Filtering Noise in Digital PID Controllers: Exploring the Electrical Components

[jaredokie]

When speaking of filtering noise of an input or output, are the texts referring to the actual electrical components of a digital pid controller? Capacitors, resistors etc...

 

[meBigGuy]

Kind of a weird question. An analog PID controller uses analog components to achieve results, as do filters. A digital PID controller achieves similar results in the digital domain. The concept "actual electrical components of a digital PID controller" is a bit off. One might approach a digital PID design by mimicing (approximating) the operation of analog components. In that case, yes, I guess, sort of.

You might want to elaborate on your question.

 

[jaredokie]

I work around pid control. I'm trying to understand the design process of temperature controllers

 

[FactChecker]

There is more to it than that. Long electrical lines can pick up unwanted electrical noise from other devices. The thermostat can read brief changes in temperature from doors opening and random convection changes that you want to ignore. A device might turn off and on, making brief temperature changes. For each application, you have to identify the possible sources of noise, decide which are too significant to ignore, and attenuate the noise. Shield cables or put digital filters in the system. The derivative part of the PID can amplify noise, so be careful with it.

 

[jaredokie]

Is the laplace transform the most commonly used transform? And does a temperature controller already have the needed heat transfer equations built into the program? For instance Eurotherm temperature controllers. All that is needed are tuning parameters right?

 

[meBigGuy]

Everyone that reads a broad general question will react to a different thing about the question. Can you ask specific questions? Post links? The answers can be yes or no or maybe for all you ask.

Lapace transforms are useful for analyzing analog systems. Z-transforms are used for digital (sampled data) systems. There are transforms between the two.

Digital systems can also use non-linear techniques that are not easily done in analog systems.

Download a manual for a specific controller and study it. Don't expect us to do it for you. But, feel free to ask questions if you cannot understand what you read.

 

[jim hardy]

keep it simple...

The controller doesn't know anything about the process, that's the job of the engineer applying it.

Controllers generally operate on an error signal, which is the difference between measured and desired values of whatever's being controlled.

a PID calculates three functions of that error:

its value multiplied by some gain (Proportional)
its time integral (Integral)
and its approximate rate of change (Derivative)
The PID controller sums the three and applies its output to an actuating device that physically affects the process being controlled.

Analog controllers are just analog computers that use resistors and capacitors to realize their transfer function, which can be expressed as a Laplace transform. Digital controllers use computer programs to do the same calculations and produce same transfer function..

Figure 3 here is a slick analog PID controller that uses just one op-amp.
http://www.postreh.com/vmichal/papers/PID-Radio.pdf

I think it's based on a 1940's design by Philbrick Nexus... a google search turns up dozens of other circuits.

Filters are similarly implemented in either analog hardware or computer software.

hope this helps