01 basic concept of process control
DESCRIPTION
01 Basic Concept of Process ControlTRANSCRIPT
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