Dynomotion

Group: DynoMotion Message: 10849 From: dms.davidstevenson Date: 1/17/2015
Subject: Measuring Current in Stepper Motors with SnapAmp
Hi Tom,

You had previously helped me with c programs to measure SnapAmp currents for both brushless and brushed servos to determine when the axis were against a stop.

I have a new project using stepper motors with the SnapAmp and would like to use a similar routine. I suspect it is quite similar to the brushless servo version, comparing A and C phases, but need a little guidance. Could you please advise what would need to be changed to use this with the steppers?

Thanks in advance,
David.
Group: DynoMotion Message: 10852 From: Tom Kerekes Date: 1/17/2015
Subject: Re: Measuring Current in Stepper Motors with SnapAmp
Hi David,

Unfortunately I don't think that is possible with a Stepper Motor.  Servo motors generate torque that is basically proportional to current.  But Stepper motors basically apply constant current and the torque is more a function of motor shaft angle.  For example if you put a large current into a stepper coil it will hold position and generate no torque.  You can tell it is generating no torque because it is sitting still and not accelerating like a DC motor would.  But then as you displace the motor shaft slightly from the holding position torque will be generated while the motor current remains unchanged.

Do you have encoder feedback?  If so there might be a way to basically use following error to determine torque.

Actually SnapAmp is capable of driving a Stepper Motor as a 4-phase Brushless servo motor but it is quite complicated to setup and tune.

Regards
TK

Group: DynoMotion Message: 10853 From: David Stevenson Date: 1/17/2015
Subject: Re: Measuring Current in Stepper Motors with SnapAmp
Hi Tom,

Thanks for the explanation. I really didn't understand how they worked.  :)   There are no encoders on these steppers... they are just for some small shuttle operations and if the position was off a few steps it wouldn't be a big deal.

Is it crazy complicated to set them up as 4 phase? Because the operations are small, infrequent movements, I am not too concerned about having them perfectly tuned. If that is something you would like to share, I would welcome the opportunity to learn how. If they were setup that way, could we use the current measurement program?

As always, many thanks,
David.

On 1/17/2015 2:42 PM, Tom Kerekes tk@... [DynoMotion] wrote:
 
Hi David,

Unfortunately I don't think that is possible with a Stepper Motor.  Servo motors generate torque that is basically proportional to current.  But Stepper motors basically apply constant current and the torque is more a function of motor shaft angle.  For example if you put a large current into a stepper coil it will hold position and generate no torque.  You can tell it is generating no torque because it is sitting still and not accelerating like a DC motor would.  But then as you displace the motor shaft slightly from the holding position torque will be generated while the motor current remains unchanged.

Do you have encoder feedback?  If so there might be a way to basically use following error to determine torque.

Actually SnapAmp is capable of driving a Stepper Motor as a 4-phase Brushless servo motor but it is quite complicated to setup and tune.

Regards
TK

Group: DynoMotion Message: 10854 From: Tom Kerekes Date: 1/17/2015
Subject: Re: Measuring Current in Stepper Motors with SnapAmp
Hi David,

Driving Steppers as brushless motors isn't that difficult.  It is similar to 3-phase brushless where commutation and tuning is involved except the commutation is more critical and there is more cogging and torque ripple to deal with.

But encoders are required so that isn't an option for you anyway.

Regards
TK

Group: DynoMotion Message: 10856 From: David Stevenson Date: 1/17/2015
Subject: Re: Measuring Current in Stepper Motors with SnapAmp
Hi Tom,

Fair enough. I guess I need to figure out a plan B.

Thanks for your help,
David.

On 1/17/2015 3:35 PM, Tom Kerekes tk@... [DynoMotion] wrote:
 
Hi David,

Driving Steppers as brushless motors isn't that difficult.  It is similar to 3-phase brushless where commutation and tuning is involved except the commutation is more critical and there is more cogging and torque ripple to deal with.

But encoders are required so that isn't an option for you anyway.

Regards
TK