Transcript Chapter 4. SYSTEM DESIGN

Chapter 4. SYSTEM DESIGN
Requirement Analysis
“ What is the problem to be solved? ”
System design
“ How will that problem be solved? ”
► Documentation (system specification)
In this chapter, we show how
the engineering process is
applied to elaborate the
techniques and structure for
doing system design.
Design for Electrical and Computer Engineers
Chapter 4. SYSTEM DESIGN
4.1 The Importance of System Design
• In system-engineering phase of a design,
→ Most of innovation and novelty originates.
→ Potential for outstanding performance is created. (cost, maintainablity, durability…).
• System designer (senior engineer) assigns tasks to circuit designer (junior engineer)
- Many engineering students are so excited that they skip or rush through the system engineering phase.
• Reasons for System-Level design.
1. To decide whether or not the problem is tractable.
2. To determine the performance limits of the design and whether or not these
limits are acceptable.
3. To get good estimates of the costs early in the project before investing too
heavily in the design of the product. (design cost and manufacturing cost)
4. To reduce the risk of the design not functioning properly.
5. To increase the reliability of the product.
6. To reduce the overall cost of developing the product.
7. To provide a framework for the organization and coordination of a team of
engineers to work on the design.
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Chapter 4. SYSTEM DESIGN
4.2 System Block Diagrams
• The dissection of a complex problem into smaller, more manageable problem
is the essence of engineering.
• Systems Engineering (= System design)
is the activity of building an entity (device or algorithm).
• System : A group of interconnected elements (Well defined function)
system
Ex : Automobile
subsystem
body
frame
engine
cooling system
exhaust system
suspension
component
(not subsystem)
muffler
tail pipe
exhaust pipe
manifold
• Block diagram
→ The diagram that shows exactly how the subsystems are connected.
• Block can have any shape
Ex : Amplifier
Multiplier
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4.2 System Block Diagrams
• Ex : A block diagram of 12-volt battery charger
• The diagram shows five function
→ Transformer, Full wave rectifier, Current limiter,
Power-on light and Ammeter
may need waveform
wire name
Block name
• The block diagram evolves as the system design progresses.
• The drawing of the block diagram is tightly linked to the system design process.
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Chapter 4. SYSTEM DESIGN
4.3 The System Design Process
• When we enter the system design stage,
we have Input the requirements specification.
• The output of the system design stage is
→ System specification
• A preliminary step is to determine
if a design is necessary ? : Search and learn
about what is available. How?
→ Search libraries and World Wide Web.
→ Search inside or outside experts.
synthesis
analysis
• If the problem has not been previously solved,
→ A system design is required in which the
solution is initially imagined in block
diagram form.
=> Systems engineering involves
: Conceptualization, Synthesis, Analysis,
Refinement, Documentation
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Chapter 4. SYSTEM DESIGN
4.3 The System Design Process (continued)
• Conceptualization
→ The objective is to develop a hazy perception of a solution.
→ Concepts are primitive solutions that do not have definite form or
character and lack in organization and structure.
• Synthesis
Block diagram
→ The objective is to create a well-defined structure for the concept.
→ The structure must be defined in sufficient detail to support analyses in areas of cost,
performance, and risk.
Ex : Synthesized block diagram of a simple
modulator of telephone network.
Ex : Battery Charger
Design for Electrical and Computer Engineers
Chapter 4. SYSTEM DESIGN
4.3 The System Design Process (continued)
• Analysis
→ The objective is to determine if the synthesized system will meet the performance and cost
objectives in the requirements specification.
→ A second objective is to determine the risk involved in carrying the design through the
detailed design and implementation stages.
Scientific methods for determining the performance of a system, including:
1. Develop a mathematical model for each of the blocks and analyze the system mathematically.
2. Simulate the system on a computer.
3. Lash together a laboratory version of the system using as many off-the-shelf components as
possible, then verify the performance through laboratory measurements.
Discrete components
(tr, Op-amo, …)
• Refinement
→ The objective is to modify the synthesized concept based on the information gained
through the analysis.
→ The engineer is in a position to synthesize a better structure after analyzing a first attempt.
→ Require several iterations of synthesis and analysis.
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Chapter 4. SYSTEM DESIGN
4.3 The System Design Process (continued)
• Documentation
→ Document the function of each block and explains how the blocks work together as a system.
→ This is an important part of the analysis and refinement activities.
→ Elements such as timing diagrams with latency specifications must be included.
Good design engineers, especially good systems engineers, develop creative
thinking skills through experience and practice.
4.3.1 Conceptualization
→ A new design begins with a concept or idea that is then methodically developed into a solution.
• There are two sources of ideas or concepts.
→ One is external : The engineer looks at concepts others have used to solve similar problems
and uses a concept similar to one of them.
→ The other is internal : The engineer thinks of an original concept, usually drawing on past
experience and knowledge of scientific principles.
• Thinking of an original concept is more difficult and more time consuming.
→ But, the original concept leads to a very high-performance or economical solution.
• Original concepts come out of creative thinking.
→ The engineer relaxes and searches his mind, recalling one by one the scientific principles that
have been committed to memory.
Design for Electrical and Computer Engineers
Chapter 4. SYSTEM DESIGN
4.3 The System Design Process
4.3.1 Conceptualization (example)
• The problem of measuring the speed of a baseball
1. A radio wave experiences a Doppler shift when it is reflected from a moving object.
2. The velocity of an object is inversely proportional to the time it takes to travel a set distance.
3. The size of the image in a video camera depends on the distance between the object and the camera.
4. The momentum of an object is proportional to its velocity.
5. Bernoulli’s law relates velocity to pressure.
6. The trajectory of a projectile in a gravitational field is parabolic, with the curvature depending on the
horizontal velocity of the projectile.
Which one is better? (Performance, cost, maintainabilty, reliability.)
To determine, you need knowledge.
That’s why you study hard!!!
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4.3.2 Synthesis : the process of bringing structure to the initial concept.
• There are two conflicting forces that drive the design process.
→ One is the need to have design completed quickly.
→ The other is the need for a novel solution that offers either a cost or performance
advantage over the competition.
• The goal is to interpolate or extrapolate a reference design to produce one that
solves the problem.
→ Adapting an existing design to fit the problem at hand is often done with
straightforward, logical reasoning.
• But, synthesis driven by linear thinking does not offer great hope for drastically
reducing cost or improving performance.
→ However, it is the most economical and reliable approach an the most commonly used.
4.3.3 Analysis : taught in most engineering classes.
• There are many tools available for analysis
→ Such as MATLAB for mathematical analysis and SIMULINK for the simulation
of block diagrams.
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4.3.4 The Synthesis/Analysis Cycle
• Refinement of the design and elaboration of the block diagram take place through an
iterative cycle of synthesis and analysis.
• To be successful, an engineer must be willing to synthesize structures for concept after
concept until one is found whose deficiencies can be corrected.
• The analysis of the structure is put on paper. This will be a logical argument supported
with mathematics, computer simulation, and laboratory tests.
When the structure is not good enough and the engineer is out of ideas for
improvement, if he or she decides to push on, there are three ways to proceed:
1. Go back to the structure first synthesized from the concept and look at modifying
it in a different way in hopes that the synthesis/analysis iterations will lead to
a structure that meets the cost and performance objectives.
2. Synthesize a new structure from the same concept.
3. Use creative thinking to conceive a new concept and then synthesize a structure used
on this concept.
Design for Electrical and Computer Engineers
Chapter 4. SYSTEM DESIGN
4.3.4 The Synthesis/Analysis Cycle (continued)
Better, but not good
enough. Discard.
Better, but not good
enough! Discard.
Revise a little
in (-) direction
Looks bad,
but try to
keep on.
Starting idea
Problem Solved!!
Happy!!
Revise a little
in + direction
Getting better!
Try more!
Start with entirely
new concept!!
Need Creativity!!
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4.4 Block-Diagram Basics -
How to build a block diagram?
Smudge: 더럽히다. 흐리게 하다
Concepts are discarded,
clumped paper are tossed into a waste
box,
or smudged areas checker the
background of the whiteboard.
Block diagrams
- the fruit of the systems-engineering stage of the design process.
- reflect the effort spent on and the quality of the system design.
- affect such things as the time to complete the paper design, the time
to debug the prototype, and the reliability of the finished product.
- a block should have a single purpose
- should be specified in such a way that detailed design of a block
can be completed by an individual engineer.
- Complex designs may require more than one layer of block diagram.
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Capstone (절정, 극치, 건축에서 마지막에 올리는 돌) Design
What is capstone design?
Capstone Design applies the engineering sciences to the design of a system,
component or process. Students choose the particular design project with
approval of appropriate faculty. Design teams are organized. Each project
includes the use of open-ended problems, development and use of design
methodology, formulation of design problem statements and specification,
consideration of alternative solutions, feasibility consideration and detailed
system descriptions. It also includes realistic constraints (such as economic
factors and social impact).
Capstone Design
공학계열의 학생이 이론과 실습위주의 교육에서 벗어나 실무
에서 접하는 문제를 해결할 수 있도록 학부에서 배운 지식을 바
탕으로 팀을 구성하고 제품을 기획, 설계, 제작하는 종합설계과
목이다.
Design for Electrical and Computer Engineers
Chapter 4. SYSTEM DESIGN
4.4 Block-Diagram Basics
• Suggestions using block diagrams to express design ideas:
1.
2.
3.
4.
5.
6.
7.
8.
The function of a block should be implementable with a single technology.
Common functions should be grouped into one block
Blocks should be defined so as to simplify the interfaces between them.
If possible, avoid feedback loops in the block diagram.
The engineers must also specify the voltage levels for the one/zero states.
Interface parameters are often included in block diagrams of analog design.
In RF block diagrams, signals between blocks are commonly specified.
Specification of timing and sequencing signals is a common requirement in
digital circuit design.
• Thoroughly annotating the block diagram cannot be overstated.
• Blocks and interconnecting signals.
→ Must take the time to develop good descriptive labels.
Design for Electrical and Computer Engineers
Chapter 4. SYSTEM DESIGN
4.5 Documentation
• Documents produced by the systems-engineering stage of the design process
= the system specification.
• The system specification serves several purposes:
1.
2.
3.
4.
5.
Used to complete the detailed design and implementation of the blocks in the block
diagram.
Stores the details of the systems-engineering effort.
Used as a reference for the design of future generations of the product.
Source of information for the engineers designing fixtures to test the final product.
Source of information to help marketing engineers develop manuals, and other
literature for advertising and technical support.
• To write the system specification
→ Everyone who needs information from the document can read and understand it.
Design for Electrical and Computer Engineers
Chapter 4. SYSTEM DESIGN
4.5 Documentation
System Specification: Table of Contents
1.
2.
3.
4.
5.
1.
2.
3.
4.
5.
The concept
Inputs/outputs and system block diagram
Specification of the blocks
3.1 Transformer
3.2 Full wave rectifier
3.3 Current limiter
3.4 Power-on light
3.5 Ammeter
System description
System analysis
The concept - Explain the principle of operation.
The block diagram - Comprises a well-annotated block diagram along with specification
of the inputs and outputs of the system.
Functional description of the blocks - Logically be divided into subsections, with a
subsection devoted to each of the blocks.
Description of the system - Describes how the blocks in the block diagram interact with
one another to make the system work.
System analysis - Consists mainly of the results of mathematical analysis and simulation
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4.5 Documentation – example of system specification
1. The concept
The battery charge is powered from a standard 110V household wall outlet. It produces positive voltage pulses that
exceed 12V and drives current into a 12 V automobile battery. The battery charger is based on a conventional
transformer and a full wave rectifier. It is not a DC power supply. Charging current is supplied only when the
rectified voltage is greater than the battery voltage.
2. Inputs/outputs and System specification
The input is the standard three-terminal wall outlet. The three wires are hot, neutral, and ground. The electric
potential between hot and neutral is 110 V RMS AC minimum and 120 V RMS AC maximum.
The output is two wires that end with spring-loaded claps that can grip battery terminals. One wire is
ground, also referred to as black: the other has electric potential in the form of a full wave-rectified 60Hz
sinusoid and is referred to a+14 or red. With no load, the peak value of the electric potential of red with respect
to black is at most 18 volts. The output current is short-circuit protected with a current limiter. The peak current
is limited to 10 amperes.
Typical waveforms for open-circuit and battery-loaded red-to-black electric potential are shown in Figure · ·
·. Typical waveforms for short-circuit and battery-loaded red-to-black current is shown in Figure · · · (the figures
are omitted, but the loaded red-to-black current could be that in Figure 4.3).
3. Specification of the blocks
3.1 Transformer: This block is a transformer with floating secondary. The input is the three-terminal wall outlet.
The output is two wires, which are referred to as “18V peak” and “grnd” in the block diagram of Figure4.2. The
Thevenin equivalent circuit of the output is a 60Hz voltage source with 0.118 times the electric potential of the
input in series with a 0.1Ω resistor.
Design for Electrical and Computer Engineers
4. System description – how the system of blocks works together
The transformer converts 110V RMS AC to 13V RMS AC. This is followed by a full wave rectifier which has a twowire output, one of which, the on labeled “+16” in Figure 4.2, is always positive with respect to the other wire. Since
the secondary of the transformer is floating, one wire of the output of the rectifier can be and is connected to ground.
The “+16” wire is fed through the current limiter for short circuit protection. The “power-on” light is located after
the current limiter. The light will be on only when the charge is plugged in and the transformer, full wave rectifier, and
current limiter are operational. The ammeter displays the DC current that flows through the red output.
5. System analysis
- The analysis of the final structure is recoded.
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Chapter 4. SYSTEM DESIGN
4.6 Example - Design a Flicker Analyzer
• A power utility(공익기업, 한전) has determined that an important measure of the quality
of the 60 Hz, 120 VAC supply in residential houses is the variation in supply voltage
over time. The variation is referred to as flicker. The power utility requires the design
of an instrument to measure it.
• Flicker analyzer
- Input =Voltage at the wall outlet
- must determine the RMS voltage of the flicker to an accuracy of ±0.01 VRMS.
• The wall-outlet voltage can be modeled by
 (t )  A cos(2 60t   )   f (t )  {some 60Hz harmonics}
Where, A : amplitude of supply voltage, between 155~170V
 f (t ) : variation supply voltage or “flicker”
• The instrument must also display the magnitude spectrum of  (t ) for
frequencies from 0.1 Hz~25 Hz.
• The power utility knows that  f (t ) is bounded by ±2 V, will never exceed
0.9 V RMS, and will not have frequency component over 25 Hz.
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Chapter 4. SYSTEM DESIGN
Example of System design process for flicker analyzer
Conceptualization
1.Decide whether to approach the problem from
the analog domain or the digital domain
2. How to display.
Synthesis
System design is to synthesize a solution in
terms of functional blocks.
Analysis
Mathematical analysis.
Refinement
Two approaches can be taken to refine the system
1.To increase the resolution of the A/D converter.
2. Estimate the spectrum of the quantization noise
and subtract it from the final spectrum.
Design for Electrical and Computer Engineers
Chapter 4. SYSTEM DESIGN
4.6 Example
Conceptualization
1. Decide whether to approach the problem from the analog domain or the digital domain.
→The calculation of the power spectrum will be very difficult in the analog domain
⇒Digital solution will be pursued.
2. How to display the power spectrum
→Monitor or matrix LCD panel, Laptop computer? → Laptop : cheap, DSP possible
⇒ Laptop computer will be used
3. Design Concept
→Digitize the input, send the digitized input to a laptop computer, then use the laptop to do the
signal processing and display the output.
Synthesis – What to consider
1. Direct connection?
2. How to connect with laptop?
3. How to separate frequencies?
4. Any problem in sampling?
5. Maximum input to ADC?
6. 8 bit or 12 bit?
7. Safety? Ground?
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Chapter 4. SYSTEM DESIGN
4.6 Example - Synthesis
• The output of the divider, which includes the
effect of implementation error can be expressed
as
divider (t ) 
v(t )

75(1   d ) d
: accounts for the
implementation error
• The anti-aliasing filter
→ Included to remove the 60Hz harmonics
→ Highest-frequency component is less than the Nyquist rate.
→ The pass-band gain of the anti-aliasing filter is (1+εa)
where, εa accounts for the implementation error.
→ The stop-band attenuation is set at 10dB at 120Hz, and at
60dB at 180Hz and all subsequent harmonics.
→ The maximum tolerable pass-band error will be determined
during the analysis.
→ The value for the stop-band attenuations will also be checked
and corrected during the analysis.
• The level shifter
→ Adds 2.5(1+ εd) VDC to shifter the AC signal in the
range 0 to 5 volts.
→ The maximum value for εl , which accounts for the
implementation error, will be determined in the analysis.
• The A/D converter block
→ Samples the signal, converts the sample to an 8-bit binary
number, and send each digitized sample to the laptop is
RS232 format.
→ An 8-bit A/D converter was chosen because they are readily
available and inexpensive.
→ Whether or not 8bit is sufficient resolution will be determined
in the analysis.
•The oscillator circuit
→ Generates the square clock needed by the sampler and A/D block.
→ The sampling frequency is chosen to be 200Hz.
→ This gives a Nyquist frequency of 100Hz, which is well above the maximum frequency in the flicker and
the fundamental 60Hz voltage.
→ Whether or not this sampling rate is a good choice will be determined in the analysis.
Design for Electrical and Computer Engineers
Chapter 4. SYSTEM DESIGN
4.6 Example
Analysis
• Mathematical expression for voltage at the output of the anti-aliasing filter is
(1  d )
(1  d )
(1  a )cos(2 60t  ) 
(1  a ) f (t)
75
75
• The anti-aliasing filter has removed the 60Hz harmonics
→ Do not show up in the equation
• To determine the maximum value a (t), (1  d )(1  a ) must be known.
• The implementation errors εd and εa must be kept small enough to satisfy the requirement.
( υf(t) should be measured to an accuracy of 0.01V RMS)
• The governing equation.
a (t)  A
(1  d )(1  a )0.9VRMS  0.9VRMS  max error

Refinement
(1  d )(1  a ) 1.011



• Two approaches can be taken to refine the system.
→ Increase the resolution of the A/D converter.
→ Estimate the spectrum of the quantization noise and subtract it from the final spectrum.
With the first approach 10-, 12-, 14-, and 16-bit A/D converters are available.
• The analysis showed that a 16-bit A/D is needed, and also revealed why it was needed.
→ The signal of interest, which is the flicker, is relatively small compared to the 170
volts peak, 60Hz sinusoidal interferer.
• However, RS232 format sends only 8 bits a time. → 8 bits+8 bits
→ (00000000+8+8)+(00000000+8+8)…….
→ How can we make (8+8)+(8+8)…..?
Design for Electrical and Computer Engineers
Chapter 4. SYSTEM DESIGN
• 16-bit A/D is expensive. → How about using 8 bit A/D and 60 Hz notch filter?
→ Inductors and capacitors for 60 Hz notch filter would be too large and expensive.
→ 16 bit A/D is an answer!
• But, 60 Hz noise should be suppressed for high accuracy. The design engineer needs
→ Novel way of suppressing the 60Hz sinusoid.
→ This requires a new concept developed with creative thinking.
• One novel concept is;
→ Set the sampling rate to 120Hz and synchronize the sampling to the zero crossings of the 60Hz sinusoid.
→ The 60Hz interferer is effectively eliminated, as the interferer is zero volts at every sample time.
• Such a system
→ Require a simple clipping circuit to prevent the input to the A/D converter from exceeding limits.
→ Required a phase-locked loop to lock the sampling clock to the zero crossings of the 60Hz sinusoid.
(for synchronizing)
Analysis
Refinement
Design for Electrical and Computer Engineers
Chapter 4. SYSTEM DESIGN
Synthesis/Analysis cycle
that we followed is
Performance or
cost threshold
4.7 Summary
• Conceptualization, synthesis, and analysis
→ Create an orderly sequence of structures (block diagrams).
• Many of the ideas used in synthesis are born while performing the analysis.
Two type of thinking
• Linear thinking
→ Logical, deductive reasoning. Used to expand or adapt an existing solution to fit problem.
→ Linear approach is sometimes the obvious choice.
• Creative thinking
→ deeper and less constrained, searching the imagination for a novel solution.
→ personal improvement and the potential for revolutionary results
• System engineers must act responsibly, be aware of cost and time constraints, and
work to ensure the profitability of their firm!
Design for Electrical and Computer Engineers