Wireless technology continues to expand into the world of building controls as part of increasingly sophisticated, highly integrated building-automation systems (BASs). A key driver of this trend is the advent of wireless control networks enabled by mesh technology. With each implementation, the technology is proving it can deliver secure, reliable solutions that provide optimum control and unprecedented flexibility.
NO HARD-WIRED TRUNK
Wireless-mesh-technology devices include application-specific controllers (ASCs), point modules, variable-frequency drives, and meters. These devices sit on a field-level network (FLN) distributed throughout a facility and connected to a supervisory controller that performs central-monitoring and control functions. With wireless mesh technology, there is no need for a hard-wired trunk for these devices to communicate.
Because they are relatively power-hungry, FLN devices must be line-powered. That means their related radio-frequency (RF) electronics are able to scavenge power from the same source. Because they always are on, FLN devices are able to both receive messages and route messages to neighboring devices.
Recently, there has been a push to add battery-powered devices (e.g., room-temperature and other ambient sensors), which are easier for system designers to “hang,” to BAS control networks. Because they are battery-powered, these devices must conserve energy. They do that by “sleeping” most of the time, awakening periodically to see if an event (e.g., a change in temperature) has occurred and, if one has, transmitting a message to a line-powered node. The device then returns to sleep mode, resting until the next cycle.
Because battery-powered devices are asleep most of the time, they do not transmit messages to neighboring devices. However, because they can communicate with multiple line-powered nodes, there are multiple communication paths, ensuring reliability.
Mesh topologies swiftly are changing perceptions of wireless' reliability. As a result, more and more BAS engineers and designers are installing systems that would not have been economically or physically possible just a few years ago.
Efficient heating and cooling schemes require accurate inputs, which often are dependent on the final configuration of a space. Wireless sensors can be positioned after a space is finished. Rather than on a wall that is bathed in sunlight all morning or in shadow all afternoon, wireless sensors can be placed where they will provide the most accurate inputs for the ambient conditions of a space.
A wireless-mesh-controller network can be implemented two ways:
The controller communicates wirelessly through an external radio mounted in close proximity.
RF electronics are embedded in the controller.
Typically, the location of a controller is dictated by the location of the equipment to which it is mounted. The biggest advantage of deploying an external radio is that, if necessary, the radio — and its antenna — can be located away from the controller to optimize the number of communication links to other radios or circumvent obstructions. Embedding RF electronics in a controller eliminates the need to mount and power an additional device; however, it restricts location of the antenna, which can be mounted no farther than approximately 1 ft from the controller to minimize attenuation losses.
Following are three case studies involving use of external IEEE 802.15.4-2006, Wireless MAC and PHY Specifications for Low Rate Wireless Personal Area Networks (WPANs)-compliant physical radios and wireless communications based on the ZigBee standard. Radios are mounted at each ASC, with one radio for each network of ASCs at the higher-level building controller. Each radio has two switches: one for selecting from 16 2.4-GHz frequency channels and another for selecting a unique network identification. Installation and startup consisted of ensuring the two switch settings were correct; mounting the radios; connecting ANSI/TIA/EIA-485-A-98, Electrical Characteristics of Generators and Receivers for Use in Balanced Digital Multipoint Systems-compliant communication ports to the controllers; and connecting the power.
Because traditional wireless local-area networks (LANs) tend to be located in close proximity to radios, operate at relatively high power levels, have long “on-air” times, and use a good portion of the unlicensed 2.4-GHz frequency band, they may be subject to interference. Fortunately, the duration of on-air time for wireless HVAC devices is comparatively short.
Research reveals the most effective way to avoid interference between wireless HVAC devices and wireless LANs is to use frequency channels in the 2.4-GHz band on which wireless LANs do not typically operate (IEEE 802.15.4 channels 15 and 20) or never operate (IEEE 802.15.4 channels 25 and 26). Research and field experience have shown that if the same frequency must be used, a little physical distance between wireless LAN devices and wireless HVAC devices eliminates the possibility of interference.
All of the radios in each of the three case studies are powered from the same source as the controllers. In two of the case studies, this is the same power source as the mechanical or electrical system being controlled. Therefore, a dedicated power trunk did not have to be pulled to supply the ASCs and radios to realize the full benefits of wireless.
With a mesh-based network, installers do not need to ensure that ASCs and their associated radios are within range of one another or that no obstructions exist or will exist between them.
Often, when engineers can review a design drawing and verify that the project's characteristics are conducive to a mesh network, installation is as simple as “plug and play.”
Assembly line stays online
Approximately five years ago, managers of a 26-year-old, 400,000-sq-ft heavy-equipment-assembly plant near Asheville, N.C., were planning a major HVAC- and lighting-system upgrade. A traditional hard-wired network would have required the running of wire and conduit over long distances — in many cases, more than 100 ft between controllers — throughout the plant's 30-ft ceilings. That raised concerns about both safety and assembly-line disruptions. Additionally, a hard-wired project was cost-prohibitive. Wireless mesh technology allowed system designers to address the plant managers' concerns and fit the project within the plant owner's modest budget.
The challenge was to network and control from a central location equipment (gas-fired unit heaters, rooftop air-conditioning units, office split systems) that required time-consuming manual on/off operation.
Eighty-six controllers and associated radios were installed in the ceilings of the complex's four buildings for fan and gas-heat control, while six controllers and associated radios were installed in each electrical penthouse to operate the existing lighting circuits. For each wireless network, a radio was mounted at one of the supervisory building controllers. Four building controllers and associated radios are located on the plant floor, while a fifth building controller and associated radio are located in an equipment room off of the plant floor.
With wireless networking, each controlled component is accessible from multiple locations determined by the plant's facility staff. The staff can monitor and control space temperature at each unit heater from any BAS workstation, while facility managers can determine the extent of local control floor personnel have.
The BAS in a hospital in Davis, Calif., was in need of an upgrade. Everything from sensors to workstations needed to be replaced. New Ethernet-based supervisory controllers and workstations were deployed, using the hospital's existing Ethernet infrastructure. All of the approximately 220 terminal-box controllers and a number of differential-pressure monitors, zone humidifiers, and fire/smoke-damper monitors were connected via a new network infrastructure so they could communicate on the BAS.
With the existing network cabling incompatible with the new BAS, the hospital opted to use a wireless network to connect devices.
Old-school upgrade, new-school technology
Recently, a school district in South Carolina embarked on a four-year program to renovate its 46 schools. The program began at a 45,000-sq-ft, single-story elementary school built during the 1950s and expanded over the next several decades. The 25-classroom school is a typical K-12 facility, with long wings, cinderblock walls, and suspended ceilings hanging approximately 4 ft below the roof deck. Some of the block walls are filled with concrete.
Temperature was controlled via a central panel in the electrical room. If a teacher turned on one override for a zone, he or she might also have turned on 12 or 15 heat pumps. To check temperature and humidity levels after hours, a member of the facility staff would have to drive to the school and go into individual classrooms. Another problem was that the county's infrastructure-improvement budget did not support the construction costs associated with cutting through walls to run new cable and busway.
The goal was to move from an electromechanical control system to a direct-digital-control system that managed the heating, air-conditioning, lighting, and energy use of the entire building. The challenge — install and deploy the new BAS in only three weeks (to work around the summer-school schedule) while minimizing the impact on the existing structure — was significant. The solution? A wireless ASC network.
The school district completed the installation in one week and within its limited budget. Additionally, the number of wall and ceiling penetrations was significantly lower than it would have been with a hard-wired network.
Post-installation checkout revealed that two radios located at the end of a wing had not established robust communications with the rest of the network. A “repeater radio” installed between the two radios corrected the problem immediately.
More efficient control is saving the district money by reducing energy costs. Engineers now can perform remote monitoring of day-cooling set points, night-cooling set points, night-heating data, and numerous other points via the room controllers. Moreover, with the wireless system, the district can add a room controller, move a classroom, or add a water heater without having to run cable; all that it has to do is install a wireless device exactly where it is needed.
As with the assembly plant and hospital, various RF systems and devices — including wireless LANs, security systems, inventory-control systems, microwaves, and cell phones — were in use in the school. However, at no point have there been any interference issues.
FOR BEST RESULTS
When the “RF-friendliness” of an existing facility is unclear, consider an informal site survey. Such a survey might consist of simply powering up a few radios and locating them throughout the facility in a manner approximating their positions in support of a mesh-controller network.
If, during final deployment, a system cannot establish an adequate number of communication links, install additional “repeater radios” in “holes” in the mesh, or reposition existing radios and/or antennas.
ABOUT THE AUTHOR
Jeff Raimo is a product manager focused on the integration of new wireless technologies. He has a bachelor's degree in aerospace engineering and a master's degree in business administration from the University of Illinois.
In a wireless mesh network, messages are passed from device to device without having to be routed through a central switch point. The network is smart enough to determine the optimal route for messages; if that route is obstructed, the network automatically shifts to the next-most-efficient path.
Once devices are installed and communication is established, a wireless mesh network essentially becomes “invisible” to a building-automation system (BAS) — that is, the BAS does not distinguish between hard-wired and wireless devices; all are recognized as integral components.
Through evaluation of certain project characteristics, an engineer can determine whether a wireless mesh network is appropriate for his or her application. Key questions include:
Will wireless devices be in a grid-like arrangement or a long, narrow configuration? A grid-like arrangement is more conducive to mesh topology.
Will wireless devices be in a dense configuration without radio-frequency (RF) obstructions between them so that each wireless device is within communications range of two or three other wireless devices? Generally, the denser the configuration and the fewer the RF obstructions the better.
Are other wireless devices used in the facility? If yes, what frequencies are being used? An engineer may want to consult with the information-technology department to determine if a frequency-channel-management plan is in effect.