to infer the position of that core by measuring the inductance at any given time. But how to do this whilst driving it? I guess measuring current ripple is one option? (measure current at start and end of pwm on time)Basically, i'd like to control the position of a solenoid, but not add any separate position sensor!!Anyone done this or have any experience in this area?

author: Paul Price
author_rank: Super Contributor
timestamp: February 19, 2016, 10:03:17 pm
content: Good luck with that idea. Forget trying to measure the inductance.The strength of pull of the solenoid will follow an exponential relationship to driving current and solenoid position. It can be extremely difficult to find some setting of drive to set a solenoid position that is not influenced by orientation of the device, the variation of friction. the spring to return the solenoid, and, mostly, the idea that you are working with such a delicate balancing act, the possibility of oscillation. The current will be high in achieving the position you want, but keeping the position you want will be extremely difficult to set because of the combination of the momentum of the solenoid core, the inductive nature of the solenoid coil that opposes rapid current change, variations in friction and the mentioned exponential relationship between current and core travel.Your best hope, depending on the mechanics of what you are trying to accomplish is to attempt by experimentation to find some current setting steps or ramp that might achieve the desired results if what you need to do in positioning can be done slowly and somewhat sloppily.Depending on what you ask of the solenoid in core-positioning, accomplishing what you want can range from being simple to inconsistent to seemingly impossible. So much will depend on the skill of making a mechanical layout that works and the accuracy, repeatability, and the speed required to accomplish the specific mechanical positioning job to be done.Some things are better accomplished with stepper motors or motor/gears/cams/clutch/sensors/limit switches setups.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 19, 2016, 10:33:05 pm
content: The I(t) curve during turn-on will tell you about this.Tim

author: pmbrunelle
author_rank: Regular Contributor
timestamp: February 20, 2016, 05:12:32 am
content: Maybe with some extra superimposed current ripple, and some sense windings, you could essentially make an LVDT.

author: max_torque
author_rank: Super Contributor
timestamp: February 20, 2016, 12:15:28 pm
content: After a bit of playing around in Matlab simulating a typical solenoid (i took a force vs position curve from a datasheet of a small linear pull solenoid, and force is directly proportional to the change in at that position inductance) it looks to be do-able!Luckily the application doesn't care about instantaneous position, only "average" position, so we can deliberately drive the solenoid with a "lot" of ripple (ie at a low fundamental PWM freq), which will also act as mechanical dither and reduce static friction. Another trick will be to use an oversized solenoid, but operate it with the core position biased towards open, ie in the range where the exponential change in inductance is less critical.Due change in winding resistance with temperature i need to measure the applied current, because i can't rely on just applying an open loop forward voltage, so that current measurement system can also be used to identify the AC component of current. Even something like an 8b AVR is going to be able to be able to measure a single ADC channel at say 1Khz, and with a fundamental pwm frequency down at say 100Hz, that's a decade of signal bandwidth above the fastest possible control frequency.As suggested, the mechanical architecture will be important, but we can add mass and possibly even an eddy current damper to get the fundamental mechanical frequency down to a nice low and hence controllable valueCould be an interestingly little project