motion control

geofra

Guest
G
I have a customer who has an old milling machine and wants to control the motion automatically. I would appreciate it if you coul give me some ideas regarding the approach of this project. I want to use SIEMENS s7-200 PLC and TD200 for HMI.
 
BEEN THERE...DONE THAT....BE CAREFUL. I DON'T KNOW WHAT YOU MEAN BY AUTOMATIC. IF IT IS JUST AN X-Y-Z AXIS MOVEMENT, THERE ARE A LOT COMPANIES OUT THERE THAT MAKE SERVO PACKAGES THAT WILL RETROFIT TO OLD MILLS AND THAT WOULD BE EASIER THAN ADDING A PLC.
WHAT I MEAN BY BEING CAFEFUL IS THAT ALOT OF PEOPLE THINK THAT IF THEY TAKE AN OLD MACHINE AND SLAP SOME NEW ELECTRONICS ON IT THE ACCURACY WILL GO FROM .1 TO .00000001, THAT USUALLY DOESN'T WORK.
IT'S STILL AN OLD MACHINE.
 
You need to get a clear definition of what the customer expects to accomplish by doing the rebuild, especially what capabilities he expects. I don't know the details of what Siemens has to offer, but a couple of differences between CNC control and PLC-based motion control are in the coordination of axes and the ability to execute part programs written in G-codes.

When a CNC coordinates axes (called interpolation) the coordination is peer-to-peer. In PLC motion control, the coordination is master/slave. The difference is, if one of the coordinated axes in the CNC can't maintain the feedrate due to external forces, the other axis will slow down to compensate. In a PLC, if the slave axis can't keep up with the master, you get a fault indication.

A CNC will give you both linear and circular interpolation, the PLC may be limited to linear.

As has been mentioned, the customer should be doing a mechanical rebuild at the same time if he's looking to improve accuracy.
 
I've seen and worked on old milling machines retrofitted with PLCs.

It's kinda like pulling a car up a hill with a horse when it's lacking the horsepower...

Many companies offer retrofit electronic devices. Do not re-invent the wheel.

A PLC is not really the best choice for this kind of job.

I've retrofited PLCs on CNC-should-have-been-equiped-with machines and all those times they where doing a specific work with only a few parameters. Not multi patherns, programmable ones.

Remeber that the biggest factor for precision is, like many before me mentionned, the mechanical capacity of the machine. Do not put you balls on the table and garanty the precision unless you really know what it's capable of.
 

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