Stepper motor problem solved

[Don French]

Last night I was trying to get a stepper motor working with a Big Easy Driver board.   Three people were there helping, Kelly, Dave (Wrdralac?) and Bill Ryder.  I only have email addresses for Bill who I already wrote to.  So to Kelly and Dave, I just wanted to tell you that the problem was apparently with the BED although I am not sure WHAT the problem is.  Anyway, switching to a different board fixed it.  Sucker still puts out some serious heat though and the heat sink is needed.  On that subject, Kelly - I absent-mindedly walked off with your block of aluminum.  Let me know how to best get it back to you. 

Sure is sweet watching that baby humming along nice and smooth!


-- Don

[Jim Wygralak]

That was me, Jim, not Dave.
Glad to hear you got it going. Sorry to hear that it's a hardware failure.

One thing to keep in mind, ALWAYS power off the driver before disconnecting the stepper motor. The drivers I use warn very strongly about this.

[Don French]

Jim, sorry about getting your name(s) wrong.  I think I always did power off the driver but maybe not.  Anyway, it isn't an expensive piece of hardware.

-- Don

[Kelly Murphy]

Just hold on to it until next week and I’ll get it from you then, I should be in on Wed. 

 

I’m wondering, based on the heat generated, if the original board had a heatstroke and went bad.  That 2x2x1/2” block of aluminum got really hot last night.

[Don French]

Maybe but this thing is supposed to have thermal shut-down protection.

-- Don

[Kelly Murphy]

I was also looking closer at the schmetacics, chip layout and the picture of the BED on sparkfun’s site.  It looks like the order of the pinout is a, -a, -b, b.

[Don French]

I wonder if we tried that.  It happens that this is the way I hooked it up this time.  I wonder why the silkscreen on the board doesn't specify that - or the tutorial.  I will try the other BED hooked up that way in a little while.  Maybe that was the only  problem. 

-- Don

[Don French]

Kelly,

I understand the steppers a bit better now and discovered that reversing the connection between A and A- or B and B- has only the effect of changing the direction the motor turns.  Reverse pairs and there is no effect.  But it has no effect on the heat problem or the stuttering of the motor.

The heating problem arose because those motors have very low coil resistance and so pull a lot of current - which can supposedly be mitigated by turning the current-limiting potentiometer down.  I haven't tested that theory yet because I switched to this motor which has 20 times the coil resistance but  really needs 24V to operate reliably.  I would like to find one that runs well on 12V and doesn't double as a space heater.

The stuttering was due to the thermal protection repeatedly shutting it down.

-- Don

[John Scherer]

Sent from my iPhone

On Nov 11, 2012, at 10:18 AM, Don French <dcfr...@gmail.com> wrote:

Kelly,

I understand the steppers a bit better now and discovered that reversing the connection between A and A- or B and B- has only the effect of changing the direction the motor turns.  Reverse pairs and there is no effect.  But it has no effect on the heat problem or the stuttering of the motor.

Yep, that's normal

The heating problem arose because those motors have very low coil resistance and so pull a lot of current - which can supposedly be mitigated by turning the current-limiting potentiometer down.  I haven't tested that theory yet because I switched to this motor which has 20 times the coil resistance but  really needs 24V to operate reliably.  I would like to find one that runs well on 12V and doesn't double as a space heater.

The other thing to look at is hold current.  Typically, stepper motor controllers, when not moving the motor, will put the motors in a hold state with full current applied.  This is normal and is done to provide maximum hold torque.  Most drivers provide a way to adjust the hold current levels lower if you don't need that much hold torque. 

[Jeremy L]

The heating problem arose because those motors have very low coil resistance and so pull a lot of current - which can supposedly be mitigated by turning the current-limiting potentiometer down.  I haven't tested that theory yet because I switched to this motor which has 20 times the coil resistance but  really needs 24V to operate reliably.  I would like to find one that runs well on 12V and doesn't double as a space heater.

The other thing to look at is hold current.  Typically, stepper motor controllers, when not moving the motor, will put the motors in a hold state with full current applied.  This is normal and is done to provide maximum hold torque.  Most drivers provide a way to adjust the hold current levels lower if you don't need that much hold torque. 

<Jumping in without much background info>

If there's excessive heat, perhaps the motor is trying to "hold" the position of each step, fighting the momentum to turn faster?  In the logic world, a stepper holds a series of discrete positions.  In the mechanical world, it is either holding one position stationary, or it is moving a smooth linear motion with inertia.  The signal to hold a position involves fighting momentum to turn beyond the step position, which I would expect to appear as heat.

Granted, this would be a detail that presumably the steeper driver already has worked out.  But if the driver is just given a series of pulses, with no velocity information, then it has no way of knowing the intended motion, and might have to resort to discrete positions.

Just a theory.

Jeremy