Modicon 984 PLC weirdness

Hi guys,

I have two Modicon 984 connected via Modbus Plus. The first PLC has a BMOT card attached to it to control a linear slide.

I made some changes to the program off line and downloaded the code after clearing the PLC memory using the "Initialise Logic" command which wipes the memory completely.

I downloaded the code and restarted the PLC controller and my linear slide start acting funny. It's full stroke is 9" and normally homes in one continuous movement. However, it would start homing moving 1" at a time and then stop before moving another inch.

Eventually I disconnected the power to the CPU and when I powered it back up the 2nd time the linear actuator worked properly again.

This isn't the first time that this has occurred. We have 33 of these slides and they go funny the odd time that we download a program.

Has anybody any ideas what I might be doing wrong?

Thanks,

Colm

Sounds like a typicle, "we don't employ a recovery routine" type senario. When you make changes to a motion device, or the program that controls that motion, the encoder count is not updated. You should, as a method, incorporate a recovery or initialization routine for your motion devices.

Bitmore has a valid point here. Your program shoud have some sort of initiliation routine at startup to correct/reset the positions of your machine.

Hmm

Hi guys,

Thanks for the replies so far. The thing is, once the program is downloaded, we have a homing routine that moves the slide back to a homing sensor which then resets everything. As my slide is moving back to the sensor it does it's "funny" movement thing (as mentioned above). Then when it has eventually homed and everything is reset, it continues to do the "funny" movement.

At the moment, I'm not convinced that it is the re-initialisation of the program but I will check it out this morning to rule it out.

Thanks,

Colm

Don't forget that the BMOT module is running its own resident motion program which will not be initialised by clearing the PLC memory.

Are you halting this resident program first?

It may be be getting screwed up by suddenly losing its interface to the PLC and only recovering when you reboot it by powering down the backplane.

What type of information is being sent over the network connecting your two PLCs? Restarting only one of them with a new program (and wiped data storage) may not automatically refresh the required intialization data.

Hi Jim,

The only information being transferred betweent the two PLCs is status information. The information doesn't effect the station where I am seeing my linear slide faults. I have the one of the lines booked for a day next week so I will be able to investigate further, carry out some trials and come up with a reason for my problems (fingers crossed).

Thanks for all the suggestions everybody.

Regards,

Colm

I was thinking more about the symtoms, have you changed any velocity or acceleration limits or commands? Could it be that resetting the PLC has also defaulted these limits in the other controller, or has your program changes exceeded these limits already set previously in the other controller?

just a bitmore