During one of my lab session I faced this problem and couldn't find any good answer. As far as I know for a stepper,

  1. Reversing the direction of current will change the direction of rotation.
  2. Number of pulses applied at each pole will determine the position.
  3. Frequency of the pulses will determine the speed.

But I dont know how to achive these with a servo. For my understanding servos control speed and direction of rotation on their own.

It would be great if some one can tell whether my understanding is correct or not.

  • \$\begingroup\$ Servo motors are usually simple DC motors with a rotary encoder attached. The encoder provides information about the "position" which can be used to compute speed and direction. en.wikipedia.org/wiki/Rotary_encoder \$\endgroup\$ Feb 2, 2015 at 10:22
  • \$\begingroup\$ @RolandMieslinger But it doesn't say how to change the direction or speed. I can change the position by providing the correct PWM signal. But how to decide the direction or speed? \$\endgroup\$ Feb 2, 2015 at 10:56
  • \$\begingroup\$ The first question is what is meant by a "servo motor" ... a model aircraft servo is usually a cheap DC brush motor as @Roland says : however, in industrial robotics, "servo motor" usually means something more like a BLDC or PMSM with dedicated positional controller. \$\endgroup\$
    – user16324
    Feb 2, 2015 at 12:07
  • \$\begingroup\$ This helped for my TMA 2 of Microprocessor and interfacing of OUSL. thanks :P Upvoted! \$\endgroup\$ Feb 7, 2015 at 15:34
  • 1
    \$\begingroup\$ @MarlonAbeykoon :). I'm also studying at OUSL. Same TMA gave rise this problem. \$\endgroup\$ Feb 8, 2015 at 3:25

1 Answer 1


For cheap and plain RC servos you are right, they control their speed and rotation themselves. The user specifies a certain target position with a PWM signal, and the servo tries to transition into that position. It depends on the servo how fast it will move, e.g. you can't be certain about the speed. You can't even be certain if the servo did ever got into the target position, but it will try to get there as hard as it can.


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