How to produce a control system modelling document

[David-Baratheon]

Hi everyone, sorry if this is the wrong section to post this but the control system will be on some NI hardware and the process I develop now will be used for how I approach all future control system tasks in my working career so it will be useful.  I am a fresh graduate working in industry (BEng Electronics Background).

I am trying to understand the control system modelling design process a bit better.

I am planning to use a LabVIEW programmed PID controller, but first want to model the system so that I have my PI values and can confirm I will get the correct behavior from the system.

What pieces of information do I need to gather in order to put together a document modelling the control system? I presume I will need some sort of block diagram showing the system, for example a motor output and some laser sensors for measuring movement going back into the closed loop control system.

If the motion created a linear motion (e.g. a lift going up and down) rather than a rotational one, and a sensor was able to tell the distance that the "lift" had moved, how would I go about modelling that system and selecting the right PID values for the control system?

Any insight into the design and modelling process will be greatly appreciated. I have some university level experience such as a quadcopter and IMU sensor but most of us struggled to get it working as it was an embedded systems module so not much focus on the actual control system modelling part. We had a control module but it was very basic. So am trying to get my skills up to scratch in industry. I have a copy of MATLAB if required. Any tutorials, examples, brief run through of the modelling process used in industry or other advice etc will be greatly appreciated.

Many thanks.

David Baratheon

[Sam_Sharp]

There are lots of good textbooks about modelling and control (although it's been quite a long time since I read any of them).

Typically, you would produce a modelling diagram which has blocks for your plants/sensors/actuators/controllers and shows the inputs/outputs of each block and how they relate to each other for your system.

This is pretty much the simplest example I could find for this:

r is your setpoint

u is your control input

y is your output 

e is the error between your setpoint and output

C(s) and P(s) are your controller/plant models as transfer functions

Each of these blocks will contain some maths that describes how that block operates. In classical control theory, this maths is generally derived from differential equations to produce the laplace transfer function (e.g. for a first/second order equation in the frequency domain in terms of 's'). In modern control theory, state space models are used - these are matrices that describe the inputs/outputs.

Typically, once you have developed the models you would then simulate them to allow you to try out different controllers/gains and see what impact it has on the system response (e.g. step response). For PID controllers, there are some basic PID tuning guidelines you can use (e.g. Ziegler-Nichols) to get a good approximation of what values you should use.

This is a huge subject...control design, simulation, system identification, system modelling are all pretty substantial topics but hopefully this is a good starting point (or at least some keywords for you to search for to learn more).

LabVIEW does have a control design & simulation module - very similar to what Simulink does in MATLAB.