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PROCESS CONTROL SYSTEM REVIEW

CHAPTER 1 INTRODUCTION

1. Typical Controlled Variables

Temperature, Pressure, Flow, Level,Concentration,Physical property

2. Terminology in the process

The Controlled Variable (CV), the Manipulated Variable (MV)

Factors causing CV to deviate from their set points is called the Disturbance or“Load" Variables

3. Two Classes of Process Control

Regulatory control- disturbance change , Servoing control- setpoint change

4. Control Equipment Process Equipment

5. Modeling

1) Theoretical Modeling:

Physical/chemical laws (fundamental, global)

Material/energy balances

Heat, mass, and momentum transfer

Thermodynamics, chemical kinetics

Physical property relationships

2) Empirical Modeling (Black Box)

Large number of unknown parameters

Being obtained quickly (e.g., linear regression)

Model structure is subjective

Dangerous to extrapolate

CHAPTER 3 DYNAMIC RESPONSE

1. Analysis of the 1st Order Process

1) Property 1 : y increases from 0 to a new steady state MK, thus self-regulating

(Integrating process: Non-self-regulating )

2) Property 2 : Steady state gain K = y/M, The larger gain K, the more sensitive is the output to the change in the input

3) Property 3 : At t=τ, the output is y =0.632MK

4) Property 4 : The shorter the space time τ, the faster reaches the new steady state

2. Analysis of the 2nd Order Process

)(01222t bx y a dt dy

a dt y d a =++

1) 4 variables

3. Processes with Dead Time

1) Pad é approximation

The 1nd order Pad é approximation

The 2nd order Pad é approximation

2) Two plants have different intermediate variables but have the same input-output behavior!

Steady

state

transfer function

CHAPTER 5 ANALYSIS OF SISO SYSTEM

1. PID Controllers

1) The Proportional Band (PB ) is the inverse of the Gain. In the sense of proportional band PB

The higher PB, the slower (and more stable) control. The lower PB, the faster (and less stable) control.

2) Kc

The higher gain, the faster (less stable) control; The lower gain, the slower (more stable) control. The larger Kc, the faster is the change to a given error; Too large Kc will make the process unstable

3) Integral action makes the process saturation, reset windup

A. Integral term takes on a final nonzero value, thus permitting e s to stay at zero, as time progresses

B. the reset element of the controller continues to increase (or decrease) the controller output

C. even when the change in output does not cause any change in the process measurement (controlled variable).

D. with no resulting decrease in error, the output will continue to increase until it reaches its limit.

4) A small change in error may produce a big controller output occurring in changing set point (derivative kick ) , exhibiting in high-frequency noise

5) The Properties of P, I, D Control: from time standpoint

t = 0 t = t

t = T (T>t)

Proportional Mode

P-Control

Persistent Mode I-Control

Predictive Mode D-Control

Historical period Current Time

Future Time