Re: DC Motor Control: H-Bridge +5A, 48v

On Fri, 13 May 2005 14:44:37 -0400, R Adsett
<radsett@xxxxxxxxxxxxxxxxxxxxxxxxx> wrote:

>In article <lfd281hl7ib4d2b07t7q3nt7kk5n5hl9eu@xxxxxxx>,
>cdsmith69NOSPAM@xxxxxxxxxxxxx says...
>>
>> When we would run the lift motor at about half PWM, which
>> resulted in maybe 250 amps or so, the image on the CRT of that
>> old WYSE terminal would shake up and down about 1/4 inch, even
>> though it was on a table a couple feet away.
>
>Seen that too. It sounds like we had similar experiences at roughly the
>same time. Any chance your lift truck was a reach? :)

Nope. Just a stand up counterbalance.


>Speaking of current limits. The controller was designed, over my
>objections, to use the NMI to signal an over-current.

Already sounds bad...

>The board was eventually redesigned to put a
>one-shot on the NMI. Ugly, but workable.

Our boards all had the overcurrent implemented totally in
hardware so such things couldn't happen. But there was a logic
signal back to the controller so that it could tell when the
overcurrent was tripping.

The overcurrent was controlled by a one-shot. So if you
continually pushed the forklift into an overcurrent condition, it
would make a ratchet like sound as the overcurrent re-tripped on
the next PWM cycle after the one-shot. The problem is that while
that is happening the average power was far less than what the
controller could do right at the edge of current limit, so the
only way out of it would be to back off on the control handle and
ease into it again.

Later we were designing a new controller that did cycle by cycle
current limiting. That worked so much better there was no
comparison. It would just take the controller to maximum power
and sit there. You could even slam the PWM on to 100% and just
watch each cycle current limit a little later as the motor sped
up. But that design never made it to production, as it was in
progress at the time of the Great Downsizing that left the
company with 1 employee.

.

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