Pulse Width Position Servo
I have described elsewhere in these pages how normal PWM works in
controlling the speed of a motor. PWPS is an entirely different system
and two should not be confused: the similarity of names is unfortunate
and I suggest you forget everything you know about ordinary PWM motor speed
control so you can follow this explanation.
What is PWPS?
PWPS (you won't often find it referred to as this, but usually as PWM -
which is confusing) is a control system which is extensively used in radio
control to vary the rotation of (or position of), for instance, a rudder.
It does this by using a small motor to rotate a shaft to which is attached
a standard potentiometer. This output shaft is also coupled to the rudder
- it is the angular rotation of this output shaft which is being controlled
by the width of the input pulse being fed from the radio transmitter. These
variable width pulses are fed at intervals.
The angular rotation of this output shaft is monitored via a feedback
pot inside the servo mechanism, this pot then causes the output shaft to
be 'slaved to ' the transmitted pulse. The word 'servo' in fact means nothing
other than slave and comes from the Latin Servus - a servant.
System parameters
The system is defined as being centred at a pulse width of 1.5mSec. Pulse
repetition rate is not important, a typical rate might be 400Hz (2.5mS
pulse spacing) or 50Hz (20mS spacing). Rate of change of pulse width with
rotational angle is 10µS per degree with a range of 90° (±45°).
Therefore maximum pulse length is 1.95mSec and minimum is 1.05mSec (1.5mSec
±450µsec).
Now if you were trying to work out how to give a signal which depends
on the rotation of a pot, one way you might chose is by using the pot to
control the pulse width of a common mono-stable.
This is exactly what PW PS does. Every time the receiver gets a pulse
from the transmitter, it fires a monostable. The monostable's pulse width
is determined by the pot on the output shaft. The receiver compares the
received pulse with the monostable's pulse. If the received pulse stops
first then the receiver causes the motor to rotate anticlockwise so the
monostable's pulse length is reduced. If the local pulse stops first then
the motor is rotated clockwise, to increase the local pulse length.
The commonest 'home constructor' chip for this application used to be
the ZN409 (GEC-Plessey Semiconductors). The ZN 409 contains the receiver
monostable (which needs external timing components) and most of the drive
components to control a small, 6v motor. It also contains other bits of
circuitry to tailor the motor response and to include a dead band, to reduce
overshoot and hunting.
This chip has been rendered obsolete because manufacturers nowadays
use dedicated microcontrollers for the job.
Interfacing
It is of course quite simple to interface such a system to a 4QD controller.
You would fix a second pot (10K) onto the output shaft and feed this to
a joystick interface board. The JSI will drive a standard NCC
series controller. However see also Radio control.
Average value control?
If the pulse repetition time is high (400Hz, as above) then the 1.5mS is
a 60% duty cycle. 90° forward is 96% duty cycle and 90° reverse
is 24% modulation. A 6v signal with 24% duty cycle has an average of 1.4v,
60% gives 3.6v and 96% gives 5.8v. With this system the pulse train could
feed direct to, say, a 2QD controller and the ramping circuit in the controller
would average out the pulse train, converting it directly to a demand speed
value.
However life with your r/c signal will probably not be that simple:
pulse repetition frequencies vary a lot and are not neccessarily stable.
For instance a dual channel transmitter might have a 20mS rate, transmitting
first a pulse for one channel, then for the second channel. A 1mS pulse
every 20mS is only 5% duty cycle and there is not going to be enough variation
to use this signal directly.