Electronics > Beginners
Can standard controllers be combined in automation?
bonzer:
Hello! I have a curiosity.. When dealing with control regulators: my question could sound strange because I'm at the beginning of an electronics engineering degree and not automation but I was thinking about automation control. I know they usually use standard blocks like P, PI, PD, PID. But I don't think you can do any function you want with them. But they are simple and confortable when dealing with discretizations. But if I want a general function because I find it particularly helpful for my system. For example (s+x1)/(s+x2) where x2>x1 how do they deal with that? Is it all just programmable for example in PLC? (I know nothing about PLC) . Or they use some combinations, for example: this function could be acquired with a block of a PD controller if it has an external proportional constant feedback (like a P controller) so this closed loop system would give me that type of function.
In analog electronics you can place capacitors and resistors with opamps to get almost any transfer function you want.
But in automation where it looks like they usually use blocks? Anyway this is my curiosity. I also see that discretization methods are usually made for standard control blocks.
Mattjd:
I'm not sure I fully understand your question and I've never dealt with programmable logic controllers but I do not believe they're the same controller as in a PID controller.
The discretization of a PID controller is done the same way as for a filter. The proper way is with dsp techniques, but that can be overkill in a lot of cases.
The general method is create the transfer function for your system, converting any mechanical components to their equivalent electrical counter part. Once the transfer function is made, a PID controller is tuned to obtain the desired rise, fall, settling times, over shoot error, etc.
With the s domain equivalent obtained, and gain parameters found, the discretization occurs by applying one of a few different transformation techniques. The most popular is the bilinear transformation, this will convert your complex algebraic equation of variable s into a complex recurrence relation of variable z. The inverse z transform can then be taken which provides you with a finite difference equation which is the discrete counter part to an ordinary Differential equation.
Sedly:
There are quite a few types of PLCs as well as distributed control systems. Many are quite sophisticated and feature a selection of programming languages from ladder logic to sequential function blocks to structured text. Block functions such as PID blocks as well as math functions and multi-dimentional arrays are available. They are quite capable and support sophisticated I/O as well as communications. They can easily perform the type of trnansform you outlined.
The hardware tends to be expensive and the programming tools are proprietary with significant license fees. These are the tools used by Controls and Automation engineers.
Medium:
--- Quote from: Sedly on November 15, 2019, 05:23:48 am ---The hardware tends to be expensive and the programming tools are proprietary with significant license fees. These are the tools used by Controls and Automation engineers.
--- End quote ---
Programming PLCs is a big part of my daily job, and thanks to the availability of high level language support I practically do not need any knowledge of electronics to automate entire plants*. (Provided that anything that happens outside the PLC is handled by someone else.) PLCs are practically a µC on steroids. In industry application, no one would really implement functions in dedicated hardware these days anymore. Even simple small machines have at least some type of µC to do most of the logic stuff. They cost pennies and are reliable enough.
*) And in fact, I have next to none. I'm an IT guy (application development) that ended up doing automation for a living.
Berni:
This is why they are called Programmable Logic Controllers.
As long as they have a means of analog input and output they will also have math functions to implement anything you want. Its essentially a super robust microcontroller with some fancy underlying firmware and a IDE to program it in.
Just like a computer it will want everything is discrete form in order to run it real time. So no plopping down integrals and derivatives, you need to turn it into a series of additions and multiplies first. But in most cases of process control the PLC will be much faster than the sluggish industrial process it is controlling, so simply doing your integration as i = i+x and derivative as d = x-i tends to be close enough.
None of this is done analog anymore since PLCs offer so many advantages. A big thing with PLCs is also that they can be wired up on a network. This means your process control room can simply be some ethernet cable running into a PC running SCADA software, so that operators click virtual buttons on a screen. As opposed to having to wire up tens, hundreds or even thousands of wires into individual gauges and switches on a huge custom built control panel.
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