1. Control Systems

8/10/2019 1. Control Systems

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FEEDBACK AND

CONTROL SYSTEMEE 179


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Control Making subject X do task Y.

Manual Control

Has human-machine

interface. e.g.:

Driving a car

Manipulating a crane

Turning a voltage supply

to a desired level.

Automatic Control

Machine-machine

interface. e.g.:

Rice cooker

Thermostat

Disk drives

Aircraft control Satellites

Moon landing


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Control Systems

Consists of subsystems and processes assembled for thepurpose of controlling the outputs of the processes.

Example:

Automatic Aircraft Landing System (ALS): Radar Unit (sensors)measures the aircraft position

Controllerdecides the aircraft commands based on the radar output.

Aircraftexecute commands sent from the controller.

Radar Controller Transmitter


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Response Characteristics

Control Systems provides output/response from a given

stimulus

Ex. When the 4thfloor button of an elevator is pushed

from the ground floor, the elevator rises with a passenger-

comfortable speed and accuracy.


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Response Characteristics (cont.)

2. There is a difference in the input and the output.

After the transient response, the physical system approaches to steady-

state response.

Caused by the accuracy of the elevator leveling with the floor.

The difference is called the steady-state error.

Steady-state errors:

Does exist not only in defective systems.

Often inherent in the designed systems.

The control systems engineer determines if error is tolerable or not.


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System Configuration:

Open-Loop Systems

Does not compensate for disturbance

Only commanded by the input More stable

Can only perform well if calibrated

Easier to build


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System Configuration:

Closed-Loop Systems

Can compensate for disturbance

Input signal is subtracted by the Output signal to produce the

actuating/error signal

Less sensitive to noise

Requires output transducers

More complex and expensive than open-loop systems


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Computer-Controlled Systems Controller or compensator is a digital computer Multiple loops can be controlled or compensated through time

sharing

Adjustments (ex. calibration or changes in the design) can bemade in the software.

Additional intelligent functions such as scheduling can be addedto the system through programming.


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Analysis and Design Objectives

Analysis: determines the systems performance.

In the elevator example, we found out responses from the input.

Comparing these responses to a specification is a form of analysis.

Design: systems performance is created or changed We can design a system based on a desired specification.

Redesigning maybe required if the responses does not meet the

specifications.


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Analysis and Design Objective: Transient

Response Slow transient response in the elevator example makes

passenger impatient.

Very rapid response makes them uncomfortable

Oscillating response for a certain duration results todisconcerting feeling.

Not meeting a transient response specification may result

to physical damage.

Parameters and components can be adjusted or

redesigned to yield desired transient response.


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Analysis and Design Objective: Steady-

State Response Response after the transients have decayed to zero

The concern is on the accuracy of the steady-state

response.

Imagine in the elevator example, the input is 4

th

floor andthe elevator stops near 4thfloor (in between 3rdand 4th

floor)

Steady-state error can be reduced by designing acorrective action.


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Analysis and Design Objective: Stability

Analysis and design of transient and steady-state

responses are useless if the system is not stable.

The total response of a system is the sum of the natural

and forced responses.

A system is stable if:

The natural response approach to zero. Leaving only forced

response.

Or Oscillate.

If the natural response grows without bounds, the

systems is unstable.


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Analysis and Design Objective: Others

Factors affecting hardware selectionhardware specs

must be considered in the design.

Financial considerationbudget may affect the design.

Robustnesssystem is not sensitive to parameter

changes.


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Design Process

Develop a

Mathematical

Model

From the

schematic


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(Case Study) Antenna Azimuth: An

Introduction to Position Control SystemsPosition Control

Converts a position input command to a position output

response.

Used in antennas, robot arms, and computer disk drives.


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Introduction to Position Control SystemsStep 1. Transform the requirements to physical system.

What is to be designed?

What are the design specifications?


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Introduction to Position Control SystemsStep 2. Draw a functional block diagram.

Translate the qualitative description to functional block

diagram

Identify components and their interconnections.

Includes possible

hardware descriptions.

a detailed layout if

possible.


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Introduction to Position Control SystemsStep 4. Develop a mathematical model.

Use physical laws on the schematic. Electrical systems:

KirchoffsVoltage Law

KirchoffsCurrent Law

Mechanical System:

Newtons Law

These laws leads to mathematical models that describesthe input and output relationship of dynamic systems.

ex. A linear, time-invariant differential equation.


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Introduction to Position Control SystemsStep 5. Reduce the block diagram. A large system may have multiple interconnected subsystems each described

by a mathematical model.

In order to simplify the analysis and design, the system should be reduced to

a single block that represents the system from its input and output.

A system with multiple

subsystems


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Introduction to Position Control SystemsStep 6.Analysis and Design.

Skip the step 5 if the interest in the performance of each

subsystem.

System response and performance are analyzed and

compared to the response specifications and performance

requirements.

If the above fails, the designer redesigns or add

hardware/software to achieve desired performance.

Test input signals are used.

Standard inputs signals: impulses, steps, ramps,

parabolas, and sinusoids.


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Documents

1. Control Systems