Stepper Motor Torque Control

Can the torque on a stepper motor be controlled?

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You need to provide more info as to the application and size of the equipment. One way for true torque precision regulation (+/-) would be a 4 quadrant DC servo drive system with torque feedback.
To some extent for protection, it can be limited. It moves a motor pole at a time, but current could be limited as not to exceed some torque value. Also through a particle brake set to a torque value limit. A stepper is normally used as to rotate a given amount and / or at a given rate.
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No, not without slipping steps, in which case the stepper motor is not operating properly.
-Robert Scott Ypsilanti, Michigan (Reply through this forum, not by direct e-mail to me, as automatic reply address is fake.)
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Some microsteppers drive each phase with current- mode DACs, which controls the torque exerted by each phase winding against the magnetic detents, causing the motor to equilibrate at what is otherwise not a stable position. External torque will drive the stepper off that metastable position, unless there is a position feedback loop.
And, yes: More voltage in the power supply will allow the stepper to move loads with more inertance.
And, no: For a DC motor, torque and speed uniquely define an operating point for the motor. For a stepper motor, the inertia and friction of the load, the inertia of the motor, the inductance of the motor, and the dynamics of the phase drive circuits all interact with each other to determine whether a stepper can drive the load at a given speed.
Actually, speed is an irrelevant measure for a stepper. Because of its incremental motion, step rate is more meaningful.
And torque, as measured by something like an ordinary motor dynamometer, is also meaningless or misleading when applied to a stepper. Instantaneous torque, as a function of angular position, is more meaningful.
-Mike-
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I have an application that I want to use a constant speed device to feed wire onto a spool driven by a constant torque motor (the rotational velocity of the spool will need to decrease as the diameter of the package increases). I need to position the spool when the package is full. I need to build 24 of these and am trying to find the most cost effective method.
thanks

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I just don't see a stepper as being the right answer, unless it's a really small spool. Here's three alternate suggestions, and a comment on using a stepper if it's right:
If you put position feedback on the spool you can servo a direct-drive DC motor to position, and getting a constant torque is easy (drive with constant current). This is conceptually simple but that spool's probably going to have a lot of inerta, so your control rule on the motor needs to be well thought out, and direct-drive motors are big and you burn up a lot of energy as heat.
If your torque will never, ever change you could use a slip clutch on the spool and a gear motor or some such to position the spool when you're done (assuming that the wire gets cut and the external torque on the spool goes to zero). Just drive the gear motor faster than the spool will ever go during run and let the clutch slip, then use closed-loop control to locate the spool when you're done.
Don't you want a constant tension on the wire rather than constant torque? Should you have a tensioner with the appropriate feedback (and lots of movement) and control your spool with a gearmotor? This might be the easiest way, and you could back it up with a slip clutch to prevent those embarrasing broken wires. The nice thing here is that it's easy to drive a gear motor at a more or less constant rate, and that's more or less what you'll need to do when you're in steady state.
If your whole assembly is really small then you could replace the above assembly with a stepper -- use the stepper for positioning, put an index on the spool and use a tensioner with feedback. This gives you easy position control and the stepper will give you a built-in slip clutch action (probably more than you want -- but I don't like steppers).

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As you can probably tell I am not an electrical engineer, I'm a mechanical engineer. This design is to replace an existing machine that is loaded with mechanical systems that I know could be better served with closed looped servo motors. I know very little about steppers, but from what I have researched, it appeared they would perform the same function for less capital investment. The biggest unknown for me was the torque question.
You are right about the tension control but the package is only 3 lbs and the tension vs torque did not seem worth the additional complication.
Thanks

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TEM wrote:

Stepper motors are better suited for torque limits (with jumpy cogging when slipping) rather than precision and smooth torquing at a wide range of speeds. They are well suited to slow, precise average speed operation if you can easily tolerate the vibration of the stepping process. Microsteppng can reduce this vibration to essentially that of a synchronous motor (which is really what a stepper motor is).
Have you considered an eddy current clutch as the torque control element? They are excited with variable DC and have very smooth torque adjustment. Their torque is proportional to both the DC excitation and the speed differential across them. They can be used to apply drag torque to a rotating shaft from a stationary reference (variable torque braking), or transmit a variable torque from a constant speed source. They cannot transmit torque with not rotational velocity difference (they cannot act as a stationary brake).
--
John Popelish

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Or an ECM and controller. Also look at Bodine web site. You may also need an encoder as to position it. The whole thing can be done with a PLC with servo card, drive motor and feedback.

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This may help you.
http://www.placidindustries.com/apptension.html

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Are you sure its constant torque you need? Isn't it constant tension on the thread being spooled?
Walter

velocity
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Ideally constant tension would be better. I am more concerned with positioning the spool after the spool is full than constant tension.

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This is a lot of good information.
Thanks

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