Here’ s a
design circuit for servo signal is a simple digital
pulse. It spends most of its time at a logic low (0 V). About every 20mS
it goes logic high (3 to 6 VDC) and then quickly goes low again. It
is this tiny window of logic high time, called the pulse width, that gets the
attention of the servo. Here’s the figure of the circuit;
Please refer to the drawing. The period
labeled "A" is called the frame rate. In the example it is repeated
every 20mS (50 times per second), which is quite typical for most radio
systems.
Modern
servos define center as a 1.5mS pulse width, as shown by detail "B"
in the drawing. Full servo rotation to one side would require that this pulse
width be reduced to 1.0mS. Full rotation to the other side would require the
pulse width to increase to 2.0mS. Any pulse width value between 1.0mS and 2.0mS
creates a proportional servo wheel position within the two extremes. The frame
rate does not need to change and is usually kept constant.
The servo will not move to its final
destination with just one pulse. The servo amp designers had brilliantly
considered that multiple pulses should be used to complete the journey. This
little trick reduces servo motor current draw and it helps minimize erratic
behavior when an occasional corrupt signal is received. To move the servo, you
must repeat the pulse every few milliseconds, at the chosen frame rate. Modern
R/C systems use a 40Hz - 60Hz frame rate, but the exact timing is not critical.
If your frame rate is too slow, your servo's movement will become rough.
If the rate is too fast the servo may become very confused.
