Digital Control SystemIntroduction

SystemA system is a combination or an arrangement

of different physical components which act together as an entire unit to achieve certain objective.

Control SystemA control system is an arrangement of

different physical elements connected in such a manner so as to regulate, direct or command itself or some other system.

What is control system?A control system is a system that is designed

to control some physical quantity (temperature, pressure, speed, airplane heading, etc.)

What does a control system do?In order to control a physical quantity, and

control system must sense (measure) that physical quantity (controlled variable, output of the system, etc.).

What is in a control system?A control system will have some sort

of sensor or measuring device that is used to measure the value of whatever it is you are trying to control.

After measuring the controlled variable, the system needs to determine how well the system is performing at that instant of time.

the system computes a control effort (If you are controlling the speed of your car, the control effort is the throttle setting.) that is applied to the system to control the output.

What is Digital Control system?Using digital computers within a control loop

Open Loop SystemA system in which output is dependent on

input but controlling action or input is totally independent of the output or changes in output of the system, is called an open loop system.

AdvantagesSuch systems are simple in constructionVery much convenient to measure the output.No problem of stability.Simple to design and economical.

DisadvantageSuch systems are inaccurate and unreliable.They cannot sense environmental changes.They cannot sense internal disturbances in

the system.

Examples An electric switchAutomatic washing machineAutomatic toaster systemTraffic Light Controller

Temperature control SystemFirst, if you are trying to control a

temperature, you need to know what the temperature is.Thus, the first thing you have to do to control a

quantity (temperature) is to measure that quantity to see what value it has.

Secondly, after you measure the temperature, you have to compare the measured

temperature with the temperature you want to have - otherwise known as the desired temperature. 

Is it too high?  Is it too low?  How much higher (lower) is it compared to what you want?

Finally, after you have compared where you are at (i.e. measured the temperature) and compared it with what you want (i.e. the desired temperature) you should take control action. 

There are numerous ways you can control a variable.

Closed Loop Control SystemA system in which the controlling action or

input is dependent on the output or changes in output is called closed loop system.

Advantages of Closed loop system: Accuracy is very high.It senses environmental changes as well as

internal disturbances.Reduced effect of distortions.

Disadvantages:Such systems are complicated and time

consuming to design and hence costlier.Tendency to overcorrect the error may cause

oscillations without bound.Stability problems are more.