The Art of the Helicopter John Watkinson - Karatunov.net

56 The Art of the Helicopter
Fig. 2.35 (a) A simple feedback system compares the actual load position with the desired position. To prevent
hunting, the load velocity may be measured with a tachometer (b) or by differentiating the actual position signal.
This is not acceptable in any precision system. The problem of hunting is due to
considering only the position of the load. The solution is to consider the load’s velocity
as well. In this way the control system knows how much kinetic energy is stored in
the moving load so that it can be brought to rest by applying a suitable reverse drive.
Figure 2.35(b) shows that in this example the load velocity can be measured with a
tachogenerator connected to the motor shaft. Many servo motors are designed with
an integral tachometer.
If the control potentiometer is set to a new position, the position error will drive the
motor to cancel the error. As the load gathers speed, the load velocity signal will be
subtracted from the position error. As the load approaches the desired position, the
position error becomes smaller, and when the position error is less than the velocity,
the signal driving the motor will actually reverse polarity so that the load is retarded.
This retardation process damps the hunting tendency. Instead of having a separate
tachometer the velocity of the load can be obtained by differentiating the actual position
signal.
The performance of a servo may be tested by measurement of the response to a step
input. The speed of response can never exceed the limits set by the motor power and