Electronic devices are integral parts of almost every light source or luminaire released to the market these days. Electronic circuits are used in power management, switching and dimming control, color mixing, system monitoring and the user interface. Digital and analog integrated circuits are also used in electronic ballasts or high-intensity discharge lamps, compact fluorescent lamps, LED lighting, incandescent lamp dimmers and control equipment.
To use the electronics inside the lamp in a smart way, and make it perform tasks other than simple on-off switching, an interface is needed to communicate between devices (such as luminaires and sensors) and with a controller. New interfaces are being promoted that require no dedicated cabling, and that are therefore cheaper and easier to install. But are they ready for implementation today?
Why Traditional Lighting Interfaces Are Outdated
One of the easiest ways to implement dimming control for lighting fixtures is with the traditional 0V to 10V protocol. This simple technique is easy to implement, but has its disadvantages. First, every light source must be controlled separately. In a system with many lamps, this requires a large amount of cable.
Second, the protocol only supports one-way communication, and only supports dimming – the system cannot gain information about the lamp. To address these deficiencies, digital network technologies for lighting were developed: first, DMX512A-RDM), which daisy-chains up to 512 devices on a single bus with two-way communication. Similarly, Digital Addressable Lighting Interface (DALI) supports a network of up to 64 devices, each with individual and group addresses. Broadcast messages can control scenes or groups of light sources.
DALI offers a data rate of 1200baud, whereas DMX512A offers a data rate of 250kbaud. DALI, then, is suitable for lighting systems in buildings in which the digital management of light sources is required, such as modern office buildings and hotels, or where a monitoring feature is needed. DMX512A is ideal for professional lighting environments.
But both DALI and DMX512A networks still require their own cabling infrastructure separate from the power. This adds significant cost to lighting installations. It might even be impracticable to add control cabling to existing structures that already have power lines running to the lighting equipment.
Great expectations, then, have been raised for the introduction of lighting control protocols that require no additional cabling.
How to Eliminate Separate Control Cabling
Traditional lighting wired interfaces are mature, and a proven solution for their intended applications: dimming, stage lighting and lighting management in large buildings.
But their limitations make them unsuitable in some circumstances:
- As above, where power cabling already exists but there is no provision for control cabling
- Where a large number of lights needs to be operated by a single control point
- Where two-way communication at a high data rate is required, for instance to transmit data from temperature or light sensors
- Where color-mixing is required
Two different technologies that might provide the answer appear to be gaining commercial support today: wireless communication using unlicensed ISM frequency bands; and power line communication, carried on existing mains cabling.
But is the technology really ready to be adopted by the lighting industry?
Wireless Lighting Control
What are the particular operational requirements of a wireless lighting network? It must:
- Support many nodes over a long range
- Operate at low power, including in stand-by mode. This is especially true of systems using wireless switches or sensors unconnected to the mains
- Connect devices quickly at start-up
- Add little to the total cost of the lighting equipment
These requirements disqualify Bluetooth and WiFi, the two most well known wireless protocols. They offer a limited range and number of nodes and consume too much current. It also takes a long time for devices to connect at start-up.
Fortunately, there is a wide choice of other RF protocols, each of which has its own combination of strengths and weaknesses. Manufacturers of lighting control equipment will certainly be able to find one which meets the requirements listed above, and which will operate in the intended application’s conditions.
There are broadly two types of applications, each of which entails different operating conditions: outdoor lighting and indoor lighting. For outdoor applications such as street lighting, the protocol must provide long range between nodes, while supporting a long, thin network.
In indoor applications, the operating range between nodes is less important as they are closer together. But the radio protocol should be able to form a tree or mesh topology (see Figure 1). This allows the overall range of the network to be extended, as each node effectively acts as a repeater.
The mesh topology is the more sophisticated; it provides multiple paths for the signal and, together with the self-repair feature often found in mesh protocols, keeps the network operating even when some nodes fail. Mesh network management, however, is relatively complex and therefore consumes more processor resource; for this reason, lighting control networks will generally find the simpler tree topology more suitable.
The range between nodes depends mainly on transmit power, receive sensitivity, frequency and propagation conditions. All other things being equal, a lower frequency (<1GHz) system will have greater range and a better ability to penetrate masonry and other obstructions, but will consume more power. 2.4GHz systems offer contrasting benefits: smaller size, lower power consumption, higher data rate and faster network management.
Manufacturers of lighting control equipment, then, have a variety of factors to consider when evaluating RF technologies:
- Protocol features and choice of topology v complexity and drain on processor resources
- Frequency v range and power consumption.
Having decided on these high-level requirements, the equipment designer must then consider specifically which protocol to adopt. The ZigBee standard is the most popular wireless network type which meets the requirements for lighting networks. Key specifications include operation at sub-GHz and 2.4GHz frequencies; up to 250kbits/s data rate; 100m range (open air at 0dBm power at 2.4GHz); and support for tree and mesh networks.
|California Eastern Labs||California Eastern Labs||Synapse||NXP||RFM||RFM||Microchip|
|Architecture||Module||Module||Module||Transceiver and Module||Module||Module||Transceiver and Module|
|Chipset||MC1322xV||Ember’s EM250||8051||32-bit RISC||8051||Ember’s EM357||External PIC MCU|
|Communication||UART||SPI, I2C and UART||UART||SPI||UART||SPI, UART||SPI|
|Software Stack||ZigBee and ZigBee PRO||ZigBee Pro||ZigBee||ZigBee Pro||ZigBee||ZigBee Pro||ZigBee|
|VCC||2.1V to 3.6V||2.1V to 3.6V||2.1V to 3.3V||2.3V to 3.6V||3.3V to 5.5V||2.4V to 3.4V||2.4V to 3.6V|
|Dimensions in mm||25 x 36||25 x 36||34 x 34||18 x 30||12 x 26||27.2 x 17.45||17.8 x 27.9|
Table 1. There is a Wide Choice of ICs and Modules Supporting ZigBee
ZigBee solutions are offered both in ICs and modules (see Table 1). Commercial use of this specification is not free of charge – manufacturers must belong to the ZigBee Alliance, and end products must be certified, a requirement which guarantees that the equipment from different vendors will be interoperable.
The development process can be accelerated by starting from a lighting control reference design. Particularly useful development boards are available from Future Electronics:
- ’Future Electronics’ Wireless Lighting and Control Reference Design implements wireless control of red, green, blue and white LEDs over an 802.15.4 network. The kit includes a controller board, remote light boards, and transceiver boards.
The licensing cost of ZigBee can be avoided through use of proprietary protocols developed by chip manufacturers (see Table 2). NXP offers the JenNet protocol, which works in the 2.4GHz band and can form tree, star and linear self-repairing networks with up to 500 nodes. Microchip’s free MiWi stack runs on PIC microcontrollers and on Microchip transceivers at both sub-GHz and 2.4GHz frequencies.
Sharing the Power Cabling
The other way in which to implement a sophisticated lighting control network without dedicated control network cabling is through Power Line communication (PLC), which can be hosted on power cabling.
|Vendor||ZigBee Alliance||NXP||Microchip||Synapse Wireless||RFM|
|Topologies||Mesh||Star, tree, linear||Mesh||Mesh||Mesh|
|Platform||NXP modules||PIC MCU's and modules||Synapse modules||RFM modules|
Table 2. Different Short-Range RF Protocols Offer Different Features and Performance Levels
Utility companies have pioneered the use of PLC, as it offers a way to access meters, and other appliances, over the power grid. But PLC can also be used to provide low data-rate communication within a building, and therefore to control light fixtures connected to the mains.
The range of this technology depends on the modulation scheme used, the line impedance and noise, but PLC is most likely to be worth evaluating when the distance between nodes is too great, or conditions are too hostile (e.g., buildings with metal walls) for wireless signals.
The biggest obstacle to the adoption of PLC by the lighting industry is that there is no generally accepted PLC specification for lighting control or home automation. The most popular standard is KNX, which can use the power line as well as other media for home and building automation functions; but it is limited to a low data rate of 1.2kbits/s.
Alternatively, lighting control equipment manufacturers can use technology developed by manufacturers of PLC modems and systems-on-chip (SoC). For instance, the IT700 modem from Yitran uses advanced DCSK modulation, providing robust communication with speed up to 7.5kbits/s, and its Y-Net protocol stack for command and control applications.
Microchip has released a PLC soft-modem reference design based on its dsPIC33F family of microcontrollers, offering a data rate up to 7.2kbits/s. Cypress Semiconductor also offers a range of PLC devices which integrate Cypress’ Powerline Network Protocol stack on the same device as the PHY.
Given the high cost of installing a parallel cabling infrastructure to support DALI and other established lighting control standards, it is high time that alternatives requiring no additional cabling entered the mainstream.
For pioneering lighting equipment manufacturers, PLC offers an opportunity to establish a position in a market while it is still developing. By contrast, RF technologies are already being widely used in building automation, and provide data rates and network features that are clearly adequate for the needs of lighting control applications.
Manufacturers that can give confidence to end users that the RF links will be robust across a complete installation stand to benefit as lighting specifiers look to implement more control applications in order to improve the user experience and to reduce energy consumption.