The Art of the Helicopter John Watkinson - Karatunov.net

Technical background 55
of oscillation to occur at right angles to the main oscillation. Further electrodes and
suitable electronic circuits can measure the amplitude and sense of the Coriolis effect
that is proportional to the angular velocity.
A piezo-electric gyroscope may be only 20 mm long and weighs a few grams. The
power consumed is a few milliWatts. They were initially developed for image stabilizers
in consumer video camcorders. These gyros have already eclipsed the mechanical
gyro in model helicopters on account of the low power and weight, and may find certain
applications in full-size machines in due course. The small size allows completely
redundant units where two or more sensors can be used in case one fails.
In the laser gyroscope, shown in Figure 2.34(b), light from a laser is split into two
components and sent in both directions round an optical path. As the distance in both
directions is the same, the two components will emerge in the same phase. However, if
the optical path rotates about the axis shown, light travelling in one direction will take
a slightly longer time and that travelling in the other direction will take a shorter time.
The two components will no longer be in phase and the phase difference is proportional
to the angular velocity. This can be sensed electronically. These alternative gyros both
have the advantage of freedom from wear.
2.19 Feedback
Control signalling conveys the desired position of a flight control or a subsidiary control
from one part of the airframe to another. The next essential is to ensure that when
a command is received it is carried out accurately. Feedback is a useful tool to reach
this goal. Feedback is a process that compares the current condition of a system with a
desired condition and tries to make that difference smaller. This is exactly the characteristic
needed to make a remote load follow a control signal, hence the extensive use of
feedback in control systems. Feedback may be implemented in mechanical, hydraulic
and electrical systems and in the case of the latter may be implemented with analog or
digital techniques, although the principles remain the same in each case.
Figure 2.35(a) shows a basic electrical feedback servo system. The desired position
of the load is defined by setting a control potentiometer that creates a proportional
electrical voltage. The actual position of the load is measured by a second potentiometer.
The actual position voltage is subtracted from the desired position voltage to create
a signal called the position error. This is a bipolar signal whose polarity indicates the
sense of the error and whose magnitude indicates how far the load is from the desired
position.
The position error is amplified and, in this example, used to power a DC electric
motor that drives the load and the feedback potentiometer. The polarity of the system
must be such that the motor runs in a direction to cancel the position error. In other
words the system has negative feedback: the information regarding the position of the
load flows to the error sensor and the error information flows to the load forming a
closed loop or feedback loop.
If the control potentiometer is set to a new position, a position error will result which
will drive the motor until the error becomes zero again. The power to the motor will
then be zero. In most cases this will not cause the motor to stop. The motor and the load
have inertia and once in motion may continue even if the motor power is removed. As
a result the load may overshoot the desired position. This will cause a reverse position
error so the motor is driven back. Again the load may overshoot and an indefinite or
decaying oscillation known as hunting takes place.