I know that RC applications, such as a drone, use PWM signals to drive the motors. This PWM signal is mostly 50 Hz (0.02 s). The pulse itself varies from 1 ms to 2 ms. So a 1 ms pulse corresponds with a minimum motor rotation and a 2 ms pulse with a maximum motor rotation. So basically the other 18 ms of the period the signal stays idle.
Why does the PWM signal have such a format? Why is the active part of the signal not spread over 1 ms and 10 ms? What is the advantage of using such small pulses?
Best Answer
The reason for the long gap is so that the transmitter can send all of the other servo positions.
In the days of clothes-pegs and crashed aircraft from frequency collisions, radio control was done with AM at 27 MHz.
The transmitter would send a sync pulse, and then a series of 1-2 ms pulses, one for each servo. The earlier ones delayed the later ones, didn't matter much. These are just RF pulses, no special modulation.
The receiver would receive the pulse stream, synchronise on the first one and then direct each successive pulse in turn to a different servo socket.
So to allow for maybe 8 channels set to 2 ms, and have some gaps, you need about 20 ms. With an 8 channel transmitter, the duty cycle on the combined RF channel would have been over 50%.
This servo protocol, of 1-2 ms every 20 ms, has just stuck around from then.
This site about making a PC digitiser for your remote control has some oscilloscope graphs showing four or five channels.