Re: [CAD_CAM_EDM_DRO] Re: Axis Stall Problems

Wayne C. Gramlich wrote:

>--- In CAD_CAM_EDM_DRO@yahoogroups.com, "Paul Kelly" <tech@c...> wrote:
>
>
>>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 understand what you are saying, but the measured current
>does indeed average around 1 Amp.
>

But, are the individual pulses all around the same width, or is it
making a very long average of dozens of narrow and wide pulses
averaging out to 1 A? I think Paul is on to something here.
I have several drives that I've worked with that run at about a
20 KHz rate, but they are all capable of varying the duty cycle in
microsecond increments. if you are really only taking one sample
every 50 us, then the current rise and fall rates might be very rough.
This may work fine at low speeds, but when the speeds pick up,
the PIC may not respond to coil current relative to the new coil
polarity needed in a timely manner. Obviously, when the step rate
reaches 5000 steps/sec or so, then things will really break down,
as there are only four sample per step. (Or, maybe, things will
actually run well there!)

>
>
>
>I've looked at the A2977 and there are three problems (for me):
>
> 1) 35V is on the hairy edge for 24VAC transformer.
> Finding a smaller transformer is a pain.
>
> 2) I can not find any Allegro spec. sheet that
> explains how to heat sink their product. At
> least with the L297, I can just buy a heat sink
> that clips right on.
>
> 3) Surface mount. I do not really have adequate
> equipment to mount the chips.
>
>
>

You might use the 2977 as a driver for external FETs, for higher
voltage and current.

Surface mount is not that hard, if you are making boards. I have moved
almost exclusively to surface mount. My tools are curved-tip tweezers,
a soldering iron with a small tip, and solder in the .015" diameter. I also
use a jeweler's loupe, flip-down head magnifier, and a microscope for the
high density stuff. But, for plain SO-type chips, the tweezers and
soldering
iron are fine. Obviously, you can't use SMT chips directly in the plastic
breadboards, or for wire-wrap.

Jon