COMMUNICATION PROTOCOLS IN BUILDING AUTOMATION SYSTEMS
Building Automation Systems (BASs) provide means to control as well as monitor various operational aspects of buildings. Activities that typically require human interface such as data logging, equipment health monitoring, data analysis etc. can be automated by a BAS system to reduce human error – as well as enhance efficiency and accuracy of data. With the cost of manpower increasing, and the need to operate and manage buildings at the lowest possible cost, BASs are finding their ways in more and more building designs. A BAS while costing more at the build stage helps lower operating costs for the owners and the payback periods for the Capex are in the range of 2 to 4 years.
A typical BAS installation enables equipment and systems of a building such as the generators, HVAC plants, domestic water systems etc. to be ‘connected’ to a management system through sensors. Sensors capture system data such as pressure, temperature, flow etc., as well as the state of the equipment – on or off, speed etc. The sensors convert the data to electrical signals that are transmitted to the analysis device, usually a microprocessor or a computer where the data is again converted to enable the device to ‘read’ the data. The output of this data collection activity is the physical value the operator sees on the computer screen of the controller. Data can be logged in the computer’s memory or output to standard printers for review.
A BAS can be a simple monitoring tool where only system data is captured and analysed, but no active control of the equipment is provided. Advanced BAS have both monitoring and control abilities, which enable remote start/stop as well as operations of the systems.
BAS communications protocols
While a BAS has a number of components, the sensors, actuators and the controllers are the most important and critical elements. The sensor senses the physical quantity and the controllers manipulate the system through actuators to the required end points. There are a plethora of manufactures of both sensors and controllers, and hence, for the sensors, actuators and controllers to function correctly in a system, a common method of understanding data is essential. A communication protocol is a means for a device manufacturer to make sensors that can send out data in a way that the controller is able to read the data and take suitable action.
There are two main classifications of communication protocols – Proprietary and Open protocols. The proprietary systems are designed by manufactures that have the entire gamut of BAS systems inhouse or where there are specific building requirements. Open systems are the more common ones as they allow a larger number of options for the end users. The most commonly used protocols in the building automation industry are the BACnet, Modbus and LONWORKS.
BACNet (Building Automation and Control Network)
This protocol was specifically designed for the building automation sector, and the HVAC systems, and hence is one of the most commonly used protocols. The BACNet protocol is an ASHRAE/ANSI standard (135P) and was introduced in 1995. BACnet devices have a microprocessor that associated software to use the BACnet protocol for system application. The BACNet protocol uses ‘Objects,’ ‘Properties’ and ‘Services’ to represent data of the system.
- Objects: Each object has an identifier and a number of properties associated with it. The properties are used to monitor and control the object. 54 standard objects are defined in the protocol, such as Analog Input, Analog Output etc.
- Properties: The properties define a BACNet object. An Object can have multiple properties and each property has an identifier and a value. The property can be read only, where other objects read the value of the objects property and read/write where the property value can be changed – e.g. changing set point of a VAV controller.
- Services: This is the process by which one BACnet device request for information or gives instructions to other BACNet devices to carry out tasks. The BACNet protocol has a list of 32 standard services and categories into Object Access, Device Management, Alarm and Event, File Transfer and Virtual Terminal.
The BACNet protocol is very versatile and widely used in the industry as it can be used on most common network systems such as Ethernet, ARCNET and IP. The IP protocol allows BACNet devices to connect over standard internet networks, thus, allowing remote monitoring and operations.
How BACnet works
The BACnet protocol uses an Object-oriented approach (Object-oriented programming allows for simplified programming. Its benefits include reusability, refactoring, extensibility, maintenance and efficiency). As a result, this approach standardizes the representation of processes and data. Secondly, BACnet provides the ability to control and monitor any building automation process, to meet the needs of users, integrators, and equipment vendors. As a result, the BACnet protocol uses mobile and cloud-hosted devices, head-end computers, general-purpose direct digital controllers, and application-specific or unitary controllers with equal effect.0
In a typical HVAC application BACnet would handle lighting, heating/cooling with a programmable thermostat, smoke detection and some form of security/safety applications. In these applications, motion detectors can be used to more efficiently measure human presence and activity and with a timer to make sure lights are off after hours. To further simplify things, with BACnet, devices from various manufacturers work in conjunction with each other with little or no additional integration. This leads to cost savings over proprietary systems, but also tremendous savings in resources, installation costs, maintenance costs, and energy savings.
Today, the majority of users prefer to use Ethernet. For example, the Control and Information Protocol (CIP) used in industrial application, BACnet uses objects to represent data on a network. These objects are defined by the BACnet specification and have both required and optional data. BACnet is an unconnected, peer network where any device can send service requests to any other device. Unlike connected protocols where devices have ongoing data transfers, communication in BACnet is unscheduled without any time critical operations. With ease of use in mind, BACnet is a certifiable standard, though certification is not required.
What are the 2 Protocols?
There are two distinct BACnet protocols being BACnet IP and BACnet MSTP.
BACnet IP:
The BACnet/IP allows users to transfer data to and from devices over Ethernet using BACnet/IP Protocol. BACnet/IP communication is implemented by defining a new protocol layer called the “BACnet Virtual Link Layer” or BVLL. There are many advantages to this approach. First it is extensible to other, future transport mechanisms such as IPv6, ATM, Sonet among others. Secondly, the concept of defining an extensible mechanism for peer-to-peer management of BACnet messages means that other manipulations. This includes such things as encryption/decryption and compression/decompression that can be performed outside of the process of generating BACnet APDU/NPDUs, i.e., without altering the existing standard. Routing between BACnet/IP and non-BACnet/IP networks is specified, including the case where IP and non-IP BACnet devices reside on the same LAN.
BACNet MSTP:
The MS stands for Master – Slave although in practice there are not many slaves out there and the TP stands for Token Passing. This method of BACnet the most common use to connect field devices to controllers and routers, to control applications. Up to 128 devices can be installed on a single network in the physical layer using RS485 with a max physical length of 4000 feet, and speeds up to 115k baud. Nevertheless, if more length is needed, it can be increased by the use of repeaters. You can compare to Ethernet, where the spec allows a max of 100 meters (330ft) on a single, unrepeated segment. Common baud rates are 19200, 38400 and 76800 and all devices must operate at the same baud rate. More and more devices can auto sense the baud rate and configure themselves correctly.
However, a disadvantage of the token system is that any one device gets a limited use of the bandwidth. Thus, a device may need to keep an internal queue of application layer messages it wants to send waiting to use the token. There are some vendor systems which fill their queue and then drop subsequent messages without notifying the user of the problem. Finally, limited access, combined with the overhead, makes it easy to use up all the bandwidth on the network. This happens if there are many devices with many objects and many properties of interest.
Key Points
- Developed by: ASHRAE
- Use: Communication across devices
- Markets: Industrial, Transportation, Energy Management, Building Automation, Regulatory and health and safety
- Examples: Boiler Control, Tank Level Measurements
- Proprietary: no
- Transmission Modes: Ethernet, IP, MS/TP, Zigbee
- Standards: ANSI/ASHRAE Standard 185 ;ISO-16484-5; ISO-16484-6
- Costs: Low; No charge for usage or licensing fees
- Network Interfaces: Existing LANs and LANs infrastructure
- Testing: BACnet Testing Labs
Advantages:
- Scalability between cost, performance and system size
- Endorsement and adoption by nearly every major vendor in North America and many other countries
- Robust internet working including multiple LAN types and dial-up
- Unrestricted growth and the ability to add new innovations and new features anytime
Disadvantages:
- Limited the number of field devices that can connect to a master station except Ethernet TCP/IP
- MT/TP-Wire Length
- Ethernet-Infrastructure
- New standard has security standard but not implemented in all devices
KNK Protocol
The KNK communication protocol is an approved European (EN) and International (ISO) Standard, which is also widely found in building automation system applications. The protocol can be used over simple Twisted Pair lines as well as RF and IP networks.
The KNK protocol has two configuration modes which manufactures provide
- S Mode (System Mode), which is used for complex installations requiring a high level of customisation and features.
- E Mode (Easy Mode) provides basic functionalities to the devices and can be programmed without any specialised knowledge and tools. This mode is used for simple systems.
A KNX system has two main components – The sensor and the actuator. Data between Sensors and Actuators are shared in the form of ‘data telegrams.’ These telegrams are sent out to Group Addresses which are logical names for ‘topics.’ The topics link the output of a sensor to the input of an actuator. Sensors emit the telegram and multiple sensors listen into the topics.
A KNX system can be used without a controller during normal operation – as a processor is not required for the group to listen to the telegrams. This allows the system to be robust as well as reduces cost.
An Engineering Configuration Tool (ETS) is used to configure KNX devices that use the S Mode configuration. Figure 2 shows a typical KNX installation.
KNX has been developed for use on a large number of building management systems and covers:
- lighting
- blinds & shutters
- Heating, Ventilation And Air Control (HVAC)
- Audio/Video Control (AV)
- operation and visualisation
- security
- remote access
DALI protocol:
DALI (Digital Addressable Lighting Interface) is very useful for lighting control in building automation systems. DALI provides effective control over lighting by addressing each device separately. It supports 256 levels of brightness.
High signal-to-noise ratio leads to reliable communication. Communication remains bidirectional so that we can take feedback of operating state of the lamp. Its flexibility can be known by the fact that you can easily change the lamp in case of lamp failure. We can also integrate Emergency lighting with the help of this protocol. Up to 64 devices can be connected on a single DALI network . Multiple networks can be connected through gateways. It is a worldwide standard.
Conclusion
BAS are essential components of modern buildings, with many Green certification systems mandating them. The BAS systems enable better control of the various equipments and systems in a building, resulting in higher efficiencies and reliability. Which system to use, BACNet or KNX? Both are reliable and robust systems, independent and open systems, and hence are supported by a large number of manufactures across an exhaustive product range.
The choice of communication protocol to be used is usually decided by the equipment and control elements that are installed and the level of flexibility that is required. Building automation products typically use the BACNet protocols – while the building management systems tend to deploy the KNX protocol.
Good content
ReplyDelete