Siemens s7 Standard PID Runtime software for Temperature control


Thread Starter

Les Bond

I am trying to decide whether I should use the Siemens Standard PID runtime software for temperature control rather than using a dedicated controler on a profibus network.

I am worried that the Siemens package will not be accurate enough for my application.

I would like to hear from anyone using this package in regard to accuracy of temperature control relative to any given setpoint with in the range.

david mertens

I don't think a temperature control problem would be a reason to use an external controller, of
course, I don't know the exact application you are using the controller in, but temperature
control is mostly a slow process with no need for ultra fast stand alone controllers. The accuracy of a temperature controller in software is comparable with that of a digital external
controller (if you use the right input and output modules and compare with normal digital
controllers), and because you mention connecting it with profibus, I suppose we are talking about
a digital external controller. So I would go for the software implementation as it is far cheaper. This would also give you more flexibility in handling constraints and interlocks and special "wishes" like cascaded control or start up behavior.

What type of temperature control loop is this?
How fast of a response?, how fast do you need to calculate in loop? Last but not least- S7300 or
I have use the standard FB41 and it worked great.

Hi Les,
I've done stacks of temperature/ pressure control with various plcs over the last 15 years or so
and i know you can achieve what you want with a standard plc incorporating analog input/output
I've written software that can be adapted to pretty much any plc (already tried and tested with
allen bradley / mitsubishi / siemens (s5 and s7) that can hold temperature within +/- 0.1 degree
c (dependent upon the reaction speed of the cooling / heating plant of course)
Can advise you what to do / where to go with it if you want.
Feel free to contact me at "[email protected]", mailto:[email protected]
As with most analogue control problems in PLCs, it is rarely the CPU or software which limits the 'accuracy' of the control. By the time you take in to account the accuracy and speed of response of the sensor and actuator, never mind the process itself, the CPU forms a relatively minor part of the problem.

Some factors worth bearing in mind for determining whether to use a dedicated PID controller, either as a Profibus device, or as a co-processor module in the PLC, as opposed to software, could be -

i) redundancy - is it important to retain PID control regardless of what your CPU is doing?

ii) integration - how tightly knit is the PID control in to the rest of your control strategy?

iii) HMI - most standalone PID controllers have a faceplate display and buttons. Most PLC CPUs don't. Is this a factor? (good or bad!)

iv) Rarely, speed - how much else is the PLC doing, and how many loops do you require it to control? In temperature applications speed is usually not an major issue (yes, I know, big generalisation!). In positioning or speed control, responses in milliseconds may be required, whereas CPU scan times may be an order of magnitude greater.

Overall, I think you should consider more than just 'accuracy' when selecting the appropriate PID control method.

Donald Pittendrigh

Hi All

I have recently completed a project using amongst other things an S7 PID algorithm for speed control of a hydraulic system. The PID cycles every 10ms, uses switched controller limits and gains for different speed "areas" of the machine trajectory and various other fancy tricks. The actaul speed value is provided by a Fraba encoder on Profibus and the setpoint is an analog output on ET200m. It was not without problems to get the required performance out of the machine however it is now working well and without the flexibility of the software controller I doubt it would have been a practical excercise. I dont think that in the case of level control you need to worry too much about acheiving your goals.

Donald Pittendrigh