PLC5 Devicenet scanner/VFD start/stop control


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I am attempting to utilize a AB 1771-SDN scanner in a PLC 5 to read a DeviceNet encoder count and consistently start and stop a VFD via DeviceNet. The VFD basically drives the encoder shaft. I want start the VFD until X counts are reached stop the VFD reset encoder, start VFD until X counts reached stop VFD reset encoder etc. etc. Encoder is reset using explicit msg via Block xfer. VFD start/stop done via DeviceNet as well. Encoder always reaches count "setpoint" but overrun is inconsistent. Expected some overrun but thought it would at least be consistent. Not a high speed app. Any thoughts would be greatly appreciated. AB's answer was to hardwire start/stop to VFD.
This is due to PLC scan time variation, where the PLC scan is in relation to the encoder read block at the time and variations in reads on the Device Net network. If you are using an analogue output to drive the VSD there will be time variations there as well.
If you are looking for accuracy, use an absolute encoder or incremental encoder direct into a high speed counter card with it's own output to drive the VSD.
I have not seen a high speed counter card with an analogue output but digital should be OK to start and stop the VSD. If you need to control speed, you will need an analogue output for that.
Does this mean the motor rotates after the stop was issued? In that case you may look at a brake?
How are you getting the counts, cyclic or polled? I believe with the PLC 5 that with all the block transfers going on, encoder - SDN - PLC - SDN - VFD, the scan timer could really affect the performance on your setup.
If you are using the AB devicenet encoders, lower the resolution of them. If you have the resolution to high, the counts will be farther out and more inconsistant. We had the same trouble with ours, but we slowed down our VFD when we got close to target and lowered the resolution and now they work fine.