RE: [CAD_CAM_EDM_DRO] Re: Axis Stall Problems

I run my 297+298 driver (circuit pretty much straight from the datasheet) at
18kHz.

Thinking about your setup (I've built drives very similar to this and had
similar problems) I reckon 20kHz sample rate is about a factor of 10 too
slow.

A typical PWM drive starts with a square wave at the PWM frequency. Turns on
the drive transistor for a minimum period (ie ignores the current spike from
switching transients), then uses a comparator to turn off the drive when the
current reaches a threshold. All this is happening VERY fast. I can
remember having to change an op amp I was using to amplify a current sense
voltage because it wasn't fast enough.. If you are switching at say 10KHz
and sampling at 20kHz then you effectively only have two current settings.
(it's a bit more complex I know..)

I'm not trying to rubbish your (obviously) hard work BTW. It all looks
fairly well thought out. My advice is still to buy a 297, build a drive and
test..

Slightly off topic. I've just build some three axis Allegro 2977 based
boards. These little microstep chips work really well. Only good for 35V and
2-2.5A though.... If you end up deciding to rebuild your drives, get in
touch and I'll send you some schematics for them..


PK

-----Original Message-----
From: CAD_CAM_EDM_DRO@yahoogroups.com
[mailto:CAD_CAM_EDM_DRO@yahoogroups.com] On Behalf Of Wayne C. Gramlich
Sent: Sunday, 13 November 2005 8:29 AM
To: CAD_CAM_EDM_DRO@yahoogroups.com
Subject: [CAD_CAM_EDM_DRO] Re: Axis Stall Problems

--- In CAD_CAM_EDM_DRO@yahoogroups.com, "Paul Kelly" <tech@c...> wrote:
>
> How quick is your current control loop?
> The 16 series A2D converters have big input caps and its tricky to scan
> multiple inputs quickly. A 297 IC is a cheap thing....

The microcontroller appears to be taking samples at 40KHz.
Split between both axes, that is 20KHz sample rate per
axis.

The reason for choosing a PIC16F676 is two fold.

1) They are even cheaper than an L297 which has gotten
kind of pricy. The L297 is $5-$7 and the 'F676 is
about a $1.50. Mind you if the sample rate is the
problem, I'll gladly pay for the L297.

2) Also, with the 'F676 I have the opportunity to
do a microstepping drive. The L297 is limited as
to what kind of drive it supports.

What sort of chop rate should I be looking at for an L298?

[snippage]

> 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.

[snippage]

-Wayne





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