The Art of the Helicopter John Watkinson - Karatunov.net

Fig. 7.27 Transducers used in signalling. At (a) the potentiometer or variable resistance. At (b) the synchro
outputs three AC signals as a function of the rotor angle. The LVDT (linear variable displacement transducer)
shown at (c) is a form of linear synchro. It has the advantage over the potentiometer that there is no sliding
contact and no wear mechanism.
Figure 7.27 shows some methods used for control signalling. At (a) is the potentiometer.
This is a resistance that outputs a DC voltage proportional to the position of the
moving contact or wiper. The potentiometer may be rotary or linear and is simple, but
the sliding wiper introduces a wear mechanism. The remaining devices are more complex
but wear free. At (b) is the AC operated synchro generator that signals rotational
angle to a synchro motor using three wires. The synchro is ideal for measuring the angle
of a joystick or side-arm controller or the reading of a compass. In the synchro there
is a rotor carrying a single coil which is fed by sliprings and brushes. Surrounding the
rotor is a set of three coils mounted at 120 ◦ in a star arrangement. The synchro works
like a transformer in that it must have an alternating supply. Traditionally this will be
the 400 HzAC used in aircraft and the supply is fed to the rotor via the sliprings. The
current flowing in the rotor produces a magnetic field at right angles to the rotor shaft.
The voltage induced in each of the secondary windings depends on the angle of the
rotor. If the field of the rotor were parallel to a given secondary, the induced voltage
would be a maximum, but if the rotor were at 90 ◦ to that secondary the induced voltage
would be zero. The induced voltage in a given winding is proportional to the cosine
of the angle between the rotor and that winding. With three windings there must be a
unique combination of voltages for any position of the rotor.
At (c) is the linear equivalent of the synchro which is the LVDT (linear variable
displacement transducer). This is also fed with an AC excitation signal. The moveable
core alters the magnetic coupling between a pair of input coils and the output coil. The
input coils are connected in anti-phase and with the core in the central position the flux
from these two is cancelled out. If the core moves off-centre the amplitude of the output
increases in proportion to the displacement. The phase of the output represents the
direction. A phase-sensitive amplifier can extract a voltage proportional to the position
of the core. As the LVDT signals linear position, it is ideal for measuring the extension
of an hydraulic ram or of a ball screw actuator.
The synchro motor is almost identical to the synchro generator. The main difference
is that it contains a rotary damper to prevent it spinning like an electric motor. The
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