multiple drive load sharing

[jeffdc]

For a system such as a pipe straightener where you have several rolls/motors/drives on top and several on the bottom, what is the best way to setup load sharing? Would it be best to have 1 drive as a master in speed mode, with all other drives in some kind of torque follower mode? Or, would it make sense to split them into pairs where top drive 1 master, bottom drive 1 follower, top drive 2 master, bottom drive 2 follower, etc., and maybe put droop on the top drives to shift some load between the pairs?

[paulengr]

This is highly dependent on the mechanical system. Imagine for a moment a vehicle lift with 4 legs or say a platform with several drive wheels. In this case the mechanical system has minimal coupling between legs and it's best to operate an independent virtual axis. The 4 drives operate in speed follower mode. If there is mechanical coupling such as several motors driving a common gear box, then torque sharing is required. One drive is indeed the master and all of the others operate in torque follower mode. This is the easiest and frequently works well. The problem comes in when you get resonances in the overall electro-mechanical system which cause various strange oscillations and vibrations. If this is a problem, most of the higher end servo drive systems (AB, Siemens, ABB definitely) offer high bandwidth fiber coupling systems where the system bandwidth is raised high enough to eliminate this problem. In the case of one particular system I'm familiar with (the swing mechanism for large electric excavators), the DC motors use cross coupling (fields in parallel, armatures in series) but this still isn't enough. An additional discrete RLC network is used to reduce the resonance issue. Theoretically the problem couple be eliminated in the drives but the original (1960's-1970's vintage) drives were analog op amp or magnetically coupled systems and the RLC network was the only practical fix.