Process Control Engineering
In the operation of technical systems we have various degrees of freedom, which we use in order to come closer to our operating goals. In process technology we try to optimize the performance of a procedure; to do this we develop strategies or guidelines
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Process Control Engineering
In the operation of technical systems we have various degrees of freedom, which we use in order to come closer to our operating goals. In process technology we try to optimize the performance of a procedure; to do this we develop strategies or guidelines according to which we use these degrees of freedom; subsequently, we apply these strategies to the operation of the system, by either automation or manual interference. Process control engineering has the task of converting our operating goals and strategies into practical application. To do this, we use mathematical algorithms (strategies), technical elements (actuators, control members), and sensors (measuring systems). A basic understanding of process control helps us: • to apply simple operating strategies successfully to the manual control of systems; • to search for possible ways of optimizing operation, e.g., with the help of simulation; • to recognize possibilities and obstacles in drafting control strategies for more advanced control systems; • to communicate successfully with control engineers. In addition, process control technology provides a general understanding of problems in the operation of plants. Here only a short introduction to the simplest principles of process control technology is given.
13.1 Examples of Operating Strategies The following examples show different aspects of control engineering on the basis of descriptive examples. 315
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13 Process Control Engineering
13.1.1 Adjusting the Water Temperature of a Shower If we want to adjust the water temperature of a shower in a hotel, we have two degrees of freedom in operating the unknown mixing tap: the flow rate Q and the position of the temperature lever S. We start with the highest possible flow Qmax and choose the highest possible temperature Tmax. As soon as we feel an increase of the temperature, we reduce T and possibly also Q. Since the showering system is unknown to us, the first correction will not be sufficient. A second one follows, possibly before the first one has fully affected the temperature (the lag in the controlled system is relatively long, i. e., it takes some seconds between the change of the position, the filling of the hose, and the reaction of the measurement system – our feeling of the temperature on the skin). After several corrections, we succeed in obtaining the desired temperature. The controlled variable settled down after several oscillations. Our strategy worked satisfactorily depending upon our patience, or we got scalded under too hot a shower. At home, we have experience with our own shower; we are experts. We successfully reach the desired temperature with only a single correction. We can even consider a preheated, just used shower without any problems. The specific characteristics of a plant (time constants, degrees of freedom, state variables, measured values, etc.) determine the reaction of the plant to our operating strategies. Expert knowledge (control engineering) improves these strategies and helps to achieve our
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