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Control Algorithm, continued

Open-loop Proportional

Because open-loop proportional control does not involve feedback, its analysis is more straight forward compared to the others. As the open-loop response plot shows, the control voltage is negatively proportional to the optical signal on the photosensor. The greater the optical signal, the lower the control voltage.

Open-loop control has one adjustable parameter - the constant of proportion between the control voltage and the optical signal. The constant of proportion is the slope of the open-loop response curve. When commissioning an open-loop proportional control, the constant of proportionality must be determined by the photosensor signal that is related to the appropriate dim level.

An open-loop proportional control algorithm can be used in a closed-loop configuration, but it is not well suited for closed-loop applications because the system will never be able to deliver the maximum electric lighting level. This deficiency amounts to wasted electric light capacity. According to the open-loop response curve, whenever the optical signal on the photosensor is greater than zero, the control signal is less than maximum and so the electric light is dimmed whenever any light is present.


 
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