Re: Axis Stall Problems

--- 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. 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. So I'd like to help you with some info you may not have.
Right now I have a system using the L297/L298 combo that works fine.
I don't know if you have seen a actual scope trace of what the
current-limiting of this pair looks like or not, but here is one:

http://members.cox.net/tararatski/L298_current_sense.jpg

The top trace is the voltage across the .5 ohm sense resistor. The
bottom is the voltage at the sense input of the L298 (pin 14 in this
case). The L298 pin 14 signal is filtered by a 10K series resistor
from the sense resistor and a 680pf cap to ground. The scope is set
at 10 microseconds per division and .5 volts per division. The L298
oscillator is about 18KHz (in fact with the 56 microsecond period,
it's 17,857 Hz). The stepper is a Powermax II M21NRXA, 8 wire,
running the coils in series. The specs for this configuration are
phase resistance .92 ohm, inductance 2.8 millihenry. I took these
with my digital camera, so it's hard to see the slope of the top
trace exactly, but there is a little rise after the ringing stops.
The reference voltage into the L298 is .240 volts, and it trips at
just about that. This circuit has a "holding current" reference
voltage that kicks in after a small delay without a step pulse, and
the traces show the "holding current" configuration (with about .48
amps current into the still motor).

Hope these traces help you figure out what to do next.

Irby