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Disclaimer: This document is written in the hope that you can utilize for your own education to gain knowledge of PLC systems (should you decide to utilize this document). Although I believe the information in this document to be accurate, it is YOUR responsibility to verify this information before implementing it in any way, especially when damage to personnel or equipment could result. By continuing to read this document, you agree to hold no one who writes, modifies, or distributes this document liable in any way (even negligence). Due to the wide variety of plant applications, some of the examples in this document may be prohibited at your location, or could cause damage to equipment, or harm personnel.
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Hardware


A ControlLogix servo system consists of several basic components: A ControlLogix system with motion module, a drive, a motor, and an encoder.




The ControlLogix chassis with Motion Module:

The ControlLogix system sends commands to the motion drive to turn the servo on, and to control the motion of axis.







The Servo Motor:

This is a special motor designed to utilize the advantages of both AC and DC Motors. Usually you will find a feedback device mounted to the motor such as an encoder.






Feedback Devices:

A common feedback device is the Optical encoder. A light source passes through a grating and is received by a detector. As the grating turns, the detector senses the pulses of light. The number of light pulses (or pattern of pulses) can be used to indicate the location of the load.


Two channel encoders have two detectors, and can be used to indicate the direction of rotation. The pulses from each channel are offset, so direction of rotation can be determined by which channel receives a pulse first. This is referred to as an 'A' and 'B' channel. Many encoders also have another channel 'Z' to indicate a 'Zero' position.


This is a picture of an encoder:



Absolute encoders are more stable and provide more information to the controller about it's position. The pattern of light received at any given time indicate the exact position of the encoder. Below is an image showing the difference between the absolute encoder disk and the incremental encoder.





Another type of feedback device is a resolver. The resolver can provide direction, speed, and position and can provide a resolution up to 200,000 counts in one revolution.


A tach generator can provide information about the speed of the motor by sending a voltage to the controller that is proportional to the speed of the motor shaft.


The Drive:

The drive interprets commands received from the ControlLogix, and has the responsibility of carrying out the commands. It does this by sending voltage to the motor, and reading the motor's position through a feedback device such as an encoder.


The Ultra 3000 Sercos Drive