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Physical Limits to Modularity. Daniel Whitney MIT October 27, 2005. When do designers have freedom to define modules and assign functions to them? What limits this freedom? Will some kinds of systems always be harder to make modular than others for reasons we cannot change?.

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physical limits to modularity
Physical Limits to Modularity

Daniel Whitney


October 27, 2005

  • When do designers have freedom to define modules and assign functions to them?
  • What limits this freedom?
  • Will some kinds of systems always be harder to make modular than others for reasons we cannot change?
background 1 whitney tries to explain why meche is hard
Background 1: Whitney Tries to Explain Why MechE is Hard
  • Draper proposal to DARPA in 1989 to study complex electro-mechanical-optical (CEMO) systems
    • Missile seeker heads
    • Polaroid cameras
  • DARPA’s reply: get smart, do mechanical design the way VLSI design is done
  • It’s not that easy, but how to counter this argument?
background 2 how come whitney can build a computer
Background 2: How Come Whitney Can Build a Computer?

TTL Cookbook, 1979

integrality and modularity
Integrality and Modularity
  • Modular systems are, ideally, those in which
    • Functions can be associated simply and directly with modules more or less one-to-one
    • Only predefined interactions occur between modules
    • Interactions occur at, and only at, predefined interfaces
  • Integral systems differ as follows:
    • Functions are shared among modules
    • Interactions that were not defined can occur, and they can occur at undeclared interfaces
physical limits to modularity1
Physical Limits to Modularity
  • Distinction based on power level in the system
  • Information processing systems are easier to modularize
  • Elements do not back-load each other due to huge impedance mismatch - no worry about wasting power (until recently)
  • Power processing systems contain unavoidable interactions at undeclared interfaces
  • Side effects in high power systems occur at the same power level as main effects: vibration, heat dissipation, crack growth
  • These side effects either constitute or generate integrality
integral modular situations
Integral/Modular Situations*

Each function is

realized by

Many functions are

realized or shared by

(read down!)

Cast or molded parts







function sharing◊



One part




Most assemblies

Car door

Integral -




“chain*” or “holistic#”


Many parts

Mixed architectures are the most common, in which some functions are realized

by some options and others by other options

*Tim Cunningham, PhD thesis; #Ulrich and Ellison; ◊Ulrich & Eppinger

low power items vlsi
Low Power Items - VLSI

Function is logical

and can be represented

logically and symbolically

Side effects are

few or can be

isolated logically

Freedom to

create modules

to do single fct

The symbols can be

converted to a picture

Low power

No back-


Modules are indep

in operation

Modules are indep

in design

The picture

is the design

Modules can be

designed and

verified in advance

of any known use

Design can be

validated logically

Interfaces are

indep of fct

Main fct carriers

can be standardized

System design and validation

are indep of module design

A construction process

exists that eliminates

most assembled interfaces

Systems with a huge number

of parts can be designed with

good confidence that they will work

high power items jet engine
High Power Items - Jet Engine

Function is physical

and cannot be represented

logically and symbolically

Side effects are

high power

and can’t be isolated

Modules display

multiple behaviors

in multiple energy domains

The design can be

converted to a picture

High power

Severe back-


Module behavior

changes when

combined into system

Modules are indep

in design

The picture

is an incomplete abstract

representation of the design

Modules must

be validated physically

Modules must be

designed anew

specifically for

their function

Interfaces must

be tailored to fct

Separate module and system

validation steps are needed

Main fct carriers

can’t be standardized

A construction process

exists that eliminates

most assembled interfaces

Systems cannot be designed with

good confidence that they will work

counter example proves the rule
Counter-Example Proves the Rule?
  • Microprocessors increasingly give off huge amounts of heat - frying pans with computing power
  • Inability to get rid of this heat is THE blockage to following Moore’s Law, not lithography or other traditional barriers - processors self-destruct
  • Heat dissipation equates to short battery life and hot laps, threatening the laptop computer market
  • The campaign to mitigate heat has tied together electrical and mechanical designers, microprocessor designers and computer makers and software designers
  • These are symptoms of integrality
  • Conclusion: high power drives traditional modular item into integrality
evidence at intel
Evidence at Intel
  • Patents on fans
  • Investments in software and heat transfer solutions
  • Close cooperation with PC designers
  • Major shift in marketing strategy to de-emphasize processor speed
  • Ref: SDM Thesis by Sam Weinstein, March 2004, LFM thesis by Tom Evans, June 2003

Business Week OCTOBER 6, 2003   •  Editions: N. America | Europe | Asia | Edition Preference


Japan: Fuel-Cell Nation

NEC, Toshiba, and Sony are developing ever-smaller cells to replace batteries

When it comes to gadgets, the Japanese are a tough lot to please.

So when NEC Corp.'s stand at the WPC Expo computer trade show

in mid-September started to draw crowds like Tokyo's Shinjuku Station at rush hour,

it was clear that something extraordinary was on display.

The attraction turned out to be a fuel-cell-powered laptop. The 2-kilo computer

can go five hours before its cell needs to be refilled with methanol. That's better

performance than all but the hardiest of laptops running on conventional batteries.

NEC plans to start selling an even lighter fuel-cell-powered machine late next year,

and a year after that says it will release a model that can operate 40 hours on a

single fill-up. "We're confident that fuel cells will become the main power source

for mobile devices," says Yoshimi Kubo, the senior research manager overseeing

the development of NEC's new laptop. "It'll be a very big and lucrative market."


Fan and Heat Sink 100 x Volume of Processor Chip:

OK for old generation processors in desktops, totally inadequate for today’s most powerful ones in laptops.

car transaxle case
Car Transaxle Case
  • Single part does many things: it is integral
  • Designers would not switch to modular design


car transaxle case integral vs modular
Car Transaxle Case: Integral vs Modular

* Modular: each pore is plugged separately

high and low power domains
High and Low Power Domains
  • VLSI designers want modularity because it permits them to conquer complexity
    • Integrality would smother them in testing costs
  • High power system designers exploit integrality and function sharing to achieve efficiency and elegance
    • Modularity would saddle them with unreliable Rube Goldberg things
    • Extra interfaces take up space and weight
what makes something inherently integral
What Makes Something “Inherently Integral?”
  • It has multiple Key Characteristics (a measure of complexity?)
  • KC delivery is distributed within the product, giving rise to “chains of KC delivery”
  • The KCs are coupled and may conflict
    • car door outer panel alignment to frame (appearance)
    • car door inner panel alignment to frame (noise, leaks)
  • Inter-module couplings are very strong
    • load paths in aircraft structure
    • data exchanges in time-critical computing tasks
  • As a result, the product may appear modular but it is not
business impacts of modularity
Business Impacts of Modularity
  • Design for users and user interfaces
  • Divide and conquer for diagnosis
  • Structure and modularity for repair and upgrade
  • Baldwin-Clark modularity categories
    • Modularity in design
    • Modularity in manufacture
    • Modularity in use
    • These are rarely the same
some educational implications
Some Educational Implications
  • EEs are given their components
    • Linear, independent, pre-tested, single function
    • They can start designing systems as sophomores
    • Separate component experts exist (chemists, solid state physicists)
  • MEs must learn to design components first
    • Non-linear, coupled, designed to suit, multifunctional
    • They don’t see system design until they are seniors
      • This happens in basic servo theory
      • Mechanical assembly is not taught
some principles
Some “Principles”
  • Power levels can be determinative in limiting modularity choices at the physical design level
  • “Business issues” further limit or shape modularity choices: customization, reuse, common architecture, legacy
  • Business and physical domains are coupled
  • Hidden integrality in one domain (business or physical) can scramble sought-after modularity in the other domain (2001 SDM thesis by David Craig)