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Continuous vs Discrete Control

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Text of Continuous vs Discrete Control


Continuous versus discrete control system

Contents Continuous versus discreet control system Comparison between continuous and discrete control systems. Continuous control systems. Categories of continuous control system. Discrete control systems. Types of discrete control system.

Continuous versus discrete controlContinuous control: control system in which variables and parameters are continuous and analog Discrete control: control system in which variables and parameters are discrete. They are mostly binary discrete.

Comparison between continuous and discrete controlComparison factor Continuous control in process industries Discrete control in manufacturing industries

Typical measure of product input Typical quality measures Typical variables and parameters Typical sensors Typical actuators Typical process time constants

Weight measures, liquid volume Number of parts, number of measures, solid volume measures products Concentration of solution ,absence of contamination , conformance to specification Temperature , volume flow rate , pressure Dimensions , surface finish , appearance , absence of defects , product reliability Position , velocity , acceleration , force

Flow meters , thermocouplesValves , heaters , pumps Seconds, minutes , hours

Photoelectric sensors , strain gauges Switches , motors , pistonsLess than a second

Continuous control systems Objective is to maintain the value of an output variables at a desired level Operation is similar to the feedback control system Consists of many feedback loops which control and coordinate to maintain the output variable at the desired value For example Control of a chemical reaction

Categories of continuous process Regulatory control: Objective is to maintain process performance at certain value or within a tolerance band of that level Performance measure of the process is called index of performance Problem with regulatory control is that action is taken when disturbance has effected the process Due to error , the output of the process is different from the desired value

Feed forward control: Strategy is to foresee the disturbance by sensing them and compensate for them before they effect the process In ideal case compensation is completely effective In real complete, compensation is impossible because of imperfection of feed forward control, actuators operation etc. Regulatory and feed forward control more closely associated with the process industries

Steady-state optimization. The term refer to the class of optimization techniques in which the process1. exhibits the following characteristics There is a well defined index of performance such as product cost, production rate. The relationship between the process variables and the index of performance is known The value of system parameters that optimize the index of performance can be determine mathematically. When these characteristic apply the process moves towards optimal state . The control system is open loop. It works successfully when there is no disturbance .


Adaptive control : A self correcting form of optimal control is called adaptive control. Adaptive control combines the feedback control and optimal control. It measures the relevant process variables during operation It use a control algorithm that attempts to optimize some index of performance . Adaptive control system performs three function

Identification function: The value of the index of performance is determined on the basis of the measurement taken from the process . As environment changes with time so system performance also changes. Identification functions must be more or less continuously over the time during operation.

Decision function. Decision function decides what changes should be made to improve performance This function is implemented by the help of adaptive systems programmed algorithm. Depending upon algorithm one or more input parameter to the process are change.

Modification function. Modification function implements the decision. Decision is a logic function and modification is concerned with physical changes in the system It involves hardware rather than software. System parameters are altered using actuators to drive the system towards more optimized state.

On-line search strategies. on-line search strategies can be used when decision function cannot defined properly . Therefore it is impossible to decide on the changes in the internal parameter of the system to produce desired performance . So small experiments are performed . Small changes are made to observe the effect of the changes on the output variables Based on the results large change are made to improve performance. On-line strategy is more used in continuous process.

Other specialized techniques. Includes strategies that are currently evolving in control theory and computer science. Examples are experts systems and other artificial intelligence method for process control.

Discrete control systems Parameters and variable of the systems are changed at discrete moments in time. The changes are defined in the program of instruction . The change of system occur when the state of the system changes or time is elapsed. Discrete control is associated with batch processing. The objective of batch processing is to manage the sequence and timing of the processing steps

Event drive changes: It is executed by the controller when due to some event, state of the system is changed . The change can initiate or terminate the operations such as start or stop a motor.

Examples :Counting of parts moving along a conveyor belt with The help of an optical sensor.

Time drive changes: It is executed by the controller either at specific point in time . The change consists of starting or stopping of something . Time when the change occurred, is important. Example In factories with specific starting and ending time of the shift is set to sound a bell at specific moments during the day.

Types of discrete control system. Combinational logic control:it is used to control the execution ofevent-drive changes .

Sequential logic control:it is used to control to manage timedrive changes .

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