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Chapter 7: Common Control Loops - A Real-Time Approach to Process Control, 3rd Edition [Book]
Modernize SPC with a 21st Century Solution To optimize your processes and produce the highest quality products, you need an unprecedented degree of insight into the behavior of every part of your manufacturing operation. User friendly Budget friendly IT friendly Mobile friendly Value Priced: Make quality your biggest advantage, not your biggest investment. Learn how to make quality your biggest competitive advantage. They highlight the three most known simulation techniques—the first two considered real-time simulations:.
The advantages of RCP over a controller prototype are that developing the controller in a real-time simulator is faster and more flexible. Considering the HIL technique, usually a physical controller is connected to a virtual plant executed on a real-time simulator. The main advantages of HIL is allowing test of controllers in conditions unavailable on real plants. SIL usually does not require real-time and deterministic responses as both controller and plant can be simulated on the same simulator.
They provide an illustration of a real-time simulation system in a Hardware-in-the-Loop configuration:.
Read the whitepaper for more on how they build the process model and method for modeling, characterizing, and developing control strategies for the simulation. The use of HIL simulations in design of control systems and operator training environment is effective since the designer knows well the behavior or the dynamics of the process to be controlled. The HIL architecture, as the one proposed in this paper, allows several studies to be conducted, for example the design and implementation of new control strategies.
In addition, and maybe more important, is the possibility of building operator training centers fast and safely, reducing training costs and time to operate an industrial plant. The advantage of using a real-time simulator for OTS is that the user can acquire a feeling for the controller and plant that correctly and reliably represents the real system, without the delays and limitations commonly found in training environments based on recorded scenarios.
Very few and specialized control devices still use plain hardware or programmable hardware using a field programmable gate array to accomplish the job for which they were designed.
Nevertheless, to design and develop software applications for a controller or automation device, special skills and a good understanding of the automation and control problem are required. Many designers and programmers build web servers, information systems, business process management systems, and many other important and valuable infrastructures based on computing platforms.
But when it comes to industrial process controllers, the designer recognizes that a new design and programming paradigm is required. This is well accepted throughout the automation and control industry.
Similarly, many concepts involving signal acquisition, transducers, control setpoints, and others are related to process-state variables, measurements, and control. But mostly, they are closely connected with real-time processing. The concept of real-time processing must be considered when any engineer designs, develops, and deploys a new automation and control system.
This is what differentiates automation software designers from designers of any other application software. Automation controllers must be real-time systems because they must control physical processes or plants that demand real-time control. At this point, the automation controller designer must have a very good idea of what a real-time system is. Hermann Kopetz, in his book, "Real Time Systems: Design Principles for Distributed Embedded Applications" , stated that "A real-time computer system is a computer system where the correctness of the system behavior depends not only on the logical results of the computations, but also on the physical time when these results are produced.
By system behavior we mean the sequence of outputs in time of a system. For any physical plant to be governed, it requires a controller capable of acquiring some input signals and producing some specific output signals within a very specific time frame. If the controller outputs occur outside such time frame, those outputs will no longer be valid, and will produce malfunctions or even a catastrophic failure in the physical plant.
How can we determine that a computer system is a real-time system? Suppose there is a very simple system, with just one input and one output. Such system generates an output signal every time it receives an input signal see Figure 1. Now, suppose that we repeatedly but not periodically apply the same input signal to this system. We may expect that this system will generate the same output signal every time, at exactly t seconds after the input signal is applied.
Real-Time Statistical Process Control
Unfortunately, this is not what happens in actual controllers. The same output signal will be generated by the controller every time, but the time t that it takes the controller to produce each output may increase slightly see Figure 2. It strongly depends on how the controller hardware was designed, which components were used, and how the application software running inside it was designed and developed. The former depends on the function that the controller is supposed to perform, while the latter depends on how the designer implemented this function inside the controller.
Therefore, the controller designer must know the maximum that the physical plant would allow without causing failures or damages, and consequently he or she must build the right real-time controller that takes into account such constraint. Depending on the physical plant and the type of control to be performed, the controller may be classified as "hard real time" or "soft real time. Each physical process has its own "latency," that is, the mean time the process reacts to a change in one or several of the inputs. This latency is tied to the physical, chemical, and electrical laws governing that process.