What Is a Servo Drive?
A servo drive is a device that controls a servo motor based on the values set by the controller in a servo system.
A servo system is a system of motor drives indispensable to industrial equipment for high-speed and precision control, and it is no exaggeration to say that servo drives make this possible. The main role of servo drives is to supply power to the servo motor, so it can do its job according to the load torque of the motor.
The servo drive can be divided into two parts: the part that performs power conversion and the part that detects the motor status and performs control operations.
Uses of Servo Drives
Servo drives are used in conjunction with motors in servo systems. They have a wide range of applications, from industrial equipment to machine tools.
The industrial robots used in automobile manufacturing plants, for example, are required to accurately repeat predetermined movements. To achieve this, it is necessary to accurately supply power commensurate with the load on the motor required for the operation to be performed. Servo drives monitor the condition of the motor while supplying the precise power needed to achieve this movement and torque to the determined position.
Other suitable applications where precision movement is required include semiconductor manufacturing equipment and medical equipment.
Principle of Servo Drives
The principle function of servo drives is amplification. They provide electrical feedback while closely monitoring the motor status — including rotation angle, speed, and current — with sensors, to facilitate precise motor control. A Programmable Logic Controller (PLC) or similar device is generally used as the controller for the amplification, setting the set values, and transmitting the information.
The servo drive supplies the necessary power to control the target set values, but for more accurate control, it is necessary to monitor whether the servo motor operation has reached the set values and to provide appropriate feedback. For this purpose, servo drives usually have a built-in sensor called an encoder.
The encoder comprises a disk with a slit and a built-in photodiode to detect the angular velocity and speed of the motor. Since the disk rotates in conjunction with the motor rotor, the speed and number of rotations can be monitored by detecting optical signals passing through the slits.
The photodiode detects these optical signals, converts them to an electrical current, and feeds it back to the amplifier. The amplifier generally uses the Pulse-width modulation (PWM) method to drive the motor by converting alternating current to direct current and then to an arbitrary frequency. This block consists of a converter section, a smoothing circuit section, and an inverter section. Encoder information is usually fed back to the inverter section.
Additional Information on Servo Drives
1. The Difference Between Inverters and Servos
In terms of motor control, inverters and servos are the same, but they have very different characteristics, and their applications can be broadly classified into two categories.
Inverters
Inverters have a wide range of applications, ranging from social infrastructure equipment such as air conditioners, elevators, and escalators, to industrial equipment and home appliances. Inverters are characterized by their ability to control the speed of motors and provide smooth and stable operation, especially when continuous operation with low power consumption is required.
Servo
Servos are valued for their high precision, facilitated by their ability to precisely adjust and control speed. Servos are preferable when high-speed, high-precision control of machine motion position is required.
2. Feedback Control of Servo Drives
Closed-loop control is normally used for servo drives based on sensor information from the encoder. However, in practice, due to workpiece shrinkage and misalignment between axes, deviation from the desired motion may occur even with feedback control. For this reason, there are servo drives that provide various phase compensation functions.
Additionally, depending on control timing, there are situations where fine adjustments to angular velocity are crucial.
For example:
- Shortening the time it takes to reach the required rotational speed and phase angle during start-up and shut-down.
- Ensuring a consistent rotational speed during normal operation.