Updated on March 4, 2025

Function

Function is an action performed by one material object (function carrier) to change or maintain a parameter of another material object (object of the function).

Overview

Function in TRIZ plays a particularly crucial role. One of the main paradigms of the methodology is separating the function from its carrier. In practice, this means that as long as the function is being performed, it does not matter which component performs it. This approach helps us overcome limitations and discover innovative solutions.

Elements of the function

Function consists of 3 elements:

  1. a function carrier, i.e., a substance and/or a field performing an action,
  2. a function object, i.e., the addressee of the action,
  3. a verb describing the action being performed.

Functions are identified during function analysis, specifically its last stage – function modeling.

Conditions for a function existence

For a function to be considered as performed, the following conditions must be met:

  • both the function carrier and the object of the function are substances and/or fields,
  • the function carrier interacts with the object of the function, and
  • the action performed by the function carrier changes or maintains (keeps unchanged) some parameter of the object; if such a parameter cannot be identified, it means that there is no function.

Note that one function can change only one parameter of the function object. If the function carrier changes more than one parameter of the function object, it means that it performs more functions.

Some examples of functions can be the following:

  • the hammer moves the nail (changes its position in space),
  • the glasses focus light (changes its direction),
  • the book informs the reader (changes the scope of their knowledge).

In the table below, the left column shows some examples of incorrectly formulating functions. The right column contains correctly formulated functions.

incorrect formulation of functions
(no parameter of the object is changed = no function)		correctly formulated functions
the sunglasses protect eyesthe sunglasses direct light
the vacuum cleaner cleans the carpetthe vacuum cleaner removes dust
the dryer dries clothesthe dryer removes water
the ruler measures the lengththe ruler informs a person
the lamp illuminates a bookthe lamp generates light
the drill makes a whole in a boardthe drill changes the shape of a board

Evaluation of functions

Functions are evaluated in terms of:

Functions are evaluated during function modeling, and the results of this evaluation are used in subsequent steps of the analytical process. Categories and performance levels are essential for identifying the function disadvantages of the system, which serve as inputs for the cause-effect chain analysis (CECA). Function ranking is crucial for trimming.

Categories of functions

In modern TRIZ, two categories of functions are identified:

  • useful functions, and
  • harmful functions.

Useful function is a function performed by a function carrier that results in a positive (required) change or preservation a parameter of an object of the function.

Harmful function is a function performed by a function carrier that results in an inacceptable change or inacceptable preservation of a parameter of an object of the function.

Simply put, if the function carrier changes the object’s parameter in a desired direction, the function is useful; if it changes it in an undesired direction, the function is harmful.

Functions themselves are neither harmful nor useful. Their category must always be considered in the context of the specific situation. The function hot air melts plastic would be useful if the goal is to increase the plasticity of the plastic, but if the hardness of the plastic needs to be preserved, it would be a harmful function.

Level of performance

Level of the function performance is determined only for useful functions. For harmful functions it is ignored, and they inherently represent function disadvantages of the system.

The level of performance determines whether the useful function is performed normally (i.e., within the intended parameters, exactly as it was designed) or is performed excessively or insufficiently. The excessive and insufficient performance of the function are viewed as function disadvantages of the system.

The performance level of a function can be illustrated by the example of a bicycle rim brake. Its purpose is to hold the tire to enable smooth and effective stopping of the bicycle. If the brake holds too tightly, the stop is too abrupt, which can lead to an accident. If the brake holds too loosely, the braking distance dangerously increases, also raising the risk of an incident.

Function ranking

Function ranking is conducted when it is necessary to develop an innovation strategy. It involves identifying the type of the function and then assigning it an appropriate number of points based on that type.
While categories and level of performance are the same for all types of engineering systems, function ranking is completely different for devices and processes. They are as follows:
function ranks for devicesfunction ranks for processesscore
basic functionproductive function3
additional functionproviding function2
auxiliary functioncorrective function1
Function ranking plays a particularly important role in trimming for processes because the type of the function is the only factor determining which trimming rule will be applied.

Practical tips on formulating functions

Here are some practical tips on how to formulate a function correctly:

  1. Formulate the function in the affirmative form – not to do something is not a function. It relates both to useful and harmful functions.
  2. Do not use declarative terms (e.g. to provide, to protect, or to seek); they do not change/maintain any parameter of the object, hence do not constitute any function.
  3. Be as specific as possible.
  4. If you are unable to determine the action with one verb, you can use the formula: component A changes (or maintains) a parameter X of component B.
  5. Remember that function carrier cannot perform any function on itself. If you come across such a situation, split the object into two parts – one part will perform the the role of function carrier, the other one the role of function object.

Function as a model of a problem

A generalized function can be used as a primary criterion for identifying technologies that exist worldwide. By reducing functions to universal terms, it becomes possible to identify technologies performing similar functions in both related and unrelated industries.

The tool designed for this purpose is function-oriented search (FOS).

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