control in instrumentation - ferdowsi university...

lecture 2

Ali Karimpour Sep 2015

Ali Karimpour

Associate Professor

Ferdowsi University of Mashhad

CONTROL IN

INSTRUMENTATION

References:

1- Modern Control Technologies: Components and Systems, 2nd Edition, by Kilian,

Delmar Publication Co, 2005

2- Power Points of Dr. Hamed Molla Ahmadian

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Lecture 2

An Introduction to Control in

Instrumentation

Topics to be covered include:

v Introduction

v Block Diagram

v Open-loop and Closed-loop Systems

v Transfer Functions

v Analog and Digital Control Systems

v Classification of Control Systems

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An Introduction to Control in

Instrumentation

v Introduction.

v Block Diagram.

v Open-loop and Closed-loop Systems.

v Transfer Functions.

v Analog and Digital Control Systems.

v Classification of Control Systems.

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Introduction

Control System An interconnection of components forming a system

configuration that will provide a desired response.

Control is the process of causing a system variable such as temperature to

conform to some desired value.

System An interconnection of elements and devices for a desired purpose.

Controller In most cases, electronic circuits provide the intelligence, and

electromechanical components such as sensors and motors provide the

interface to the physical world.

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Introduction: Sensor and Actuator

A simple Governor

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Introduction: Sensor and Actuator

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Water-level float regulator (before BC)

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Introduction: Earlier Control Systems

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Human System

i. Pancreas

Regulates blood glucose level

ii. Adrenaline

Automatically generated to increase the heart rate and oxygen in times of flight

iii. Eye

Follow moving object

iv. Hand

Pick up an object and place it at a predetermined location

v. Temperature

Regulated temperature of 36°C to 37°C

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Introduction: Transportation

Car and Driver

v Objective: To control direction and speed of car

v Outputs: Actual direction and speed of car

v Control inputs: Road markings and speed signs

v Disturbances: Road surface and grade, wind, obstacles

v Possible subsystems: The car alone, power steering system, breaking

system 8

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Introduction: Transportation

Functional block diagram:

Time response:Measurement, visual and tactile

Steering

MechanismAutomobileDriver

Desired

course

of travel

Actual

course

of travelError+

-

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Sensors are the eyes of control enabling one to see

what is going on. Indeed, one statement that is

sometimes made about control is:

If you can measure it, you can control it.

As an example consider the altitude sensor in an

aeroplane or the temperature in a room.

Introduction: Sensor and Actuator

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Once sensors are in place to report on the state of a

process, then the next issue is the ability to affect, or

actuate, the system in order to move the process

from the current state to a desired state.

As an example consider the ballet in an aeroplane or

the fan in a room.

Introduction: Sensor and Actuator

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In summary:

Sensors provide the eyes and actuators the muscle

but control science provides the finesse.

Introduction: Sensor and Actuator

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Better Control(Computing)Provides more finesse by combining sensors and

actuators in more intelligent ways

Better ActuatorsProvide more Muscle

Better Sensors

Provide better Vision

Introduction: Sensor and Actuator

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An Introduction to Control in

Instrumentation

v Introduction.

v Block Diagram.

v Open-loop and Closed-loop Systems.

v Transfer Functions.

v Analog and Digital Control Systems.

v Classification of Control Systems.

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Block Diagram

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v Block Diagram of a Control System

v Open Loop Control

v Closed Loop Control

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v An open-loop control system utilizes an actuating device to control the

process directly without using feedback.

v A closed-loop feedback control system uses a measurement of the

output and feedback of the output signal to compare it with the desired

output or reference.

Actuating

DeviceProcess Output

Desired Output

Response

Desired

Output

Response

Measurement

OutputController ProcessComparison

Open-loop and Closed-loop System

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Open-loop and Closed-loop System

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Open-loop System

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Open-loop and Closed-loop System

Closed-loop System

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An Introduction to Control in

Instrumentation

v Introduction.

v Block Diagram.

v Open-loop and Closed-loop Systems.

v Transfer Functions.

v Analog and Digital Control Systems.

v Classification of Control Systems.

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Transfer Functions

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Physically, a control system is a collection of components and

circuits connected together to perform a useful function.

Each component in the system converts energy from one form to

another; (for example, temperature sensor or a motor)

A transfer function (TF) is a mathematical relationship between

the input and output of a control system component.

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Transfer Functions

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A System With three TF:

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An Introduction to Control in

Instrumentation

v Introduction.

v Block Diagram.

v Open-loop and Closed-loop Systems.

v Transfer Functions.

v Analog and Digital Control Systems.

v Classification of Control Systems.

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Analog and Digital Control Systems

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• A program executes over-and-over (iteration or scan).

• The total time for one pass through the program may be less than 1 millisecond.

• The digital system only “looks” at the inputs at a certain time.

In an analog control system, the controller consists of traditional

analog devices and circuits, that is, linear amplifiers.

• Older times it was the only available technology.

• Change in either set point or feedback is sensed immediately

In a digital control system, the controller uses a digital circuit. In

most cases, this circuit is actually a computer, usually microprocessor-

or microcontroller-based.

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Analog and Digital Control Systems

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A digital control system

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Analog and Digital Control Systems

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Analog Control

v Advantages:

1. Speed of Operation

2. Simplicity for many cases

3. No need DAC and ADC

v Disadvantages:

1. Noise

2. Hardware Implementation and Limitations

3. Low Flexibility

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An Introduction to Control in

Instrumentation

v Introduction.

v Block Diagram.

v Open-loop and Closed-loop Systems.

v Transfer Functions.

v Analog and Digital Control Systems.

v Classification of Control Systems.

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Process control refers to a control system that oversees some

industrial process so that a uniform, correct output is maintained.

Classification of Control Systems

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Control system for a boiler of a thermal plant

Classification of Control Systems

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A typical industrial control problem will usually

involve many different actuators - see below:

Classification of Control Systems

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Classification of Control Systems

Manual Control

Automatic Flow Control

Automatic Color Control

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Classification of Control Systems

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v Approaches of Multi process Control:

u Individual local controllers

u Direct computer control

u Distributed computer control

Classification of Control Systems

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v Sequentially Controlled Systems: A sequentially controlled

system controls a process that is defined as a series of tasks to be

performed that is, a sequence of operations, one after the other.

Classification of Control Systems

• Time-driven (open-loop control)

• Event-driven (closed-loop control)

v Examples: washing Machine, Traffic Control and Packing Materials.

v Many Tasks one after one control by PLCs.

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Classification of Control Systems

• Such a system typically includes a motor, mechanical parts that move,

and (in many cases) feedback sensor(s).

• Examples: Automatic assembling machines, industrial robots and

numerical control machines.

v Motion Control: Motion control is a broad term used to describe an open-

loop or closed-loop electromechanical system wherein things are moving.

• It has some categories such as:

• Servomechanisms,

• Numerical Control,

• Robotics.

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Classification of Control Systems

Servomechanism is the traditional term applied to describe a closed-

loop electromechanical control system that directs the precise

movement of a physical object such as a radar antenna or robot arm.

A remote antenna

Positioning system.

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Classification of Control Systems

Numerical control (NC) is the type of digital control used on machine

tools such as lathes and milling machines.

These machines can automatically cut and shape the work piece

without a human operator.

Numerical control

milling machine.

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Classification of Control Systems

Industrial robots are classic examples of position control systems.

In most cases, the robot has a single arm with shoulder, elbow, and

wrist joints, as well as some kind of hand known as an end effector.

A pick-and-place robot

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Exercises

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1- In a certain system, an electric heating element was found to increase the

temperature of a piece of metal 10° for each ampere of current. The metal expands

0.001 in./deg and pushes on a load sensor which outputs 1 V/0.005 in. of

compression.

a. Find the transfer functions of the three components and draw the block diagram.

b. Calculate the overall transfer function of this system.

2- The iteration time of a digital controller is 1 s. Would this controller be

appropriate for the following?

a. A robot that paint sprays cars

b. A solar panel control system that tracks the sun across the sky

3- Give an example (other than in this lecture) of the following:

a. A time-driven control system

b. An event-driven control system

c. A combined time- and event-driven control system

4- Give an example (other than in this lecture) of a servomechanism.

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Exercises

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