Application: ControlLogix, Conveyor, DeviceNet, Encoder, Robots

Hello,
I have an application in which 2 robots operate (no touch) on an object moving on the conveyor. The robots are 7-axis ones and their bases move along the object.
They must continue their work even when the conveyor is stopped or accelerating/decelerating.
My initial idea is to have A-B ControlLogix5555 system with DeviceNet scanner in the chassis. The robot manufacturer provides DeviceNet interface. The encoder is multi-turn, absolute, with DeviceNet interface, too. The idea is to read the conveyor position using the encoder, send it to the PLC, PLC sends the current position to the robots, so they increment their position accordingly.
I am afraid the overall system speed is going to be too slow and the robots are not going to be enable to "catch up" with the object. Let's see: The conveyor max. speed is approx. 30ft/min = 6in/sec. The PLC reads the encoder position, performs some basic math operations, sends it to the robots, they read it and reposition their bases. Let's assume, that the PLC's scan time is 20ms and all the read/calculate/write operations (including the scan time) will take up to 100ms. This means, the resolution achieved is 0.6in/sec. It is below my expectations. It is going to affect the quality of the work performed to a great extent.
Another solution would be to use a dedicated small, very fast controller with DNet card, to handle the robots and the encoder only and to pass other data to and from main PLC via Ethernet/IP.
One more solution, suggested by the robot manufacturer, is to buy from them two encoder cards for the robots and an encoder with its signal splitter. Unfortunately, this solution presents a huge cost impact to the project.
General note: DeviceNet speed can be set to 500kbps. The distance is not an issue here.
Has anyone had experience with similar cases? Any ideas, examples, links, brand names, comments are welcome.
Thank you, Gregory Pal
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Why not use M02AE and M02AS? The AE uses a quadrature encoder and the AS uses SSI absolute encoders or SSI MDTs. Both use the same motion blocks so the programming should be easy. It may be better to use a Control Logix 5563 because it has a hardware floating point unit.
Peter Nachtwey
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Thank you, Peter, for your answer. The robot manufacturer dropped the price a little bit a we are going to go with their encoder and cards. They are also going to provide us with the signal from the encoder using some sort of a splitter. It appears, that we are going to end up with M02AE or AS anyway... And 5563, too.
Good luck and happy controls. Gregory Pal
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