Separating the process control men from the boys.

A high level, block diagram view of a closed loop feedback control system in the form of a PID Loop is shown below.

Nowadays you can simulate a entire factory on a $1000 laptop.

That means that there is no longer any excuse for you to learn PID control or PID Loop Tuning on your actual plant and risk breaking expensive equipment or, worse, bringing your plant down for a few minutes, hours or days.

Simulation is so quick, easy and cheap to implement that you should be spending 80% of your time simulating and 20% implementing. This will let you choose your control strategy and iron out 95% of the bugs in your code or tuning constants at your desk.

If you are embarking on a PID Control project, then you can break your simulation into phases depending on your budget and your project size.

PID Simulation Phases

1) Simple Simulation of the main process characteristics.

This stage is essential for any size project - no excuses. You can get a good PID Loop Simulator for under 200 bucks, and make all your major tuning mistakes on that - before you even touch your plant. Here is the process in a nutshell:

a)You do all your pre process checks (the PID Tuning Blueprint can walk you through these) and then identify your major open loop process characteristics, such as Process Gain, Lag and Deadtime.

b)You configure your simulator with the characteristics you have just measured and check that the simulated response is similar to the real thing.

c)You calculate your tuning constants using a good, open loop, loop tuning procedure,configure the simulated PID controller and select its PID Algorithm.

d)The simulator shows you how your simulated process will respond to those tuning parameters based on setpoint and/or load changes.

e)You spot any mistakes in your tuning well before you touch your plant, and gain confidence in the tuning technique that you have just learned.

This is also a great PID learning tool for playing with the PID controller setup and seeing how the different parameters affect the process performance.

2) Hardware-in-the-loop PID simulation

Whereas step 1 can be done for a coupla hundred bucks, you are probably going to have to spend $5-$10k on software if you want to do this kind of simulation.

With this phase of the simulation, you use your real control hardware, and real control system code, but instead of wiring the controller into the plant you wire it into a PC that is running a simulation of the plant.

Why do this? Well it lets you debug all your code/tuning on the actual hardware (PID Controllers)that will do the controlling and gives you the opportunity to fix and vendor specific peculiarities at your desk with a nice cup of coffee, instead of out on the plant.

If you do this part right, your onsite commissioning time can easily be reduced by an order of magnitude.

If you want to discuss how to do this kind of simulation - drop me a line and I can point you in the right direction.

Just bear in mind this warning (courtesy of my old Control Eng Lecturer):

"Simulation is like masturbation, the more you do it, the more you think it's the real thing..."