powerflex 40 latency

[kchapman]

i have two groups of powerflex 40 & 40p drives communicating with a compactlogix plc via ethernet. i am seeing some occasional latency in the execution of commands to those drives. here is the layout:

compactlogix plc with a 1768-ENBT

panelview plus 1000 oit

two 22-comm-e modules in multi-drive mode (three powerflex 40's on one, two powerflex 40p's and two powerflex 40's on the other)

hardy 4050 weigh scale

labelpak label printer

above hardware connected to an N-Tron 8-port managed switch (508tx-a)

any insights as to where i should look to find the cause would be greatly appreciated.

 

[robertmee]

What do you have the RPIs set at for each drive in Controllogix?

 

[kchapman]

the rpi for the 22-comm-e modules is 100 ms. the rpi for the hardy is 50 ms.

 

[Ken Roach]

What do you mean by "occasional latency" ? Do you mean that the drive doesn't start or stop within 200 milliseconds of when you command it to (worst case based on your RPI's), or is it some other function of the system ?

Compared to PowerFlex 7/700 drives, PowerFlex 4/40 drives are very slow to react to network commands no matter what network they are connected to. In Multi-Drive mode you're loading the serial link further.

My rule of thumb, confirmed by network analysis, is that when an Ethernet command arrives at a 22-COMM-E, the drive reacts about 35 milliseconds later, and the reply confirming that command is sent about 70 milliseconds after the command arrived. I have not done network performance tests in Multi-Drive mode.

 

[Ken Roach]

Just as a wild guess: Go check the logic for the Stop/Start commands on the drive.

Rookie Mistake #1 for any A-B drive on any Logix network is to write the Stop bit logic "if Start is false, then Stop is true".

This is because the output data to the drives is picked up by the network module asynchronous to program execution. One time out of a hundred or one time out of ten thousand, the network card might pick up the data in between the Start command rung and the Stop command rung, thus sending out both Stop and Start bits true. Stop trumps Start, so the drive doesn't start as the program thinks it commanded it to do, and I get a call that says "there's something wrong with the network".

The very simple workaround: Program the Stop bit to only be true until the drive's "Running" bit confirms the drive has stopped. This means the Stop bit will be zero most of the time, minimizing the chance that it will get picked up in between rungs. This also prevents the drive communication module's "Hold Last State" feature from asserting Stop and overriding local control in a network failure event.

Merely guesses and things I've seen before.

 

[kchapman]

by "occasional latency", i mean that most of the time, the drives perform as expected. but there are instances when the start command is given (or in the case of the 40P's, the signal to execute the next step in the positioning logic), and there is a delay before the command is actually executed. this delay can be up to a full second in duration.

 

[kchapman]

Rookie Mistake #1 for any A-B drive on any Logix network is to write the Stop bit logic "if Start is false, then Stop is true".

that is the way i wrote the logic. i will try what you're suggesting, ken. thanks for the advice.