A servomotor is a basic electrical device that is used to rotate or move the gadgets with great precision and accuracy at different angles and with different velocities. Servomotors are also called ‘control motors’.
Servomotors have high-torque capabilities. Unlike large industrial motors, they are not used for continuous energy conversion but only for precise speed and precise position control at high torques. It is a closed-loop feedback-controlled system
These motors look like the usual electric motors. Their main difference from industrial motors is that more electric wires come out of them for power and for control.
The servomotor wires go to a controller and not to the electrical line through contactors. Usually, a tachometer (speed indicating device) is mechanically connected to the motor shaft.
The basic principle of operation of servomotors is same as that of other electromagnetic motors. However, their construction, design and mode of operation are different.
There are practically two types of servo motor depending how it is powered
- AC servo Motor
- DC servo motor
DC Servomotors
These motors are either separately-excited de motors or permanent-magnet dc motors.
Construcction
- A DC servomotor consists of a small DC motor, feedback potentiometer, gearbox, motor drive electronic circuit and electronic feedback control loop.
- The stator of the motor consists of a cylindrical frame and the magnet is attached to the inside of the frame.
- The rotor consists of brush and shaft. A commutator and a rotor metal supporting frame are attached to the outside of the shaft and the armature winding is coiled in the rotor metal supporting frame.
- A brush is built with an armature coil that supplies the current to the commutator. At the back of the shaft, a detector is built into the rotor in order to detect the rotation speed.
- With this construction, it is simple to design a controller using simple circuitry because the torque is proportional to the amount of current flow through the armature.
Working
The input signal for the motor is termed as a feedback signal. On the basis of the feedback signal, the motor starts working.
Comparator signal is called a logic signal of the motor. The motor would be ON for the desired time when the logical difference is higher and the motor would be OFF for the desired time when the logical difference is lower.
Basically, a comparator is used to decide that motor would be ON or OFF. Proper functioning of the motor can be done with the help of a good controller.
Driver is used to provide necessary supply to the motor.
Controlling of Servomotor
- The servo motors can be controlled by the method PWM i.e., Pulse Width Modulation. These send electric signals of inconsistent width to the motor.
- The width pulse is varied in the range of 1 millisecond to 2 milliseconds and transfer this to the servo motors with repeating 50 times in a second. The width of the pulse controls the angular position of the rotating shaft. In this, three terms are used which shows the controlling of the servomotor i.e., a maximum pulse, minimum pulse and repetition rate.
For example, The servo moves with the pulse of 1 millisecond to turn motor towards 0˚ whereas a pulse of 2 milliseconds to turn motor towards 180˚ Between the angular positions, the pulse width interchange by itself. Therefore, the servo turns to the 90˚ with the pulse of width 1.5 milliseconds.
- There are three wires or leads in every servo motors. The two wires used from positive supply and ground supply whereas the third wire is used to control the signal.
The schematic diagram of a separately excited d.c. motor alongwith its armature and field MMFs is shown in the following figure.
The torque/speed characteristics of DC servomotor is is shown in figure.
The speed of d.c. servomotors is normally controlled by varying the armature voltage. Their armature is deliberately designed to have large resistance so that torque-speed characteristics are linear.
Advantages of DC Servomotor
- Precise Control and Accuracy
- Stable Operation
- Fast Response
- Lightweight and portable
- Four Quadrant operation possible
Disadvantages of DC Servomotor
- Due to the complex circuit, reliability is less
- Due to closed-loop components i.e. amplifier, gearbox, etc. it is costly
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