Designing Appliances that Communicate
While people are more connected than ever, the various systems and appliances in our homes remain islands unto themselves, with no effective means to communicate. However, the need for our appliances to be networked is rapidly emerging, and fortunately, so are the means with the advent of ZigBee low-cost wireless platforms.
Growing energy demand, shrinking supplies, increasing costs and climate change are driving demand for more eco-friendly appliances. Under the Energy Star banner, appliance designers have engineered dramatic reductions in appliance energy consumption. Attention is now turning toward the “peak energy demand” dilemma, where approximately 10 percent of total electric generating capacity exists only to be used less than one percent of the time. If energy demand can respond dynamically to the available energy supply, huge cost and reliability gains can be achieved within the energy grid. This benefit is not lost on government regulators: the U.S. Federal Energy Policy Act of 2005, California’s Title 24, and similar initiatives across North America and Europe, are driving requirements for demand response systems in the home.
Demand response systems use Advanced Metering Infrastructure (AMI) networks that provide real-time, two-way communications between electric, gas, and/or water meters and their associated utilities. These increasingly include wireless Home Area Networks (HANs) that connect communicating thermostats, load switches, lighting systems and in-home displays to the meters.
Pilot projects in Texas and California have already begun, with rollouts planned for millions of homes starting early 2008. During periods of peak demand, AMIs and HANs work together to throttle high-load devices in participating homes, such as changing the thermostat setting of the HVAC system. Utilities save big by not having to build new power plants. Homeowners save money through lower bills and attractive rebates. And communities avoid the ravages of rolling blackouts. Utilities may institute “time of use” pricing schemes, where the AMI/HAN is used to communicate the current price of energy to the consumer. Smart, communicating appliances connected to the HAN can then be set to operate only during low-cost energy periods.
HANs for energy management come at a time when wireless home automation products that control entertainment, lighting, climate and security systems are taking hold. “Whole house” automation systems are expected to become standard in upscale homes within seven years, and will make significant inroads even in average homes. And various broadband and wireless telecom service providers are beginning to offer “home awareness” services that monitor connected home systems over the Internet or cell phones.
The ZigBee Wireless Standard
But is the technology for smart appliances ready for primetime? Just as Wi-Fi grew to meet the demand for wireless data networking, and Bluetooth for wireless cell phone connectivity, ZigBee has emerged as the standard for wireless device networks.
ZigBee is a wireless networking standard designed specifically for highly reliable, low-power and low-cost control and monitoring applications. Similar to the way Wi-Fi specifications leverage the IEEE 802.11 standards, ZigBee is built on top of IEEE 802.15.4, and enables devices to self-assemble into wireless mesh networks that can operate for years on low-cost batteries. The 15.4 standard defines the physical and MAC layers, typically operating at 250 kbps on one of 16 selectable channels in the 2.4 GHz band, which is uniquely unlicensed in most of the world. ZigBee further specifies a complete and reliable network stack that defines how the mesh network forms and operates, including device association and addressing, routing, security, and management. ZigBee also defines application profiles that specify device types and messages for various applications – such as lighting controls, HVAC controls and so on.
ZigBee is designed to be easy to incorporate into a wide range of devices, and be easily deployable. However, this does not mean ZigBee is a simplistic protocol. In comparison to earlier proprietary solutions aimed at home networking, ZigBee is highly scalable, supporting thousands of devices in a very robust and reliable self-configuring and self-healing mesh network. ZigBee also provides strong security capabilities to prevent mischief, and is extremely tolerant of interference from other radio devices, including Wi-Fi and Bluetooth. In fact, typical home automation/entertainment products often build in both Wi-Fi and ZigBee in the same device.
ZigBee defines three different types of nodes: ZigBee Coordinator (ZC), which is responsible for initial configuration and continuing control of the network; ZigBee Router (ZR), which can relay and/or respond to messages in the network, and a ZigBee End Device (ZED), which can send and receive, but not relay messages. There is one coordinator in each ZigBee network, and in typical home networks, this may reside in the electric meter, home gateway, or central home automation controller. Any device may be a ZigBee router, though these are generally line-powered devices as they need to be continually active in order to forward messages through the network. The simplest ZigBee devices are the ZEDs, which may implement various “sleep” modes in order to allow a very long operating life with low-cost batteries.
Implementing ZigBee in Appliances
Even though the ZigBee protocols are quite sophisticated, ZigBee can be fully implemented with low-cost analog/digital hardware and software running on a small microcontroller. Most designs use single-chip SoCs (system-on-chips), that integrate the IEEE 802.15.4 radio, MAC, embedded microcontroller core, AES encryption engine, RAM, Flash, and peripherals for SPI, UART, I2C, GPIO, ADC, and timers. Very few external components are required. The ZigBee stack runs as software on the core and is stored in the integrated Flash memory. The device application (such as a wireless light switch, temperature sensor, load switch, etc.) is also compiled to the embedded core, sharing cycles and memory with the ZigBee stack.
Sometimes a separate microcontroller is desired for the device application, such as when ZigBee is being added to an existing design, or when the application is reasonably complex. Here a ZigBee network coprocessor may be used. Here the device application interacts with the ZigBee stack (fully implemented in the coprocessor) via a simple serial (SPI or UART-based) interface. Hence ZigBee connectivity may be added to existing “smart appliance” designs in a relatively simple and straightforward way, and at low additional cost.
Many designers with deep experience in embedded microcontroller and software development may not have experienced the challenges of implementing RF radios in their designs. A poor RF design will dramatically impact the range and reliability of the final product. Fortunately, most ZigBee suppliers provide complete and proven reference designs for a wide range of different application scenarios, greatly simplifying this part of the design. Partners of these suppliers can also offer design services or even complete, low-cost modules that make implementing ZigBee that much simpler.
To assure interoperability with other ZigBee devices at the protocol level, and to earn use of the ZigBee Alliance logo, designers must start with a “ZigBee Compliant Platform,” consisting of the SoC or coprocessor hardware and the software stack, that has been tested by one of the Alliance-designated test houses. Use of the ZigBee-defined application profiles, such as the Home Automation profile, additionally assures interoperability at the device message level, and allows designers to have their end products tested as “ZigBee Certified Products.”
While “smart” appliances have proven to be a boon for consumers, smart appliances that communicate will enable societal benefits as well as greater convenience to consumers. With the emergence of complete platforms implementing the ZigBee wireless standard, designers can now deliver these benefits simply and cost-effectively. ZigBee has prevailed over earlier proprietary offerings for HAN and advanced home automation applications not only for technical superiority, but also because it is an open, multi-vendor standard provides designers many choices of platforms to use.