how to make a stepper motor step gradually?

Posted by CosminB [BRT] on June 2, 2006, 6:17 am

Hello,

I have a stepper motor which rotates a platform at 3.6deg/step. There's
an object on the platform. At each step I have to take a photo of the
object. The motor is in full step mode. The problem is, when I'm
performing a step for the next photo, the platform quickly moves (quite
brutally) to the new position, and the object slips on the platform
(not enough adderence I suppose). What I want to know is if there's an
easy method to make the platform/motor move slowly to the new position.
I don't want to do microstepping or the like, I need the simplest idea.

Could this be done with capacitors? Any other ideas?

Thanks,
Cosmin.

Posted by Tim Auton on June 2, 2006, 8:34 am

>I have a stepper motor which rotates a platform at 3.6deg/step. There's
>an object on the platform. At each step I have to take a photo of the
>object. The motor is in full step mode. The problem is, when I'm
>performing a step for the next photo, the platform quickly moves (quite
>brutally) to the new position, and the object slips on the platform
>(not enough adderence I suppose). What I want to know is if there's an
>easy method to make the platform/motor move slowly to the new position.
>I don't want to do microstepping or the like, I need the simplest idea.
>Could this be done with capacitors? Any other ideas?

You could try just reducing the drive voltage to the stepper to reduce
torque. That may be enough, but it could en up being marginal (missing
steps with a heavy load). A superior solution is 'microstepping'. It
involves ramping the applied voltages to the windings using PWM and
can make for much smoother motion and greater positioning accuracy
(though there are practical limits on how much additional accuracy can
be gained).

Tim
--
-

Posted by Rich Webb on June 2, 2006, 9:53 am

On Fri, 02 Jun 2006 13:34:50 +0100, Tim Auton

>>I have a stepper motor which rotates a platform at 3.6deg/step. There's
>>an object on the platform. At each step I have to take a photo of the
>>object. The motor is in full step mode. The problem is, when I'm
>>performing a step for the next photo, the platform quickly moves (quite
>>brutally) to the new position, and the object slips on the platform
>>(not enough adderence I suppose). What I want to know is if there's an
>>easy method to make the platform/motor move slowly to the new position.
>>I don't want to do microstepping or the like, I need the simplest idea.
>>Could this be done with capacitors? Any other ideas?

>You could try just reducing the drive voltage to the stepper to reduce
>torque. That may be enough, but it could en up being marginal (missing
>steps with a heavy load). A superior solution is 'microstepping'. It
>involves ramping the applied voltages to the windings using PWM and
>can make for much smoother motion and greater positioning accuracy
>(though there are practical limits on how much additional accuracy can
>be gained).

Depending on the setup, attacking the problem at another point might be
easier. For example, using a belt-driven system with the small pulley on
the stepper, large on the turntable, and a "soft" resilient belt.

--
Rich Webb Norfolk, VA

Posted by slacker on June 5, 2006, 6:38 am

more specifically i think you mean you want to reduce the current. in dc
motors the torque is proportional to current, speed is proportional to
voltage.

although by reducing voltage i suppose in effect, you are reducing the
current.

if you reduce the torque enough you should be able to slow the angular
acceleration down at each step.

Tim Auton wrote:

>> I have a stepper motor which rotates a platform at 3.6deg/step. There's
>> an object on the platform. At each step I have to take a photo of the
>> object. The motor is in full step mode. The problem is, when I'm
>> performing a step for the next photo, the platform quickly moves (quite
>> brutally) to the new position, and the object slips on the platform
>> (not enough adderence I suppose). What I want to know is if there's an
>> easy method to make the platform/motor move slowly to the new position.
>> I don't want to do microstepping or the like, I need the simplest idea.
>> Could this be done with capacitors? Any other ideas?

>
> You could try just reducing the drive voltage to the stepper to reduce
> torque. That may be enough, but it could en up being marginal (missing
> steps with a heavy load). A superior solution is 'microstepping'. It
> involves ramping the applied voltages to the windings using PWM and
> can make for much smoother motion and greater positioning accuracy
> (though there are practical limits on how much additional accuracy can
> be gained).
>
>
> Tim

Posted by on June 2, 2006, 6:45 pm

CosminB [BRT] wrote:

> (not enough adderence I suppose). What I want to know is if there's an
> easy method to make the platform/motor move slowly to the new position.
> I don't want to do microstepping or the like, I need the simplest idea.

There is not an easy electronic solution to my knowledge, since you
rule out microstepping.You can consider mechanical solutions to damp it
a bit; for example, if you decouple the platform and motor, recouple
them through a helical spring, and add oil-damped vanes to the
platform, you might achieve the desired effect (at a loss of positional
accuracy).

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