Electronics > Projects, Designs, and Technical Stuff
Arbitrary (saturable) coupled inductors in LTSpice
ahbushnell:
Cool,
Very good.
uer166:
So I'm a bit stuck now, just as I thought everything was figured out. In general the sensor that was implemented is able to measure down to 0.2mA resolution of DC current with various cores.
What the issue is: some cores seem to exhibit highly asymmetric magnetic properties when saturated clockwise vs. counter-clockwise. I'm relying on the symmetry for the sensing, so it blows it all out of the water. I've seen resultant DC offsets of anywhere from 1mA to 20mA caused by this effect.
Some observations:
* It's a consistent core-to-core variation. Cores that are symmetric are always symmetric, ones that aren't, never are
* In some cores there's a huge dependence on mechanical stress. If I squeeze a core it can change the level of asymmetry dramatically
* I tried various materials, including Metglas 2714A and some Chinese suppliers. I thought it was a quality issue at first, but even expensive Metglas fails?
* The asymmetry is somewhere between highly temperature dependent and temperature dependent, so can't simply calibrate it out
* The integral of the current going through core seems to still be same one way vs. the other, only shape is different with an inflection point
I've read about https://en.wikipedia.org/wiki/Exchange_bias and similar things, but found no practical info as to how to combat it, or how to specify a core that is fully symmetric.. |O |O |O
It's possible that I'm going about it the wrong way and the usual peak detection circuit usually used for fluxgate sensors is more appropriate. Right now I'm sampling the current approximately in the coercive force regions to detect asymmetry caused by another current source.
uer166:
I know it's been a while, thanks for helping out with this thing, the simulation was key to make the sensor work! Also a variation of this made a nice fluxgate magnetometer that picks up earth's field pretty well, useful in a compass application. Would you be able to explain how you coupled the inductors via the .subckt statement? I've tried to draw the V1, E1, F1 sources with their connections as per the statement but still really confused by it..
Jay_Diddy_B:
--- Quote from: uer166 on November 29, 2019, 03:20:29 am ---I know it's been a while, thanks for helping out with this thing, the simulation was key to make the sensor work! Also a variation of this made a nice fluxgate magnetometer that picks up earth's field pretty well, useful in a compass application. Would you be able to explain how you coupled the inductors via the .subckt statement? I've tried to draw the V1, E1, F1 sources with their connections as per the statement but still really confused by it..
--- End quote ---
Let me try and build it up.
The coreBwinding Model
This is what the circuit looks like described by these directives:
params: n=1 is the default number of turns. The default is one turn.
n is the number of turns on that winding.
.global coreB
This is essentially a global variable that is shared between all the sub-circuits. This how the windings are coupled together.
This circuit is an ideal transformer winding. The left side is a single turn winding. The right side is n times bigger in amplitude. F1 draws n times the current from the single turn side.
WBx
This is used to define a winding using the coreBwinding subckt.
The parameters include setting n, the number of turns and series and parallel resistors.
Winding model
The model includes the parallel and series resistors as shown above.
Two winding transformer
You can build a two winding transformer like this:
This transformer is 1:1 and is placed in a test circuit. The symmetry is obvious. The model will work equally well if the primary and secondary are swapped.
Non-Linearity
As shown so far the model does not include any non-linearity and has no core saturation.
The core can be added by connecting the non-linear inductor between coreB and ground.
Regards,
Jay_Diddy_B
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