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Commissioning, continued

Commissioning Integral (reset) photosensors

Integral (reset) photosensors have only a single parameter adjustment which is the maintained photosensor signal level. Adjustment is made with the electric lights on and either with or without daylight present. Once set, the control algorithm attempts to maintain signal level at the set value. It becomes clear that for this control scheme to be useful for maintaining a constant illumination level, the photosensor signal must be proportional to the illumination level (i.e., workplane illuminance). Once installed, nothing else can be done during the commissioning process to make the system work other than setting the maintained signal level.

Commissioning open-loop proportional control photosensors

The single adjustment setting on a proportional control can be set at any time that daylight is present. However, since the distribution of daylight is not constant, the adjustment should be made at a time when the daylight distribution is representative, or typical, of what it usually is. For example, do not commission during a period when direct sunlight is streaming in through the window at low angles late in the day because that sort of direct sunlight is not typical and would most likely be blocked by window shades. The extent to which daylight at any one time is representative of the entire day or year illustrates the limitation of the open-loop proportional control algorithm.

Commissioning closed-loop proportional control photosensors

A true closed-loop proportional control algorithm should have at least two independent parameter adjustments, one that sets the maintained lighting level and another that sets ratio of the photosensor optical signal to the desired lighting level (which is usually the workplane illuminance). These parameters are usually set in a two-step process. First, the maintained illuminance level is set by making an adjustment with only the electric lights on (no daylight). This is usually done at night unless the room has opaque window blinds that close tightly enough to exclude daylight. Second, with daylight present and electric lights on, the ratio between the photosensor optical signal and the desired light level is set by dimming the electric lights until the desired light level is achieved. The desired light level might be equal to the maintained illuminance level originally set for the first parameter, or it may be set to a greater value allowing the light level to increase as daylight enters the room.

The need to commission at two different times incurs extra expense and hassle and it could be seen as a disadvantage of the closed-loop proportional control algorithm. However, there are ways for the two parameters to be set at nearly the same time, without the need to exclude daylight. This can be done taking photosensor signal and light level readings in the presence of daylight with the electric lights on and with them off and subtracting one from the other to determine the photosensor signal and light levels with electric lights only. Even though this reduces commissioning time, no photosensor product yet offers this feature.


 
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