8.1.2 Control Mechanism

The control signal idles at 0V. A square pulse peaking at 5V with a width of 1ms to 2ms (for most R/C servos) specifies the desired angle. 1ms specifies one extreme, while 2ms specifies the other extreme. The angular difference corresponding to the 1ms is about 180 degrees. In other words, if we consider the angle at 1.5ms is 0, 1ms specifies an angle of -90 degrees, while 2ms specifies an angle of +90 degrees.

Upon reception of the falling edge of this 1ms to 2ms pulse, the R/C servo control circuit compares the potentiometer output with the command. If there is any difference, the control circuit turns on the DC motor to correct the error. The correction lasts about 20ms, then the R/C servo control mechanism idles again.

Note that the correction may be strong or weak, depending on the difference between the control signal and the potentiometer output.

If an R/C servo is under sufficient load that the angular position changes without active correction, the pulses should be spaced no more than 20ms apart. If the control pulses are too far apart, it is possible for the R/C servo to creep a little bit before correction is made by the next control pulse.