The Art of the Helicopter John Watkinson - Karatunov.net

Fig. 7.35 In a pure binary system, only positive values are possible. A bi-directional system can be obtained
by adding an offset so that neutral is half-way up the scale.
Fig. 7.36 Trying to reduce control travel by dividing all of the sample values by a constant produces an
unwanted offset.
In practice it may be required to modify the control samples between, for example,
a joystick and the associated actuator. The control may prove too sensitive and by
processing the samples a given movement of the joystick could be arranged to produce
a smaller movement of the actuator, perhaps as a function of airspeed. A small number
might be added to the samples to perform a trim function. Two control signals might
be added together to mix cyclic and collective controls into the swashplate or to mix
the output of a stability augmentation system or autopilot into the flight controls.
If two offset binary sample streams are added together in an attempt to perform
digital mixing, the result will be that the offsets are also added and this may lead to an
overflow. Similarly, if an attempt is made to reduce the sensitivity of a control by, say,
half by dividing all of the sample values by two, Figure 7.36 shows that the offset is also
divided and the waveform suffers a shifted baseline. The problem with offset binary
is that it works with reference to one end of the range. What is needed for aircraft
controls is a numbering system operating symmetrically with reference to the centre of
the range.
Control 299