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If the speed of the motor speed depends on the duty cycle how frequent should that pulse be passed to the motor. I know that there is some minimum frequency such as 33Hz but I am not sure, and I am not also sure about the maximum frequency?

I dont have any particular servo in mind, more sort of how they work in general. A good example could be the SG90.

suyol854
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  • What kind of servo? RC model servo? –  Dec 15 '16 at 17:33
  • @Brian at least the example servo is the RC model kind. – Grebu Dec 15 '16 at 17:58
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    Do you really want to control the motor speed? These servos are specialized in control of the position / rotation angle, as they only have a input for the position. Or do you want to know how often you can update the position set point? – Grebu Dec 15 '16 at 18:02
  • From what I understand, most servos care more about the pulse width than the frequency. Note, this is not the same as the duty cycle, duty cycle is the ratio of ON time to OFF time, but the pulse width is only about the ON time, the OFF time can be totally arbitrary. E.g. I believe the ranges were something like 1mS is full one way and a 2mS pulse is full lock the other way. Now it's the length of the pulse that's important, so it doesn't matter too much if you send a pulse 30 times a second, 50 times a second 100 times (well, within reason), as long as you accurately control the pulse width – Sam Feb 22 '17 at 21:49
  • I would hazard a guess that the speed of a generic servo could be slightly modified by the supply voltage between about 4.5V and 6V. Some servos go idle if they do not receive control pulses within a certain time window, if the servo does not reach the final position before it goes idle it will appear to approach the commanded position slowly in steps as it receives new pulses. – KalleMP Feb 23 '17 at 07:29

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Typical RC servo frequency is 50 Hz, as you can see from the SG90 datasheet. This question states that servos are pretty forgiving about the frequency, but why not use the right frequency? Note that the servo input controls the position (angle) not the speed. Also, the pulse width (amount of time on) matters, not the duty cycle (% of time on).

(Edited to incorporate comment feedback.)

Community
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Ken Shirriff
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    It is not the duty cycle but the pulse width that is used to signal the commanded angle. If it was duty cycle the pulse width would have to vary with the update rate. – Kevin White Dec 16 '16 at 04:17
  • @KevinWhite in the old days of RC the "update rate" was always 50 Hz which might explain the confusion. – user1890202 Feb 23 '17 at 01:06
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If the speed of the motor speed depends on the duty cycle

Duty cycle sometimes mean the time thee signal is one, and. Other times the percentage of time the signal is on. Most servos I have used care about the time the signal IS on. The off time can be from 5ms to 40ms, without any impact on the positioning.

dannyf
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