What would cause only the Enable (EN) bit of a MAPC Instruction to stay on?

[Jon13926]

This is a program in a machine that has been running for about 7 years without any issue like this. We have a master servo (the machine's main drive) which, as long as the machine is started, runs constantly in a rotary fashion.

And then we have the slave servo (which runs an infeed belt) which can be stopped any time a safety door needs to be opened momentarily without stopping the entire machine.

In the logic, we have an MAPC_Enable XIC and a One Shot that triggers the MAPC instruction. If you stop the belt to open the door, you lose the MAPC_Enable. Once you shut the door and reset, you regain the MAPC_Enable and the MAPC Instruction runs again.

This MAPC is set to run continuous. Normally, we have a DN bit and a IP bit at all times. All of a sudden, about a week ago, we started getting a fault on the HMI for "Servo Unresponsive". This fault doesn't originate from the drive, it comes from the logic. If the EN bit from the MACP Instruction stays on for 5 seconds, it triggers the Servo Unresponsive fault.

As a result, one of the other maintenance guys replaced the servo motor and the encoder feedback cable. This resulted in no change.

The problem is, we don't have any other bits coming on from that instruction. The MAPC_Enable XIC comes on and triggers the MAPC instruction, the EN bit comes on (only) and nothing else happens. Looking through the instruction help and the flow chart, I saw there were some scenarios where you might have a stuck EN bit, but it's typically accompanied by another bit like the Error (ER) bit or the IP bit. We aren't getting an accompanying bit though.

This happens about 1 out of every 10 times that the MAPC_Enable goes from true to false and back to true. The other 9 out of 10 times we get the DN and IP bits like normal.

Does anyone have any idea what would cause ONLY an EN bit to come on from this instruction?

 

[zachary.wilson]

howdy, I know you said it has run for 7 years with no other issues, but is it possible the one shot, is now being used somewhere else, that could be turning it on and off so you get the en bit changing from 0 to 1 and back and forth.

Reading in the manual, it says it is on, but cleared if superseded by another command, or canceled by stop/merge/servo fault. So are you sure you are not getting it and then something else is causing it to stop before it can finish its function. you could try adding a line of logic to see if the ip bit turns on and latch an unused bit, that way even if it is on for a scan you will see. might be a starting point.

 

[James Mcquade]

Welcome to the forum.

Since the machine has run for 7 years with no issues, i have to ask WHAT CHANGED?

plc programs do not change unless modified. do you have a backup copy to do a file comparison with what is in the plc now? bubba may have changed something.
wiring changed / damaged?
valves replaced?
air line leaks / tubing size changed?
servo cables damaged? we have this a lot.
servo encoders damaged /bad -we also have this
servo cable connections at the motor and drive tight and secure? we have this, but not often.
do you have a replacement drive unit? we have to replace ours now and then.
hope this helps,
james

 

[cardosocea]

Is the system stuck mechanically? Bearings having issues, a belt too tight? Servo drive having problems?