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#### Control Algorithm, continued

##### Closed-loop Proportional

The closed-loop proportional control algorithm is identical to the open-loop control algorithm with the addition of an offset to the dimming curve. The offset prevents dimming until the optical signal on the photosensor reaches the desired set point level. Having this offset enables a proportional control algorithm to be used effectively in closed-loop systems. The closed-loop proportional control algorithm has two adjustable parameters: the constant of proportion between the control voltage and the input optical signal (the slope of the open-loop response curve), and the offset (the input optical signal where dimming starts). For easy, systematic commissioning of the photosensor these two parameters should be independently adjustable.

##### Advantages of the closed-loop proportional control algorithm
Because it works in a closed-loop system using feedback, the closed-loop proportional control algorithm can react to and compensate for changes in the illumination level in the room. Therefore, it has a better chance of maintaining constant workplane illuminance.

As with the open-loop proportional control algorithm, the closed-loop proportional algorithm allows flexibility in setting the constant of proportion between dim level and input optical signal. This flexibility allows for increasing light levels on the work plane as daylight enters the room as well as for a constant work plane illuminance. Although design strategies that allow for increasing light levels decrease energy savings, studies have shown that occupants often prefer higher illumination levels when daylight is present under certain conditions.

The closed-loop proportional control algorithm solves the problem of closed-loop control when dealing with two different light distributions and light source spectra, such as electric lighting and daylighting. To the extent that daylight can be considered as a single lighting distribution and spectrum, light level control is exact. Of course the relative distribution and spectrum of daylight within a room is not constant, so in practice the control with daylight will not be exact. However, the failure of integral(reset) control algorithms is caused by huge differences between electric lighting and daylighting distributions within a room (workplane/ceiling illuminance ratios are typically different by factors of five or more), not by the relatively minor differences in the distribution of daylight throughout the day and year which are comparatively much less. The spectral differences between various daylight conditions are also much less than the differences between daylight and fluorescent light sources.

##### How closed-loop proportional control works
Closed-loop control uses feedback to compare the photosensor signal against a target value (set point) from which an error signal is generated. The error signal determines the amount and direction of change in the control signal controlling the electric light level. Negative feedback is used so that a positive error signal leads to a reduction in electric light level while a negative error signal leads to an increase. Closed-loop proportional control deals with the differences in the distribution and spectra of electric light and daylight by having the target value increase as the photosensor signal increases. Therefore, as daylight enters the room causing the photosensor signal to increase, the target value (set point) becomes greater. This action allows the ceiling illuminance, where the photosensor is located, to increase proportionally more than the task illuminance. For this reason, the closed-loop control algorithm has been referred to as "sliding set point control" in the literature.