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
(Reply through this forum, not by direct e-mail to me, as automatic reply
address is fake.)
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.
More voltage in the power supply will allow the stepper to move loads with more
For a DC motor, torque and speed uniquely define an operating point for the
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.
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.
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).
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.
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
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