Introduction to Functional Modelling

A functional Model is used to represent a system and its elements in terms of their functions.

In functional models, system elements are defined in terms of their functions using a functional taxonomy. Their interactions are modelled as the exchange of flows using a functional flow taxonomy. The failure mode of a flow is the deviation of the functional outputs from their nominal state. These item failure modes are propagated through the system model during automated failure analysis of the system.

The sections below describe the terms used in Functional Modelling and steps to use Functional Modelling tools in MADE.

Functional Modelling Definitions

• Functions: A description of an operation to be performed by a device, artifact or item, expressed as a verb.
• Flows: A change in material, energy or signal with respect to time, expressed as the object of a function.
• Failures: The event or inoperable state in which any item or part of an item does not or would not perform as previously specified.
• Functional Taxonomy: A standardised list of terms to define system elements in a functional system model. It aims to make the model and analysis results clear, consistent, and reusable. The section below further elaborates on functional taxonomy terms.

Functional Taxonomy Definitions

The three types of terms used to construct a functional definition are:

• Function Verb: An action of the item on the output flow (e.g. Provide, Reduce, Transmit etc.)
• Flow Noun: The material, energy or signal that is being acted upon (e.g. fluid, mechanical energy).
• Flow Property: The property of the flow that is required to fulfil the performance specifications of the item (e.g. flow rate, rotational velocity).

The order these terms appear in the model and FMEA report depends on whether the default taxonomy is used or a display name is overlaid.

For example, 'Refine Air Mass Flow Rate' uses the format [Function][Flow][Flow Property]. 'Refine' and 'Mass Flow Rate' are default function and flow property taxonomy terms respectively, however 'Air' is a display name which overwrites the flow taxonomy 'Gas'.

The 'cause and effect' relationships between input flow properties and output flow properties of an item is represented by 'causal connections'. These causal connections are used to determine simulation and failure mode responses.

Function Verbs

A function verb can represent the merging or distribution of flow properties, transformation of one flow into another flow type, or a change between the properties of the same flow.

Standardised function verbs are provided in a hierarchical tree-structure and may be selected from any level within the list. Descriptions and synonyms are provided to help the user identify the appropriate function verb.

Flow Nouns

Functional modelling flows are classified into material, energy or signal.

Each flow has specific properties, which are classified even further. Flow properties are displayed when a flow is used to construct a functional definition.

Accessing the Functions Editor

Opening the Functions Editor

The functions editor allows the user to define the function of an item and map the effect of incoming flow properties to outgoing flow properties using causal connections.

The user opens the Functions Editor by:

• Right-clicking an item in the Project Explorer and selecting Functions from the menu
• Right-clicking an item in the System model and selecting Functions from the menu

Building Functional Definitions

Functional Definitions are created and edited in the Functions editor.

The procedure for creating a Function is:

1. Select an item and open its Functions Editor
2. Drag functions from the Functions Taxonomy into the empty space of the box to the right
3. Select the Flows Tab
4. Drag a flow from the Flows Taxonomy into a Function on the right
   • Incoming Flows on the left side of a Function
   • Outgoing Flows on the right side of Function
5. To activate Flow Properties:
   • Tick the required Flow Property check box
   • Creating a causal connection: Left-click and drag an Incoming Flow Property to Outgoing Flow Property

Connecting Flows in the System Model

After defining item functions, these item flows need to be connected together to complete the functional model. This is performed on the System Model.

The System model is used to describe the functional flows connecting subsystems and components. It describes the functional relationship between the items in the system, including feedback relationships and System level inputs & outputs using the "MUX Bar".

Functional Rules

When connecting out flows to in flows:

• The target flow must be the same flow property as the source flow
• The activated target flow properties must also be activated in the source flow

See also:

• Failure Mode definition (Glossary)
• Failure Diagrams
• Function Taxonomy (03-02-02-FunctionTaxonomy.xml — extracted separately)
• Function Flow Taxonomy (MADe_Flow_Structure.xml — extracted separately)