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Old August 28th, 2006, 05:58 AM   #1
drawson
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Load Sharing

I am currently commissioning a system of DC drives that I would like to have load sharing capabilities. I am using a L63 ControlLogix controller, controlnet as my communications medium and AB 1395 DC drives. I am looking to create a simple load sharing scheme using ladder logic and RS Logix 5000 to accomplish this. Is there a generic load sharing scheme I can cut and paste into my program, or any simple methods of achieving load sharing through ladder programing??
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Old August 28th, 2006, 06:43 AM   #2
PeterW
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If I gather correct to what you want, I would set up a run-time count for each drive you want to load share on. On each start, the drives that start are the ones with the least run-time.

If you do a search, I'm sure there was another thread on the same thing about a month or so back.
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Old August 28th, 2006, 06:22 PM   #3
kamenges
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Is PeterW's description what you mean by load sharing? Or do you want to run booth motors at the same time driving a common load and you want them both to deliver the same power?


Keith
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Old August 28th, 2006, 09:35 PM   #4
Dave_in_Oz
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Can you tell us a bit more about your application? If it is for something like a rolling mill or a wire drawing machine I would steer clear of using a plc to calculate the power sharing.
Is the hardware already in place? if not I would opt for either a system with SSD 590, Sprint PLX or Control Techniques Mentor II
(with MD29) and do all the power sharing within the drives, using their onboard software blocks. For things like speed references/ratios most decent HMI's such as Horner, Mitsubishi E-Terminals, Red Lion G3 and SSD TS8000 have protocols to connect directly to these drives via a standard or optional comms port, Leaving a second (or third) port on the HMI to connect to a PLC if you still need one.
Just my 2 cents worth, many a time I have renewed control systems that had empoloyed bespoke PLC algorithims/code that could be simply done within an intelligent drive.

Last edited by Dave_in_Oz; August 28th, 2006 at 09:40 PM.
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Old August 28th, 2006, 10:22 PM   #5
drawson
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A better Description of the Application

Sorry for the vague post it was 4:00AM when I posted. Here are some more details.

The machine is a cross roll straightener for straightening steel pipe. It consists of six rollers in total. The bottom three rolls are driven by a single gearbox and motor with three output shafts and the top rolls are setup in the same configuration. So we have two drive motors (one for the top rolls and one for the bottom rolls). Each motor is a Siemans 300 hp DC motor. We have Rockwell 1395 DC digital drives networked via control net. There are also entry and exit conveyors that move the pipe into the straightener. They are driven by Powerflex 700 AC drives (can't remember the HP but it is small, maybe 15hp each) which are also on the controlnet.

The top and bottom drives are supposed to be setup in a power sharing configuration so that they supply approximately equal power to the machine and then also match line speed. We purchased the system, engineering and software directly from Rockwell. Unfortunately the "expert" technician they sent out can't even get the top roll drive to stay on line.

I'm under the gun to get this machine back into full service and I am looking for a simple way to accomplish the load sharing. I would prefer not to learn function block and structured text programming.
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Old August 28th, 2006, 11:54 PM   #6
kamenges
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This kind of depends on how much dynamic response you need. If you are just trying to get in the ballpark and make sure that one motor doesn't do all the work or that they push different direction it's not too bad. If you need highly dynamic response it gets a bit tougher.

The easiest thing that quickly comes to mind is to dynamically torque limit the upper drive. Set the torque limit to the average of the filter torque feedback values of the upper and lower drives. For this to work right you will need to add a small overspeed to the upper roll drive when you are reasonably confident there is a pipe between the rolls. If there is no pipe between the rolls let the torque limit go to max and drop the overspeed.

If you need something a bit more dynamic you may be able to switch the upper roll drive into torque mode when a pipe is present and use the velocity loop output of the bottom drive as a torque command to the upper drive. You can dynamically switch control modes on the 1395 so you can switch back to velocity mode if a pipe is not present. This will give you very good dynamic response but has some issues you need to be careful of. I wouldn't dive into this one unless other methods didn't work first.

Are you sure the present control theory as delivered is flawwed? It sounds like the Rockwell tech has been battling hardware issues.

Keith
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