RE: [CAD_CAM_EDM_DRO] stepper issues

Hi Chuck,

You're dealing with an inductor that takes a logarithmic amount of time
to build up current and it's the current (amp x turns) in the motor that
creates the torque. Step too soon before the current is built up to
maximum and the motor doesn't develop enough torque.

Now add to that the result of the motor turning creating back emf and
the faster the motor turns the more the back emf resists current
build-up in the winding. That's what also limits the top speed of a DC
motor for a given applied voltage.

The way the high end stepper controllers work is to change the log curve
for current build-up by increasing the applied voltage and then limiting
the current when the motor current spec is met. The limiting is either
done with a resistor in series with the power (RL) or with a chopping
circuit. The UCN5804 is usually RL limited. With 9 volts and a 5 volt
motor you don't have a lot of head room so you are limited in your
stepping rate.

If you have a scope available, you can put a small series resistor in
series with the motor winding and use that to measure the current.
Apply the voltage to the winding and note the time when it reaches max.
That will give you an idea of how fast your stepping pulses can be
before you start to lose torque.

Speed the stepping rate up and watch the current drop. Here are a
couple of scope photos that demonstrate a doubling of stepping speed
results in current just barely getting to max.

http://www.autoartisans.com/images/123RPM.JPG
http://www.autoartisans.com/images/246RPM.JPG

The yellow trace is low when current is turned on. It goes on and off
when max current is reached.
The blue trace shows current in the winding.

So in your case you need slow down the motor to avoid it skipping or
locking up.

John Dammeyer

Automation Artisans Inc.
http://www.autoartisans.com
Ph. 1 250 544 4950

> -----Original Message-----
> From: CAD_CAM_EDM_DRO@yahoogroups.com
> [mailto:CAD_CAM_EDM_DRO@yahoogroups.com] On Behalf Of Chuck Merja
> Sent: Saturday, November 17, 2007 8:59 AM
> To: CAD_CAM_EDM_DRO@yahoogroups.com
> Subject: RE: [CAD_CAM_EDM_DRO] stepper issues
>
>
> Thanks for the quick reply - It looks like we are driving
> this motor at
> about 37 steps/sec - so that's like 26 milliseconds between
> pulse edges.
> Haven't measured width - maybe I should? Seemed to stall easier at 37
> steps/sec than at 33 steps/second, but still stalled at both
> speeds - AND at
> slower speeds.
>
> Motors are .7.5 deg, 100 ohm 22July91, 42SPM-24DCZD, not
> "DCZA" which I'm
> having trouble finding a datasheet for. The "A" version is
> 5-6 V, son't
> know about "D" version, but I'm trying to drive at 9V - are
> we asking for
> trouble?
>
> It actually looks like I should speed this motor up, rather
> than slow down.
>
> Chuck Merja
> <http://www.srvscience.org> www.srvscience.org
> www.3rivers.net/~chuckm/index.htm
>
> Lemelson-MIT Inventeams - http://web.mit.edu/invent/n-main.html
>
> US FIRST Robotics - http://www.usfirst.org/
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> From: CAD_CAM_EDM_DRO@yahoogroups.com
> [mailto:CAD_CAM_EDM_DRO@yahoogroups.com] On Behalf Of hannu
> Sent: Saturday, November 17, 2007 7:27 AM
> To: CAD_CAM_EDM_DRO@yahoogroups.com
> Subject: Re: [CAD_CAM_EDM_DRO] stepper issues
>
> How fast does it run in hz ? The step rate I mean.
> You may be into mid-band resonance. Probable even - old round
> steppers,
> yes ?
> How many rpm does the motor turn at 9.6 V ?
>
> If you slow down your step rate, resonance will go away.
>
> I think I remember that for your use, its more important to
> get accurate
> than to get fast.
>
> ebiz_59 wrote:
> >
> > Thanks for all you help so far. I'm working with a high school
> > freshman to get a stepper motor to run a certain number of steps and
> > then stop - calibrate a small erector set car to run
> between 5 and 10
> > meters (we'll be told specific distance at the competition)
> and then stop.
> >
> > We have combined 2 basic circuits to accomplish this task - a 555 as
> > monostable (adjustable time) dumping into a 555 astable to
> give steps
> > while monostable is saying "go" and the astable dumps into
> the trigger
> > of a 5804.
> >
> > We get this circuit to drive a stacked can motor - we need
> like 30 to
> > 45 seconds of run time. We don't have it hooked to the device yet -
> > the motor is just driving another free running gear. The
> motor starts
> > alright, but sometimes well into the run, it gets confused and
> > stalls/reverses.
> >
> > A few notes about stuff we might have done wrong -
> > 1 - we didn't use pullup/down resistors for the full, half,
> direction
> > pins - we just tied them to +5 or gnd.
> > 2 - we hard wired this on an soldered experiment board and
> have wires
> > going everywhere. We initially did 3 circuits on 3 separate
> > breadboards and had the same problem. I thought it might be the
> > breadboards and the fact that we could only put about 7 V
> to the motor
> > circuit, but we get the same results with a wired cicuit all on 1
> > soldered breadboard and 9.6 V battery running the motors and dropped
> > down through a 7805 to run the 555's and 5804.
> > 3 - we are limited to 9.6 V max by the rules
> > 4 - we didn't use a PIC for a couple reasons - I wanted him to learn
> > about 555's, we didn't have PIC programmer in stock so I thought I'd
> > have him work on the 555 path while I researched and acquired
> > programmer - still hope to do this, but the competition that sparked
> > this project is Tues, Nov 20, so we are trying desparately
> to get this
> > circuit to work.
> >
> > We'd sure appreciate your help in where to look.
> >
> > Thanks again for your help so far, and for any you can give over the
> > next few hours/days.
> >
> > Chuck
> >
> >
>
>
>
> [Non-text portions of this message have been removed]
>
>
>
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