Using an inductor to decouple a motor driver

In the spirit of "wanting to know why", I did some research today on decoupling/bypass capacitors, and came across mentions of a decoupling
inductor, such as:
http://www.williamson-labs.com/480_dec.htm
The general idea is you can put an inductor in series with a supply to isolate it from power supply noise. Commonly, an electrolytic cap is added to ground before and after the inductor, and this is called a pi filter.
However, no where could I find mention of how to calculate what size inductor you needed.
This seemed like it would be useful in robots, where you commonly have noisy PWM generators and motors working together. On the other hand, the fact that I haven't seen it used suggested there might be a reason not to.
Is this a good idea, or bad idea ? Is it just that separate batteries are usually easier ? If it is a good idea, how do you decide what size inductor to use ?
-Chris
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The resistance of the capacitor: Xc = 1 / ( 2 * PI * f * C ) f: the frequency of the signal (Hz), C: the capacity of the capacitor (F). For an inductor: Xl = 2 * PI * f * L L: induction of the inductor (H)
In a decoupling circuit these are used to construct a frequency dependend voltage divider. This is pretty much a simple low-pass filter.
I doubt this is going to work for PWM as PWM signals have a width spectrum (a block wave is composed of a very large number of sinus waves with increasing frequency and decreasing amplitude) and a low-pass filter would deform the wave shape. You could try to isolate the entire PWM + motor circuit with filters or seperate batteries.
-Patrik
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RedX wrote:

Actually, you don't calculate it as a low pass filter. A PWM signal is on or off, you calculate the inductor by the integration of current into the inductor. An inductor "resists" the rise in current of a circuit. So, when the PWM signal turns on, current into the coil will integrate linearly until the signal is turned off.
There are lots of tricks you can play too. Motor speed is limited to the voltage of the system (supply voltage has to exceed the motor's back EMF to drive current into the motor), you can use a step up transformer or a boost circuit to get higher voltage than the supply voltage and more more speed.
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: For an inductor: Xl = 2 * PI * f * L : L: induction of the inductor (H)
OK, I want to know what size inductor to use -- I want L: Flipping it around, we get:
L = Xl / (2 * PI * f)
Assumeing f is the PWM frequency, this still looks like 1 equation in 2 unknowns. What am I missing ?
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