Closed-loop process control with Programmable Logic Controllers

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Programmable logic controllers are now firmly established as the backbone of most factory automation systems. PLCs are versatile, extremely reliable, inexpensive, and easy to use.

PLCs are easy to use because they mainly use logic and sequential control algorithms, easily expressed in ladder diagrams. It’s great for systems with discrete emphasis or can be structured into logical states.

Most PLC applications are in this area since they apply to many systems. As far as these applications go, “control” means defining logical states and conditions and then moving the process from one logical state to the next, i.e., defining the process sequence. Digital signals can be used to measure discrete variables, and analog signals can be used to measure continuous variables.

Discontinuous variables include ON/OFF switches (make-or-break devices) and valves in closed or open positions; continuous variables include temperature at setpoint value, servo valves opened to defined positions, and percentages.

Then again, there are a lot of systems that run continuously instead of discretely. Logic controllers aren’t suitable for these systems; you need a closed-loop controller. 

A feedback control system is a continuous control system that ensures that a specific desired state is maintained, while a logic controller deals with discrete logical statuses of the process. 

Using today’s state-of-the-art feedback control technology, you can ensure continuously stable and optimized operation in complex processes by using advanced control algorithms.

PLCs are best suited for essential process control, whereas multiloop controllers are better for more complex applications.

There’s a combination of logic and feedback control needed here. Most industrial processes are both discrete and continuous in real life. 

PLCs and multiloop controllers can automate these processes. But these hybrid setups also have inherent disadvantages, like complex data communication between the components, redundant double installation of some functions, and a high price.

Is it possible to combine the advantages of a PLC with the more advanced functionality of a multiloop controller without the disadvantages?

A feedback control system usually requires a lot of technical and theoretical knowledge, which limits it to highly qualified process control engineers. The ideal approach combines the functionality of PLCs and feedback controllers while keeping the simple operation and programming offered by PLCs.

PLC + IDR BLOK = PLC with integrated multiloop controller

Using IDR BLOK software, MELSEC A series controllers from MITSUBISHI ELECTRIC can now be configured for direct digital feedback control applications.

This program doesn’t require any specialized knowledge. Users create applications by arranging and linking “building blocks,” each serving a specific purpose.

This block structure and graphical programming approach reflect the basic process of closed-loop control system engineers: Data Acquisition, Status Information, Control, and Setup. The result is intuitive learning, a steep success curve, and efficient programming.

In turn, that means lower programming and system installation costs.

But IDR BLOK isn’t just for that – it also helps you solve feedback control problems visually or “topologically.” In the development of control algorithms, it replaces abstract theory with concrete topology: programs are written by physically arranging function blocks on the screen.

This has two significant effects: Firstly, the control algorithms and the program can be developed simultaneously, eliminating an entire step in the application development process. 

Additionally, it reduces the qualifications required to write and develop control algorithms: a trained technician can use it effectively for a wide variety of “standard” tasks without knowing much about logic control systems and feedback.

A ladder diagram language program and an IDR BLOK program can also run on the same PLC together. With this combination of sequential and feedback control elements, ordinary PLCs can solve many problems.

Writing and testing the PLC programs

IDR BLOK applications are written by connecting blocks from the block library (Fig. 1). The blocks are arranged in groups referred to as “aloops.”. Each logical loop has a priority and an execution period. 

Each block gets executed at a different frequency based on the logical loop to which it belongs. Each logical loop can have its independent control loops, called PIDs.

Each block has inputs and outputs, which can be linked to others’ information. Printouts of the control program schematics are made straightforward by arranging them in pages with header data so the user can print them out quickly. To save time, the documentation header is generated automatically, so all you have to do is a comment.

A program gets tested online when it’s ready. All variables, including feedback control parameters, are displayed as trends or numeric data tables in this mode, and all control parameters can be changed online during the test. Safe testing makes it easy to optimize parameters and reduces the need for specialized knowledge.

Block types

The names of the different function blocks reflect the philosophy inherent in the program package, which the sequence Data Acquisition, Status Information, Control, and Setup express.

The blocks are classified into six:

  • Getting process data and converting it into technical units
  • Blocks for executing different decision functions, like selecting input signals and searching for maximums and minimums.
  • Blocks for calculating and executing logic
  • These are control blocks for executing control algorithms like PID controllers, two-layer controllers, and three-layer controllers.
  • Sending commands to the switches with output blocks
  • For executing additional tasks, like limiting the block output value, exchanging data with the ladder diagram program, etc.

How does this solution benefit the user?

These systems have a few key benefits:

  • Better and simpler integration of the logic controller and feedback system
  • Elimination of multiple loop controllers and complex links between single loop controllers (PLC systems only)
  • Cheap
  • Reliability boost

Besides standard PID controllers, the program can also implement optimum value, cascade, and ratio controllers.

What’s up

This combined approach can solve problems in various applications, including cooling and steam generation plants, vulcanizing plants, exhaust burn-off plants, pump stations, water treatment plants, and even power generation and distribution plants. IDR BLOK applications are already used in the water industry, breweries, car factories, tire and rubber plants, and ski production facilities.

Most applications generally involve heat generation and various continuous and batch processes. The control parameters are temperature, pressure, and flow rates/volumes (steam, water, air, etc.).

PLC Example application

This plant for burning off exhaust gases provides a clear illustration of the capabilities of the system. The plant equipment is from LIVING Vransko, and the logic and feedback control systems were delivered by INEA Domzale (Slovenia).

The exhaust gases in rubber vulcanizing works usually contain many explosive and toxic substances. These gases are burned off in plants like this, drastically reducing pollution emissions from the production process.

The burn-off plant has three primary functional stages:

  1. Exhaust gas and air input (from the vulcanizing system lines and fresh air intakes)
  2. Air/flue gas heat exchangers and a combustion reactor with natural gas fuel input and a chimney stack (for burning off the exhaust gases)
  3. Oil/gas heat exchangers (for cooling the flue gases and recycling the waste heat)

The process

A blower transports exhaust gases from vulcanizing lines to a combustion reactor, where natural gas fuel is used to burn them off. 

In the first stage, their heat is used to preheat the incoming mixture of exhaust gases and air; in the second stage, they are passed through two heat exchangers connected to cool the flue gases. 

Afterward, a second blower transports them into the chimney stack and the surrounding environment.

All process stages are fully automated, including start-up, readiness for hot operation, heating, burn-off, cooling, shutting down, and transitions between them.

Clearly, this shows how important it is to execute sequential control algorithms (all transitions) and feedback control algorithms (all stationary processes) simultaneously.

It’s a Mitsubishi MELSEC A2N programmable logic controller (multiloop controller!) with a MAC control unit, two frequency inverters, and an industrial monitoring computer running SCADA FactoryLink.

It has 95 measured variables in 12 control loops (with a total of 250 I/Os) – 25 analog and 70 digital variables. Temperature, pressure, fresh air flow, flue gases, and oil flow are control variables. 

An excerpt from the IDR BLOK program shows how the four control loops are interdependent and linked in two cascades and optimum value algorithms. During the heating phase, the fireclay lining of the walls must be heated at a controlled rate, which is done by holding the combustion temperature in the combustion reactor.

Conclusion

For developing and programming feedback control implementations, IDR BLOK is a great tool. In both traditional and hitherto unexplored fields, this software significantly extends the functionality of MITSUBISHI ELECTRIC’s programmable logic controllers. 

The combination of PLCs with this specialized software has already proven its great potential in various applications, including building systems automation, industrial energy generation, and process control. This may start a significant new trend in factory automation, but IDR BLOK sets a new standard!

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