The Art of the Helicopter John Watkinson - Karatunov.net

Technical background 59
shut down and replaced by pilot action. For example if the autopilot failed, the pilot
could still use the outputs of the navigator and the power assistance, but he would have
to move the controls to stabilize the attitude of the machine on the course stipulated
by the flight director.
Feedback loops have many advantages, but they do behave badly in the event of
certain failures. If the position feedback sensor of Figure 2.35 failed or jammed, the
movement of the load would not reduce the servo error and the servo would inadvertently
keep driving the load in the direction that would normally reduce the error, until
the load reached a mechanical limit or stop. This is a characteristic of feedback loops,
which is that when they fail they generally go to one control limit. This is known as a
hardover.
Helicopters have crashed because a stability augmentation system had a hardover
failure. Subsequently systems were designed with limited authority so that even if a
hardover occurred, the amount of incorrect control travel or the force produced was
not so great that the pilot could not overcome it. The end stops of the servo could be
brought closer to the neutral position and a slipping clutch could be fitted in the drive
train. Today part of the certification process is to prove that the pilot can overpower the
servo so that control can be retained if a stability augmentation system has a hardover
failure. If this is impossible, the system will have to be made failsafe by incorporating
redundancy.
Certain helicopters use a redundant system in which the autopilot or stability augmentation
signals are generated in two independent systems and fed to two motorized
servos that drive the controls through a differential gear. In the event that one of the
servos experiences a hardover failure, the tachometer of the failing system will output
an unusually large signal. Detection of the hardover condition will shut down the failing
servo by removing power and apply a shaft brake to lock the motor. The remaining
servo will be able to retain control through the differential gear and the performance
will be identical if the signal which shuts down the failing servo also doubles the travel
of the surviving servo.
In a fully powered feedback servo system, the pilot simply moves a transducer and
the feedback loop will do whatever it can to carry out the command. It will still try
even if enormous resistance is met. This may result in overstressing and a solution is a
system of force feedback. The control stick is not centred by a spring, but by a force
motor. The resistance felt by the pilot comes from the force motor that produces a force
proportional to the force being exerted by the servo. In this way the pilot produces a
small force that is used to control the machine, but he also experiences a scaled down
replica of the resistance to the control efforts.
This gives the pilot a good sense of how much stress the machine is under. Force
feedback systems of this kind are also known as artificial feel systems. Artificial feel of
this kind can also be used with autopilots. If an altitude error is suddenly experienced
due to turbulence, the autopilot could react very quickly and powerfully, but to do so
would stress the airframe and cause discomfort to the passengers. It may be better to
take longer to cancel the error by reacting more gradually.
Feedback is used in servos designed for radio control of models. These work on the
principle of pulse width modulation where the length of a pulse, typically 1 millisecond
at the neutral position, is increased or reduced as a means of control signalling. A servo
operating on pulse width signalling can use feedback based on comparison of pulses.
The feedback potentiometer controls a pulse generator and the servo error is obtained
by determining the difference in pulse width between the input and the feedback pulses.
The error pulses are used directly to drive the motor amplifier. As the error becomes
smaller, so does the width of the pulses, reducing the drive to the motor. Velocity may be