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Improved Control Algorithm Appears in a Next-Generation Daylighting Product

LS-301 Control sensor

Figure 1. The LS-301 dimming photosensor from The Watt Stopper uses an algorithm developed by EETD researchers.

The commercial market has recognized the energy-saving potential of this algorithm, first proposed in 1984.

Seeking ways to save billions of dollars in lighting energy by using daylight to reduce electric lighting needs, researchers at the Lawrence Berkeley National Laboratory (Berkeley Lab) Environmental Energy Technologies Division (EETD) proposed, in 1984, a method to make daylight sensors more reliable. Now, this method, the sliding setpoint algorithm, has been incorporated in a commercial product, the LS-301 Dimming Photosensor recently released by The Watt Stopper Inc., (see Figure 1).

Proposed by EETD researcher Francis Rubinstein, the sliding setpoint algorithm was first presented in Energy and Buildings and later refined in a paper published in the Journal of the Illuminating Engineering Society with co-authors Greg Ward and Rudy Verderber. The researchers, who were part of the Division's Lighting Group, were striving to correct the tendency of daylight sensors to over-dim electric lights in response to increasing daylight in a room. (Verderber is now retired, and Ward is no longer at Berkeley Lab.)

"We had determined that the standard control algorithm used by most manufacturers of daylight photosensors at the time did not work well, and we developed the sliding setpoint algorithm to solve these problems," says Rubinstein, who is still a scientist in the Lighting Group. "We spent quite a bit of time experimenting with different photocell configurations in a scale model located on the roof of a lab building."

Jerry Mix, president of The Watt Stopper, praises EETD's research: "The work of Francis Rubinstein and his colleagues at Berkeley Lab has provided us with great insight into the complex problem of dimming lights in response to daylight. We believe that their concepts, now implemented in real products, will go far to realize the dream of reducing energy use while providing highly desirable control."

Daylighting Saves Energy

Back in 1983, Rubinstein estimated in a published paper that daylighting with an effective lighting control system could save hundreds of millions of dollars per year in lighting energy costs. "That potential for energy savings still exists," he says today, "and it is still unrealized."

To use daylighting effectively as an energy-saving strategy, daylight photosensors need to be able to accurately track the daylight entering a room and to dim the electric lighting in proportion to the amount of daylight detected. As daylight increases, a control system receives input from the photosensor and automatically dims or turns off the overhead lights, saving lighting energy in the process.

The problem is that most daylight photosensors are easily fooled by the amount of daylight entering the room. When these photosensors use the standard "constant setpoint" control algorithm, they respond too strongly to the light coming in from the window and turn down the electric lights too much, so a constant total illumination level is not maintained at the desk surface. (Algorithms allow control systems to transform the signal from a sensor into a control signal to change the light level.)

The sliding setpoint algorithm "effectively reduced the sensitivity or gain of the control system," Rubinstein explains. The improved algorithm allows the lighting control system to keep illumination in a space roughly constant regardless of daylight level, so, as daylight levels increase during the day, building occupants do not experience annoying dips in total illumination. (see Figure 2).

Closed and Open Loop Control Systsem

Figure 2. Closed and Open Loop Control Systems for Daylighting

With the sliding setpoint algorithm, the control system adjusts the electric light level so that the total light level detected by the photosensor is a linear function of the daylight component of the photosensor signal.

"In our research, we characterized the existing control algorithm and the improved sliding setpoint algorithm and showed how they both performed," says Rubinstein, who coined both terms. "We also built circuits to show how the sliding setpoint method would work and tested the system in a daylighted scale model at Berkeley Lab. The concept sat on a shelf for quite some time, probably because the sliding setpoint algorithm required two calibrations of the photocell -one that set the desired light level at night, when there's no daylight, and one during the day." This additional calibration required that manufacturers change their hardware and commissioning process. ("Commissioning" means setting up a system and making sure it works according to specifications.)

The Wattstopper Inc. Develops Improved Technology

Searching for ways to improve on its existing daylighting controls, The Watt Stopper Inc. turned to the papers authored by Rubinstein and his colleagues. The Santa Clara, California-based company has worked with Berkeley Lab researchers in the past, for example, on an improved lighting control system for hotel bathrooms (Berkeley Lab Research News, March 17, 2003).

Combining the sliding setpoint algorithm with wireless technology, engineers at the company have created a new dimming photosensor called the LS-301, which became available on the market in August 2004. "Until now, the setup and adjustment of a sliding setpoint device has been daunting. Now, we have combined technologies to create a device that is user friendly," says Mix.

The technology consists of the daylight sensor itself and a handheld commissioning control unit. The commissioning unit is used by the installer once per sensor to establish the setpoints, one during the day, and one at night. Previously, sensors, usually mounted in ceilings, were adjusted by installers on ladders. The commissioning remote helps them work more rapidly and safely. A second handheld remote allows the room's occupants to adjust lighting levels within the range of the setpoints they desire.

The Watt Stopper also designed their photosensor with an improved detector that responds to different wavelengths of light in the same way as the human eye. That is, the spectral response of the sensor matches the photopic response of the human eye.

"I'm pleased to see that Wattstopper has taken our ideas and added their own innovations to provide a practical solution to a real-world problem. Companies like Wattstopper, which invest in daylighting control technologies, are improving the quality of the lighted environment while helping consumers conserve energy and natural resources," says Rubinstein.

Mix adds, "We value our continuing work with Berkeley Lab to provide control solutions that are simple to install and adjust. We believe that the greatest deterrent to the success of the daylighting controls is whether or not the devices are initially set up and adjusted correctly by the installer. We now have the ability to build greater intelligence into the controls to automate the initial adjustment."

The Watt Stopper and Rubinstein are now working together to develop a next-generation version of this technology that will further simplify the commissioning process. The continuing work, funded by the California Energy Commission's Public Interest Energy Research program, aims at making a sliding setpoint device that can be adjusted in one visit.

— Allan Chen

For more information, contact:

  • Francis Rubinstein
  • (510) 486-4096; Fax (510) 486-4089

For more information about The Watt Stopper Inc., visit their web site.

This research is funded by the U.S. Department of Energy, The Watt Stopper, Inc., and the California Energy Commission's Public Interest Energy Research program.

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