March 2012
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AutomatedBuildings.com

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Deploying a High-Performance Wireless Monitoring and Control Network

A reliable wireless system should be able to transmit data through various types of building materials, tolerate interference from other devices, and recover from disruption to the network.

Harry Ostaffe
Harry Ostaffe

Vice President
Marketing and Sales
Powercast Corporation


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Much has been written about the benefits of wireless technology for building automation, HVAC control, and energy management.  Over the past several years, wireless technology has improved greatly in terms of performance and lower power consumption.  Product innovations have also accelerated as regulatory certification has become less burdensome with the FCC’s approval of  pre-certified wireless modules.

The most obvious benefits of wireless solutions are rapid deployment and low installation costs, especially for retrofit applications where wiring is prohibitively expensive.  Facilities constructed with lots of concrete block (e.g. K-12 schools), with asbestos, or projects where running conduit is required, are all great candidates for using wireless technology to reduce costs.  Wireless is also a great solution where installation is time sensitive or where occupant disruption has to be kept to a minimum.

Wireless Applications

Wireless can be used for a wide range of applications from simple point-to-point links to a facility-wide network, for real-time control or just monitoring and data logging.  Example wireless sensors include temperature, humidity, light level, differential pressure and pulse meters.  And new, low-power wireless CO2 sensors are now enabling rapid wireless retrofits for demand-controlled ventilation (DCV).  DCV can deliver substantial energy savings with a relatively short payback timeframe, as documented in a study by the US. Department of Energy.  A wireless solution makes DCV implementation faster and easier than wiring.

Temporary data logging is an often-overlooked wireless application, such as during an energy service company (ESCO) pre-investment audit.  In this case, wireless data logging with the ability to push sensor data to the cloud in real-time, is far more useful and cost effective than repeated trips to collect data loggers, download their data, and redeploy them.  Instead, ESCOs can use a wireless system for pre-investment audits, and then leave it in place for monitoring during the life of the performance contract.

Wireless Technology and Robust Performance

The three most important considerations when designing and deploying wireless-based professional HVAC and energy-management systems are: battery life, range, and reliability (see Figure 1).

 Figure 1: Key wireless deployment parameters

Figure 1: Key wireless deployment parameters

Battery Life
Wireless devices with short battery life result in higher maintenance costs and potentially reduced system efficiency, with subsequent lost energy savings, when such devices fall out of the network because their batteries aren’t replaced.  Many wireless sensors on the market today are specified with 3-5 years of battery life, though some systems may reach 7 years if they’re configured for longer sampling periods.  Over time, the repeated cost of battery replacement negates the benefits of wireless technology, and even limits the scale to which facility managers are willing to deploy a wireless system.

One solution to the problems surrounding batteries is to harvest energy from other sources, such as solar or artificial light, to create a perpetual power source. The challenge is that indoor solar harvesting is not reliable in all scenarios because it is an intermittent energy source, and not available in places that receive no light.  HVAC system designers and facility operators need sensors to provide reliable data all the time, not just intermittently when a device receives harvested power.  Adding a battery improves performance, but rechargeable batteries do not have the same operating life as high-quality primary batteries.

Consider a battery-based system that could operate for 25 years or more, sending data every minute. This kind of system would provide more reliable performance than one based on indoor light harvesting, and would eliminate the issue of battery replacement, because the control system is likely to be replaced before the battery is depleted.

Range
Range can impact both system cost and battery life.  Lower frequencies, higher transmission power, repeaters, and none-to-node meshing are various techniques used to extend range.  Some component manufacturers and system providers promote various flavors of mesh protocols as a way to extend range, by having data hop from one sensor node to another until it reaches the destination gateway.  There are benefits to this approach, but they come at the expense of latency and battery life.  For example, the data from a sensor node that is five hops from a gateway will take 5 minutes to get there if each node wakes up and listens for data once per minute.  Listening for data from another node is an expensive use of finite battery energy.  Applications requiring real-time, or near real-time performance may not be able to tolerate this type of delay.  For HVAC control, such a delay means greater swings in environmental conditions which impacts occupant comfort, and reduced system efficiency leading to higher energy consumption.

Line-powered repeaters can forward data in real-time, which is a better approach to extending range in commercial building control applications.  Repeaters are easily installed in utility closets and require minimal configuration.  Repeaters should be capable of extending range to unlimited distance and form a network of various topologies including linear, ring, and mesh (see Figure 2).  A repeater network maximizes the network’s range while preserving battery life of the sensor devices.  Repeaters that use smart forwarding techniques will not forward duplicate packets from multiple sources and thereby reduce the overall amount of network traffic, which also improves wireless performance.

 Figure 2: A repeater network links various topologies (linear, ring and mesh) to extend range.

  Figure 2: A repeater network links various topologies (linear, ring and mesh) to extend range.

Reliability
Reliability, or the ability of wireless transmissions to reach their intended destination, is critical.  A reliable wireless system should be able to transmit data through various types of building materials, tolerate interference from other devices, and recover from disruption to the network.  Lower frequencies propagate through building materials better than higher frequencies, and users can employ various modulation and frequency agility mechanisms to minimize or overcome the effects of interference.  Some systems simply increase the transmission power to force the signal through, but this solution potentially interferes with other signals, and also reduces battery life.

Control Solutions, Inc An alternative to achieve reliable signal performance is to combine both robust modulation and frequency agility.  From a wireless network perspective, using a line-powered repeater network in a mesh or ring configuration enables multiple paths back to the gateway.  A properly-designed repeater network is inherently self-healing in the event of a disruption.  Additionally, implementing multiple wireless gateways such that redundant gateways can receive the same information enhances overall system performance.  These gateways can be in different areas, fed by different power circuits, and connected to different network routers.  In the event of a failure, the control system can switch over to the backup gateway.

Example Wireless Sensor System for HVAC, Building & Industrial Automation

Powercast has developed a wireless sensor system intended to provide good range, link and system-level reliability, and the longest battery life available in the building controls industry.  The company’s goal was to provide the convenience and flexibility of wireless with the predictability and longevity of wired solutions.  The Powercast Lifetime Power Wireless Sensor System offers:

•    25+ year battery life – using proven battery technology and transmitting at one-minute internals
•    Long range – including unlimited network range, and real-time network performance
•    High reliability – supports frequency agility, self-forming/healing network, and redundancy

Wireless Sensors
Most wireless sensors for our system are available in small, 4”x2”x1” enclosures and are suitable for wall-mounting in commercial office buildings (see Figure 3).  They can be configured off-site and deployed rapidly on-site.  Sensor types include:

•    Temperature
•    Humidity
•    Light level
•    External temperature probes
•    External dry contacts
•    Carbon dioxide (CO2) – self calibrating
•    Differential pressure

 Figure 3:  Sensors come in small, 4”x2”x1” enclosures suitable for wall-mounting.

Figure 3:  Sensors come in small, 4”x2”x1” enclosures suitable for wall-mounting.

We designed our wireless sensors for very low power consumption, which results in their ultra-long battery life. We have also performed several simulated, worst-case, accelerated battery life tests which confirmed operation for longer than 25 years. The company has produced a test report (free download) to document the test procedure and results. 

Wireless Gateway
Supports up to 100 devices and connects to any BAS or IT system using standard protocols including BACnet, Modbus, LonWorks, Metasys N2, SNMP, XML, and more.  Also supports data logging to a local PC or connectivity to the cloud for remote monitoring.

Wireless Repeaters
Support linear, mesh, ring, or hybrid topologies with real-time latency performance.  The repeater network is inherently self-forming, self-healing, and capable of unlimited hops.

More information is available at www.PowercastSensors.com


About the Author
Harry Ostaffe is vice president of marketing and sales at Powercast Corporation, and has over 23 years experience in the fields of broadband and wireless networking, industrial controls, and computing through various management and engineering positions at Ericsson, Marconi, Lucent Technologies, AT&T Network Systems, Bayer and IBM.  Harry received an MBA from Carnegie Mellon University and a B.S. in Electrical Engineering from Penn State University.
www.LinkedIn.com/in/HarryOstaffe


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