CH3040: Process Control & Instrumentation

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1 CH 3040 Process Control & Instrumentation Arun K. Tangirala January 05, 2011 Department of Chemical Engineering What does control mean? Common sense and dictionary meaning of control needs no explanation Technical definition (one form of definition): Control is the task of maintaining a process variable at its desired point (or within desired limits), typically by adjusting another variable How different is control from design? Process design takes into account only steady-state considerations Control is concerned with the transient behaviour of a process Control can be viewed as the task of maintaining the process at a desired state or taking the process from state A to state B Control design is about choosing a good or optimal path for the regulation (at desired state) or transition (from state A to state B) 2

2 Why is control important? Safety Product specs. Safe operation of the process is foremost for the well-being of the people and for the development of the plant. Certain quantities of interest such as temperature, pressure should be within limits. Desired amount and quality of the products should be maintained. Requires productivity targets and purity specifications are met. Environmental Regulations according to laws should be satisfied. For example, pollutant levels from an industry should not exceed a limits Constraints Economics Operational constraints should be satisfied throughout the process operation. For e.g., tanks should not overflow or go dry, distillation columns should not be flooded Operation of a plant must adapt to market conditions. This imposes setpoints on the product. Besides, it is required that the plant is running at optimum operating levels (e.g., minimum cost) 3 Ensuring stability It is important to explicitly state this requirement Stability can be informally understood as the condition where the process variable is not running away (unbounded) for all (bounded) perturbations An aeronautical system in the absence of any control is unstable An inverted pendulum cannot remain vertical even due to a slightest perturbation Stable processes are known as self-regulating processes They can return to equilibrium state despite perturbations (given sufficient time) In controller design, the primary requirement is that the controlled process should be stable For unstable systems, the primary purpose of control is stabilization For stable systems, it is a requirement that has to be satisified 4

3 Two fundamental control problems Primarily only two control problems arise Regulatory control (Disturbance rejection): The control objective here is to suppress the effect of disturbances. Disturbances cause deviations in output (or bring in undesirable addition of energy). The role of controller is to minimize the deviations of the output from the desired value. Servo control (Set-point tracking): The control objective here is to take the output from its current value to another desired value. The role of the controller here is to ensure that output tracks the setpoint changes satisfying some performance specifications (e.g., minimum time) In general, any control problem is a mix of both. However, for controller design, the regulatory control problem is given preference since set-point changes are relatively less frequent when compared to disturbances 5 A typical control system Measured disturbances (feedforward path) Disturbances Set-point Controller Manipulated variable Actuator Process Process variable Feedback path Sensing Element In any control system, one should make it a point to identify the fundamental elements, namely, the sensing element (sensor), the controller (which decides the input moves), the actuator (the physical device which realizes the control actions, e.g., valve) 6

4 Examples Driving a vehicle Liquid level control system Regulation of blood pressure Flight control Reactor temperature control Composition control in a distillation column 7 Control Objectives: Elements of controller design What are the operational objectives that a control system is called upon to achieve? Usually the objectives are a mix of the regulatory, servo and/or performance optimization requirements. Select Measurements: What variables should be measured in order to monitor the performance of the process? Sometimes the lack of measurements can force one to modify the control objectives or search for alternative measurements. If the measured variables are those which are being controlled, then they are primary measurements If the measured variables are used to infer controlled variables, then they are secondary measurements. 8

5 Elements of controller design Choice of manipulated variables: These are very important quantities to choose. The choice of manipulated variables depends on the degrees of freedom available and their relationship with the controlled/performance variables. The choice is important since it affects the performance of the control scheme. Control configuration: It is the information structure through which the available measurements connect to the chosen manipulated variables (for e.g., feedback or feedforward). The best control configuration has to be chosen. Control Law: How are the manipulated variables adjusted for changes in controlled variables or under external influences? The law or the technique determines the performance as well as limitations of the control scheme. 9