Aircraft Servomechanisms

Servomechanisms can be classified according to two main categories:

• Open loop systems.

• Closed loop systems.

OPEN LOOP

In an open loop system, the input demand generates an electrical equivalent of the demand position. This signal is amplified to the required power level and applied to a motor to position the load. The speed of response and the final position of the load depend on the following factors:

• Any variations in load conditions.

• Frictional forces within the motor and its load, and any mechanical interconnections.

• Variations in power supplies.

• The value of the demand voltage.

• Variations in amplifier gain.

As the open loop system suffers from the variable factors shown above, the output is unlikely to follow the input precisely and cannot provide the close tolerance required.

CLOSED LOOP

If the errors in the output of a system are detected and fed back to the input so that the necessary corrections can be made to eliminate the error, the system is said to be a closed loop system. A Closed loop system is shown below.

The essential features of a closed loop system are:

• Information concerning the behaviour of the load is fed back to the input. This is called feedback.

• The position of the output (feedback) is compared to that demanded by the input. Typically in a summing amplifier.

• The production of an error signal proportional to the difference between the demand and feedback signals.

• Power amplification of the error signal to control the load.

• Movement of the load in such a direction as to reduce the error signal to zero, at which point the output is the same as that demanded by the input.