What is a Servo Drive and How Does It Work?

Copley servo drive XEC-XPC

What is a servo drive?

A servo drive is used to deliver power to a servo motor with feedback designed to accurately control position, speed, and torque in robotic and machine automation systems. Motion control master software may command a servo drive to move to a desired position with a desired profile velocity. It may also command a target velocity or torque. Feedback sensors are used to measure actual position, velocity, and torque (current) as feedback to close the loop between commanded and actual values. A robust control loop, such modern nested loop or a classical PID, is used to close the feedback loop with proportional, integral and optional differential gains.

Copley Controls servo drives come with features and formats designed to fit the application and OEM requirements ranging from semiconductor, automatic test, general automation, integrated cabinets, robot controllers, distributed systems, robotic joints, medical equipment, deep sea, aerospace, defense, warehouse, and AGV’s.

Power is typically fractional horsepower but there are variables:

  • Fractional horsepower ranges from 1W (one Watt) to several KW (kilowatt)
  • Horsepower-given AC line voltages are 100/120/200/208/240VAC or in some cases even 200/480VAC.
  • DC power supply voltages are 12V/24V/48V/75V/150V or in some cases even 350V/700V.
  • Current control has a range of 1mA to 30A or in some cases 40mA to 100A.

Features include: Standard low-cost ARM processor functions like incremental encoders and advanced FPGA processor features like dual absolute encoders.

Control architectures include: Standalone push button, Multi-axis, Analog, Digital, Electronic gearing, Camming, RS232 serial , ASCII/binary

Included Networks: Serial Multi-drop, CANopen, EtherCAT, Ethernet IiOT, UDP, TCP-IP, Modbus-TCP, Ethernet IP

Environments include: Industrial, Extended environments, Ruggedized MIL-810

How Do Servo Drives Work?

A servo drive receives a command signal from a controller. Modern servo drives rely on a microprocessor to analyze the incoming signal through special algorithms, filters, and limits to perform control of position, velocity, and torque (current). The drive will protect the motor from overheating using the I2T current limiting algorithm.

Servo motors with feedback allow for performance tuning and precise control in real-time. Manufacturers can control motion, vibration, settling, and performance of a system to gain maximum accuracy and throughout.

What is the Difference Between a Servo Drive and Stepper Drive?

While stepper motors are best suited for low-speed open-loop applications some stepper motors do have encoders used to verify position. The encoder is normally not used to perform closed loop servo control like a servo motor but could be used to perform encoder corrections to make-up for lost open loop steps. A stepper drive can apply open loop current in a micro stepping mode to hold open loop position between full steps for higher resolution with great stiffness.

However, that does not allow for servo control around the open loop position like a servo motor excepting when encoder corrections are applied to make up or lost steps. The disadvantage of open loop control is that the amount of current applied is larger than is required in a servo mode. It’s more inefficient, producing more heat and also, due to the low-cost construction of the motor, will produce less open loop positional accuracy.

Servo motors work in a closed-loop system with a servo drive designed to control high speeds with high accuracy. With a servo drive, a servo motor will only use the current required to control position in a tuned for stiffness servo loop, so it offers greater efficiency and lower heat.

It is possible for a stepper drive to perform servo mode control. However, that does not outperform the stiffness and holding characteristic of the open loop stepping mode with encoder corrections. Further disadvantages come with the poor reliability feedback devices traditionally found on some low-cost stepping motors as compared to servo motors with more reliable, redundant, and accurate feedback methods.

Types of Servo Motors

Historically DC brush motors with tachometers were used to control velocity with a servo drives to close the velocity and current loops. However, this type of feedback has been replaced by incremental encoders and modern digital servo drives used to close the position, velocity, and current loops. One disadvantage of the DC brush motor is that the brushes will eventually have to be replace or the motor swapped out. The maintenance time depends a lot on environmental conditions so may vary from motor to motor.

Today BLDC (Brushless DC) motors with traditional Halls and Incremental encoders or resolvers, or BLAC (Brushless AC) motors typically with absolute encoders are more common because of the significant reduction in mechanical wear possible with when compared with brush motors.. Additionally, brushless motors are faster and offer higher torque, allowing for higher performance capabilities.

Servo motors also come in rotary and linear formats. A Brushless linear motor is typically a BLDC or BLAC motor rolled out flat to make a brushless linear motor. A voice coil linear motor is a two-wire motor like a DC brush motor with no brushes.

Choosing between the different types of motors depends on the application. As an engineer we can get support from our motion control distributors to help us select the correct motor for the mechanical system or automation job requirements.

Why You Should Use a Servo Drive?

Servo drives offer advanced position control, an ideal solution in automated environments. The automatic adjustments allow the motor to manage its own performance and maintain peak outcomes, regardless of application.

Servo motors offer higher torque and speed than traditional stepper motors. You’ll have greater capability switching out your motor type to a servo system, where there are no caps on speed or performance.

They offer precise control with minimal unwanted motion. Many of these options — particularly AC servo models — are quieter and more efficient. Plus, with position control and automatic adjustments, the motor itself will keep tabs on the required motion through the closed-loop feedback system.

Copley Controls offers elegant solutions for motion control, designing high-performance motors and their components. You’ll find OEM solutions for AC servo motors as well as DC in both brushed and brushless designs.

Whether you need a basic stepper motor or are looking for more advanced motor solutions, Copley Controls has a solution designed perfectly for your application.

MSI TEC is a Copley Controls distributor. Contact us for pricing and product selection assistance.

Originally published on Copley Controls website here. 

Visit the Copley website at https://www.copleycontrols.com .   See their full product line here.


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