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In article <5470@umbc3.UMBC.EDU> mittal@homer.engr.umbc.edu (Samir Mittal) writes:
>I am new to robotics and I have this elementary question.
>Are there special type of motors in which we can specify 
>the torque that we want? In most of the servo or stepper motors
>one inputs voltage which is able to regulate angular speed . Is it 
>possible to input current that will regulate the torque exerted by
In theory, most DC and "torque" motors produce torque in direct proportion
to their armature _current_, which means you would need to servo the
armature current to a command voltage from an A/D, or something similar.
While you are accelerating the load, however, the motor is acting like
a generator (_is_ a generator), and producting a "back EMF" which is
directly proportional to the motor (and load) speed.  In order to achieve
a constant armature current, you have to apply progressively more _voltage_
to overcome the back EMF, which is often the limiting factor in the loop.

One way to overcome the low-speed effect of back EMF is to put the armature
in the feedback path of a DC power amplifier, or equivalently establish
a "current loop" servo.  A more efficient way to do this is to use a
switching amplifier (such as a PWM amplifier), which is designed to apply
relatively high voltages to the motor for  brief, variable time slices.
In this case, the PWM amp. is (at least roughly) matched to the motor's
inductance.  See manufacturer's literature.

Hope that was a useful answer.



-- 
MILLS,JOHN M.
Georgia Institute of Technology, Atlanta Georgia, 30332
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