Linear v non-linear

Good morning

Can anybody explain the advantages/disadvantages of linear v. non linear conversion of a PID loop input.

Data registers 0-4095

Temp range 40 to 200 F. Set point 180 F.

Temp control on/off.

Thank you for your time

what type of raw material you need to control/produce with this temperature SP ?

Water tanks. Big washing machine for machined engine blocks

http://www.virtualnerd.com/tutorials/?id=PreAlg_13_01_0006

from this you can select characteristic you needed,its up to you.

Can anybody explain the advantages/disadvantages of linear v. non linear conversion of a PID loop input.

Click to expand...

Can you expand on this? What type of sensor is connected to the PLC?

The PID function is a linear function and will only work optimally on a linear process or a quasi-linear segment of a non-linear process unless you change the PID gains as you move through segments of the process.

However, there are sensor feedback signals that return a non-linear feedback of a linear function. One example would be using differential pressure across an orifice plate to determine flow. In that case the pressure feedback value would need to be "linearized" in order to provide the PID function with the linear feedback it would expect to see.

Having said that I have on a few occasions intentionally provided non-linear feedback of a linear process to a PID function. In those occasions I had a process that could "wander" around the setpoint without creating any issues but absolutely needed to stay away from the process limits. By squaring the error to the PID function I could keep the system stable, albeit mushy, around the setpoint but at the same time aggressively correct for large deviations in process variable. But this situation is by far the exception as opposed to the rule.

Keith

So what you are saying is that ALL inputs to a PID loop are linear and that non linear inputs need to be converted into a linear equation before the PID loop can process it. I think I understand that.

I'm setting up a PID configuration for temp control with a J type probe and an analog card.
It's pretty straight forward with a simple on/off control, but setting up the PID parameters, one of the input settings was linear conversion and the default setting was "disabled" . So I wanted to understand.

I called the distributor and he explained that setting up the analog card as a J type input automatically disabled the linear conversion. When I set J type, the conversion formula was automatically loaded.


Thank you all for your time and knowledge.