Photosensor-based Control

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Introduction

Photosensor control of electric lighting systems ensures that energy savings are achieved when daylight is available to illuminate a space, area, or task. Photosensors are very common for use with exterior lighting systems where the absence of daylight causes a lighting system or luminaire to be turned on. For interior applications, a photosensor can be used to switch electric lighting on and off, or to dim the lighting equipment in response to daylight. The layout and control algorithms for photosensor-based lighting control can be categorized as either open-loop or closed-loop. Each of these control configurations can be configured to switch or dim the electric lighting system. Closed-loop dimming control has more than one basic control algorithm. These are discussed in more detail below.

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Lead Author(s): Rick Mistrick

Calibration to a Critical Work Plane Point

Figure 1 – The dimming level required from the first two rows of luminaires adjacent to a south-facing window for different points across the work plane with the interior row on at full output. The colored region of the room receives more than 100 lux from this lighting zone. With this region, the critical point is located at the blue X.

Before discussing the different types of photosensor control, it is important to understand how photosensor-based control systems are calibrated. The lighting zone controlled by a photosensor might encompass all of the lighting in a room, or simply a portion of it. In either case, the area illuminated by this zone should closely correspond to the area within the space that is illuminated by daylight. A photosensor-based lighting control system should provide the desired target illuminance level at all task locations within a space. In a space such as a classroom, the entire room area or the portion where the desks and other important tasks are located may be considered the task area. In a private office with a known desk location, the goal may be to provide this surface with an appropriate illuminance while still maintaining a general room ambient level and room surface brightness elsewhere within the space. The primary work surface might be positioned in the center of the space or along a wall. In calibrating a photosensor control system, it is important to configure the system to provide the appropriate light output at the “critical” task location. This location is the task position that requires the highest light output from the controlled lighting zone (assuming that dimming control is being applied). Typically, in a room such as a classroom with sidelighting (i.e., windows) where a broad task area is being considered, this point is often located just beyond the interior boundary of the photosensor controlled lighting zone (assuming that a secondary interior lighting zone not under photosensor control may also exist) and near a side wall where daylight levels are generally lower (as compared to those in the center of the space). Figure 1 provides an example of the dimming levels required to maintain a target illuminance at different locations across a classroom space where two of the three rows of luminaires in a space will be dimmed.

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Lead Author(s): Rick Mistrick

Dimming Control

Open-Loop Control

Open-loop control.jpg

In an open-loop system, the sensor is located so that it reads only daylight and receives no signal from the electric lighting system (or one that is negligible in comparison to the daylight reading). An exterior sensor is a prime example of an open-loop sensor. A sensor that faces out a window, or one that is positioned within a skylight well, is also considered to be an open-loop photosensor. The goal with open-loop control is to position the sensor so that its reading is proportional to the amount of daylight admitted to a space. If shading devices are applied to a window, the sensor must be located on the interior of these shading devices to maximize the correlation between the photosensor’s signal and the work plane illuminance. The more consistent the daylight distribution is across a space in relation to the relative magnitude of the photosensor signal, both without and with operable shading devices applied, the better the open-loop photosensor control system is likely to perform.

Open-loop control systems are most common in toplighting applications (skylights), but relatively rare in sidelighting applications (spaces with windows), except for a few systems that position a photosensor near a window with a view of the window and the adjacent floor, causing the daylight signal to significantly dwarf that of the electric lighting system. Still, such a system will receive some contribution from the electric lighting system at night, which could cause the electric lighting system to dim if not properly addressed.

The most common open-loop dimming algorithm is a simple linear relationship. Higher photosensor signals result in reduced lumen output from the dimmed zone, while no signal results in full light output. With many photosensor control systems, there is an option to either dim the controlled zone to minimum output and hold it at that level, or dim to minimum then turn off the controlled lighting zone completely to save the energy required to operate the lamps at minimum output, which is often about 20% of the full ballast power for a fluorescent system.

Closed-loop Control

Closed-loop control.jpg

Closed-loop control occurs when a photosensor is located within the space it controls such that the signal the photosensor receives is from a combination of daylight, the electric lighting system it controls, and possibly other electric lighting equipment outside the daylight zone that it does not control. For this type of control to function properly, the ratio of the photosensor signal from daylight to the illuminance at the critical point (the calibration point) from daylight (which will be referred to as S/E) must be greater than or equal to the same ratio for the controlled electric lighting zone. To achieve this, a photosensor should be located so that it receives as little light as possible from the luminaires directly. Placing a ceiling-mounted photosensor directly over an indirect luminaire is very likely to result in a S/E ratio for electric light that is greater than that for daylight and make proper photosensor control impossible to achieve. In the case of closed-loop photosensor-based switching, the act of switching the lighting system off will result in a drop in the photosensor signal, while turning the lighting system on will increase the signal. For this reason, the magnitude of the dead band must be at least that of the electric lighting control zone photosensor signal. A time delay is also usually incorporated into this type of control.

With closed-loop dimming there are two basic control algorithm options – integral reset (also known as constant setpoint), and linear proportional (also known as sliding setpoint).

Integral Reset

Integral reset control is the simplest means of closed-loop control, but somewhat difficult to achieve well. This method of control attempts to maintain the photosensor signal at a constant value. This would be the type of control desired if a photosensor is mounted on a desk facing upward. Each unit of illuminance of electric light is replaced by a unit of daylight illuminance, so when 200 lux of daylight is added, 200 lux of electric light is removed. The problem with this control algorithm when the photosensor is mounted on the ceiling is that a ceiling sensor does not read the illuminance at the critical work plane point, but instead receives an integrated signal from a variety of directions. For the electric lighting system, this signal has a relationship to the critical work plane illuminance that is constant. Under daylight, this signal can vary depending on the directionality of the daylight entering a space. For example, if a bright sunlight patch falls on the floor near a window, this high luminance will contribute to the photosensor’s daylight signal if it is within the photosensor’s field of view, and the daylight S/E will increase since this patch contributes proportionally more light to the photosensor than to the work plane critical point (the system’s calibration point), which only sees reflected light from the ceiling and walls, and does not have a direct view of the sunlight patch.

For this reason, S/E for daylight tends to be much higher than S/E for electric light, particularly for any photosensor that views surfaces near a window or a window directly. Given the fact that S/E for daylight is usually higher than S/E for electric light, a one-for-one trade-off between daylight and electric light at the photosensor’s signal generally does not adequately maintain constant light output at the work plane. For an equal tradeoff to occur, S/E for both daylight and electric light must be equal. For a ceiling-mounted photosensor, this is more likely to occur if the photosensor has a narrow field of view and is positioned away from a window to avoid it from viewing the floor near the window, which will be brighter under daylight conditions than under electric lighting conditions and increase the daylight signal.

If S/E is higher for daylight than for the controlled electric lighting zone, an integral reset photosensor controlled lighting system should not be calibrated to apply the nighttime signal of the electric lighting system as the setpoint. Calibration must be performed under a representative daylight condition, which requires knowledge of the work plane illuminance under that daylight condition. The daytime calibration can help prevent significant overdimming (providing too little electric light on the work plane) as daylight is added to the space. If overdimming still occurs, the setpoint should be increased. With a higher settings, the control system will not dim until daylight reaches a preset level, then it will dim the controlled zone rather quickly down to its minimum level (over a somewhat narrow range of daylight illuminance). A narrow distribution photosensor mounted deep enough into a space should also provide a much closer relationship between S/E for both daylight and electric light.

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Lead Author(s): Rick Mistrick

Switching Conrol

Photosensor-based Switching

Photosensor based switching of electric lighting turns a controlled electric lighting zone off when the photosensor signal exceeds a specified level, and on again when it drops below another preset level. The difference between these two signal levels is referred to as the dead band, since a signal within this range results in no change in the controlled lighting zone status, and the controlled lighting zone may be either on or off. To calibrate such a system, both the On and Off signal values must be entered to the system. In some cases, this process may be semi-automated. To properly establish appropriate on and off signal levels at calibration time, it is necessary to determine the approximate signal level that would be present when the target illuminance is achieved through daylight. In the case of a closed-loop system (where the photosensor is within the space), a portion of the signal from electric light will be lost when the controlled lighting zone is switched off. The dead band must be greater than this electric light signal, or the control system will oscillate between a lights-off and a lights-on signal condition. The photosensor signal that is obtained when daylight reaches the target level can be approximated knowing the photosensor signal from the electric lighting system and the ratio between the photosensor signal due to daylight and the daylight illuminance level at the critical work plane point that is being used for system calibration. The off-signal should be equal to the electric light signal, SE, plus an approximate value of S/E for daylight (SD/ED) multiplied by the amount of daylight necessary to bring the interior illuminance to the target level, ET, plus some extra amount to provide some additional space within the dead band. It has been recommended that this extra amount should be at least 20%-25% of the daylight signal that is needed to achieve the minimum desired task illuminance. If a 25% value is being applied, then the multiplier MDB in the equation below would be equal to 1.25. Note that if the critical task location receives illuminance from a lighting zone that is not controlled by the photosensor (we will call this contribution ENS, where NS stands for Non-Switched zone), the targeted daylight contribution is less than the desired task illuminance at that point.

In addition to the dead band, a time delay is also often included to prevent frequent oscillation of the controlled zone lighting condition.

Soff > SE + [SD/ED] x (ET – ENS) x MDB

SE in the above equation is the sum of the signal from both the switched and non-switched zone (SE = SSW + SNS).

As the daylight condition drops, the electric lighting should be turned on when the daylight illuminance drops below the target level. If SNSE is the signal from any non-switched electric lighting system in the space, then the ON signal would be equal to

SON @ SNS + [SD/ED] x (ET – SNS)

Open-loop switching.jpg
Closed-loop switching.jpg

ET in the above equations is the minimum illuminance condition that is desired at the task location. Note that different daylight conditions will provide different S/E ratios, in which case the task illuminance at which both the controlled lighting zone is switched off and on will vary across different daylight conditions. The goal is to calibrate the system so that under the worst conditions, an appropriate task illuminance value is provided when the switched zone is Off.

The figures to the right show the relationship between the on-off photosensor signal points and the dead band as the photosensor is used to switch the electric lighting control zone on and off for both open-loop and closed-loop control. Note that in the case of closed loop control, the signal received by the photosensor is significantly reduced by the loss of the signal from the controlled electric lighting zone. The dead band, the difference between the On and Off signals, must therefore be much larger in the case of closed-loop switching control.

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Lead Author(s): Rick Mistrick

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