I have found some information from the book,
programming the propeller with SPIN; a beginners
guide to parallel processing.(page 204)
Futaba servos need to have a pulsed signal sent to them approximately
60 times a second on a regular basis to accurately maintain their commanded
position. If this is not done, the operation of the motors becomes jerky and
irregular. The requirement also means that there can be a minimum worst-case
delay of about 1/60th of a second whenever a command is sent to an R/C servo.
For all practical purposes, a lag this long is not critical in a motor application.
However, you need to have in mind that this delay does exist. It takes
1/60th of a second(worst case) for the motor to start responding to the
last command sent to it.
The servos have to be reminded of their position about 60 times a second in
order to maintain proper operation, so , we need a way to pulse the servos
on a regular basis 60 times a second.
This programming is assumed
to be in every robot that uses these types of servos.