control chap1
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TRANSCRIPT
CONTROL SYSTEMS THEORYAssessment Mini Project 20%Lab Report 20%Test 20%
Final Exam 40%Total 100%
Teaching planWeek Chapter
1 Introduction to control systems
2-3 System representation
4 Response analysis in time domain
5-7 Stability analysis in time domain
8 Test
9-10 Controller design
11-12 Response analysis in frequency domain
13-14 Stability analysis in frequency domain
CONTROL SYSTEMS THEORY
Basic Control Systems Concept
Chapter 1STB 35103
Objectives
To understand the concept of control systems engineering.
To evaluate the characteristic responses of a certain automatically controlled system.
Introduction
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 assembled for the purpose of controlling the output of the process
Control System
Subsystem
Process
Control Output of the process
Purpose of building control systemWe build control systems for four primary reasons
1. Power amplification
2. Remote control
3. Convenience of input form
4. Compensation from disturbances
Early elevators were controlled by hand ropes or an elevator operator. Here, a rope is cut to demonstrate the safety brake, an innovation in early elevators;
Photos courtesy of United Technologies Otis Elevator.
Figure 1.1 aElevators
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.
Figure 1.1 bElevators
Other examples ASIMO-created by Honda
Other examples Car cruise controlhttp://auto.howstuffworks.com/cruise-control3.htm
CablesElectronically-
controlled Vacuum actuator
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
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
Advantages & disadvantages?
Response characteristic
Input: Our desired response
Output: The actual response that we get from the system
E.g. Using the elevator.
Response characteristicElevator:
We need to push the button of our desired floor. Elevator rise to the floor with speed and floor-leveling accuracy that is comfortable to the passenger.
Input - Push of the floor button - can be represented by a
step command - represents what we
would like the output to be after the elevator stops
Figure 1.3Elevator input and output
Response characteristicTransient response:The state changes through a path that is related to the physical device and the way it acquires and dissipate energy.
E.g.The elevator undergoes a gradual change as it rises from the ground to our selected floor. We call this part of the response the transient response.
Figure 1.3Elevator input and output
Response characteristicSteady state response:Approximation to the commanded or desired response.
E.g.This response occurs when the elevator reaches our desired floor.
Response characteristicSteady state error:The difference between the input and the output. Often 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.
Type of systemsControl system configuration can be categorized into two:
1)Open-Loop
2)Closed-Loop
Control SystemInput Output
Control SystemInput Output
Type of systemsOpen-loop system
A generic open-loop system
Input transducer: Converts the form of input to that used by controller
Controller: Drives a process or plant
Type of systemsOpen-loop system
A generic open-loop system
Plant: Air conditioning system
Controller: Remote control
Input: ???
Output: ???
Type of systemsOpen-loop system
A generic open-loop system
Open loop-system characteristic
Cannot compensate for any disturbances that add to the controller’s driving signal.
Signal that drives plant is corrupted by disturbance. The output is also corrupted.
Open loop control Missile launcher system
Type of systemsOpen-loop system
Advantage ofOpen-loop system
Simple Cheap
Type of systemsOpen-loop system
Solution
Use closed-loop system
Disadvantage ofOpen-loop system
Sensitive todisturbance
Inability to correctfor disturbance
Type of systemsClosed-loop system
A generic closed-loop system
Type of systemsClosed-loop system
Input transducer:
Converts the form of the input to the form used by controller.
Output transducer (sensor):
Measures the output response and converts it into the form used by the controller.
Type of systemsClosed-loop system
Output transducer (sensor):
Measures the output response and converts it into the form used by the controller.
E.g.
Valves of a temperature control system:
Input position
+ electrical signals
output temperature
Type of systemsClosed-loop system
Valves of a temperature control system:
Input position potentiometer (variable resistor)
output temperature thermistor Electrical resistance changes with temperature
Type of systemsClosed-loop system
A generic closed-loop system
Type of systemsClosed-loop system
First summing junction adds the signal from the input to the signal from the output which arrives via the feedback path (return path from output to summing junction).
Type of systemsClosed-loop system
Actuating signal: Input and output have different gain
Error: Input and output transducers have unity gain (transducers amplifies its input by 1)
Type of systemsClosed-loop system
Closed-loop system compensates for disturbance by:
•Measure output response
•Feeding the measurement back to summing junction
•If there is difference between two responses, the system drives the plant. If no difference, the system does not drive the plant.
Closed loop feedback control Missile launcher system
Type of systemsClosed-loop system
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
Advantage ofClosed-loop system
Less sensitive to noise, disturbances
and changes inenvironment
Transient responseand steady-state error
can be controlled easily
Type of systemsClosed-loop system
Disadvantage ofClosed-loop system
More complex More expensive
Type of systemsOpen loop vs. Closed-loop system
How do we choose OL or CL? Criticality of application
The need to monitor output The need to control the output The need for reduced error or zero error Cost / budget Safety
Analysis and design objectives
3 majorobjectives
TransientResponse
Steady-stateresponse
Stability
Analysis and design objectivesTransient response
A very important aspect in control systems.
E.g.
Elevator. Slow transient response makes passengers impatient. Rapid transient response makes them uncomfortable.
Analysis and design objectivesSteady-state response
This response resembles the input. We are concerned about the accuracy of the steady-state response.
E.g.
An elevator must be level enough with the floor for the passengers to exit.
Analysis and design objectivesStability
Total response of a system is the sum of natural response and forced response.
Total response = Natural response + Forced response
A useful control system has a natural response that
1. Eventually approaches zero, leaving only the forced response.
2. Oscillates.
forced naturalc t c t c t
Design process
Determine a physical system
and specifications from the
requirements
Draw functional
block diagram
Analyze, design and test to
ensure requirements
are met
Transform physical
system into a schematic
Reduce block
diagrams ifnecessary
Obtain block diagram,
signal flow diagram,
state-space representation