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I want a gantry axis with two linear motors, one on either side of the gantry beem. A very high bandwidth (high-gain) system is prescribed for tracking performance. Should I use two separate position controllers or some sort of master slave configuration? Both motors have linear encoders for commutation. What I am really looking for here is some insight into the practical considerations from anyone who has done this. I can handle theoretical analysis of the performance of the system.
There are two possible scenerios considered.
1) Two seperate position control loops, each using linear encoder feedback from its respective motor.
2) One controller, using encoder feedback from one motor but providing the same current/force command to each of the two motor drives.
A third scenerio is to use two controllers with one feedback feeding both; but this is not very different from the second.
In terms of control loop performance and stability, my analysis says to use two seperate loops (scenerio 1)) -- here's why. If you consider that gantry beam is a spring then one of the motors is operating from a noncollocated sensor when you use one feedback for position control. If the resonance is below or near the desired position bandwidth (BW) then the noncollocated motor oscillates (maybe even unstable). This has been confirmed in simulation. With two seperate controllers the beam does not have to be stiff for stability, even at high BWs (it can be light also). This also has been confirmed in simulation and modelling -- AND IT IS HAS BEEN CONFIRMED ON A SMALL TEST SYSTEM.
My first concern is this: When I put such a machine into service, do I run the risk of running with "hot" motors because the motors may drive against one another due to DC offsets, small encoder mismatches, etc? Remember the controllers are high gain and have integral action. Therefore, small offsets can mean large currents. HAS ANYONE EXPERIENCE WITH THIS?
My second concern is this: WHAT PRACTICAL IMPLEMENTATION CONCERNS HAVEN'T I CONSIDERED?
There are two possible scenerios considered.
1) Two seperate position control loops, each using linear encoder feedback from its respective motor.
2) One controller, using encoder feedback from one motor but providing the same current/force command to each of the two motor drives.
A third scenerio is to use two controllers with one feedback feeding both; but this is not very different from the second.
In terms of control loop performance and stability, my analysis says to use two seperate loops (scenerio 1)) -- here's why. If you consider that gantry beam is a spring then one of the motors is operating from a noncollocated sensor when you use one feedback for position control. If the resonance is below or near the desired position bandwidth (BW) then the noncollocated motor oscillates (maybe even unstable). This has been confirmed in simulation. With two seperate controllers the beam does not have to be stiff for stability, even at high BWs (it can be light also). This also has been confirmed in simulation and modelling -- AND IT IS HAS BEEN CONFIRMED ON A SMALL TEST SYSTEM.
My first concern is this: When I put such a machine into service, do I run the risk of running with "hot" motors because the motors may drive against one another due to DC offsets, small encoder mismatches, etc? Remember the controllers are high gain and have integral action. Therefore, small offsets can mean large currents. HAS ANYONE EXPERIENCE WITH THIS?
My second concern is this: WHAT PRACTICAL IMPLEMENTATION CONCERNS HAVEN'T I CONSIDERED?