ME886.3
Topic 1: System and System Design (II)
1
Science-based design: making a
better system
Introduction: System and Design
 A system is a set of elements that are connected
in a way that meets a principle
 An element may further be decomposed into
smaller elements; in this case, the element may
be called subsystem
 Design is a decision-making process starting
from requirements to system structures
Introduction: System and Design
 Requirement consists of functional requirement
(FR) and constraint requirement (CR).
 FR: A function or functions a device or system has to
achieve or a role or roles a device or system has to play.
 CR: A condition or situation where a device or system
achieves a required function.
 Specific attributes are prescribed the function or role. For
example the function is to support an object. The attribute
of this function may be 50 KN – “support an object that is
less than 50 KN.
Introduction: System and Design
o FR(upper id, current id).{Attribute}
FR(1, 2).{Load}
Specify how good with a
function performed
o 1: FR at Level 1,
o 2: The second sub-function,
o {load}: one attribute that is “load”.
Attribute:
o Name | expression |
Ex:
Load: >50 KN
Load: <50 KN & > 10 KN
Accuracy: ~0.05
Speed: <50 rmp
Introduction: System and Design
o CR has two types in terms of the stringency to meet:
1. A condition must be met; for instance, the pedal of a bike
must be in black color.
2. A condition has a range; for instance, the cost of a system
under design must be less than X dollars.
o CR can be global and local:
1. Global: a CR is applicable to a group of FRs.
2. Local: a CR is only applicable to one FR.
Introduction: System and Design
Ex:
o Global constraint: The total weight of a system is less than 5 N.
o Local constraint: color of a pedal must be red.
As regards “using foot to trigger”, it can be
either specified with FR or with a separate
CR.
To a brake pedal, its function is to trigger the braking mechanism. Suppose
that the pedal is required to have a block color.
FR: trigger the braking mechanism.
CR (local): device should be in a black color.
Introduction: System and Design
As regards “using foot to trigger”, it can be either specified with FR or with a
separate CR.
FR: triggering the braking mechanism with the foot.
FR: triggering the braking mechanism.
CR: with the foot.
Formalism: CR (expression for scope)
Ex:
CR1 (FR1): one CR (CR1) is applicable to FR1,
CR2 (FR1): another CR (CR2) is applicable to FR1 as well,
CR1 (SB-S1): a CR is applicable to sub-system 1,
CR1 (S1): a CR is applicable to system 1.
Introduction: System and Design
Design should be said:
To find a mapping from the R space to S space,
where
R: requirement (function, constraint)
S: structure
If design is conceptualized
as an optimization
problem, it is a constrained
optimization problem.
But not
To find a mapping from the F space to S space,
where
F: function
Introduction: System and Design
Idea
Concept
Formalization or
mathematics
o On a side note, any theory is
developed by following the
above left logics.
o The left logics is further
triggered by our curiosity to
respond to a question or to
solve a problem.
Introduction: System and Design
Design parameter (DP):
Description of the structural element to meet both the
functional and constraint requirements
DP: Concave topology
Science-based Design
1. General design phase theory
2. Design theory and methodology
3. Design modeling and optimization
Towards a Better System
2. General design phase
Technical specification of requirement
Concept design
Embodiment design
Detail design
Technical specification of requirements
Task: Develop a technical specification.
Technical specification: a document of customer’s
requirement in the technical term.
Example
Customer’ s
requirement:
Need a drill to
penetrate common
steel
Technical specification:
Be able to penetrate into
the material with
HRC>30
Concept design
Task: Develop the working principle of device under design
from the technical specification
Example
Working principle is DP
Requirement:
Working principle:
Rotation to
translation
o Crank-slider, or
o Rack-and-pinion
Remark: for machine design, it refers to kinematic design
(principal dimension is given but not material and volume)
Remark:
Making a statement of what without including the
means to achieve it
Realize the rotation to translation through a linkage
Means
The means restrict the possibility of
having the means of rack and pinion
Embodiment Design
Requirement:
FR: To materialize a crank-slider kinematic linkage
for load of 50 KN
CR: To make device as light as possible
Embodiment crankslider linkage:
Layout is DP
Detail Design
Task: Develop a complete specification for each individual
component ready for manufacturing
Requirement:
FR: To specify geometrical details ready for manufacturing
CR: To be easy for manufacturing and assembly
Detailed description of a
piston component ready
for manufacturing
DP at this level: detailed information of component for manufacturing
Total FR may be fulfilled at
different design phases
FR
1
3
2
4
Concept design
Embodiment design
Design design
Science-based Design
1. General design phase
2. General design theory and methodology
3. Design modeling and optimization
Towards a Better System
3. General design theory and methodology
General Axiom 1:
Making what to do and how to do separately. In particular
while describing what it is, not be affected by how to realize it
Example (Rotation to translation)
Requirement statement:
When a rotary motor turns, it connects with a rod which
further transfers motion to a slider to realize a translation
…….
The above statement has violated Design Axiom 1
3. General design theory and methodology
Complexity
General Axiom 2:
Divide and conquer
Problem -0
Problem -2.1
Problem -1.2
Problem -2.2
Simplicity
Problem -1.1
3. General design theory and methodology
General Axiom 2:
Divide and conquer
Requirement:
Design X to hold hot (~100oC) water with hand
X holds hot water
X holds room
temperature
water
23
Hand grasps X
X resists heat transfer
3. General design theory and methodology
X resists heat transfer
Hand grasps X
X holds room
temperature water
material
24
3. General design theory and methodology
General Axiom 3:
Generalization of design
Generic design
problem
Current design
problem
Other design
problems along
with their
solutions
A solution to the
current design problem
Remark: The above process is in fact an analogy
process to lead to innovation
Science-based Design
1. General design process
2. General design theory and methodology
3. Design modeling and optimization
Towards a Better System
4. Design modeling and optimization
1. Mathematical model for design objects at a
different level (conceptual object, embodiment
object, detailed object)
2. Variables defined for structural features, and
mathematical relations defined for the whole
structure along with its behavior
3. Varying of the variables to seek the best behavior
5. Designer’s cognitive process
1. Team-based brain storm and white-box
communication
2. Diversification in team composition and in task
management
3. Confidence
4. Removal of unnecessary constraints
6. Summary
 Design is a marriage of human designers and
science-based processes,
 Science-based processes are: general design
process, design theory and methodology, and
modeling and optimization,
 Design theory and methodology has three
general axioms.
 Designer’s cognitive process includes four
guidelines,