CONTROL SYSTEM

CONTROL SYSTEMGOAL:Maintaining process variables (temperatures, pressures, flows, compositions, levels) at a desired operating value.Processes are dynamic in nature, and changes are always occurring. The important variables those related to safety, product quality, and production rates will not achieve design conditions. 1.1 PROCESS CONTROL SYSTEM

Manual process control Automatic process control

Introduction to Control Systems

Automatic Control has become an important and integral part of modern manufacturing and industrial process

Automatic control is essential in such industrial operations as controlling pressure temperature, humidity, viscosity and flow in process industries

Process The device, plant, or system under control. The input and output relationship represents the cause-and-effect relationship of the process.Process The device, plant, or system under control. The input and output relationship represents the cause-and-effect relationship of the process.Open-Loop Control Systems utilize a controller or control actuator to obtain the desired response.Controller

ProcessInput Output Controller

Feed Back Manual Process Control:

Closed-Loop Control Systems utilizes feedback to compare the actual output to the desired output responseController

Process

Input Output Feed back Open Loop Control SystemClosed Loop Control SystemSimple & EconomicalAccurateEasier to constructSensitivity is small to make system stableGenerally stableless affected by noise.Open Loop Control SystemClosed Loop Control SystemInaccurate and unreliableFor stable system design is Complex and costlierChanges in the output due to external disturbance are not corrected automaticallyThe feed back in the closed loop system may lead to oscillatory response.Feedback reduces the overall gain of the systemAdvantages & Disadvantages of Control SystemAdvantages

Disadvantages8 The block diagrams of feedback control systemFig 1-4 Block diagrams of closed-loop Automatic control systems

The advantage of feedback control1.4 CONTROL STRATEGIESCompensate for all disturbances The result of any disturbance entering the process is to make the controlled variable deviate from the SET POINT. Once the controlled variable deviates from the set point, the controller changes its output to return the controlled variable to SET POINT(its desired value). The feedback control loop does not know, nor does it care, which disturbance enters the process. It only tries to maintain the controlled variable at set point, and in this way compensates for all disturbances. The feedback controller works with minimum knowledge of the process. Actually, the only information it needs is in which direction to move, and how much to move is usually adjusted by trial and error. The characteristics of feedback control Why Negative Feedback

the negative feedback has more advantages.they are

1.it has more stability.

2.it increases the bandwidth.

3.it reduces the noise.

but only the disadvantage of negative feed back over positive feed back is it decreases the gain.TRANSFER FUNCTIONTransfer Function(also known as thesystem functionornetwork function) is a mathematical representation, in terms of spatial or temporal frequency, of the relation between the input and output of alinear time-invariantsystemTRANSFER FUNCTION OF CLOSED LOOP CONTROL SYSTEM

W(s) = K E(s)

Y(s) = G(s) W(s)

E(s) = U(s) Y(s)

W(s) = K( U(s) Y(s))

Y(s) = G(s) K (U(s) Y(s))

Transfer Function Y(s)/U(s) = K G(s) 1 + KG(s) TRANSFER FUNCTION OF CLOSED LOOP CONTROL SYSTEM

Controller

Final control

element

Process

or plant

SP

Disturbance 1

Sensor/

Transmitter

Disturbance 2

Summing

junction

-

Output

Or

Controlled

variable