TOPIC

Continuous versus discrete control system

Contents

Continuous versus discreet control systemComparison 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 control

Continuous control: control system in which variables and parameters are continuous and analogDiscrete control: control system in which variables and parameters are discrete. They are mostly binary discrete.

Comparison between continuous and discrete control

Comparison 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 measures, solid volume measures

Concentration of solution ,absence of contamination , conformance to specification

Temperature , volume flow rate , pressure

Flow meters , thermocouples

Valves , heaters , pumps

Seconds, minutes , hours

Number of parts, number of products

Dimensions , surface finish , appearance , absence of defects , product reliability

Position , velocity , acceleration , force

Photoelectric sensors , strain gaugesSwitches , motors , pistons

Less 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 process

exhibits the following characteristics1. There is a well defined index of performance such as product cost, production

rate.2. The relationship between the process variables and the index of performance is

known3. 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 system’s 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 of event-drive changes . Sequential logic control: it is used to control to manage time-drive changes .