In our last posting we discussed how we made our “Hybrid Drive” act like a torque motor.
One issue that you do have be aware of is the fact that at some point in the stepper’s speed-torque curve the torque is going to begin to fall off as a function of speed just as it did with the AC torque motors.
The step rate becomes so fast that the drive can’t get the full current into the winding and the torque begins to fall off. You’ll need to look at the maximum web operating speed and see if it’s less than the stepper’s torque-fall-off break-point. If the required speed is higher than the break-point, then maybe you can live with slightly less torque or you can increase the diameter of the pinch roller that the stepper is driving so it operates at a slower speed.
You’ll have to increase the max current value too to compensate for the larger pinch roller diameter.
Could we use a standard stepper drive to act as a torque motor?
I remember when a customer was trying to use a standard stepper as a clamp.
The clamping position varied by several full steps and the customer was going to program the stepper’s control unit to go slightly farther than the maximum clamping distance. That way it would ensure that contact was made no matter what the variation in the clamping distance was. Does that sound good to you? I hope not, because you can’t use a standard stepper in this fashion.
Since the clamping variation was several full steps in this application the point of contact that the clamp and the work piece meet varies. This means that the torque generated by the stepper motor when contact is made and the stepper control finally stops sending steps can be anywhere in stepper motor’s torque sine wave between 0 and 360 degrees (four full steps). If contact happened to be between 0 and 180 degrees, (up to plus two full steps) then the system would apply a positive torque anywhere from zero to the maximum rating of the motor. It would clamp, but with an unknown amount of force.
If contact happened to be between 180 and 360 degrees, (higher than 2 full steps and less than 4 full steps) then the motor would jump back away from the work piece and not even touch it. I’m sure you remember that there are four stable full step conditions and the motor is happy to be in any one of them. If you don’t remember that, then go way back in this blog and read that discussion.
I predicted that this application wouldn’t work and that depending on how far beyond the clamp contact point the motor would try to go the motor shaft/clamp would bump into the work piece, jump backwards to its new stable position and charge right back into the work piece and repeat this “wood pecker” action until the incoming step stream stopped.
The customer tried it anyhow and called back a few days later, which was nice that they closed the loop with me, and said “you were right, it didn’t work.” This application was a while ago.
Today we can use our “Hybrid” drive in “torque mode” to achieve success with exactly this type of application.
More “Hybrid” motor technology discussions next time.