Big things with small origins

It started as a side meeting at the light+building 2012 trade show in Frankfurt, Germany. A group of experts from the BACnet community and the OPC Foundation met for a first informal get-together. Very soon it became clear to all that both markets increasingly require data exchange from building automation to industrial automation processes and the ability to connect machines to building management systems. After a few teleconferences half a year later Matthias Damm and Frank Schubert were elected as leaders of the newly founded working group “BACnet OPC UA Information Model” and the group started on the specification work.

Figure 1: Foundation of the working group at the University of Dresden, Fall 2012 (from left to right)

Klaus Wächter, Siemens AG, member of the BACnet Interest Group executive board
Matthias Damm, Ascolab
Frank Schubert, Beckhoff Automation
Stefan Hoppe, Vice President OPC Foundation

The driving forces - www.opcfoundation.org                www.big-eu.org

The basic idea

In several face-to-face meetings, the group discussed potential use cases and decided to focus on connecting building automation systems to enterprise systems first.

The most commonly expected use cases include:

• Observation
• Operation
• Engineering

Data from building automation systems (where the source is BACnet)

• Energy metering data, energy analysis
• Real-time information (weather, temperature)
• Maintenance tickets (alarms)

brought to enterprise systems
(where the destination is OPC UA)

• Accounting, billing
• Operation surveillance
• Energy monitoring

General overview about communication

In every technical communication model it’s essential to clearly specify three different elements; the network media, the services to access the data and the data content itself.

Network media: The BACnet OPC UA Information Model does not mandate using specific network media. While BACnet supports a total of 8 different physical network layers including IPv4, IPv6 and serial networks using EIA-232, EIA-485 or wireless connections using Zigbee, OPC UA uses TCP/IP communication. A gateway application implementing this profile may choose any of the supported BACnet layers, even though it is expected that the majority of implementations will likely use IP.

Services: The BACnet OPC UA Information Model does not mandate how a gateway application implementing the BACnet OPC UA Information Model will gather the data from BACnet. An application may even fall back to polling the data points if retrieving the data using COV (change of value) is not possible at all or temporarily not available due to device limitations or a higher number of clients.

Data: The main focus of the BACnet OPC UA Information Model is actually the data mapping and representation of BACnet objects into OPC UA data structures. Since both use an object-oriented data model, specifying the mapping procedure was not complicated at all.

Data representation in BACnet and OPC UA

Both OPC UA and BACnet specify objects to model application data. Unlike in other standards or more simple communication protocols not only the values are provided. Additional information like the name of an object, a description text, the engineering units, alarm limits, operating hours or change-of-state count, etc. is provided. The BACnet OPC UA Information Model describes a gateway interface between the two object models.

Scope of the mapping BACnet ➞ OPC UA

• BACnet Objects/Properties ➞ OPC UA Object Types
• BACnet Events ➞ OPC UA Alarms & Conditions
• BACnet Logging ➞ OPC UA Historical Access
• BACnet Data Structures ➞ OPC UA Structure Data Types
• BACnet Units ➞ OPC UA Engineering Units

BACnet defines a set of object types where all common properties are repeated for each type. Type hierarchies and inheritance are not used in BACnet.

Since OPC UA supports type hierarchies, inheritance and aggregation, these concepts are used to avoid duplicated definitions in the OPC UA representation of BACnet. The example of a BACnet analog input object type is used in this overview chapter to describe how the different concepts are used in this mapping specification.

• Type hierarchies and inheritance is used to map common BACnet properties to a base type or different levels of a type hierarchy.
• BACnet properties that match existing OPC UA Attributes or Properties are mapped to existing OPC UA concepts like BACnet Object_Name to OPC UA Attribute BrowseName or the BACnet properties Units, Min_Pres_Value and Max_Pres_Value to OPC UA Properties of the AnalogItemType.
• Aggregation is used if BACnet properties can be logically grouped or a group of BACnet properties is used in different OPC UA ObjectTypes across the type hierarchy.

The BACnet OPC UA Information Model overview

Figure 3 below shows the main ObjectTypes of the OPC UA for BACnet information model and their relationships. This schematic overview is not intended to be complete.
 

Figure 3:

The boxes in this diagram show the ObjectTypes used in this specification as well as some elements from other specifications. The upper grey box shows the OPC UA core ObjectType from which the OPC UA for BACnet information model ObjectTypes are derived and some VariableTypes used in the BACnet ObjectTypes.

The grey column on the left of the second level shows the main ObjectTypes that this specification introduces. They represent corresponding BACnet object types. A type hierarchy is used whenever identical components are used in different BACnet object types.

The middle grey column in the second level shows the ObjectTypes used for grouping BACnet features. These groupings are used in the OPC UA for BACnet information model ObjectTypes shown in the left grey column.

Example mapping of an Analog Input value

Figure 4below provides an example of how to map a BACnet Analog Input object type to an OPC UA ObjectType:

Figure 4:

The left grey column shows the list of BACnet properties of the BACnet Analog Input object type.

The middle grey column shows an instance of an OPC UA BACnetAnalogInput ObjectType.

The right grey column shows the OPC UA ObjectTypes used to represent a BACnet Analog Input object type.

Most of the BACnet properties are mapped to OPC UA Properties using the BACnet property name as OPC UA BrowseName. They are either Properties of the Object directly or Properties of the EventReporting Object.

Device representation

In BACnet, physical devices are represented using the Device Object Type. Beyond the representation of property data like the device name, vendor or model name and APDU parameters, the group decided to include device management services like Device Communication Control (e.g. to silencing a device), Reinitialize Device (reboot), Time Synchronization or Object Creation and Deletion (see fig. 5).

 
Figure 5
 
Mapping events and alarms

Events and alarms are conditions triggering a message to a single recipient or a group to be notified. In BACnet these messages are provided through notification class objects. A mapping gateway will likely act as a recipient for all alarms and events, transporting these messages to OPC UA systems at all times, 24/7 (see fig. 6).

Figure 6

Conclusion

Both technologies and standards, BACnet and OPC UA, perfectly fit together. The BACnet OPC UA Information Model provides a standardized and proven architecture to bridge the gap between different applications in industry and building automation. Both groups, i.e. the OPC Foundation and BACnet Interest Group Europe will continue their collaboration.

Outlook: what’s next?

After receiving very positive feedback from companies which implemented the BACnet OPC UA Information Model, the working group started a 60-days public review in June 2016. Public reviews are commonly used in the development of the BACnet standard in order to allow feedback from parties not being able to attend the face-to-face meetings. The comments received within the public review period will be incorporated into the document and if no substantial issues are recognized the BACnet OPC UA Information Model will be released as Version 1 in fall 2016.

In a later stage the group may establish further meetings to specify connecting industry processes from OPC UA to building automation management systems to allow for other use cases where BACnet is the leading system and OPC UA provides the data to be connected.

Where to get the specification

The BACnet OPC UA Information Model is provided free of charge and license-free. The specification document is available from the BACnet Interest Group Europe website.

It is neither planned to include this profile into the BACnet-standard, nor into the OPC UA specifications, though.
It will be available as a separate document providing a recommended architecture to interconnect these two protocols.

http://www.big-eu.org/fileadmin/downloads/BACnet_OPC_UA_Information_Model.pdf

Contact the working group

Feedback and support is always welcome, feel free to contact us!
Matthias Damm:    Matthias.damm@ascolab.de
Frank Schubert:    f.schubert@beckhoff.com

Acknowledgements

The working group received great support from the OPC Foundation, the BACnet Interest Group Europe and the ASHRAE (American Society of Heating, Refrigerating and Air-Condition Engineers), not to forget the companies which sent their experts as volunteers to the working group meetings. Thanks to all supporting parties and individuals!

About the Author
Frank Schubert, Marketing and Training Building Automation at Beckhoff Automation GmbH & Co. KG in Verl, Germany, serves in numerous working groups in the BACnet community, including ASHRAE SSPC135 (aka “the BACnet committee”) and the advisory board of the BACnet Interest Group Europe. He was one of the early adopters of BACnet in Europe starting in 1997 and conducts training seminars for the German Engineers Association (VDI) and BACnet Academy since the year 2000.