Servomechanism is a powered mechanism producing motion or forces at a higher level of energy than the input level, e.g. in the brakes and steering of large motor vehicles, especially where feedback is employed to make the control automatic.
A servomechanism, sometimes also called Servo, is an automatic device that uses error-sensing negative feedback to correct the performance of a mechanism and is defined by its function.
Components of Servomechanism:
A servo system mainly consists of three basic components
- A controlled device
- A output sensor
- A feedback system
Working of Servomechanism:
Servomechanism is an automatic closed loop control system. Here instead of controlling a device by applying variable input signal, the device is controlled by a feedback signal generated by comparing output signal and reference input signal.
Carefully observe the figure above and think. When reference input signal or command signal is applied to the system, it is compared with output reference signal of the system produced by output sensor, and a third signal produced by feedback system. This third signal acts as input signal of controlled device. This input signal to the device presents as long as there is a logical difference between reference input signal and output signal of the system. After the device achieves its desired output, there will be no longer logical difference between reference input signal and reference output signal of the system. Then, third signal produced by comparing theses above said signals will not remain enough to operate the device further and to produce further output of the system until the next reference input signal or command signal is applied to the system.
Hence the primary task of a servomechanism is to maintain the output of a system at the desired value in the presence of disturbances.
Now we will discuss Servo Motor.
A servo motor is a DC, AC, or Brushless DC Motor combined with a position sensing device e.g. a digital decoder. Thus any motor controlled using Servo Mechanism is Servo Motor.
Servos are extremely useful in robotics. The motors are small and are extremely powerful for their size.
Now we will discuss Servo Motor Control Methodology using DC Motor and we assume that speed of DC Motor is controlled by controlling the Armature Terminal Voltage.
Servo Motor Control:
For understanding servo motor control let us consider an example of servomotor that we have given a signal to rotate by an angle of 45° and then stop and wait for further instruction.
The shaft of the DC motor is coupled with another shaft called output shaft, with help of gear assembly. This gear assembly is used to step down the high rpm of the motor’s shaft to low rpm at output shaft of the servo system.
The voltage adjusting knob of a potentiometer is so arranged with the output shaft by means of another gear assembly, that during rotation of the shaft, the knob also rotates and creates an varying electrical potential according to the principle of potentiometer.
This signal i.e. electrical potential is increased with angular movement of potentiometer knob along with the system shaft from 0° to 45°. This electrical potential or voltage is taken to the error detector feedback amplifier along with the input reference commends i.e. input signal voltage.
As the angle of rotation of the shaft increases from 0° to 45° the voltage from potentiometer increases. At 45° this voltage reaches to a value which is equal to the given input command voltage to the system. As at this position of the shaft, there is no difference between the signal voltage coming from the potentiometer and reference input voltage (command signal) to the system, the output voltage of the amplifier becomes zero.
As per the picture given above the output electrical voltage signal of the amplifier, acts as input voltage of the DC motor. Hence the motor will stop rotating after the shaft rotates by 45°. The motor will be at this rest position until another command is given to the system for further movement of the shaft in desired direction.
From this example we can understand the most basic servo motor theory and how servo motor control is achieved.
Thus we can conclude that, the shaft of the servo is connected to a potentiometer. The circuitry inside the servo, to which the potentiometer is connected, knows the position of the servo. The current position will be compared with the desired position continuously with the help of an Error Detection Amplifier. If a mismatch is found, then an error signal is provided at the output of the error amplifier and the shaft will rotate to go the exact location required. Once the desired location is reached, it stops and waits. Some of the applications of Servomechanism are Position Control, Speed Control etc.
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