Which one is better to design a process plant which has several PID controlling loops:

1) Connect all loops to the plant PLC, do some PID programming, and let the PLC control the loops directly.

2) Utilise single-loop controllers (with PID capability) for every single loop, then integrate them to the plant PLC using communication scheme such as MODBUS, etc.

 

Mr. Anonymous,

Some positive points for the PLC solution:
. You may have a faster PID update cicle (compared to low-cost single loops). This may be important to flow and other fast loops.
. You may have a better "non-continuous" conditions management, including some logic to treat special batch conditions.

Some positive points for the single loops:
. Reliability: a failure in one controller will not stop all the system
. Maintenance: every controller have one input, one output, display, keyboard, and you don't need special cable or software to program/debug it. An instrumentist can fix a problem at saturday 3AM.

Regards

Marcos T. B.

 

I have replaced single loop controlers with PLC's for years. I need to switch between different controlers during the process.I also pass values and control bits between loops. The input and output configurations vary but I maintain a structured data block for each controler. This makes HMI screens and historical base management easy for multiple loops. With newer PLC's this is also possible with multi-instance processing.In many cases the HMI database can be populated straight from the PLC. I am now starting a project with S-7 and IFIX and am searching for peoples experience with Siemens PID controlers. In the past I have used other brands or written my own.

 

Siemens purchased Moore Automation over five years ago. They still market their 353 Controllers which I found to be the most robust and advanced devices available. That said, they are a bit pricier than some others on the market. At an LNG plant that I previously worked at, they were the sole control platform on several heat exhangers and boiler controls.

I was recently told that Emerson (Fisher-Rosemount) is no longer making their model. I have heard good things about the Foxboro products, but have never seen them in action. I hope that this helps.

 

I would suggest using loops in the PLC, it is more cost effective, easy to do and scalable. In the Boral cement plant i work at, i add and remove loops all day. If the process engineer wants to cascade a temperature and flow loop its simple to perform in the PLC without tedious wiring and saves days of work at a time. We have hundreds of loops running in the PLC to SCADA, using S7 and TI with CITECT. If you want an example just e-mail me and ill send you a S7 example.

[email protected]