The Functional Basis: Reconciled Function and Flow Taxonomy (Hirtz et al. 2002)

What this document is

This is the primary academic source for the standardized "function verb / flow noun" vocabulary that MADe's Functions Editor is built on. It is NIST Technical Note 1447, published February 2002: "A Functional Basis for Engineering Design: Reconciling and Evolving Previous Efforts," by Julie Hirtz and Robert B. Stone (University of Missouri-Rolla), Daniel A. McAdams (University of Missouri-Rolla), Simon Szykman (NIST), and Kristin L. Wood (University of Texas at Austin). Published in Research in Engineering Design, Vol. 13, No. 2 (2002), pp. 65-82.

The paper's stated purpose: engineering design needs a standardized, minimal vocabulary of function and flow terms so that functional models are repeatable and comparable across designers — the same motivation that later made this vocabulary attractive as the backbone of a commercial FMEA/RCM tool like MADe.

Origins: two independent efforts merged into one

Per Hirtz et al. 2002, the "Functional Basis" is a reconciliation of two separately-developed vocabularies:

1. The NIST taxonomy (Szykman et al., 1999) — developed as part of NIST's Design Repository Project, aiming for an exhaustive, atomic classification of engineering functions and flows to support design-knowledge exchange and design repositories. It contained over 130 functions and over 100 flows, organized as a hierarchical taxonomy following the flow classification set forth by Pahl and Beitz (1984): material, energy, and signal.

2. The (original) Functional Basis (Little, Wood & McAdams 1997; Stone & Wood 1999/2000) — developed empirically through study of over 100 existing products, aiming for a minimal, near-orthogonal ("basis," in the mathematical sense — spanning the space of design concepts with as little redundancy as possible) vocabulary usable directly by designers. It contained 8 primary function classes and 3 primary flow classes (material, energy, signal), each broken into two further levels of detail.

Both efforts trace their intellectual lineage further back to:

• Value Engineering (Miles, 1972) from the 1940s, which first used active verb-object descriptions of product function to target high-cost functions for redesign.
• Pahl and Beitz (1984), who defined five generally valid functions and three flow types at a high level of abstraction — the direct ancestor of the material/energy/signal split still used today.
• TIPS / TRIZ (Altshuller, 1984, "Theory of Inventive Problem Solving"), a Soviet-era classification of ~30 functional descriptions derived from studying over 2 million patents.
• Collins et al. (1976), who built a 105-term function vocabulary specifically to catalog helicopter failure modes — an early precedent for using function vocabularies in reliability/failure contexts, directly foreshadowing FFIP and MADe's use case.

The reconciliation method

The authors merged the two vocabularies using a three-step algorithm: Review (define every term precisely) → Union & Intersection (find the common "core" set, set aside disputed terms as "suspense") → Reconciliation (place each suspense term at the correct level using rules: if a suspense term is a subset of an existing term, push it down a level; if a superset, promote it and demote the old term; if synonymous, merge as a "correspondent"). Every reconciled term had to be mutually exclusive with its siblings at the same level and, collectively, the terms at a level had to completely cover the category above them — i.e., any flow must classify into material, signal, or energy with no gaps.

The resulting three-level hierarchy

The reconciled Functional Basis organizes both functions and flows into three levels of increasing specificity:

• Class (primary) — the broadest category (e.g., Material, Energy, Signal for flows; Branch, Channel, Connect, Control Magnitude, Convert, Provision, Signal, Support for functions).
• Secondary — a refinement within the class (e.g., Import, Export, Transfer within Channel).
• Tertiary — the most specific level, often tied to a physical mechanism (e.g., Transport / Transmit within Transfer).

A fourth, informal column of "Correspondents" lists synonymous terms from other vocabularies (e.g., "allow," "capture," "input" all map to Import) — useful for mapping legacy or third-party terminology onto the standard basis without expanding the core vocabulary.

The reconciled Flow set (Table 3 of the paper)

Class (Primary)	Secondary	Tertiary (examples)
Material	Human, Gas, Liquid, Solid, Plasma, Mixture	Solid → Object, Particulate, Composite; Mixture → Gas-gas, Liquid-liquid, Solid-solid, Solid-Liquid, Liquid-Gas, Solid-Gas, Solid-Liquid-Gas, Colloidal
Signal	Status, Control	Status → Auditory, Olfactory, Tactile, Taste, Visual; Control → Analog, Discrete
Energy	Human, Acoustic, Biological, Chemical, Electrical, Electromagnetic, Hydraulic, Magnetic, Mechanical, Pneumatic, Radioactive/Nuclear, Thermal	Electromagnetic → Optical, Solar; Mechanical → Rotational, Translational

Each Energy secondary category also has a power-conjugate pair (an "effort" and a "flow" variable, borrowed from bond-graph modeling, where effort × flow = power) — e.g., Mechanical/Rotational → Torque (effort) and Angular velocity (flow); Electrical → Electromotive force (effort) and Current (flow); Thermal → Temperature (effort) and Heat rate (flow). This effort/flow pairing is exactly the representation MADe uses internally when it converts a hardware block diagram into a bond-graph simulation model (see made-functional-basis-connection.md).

The reconciled Function set (Table 5 of the paper)

Eight primary (class) function categories, each with secondary and tertiary sub-functions:

1. Branch — Separate (Divide, Extract, Remove), Distribute
2. Channel — Import, Export, Transfer (Transport, Transmit), Guide (Translate, Rotate, Allow DOF)
3. Connect — Couple (Join, Link), Mix
4. Control Magnitude — Actuate, Regulate (Increase, Decrease), Change (Increment, Decrement, Shape, Condition), Stop (Prevent, Inhibit)
5. Convert — Convert (a single, deliberately broad secondary/tertiary term — "any type of flow conversion is valid... convert electricity to torque will be more common than convert solid to optical energy")
6. Provision — Store (Contain, Collect), Supply
7. Signal — Sense (Detect, Measure), Indicate (Track, Display), Process
8. Support — Stabilize, Secure, Position

Every function definition in the paper is illustrated with a worked example from a real product (a coffee maker, a power drill, a vacuum cleaner, an automobile seat, etc.) — the same "one verb + one or more flow nouns" pattern MADe's Functions Editor exposes directly in its UI (e.g., "Transmit Rotational Energy" for a drive shaft).

Why this matters for MADe

MADe's Functions Editor lets an engineer describe a component's behavior as a two-part statement: one verb selected from the Functions list, plus one or more nouns selected from the Function Flows list (Material / Energy / Signal, each with the secondary/tertiary breakdown above). This is not MADe's own invention — it is a direct software implementation of Hirtz et al.'s reconciled Functional Basis, cited explicitly in PHM Technology's own 2008 technical paper as "the functional ontology developed by Stone and Wood [2000]" (see made-functional-basis-connection.md for the primary-source confirmation). Understanding this taxonomy's origin as an academic effort to create a minimal, mutually-exclusive, fully-covering vocabulary explains why MADe's verb/noun lists are fixed and standardized rather than free text: the whole point of the Functional Basis was to eliminate the ambiguity of synonyms and ad hoc terminology so that functional models (and, in MADe's case, failure propagation logic built on top of them) are computable and comparable.

Limitations to note

• This paper documents the 2002 academic state of the taxonomy. MADe's actual shipped Functions/Flows lists (as seen in the 2008 PHM Technology paper's screenshot) are close to, but not necessarily byte-identical to, Table 3/Table 5 here — PHM Technology may have made small implementation-specific additions or omissions (e.g., the MADe screenshot shows "Vibrational" as an Energy flow type alongside the ones listed above, which is not in the 2002 reconciled table verbatim).
• The full appendices (A: Flow Definitions, B: Function Definitions) in the source paper contain a worked example and one-sentence definition for every single term; only representative highlights are reproduced above. Consult the full NIST Technical Note 1447 PDF for the complete, precise definition of any specific term.