1 A Introduction to Process Control

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Introduction to Process Control

Practical Industrial Process Control:

Understanding, Tuning & Autotuning Control Loops Understanding, Tuning & Autotuning Control Loops

Edi Leksono

Department of Engineering Physics Institut Teknologi Bandung June 2003

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Introduction to Process Control

Training Objectives

 Introduction to process control

 Elements of process control loop

 Dynamic modelling

 Analysis of dynamic systems

 Design of P, PI, PD and PID for specific process objectives or product specifications

 Design of feedback, feedforward, cascade, feedforward/ feedback, feedforward/feedback + cascade controls

 Tuning & Autotuning

 Practical Troubleshooting

GOALS

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Introduction to Process Control

Road Map of the Training

Controller

Controller Process_Process

Sensor + TransmitterSensor +

Transmitter

+

Actuator

 First, we will visit all the block elements of the control system,especially the controller

 Then, analyze the whole system all together

 Then, consider the variations of the elements

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Introduction to Process Control

Time Table

Day

Time Day 1 Day 2 Day 3 Day 4 Day 5

08.00-09.30

Opening, Introduction to Industrial Process

Control

PID Control I of PID Controller Tuning Methods

+ Lab. III Cascade Control I

Feedback/ Feedforward Control + Lab

10.00-11.30 Actuators PID Control II Methods of PID Autotuning Controller + Lab.

Cascade Control II + Lab.

Feedback/ Feedforward + Cascade Control

+ Lab

12.30-14.00 Sensor/Transmitter, _Filtering of PID Controller Tuning Methods + Lab. I

Practical

Troubleshooting I Feedforward Control I Demo

14.30-16.00 Process Dynamic

Modelling

Tuning Methods of PID Controller

+ Lab. II

Practical

Troubleshooting II Control II + Lab.Feedforward

Final Test, Closing

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Introduction to Process Control

Introduction to

Process Control

Edi Leksono

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Introduction to Process Control

Session Outlines & Objectives

Outlines

 The importance of process control

 Basic concepts of process control

Objectives

 Understand what process control is

 Know the terms of process control system

 Identify the elements of process control system

 Understand the importance of process control

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Introduction to Process Control

Definition (1)

Process

Raw Materials Products Energies Out

Energies Out

 Process

• A series of interrelated actions which transform material

It covers all resources that are involved in the process and talks about process “inputs” (e.g. resources, raw material) and “outputs” (e.g.

finished product)

 Control

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Introduction to Process Control

Definition (2)

Corrective Action

Knowledge Data

Process

Information

 Process Control

• To maintain desired conditions in a physical system by adjusting selected

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Introduction to Process Control

Brain: Control calculation

Eyes: Sensor

Steering wheel: Actuator

Daylife Example: Driving a Car

 Control Objective (Setpoint):

• Maintain car in proper lane  Controlled variable:

• Location on the road  Manipulated variable:

• Orientation of the front wheels  Actuator:

• Steering wheel  Sensor:

• Driver’s eyes  Controller:

• Driver

 Disturbance:

• Curve in road  Noise:

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Introduction to Process Control

Feed Condensate Product

Stream Steam

TT

TC

Industrial Example #1: Heat Exchanger

• Maintain temperature  Controlled variable:

• Outlet temperature of product stream  Manipulated variable:

• Steam flow  Actuator:

• Control valve on steam line  Sensor:

• Thermocouple on product stream  Controller:

• Temperature controller  Disturbance:

• Changes in the inlet feed temperature  Noise:

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Introduction to Process Control

Industrial Example #2: Liquid Level Control

• Maintain level

 Controlled variable:

• Fluid level in the tank  Manipulated variable:

• Fluid flow  Actuator:

• Control valve on fluid line  Sensor:

• Level transmitter on the tank  Controller:

• Level controller  Disturbance:

• Changes in the inlet feed flow  Noise:

• Measurement noise

Fluid

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Introduction to Process Control

 Sensor

• Measure process variable

 Transmitter

• Convert the measured process variable into standard signal

 Controller

• Drive actuator by giving an appropriate controller output signal

 Actuator

• Adjust manipulated variable based on the value of the controller output

signal

 Process

• Physical system to be controlled

Elements of Process Control Loop

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Introduction to Process Control

The Terms I

 Control Objective (Setpoint, SP)

 Controlled Variable (CV) or Process Variable (PV)

 Measured Process Variable (PVm)  Controller Output (CO)

 Manipulated Variable (MV)

 Final Control Element (Actuator)

 Sensor/Transmitter

 Controller

 Disturbance Variable (DV)

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Introduction to Process Control

24 hours process operation? Hmm… I think, to achieve those, we need to continuously

monitor & control the process 24 hours a day,

7 days a week!!!

Goal of Process Operation

 Safety & Reliability

 Product Specification

 Environmental Regulation

 Operating Constraint

 Efficiency

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Introduction to Process Control

Safety and Reliability

 The control system must provide safe operation

• Alarms, safety constraint control, start-up and

shutdown

 A control system must be able to “absorb” a variety of disturbances and keep the

process in a good operating region

• Feed composition upsets, temporary loss of utilities

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Introduction to Process Control

Time Im pu ri ty C on ce nt ra tio n Limit Time Im pu ri ty C on ce nt ra tio n _Limit

Old Controller New Controller

 Quality

• Products with reduced variability

 For many cases, reduced variability products are in high demand and have high value added (e.g. feedstocks for polymers)

 Product certification procedures (e.g., ISO 9000) are used to guarantee product quality and place a large emphasis on process control

Product Specification

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Introduction to Process Control

Environmental Regulation

 Various government laws may specify that the

temperatures, concentrations of chemicals, and flow rates of the effluents from a process be within certain limit

Examples:

• Regulations on the amounts of SO2 that a process can eject to the

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Introduction to Process Control

Operational Constraint

 All real process have constrained inherent to their operation which should be satisfied throughout the operation

Examples:

• Tank should not overflow or go dry

• Distillation column should not be flooded

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Introduction to Process Control

Efficiency

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Introduction to Process Control

Maximizing the Profit of a Plant (1)

 The operation of a process may many times involves controlling against

constraints

 The closer that you are able to operate to these constraints, the more profit

you can make

Example:

• Maximizing the product production rate usually

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Introduction to Process Control

Time Im pu ri ty C on ce nt ra tio n _Limit Time Im pu ri ty C on ce nt ra tio n Limit

New Controller Improved Performance

Constraint control example: A reactor temperature control

 At excessively high temperatures the reactor will experience a temperature runaway and explode

 But the higher the temperature the greater the product yield

 Therefore, better reactor temperature control allows safe operation at a higher reactor temperature and thus more profit

Maximizing the Profit of a Plant (2)

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Introduction to Process Control

 1960s Pneumatic analog instrumentation, controllers, and computing modules

 1970s Electronic analog instrumentation, controllers, and computing modules

• Direct digital control with special algorithms programmed in main frame computer

 1980s Electronic analog instrumentation and digital distributed control systems (DCS)

• Supervisory and model predictive control configured in special purpose computers

 1990s Smart analog instrumentation, valves, and digital distributed control systems

• Supervisory and model predictive control configured in special purpose computers

• Neural networks, online diagnostics, and expert systems in special purpose computers

• Real time optimization using model libraries in special purpose computers

 2000s Field bus based digital smart instrumentation, valves, and control systems

• Digital bus takes full advantage of smartness and accuracy of instrumentation and valves

• Some fast PID controllers such as flow and pressure go to the field transmitter or valve

• Model predictive control, neural networks, online diagnostics, and expert systems are integrated into the graphically configurable field bus based control systems and move to PCs

• APC Infrastructure, interface, and engineering costs decrease by an order of magnitude

• APC projects use consultants more for front end and commissioning than for whole job

• APC software tools are easy enough for the average process and control engineer to use

The History of Process Control

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Introduction to Process Control

Common Types of Control Strategy

 Manual vs. Automatic

 Servo vs. Regulator

 Open-loop vs. Closed-loop

 Control strategies

• Feedback Control

• Feedforward Control

• Cascade Control

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Introduction to Process Control

Emergency cooling Temperature indicator

Should I adjust the valve or should I run?

Question: Why manual override has to be included in every automatic control systems?

Manual vs. Automatic

 Manual

• Human has to adjust the MV to

obtain the desired value of the PV based on observation and prior experiences

 Automatic

• The computer (or other device)

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Introduction to Process Control

75.5 C… 75.3 C… 75.4 C…

o o o

7.00 AM: 80 C… 8.00 AM: 70 C… 9.00 AM: 60 C…

o o o

Question: How to achieve both objectives simultaneously?

Regulator vs. Servo

 Regulatory control

• Follow constant setpoint,

overcoming the disturbance



Servo control

• Follow the changing

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Introduction to Process Control

DV PV CO Process Process Decisions Controller Controller SP DV PV CO Decisions Controller Controller Process Process SP

Open-loop vs. Closed-loop

 Open-loop

• Process is controlled based on

predetermined scenario

Ex.: When food is done in an oven, timers on outdoor lights

 Closed-loop

• The information from sensor is

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Introduction to Process Control

 Feedback Control

• Corrective action based on process variable (PV)

Advantage

Requires no knowledge of the source or nature of disturbances, and minimal detailed information about how the process itself works

Disadvantage

Controller takes some corrective actions after some changes occurs in process variable PV

DV

SP

PV

Feedback Controller

CO

Process

Control Strategies (1)

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Introduction to Process Control

 Feedforward Control

• Based on the measurement of disturbance (DV)  feedforward controller can respond even before any changes occurs in PV

Advantage

Controller takes some corrective actions before the process output is different from the setpoint  theoretically, perfect disturbance rejection is possible! Disadvantage

 Requires process model which can predict the effect of disturbance on PV

 If there are some modeling error, feedforward control action will be erroneous (no corrective action)

 Feedforward controller can be quite complex

DV

SP Feedforward CO PV

Controller

Feedforward

Controller ProcessProcess

Control Strategies (2)

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Introduction to Process Control

DV

SP _Feedforward/ CO PV Feedback

Controller

Feedforward/ Feedback Controller

Process

Control Strategies (3)

 Feedback/Feedforward Control

• Feedforward controller will adjust CO as soon as the DV is detected

• If the feedforward action is not enough due to model error, measurement

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Introduction to Process Control

DV

SP _CO _PV

Outer Feedback Controller Outer Feedback

Controller Inner FeedbackInner FeedbackController_Controller Process_ProcessInnerInner Process_ProcessOuterOuter

DV₁

CO Inner loop

Outer loop

Control Strategies (4)

 Cascade Control

• The disturbance DV1 arising within the inner loop are corrected by the

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Introduction to Process Control

DV

SP _CO _PV

Outer Feedback Controller Outer Feedback

Controller _{Inner Feedback} Controller Inner Feedback

Controller Process_ProcessInnerInner Process_ProcessOuterOuter

DV₁

CO Inner loop

Outer loop

Control Strategies (5)

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Introduction to Process Control

DV PV CO Decisions Controller Controller Process Process DVs PVs COs Decisions Controller Controller Process Process … … … … … … …… … …

SISO

MIMO

SISO vs. MIMO

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Introduction to Process Control

 Closeness to setpoint

 Short transient to one setpoint to other setpoint

 Smaller overshoot and less oscillation

 Smooth and minimum changes of variable manipulation

 Minimum usage of raw materials and energy 1

2

1, 2

1 2

Regulator Servo

Performances of Process Control System

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Introduction to Process Control

 Manual control

 Automatic control

 Open-loop control

 Closed-loop control

 Feedback control

 Feedforward control

 Cascade control

 Servo control

 Regulatory control

 SISO control

 MIMO control

 Transient response

 Overshoot

 Oscillation

The Terms II

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Development of a Control System (1)

1. Open Loop Analysis

• What kind of system is considered?

2. Performance Specifications

• How is the system required to behave?

 The desired performance must be expressed in terms of the different performance measures that are chosen

 Often, depends on the type of control problem to solve

3. Control Configuration

• Which signals are used to calculate the control signal?

 Depending on the plant the desired performance specifications and the allowed complexity of the control system

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Development of a Control System (2)

4. Control Law

• Which algorithm is used to calculate the control signal?

5. Parameter Design (Tuning)

• Which are the parameters of the algorithm to calculate the control signal?

6. Evaluation

• How will the controlled system behave in theory?  simulation!

7. Implementation and Verification

• How will the control system be realized?

• How does the controlled system behave in practice?

• The controller will be implemented and one will verify whether the system

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The Terms III

 Control law (algorithm)

 Parameter design (tuning)

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Session Summary

 Control has to do with adjusting manipulated variables of the process to maintain controlled variables at desired values

 All control loops have a controller, an actuator, a process, and a sensor/transmitter