I find it difficult to believe that any large inductor can have a self resonant frequency anywhere nearly as low as 300 Hz all by itself.
Where is all this extra phase shift coming from to reduce the existing 90 degree phase margin at 300Hz ?
In principle controlling the current in a large high Q inductor isn't inherently more difficult than controlling the voltage across a large high Q capacitor, which is what power supplies typically do. Consider that if your controller has a voltage output, then a high Q inductor basically acts an an integrator, so your compensator may not need an integral term at all. Just look at the whole open system (including the amplifier and the inductor itself) and its transfer function and that should tell you what your controller will need to look like.
If your inductance is unknown, then that's a challenge, much like it would be a challenge to design a power supply controller without knowing the output capacitance. Unless you resort to something elaborate like nonlinear or adaptive control schemes, you will either have to design with very large stability margins, or make the controller tunable. At work we have a lab high power amplifier meant for driving arbitrary impedance, but it requires that you change jumpers around depending on the approximate impedance being driven and the required bandwidth.
If I wanted to control the current of an unknown and potentially very large inductance how do I approach that? Or more generally, how can you control any system with unknown and potentially large inertia (like a voltage source and large C).
So where does that leave me?
A) Am I misunderstanding my options?
B) Can I live with razor thin phase margins in this application?
C) Is there no generalized solution to potentially infinite inertia?
If I wanted to control the current of an unknown and potentially very large inductance how do I approach that?
While inductance may be high it may also be low or zero.
One way to make things easier is to add built-in inductance to your amplifier output. That way at least you know there is a minimum on the total inductance. Again this is comparable to the output capacitance built into voltage regulator circuits. If there's 1000uF built into a power supply, then you can be pretty sure that it will be fine driving an extra 0-100uF.Right or the speed of a large mass etc. My hypothetical is how I might approach things if I know nothing about my load inductance.
Good point.If your load inductance gets very large, then the voltage range of your driver will probably be the limiting factor rather than small signal response.
The original question was:
Now it has changed to :
And that is a very different thing.
One way to make things easier is to add built-in inductance to your amplifier output. That way at least you know there is a minimum on the total inductance. Again this is comparable to the output capacitance built into voltage regulator circuits. If there's 1000uF built into a power supply, then you can be pretty sure that it will be fine driving an extra 0-100uF.
If your load inductance gets very large, then the voltage range of your driver will probably be the limiting factor rather than small signal response.
Good point.
Its no good designing for a 300 Hz bandwidth if there is insufficient voltage swing available, and that may require many hundreds of volts to achieve with a large inductor.
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