Abstract:
A daylight harvesting light fixture has a housing defining a light aperture, a light emitting diode (LED) in the housing for providing illumination through the aperture, a power supply for supplying drive current to the LED; a photosensor for sensing ambient illumination through the housing aperture, and a control circuit programmed and connected for a) turning off current to the LED for a dark interval imperceptible to the human eye; b) deriving an ambient illumination level signal based on the photosensor output while the LED is turned off; c) calculating an LED light output complementary to measured ambient illumination to achieve a target ambient illumination; and d) restoring drive current to the LED at a level adjusted to produce the calculated complementary LED light output. This sequence may be repeated periodically to maintain a target ambient illumination.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention concerns lighting control systems and more particularly relates to improvements in daylight harvesting light fixtures and electronic control systems for the same. 
       STATE OF THE PRIOR ART 
       [0002]    Energy for interior illumination can be conserved by limiting artificial light to no more than needed for supplementing available daylight to achieve a desired or target level of interior lighting. Since available daylight illumination varies continuously over the course of the day it becomes necessary to adjust the artificial light complement accordingly in order to maintain a constant combined level of illumination. This technique of supplementing available daylight with electric light has come to be known as daylight harvesting. Substantial energy savings can be realized by this method. 
         [0003]    Current daylight harvesting technology is limited to use of daylight photosensors external to the electric light fixtures in an effort to minimize introduction of artificial light into the measurement of available ambient daylight. The location of the daylight sensors depends in part upon the type of control being used, whether open loop or closed loop or some combination of these. Open loop systems attempt to isolate the photosensor from the electrical illumination by mounting the sensor outside the interior space being illuminated, such as on a roof or exterior wall of the building, or sometimes on an interior wall but aiming the sensor at an exterior window or skylight. In closed loop systems the photosensor is situated for measuring overall interior illumination and the sensor output is used to adjust the artificial light level to achieve the desired target level of overall interior illumination. Such a closed loop sensor might be installed on a ceiling facing down towards work surfaces such as desktops to measure the total illumination on the work surfaces. 
         [0004]    Both types of systems require careful calibration of the photosensor output and the respective control modules to correctly isolate the effect of changes in available daylight upon illumination of the important parts of the interior space. It is also important to compensate for spillover direct illumination of the sensor by the electric interior lights which may distort the measurement of target interior illumination by the sensor. 
         [0005]    Wall and ceiling mounted photosensor modules external to the electric light fixtures complicate installation and maintenance of daylight harvesting illumination. A need exists for daylight harvesting lamp fixtures having daylight sensors integral to the light fixture. Such installation presents difficulties which to date have remained unsolved. Mounting the photosensor inside the fixture exposes it to direct illumination by the lamp, saturating the sensor. Conversely, installing the photosensor to one side of the aperture of the light fixture and away from direct exposure to the lamp places the sensor behind ornamental trim which typically surrounds the aperture of the light fixture, and would require an opening in the trim in alignment with the photosensor. This is a significant disadvantage as many interchangeable ornamental trim pieces exist in standard sizes which could not be used without the special opening for the sensor. Even if an opening is provided, the orientation of the trim piece then becomes restricted by the location of the sensor, so that square trim pieces, for example, could not be aligned with walls unless care was taken to install the entire fixture in correct orientation. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]      FIG. 1  is an elevational view of a room illuminated by one ceiling mounted light module and natural daylight admitted through a window; 
           [0007]      FIG. 2  is a bottom plan view of a typical daylight harvesting LED lamp module according to this invention; 
           [0008]      FIG. 3  is an elevational cross section of the light module of  FIG. 2 ; 
           [0009]      FIG. 4  is a block diagram of a daylight sensing control circuit according to this invention; 
           [0010]      FIG. 5  illustrates the daylight sensing and LED output adjustment sequence of the control circuit of  FIG. 4 ; and 
           [0011]      FIG. 6  is a timing diagram of the photosensor ambient illumination reading relative to the LED dark interval in the sequence of  FIG. 5 . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0012]    With reference to the accompanying drawings, in which like elements are designated by like numerals,  FIG. 1  is a schematic representation of an enclosed interior space or room R, with an end wall E in which a window W admits natural daylight D. A daylight harvesting lamp module  10  according to this invention is installed in ceiling C above a floor F for illuminating a space S (the illuminated space) generally underlying the lamp module  10  in room R. At different times of day space S in room R is illuminated by either or both of natural daylight D admitted through window W and artificial light L produced by lamp module  10 . 
         [0013]    Illumination of space S includes artificial illumination provided by lamp module  10  in combination with ambient illumination. Ambient illumination is defined here as existing illumination in space S without any contribution from module  10 . In a simple situation where a single module  10  is the sole source of artificial light in space S, ambient illumination may consist entirely of natural daylight D streaming through openings such as windows or skylights. In more complicated environments, ambient illumination may include artificial illumination contributed by light fixtures other than module  10 , alone or in combination with natural daylight D entering the space S. 
         [0014]    The daylight harvesting lamp module  10  is shown in greater detail in  FIGS. 2 and 3 . Lamp module  10  has a module housing  12  which may be of die cast aluminum for heat sinking purposes, with a module cavity  14  which has a closed end  16  and a light aperture  18 . An electrically powered light source such as a relatively high intensity LED (light emitting diode)  20  is mounted in thermal contact with housing  12  at the closed end  16  in housing cavity  14  for directing illumination through light aperture  18 . A transparent or translucent lens  24  closes aperture  18  as shown in  FIG. 3  to keep the interior of cavity  14  free of dust, and optionally may also serve an optical function such as more evenly diffusing or condensing the light output of LED  20 . 
         [0015]    A photosensor such as phototransistor  22  installed in module cavity  14  is oriented for sensing through aperture  18  the level of ambient illumination in the illuminated space S. An annular circuit board  32  carries photosensor  22  with its lens facing light aperture  18 , and other electronic components which make up the module control circuit, collectively indicated as element  34  in  FIGS. 2 and 3 . Photosensor  22  is chosen to be responsive to light wavelengths visible to the human eye, including natural daylight. Photosensor  22  is further selected to have relatively fast response, e.g. about 200 microseconds or less, to changes in light input. 
         [0016]      FIG. 4  is a block diagram of the electronic control circuit of module  10 , generally designated by numeral  50 . The module control circuit  50  includes a high intensity LED  20  powered by LED driver circuit  36 . LED driver  36  converts AC mains power, such as  110  VAC power, to lower DC voltage  44  for powering LED  20 . 
         [0017]    Control circuit  50  also includes a microcontroller unit MCU  30  which executes a control algorithm installed as resident firmware. A switch such as FET semiconductor  38  is connected in the return leg of LED  20 . The gate of FET  38  is connected to a GPIO (general purpose input output) control output  34  of MCU  30  for switching the FET  38  between on and off states, thereby turning LED  20  on and off under control of MCU  30 . 
         [0018]    During operation of module  10  with LED  20  turned on, the photosensor  22  is driven to saturation by internal reflections of LED light inside cavity  14 , including reflection from lens  15 , which is not perfectly transmissive and reflects a small percentage of LED light back towards the closed end of cavity  14  and photosensor  22 . In order to derive an output from photosensor  22  representative of ambient illumination in space S, MCU  30  rapidly turns off LED  20  responsive to its control algorithm by switching FET  34  to a non-conductive state for a time interval (the dark interval) which is kept sufficiently brief so that the interruption in light output of LED  20  is imperceptible to the human eye, that is, it is not noticeable to a person in the space illuminated by module  10 . The dark interval of LED  20  may be, for example, less than one millisecond and preferably about 250 microseconds. When LED  20  is turned off, photosensor  22  begins to recover from its saturated state at a rate which is dependent on the value of resistor R 1 . Recovery of sensor  22  must occur within a time limited by the need to keep the dark interval imperceptibly short. For this reason phototransistor  22  is chosen to have a relatively fast fall time, preferably about 200 microseconds or less. Sensor recovery is expedited by keeping the value of R 1  relatively low. This, however, also reduces the dynamic range of the photosensor&#39;s output signal, i.e. reduces the maximum level of the sensor&#39;s output signal and would result in relatively poor resolution of the ambient light measurement. Dynamic range is improved by amplification of the sensor output signal by op-amp  40  which may have a voltage gain of e.g. 10, set by resistors R 2 , R 3 . The amplified output of sensor  22  is then received as an input by an analog-to-digital converter (ADC) input  32  of MCU  30 . MCU  30  reads or captures the sensor output through ADC input  32  at a point in time which is under control of the resident control algorithm. The control algorithm delays the reading of the photosensor signal after turning off LED  20 , that is, after the start of the dark interval, and the reading is preferably timed to occur very close to the end of the dark interval, so as to allow maximum available time for recovery of sensor  22  from its saturated state, as shown in the timing diagram of  FIG. 5 . The dark interval of LED  20  is represented by the square waveform. The output of photosensor  22  is represented by curve A which falls off from a high saturated voltage V S  at the start of the dark interval to a lower voltage V L  representative of measured ambient illumination in space S. If space S is dark when LED  20  is turned off, e.g. it is night time with no daylight or other illumination in space S, the output of photosensor  22  tapers off to near zero by the end of the dark interval. If some degree of ambient illumination exists, the output of photosensor  22  will be at some level above zero but below saturation V S . At the end of the allotted dark interval MCU  30  rapidly turns on LED  20  by switching FET  38  to a conductive state. 
         [0019]    MCU  30  is programmed with resident firmware for adjusting the level of artificial illumination provided by LED  20  so as to maintain a desired total level of illumination of space S. The adjustment is made by means of an LED dimming control output of MCU  30 , such as PWM (pulse width modulated) control output  42  of MCU  30  which controls LED driver  36 . The LED dimming adjustment is made by the MCU control algorithm in response to changes in the output level of photosensor  22  obtained at ADC input  32 , depending on the level of ambient illumination provided by available natural daylight and any other sources of illumination in space S. The dimming interface of MCU  30  with LED driver  36  is not limited to a PWM output, and may include other control formats such as a variable voltage level (e.g. 0-10V),  120  (inter-integrated circuit), and SPI (serial peripheral interface), among still others. 
         [0020]    In the simple case where S is a small space illuminated by a single lamp module  10 , the only other source of ambient illumination may be daylight D streaming through windows or skylights. An appropriate control algorithm for MCU  30  dims the LED  20  so as to add only sufficient artificial illumination L to the measured ambient illumination to achieve a desired target level of total illumination L+D of space S. As ambient illumination including natural daylight D declines, more LED light output L is needed to maintain the target level of illumination. The desired target level of total illumination may be preset in the control firmware of MCU  30  as part of the initial calibration during installation of module  10 . In larger installations with multiple modules  10  a more complex control algorithm for MCU  30  will be required, as the ambient illumination sensed by photodetector  22  will include not only available natural daylight but also illumination contributed by other modules  10  or other sources of artificial light in or near space S. 
         [0021]    The daylight harvesting light fixture and control system of this invention is not limited to a particular control algorithm for MCU  30 , and many variations of such algorithms are possible. A typical general objective of the control algorithm is to recognize changes in ambient illumination in space S based on output of photosensor  22  and to compute an LED dimming control output to LED driver  36  so as to achieve a desired blend or combination of L and D illumination levels in space S. 
         [0022]      FIG. 6  illustrates a sequence  100  of steps  102 - 110  executed by the control algorithm programmed in MCU  30 . In step  102 , power to LED  20  is turned off by turning off FET  38  thereby initiating the dark interval; step  104  is a wait state to allow recovery of the photosensor  22  from the aforementioned saturated state; in step  106  the ambient illumination output signal derived by the now recovered photosensor  22  is an input to ADC  32  of MCU  30  which captures a value of the measured ambient illumination at a time determined by the control algorithm of MCU  30 ; in step  108  MCU  30  executes the portion of the control algorithm for computing an output level of artificial illumination by LED  20  needed for maintaining a target level of total illumination of space S in combination with the measured level of ambient illumination; and in step  110  MCU  30  provides an adjusted PWM control input to LED driver  36  and FET  38  is turned on for returning power to LED  20  at the newly adjusted power level. The step sequence  102 - 110  is repeated cyclically so that the power level to LED  20  and consequent LED light output is adjusted at a sufficiently frequent rate to smoothly compensate for changes in ambient illumination, including changes in natural daylight as well changes in illumination resulting from displacements and movements in the illuminated space S. 
         [0023]    The measurement of ambient illumination daylight by MCU  30  can be repeated rapidly, at a rate sufficient to achieve a fine degree of control over total illumination in space S as available daylight changes over the course of the day, and also to compensate for changes in illumination caused by such things as the lowering or raising of window blinds. 
         [0024]    With appropriate control coding of MCU  30  the artificial illumination light output of module  10  can be reduced from a maximum light output of LED  20  to take into account available natural daylight in the illuminated space while maintaining a desired target level of illumination of the illuminated space S, thereby conserving electrical power. 
         [0025]    Multiple modules  10  installed in a particular interior space S can be networked and programmed as desired to achieve illumination patterns tailored to the particular use and the requirements of the illuminated space S.