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Controlling a Motor FS90R


Well-Known Member
I have not done any robotics & I need a motor for a project I'm doing. The FS90R motor will probably suffice, but I'm puzzled by the data sheet below. It uses PWM to control the speed & direction of rotation as can be seen in the data sheet. It gives Pulse width range, Pulse width, Dead band range & Rotation direction. But I can't see what the period of the PWM signal should be. Any assistance will be appreciated.Motor FS90R.png

Nigel Goodwin

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Most Helpful Member
It's a standard remote control analogue servo signal - so the period isn't particularly important - but it's usually 20mS (50Hz). You can make response times faster or slower by altering that, often within quite large limits.


Well-Known Member
Most Helpful Member
It looks like a typical "Radio control" servo interface.

It's not PWM as such, just a positive pulse with the duration representing the required position or movement; 1.5mS is centre or neutral, longer or shorter is one or the other direction.

The typical interval between updates is 20mS but it is not at all critical.

Pre-microprocessor RC systems used a sequence of pulses, which in effect clocked a "1" bit through a shift register and each servo was driver by one of the shift register outputs.

The intervals between the transmit pulses that shifted the decoder register were set by the various joystick pots etc., and a longer interval at the end of the sequence reset everything and a new "1" bit was loaded to the shift register...

The only thing that matters to for any individual servo is that it gets a command pulse every so often.


Well-Known Member
Thanks Nigel & rjenkingb. I decided to use a motor I extracted from an old Cassette Tape Player. However, it would not run in reverse properly. So I opened it and found it has an electronic speed control. See the photo. I was surprised that it was able to run in reverse at all given the speed control electronics. But I don't have a circuit of it & did not waste time working it out. So I cut the components off except for the trim pot & connected wires to the commutator tags.
So it now runs properly in both directions.


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