Commercial lighting is unique in that there is a diversity of tasks that take place in a typical commercial building. The resulting shift to LED and high-efficiency lighting has brought computerized complexity to simple lighting control. The hallmark of new high-efficiency lighting is the digital pulse width modulation (PWM) of the lighting pattern and the refresh cycles. Digital control is common for both high voltage direct-control lighting and low voltage lighting.
In order to address multiple applications, not only are multiple bulbs used, but also multiple colors of lighting sources in the same fixture, which in turn need to be blended in order to produce various lighting effects. These blending and control functions are handled by embedded microcontroller solutions, both at the lighting fixture and at the user control interface.
Commercial lighting tasks
Most commercial lighting today is made up of large, tubular cool white (bluish tint) florescent lights. These lighting sources include overhead lighting for desk and office space in ceiling panel units, under-counter lights that dominate cubicles and desks, and open high-wattage lights that are used in conference areas, garages, stairwells, and high ceiling applications (see Figures 1 and 2). The dominant function of these lights is “daylight simulation” with a minimum of shadows.
Figure 1: Suspended ceiling light.
Figure 2: Large-area lighting.
These lights typically feature on-off functionality, and tend to be on for more than 16 hours per day. The high ceiling lights – typically used in conference areas and large entry ways – may also include dimming control. A growing trend in the commercial spaces involves supplementing powered lighting with sunlight that is brought in through windows and open spaces. These architectural features are driving a major change in the type of lighting used.
Architectural lighting targets uniformity of light coverage for an area. However, the color, intensity, and direction of natural light changes throughout the day, though at least the intensity can be controlled by blinds and curtains. As a result, fill-in lighting – on tables and in ceilings – is used heavily. These fill-in fixtures tend to be controlled with dimmers to help optimize the room light balance.
Sensor-Based solutions and challenges
As companies move towards these mixed lighting environments, they are also addressing the “on for 16 hours a day” issue by using in-room sensors to determine what type and how much light to add if the room is being used. Rather than make these decisions with a manual “person-operated” dimmer, the systems use both remote and local embedded microcontrollers to decide what the correct lighting should be.
The key input for the microcontroller is a defined state table based on feedback from the sensors. These systems do not have large complex GUIs, but typically have simple touch screens or buttons. They may also have no display and work autonomously. The sensors convey information to the MCU regarding brightness level, motion, and contact, though some sensors may provide full photo imaging with color recognition.
New control functions
As new sensors are made available, new lighting types are also employed. The rise of LEDS and OLED/AMOLED lights has increased the complexity of lighting as well as the number of available options.
Traditional lights are available in a single color. The entire light string/tube/bulb/fixture uses a common starting circuit and produces a single base color, generally defined by the chemical combination of the gases and filament materials in the vessel. LEDs, however, are electronic lights of small size that can be mixed in arrays in different patterns to create large-area lighting sources. As a result, lights of different peak colors can be mixed together. LEDs range in color from single colors such as blue, red, green, orange, yellow, and amber to multiple color temperatures of white.
There are new modules of mixed colors – white with amber; white with red, green and blue; white with amber, yellow and blue; and multiple color temperatures of white. These mixed lights utilize separate PWM controls and drivers for each LED in the module, and can also be combined into strings for large area coverage. The mixing of the colors to provide a specific output color is straight forward and can produce a mixing table that identifies whatever driver control is needed for a particular LED in order to create a specific light color and intensity.
Microcontroller options
For lighting applications, there are four basic categories of environments where the microcontrollers are used: battery or line voltage operation, with or without a display. The MCUs determine the lighting situation and the appropriate response. The actual PWM drive of the LEDs and other lighting is handled by driver ICs such as the Infineon Technologies ILD4120 driver. The Infineon driver is a specialized buck converter that is optimized for LED applications, as shown in Figure 3.
Figure 3: Infineon ILD4120 LED controller block diagram (Courtesy of Infineon Technologies).
The majority of development kits for targeted lighting applications are based on ARM® cores. The cores are typically CortexTM-M0 and Cortex-M3 cores; however, for some advanced photo- and image-processing applications a Cortex-M4 – which includes a DSP – may be used. The Cortex-M processors, available from NXP Semiconductors at Digi-Key, are available as individual components, on application boards, or as part of whole development systems. These parts are designed in many footprints and power factors. The NXP microcontrollers are available with and without display drivers, and range from the large Cortex-M4 design (LPC4300) to the smallest Cortex-M0 (32-bit data with 16-bit instructions). The LPC1200 from NXP also features a tiny footprint of just 5 mm2 and a reduced easy-to-program instruction set (see Figure 4).
Figure 4: Cortex-M0 versus Cortex-M3 instruction set comparison (Courtesy of NXP Semiconductors).
As some of the lighting control modules are retrofit items, they often must be run from battery-operated systems. These parts need a different type of microcontroller – one optimized for power, not speed. Most of the lighting control functions take place in the two to four second range and need to acquire the data to process in less than 500 msec. In these time frames, it is possible to use an ultra-low-power ARM-based microcontroller such as the Gecko series from Energy Micro. These microcontrollers have an energy management system, low-power timers, and ultra-low-power active modes, all of which enable greatly extended battery life. Figure 5 illustrates the architecture of the microcontroller's blocks. These low-power features enable extended periods of standby or same state activity with minimum switching and computation time. Lighting applications typically do active state determination and switching on the order of a two percent duty cycle; the rest of the time is spent watching for changes in the lighting environment.
Figure 5: Gecko EFM32 low-power microcontroller blocks (Courtesy of Energy Micro).
The microcontrollers themselves can be placed in several different locations. Most often they are integrated into the lighting fixture itself, which in turn is either standalone, or connected by wires or an RF link to other lighting modules.
There are several keys in the design of these microcontrollers that are unique to the commercial lighting space. One is field programmability of the lighting states. Since a commercial space will generally have several different tenants during the lifetime of the lighting solution, the ability to update the lighting to the new tenant is essential. Typical changes include office space lighting shifting from warm (reddish tint) to cool (bluish tint), based on the preference of the new occupant. More importantly, the position of the furnishings in the room and their color/material affects the reflection and absorption of the light presented to the room, hence the lighting patterns and intensities will have to be updated. In addition, the sensor inputs will be different. Some tenants may want to use motion sensors to determine if a room is in use, while others may want to have multi-state active control (bright or dim based on the task being done in the space rather than just occupying the space). These changes impact the I/O, GUI control, and state machine load of the control electronics.
For some of the systems, the PWM flow determination is best done in the microcontroller rather than the light driver. For low-voltage applications, for transformer-coupled designs, and for high-efficiency and multi-string lighting, the complexities of the timing and color mixing are best handled directly by the microcontroller. Most microcontrollers have both digital and analog I/Os and are capable of directly controlling a lighting driver circuit that will run the lights. The advantage of direct drive is the increased flexibility made possible by the extended state options and the larger memory in the microcontroller versus those typically found in the driver electronics. The complexities of color mixing with two or more colors and the input from multiple sensors (temperature, movement, brightness, activity, ambient light, etc.) are all asynchronous, interrupt-style sensors without a set schedule for state changes. ARM-based microcontrollers can support both asynchronous and synchronous interrupts and port data.
To help develop and prototype these designs, it is best to build both a functional and form-factor applicable prototype. A number of development systems are available that include reference and interface boards, software, macros, IP, and cables. Most of these boards are from third-party ARM Cortex silicon vendors. Typical kits include Atmel’s AT91SAM evaluation kit and Texas Instruments’ LM3S2965 evaluation kit. These are complete development kits that support all options of the code and include a flexible programming environment. There are other small development systems that can be built into the correct form factor and power window for testing.
The availability of these microcontroller cores found in FPGA libraries enables rapid development of complex industrial and commercial lighting systems, adding multi-string drivers and advanced color blending/sensor schemes as part of the control function.
Summary
The commercial lighting space presents unique challenges that require the innovation of today's high-efficiency lighting solutions. The use of microcontroller-based design in fixtures and user control panels helps simplify the task so that the user can select the result they want and the rest is handled automatically. As multicolor LED modules become more prevalent and there is common control for standard and high-brightness LEDs, microcontroller-based solutions will increase in complexity, while still enabling the user to just “flip a switch” and get exactly the light they want.
