Re: [CAD_CAM_EDM_DRO] Re: Axis Stall Problems

Irby Jones wrote:

>--- In CAD_CAM_EDM_DRO@yahoogroups.com, "Wayne C. Gramlich"
><Yahoo@G...> wrote:
>
>
>> The chopper is implemented using a dedicated Microchip
>> PIC16F676 microcontroller (one for each axis) running
>> at 20MHz where a couple of A/D lines are used to measure
>> the current flowing through each L298 H-Bridge. Each
>> axis is being sampled at approximately 20KHz. If the
>> current is too high, I turn the H-Bridge off and if
>> the current is too low, I turn it on.
>>
>>
>
>
>Wayne,
>
>I like what you're doing with this concept, and have been following
>this thread. I have been thinking of a similar concept, where I want
>to be able to microstep at 100 microsteps per regular step.
>

What is the purpose of x100 microstepping? Do you really think that
you can position a stepping motor to 100 x 200 = 20000 discrete
positions? A small amount of microstepping actually leads to finer
positioning, but beyond something like 4x microstepping, the friction
defeats the declining torque produced by ever smaller microsteps.
I'm quite sure Gecko is right to stop at 10X microstepping. And,
microstepping finer than that will not improve torque or reduce resonances
further.

> I have a
>bunch of steppers that have a 500 line encoder attached - 2000 lines
>with quadrature. It would seem possible to have a control loop that
>microsteps the motor until an edge is reached on the encoder inputs,
>closing the loop around the encoder (at 2000 steps per rev.) similar
>to a servo.
>

OK, so that is the same as x10 microstepping. But, if you try to close the
loop and position down to the 2000th of a rev, I think you will find you get
a lot of rumbling and humming. The amount of torque produced by a single
X10 microstep is very small. So, the control loop will keep adding more
microsteps until the motor jumps too far, then start working the other way
until it jumps again. If your encoder only resolves 2000 counts/rev, then
the 100x microstepping to 20,000 steps/rev doesn't make any sense.

Jon