4.hardware For A Process Control System [PDF]

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Hardware for a Process Control System The physical elements (hardware) constituting a control system as it is implemented in practice for the control of real physical processess.



Hardware Elements of a Control System In every control configuration we can distinguish the following hareware elements : 1. The chemical process : It represents the material equipment together with the physical or chemical operations that occur there. 2. The measuring instruments or sensors : Such instruments are used to measure the disturbances, the controlled output variables, or secondary output variables, and the main sources of information about what is going on in the process. 3. Transducers : Many measurements cannot be used for control until they are converted to physical quantities (such as electric voltage or current, or a pneumatic signal, i.e., compressed air or compressed liquid) which can be transmitted easily. Transducers are used for this purpose. 4. Transmission lines : These are used to carry the measuring signals from the measuring device to the controller. 5. The controller : 1



This is the hardware element that has ‘intelligence’. It receives the information from measuring devices and decides what action should be taken. 6. The final control element : This is the hardware element that implements in real life the decision taken by the controller. The control valve is the most frequently encountered final element but not the only one. Other typical final control elements for a chemical processes are : Relay switches, providing on-off control Variable-speed pumps Variable-speed compressors 7. Recording elements: These are used to provide a visual demonstration of how a chemical process behaves.



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eTemperatur e recorder Desired



Ts



Fi



Ti



Thermoco uple Tank



F0



CController



Control valve Transmission line



Figure 1 : Hardware elements for the feedback control of a stirred tank heater



Use of Digital Computers in Process Control Large chemical plants such as petroleum refineries, ethylene plants, ammonia plants, and many others are under digital computer control. The effects have been very substantial, leading to better control and reduced operating costs.



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Some applications characteristic of the diverse usage of digital computers : 1. Direct digital control (DDC) : Today the chemical industry is moving more and more toward the DDC of the plants. A typical system of DDCs for an ethylene plant can include between 300 and 400 control loops. All the companies that furnish the control system for the chemical industry rely more and more on DDC. 2. Supervisory computer control : Many times the human operator does not or cannot find the best operating policy for a plant which will minimize the operating costs. This deficiency is due to the enormous complexity of a typical chemical plant. In such cases we can use the speed and the programmed intelligence of a digital computer to analyze the situation and suggest the best policy. In doing so the computer coordinates the activities of the basic DDC loops ( Figure 3) 3. Scheduling computer control : Finally, the computer can be used to schedule the operation of a plant.



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Disturbances



Manipula ted Variables



Process



Measur ed Output s



Unmeasured outputs OOperator’s Console



Ccomputer-toProcess Interface



DDigital Computer



PProcess-toComputer Interface



Figure 2 : Typical DDC configuration



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computer for Superviso ry control DDC For Unit 1



Process Unit 1



DDDC For Unit N



DDC For Unit 2



Process Unit 2



Process Unit N



Chemical Plant Figure 3 : Structure of supervisory computer control



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