PLC 90-30's & Genius users...Please help?

Hi All,



I am planning on using PLC 90-30's and Genius (customer standards) to monitor and control a medium size process plant

1. How many PID loops can the 90-30 GE Fanuc 374 CPU handle? How the CPU performance could be affected by the number of PID loops?

The application that requires ~100 analogue control loops and am considering on using a dedicate PLC only to look after them. A second PLC will look after interlocking and sequencing. Coordination between the operations of both PLCs will be done via Genius (customer standard).

2. Could communications delays over Genius adversely affect the performance of the PID control loops? Can you foresee any execution timing issue?

The analogue I-O is intended to be connected through remote I-O (Field control I-O over Genius). Most of the PID control loops do not demand a great deal of speed (e.g. level and PH control) but some might (e.g. flows, etc).

I would greatly appreciate your comments and experiences.

Thanks in advance.

The PID function in the 90-30 requires a bank of 40 data registers, generally %R addresses. For 100 loops, you will need 4000 registers, so that's no problem.

According to Appendix A of GFK-0467M, the function adds about 14 microseconds to the scan time of a CPU374 when it executes.

Genius I/O scan times are predictable. One PLC can have more than one Genius Bus Controller, so you can control the maximum Genius scan time as needed. As long as the Genius scan time is less than the time between recalculations of the PID function, you will be using fresh data for each recalculation.

The fastest rate for PID recalculation is once every ten milliseconds. Even if you manage to keep your PLC scan under 10 milliseconds, the PID function will not be executed more frequently than that.

As far as I'm aware, the only limit on the number of PID functions you can use is the number of registers available, each PID instruction uses 40 registers.

I tried entering a loop number of 100 in the PID tuning parameters window (Proficy Machine Edition) and the target validated ok.

I've never had an execution timimg problem but yes, I agree flow control loops can be notoriously twitchy - but if that were the case I don't think that the normal analogue input would be much better. You might want to use discrete process controllers (e.g. Eurotherm) for such loops, they have some pretty good features these days such as auto tune, adaptive tune, etc.

Hi Steve/burnerman,

How does the PLC execute the PID instruction? (e.g. through a time interruption routine, out of a timer, etc).

is possible to set execution priorities to the loops? ..

Regards,

When the PID block is enabled the PLC solves the equation once every scan unless the scan time is shorter than the configured execution time (10msec by default) then it could be every other scan. The equation doesn't lose any accuracy because the PLC uses the actual elapsed time since the last calculation, not the configured execution time.

As far as priorities the PID equations will be solved in the order that they appear in the program - the real inputs/outputs will be updated at the beginning/end of the scan as normal. If you need to set priorities then you will need to play around with the program flow using jumps, immediate I/O, etc.

When there is a logical TRUE condition at the upper left hand node of the PID function block, it is enabled. While it is enabled, one of the 40 data registers assigned to the PID function establishes the recalulation frequency.

What I don't know without testing is how the PLC handles the off-to-on transition of the enabling logic, whether it recalculates immediately upon enabling or whether it applies the specified time.

If it calculates immediately upon off-to-on transition of the enabling logic, then it would be easy to set up multiple PID functions in ladder logic and have a fixed number of them enabled per scan.

My guess is that the function operates something like this:

If the function is enabled Then

   If time since last calculation is greater than specified time Then

      Recalculate

   End If

End If