Meta Model Elements

The Semantic Aspect Meta Model (SAMM) specifies the elements from which an Aspect Model can be built. The following diagram shows the basic building blocks:

aspect meta model

  • An Aspect is the root element of each Aspect Model and has a number of Properties, Events, and Operations. This element is mandatory and must appear exactly once per model. It has any number of Properties, Operations and Events ([0..n]).

  • A Property represents a named value. This element is optional and can appear multiple times in a model ([0..n]). One Property has exactly one Characteristic.

  • An Abstract Entity is a logical encapsulation of a shared concept. It has any number of Properties ([0..n]). An Abstract Entity can extend another Abstract Entity ([0..1]). This element is optional and can appear multiple times in a model ([0..n]).

  • An Entity is a logical encapsulation of multiple values. It has any number of Properties ([0..n]). An Entity can extend either another Entity ([0..1]) or an Abstract Entity ([0..1]). This element is optional and can appear multiple times in a model ([0..n]).

  • A Characteristic describes a Property’s meaning in the context of the Aspect with information such as its data type, if it is a collection, that a string value is to be interpreted as one of a fixed number of valid states or that a numeric value has an associated physical unit. This element is mandatory. It must appear once per Property.

  • A Trait is used to add one or more constraints to a Characteristic ([1..n]), which is referred to as the "base Characteristic".

  • A Constraint represents a limitation that is applied to a Characteristic (for example on the numeric value range or the maximum length of string). This element is optional and can appear multiple times per Characteristic ([0..n]).

Characteristics, Constraints and element descriptions, are not part of the data an Aspect sends, but only exist in its model. A number of commonly used Characteristics and Constraints are predefined in the scope of the Semantic Aspect Meta Model, see section Characteristics for more information.

  • An Operation represents an action that can be triggered on the Aspect. Depending on the Aspect, this can, but does not have to, trigger an action on the asset that is represented by the Digital Twin, for example switching a part of a machine on or off. This element is optional and can appear multiple times per Aspect ([0..n]). One Operation has any number of input Properties ([0..n]) and at most one output Property ([0..1]).

  • An Event is a model element that represents a single occurence where the timing is important. Assets can for instance emit events to notify other assets in case of special occurences. This element is optional and can appear multiple times per Aspect ([0..n]). One Event has any number of parameter Properties ([0..n]).

  • Quantifiable is the Characteristic that makes use of a Unit. Other Characteristics that are derived from Quantifiable have a Unit as well.

  • A Unit refers to a physical unit, such as metre or degree celsius. A catalog of units are predefined in the scope of the Semantic Aspect Meta Model, see section Units for an in-depth description. A custom Unit can be added to a Characteristic, when the Characteristic is defined as Quantifiable class. It must appear exactly once per Quantifiable Characteristic.

  • A Quantity Kind describes the kind a Unit refers to, such as length or temperature. This attribute is optional and can appear multiple times per Unit ([0..n]).

See the sections Attributes that all model elements have and Declaring model elements in section Aspects for hints on using the model elements in an Aspect Model.