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Multiple Motor Control with PLC


I have an application where we require speed control and (angular) position feedback of 12 motors.

Currently the plan is to have a worm gear and a 3 phase motor connected to a VFD, which will in turn be connected to a PLC which will handle the speed control and position feedback for all 12 (depending on PLC options). By feeding back into a single PLC, (which would have ethernet/ethercat/Profinet) a communication link can be made into the central controller (running ROS - Robot Operating System).

Unless there is an easier way, the speed control would be provided by an incremental encoder and the angular feedback driven by either a potentiometer or something similar 200-6332. There are two possible locations for the encoder (providing speed control) on the motor output shaft or the gearbox shaft, as the motor is 1680rpm@50Hz and the gearbox has a reduction of 100:1, measurement of the speed might be better coming from the motor shaft. In terms of the angular feedback, (the angle of the output shaft – with a required rotation of <90 degrees) if accurate counting can be maintained on the speed control encoder the value could also be used to calculate the expected output angle however the pot,etc could be mounted directly to the output shaft of the gearbox, therefore measuring the true angle output. The reason for a pot,etc is ultimately in the effort to have an absolute measurement as this could avoid the need to re-home after every power cycle.

 At this stage I am struggling to understand which PLC would be best to support this many HSC as most of the PLCs (s7-1200, Allen Bradley Micro850) I have looked at are limited to 6/8. Or am I going at this in the wrong way and is there a better approach where some re-distribution could help provide a solution. For context, the currently selected VFD is a Bosch Rexroth EFC5610. I am aware that there are some VFDs out there which will do speed control and provide that speed measurement back via a field bus communication method however as with any project there is a limited budget. For context excluding motors/gearbox/PSU’s the control aspects have a budget of around £2500. Any ideas / suggestions would be great. 

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