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Wireless mesh nets offer bright prospects for lighting control

Wireless mesh nets offer bright prospects for lighting control

Technology News |
By eeNews Europe



A simple gateway from Ethernet/Wi-Fi/USB to low-power wireless can ensure that users have access to all features of their lighting control system through their smartphone or tablet. Texas Instruments demonstrated the concept at Mobile World Congress this year using the Android-based ZigBee Home Automation lighting application running on a mobile platform.

The low-power requirements of the network and regional nature of RF regulations make it unrealistic to design a wireless lighting network topology that requires all nodes (lights, switches, sensors and remotes) to be in the RF range of a single coordinating node in the network. The solution is an even distribution of routing nodes throughout the building to extend range without increasing power—that is, mesh networking.

Lamps constitute the ideal backbone for a wireless mesh network. A good mesh network is self-forming and self-healing, and can deal effectively with a high number of nodes in diverse topologies.

Compared with a radio at 2.4 GHz, sub-1-GHz radios yield longer range and better penetration for the same amount of power. For that reason, sub-1-GHz solutions are often preferred for outdoor applications such as street and city lighting. There is also generally less interference to deal with in the sub-1-GHz bands, as Wi-Fi gear and microwave ovens do not operate in that range. Radio interference is present in all the open ISM bands, however, and it is thus important that the radio have sufficient output power and a receiver with good selectivity (adjacent channel rejection) and blocking to filter out unwanted signals.

But there is no globally available sub-1-GHz band. That forces development of regionally specific end products—supporting, for example, 868 MHz in Europe or 915 MHz in the United States.

Figure 1: Low-power wireless lighting control.
Click on image to enlarge.

Commissioning

Commissioning with traditional lighting is intuitive but expensive; whichever lights are connected after a breaker switch will be controlled by that switch. Wireless adds flexibility in terms of which switches and remotes control which lights, but it also adds complexity in the installation process that must be mitigated by offering an intuitive procedure for connecting lights, sensors and switches.

For professional installations, a USB dongle and a good graphical PC software tool can enable complex connections to be made in an intuitive manner, but such an approach generally requires the services of a trained installer.

User-installed systems, available off the shelf, must be based on simpler, more intuitive methods. One approach, proximity-based commissioning, involves holding the switch or remote control close to the light(s) to be controlled, while pressing a button. The receive signal strength of the packets sent from the light to the switch or remote control is used to determine proximity and qualify the commissioning. The Philips SmartLink system represents a successful implementation of this approach.

Security

Wireless lighting control systems require differing levels of security, depending on the purpose and location of the system. A home lighting system used mainly to set the correct ambiance will have less strict requirements than city lighting or a building’s security lighting would.

Most low-power wireless chip sets today support 128-bit AES encryption of the packets sent over the air, which is generally sufficient to avoid sniffing or injection. Authentication and key exchange when new devices enter the network are challenging and are handled differently depending on the level of security needed and the mechanisms available.

As low-power wireless chip sets have shrunk in size, raised their integration level and dropped in cost, it has become viable to include them in a wide range of lamps to provide direct control of each. Given the long lifetimes of LED- and fluorescent-based lighting, it is practical to integrate the wireless functionality with the light source itself.

Although modern and more efficacious light sources generate less heat than traditional light sources (such as incandescent and high-intensity discharge lamps), LED and CFL lamps contain driver and control electronics and therefore still create high-temperature environments. The heat becomes particularly challenging in compact designs, where the cooling situation surrounding the lamp is generally not controlled.

LEDs must also conduct their heat away (as opposed to radiating heat, as is the case with incandescent and gas discharge technologies). That compounds the problem of keeping the heat away from the driver and control electronics. The low-power wireless and driver control ICs will sustain high temperatures during operation of the lamp.

It is thus important that the ICs support operation at high temperature to ensure correct operation, good RF and high-power-quality performance. Chip sets qualified at 85°C are typically not an option; a 125°C rating is sufficient for most applications, unless a very compact fluorescent design exposes the electronics to even higher temperatures.

The devices and related external components should also be qualified for a high-temperature-operation lifetime that is as long as the expected general lifetime of the bulb.

Energy efficiency

Standby current consumption has been a focus of conservation efforts in recent years, and the continued tightening of regional requirements extends to the standby current specs for wirelessly controlled lamps. When the lamps are in standby with the light turned off, the radio is in receive mode all or a duty-cycled portion of the time. Although low-power wireless radios, regardless of technology, typically consume less than 10 mW in receive mode, there can be substantial loss in the power supply. Care should be taken to design a power supply for the wireless device that meets the desired targets for standby current consumption. The lamp or luminaire must also meet a minimum lumens/watt requirement, so the efficiency of the lighting driver is critical.

Low-power wireless chip sets are continually shrinking in footprint. TI’s CC2530 system-on-chip, with integrated radio, microcontroller, flash, RAM and peripherals for lighting systems, fits in a tight, 6 x 6-mm package. Apart from a high-frequency crystal and decoupling capacitors, all that is required for a complete solution is the addition of a balun and antenna.

The antenna represents the widest range of design choices, and the size of the solution can vary greatly. Antennas can take the form of pc board traces, whips, wires or integrated chips with associated solution size, cost, efficiency and directionality. The lower the RF frequency, the longer the antenna; for applications with restrictions on solution size, that ratio can be an important factor in choosing between sub-1-GHz and 2.4-GHz bands.

The small solution size makes it important to ensure that both the MCU system and the radio are robust with respect to noise so that they can operate well even in proximity to switching power supplies. The power converter switching frequency will also affect the size of the power magnetics. Higher switching frequency will result in a smaller lighting driver (power converter), but at a trade-off of lower efficiency and potentially higher electromagnetic emissions.

Figure 2: The CC2531 2.4-GHz USB nanodongle for
IEEE 802.15.4 (1.65 x 0.95 cm).

To maximize freedom of placement and mobility—and to avoid pulling wires—sensors, switches and remote controls in a lighting control system are generally battery- or energy-harvesting-powered. To maximize battery life or enable devices to run off energy-harvesting sources, the radio should be used as little as possible, remaining in sleep mode most of the time.

Remote controls and switches typically only wake up on key presses and perform the required transaction before going back to sleep. Sensors typically wake up periodically, using a low-power timer, in order to perform sensor measurement via an internal A/D converter, or they use an internal low-power comparator to be awakened only when a certain threshold value is reached. The system should be designed so that the values are reported over the radio as seldom as possible. Current consumption in sleep mode will generally dominate the energy consumption of these devices and will be the determining factor for battery lifetime.

ZigBee and 6LoWPAN (IPv6 over low-power wireless personal area networks) are publicly available low-power radio standards supporting 2.4-GHz IEEE 802.15.4 radios, which have shipped in the tens of millions since their launch in 2004. The robustness, low power and simplicity of IEEE 802.15.4 radios have driven them into applications such as metering (ZigBee Smart Energy), consumer remote controls (ZigBee RF4CE) and industrial control (WirelessHART and ISA 100).

ZigBee is a mesh network software stack, based on IEEE 802.15.4 radios, with a set of application-layer profiles on top. The profiles ensure that there are standard devices defined with standard telegrams to send commands, such as “on” or “off” in the case of a light. The profiles also provide standard solutions for such aspects as commissioning and security. That ensures, for example, that lights from one vendor will interoperate with switches from another vendor.

The ZigBee Alliance has a certification process to ensure specification compliance and interoperability among different vendors’ offerings; a ZigBee-enabled wall plug or passive infrared sensor, for example, can be obtained from another vendor if needed. Currently, the Home Automation and Building Automation profiles are the ones most relevant for lighting applications, but other profiles are under development to cover other aspects of lighting control.

Figure 3: ZigBee architecture, with standards-based layers in gray and proprietary layers in red.

6LoWPAN is a header compression scheme for IPv6 packets. In combination with the RPL mesh network routing protocol, it provides an efficient IP-based stack for low-power wireless networks.

Figure 4: 6LoWPAN architecture, with standards-based layers in gray and proprietary layers in red.

These open standards are defined by the Internet Engineering Task Force and promoted by the IPSO Alliance (the latter promotes the use of Internet Protocol for “smart object” communications). 6LoWPAN also forms the basis for the ZigBee IP stack on which the Smart Energy 2.0 profile specification is built.

Thanks to native IP addressing, gateways from a 6LoWPAN network to the Internet can be made simple and transparent. The result can be a reasonably seamless interface to tablet, phone or existing IP-based solutions for lighting and building control.

The challenge with 6LoWPAN lighting control today is that application layers have yet to be defined, and standards-based solutions for such aspects as commissioning and security have yet to be developed. Since there are no application layer specifications or standards body certifying solutions, there is no clear path to interoperability.

About the author

Peder Rand leads strategic marketing efforts for Texas Instruments’ low-power RF IEEE 802.15.4 and ZigBee products. He holds a master’s degree in computer science from the Norwegian University of Science and Technology.

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