Control Systems Engineering(Chapter 1. Introduction)

Prof. Kwang-Chun Hokwangho@hansung.ac.krTel: 02-760-4253 Fax:02-760-4435

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Control System Definition

What is control system?A control systems is an integral part of our society It is used in an automatically controlled systems

Definition:A Control System consists of subsystems and

processes (or plants) assembled to control the outputs of a process.

Control System

Subsystem

Process

Control Output of the process

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Purpose of building Control System

We build control systems for four primary reasonsPower amplificationRemote control

Rover was built to work in contaminated areas at Three Mile Island where a nuclear accident occurred in 1979.

Convenience of input formCompensation for

disturbances

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Elevators

Early elevators were controlled by hand ropes or an elevator operatorHere, a rope is cut to demonstrate the safety brake, an innovation in early elevators

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Elevators

Modern Duo-lift elevators make their way up the Grande Arche in Paris, driven by one motor, with each car counterbalancing the other. Today, elevators are fully automatic, using control systems to regulate position and velocity.

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Other examples

ASIMO-created by Honda

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Car cruise control http://auto.howstuffworks.com/cruise-control3.htm

Other examples

CablesElectronically-

controlled Vacuum actuator

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Other examples

Anti-lock braking system (ABS)Major components of the typical ABS system

four speed sensors (one at each wheel)electronic control unit (ABS computer) a hydraulic control unit

http://www.samarins.com/glossary/abs.html

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Other examples

Vehicle suspension system a system that will absorb the energy of the vertically

accelerated wheel, allowing the frame and body to ride undisturbed while the wheels follow bumps in the road

http://auto.howstuffworks.com/car-suspension.htm

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Response characteristics

Input: Our desired responseOutput: The actual response that we get from the systemE.g. Using the elevator

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Response characteristics

Elevator:We need to push the button of our

desired floorElevator rise to the floor with speed

and floor-leveling accuracy that is comfortable to the passenger

Input Push of the floor buttonCan be represented by a step

command

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Response characteristics

Consider a control system for an elevator The input is a step function instructing the elevator to

go to a higher floor(4th floor) The output is a transient response plus a steady-state

response and has a steady-state error

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Response characteristics

The elevator undergoes a gradual change as it rises from the ground to our selected floorWe call this part of the response the transient response If this response is too fast or slow, passenger comfort is

sacrificedSteady state response: Approximation to the commanded or

desired response This response occurs when the elevator reaches our desired

floorSteady state error: The difference between the input and the

outputOften steady-state error is inherent in the designed system It is up to the engineer to determines whether or not that error

leads to significant degradation of systems function E.g. Accuracy of the elevator’s leveling with the floor

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Type of Control Systems

Open-Loop SystemsAn open-loop system cannot compensate for any

disturbances that add to the controller’s driving signal or to the process output

An open-loop system has no feedback path

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Type of Control Systems

Missile launcher system

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Type of Control Systems

Advantage of Open-loop systemSimpleCheap

Disadvantage of Open-loop systemSensitive to disturbance Inability to correct for disturbance

Solution: Use closed-loop system

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Type of Control Systems

Closed-Loop(Feedback Control) SystemA closed-loop system can compensate for

disturbances by measuring the output, comparing it to the desired output, and driving the difference toward zero

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Type of Control Systems

Missile launcher system

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Type of Control Systems

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Type of Control Systems

Advantage of Closed-loop systemLess sensitive to noise, disturbances and changes in

environmentTransient response and steady-state error can be controlled

easily Transient response and steady-state error can be controlled by

redesigning the controller The process of redesigning is called compensating the system

and the resulting hardware is a compensator

Disadvantage of Closed-loop systemMore complexMore expensive

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Analysis and Design Objectives

3 major objectives Transient Response must meet certain criteria

Elevator Slow transient response makes passengers impatientRapid transient response makes them uncomfortable.

Steady-State Response must meet certain criteriaAn elevator must be level enough with the floor for the

passengers to exit

The system must have StabilityTotal response of a system is the sum of natural response

and forced response: A useful control system has a natural response that

Eventually approaches zero, leaving only the forced responseOscillates with exponential decrease

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Control System Design Process

Other ConsiderationsHardware limitations FinancesRobust Design

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Mathematical Models

Model the system mathematically using physical lawsKirchhoff’s Voltage Law

The sum of voltages around a closed path is zero

Kirchhoff’s Current LawThe sum of currents flowing from a node is zero

Newton’s LawsThe sum of forces on a body is zero (considering mass times

acceleration as a force)The sum of moments on a body is zero

The model describes the relationship between the input and the output of the dynamic system

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Mathematical Models: Three Models

Linear, time-invariant differential equation.Transfer function written using the Laplace transform.State-Space modelAn n-th order differential equation is represented as n

simultaneous first order differential equations in matrix form

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Test waveforms used in control systems

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Meiling CHEN

Homework Assignment #1

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Meiling CHEN

Homework Assignment #1