Servo Systems – AC Servo Motors and Drives

What is AC Servo System?
AC Servo system consists of servo drive and motor with feedback device encoder.

What is AC Servo Drive and AC Servo motor?

AC Servo Drive is a special electronic amplifier used to control AC servo motor. AC Servo Motor is a synchronous type electric motor with feedback device encoder.

Servo motor receives power/command from AC Servo drive & this drive monitors the feedback signal from motor and continuously adjust the deviations in comparison with expected behavior to make system closed loop control.

What are the advantages of AC servo motor over Stepper Motor?

Stepper motors are used in open loop control, since there is no feedback device; whereas servo system is closed loop control.

Servo motors are used for higher torque with higher speeds, for continuous load changing environment to deliver the stable performance, which is not possible with Stepper motors.

What are the advantages of AC servo motor over DC Servo motors?

AC servo motors have several advantages over DC servo motors. AC servo motors offer more torque per weight, efficiency, reliability, and reduced radio frequency noise. Besides, in the absence of a commutator, the maintenance cost is less and have a longer life expectancy

What is full closed loop control?
Servo drives usually commands servo motor in close loop control by taking motor position feedback through encoder connected at motor.

In a full closed loop control, there is a provision to read the machine position by additional feedback device like linear scale/encoder.

In case of full closed loop control when servo drives receives command signal from controller then servo drive continuously compare the read position of machine (ex. ball screw mechanism) through linear encoder with the commanded value (target value) and takes corrective action to adjust deviation.

A fully-closed loop is used when high precision is required. Therefore, there is no need to compensate for gear backlash between the motor and mechanical system, feed screw pitch error, or error due to feed screw torsion or expansion.

What are different type of encoders in servo motors?

Servo motor has two type of encoder’s incremental type and absolute type.

In case of absolute type of encoders, there is no need of repeated homing procedure especially at every power on.

Homing procedure is an operation to define machine zero position or reference home position of machine.

As in absolute encoders there is a battery backup to servo motor encoder to retain position value, hence during power goes off, then drive reads the absolute encoder position value to know the exact position of the machine and avoids need of repeated homing procedures.

What are different power rating and type of Servo Drives and motors available in Delta?

Basic servo B2/B2L with 17-bit incremental type encoder motor available up to 3kW power rating in 230V AC power class.

Intelligent servo A2 with 17-bit & 20-bit incremental & absolute encoder motor available up to 15kW power rating in 230V AC power class as well as 415V AC power class.

High performance, intelligent servo A3/B3 with 17-bit, 20-bit & 24-bit incremental & absolute encoder motor available up to 3kW power rating in 230V AC power class.

What are different control modes in servo system?

All Delta servo series B2/B2L/A2/A3/B3 supports all three basic control modes; PT (Pulse), Analog (Speed) and Toque.

Additionally Delta intelligent servo series A2/A3/B2 supports PR mode; which enables internal homing with 35 homing methods and motion position table of 64-points/PR in case of A2 and 99-points/PR in case of A3/B3.

In PR mode there is no need of external controller to send Pulse/Analog commands to servo drives to operate servo motor, with PR mode servo drives generates PR commands internally to operate servo motor which in turns saves cost of high speed pulse output PLC controllers or Analog controllers.