Servo motor is a general term for a motor that can freely determine its displacement while incorporating an encoder. The word “servo” is derived from the word “servant” (slave), indicating that it operates as specified. Servo motor is often used in robots and processing machines.
Uses of Servo Motors
Servo motors are almost never seen in household applications, but they are widely used in industrial applications.
One example is assembly robots in automobile factories. Servo motors can be used to assemble a large number of parts quickly and accurately.
Robots with servo motors are also used for painting and welding in automotive plants. Depending on the program, they can reproduce the same motions accurately and quickly.
Precision industrial equipment, such as semiconductor manufacturing equipment and medical devices, also requires high repeatability and accuracy. Many servo motors, both large and small, are used in production equipment.
Principles of Servo Motors
Permanent magnet synchronous motors are the most widely used small AC servo motors in use today. A synchronous motor consists of a rotor, stator, bearings, and a casing.
The rotor of a permanent magnet type synchronous motor has a permanent magnet attached to it, and the stator winding surrounds the rotor in a circular pattern. Wires connected to the stator windings from the outside conduct alternating current to the stator windings.
The stator winding becomes an electromagnet when the current flows through it. As the AC current changes phase with frequency, the electromagnet also changes polarity with time.
The rotor, on the other hand, rotates in response to the phase change of the stator because the polarity of the permanent magnet is fixed. This is the principle of rotational operation of servomotors.
Most servomotors are small in size and capacity. However, induction motors are sometimes used for large-capacity servo motors.
The most important feature of servo motors is that they come with an encoder. The encoder provides feedback of motor displacement to the control equipment.
A dedicated control device is used to power the servo motors. The control device reads the encoder signal and can control the number of revolutions, speed, etc.
Dedicated control devices for servo motors are called servo drivers or servo amplifiers and are sold by motor manufacturers.
Other Information on Servo Motors
1. Difference Between Servo Motors and Stepping Motors
Stepping motors can easily and accurately control the angle and speed of rotation with pulse signals because the amount of rotation is determined by a single pulse. Although the structure is simple and inexpensive, it can be out of synchronization under sudden load and has low torque and high noise level at high rotation speeds.
Servo motors, on the other hand, use a sensor to detect rotation and send feedback signals to the driver. The control signal is compared with the feedback signal and the output is adjusted so that there is no difference, thus enabling fine control.
They rotate more smoothly than stepping motors and are stable, even under high torque, high rotation, or sudden load. On the other hand, motors and drivers are complex and expensive, and since they are controlled by comparison with feedback signals, there is a delay in response to changes in output. Servo motors are used when frequent starting and stopping is required, or when fine control is desired, such as very low speed or reverse rotation.
2. Servo Motors Torque
Servo motors torque is divided into rated torque and instantaneous maximum torque. Rated torque is the torque output at the motor’s rated output and rated rotation speed. During normal operation, the motor is used at or below this torque.
Instantaneous maximum torque is the maximum torque that can be output in a short period of time, which reduces the time required for acceleration and deceleration. It can generate 3 to 5 times more torque than the rated torque, but continued use of the motor at the instantaneous maximum torque will shorten its service life.