System Functions

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What is System Functions?

System Function is a set of functions that a system must perform, in other words, what system or product is expected to do.

Why System Functions?

It serves as a fast collection of summarized, high-level requirements and allows to explore behaviors of the system. By thinking through behaviors, we naturally identify functions that the system or product must perform.

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Process Phase



1 hour-2 hours



Markers, Paper

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Method Card Reference

DI Method Cards_Compiled Deck_Version 2.
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Best Practices



Collect User Needs

There are various User Needs Analysis methods. Most widely used are user journey nap, scenario, interviews, and questionnaires. You may fine more information on the webpages dedicated to each method.


Map User Needs to Flows (E,M,S)

Flow is a representation of quantities that are input and output by functions. There are three basic flows for design problems:

  • Energy - human, electrical, hydraulic, magnetic, mechanical etc.

  • Material - human, gas, solid, liquid

  • Signal - information (status, control)


Create Function Tree/Chain/List  for Flow/Need/Activity

A function list is a list of product functions presented in functional basis form. In this form function is characterized in a verb-object (functional basis*). 

A function tree is used to represent the system architecture that is intended to be developed in this roadmap, and to provide the context within which each subsequent design team will operated. 

A function structure is a graphical form of a functional model where its overall function is represented by a collection of sub-functions connected by the flows on which they operate. Explicitly related to customer needs.

*A functional basis that is used for creating system function is a design language where product function is characterized in a verb-object (function-flow) format.




A unifying review that summarizes the explored functional requirements.


Decomposition and breaking down function is a recognized skill of the top designers. Therefore it is important to know how to breakdown a system into key subsystems.


Develop an extensive vocabulary of functions such as with the functional basis or vocabulary. This takes practice.


Seek an "atomic" level of function i.e, atomic functions are actionable. They can be solved with single entities. (e.g. Function: translatory motion - action: install a single motor)



Function and function modelling enable and empower creativity. Therefore, use a set of key functions as the opportunity statements for ideation/develop methods such as mind mapping.


Functions/functional models are the building blocks for system architecture, seeking insides such as modularity, portfolio, etc.


In function flow, it is important to map inputs to outputs.


Functions are abstract because they are about actions while ideas start with objects (nouns).



Functional Basis: Vocabulary and Flows

Demo Video

This video gives you a walk-through on how to develop system functions through a worked example of an Autonomous Vehicle Design Challenge.




Here are examples of System Functions representations for AV case study.

References and Additional Resources

  1. Stone, R., Wood, K.,  1999. Development of a Functional Basis for Design. DECT99/DTM-8765.

  2. Hirtz, J., Stone, R., McAdams, D., Szykman, S., Wood, K., 2002, A Functional Basis for Engineering Design: Reconciling and evolving previous efforts. Research in Design Engineering 13 (2002) 65-82

  3. Otto, K. N., and K. L. Wood. "Product design: techniques in reverse engineering and new product development." (2001).