Digital Twin
The Willow Digital Twin resources are the backbone of Willow. These resources contain both the rich data and metadata to enable powerful applications. The digital twin (aka knowledge graph) is comprised of two graphs:
- Models
- Twins and Relationships
Models
API Endpoints: Models
A model is the definition of a real-world concept. A model can be thought of as a category that a Twin belongs to. It contains properties, components, and relationships that describe what this type of entity may be. For example, the Willow model of a Chiller includes properties describing the type of refrigerant and its cooling capacities, components describing its evaporator and compressor, and relationships describing where it is located and what other equipment it is fed from and/or feeding.
Models can extend from (aka inherit from) other models to be more specific. For example, the Chiller extends from HVAC Equipment and is extended by Absorption Chiller, Air-Cooled Chiller, and Water-Cooled Chiller.
The collection of all Models within Willow is known as the Ontology. To learn more and see the definitions of the models within Willow's ontology, visit the Willow Open Digital Twins repository.
Here are the top-level models in the Willow ontology from which all other models extend:
| Model | Description |
|---|---|
| Agent | Any basic types of stakeholder that can have roles or perform activities, e.g., people, companies, departments. |
| Asset | An object such which is located inside (or outside) a building, but is not an integral part of that building's structure, for example architectural, furniture, and equipment. Assets are typically products that have a manufacturer, model, and serial number |
| Building Component | A part that constitutes a piece of a building's structural makeup, for example Facade, Wall, Slab, RoofInner, etc. These are typically constructed onsite. |
| Capability | A capability indicates the capacity of a entity, be it a Space, an Asset, or a LogicalDevice, to produce or ingest time series data. This is equivalent to the term "point" in Brick Schema and generic Building Management System. Specific subclasses specialize this behavior: Sensor entities harvest data from the real world, Actuator entities accept commands from a digital twin platform, and Parameter entities configure some capability or system. |
| Collection | An administrative grouping of entities that are addressed and treated as a unit for some purpose such as a System or a Portfolio. |
| Document | A formal piece of written, printed or electronic matter that provides information or evidence or that serves as an official record, for example LeaseContract, Building Specification, Warranty, Drawing, etc. |
| Event | A spatiotemporally indexed entity with participants, something which occurs somewhere, and that has or takes some time, for example a Lease or Rent. |
| Software | A set of instructions, data, and programs used to operate a computer or device and execute specific tasks. Typically used to define Connectors. |
| Space | A contiguous part of the physical world that has a 3D spatial extent and that contains or can contain sub-spaces. For example a Region can contain many pieces of Land, which in turn can contain many Buildings, which in turn can contain Levels and Rooms. |
| System | An entity which is constructed such as a Bridge or Tunnel. |
Here is an example subset from the Willow ontology which shows how the Fan Powered Box model inherits from Asset:
Twins and Relationships
API Endpoints: Twins
API Endpoints: Relationships
A Twin is a real-world instance that represents an entity in your environment. For example, you might have several twins named Chiller #1, Chiller #2, and Chiller #3 that each reference the Chiller model. They likely have additional properties defined besides name such as their individual cooling capacities.
In the Willow Twins and Relationships graph, Twins can be thought of as nodes in the graph and Relationships can be thought of as the edges connecting the twin nodes. Relationships have names such as located in, is part of, and is fed by as well as direction from the source twin to a target twin. Optionally, some relationships can have properties just like twins. An example of this in Willow is there might be an Air Handling Unit twin, AHU #1, with an is fed by relationship to Chiller #1 which has the property substance: Chilled Water defined.
The Twins endpoints enable creating, updating, and fetching one or more twins at a time. When receiving twins, there are also options to include relationships in the same response. Additionally, there are dedicated Relationship endpoints which enable creating, updating, and fetching relationship details separately.
To see examples of how twins are commonly modeled in Willow, you can visit the Samples in Willow's ontology repository.
Here is an example from the above which shows a subset of a twins graph representing
