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Control Systems Engineering (Chapter 1. Introduction) Prof. Kwang-Chun Ho [email protected] Tel: 02-760-4253 Fax:02-760-4435
Control Systems Engineering(Chapter 1. Introduction)
Prof. Kwang-Chun [email protected]: 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 (or
controller) 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 Stability
Total 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 zeroKirchhoff’s Current Law
The sum of currents flowing from a node is zeroNewton’s Laws
The 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 equationTransfer function written using the Laplace transformState-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
Usage methods will be discussed in details at “Chapter 7. Steady-State Errors” !!
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Implementation of Control Systems
Controller is a rational polynomial of s
Transfer function of control systems
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Implementation of Control Systems
Can create an equivalent block diagram of the system
How can the controller be physically implemented ?
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Implementation of Control Systems
Can use OpAmps to built control systems
Construct analog PID circuit board (PCB) by using OrCAD including Pspice
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Implementation of Control Systems
Design of digital control systems
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Implementation of Control Systems
Design example
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Implementation of Control Systems
Use microcontroller to control the plant
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Implementation of Control Systems
Comparison between analog and digital control systems
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Homework Assignment #1
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Homework Assignment #1
