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Mode of operation
Servos – in particular model making servos – are addressed using pulse-width
modulation. On servos pulse-width modulation is also called pulse-pause
modulation. The pulse width then represents the desired value. That value is
cyclically repeated, every 20 ms on most servos, and must lie between 1ms
(at the very left) and 2 ms (at the very right). However, the resolution as well
as the specific time values vary in most cases from servo to servo. Many ser-
vos – in particular digital servos – also allow operation beyond the 1ms to
2 ms range. Then angular variations up to approximately 180 degrees may
be obtained by way of exception.
To allow the accurate detection of the motor position each servo is fitted
with a measuring device that determines the current position of the motor,
e.g. the angle of rotation travelled with regard to an initial position. Such
measurement is carried out via an angular motion transducer, e.g. a syn-
chro resolver, an incremental transducer or an absolute value transducer.
An electronic regulation circuit compares the signal emitted by the angular
motion transducer with the specified desired value. If a deviation is detected,
the motor is rotated in the direction ensuring the shortest way towards the
desired value. This results in the deviation getting smaller. The procedure is
repeated by increments of approximations until the actual value lies within
the tolerance limit of the desired value. Such positioning procedure repre-
sents the simplest case. Alternatively, that principle also allows to regulate
torque or velocity values. Which makes for instance uniform driving profiles
under varying loads possible.
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for knowledge on our homepage – www.faller.de. You will find there
some interesting background information and a lot of creative sug-
gestions for all areas of model making.
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