LED lighting control using powerline communication

For instance, LED fixtures can easily provide color tenability; i.e. the same bulb can give warm white light or cool white light, or any color in the spectrum. CFLs cannot do that today. LED fixtures can also be more intelligent through communication capabilities, enabling them to perform better control, diagnostics, and automation of functions. Given that all lighting fixtures connect to a powerline to convert electricity to light, many manufacturers are turning to the Powerline Communication (PLC) interface to serve as the primary communication and control link.

Powerline Communication
Powerlines make up the largest copper infrastructure in the world. There are power outlets at every corner of a home or office building, making it an all-encompassing network. No new wiring is required to communicate anything from basic color and brightness information to more complex information such as color scenes (pre-defined color patterns of different fixtures) and fading (transitioning between colors). In addition, with advanced light fixture discovery and binding mechanisms that are abstracted from the user, a PLC-enabled lighting control network can be established without needing to remember a single number or risking accidentally turning off ones neighbor’s light.

Powerline networks use a bus topology which provides a high level of reconfigurability and the ability to control more than one device from a single controller. This controller could manage all the lights in a room or even all the lights in the home. Additionally, the bus topology enables multiple controllers to control a single lighting fixture. This way, a lighting fixture in one room can be controlled from other rooms (i.e., turn off all the house lights from the bedroom). Such a topology also enables the controller to keep track of all the devices on the network and to serve as a backbone for expandability and “plug and play” installations where any new light can immediately become part of the network.

Figure 1 compares some of the different lighting control architectures. Traditional lighting allows independent control of one light, whereas standards built on bus topologies enable independent control of multiple lights. Note that although DALI and DMX512 can control multiple lights independently from one controller, both DALI and DMX512 require the installation of additional control wires.

Figure 1: Lighting Control Architectures
(Click on image to enlarge)

Binding the Controller to the Light Fixture
As seen from the figure, traditional lighting has dedicated wires for independently controlling each light bulb. With a bus topology, the wires are shared by multiple light bulbs, which means that the signal sent by the controller is received by all of the light fixtures. In order to differentiate between different lights, the controller “binds” with each light fixture separately and assigns it a unique address.

For example, consider a light fixture A has an address of 1 and light fixture B has an address of 2. If the controller sends a message with a destination address of 1, then the message is processed only by light fixture A and not by light fixture B. Similarly, if the message was sent using a destination address of 2, then the message is processed only by light fixture B.

Older systems require the user to manually assign an address to each light (e.g. using DIP switches or a rotary dial) and then select that address on the controller. However, this method has a few disadvantages: 1) It takes extra time to set up; 2) The user has to be careful to assign a unique address to each device; and 3) If controlling multiple lights from one controller, the user has to remember the address number for each light. A more sophisticated approach makes the controller (instead of the user) responsible for discovering new light fixtures on the network, identifying a free address on the network, assigning the address, and providing an easy-to-use interface for binding and controlling individual (or multiple) fixtures.

To discover that a new light fixture is on the network, the fixture needs to send out a signal that it is available. This is best accomplished by broadcasting a message so that all controllers on the network can be made aware of the new fixture. When a controller receives the message, it can notify the user that a new light fixture is available. If the user decides to control that fixture, the controller will send a message to that fixture with a request to bind. If the fixture is still available to bind, it will send a confirmation message. If not, it will send a denial message. Once bound, the light fixture will only process messages that are received from the address of the controller that bound to it.

There still remains the issue of how the fixture can receive a request to a bind message when it hasn’t been assigned an address yet. This can be resolved by reassigning every fixture with a unique 64-bit address (similar to a MAC or physical address). Then, when the fixture first broadcasts itself as available, it can also include its unique address in the message. The controller would then be able to send a direct message to it for binding.

Since a 64-bit address is quite long for sending normal color control messages, the controller may assign a shorter 8-bit address (known as a logical address) to the light fixture after binding to it. To ensure that a new logical address value is not already used, the controller can send out a ping message on the powerline. If a response is received, it will try a new address until no response is received.

Figure 2 shows the binding sequence for two available light fixtures and the user has decided to bind to the first light fixture. Once binding is complete, the controller can start sending color information to control the light fixture.

Figure 2: Light Fixture Discovery and Binding
(Click on image to enlarge)

Real-World Challenges
The general challenges with powerline communication are:
1) the light fixture cannot receive messages from the controller
2) the wrong controller is controlling the light fixture.

If a light fixture cannot receive messages from the controller, it is usually due to one of three reasons: 1) there is too much noise on the powerline, 2) the controller and receiver are on different powerline phases, or 3) the distance between the receiver and controller is too great. If there is too much noise on the line (e.g. vacuum cleaner, heavy-duty appliances, etc.), it is recommended to move the systems away from the noise source. If the controller and receiver are on different phases, the user should try to move one of them so that they are on the same phase.

If this isn’t possible, there are phase couplers that are available to bridge the powerline communication signal across the phases. Such coupling can be performed through a large capacitor or through a wireless connection. If the distance is too great between the receiver and the controller, there are repeaters available which can re-transmit signals until they reach the intended destination. Some implementations co-locate the repeater and light fixture on the same device so that no additional cost is incurred.

Since there can be multiple controllers on the same powerline bus, there is the possibility that the wrong controller may be controlling the light fixture. This can occur for many reasons, depending on the address assignment and binding mechanism. If the addresses are assigned manually, then two light fixtures may have been assigned the same address. This may be due to the user forgetting that they already used that address for a fixture, or it may because someone else (e.g. a neighbor sharing the same powerline) assigned the same address.

With intelligent address assignment and binding described earlier, all of the addresses are unique 64-bit physical addresses, so this error condition cannot occur. If 8-bit logical addresses are used in the intelligent address assignment, the controller will ping the network to make sure that it doesn’t assign an already used address. Even with intelligent address assignment and binding, there is the chance that a different controller binds to the light fixture than what is intended (e.g. a neighbor binds to a light that the user just plugged in). In this case, a button on the light fixture should be available to force the light fixture to unbind from its controller, which will free it to bind to the correct controller.

Color Control
Color information typically takes one of two forms: CIE color coordinates or direct LED dimming values. Direct LED dimming values contain an independent value for the intensity of each of the LEDs. For example, if there are red, green, and blue LEDs, there will be 3 dimming values. The CIE coordinates are 2-dimensional and represent any available color in the spectrum. Along with the intensity (luminous flux), the CIE coordinates are mixed into direct LED dimming values, depending on the part and binning information of the LEDs that are used. For example, two red LEDs may emit a slightly different shade of red. The color mixing algorithm will take this into consideration, so that the generated color exactly represents the color that was intended.

The color information type transmitted over the powerline depends on the user input, the level of color control precision, and implementation cost. If the user input is direct LED control, then the direct LED dimming values will be transmitted. If the user input is a specific color and intensity, then the information type depends on where the color mixing is performed. If the color mixing is performed at the receiver, the CIE coordinates and intensity will be transmitted.

This is the typical choice because the LED binning information is typically stored in the light fixture. However, with PLC, it is possible for each light fixture to send its unique LED binning information to the controller, which can store the information and load it when performing the color mixing. It will then send the direct LED dimming values.

Advanced Color Control
Now that the controller is more advanced, the color control can be more advanced than just sending direct colors to one light fixture at a time. Some interesting examples are scenes, fading, and sequencing. With scenes, specific colors can be assigned to multiple light fixtures, so that with the touch of a button, multiple lights can turn on with different colors and intensities (e.g. a color gradient). With fading, the light fixture can be told to fade to the next color over a specified duration. With sequencing, multiple light fixtures can change colors in a synchronous fashion (e.g. a lighting display, mood lighting, etc.).

Implementation
The powerline communication transceiver is typically a low-voltage, DC-powered IC. To interface this device to the powerline, a power amplifier and coupling circuit are required. The coupling circuit can be modified to support the required voltage range (e.g. 110–240 VAC for global residential applications, 24V DC for pool lighting, etc.) and therefore, the same powerline transceiver IC can be used for any desired powerline voltage range.

The implementation of the controller can take on different forms, depending on the physical area available and the level of control that is needed. For basic wall-switch installations, the lighting control interface may be a simple on-off switch, one dimmer, or multiple dimmers for individually controlling the colors of the light fixture. In addition, there would be at least one button to index through the available fixtures and one button to bind to a node. An indicator LED would be useful for displaying the status of the fixture (available or bound).

Typically, a microcontroller would be used to process these inputs and interface to the powerline transceiver. With the Cypress Powerline Communication technology, for example, the microcontroller and powerline transceiver are integrated so that the input processing, intelligent binding and advanced color control with powerline communication are performed by one device.

An innovative approach to replace clunky mechanical buttons, switches, and dimmers, is to use capacitive touch-sensing technology. With capacitive touch sensing, the controller panel would be a flat surface with a decal showing the control interfaces. When the user touches a location on the panel, the controller would interpret the touch as a button press, switch toggle, or dimmer change, depending on the location of the finger. This presents the user with a sleek, clean, and robust interface for controlling the light fixtures.

The capacitive touch approach can be taken a step further by providing 2-dimensional control. For example, controller could detect an x- and y- coordinate of where the finger is touching and convert it to the CIE color co-ordinates. This a simple approach to color control, where the panel can be a color gamut, enabling the user to change to any color with one press of the finger.

For more complex lighting control (e.g. a central home automation system), the controller may be running on a PC. In this case, the interface would be a graphical user interface (GUI) application. The GUI could show the user all of the available fixtures in the house and allow the user to perform more advanced color control schemes. The PC could then interface to the powerline transceiver through USB or wireless. An example of the possible controller implementations is shown in the Figure 3.

Figure 3: Lighting Controller Implementation
(Click on image to enlarge)

Since a user interface is not required at the light fixture, an embedded microcontroller is typically used for processing the received messages and performing the LED color control. With the Cypress Powerline Communication and High-Brightness LED Control technology, the powerline transceiver and precise LED color controller are integrated in one device. Only external LED drivers would be required to set the LED color. Alternatively, with Cypress PowerPSoC technology, the precise LED color controller and LED drivers are integrated in one device. This can easily interface to the powerline transceiver through an I2C interface.

Powerline communication is a technology which is well-suited for performing sophisticated LED-lighting control at a low installation cost with no new wires. Users will be able to control the color, brightness, fading, and scenes of their LED light fixtures with a traditional style wall-switch/dimmer, a capacitive touch sense panel, or with a PC. With advanced light-fixture discovery and binding mechanisms that are abstracted from the user, a PLC-enabled lighting control network can be established without needing to remember a single number or accidentally turning off your neighbor’s light.

About the authors
Jeff Hushley is a Staff Applications Engineer for the Powerline Communications group at Cypress Semiconductor. He has 5 years of experience working on various communication systems, ranging from high-speed digital communication (HD-SDI, HDMI) to powerline communication. He has a Bachelor's Degree in Computer Engineering from the University of Toronto. He can be reached at fre@cypress.com.

Rohan Gandhi is an Applications Engineer for the Powerline Communications group at Cypress Semiconductor. He has nearly two years of experience designing and working with Powerline Communications and lighting. He has recently completed his Bachelor's Degree in Electronics and Instrumentation from BITS Pilani, India. He can be reached at rosg@cypress.com.