The Art of the Helicopter John Watkinson - Karatunov.net

Fig. 7.28 In PCM or digital signalling, the position of a control is sampled periodically and the sample is
expressed as a discrete number.
Those who are not familiar with digital signalling may worry that sampling takes away
something from a signal because it is not taking notice of what happened between the
samples. This would be true in a system having infinite response rate, but no real control
system can be like this. All control signals from joysticks, actuators, gyros etc., have a
distinct limit on how fast they can change. If the sampling rate is high enough, a control
waveform can only change between samples in one way. It is then only necessary to
carry the samples and the original waveform can be reconstructed from them.
Figure 7.28 also shows that each sample is also discrete, or represented in a stepwise
manner. The value of the sample, which will be proportional to the control parameter,
is represented by a whole number. This process is known as quantizing and results in an
approximation, but the size of the error can be controlled until it is negligible. If, for
example, we were to measure the height of humans only to the nearest metre, virtually
all adults would register 2 metres high and obvious difficulties would result. These are
generally overcome by measuring height to the nearest centimetre. Clearly there is no
advantage in going further and expressing our height in a whole number of millimetres
or even micrometres.
The point is that an appropriate resolution can also be found for a control system, and
a higher figure is not beneficial. If a conventional pushrod control system is considered,
each bearing and joint in the system has some backlash and the cyclic stick on a typical
helicopter can always move some small distance before any motion is detected at the
swashplate. If the movement step size of a digital system is of the same order as the
backlash of a mechanical system, both systems have the same accuracy.
The advantage of using whole numbers is that they are not prone to drift. If a whole
number can be carried from one place to another without numerical error, it has not
changed at all. By describing control parameters numerically, the original information
has been expressed in a way that is better able to resist unwanted changes. The amount
of backlash in a mechanical system increases with the number of ball joints and bell
cranks in a control run. The accuracy of a mechanical signalling system would decline as
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