An open-loop control system is controlled directly, and only, by an input signal. The basic units of this type consist only of
- an amplifier [The amplifier receives a low- level input signal and amplifies it enough to drive the motor to perform the desired job]
- a motor.
Process
The open-loop control system, the input is a signal that is fed to the amplifier. The output of the amplifier is proportional to the amplitude of the input signal. The phase (ac system) and polarity (dc system) of the input signal determines the direction that the motor shaft will turn. After amplification, the input signal is fed to the motor, which moves the output shaft (load) in the direction that corresponds with the input signal. The motor will not stop driving the output shaft until the input signal is reduced to zero or removed.
This system usually requires an operator who controls speed and direction of movement of the output by varying the input. The operator could be controlling the input by either a mechanical or an electrical linkage.
In practice, the accuracy of the open loop system is unsatisfactory. There is no means of precisely controlling these factors and, therefore, the open loop system is not good enough for close tolerance control.
Many factors contribute to this e.g.
- Variations of load conditions.
- Frictional forces within the motor and its load, and the mechanical interconnection (gear trains, clutches, linkages, drives etc).
- Variation of power supply.
- Value of the demand voltage.
- Variations of amplifier gain.
