01 Basic Concept of Process Control

Basic Concept ofProcess Control

Cheng-Liang Chen

PSELABORATORY

Department of Chemical EngineeringNational TAIWAN University

Chen CL 1

A Process Heat ExchangerThe Problem

➢ Control Objectives: to keep T at TD (and q at qD)

☞ T : controlled variable TD: set point

➢ Environment: varying qs, Ps, Ta, Ti, q, E (efficiency)

☞ Ps, Ta, Ti, E: hard to handle ⇒ disturbances☞ qs, q: easy for adjusting ⇒ manipulated variable

Chen CL 2

A Process Heat ExchangerThe Tools

➢ We need one sensor to know current status of T

➢ We need one valve to adjust qs

➢ We need one method to make decision

➢ We have to check if CV = SP from time to time

Chen CL 3

A Process Heat ExchangerManual Method to Achieve Control Objective

➢ To know: reading T (influenced by Ps, Ta, Ti, E)

➢ To decide: comparing T with TD and decide adjusting action

➢ To do: implementing new qs manually by a field operator

➢ Repeat above actions for every second

Chen CL 4

A Process Heat ExchangerAutomatic Method to Achieve Control Objective

➢ To know: reading T (influenced by Ps, Ta, Ti, E)

➢ To decide: comparing T with TD and decide adjusting action

➢ To do: implementing new qs automatically by a controller

➢ Repeat above actions for every second

Chen CL 5

A Process Heat ExchangerFour Basic Elements

➢ Primary/Secondary Element (sensor/transmitter)

to know current status for CV(fast, accurate, standard)

T (oC) TE=⇒ T̃ (mV )TT=⇒ y (4 ∼ 20 mA; 1 ∼ 5V ; 0% ∼ 100%)

Example: desired zero = 50oC, span = 100oC

50oC 4 mA 1 V 0%

l TE/TT−→ l or l or l150oC 20 mA 5 V 100%

Chen CL 6

A Process Heat ExchangerFour Basic Elements

➢ Decision-making Element (operator or controller)

to calculate the trial corrective action(simple to use, acceptable performance, robust, reliable)

☞ inputs: set-point ysp (mA or %), (from TDoC)

measured PV y(t) (mA or %), (from T oC)

☞ output: control action u(t) (mA or %)

Chen CL 7

A Process Heat ExchangerFour Basic Elements

➢ Final Control Element (I/P transducer + valve)

to realize operator’s or controller’s decision

u(t) (4 ∼ 20 mA)I/P

=⇒ u′(t) (3 ∼ 15 psi)valve=⇒ 0% ∼ 100% valve opening

=⇒ qs(t) (kg/sec) ( MV)

Chen CL 8

A Process Heat ExchangerFour Basic Elements

➢ Processto wait for new value of CV

☞ inputs: qs (kg/sec) (MV)Ps, Ti, Ta, · · · (Disturbances)

☞ output: T (oC) (PV, CV)

Chen CL 9

A Process Heat ExchangerFour Basic Elements

Summary

➢ Primary/Secondary Element (sensor/transmitter)

➢ Decision-making Element (controller)

➢ Final Control Element (I/P transducer/valve)

➢ Process (the heat exchanger)

Chen CL 10

⇓

Chen CL 11

Basic Concept of Process ControlSummary

➢ Process Control:

adjusting a Manipulated Variable ( MV)

to maintain the Controlled Variable ( CV)

at desired operating value ( Set Point) ( SP)

in the presence of output Disturbances ( Ds)

Chen CL 12

Control StrategiesFeedback Control

➢ Adjusting MV if CV is not equal to SP

➢ Advantage: simple, can compensate all disturbances

➢ Disadvantage: CV is not equal to SP in most time

Chen CL 13

Control StrategiesFeed-forward Control

➢ Adjusting MV to compensate influence of multiple Ds on CV

➢ Advantage:

simultaneously consider influence of multiple Ds and MV on CVdetecting Ds ⇒ adjusting MV BEFORE CV deviates from SP

➢ Disadv.s: modeling error ?; not considering ALL disturbances ?

Chen CL 14

Control StrategiesFeed-forward Control with Feedback Trim

Chen CL 15

Incentives for Chemical Process Control

➢ Safety:temperature, pressure, concentration of chemicals

should be within allowable limits

➢ Production Specifications:a plant should produce desired amounts and quality of final products

➢ Environmental Regulations:various laws specify concentrations of chemicals of effluent from a

plant be within certain limits

Chen CL 16

Incentives for Chemical Process Control

➢ Operational Constraints:various types of equipment have constraints inherent to their

operation

➢ Economics:operating conditions are controlled at given optimum levels of

minimum operating cost and maximum profit

Chen CL 17

Design Aspects of A Process Control System

Define Control Objectives:

Q 1: What are the operational objectives that a control system is calledupon to achieve ?

☞ Ensuring stability of the process, or

☞ Suppressing the influence of external disturbances, or

☞ Optimizing the economic performance of a plant, or

☞ A combination of the above

Select Measurements:

Q 2: What variables should we measure to monitor the operationalperformance of a plant ?

Chen CL 18

Design Aspects of A Process Control System

Select Manipulated Variables:

Q 3: What are the manipulated variables to be used to control achemical process ?

Select Control Configuration: (control structure)

Q 4: What is the best control configuration for a given chemicalprocess control situation ?

☞ Feedback control ⇒ cascade ? override ? · · ·☞ Feedforward control ⇒ feedback trim ?

☞ Inferential control

Chen CL 19

Design Aspects of A Process Control System

Design the Controller: (control law)

Q 5: How is the information, taken from the measurements, used toadjust the values of the manipulated variables ?

☞ Control law (controller structure, P - PI - PID ?)

☞ Controller tuning (Kc, τI, τ

D?)

Chen CL 20

Why Laplace Transform

➢ Ex: PID Controller 4 signals ⇒ 2 signals

PID Controller: u(t) = Kc

[e(t) + 1

TI

∫ t

0

e(τ)dτ + TDde(t)dt

]+ ub

Steady States: u = Kc

[e + 1

TI

∫ t

0

e dτ + TDde

dt

]+ ub [u = ub; e = 0]

Deviation Variables: U(t) = Kc

[E(t) + 1

TI

∫ t

0

E(τ)dτ + TDdE(t)

dt

]U(t) ≡ u(t)− ub; E(t) = e(t)− 0)

Chen CL 21

Why Laplace Transform

➢ PID Controller: (cont)

Laplace Transform: U(s) = Kc

[E(s) + 1

TI

E(s)s

+ TDsE(s)]

= Kc

[1 + 1

TI

1s + TDs

]E(s)

Transfer Function:U(s)E(s)

= Kc

[1 + 1

TI

1s + TDs

]≡ Gc(s)

Chen CL 22

Why Laplace Transform

➢ Ex: Simple Process 3 signals ⇒ 2 signals

First-Order Model: T dy(t)dt + y(t) = Ku(t− d)

Steady States: T dydt + y = Ku

Deviation Variables: T d[y(t)−y]dt + [y(t)− y] = K[u(t− d)− u]

TdY (t)

dt+ Y (t) = KU(t− d)

Laplace Transform: TsY (s) + Y (s) = KU(s)e−ds

Transfer Function:Y (s)U(s)

=Ke−ds

Ts + 1≡ Gp(s)

Chen CL 23

Why Laplace Transform

⇓

➢ Dynamic relation (ysp to y): time-domain vs. s-domain

☞ Time domain: simultaneous dynamic equations

☞ S-domain:y(s)

ysp(s)=

GcGp

1 + GcGp