asjeetabhi8816

How to use incremental rotary encoder to move a three phase induction motor

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I want to use an incremental rotary encoder with my Allen Bradley PLC to move induction motor (3 Phase) in clockwise and anticlockwise direction.Please help me with the program how to use High Speed Counter Values to move motor reverse and forward.

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Which A-B PLC platform are you using?

When you say "move induction motor", do you mean as an indexer (i.e., moving the AC motor to a physical position)?  

What is controlling the AC induction motor (i.e, VFD)?

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I am using Micrologix 1400 PLC which has 6 HSC Counter inputs.

Actually i have to move an object to different positions with the help of Induction Motor.

Right Now it is controlled by PLC (Using Contactors) just to move motor forward and reverse.

But i have to specify certain 4-5 buttons which will take that object to different positions (which i will set with the help of encoder).But i am confused which instruction i should use to move that object to different positions. 

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Since the motor is being controlled by contactors and not a VFD, the encoder feedback from the motor to the PLC may cause you a lot of grief.  The problem you may encounter is position control repeatability.  

Depending on the dynamics of the load being moved and the precision required for placement of the load, you may wind up spending a lot of time tweaking the PLC program to be in control and capable.

To answer your question, the MicroLogix 1400 with its (6) high-speed counter inputs, each counter input has a register in the PLC.  Refer to the user manual, wire the encoder into a PLC HSC channel, add logic to reset or clear the register, have operator run the system and take note of the register value when in-position, store the register value, conduct repeatability tests.

Refer to page 96  http://literature.rockwellautomation.com/idc/groups/literature/documents/rm/1766-rm001_-en-p.pdf

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to do positioning you need a servo. in recent years some VFD drives (but not all) also support this functionality. to do any kind of precision you need tight loop. normally that loop is part of the drive or motion controller. You can of course use PLC  code for this but the fact remains that this is not a fast loop resulting in loss of performance (either speed or accuracy or both). using only contactors to control motor in an attempt to control position is exercise in futility. another issue is braking (slowing down). if you need an example, use any cheap motor (ceiling fan, drill gun etc) and try to operate it manually using switch and - stop at specific position. it just wont work...

if you have VFD, you can control velocity (and deceleration!!!). this allows you to slow down axis before reaching stop position. with clever code and calculating deceleration ramps etc this can get you potentially close to what you expect. theoretically the only compromise would be speed. the other issue is that induction motors are designed to operate at specific speed (within some tolerances).

 

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