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Feedback Control

Photosensor control is divided into two main types: open loop and closed loop.

  • Open loop - the photosensor does not respond to, or "see" the electric light that it controls.
  • Closed loop - the photosensor senses and responds to the electric light that it controls.

An example of an open-loop system is a photosensor mounted on the outside of a building that controls the electric light level inside the building. In such a case the photosensor is exposed only to daylight. The electric light level is determined from the daylight signal alone. Such systems can be designed to simply turn electric lights off when outside daylight reaches a predetermined level. In the case of a dimming system, a signal proportional to the outside daylight instructs the system to dim the electric light by an amount proportional to the amount of available daylight sensed by the photosensor. No feedback control is used for an open loop system.

The drawback of open-loop feedback control is that the system cannot compensate or correct for any changes in the light distribution that affect the constant of proportionality between interior light levels and outside daylight levels. For example, the system will not respond to the use of window blinds, so if the occupant draws the blinds to block direct sunlight, the system will not increase the electric light to compensate for the decreased daylight levels inside the room.

An example of a closed-loop system is a photosensor mounted on the ceiling of the room where the electric lighting is being controlled. In this case the photosensor is exposed to both the daylight and the electric light in the room. The sensing of the electric light forms a feedback loop. See our animated control feedback loop. (Requires Shockwave Plug-in. Visit Macromedia to get the plug-in).

Closed-loop systems use negative feedback to respond to changing conditions. Negative feedback is a means of error correcting or compensating whereby an increase in an input signal level causes a decrease in the output signal. Conversely, a decrease in input signal causes an increase in output signal. This is the desired action of photosensor control; an increase in the amount of light in the room causes a decrease in the electric light intensity, and a decrease of daylight requires an increase of electric light. The overall feedback loop of a photosensor system must be negative for proper operation. The control algorithm characterizes the negative feedback of a photosensor.

More information on system diagram and feedback

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