INTRODUCTION TO PROCESS CONTROL

CHAPTER 1

Compiled by : Mohammad Bin AbdullahCHE 314 Basic Instrumentation and Control

Definition of Process ControlControl SystemProcess Control Block DiagramContinuous and Discrete Process ControlAnalog and Digital ProcessingPiping & Instrumentation Diagram (P&ID)Feedback, Feedforward, and Cascade Controls SystemOff-On Control, Proportion, Integral and Derivative Control ModeResponse of Control System

Topic Covered

Definition of Process ControlWhat do you understand about :

1. PROCESS2. INSTRUMENTATION3. CONTROL

Definition of Process ControlProcess

material -------------------------------------> product mechanical, electrical, physical, chemical

Control Instrumentation = device of

control

Why Control is ImportantSafetyProduct specificationEnvironmental regulationsOperation ConstraintsEconomics

Why do you need process control?SafetyPressureTemperatureProduct quality specifications and production rateMaintain specifications of product (no oscillation)Environmental RegulationsFlow rates of effluents from plants must be within allowable limits

Why do you need process control?Operational constraintsTanks must not overflowDistillation columns must not be floodedEconomicsEconomical utilization of raw materials, energy, capital, human labor

Control SystemProcess control principleThe ProcessSelf-regulationQin = Q out Example: while trouble-shooting the automatic control faulty occurred at a tank by control room personnel

Human Aided ControlThe equipment has the local gauge/ side tubeControlled variableControlling/ manipulated variableExample: while trouble shooting the faulty of automatic control of a tank by site personnel

Control SystemProcess control principle

Automatic controlMachine, electronic, or computer replace human operations = proportional signal of sensoru = output signal of controller to actuator

Control SystemProcess control principle

Control SystemDiscrete-state Control SystemConcern with controlling a sequence of eventsExample: chemical packagingWeight of chemicalDrying rateTemperature of dryingPouring into the bagSewing the bag

Process-Control Block DiagramParts of Control System

Analog & Digital ProcessingAnalog ~ continuousDigital computer ~ discreteOn/Off Control majorityData representation

Analog & Digital ProcessingSmart sensor

device with small housing contains complete set of measurement and control including analog digital or digital analog converter. Programmable Logic Controller (PLC)

Discrete-state controlDirect digital control

Piping & Instrumentation Diagram (P&ID)Essential elements

specific symbol and tagANSI and ISAInstrument line symbol

Current, pneumatic, electrical, and digital data feedInstrument symbolPLEASE REFER APP.5 Process Control Instrumentation Technology

Positive and Negative Feedback, Feedforward and Cascade Controls Systems

Chapter 4

Chapter 4Positive FeedbackNegative Feedback

Chapter 4

Feedback controlAdvantages:Corrective action occurs as soon as the controlled variable deviates from the set point.Feedback control requires minimal knowledge about the process to be controlled; it particular, a mathematical model of the process is not required, although it can be very useful for control system design.

Chapter 4

Feedback controlDisadvantages:No corrective action is taken until after a deviation in the controlled variable occurs.It may not be satisfactory for processes with large time constants and/or long time delays.In some situations, the controlled variable cannot be measured on-line.

Chapter 4

Feedforward ControlAdvantage:Measure important disturbance variables and take corrective action before they upset the process.

Disadvantages:The disturbance variables must be measured on-line.The quality of feedforward control depends on the accuracy of the process model.

Chapter 4

Cascade Control

An exothermic chemical reactorChapter 4

Simple feedback controlChapter 4

Simple feedback controlMeasure T and manipulate coolant flowrate.T will response faster when changes in inlet temperature, Ti than to changes in coolant temperature, Tc.Therefore, the feedback controller effective in compensating for changes in Ti and less effective in compensating for changes in Tc.

Chapter 4

Chapter 4Cascade control

Cascade ControlMeasuring Tc and taking the control action before its effect has been felt by the reacting mixture.If Tc increase, increase the coolant flowrate to remove the same amount of heat.

Chapter 4

Feedback control configuration

~ one measurement and one manipulated variable in a single loop.

Cascade control

~ More than one measurement and one manipulated variable.~ Useful when the disturbances are associated with the manipulated variable.~ Two control loops using two different measurement but sharing the same manipulated variable.Chapter 4

Cascade controlThe output signal of the primary controller serves as the set point for the secondary controllerThe two feedback control loops are nested, with the secondary loop inside the primary loop. There are two controlled variable, two sensor and one manipulated variable.

Chapter 4

Simple Feedback ControlChapter 4

Cascade ControlChapter 4

Cascade Control (multi-loop)

Distinguishing features:Two FB controllers but only a single control valve (or other -final control element).

2. Output signal of the "master" controller is the set-point for slave" controller.3. Two FB control loops are "nested" with the "slave" (or "secondary") control loop inside the "master" (or "primary") control loop.

Terminology

slave vs. mastersecondary vs. primary inner vs. outerChapter 4

Chapter 4

On off, Proportional, Integral, Derivative Control modes

Topic CoveredOn off controlProportional controlProportional-Derivative controlProportional-Integral controlProportional-Integral-Derivative controlGeneral tips for designing a PID controller

Topic OutcomeAble to understand the characteristics of the each of proportional (P), the integral (I), and the derivative (D) controls.Able to use them to obtain a desired response.

IntroductionConsider the following unity feedback system: Plant: A system to be controlledController: Provides the excitation for the plant; Designed to control the overall system behavior

On off controla thermostat is a simple negative-feedback control: when the temperature (the "measured variable" or MV) goes below a set point (SP), the heater is switched on. Another example could be a pressure-switch on an air compressor: when the pressure (MV) drops below the threshold (SP), the pump is powered.

The three-term controllerKp = Proportional gain Ki = Integral gain Kd = Derivative gain

The characteristics of P, I, and D controllers The goal of this problem is to show you how each of Kp, Ki and Kd contributes to obtain:

Fast rise time Minimum overshoot No steady-state error

Open-loop step responseThe rise time is about one second, and the settling time is about 1.5 seconds. Let's design a controller that will reduce the rise time, reduce the settling time, and eliminates the steady-state error.

Self-regulated

Proportional control The proportional controller reduced both the rise time and the steady-state error, increased the overshoot, and decreased the settling time by small amount.

Proportional-Derivative controlThis plot shows that the derivative controller reduced both the overshoot and the settling time, and had small effect on the rise time and the steady-state error.

Proportional-Integral controlWe have reduced the proportional gain (Kp) because the integral controller also reduces the rise time and increases the overshoot as the proportional controller does (double effect). The above response shows that the integral controller eliminated the steady-state error.

Proportional-Integral-Derivative controlNow, with PID gain, we have obtained the system with no overshoot, fast rise time, and no steady-state error.

General tips for tuning a PID controller Obtain an open-loop response and determine what needs to be improved Add a proportional control to improve the rise time Add a derivative control to improve the overshoot Add an integral control to eliminate the steady-state error Adjust each of Kp, Ki, and Kd until you obtain a desired overall response.

You can always refer to the table shown in this "PID Tutorial" page to find out which controller controls what characteristics.

SummaryLastly, please keep in mind that you do not need to implement all three controllers (proportional, derivative, and integral) into a single system, if not necessary. For example, if a PI controller gives a good enough response (like the above example), then you don't need to implement derivative controller to the system. Keep the controller as simple as possible.