Author Topic: digital current limiting, chopping, microstepping  (Read 2632 times)

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Offline izuaTopic starter

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digital current limiting, chopping, microstepping
« on: April 13, 2008, 02:03:20 PM »
I'm trying to understand more into microstepping and the subfield of current limiting.
What I got till now is: powering the coil at a higher voltage than rated, increases speed (and ustep versatility) but takes in more current which will destroy the stepper - we can limit the current and we'll have (mostly) the same effects.

I want to simulate this in isis, but i know nothing of analogue electronics. I wasn't able to find a decent (ie working) chopper/current limiter schematic, except this one:


But this is very theoretical, if someone can help me with some values for this thing (inductance, vcc, sense R value -, so i can actually see it working (ie chopping) on a scope, it would be great.

I'd also like to understand this. I presume the Sense voltage is something among (Coil resistance + 0.7) / sense * Vcc. But i'm not sure how the coil resistance goes around.
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Offline izuaTopic starter

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Re: digital current limiting, chopping, microstepping
« Reply #1 on: April 18, 2008, 01:08:21 PM »
no one here likes steppers? :P
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Offline AndrewM

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Re: digital current limiting, chopping, microstepping
« Reply #2 on: April 19, 2008, 08:56:15 AM »
I don't know much about stepper motors, but I do know coils and EMF fields.  When it comes to increasing an EMF field, thus increasing speed, you need one of two things:  more current or more wraps of wire.  Voltage increases in a coil (without increasing the current) has the effect of activating the coil quicker, but does not make it a stronger EMF field.  Gauss rifles work on this principle using high voltage, lower current (relative to the voltage) to activate the coils of the rifle quicker.  Remember that electromagnetic coils are just inductors, and an inductors job is to retard changes in current, higher voltages help to overcome this (although I never saw it taught in any courses).
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Offline paulstreats

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Re: digital current limiting, chopping, microstepping
« Reply #3 on: April 19, 2008, 10:06:30 AM »
the part that cuts the current on that circuit is just the resistor on the sense pin, to resist more current use a higher value resistor or a bunch of resistors to help spread the heat. You can also use a potentiometer along with a power transistor instead of the resistor thereby giving you variable control. If you used a digital pot with the power transistor you can have the added ability of your microcontroller being able to step current up or down whenever necessarry (you would need to use some method for your controller to read the current draw for the digital way to work effectively)

 


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