Multiple PI controllers on S7-300 series.

[Pollard]

Good morning all, I have a new project that involves three, potentially, simultaneous batch filling operations. I have up to six chemicals to dose into three separate tanks. Each tank is filled with a preset amount of water and varying ratios of the chemicals are dosed into the water flow via a mixing block prior to the holding tank. Baseed on information from the customer, this operation will take place up to seven times an hour (I have approximately six minutes for each batch). My setpoint for the PI controllers will be generated from the water flow on each line and the feedback will come from individual flowmeters in the chemical lines. The output from the controllers are peripheral words directly operating the six dosing pumps via an analogue signal. My question is this; does anyone have experience of running multiple PI controllers simultaneously and, if so, what are the potential pitfalls?

 

[JesperMP]

The only pitfall I can think of is if the load on the CPU will be too much causing CPU scantime to increase so that the update time of the PID controllers cannot be met.

However, unless you plan to use an obsolete CPU, I do not think that 6 PIDs in a pump application will be too much for any of the current S7 CPUs.
What update time for the PIDs are you aiming for ?
What CPU (exact type no.) are you planning to use ?

 

[Pollard]

I am using a S7-313C cpu for the project. I haven't yet considered the update time. I am using PIDs on an existing project and these have an update of 50ms but I am only using two controllers.

 

[JesperMP]

I do not recommend to use a 313C for several reasons.
Mots notably the 313C and the other "compact" brick CPUs havent been updated to the most recent generation of CPUs, and there is no word on when that will be, if ever.

 

[Pollard]

Ok, thanks for the advice. Unfortunately, we already have the hardware and, as is usual in most companies, I am under pressure to keep the costs down (hence the use of a cheaper compact unit). I guess the only way to find out is to try it. I can reduce the CPU load by skipping non-critical parts of the application during the batch fill cycle. For reference, what CPU would you recommend for this application?

 

[JesperMP]

Today I will never recommend a CPU without an ethernet port.
I also totally dislike bricks with integrated i/o, like the 313C. But that is just me.

In the S7-300 range the lowest end CPU with ethernet is the 315-2PN/DP. It costs approx 1800 €.
If that is too expensive, then consider the IM151-8 CPU that works with ET200S i/o modules. It costs approx 600 €. Even at this price, it has much more memory and higher performance than a 313C.

edit: both 315-2PN/DP and IM151-8 will be somewhere between 5-10 times faster than the current 313C.

There is also the CP343-1 LEAN for adding ethernet to a lower end CPU like the 312 or 314. But the performance is much lower, while the total cost is only slightly lower than a 315-2PN/DP.

 

[Pollard]

Traditionally, our equipment has always been controlled by our own control systems using bespoke controllers and HMIs. My current projects are our first foray in to the world of off the shelf control. Our equipment is all stand-alone and we have had very few enquiries regarding any form of network connectivity, therefore ethernet is not a neccessity (though the additional functionality provided could add to the sales appeal).

 

[JesperMP]

Take my word for it.
In your situation, where you can freely design a system without having to take legacy controllers or other dependencies into account, it will be a major blunder not to think ethernet into the design.
Think not just this project. Think 10-15 years ahead.

Even if it is a standalone machine, there are benefits to ethernet that are invaluable.
Remote support.
Webserver on the machine.
Off the shelf HMI solutions with ethernet in stead of costly network cards.
Remote i/o via Profinet.
And more...