complex vs simple PID control

Hey All

In my down time between projects i enjoy building different types of control systems. I already do it all day for work but like to tinker with things not usually seen in my industries

If i were to ask you to name a few processes that are difficult to control vs simple to control what would you say? I am looking for new project ideas centered around PI or PID

Simple

Temperature control
Motor Speed Control


Difficult or non linear

Chemical Dosing - perhaps pH


Can you name some more to add to this list?
Thanks

rscott9399 said:

Chemical Dosing - perhaps pH

Click to expand...

Nothing really to add, but my coworker is dealing with pH control using PID and a dosing pump right at the moment, and it sounds like a pain.

mylespetro said:

Nothing really to add, but my coworker is dealing with pH control using PID and a dosing pump right at the moment, and it sounds like a pain.

Click to expand...

I have done it before, mine works great.
It is a complex design. I can not share because of confidentiality issues but my design worked well.

I was just thinking about other things to tinker with for weekend projects.

One thing i do want to tackle is position control.

I have always wanted to try and build a balancing robot with a PLC but the extension cord will kill me.

Individual control of pressure, flow, level can be pretty straight forward. When you have controllers that interact with each other, it can get tricky. Refinery distillation towers for example have many controllers that can affect each other. Even a simple fractionator can have two level controllers, one for the tower and one for the reflux accumulator. The tower will have a pressure/vacuum controller. There will be a temperature controller for the tower and one for the furnace(maybe cascaded). A flow controller for the reflux line. Reflux is the light distillates that come from the top of the tower that get pumped back in to improve the performance of the tower trays.

When a change in any one of these controllers happens, It can have an affect on the other controllers. For example if you increase the tower pressure, you will decrease the amount of light ends produced and the level in the tower will increase. If you increase the tower temperature, you will produce more distillate that doesn't meet product spec and lower the level in the tower and increase the level in the reflux accumulator. If the heat exchangers start to foul and lose efficiency, it will cause the furnace to work harder and the feedstock entering the tower won't be at a high enough temperature making it almost impossible to control.

What are the correct setpoints? The ones that produce the most in spec product at the lowest operating cost. Operating cost being the natural gas flow at the furnace in this example.

A note on the derivative term. I kept a running count for several years while I worked for a huge oil and gas engineering firm that makes gas turbine engines. I worked in natural gas compression station control. over the course of several years and many large projects including the engine control units, station controls, anti surge control, BOP(balance of plant) level, pressure, etc, the number of PID loops that required a derivative term was.........Wait for it.

Basically Zero.

Does that mean you don't need it? Of course not. I have used them, just not during that time and on those type of processes. I would say its pretty rare.

I can think of many things.


1. Controlling a mass on the end of a spring. A hydraulic cylinder can be like this.


2. Control the level of the second tank in a two tank system where a motor/pump pumps water into tank 1 that has a hole in the bottom and the water flows into tank 2. Tank 2 has a hole in the bottom and flow to a recirculation system back to the pump.


3. Any project with a lot of dead time.


4. A project that requires gain scheduling.


5. a fun one would be controlling a quad copter drone. There is a lot of people that have written about this but most get their drone to work by trial and error.

haha I just posted needing help with what to me is complex. Dissolved oxygen control with 3 ways of changing it but like everything in life, each output has ideal max and mins. Title of my post is Three variable PID Control if you are interested.

Iandm said:

haha I just posted needing help with what to me is complex. Dissolved oxygen control with 3 ways of changing it but like everything in life, each output has ideal max and mins. Title of my post is Three variable PID Control if you are interested.

Click to expand...

I made a business out of doing DO control. It is very complex, non-linear, has long process and transport delays, multiple min/max restrictions, huge load and process variations ....

I did not use PID.

Wrote a book on the topic: https://www.amazon.com/Aeration-Con...=1549462364&sr=8-5&keywords=thomas+E.+Jenkins