Re: Axis Stall Problems

--- In CAD_CAM_EDM_DRO@yahoogroups.com, Jon Elson <elson@p...> wrote:

>
> Wayne C. Gramlich 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.
> >
> I see a possible problem in the posted schematics. There are no
> damping components on the current sense resistors.

You are correct. That is clearly a mistake. I'll tack
some .1uF Capacitors across each sense resistor.

> Also, these
> resistors need to be non-inductive types.

They are 2W .51 Ohm non-inductive.

> Generally a small RC
> time constant is used to get rid of the inductive effects when the
> power transistors switch. If the spikes cause the PIC to think
> the current limit has been reached too early, the current will not
> build up to the desired value.
>
> > I am currently using ripple drive (only 1 coil at a time)
> > in the stepper controller with a maximum current of 1 Amp
> > through each coil.
> >
> Have you actually measured the 1 Amp current?

The last time I looked at the scope trace of the voltage
developed across the sense resistor, I saw a really boring
saw tooth with rounded sides (classic inductive ramp up
and ramp down.)

> > 1) Increase the current to 2 amps.
>
> Are your motors rated for 2 A? If so, then you do want
> to deliver rated current to the motors.

I guess I was being a little conservative. In general,
I do not like to run power components right to the hairy
edge. Also, L298's are pretty notorious for getting hot.
I'll crank the current up tomorrow.

-Wayne