topic 1 - servo technology

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Closed loop systemServo motor working principle – partsFunctional blocks of servo driveFeedback systemsCommand signalling methodsServo responseServo parametersModes of operation


Servo Control .. Basic Meaning (Closed Loop Control)

The Controller receives the ‘Command Signal’, as well as actual Feedback Signal from the Output Device. It compares both, and tries to correct the ‘Output’ by generating adequate output control signal.

OutputControl Signal

Encoder Feedback Signal

Servo Controller

Command Signal

Error Signal

Closed loop system

Motor


The ‘Servo Loop’ is a Time based Cycle consisting of:

This Loop (Cycle) keeps operating continuously. The faster the Loop time, the Servo Action (response) is better.

• Monitoring the ‘Actual Output’. • Calculating the ‘Error Signal’ w.r.t. Command Signal • Correcting the Output Signal

Typical Loop Timings … 125 µ Sec.

Servo Loop


Servo Motor … Drives the Load Servo Amplifier … Electonic Amplifier Unit .. Brain of the Servo SystemRotary Encoder … Motor Shaft Position Sensing

Power IN

Rotary Encoder (Built-in)

Servo Motor

Servo Amplifier

Power OUT

Motor Operation Command Signal

Shaft Position Feedback Signal

Servo basic elements


Why and where Servo motor is used

Widely used – because of high accuracy, high torque, high efficiency, fast response and wide range of speeds and torques.

They are brushless motors - hence lower maintenance compared to other motors.

Used mainly in motion control systems - packaging, printing, textile, semiconductor production, plastics and assembly industries.

Important part of robotics and CNC machine tools.


Servo Motor Basics

• Special electromechanical device that operates in precise degrees of rotation.

• Quickly responds to positive or negative signals from a servo amplifier.

• Fast and accurate speed, torque and direction control are the mark of a servomotor's characteristics.

• The primary industrial use of these motors is for the accurate positioning of the work piece or work tool.


Servo Motor parts


Parts of Servo Motor - Stator

1) Stator 2) Rotor

3) Brake assembly 2) Bearings


Synchronous Motor

Synchronous motor - magnetic field generated by the stator and magnetic field generated by the rotor rotate at the same frequency.

The stator flux (magnetic force) actually rotates around the motor and the rotor actually follows this rotating magnetic field.

This type of motor is called a synchronous motor because it always runs at synchronous speed (rotor and magnetic field of stator are rotating at exactly the same speed).


Servo Drive Basics

Special electronic amplifier with Vector control - used to drive electric servo motors.

Monitors feedback signals from the motor and continually adjusts for deviation from expected behavior.

It receives a command signal from a control system, amplifies the signal, and transmits electric current to a servo motor in order to produce motion proportional to the command signal.

Command signal - desired velocity but can also represent a desired torque or position.

A velocity sensor attached to the servo motor reports the motor's actual velocity back to the servo drive.

Drive compares actual motor velocity with the commanded motor velocity. It then alters the voltage frequency to the motor so as to correct for any error in the velocity.


Functions of Servo drive

REGULATORS (CONTROLS)

It rectifies AC power converting it to DC


Functions of Servo drive

REGULATORS (CONTROLS)It controls the DC output voltage and PWM voltage & current in

response to various control and feedback signals thereby regulating the motor's performance, in speed, position and torque.


• Position Feedback Device is a built-in Element of Servo Motor.

• Various Types of Position Feedback Elements are available

O/P ShaftSERVO MOTOR

Incremental Rotary Encoder

Feedback elements


• Incremental Rotary Encoder • Absolute Position Encoder • Resolver

Types of feedback devices


Current feedback

The second source of feedback information is obtained by monitoring the motor actual current. As discussed previously, this is an accurate indication of the torque required by the load.

ACCTs are used to sense the actual currents and the output of ACCTs is compared with the current reference signal and accordingly the error is eliminated.

The current limiting action of most controls is adjustable and is usually called "Current Limit" or "Torque Limit".


The Servo Amplifier is a ‘Closed Loop Control’ Device, however it needs information (Command) to be given about the type of Motion required from the Motor.

The ‘information’ (e.g. Displacement / Speed / Direction / Stop etc.) is given by External Device to Servo Amplifier through Command Input Signal.

Typical Command Signals .. Pulses / Analog Signal Voltage / Fieldbus Communication Command

Command inputs


• “Command Signal” contains information about what type of Motor Rotation is required.

* Types of Command Signals …- Discrete Signals (Digital Pulses or Analog Voltage) - Fieldbus Communication - Triggering of built-in Position Table.

Servo Motor

Servo Amplifier

Command Signal

Command inputs


This is done in Amplifier, thro’ Parameter Setting

Example:

Analog Voltage Motor Speed Relation

Command inputs – command pulses / analog


* Fieldbus Communication Module in Motion Controller (PLC) sends information to one or more Servo Amplifiers in Fieldbus Communication Mode (Digital Information)

ServoAmplifier

1 2 3

PLC

Command inputs – serial communication


Servo Tuning / Response refers to …

During Start of Motion (from rest state), or during stop of Motion (from Movement to rest state), or during Speed Change over (Increase / Decrease) …

How smoothly the Servo Motor behaves (Smooth Start / Stops without Overshoots & Corrections).

Servo Tuning & Response


(Similarly, stopping at the End Position Can be with Overshoot / Hunting or Smooth).

Jerky Start Smooth Start

Time

Speed Speed

Time

Start of Movement



“Tuning” of Servo essentially refers to adequate Pumping of Current into Servo motor depending on the Load moment of Inertia.

“Auto Tuning” feature is normally available in Servo Amplifier. When the machine is assembled, the Servo Motor is made to move the axis Fwd / Rev at different speeds for certain times. During this, the Servo Amplifier understands the behavior (response) of the Load, & then automatically tunes (Sets) itself for Smooth Start / Stop.



Advantages of Servo system

Constant torque over nominal speed rangeBest speed accuracy and stability.High intermittent torqueHigh torque to inertia ratioControl of torque is possibleHigh speedsExcellent for frequent speed reversalExcellent smoothness in movementPositioning accuracyAvailable in all sizesNot vulnerable to external factorsLow maintenanceQuiet


Disadvantages of Servo motor

• Initially Expensive but as compare to all advantages its better and cheaper than other conventional systems.

topic 1 - servo technology

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