The Art of the Helicopter John Watkinson - Karatunov.net

292 The Art of the Helicopter
rotor of the synchro motor is fed with the same AC drive as the synchro generator
by connecting the two in parallel. When the three windings of the synchro motor are
fed with signals from a synchro generator, they produce a resultant flux direction with
which the rotor aligns.
The synchro system is capable of high resolution but the synchro motor cannot
develop much torque. If a significant load has to be positioned remotely, the synchro
system will require power amplification. In this case the receiving device may be a
control transformer. This is virtually identical to the synchro motor, the main difference
being that the rotor winding is not connected to the AC supply but is instead connected
to a phase-sensitive amplifier. Figure 7.7 showed the principle. A motor driving the load
also turns the rotor of the control transformer. An amplifier senses the signal from the
control transformer rotor and drives the motor.
The three signals from the synchro generator reproduce the flux direction of the
generator’s rotor in the control transformer. If the rotor of the latter is at 90 ◦ to that flux
direction it will produce no output. However, if there is a misalignment, say clockwise,
the rotor will produce an output which is a sine wave in-phase with the signal exciting
the synchro generator. However, if the misalignment is anticlockwise, the rotor output
will be in anti-phase with the synchro generator input. The amplitude of the rotor
output will increase with the angular error. The phase-sensitive amplifier drives an
electric motor to move the load. Using the phase information from the stator, the
amplifier can drive the load in the correct direction to make the angular error in the
control transformer smaller. When the rotor of the control transformer is at 90 ◦ to
the induced field from the three windings the rotor output disappears and the motor
stops. In this way a significant load driven by a motor can be controlled remotely by
the low power signal available from the synchro generator.
7.20 Digital signalling
The potentiometer, the synchro and the LVDT are analog devices in that the signals
they produce are infinitely variable. Instead of analog signalling, it is also possible to
transmit control position numerically. In digital systems, a binary number proportional
to the position of the control is encoded. All of these signals may be carried by electrical
cable, leading to the marketing term ‘fly-by-wire’. Digital signals can also be carried
along optical fibres, leading to the term ‘fly-by-light’.
Digital signalling is a technology that arrived in aviation quite suddenly and so there
is no traditional knowledge or experience base. As a result many people regard digital
signalling as little short of black magic when in fact it is based on relatively simple
principles. One of the key concepts to grasp is that digital signalling is simply an
alternative means of carrying the same information. An ideal digital signalling system
has the same characteristics as an ideal analog system such as a synchro: both of them
are totally transparent and remotely reproduce the original applied control movement
with negligible error.
Although there are a number of ways in which the position of a control can be
represented digitally, there is one system, known as pulse code modulation (PCM)
that is in virtually universal use. Figure 7.28 shows how PCM works. Instead of being
continuous, the time axis is represented in a discrete or stepwise manner. The control
waveform is not carried by continuous representation, but by measurement at regular
intervals. This process is called sampling and the frequency with which samples are taken
is called the sampling rate or sampling frequency Fs. The sampling rate is generally
fixed and is thus independent of any frequency in the signal.