Patent Publication Number: US-8115626-B2

Title: Occupancy sensing with selective emission

Description:
BACKGROUND 
     Occupancy sensors are used to monitor the presence of human occupants in indoor and outdoor spaces. Occupancy sensors conserve energy by automatically turning off lighting and other electrical loads for a space when the space is unoccupied. Occupancy sensors also perform a convenience function by automatically turning on lighting and other loads when an occupant enters a given space. 
     Sensing technologies used with occupancy sensors can generally be characterized as either active or passive technologies. Passive technologies do not involve the active emission of any type of energy in the monitored space. Instead, passive technologies rely on the detection of energy given off by the occupants themselves, or reflected by the occupants from ambient sources. An example of a passive occupancy sensing technology is passive infrared (PIR) sensing. PIR sensing relies on the fact that the thermal energy of warm objects causes them to emit infrared radiation. The infrared radiation is sensed by a photocell which converts the radiation to electric signals for further processing. Another type of passive occupancy sensing technology is video sensing which relies on ambient light that is reflected by an occupant and detected by a video sensor such as a charge coupled device (CCD). 
     With active technologies, some type of energy is emitted in the monitored space. The emitted energy is reflected by an occupant and converted into an electric signal by a suitable sensor. An example of an active occupancy sensing technology is ultrasonic sensing. In an ultrasound system, the monitored space is flooded with ultrasonic waves that are constantly emitted by an ultrasound driver. An ultrasound sensor detects waves that are reflected by an occupant and/or other objects in the monitored space. By comparing the emitted and reflected waves, an ultrasonic system can determine whether an object is moving. Moving objects are assumed to be occupants. 
     Some occupancy sensors use a combination of sensing technologies. For example, PIR is generally more accurate for detecting large motion such as a person walking into a room in a path that is directly within the line-of-sight of the occupancy sensor. Ultrasound systems tend to be more sensitive for detecting small motion, such as a person working at a desk, and motion that is hidden from the line-of-sight of the occupancy sensor, such as behind partitions in an office or restroom. The added sensitivity, however, may cause false occupied readings. Therefore, an occupancy sensor may initially use only PIR sensing to determine that the monitored space has become occupied. Once the space is initially determined to be occupied, an occupied reading from either PIR or ultrasound may be used to determine that the space continues to be occupied. A countdown timer is typically used to keep the lights on only for a predetermined period of time unless occupancy is sensed again during the countdown time. The countdown timer is reset to a predetermined value (typically 10-30 minutes) when occupancy is initially sensed and the lights are turned on. The timer then continues to decrement toward zero. Each occupied reading from either PIR or ultrasound causes the timer to reset to the maximum value. If the timer decrements all the way to zero before another occupancy event is detected, the lights are turned off, and the sensor returns to the PIR only sensing mode. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates an embodiment of an occupancy sensing system according to some inventive principles of this patent disclosure. 
         FIG. 2  illustrates another embodiment of an occupancy sensing system according to some inventive principles of this patent disclosure. 
         FIG. 3  illustrates an embodiment of an occupancy sensing method according to some inventive principles of this patent disclosure. 
         FIG. 4  illustrates another embodiment of an occupancy sensing method according to some of the inventive principles of this patent disclosure. 
         FIG. 5  illustrates another embodiment of an occupancy sensing method according to some of the inventive principles of this patent disclosure. 
         FIG. 6  illustrates another embodiment of an occupancy sensing method according to some of the inventive principles of this patent disclosure. 
         FIG. 7  illustrates another embodiment of an occupancy sensing method according to some of the inventive principles of this patent disclosure. 
         FIG. 8  illustrates another embodiment of an occupancy sensing method according to some of the inventive principles of this patent disclosure. 
         FIG. 9  illustrates another embodiment of an occupancy sensing method according to some of the inventive principles of this patent disclosure. 
         FIG. 10  illustrates another embodiment of an occupancy sensing method according to some of the inventive principles of this patent disclosure. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  illustrates an embodiment of an occupancy sensing system according to some inventive principles of this patent disclosure. The embodiment of  FIG. 1  includes a driver  10  to emit energy in a monitored space  12  in response to a drive control signal. The energy may be in any form suitable for occupancy sensing such as ultrasonic waves, infrared light, visible light, microwaves, electrostatic energy, or any other form of waves, rays, pulses, particles, etc. A sensor  14  outputs a sense signal in response to some of the energy reflected within the space. For example, energy may be reflected by an occupant  16 , if any, or from walls, furniture, moving air masses, etc. The sensor  14  is sensitive to the particular type or types of energy emitted by the driver  10 . For example, in one embodiment, the driver may emit ultrasonic waves, and the sensor  14  may be an ultrasound transducer that converts reflected ultrasound waves into electrical signals. In another embodiment, the driver  10  may emit infrared radiation to illuminate the space with light that is not visible to the human eye, and the sensor  14  may be a video camera that is sensitive to infrared wavelengths. 
     The system of  FIG. 1  also includes control logic  18  to determine the occupancy condition of the space in response to the reflected energy sensed by the sensor  14 , and to control an electrical load  20  for the space  12  in response to the occupancy condition. The control logic  18  may selectively de-energize the driver  10 . That is, the driver may be de-energized by the control logic at times other than when the control logic itself and/or the entire system is powered down. For example, the control logic  18  may de-energize the driver  10  whenever the load  20  is de-energized. As another example, the control logic  18  may de-energize the driver  10  when potentially interfering energy is sensed in the space  12 . The driver may be de-energized completely, for example, by completely disconnecting the driver from a source of power, or by switching the drive control signal to a state that disables the driver. The driver may also be de-energized partially, for example, by reducing its energy emission to a level adequate to substantially reduce energy consumption or to substantially reduce or eliminate interference with other devices that may utilize or be sensitive to the type or types of energy emitted by the driver. 
     An embodiment of an occupancy sensing system according to the inventive principles of this patent disclosure may utilize only a single type of active occupancy sensing technology as shown in  FIG. 1 , or it may include apparatus and/or software to implement one or more additional active or passive occupancy sensing technologies in combination with the first type of active occupancy sensing. 
     The control logic  18  may be implemented in hardware, software or any combination thereof. The control logic may include one or more microprocessors or microcontrollers, discrete logic, analog circuitry, or any other suitable apparatus and/or software to implement any of the sensing and/or control schemes according to the inventive principles of this patent disclosure. 
     The connections between the control logic  18  and the driver  10 , load  20  and sensor  14  may be in any suitable form. Hardwired connections may include screw or spring terminals, pigtail leads, printed circuit (PC) board traces, etc. Wireless connections may include signaling media such as radio frequency (RF), infrared (IR), optical, etc. Signaling modes for wired or wireless connections may be analog, digital, hybrid, etc., at any suitable voltages, currents, optical power levels or wavelengths, RF power levels or frequencies, etc. 
     The control logic  18  may control power to the load  20  directly, or through a load control circuit which may be integral with the control logic, integral with the load, or distributed between both. Alternatively, the load control circuit may be embodied as one or more components separate from both the control logic and the load. 
     In some embodiments, the load control signal may deliver power to the load. For example, the load control signal may operate at 115 VAC to provide power to a lighting load or fan. In other embodiments, the load control signal may only perform a signaling function. For example, the load control signal may be implemented as a hardwired low-voltage digital signal to communicate with a heating ventilation and air conditioning (HVAC) system, a lighting control system, or any other type of system. 
     The components shown in  FIG. 1  may be arranged in any physical relation to the space  12  and to each other. Any or all of the components may be located in, near, or remote from the space. For example, electrical lighting loads may typically be located in or just above the space, whereas an exhaust fan or air conditioning (A/C) load may be located remotely from the space. The control logic  18 , driver  10  and sensor  14  may be arranged in any combination of common or separate locations and in common or separate enclosures, if any. For example, in one embodiment, the control logic, driver and sensor may be located in a common wall switch enclosure that includes power control circuitry for controlling the load. In another embodiment, the driver and sensor may be located in a separate enclosure that is mounted remotely from the control logic. In yet another embodiment, the control logic, driver and sensor may be located in a ceiling mount or wall/corner mount enclosure that sends low-voltage control signals to a relay cabinet or other apparatus for controlling the power to the load. 
       FIG. 2  illustrates another embodiment of an occupancy sensing system according to some inventive principles of this patent disclosure. For purposes of illustration, the embodiment of  FIG. 2  is described in the context of a lighting control system that includes active ultrasound sensing and passive infrared (PIR) sensing, but the inventive principles are not limited to these implementation details. 
     The system of  FIG. 2  includes a microcontroller  22  to provide centralized control and monitoring of the system. A driver  24  may include a transducer and associated drive circuitry arranged to emit sound waves into the monitored space at ultrasonic frequencies in response to a drive control signal from the microcontroller  22 . The driver may be energized and de-energized in numerous manners according to some of the inventive principles of this patent disclosure. For example, the driver may be turned full-on at a fixed output power level or full-off in a digital manner. Alternatively, the driver may be energized or de-energized to any intermediate power level. The driver may be controlled to emit ultrasound energy at any fixed frequency, or at any combination of modulated frequencies. The transition between power levels, frequencies, or both, may be made in abrupt steps, or may be ramped to provide smooth transitions between states. For example, when driver is energized, the power, frequency, or both may be ramped gradually to reduce or eliminate an audible clicking or chirping noise that may otherwise result from the sudden energizing of the driver. 
     A sensor  26  may include another transducer to convert ultrasound waves received from the monitored space into an electric signal. The sensor  26  may also include associated circuitry to process the signal from the transducer to detect the presence of ultrasound energy, for example from another source of ultrasound energy in the monitored space. The sensor  26  may also include circuitry to process the signal from the transducer to detect the presence of an occupant in the space. For example, in an embodiment that utilizes the Doppler Effect to detect motion of an occupant, the circuitry may include a demodulator to detect frequency and/or phase shift of the ultrasound signal emitted by the driver  24  and reflected by the occupant in the monitored space. In some embodiments, the same circuitry may be used to perform any or all of these functions, while in other embodiments, separate circuits may be utilized. 
     The embodiment of  FIG. 2  also includes passive infrared circuitry  28  to generate a PIR sense signal in response to infrared radiation received from the monitored space by a photocell. A load control circuit  30  controls the lighting load  32  in response to a load control signal from the microcontroller  22 . 
     The embodiment of  FIG. 2  provides a versatile platform for implementing numerous methods according to some of the inventive principles of this patent disclosure. The microcontroller  22  may be programmed to perform actions at specified times and/or in response to various user inputs or sensed conditions. Some example embodiments of methods implemented with the system of  FIG. 2  are described below, but the inventive principles are not limited to these specific details, or to the platform of  FIG. 2 . 
       FIG. 3  illustrates an embodiment of a method according to some inventive principles of this patent disclosure.  FIG. 3  illustrates various states, events and/or actions of the system as time progresses along the horizontal axis. The line OCC TIMER represents the state of a delay timer which counts down from a maximum (reset) value of 100 percent down to zero percent. When OCC TIMER reaches zero, the lights are turned off. If implemented with the system of  FIG. 2 , OCC TIMER may be embodied as a digital word that is reset and decremented by the microcontroller, or as a separate timer circuit in the microcontroller. In an analog implementation, OCC TIMER may be embodied as a voltage on a capacitor that is discharged by a constant current. 
     The symbol ↓ in the PIR DET line indicates when the signal from passive infrared circuit indicates that large motion has been detected. The symbol ↓ in the U/S DET line indicates when the signal from the ultrasound sensor indicates that small motion has been detected. 
     The line LIGHTS indicates the state of the lights where a solid bar indicates that the lights are on. The line U/S DRV indicates the state of the ultrasound driver. In this embodiment, the ultrasound drive is assumed to be either full on (indicated by a solid bar) or full off, but other states of the driver are possible according to the inventive principles of this patent disclosure. Likewise, the lights or other electrical loads may be controlled to states other than full on or full off. 
     Referring again to  FIG. 3 , prior to time t 0 , the lights are off, the OCC TIMER is at zero, and the ultrasound drive has been de-energized. In this state, the system waits for the PIR DET signal to indicate that large movement is sensed. At time t 0 , large movement is sensed, which may be caused, for example, by an occupant entering the space. This causes the system to turn the lights on, reset OCC TIMER to 100 (percent), and energize the ultrasound driver. OCC TIMER continues to decrement until time t 1  when the ultrasound sensor detects small motion. This causes the system to reset OCC TIMER. Two more small movements are detected at times t 2  and t 3  which causes the system to reset OCC TIMER in response to both events. A time t 4 , both PIR DET and U/S DET indicate that motion is sensed, which may indicated that the occupant has left the space. At time t 5 , OCC TIMER reaches zero without any further detection of large or small motion, so the system turns off the lights and de-energizes the ultrasound driver. The lights remain off, and the ultrasound remains de-energized until time t 6  when the PIR DET signal to indicate that large movement is sensed. 
     Because the ultrasound driver is de-energized during unoccupied periods, energy may be conserved because no energy or reduced energy is emitted into the monitored space during periods in which the space is known to be unoccupied. This may also prolong the life of the ultrasound driver. 
       FIG. 4  illustrates another embodiment of a method according to some inventive principles of this patent disclosure. In the embodiment of  FIG. 4 , the ultrasound driver is not energized immediately upon detection of a large motion, but instead is left de-energized until after a time delay TD that is typically shorter than the reset value of the occupancy countdown timer. The line U/S DLY in  FIG. 4  indicates the state of the time delay. Since the passive infrared sensor has detected a large movement, the ultrasound sensor is not needed to detect small movement immediately. In this embodiment, the time delay is set to roughly half the length of the countdown timer, but any other suitable time delay can be used. After the time delay, the ultrasound driver is energized to enable the ultrasound sensor to detect small motion and reset the countdown timer OCC TIMER prior to the countdown timer reaching zero indicating that the space is unoccupied. 
     Referring to  FIG. 4 , prior to time t 0 , the lights and ultrasound driver are both off. At time t 0 , the PIR DET signal indicates that a large movement is detected. Therefore, OCC TIMER is reset, and the lights are turned on. The ultrasound driver, however, is left off for a time delay of TD beginning at time t 0 . At time t 1 , the time delay is complete and the ultrasound driver is turned on. At time t 2 , the U/S DET signal indicates that a small movement has been detected, so OCC TIMER is reset and the ultrasound driver is turned off again for another time delay TD which ends at t 3 . The ultrasound driver is turned on at t 3 , and at t 4 , the U/S DET signal again indicates that a small movement has been detected, so OCC TIMER is again reset and the ultrasound driver is turned off again for another time delay TD which ends at t 5 . The ultrasound driver is turned on at t 5 , but no further movement is indicated by either the PIR or U/S DET signals before OCC TIMER reaches zero, so the lights and ultrasound driver are both turned off at time t 6 . Both are left off until another large movement is indicated by the PIR signal at t 7 . 
     As is apparent from  FIG. 4 , the ultrasound driver spends a greater amount of time in the de-energized state. This may reduce power consumption and/or prolong the life of the ultrasound driver. Moreover, reducing the amount of time the ultrasound driver spends in the energized state may reduce or eliminate potentially harmful interference with other devices such as electronic white boards, hearing aids, etc., that utilize or are sensitive to, ultrasonic energy. 
       FIG. 5  illustrates another embodiment of a method according to some inventive principles of this patent disclosure. In the embodiment of  FIG. 4  is similar to that of  FIG. 4  but shows how the PIR sensor may preempt the time delay TD for turning the ultrasound driver on. At time t 0 , the PIR signal indicates that a large movement is detected, and therefore, OCC TIMER is reset, the lights are turned on, and the time delay TD for turning the ultrasound sensor on begins as shown in the line U/S DLY. Before the time delay expires, however, the PIR signal indicates another large movement at time t 1 . Therefore, both the countdown timer OCC TIMER and U/S DLY are reset, and the ultrasound driver remains de-energized. Additional large motions are detected at t 2  and t 3  which preempt the time delay TD and reset U/S DLY. At time t 4 , the time delay TD finally expires before a large movement is detected, and therefore, the ultrasound driver is energized. Shortly thereafter, at time t 5 , the U/S DET signal indicates a small movement has been detected, so OCC TIMER and U/S DLY are both reset, and the ultrasound drive is de-energized. Another large movement is detected at time t 6 , so OCC TIMER and U/S DLY are both reset. The time delay TD expires at t 7  at which time the ultrasound driver is energized. No further movement is indicated by either the PIR or U/S DET signals before OCC TIMER reaches zero, so the lights and ultrasound driver are both turned off at time t 8 . 
     From  FIG. 5  it is apparent that this embodiment may further reduce the amount of time the ultrasound driver is energized. 
       FIG. 6  illustrates another embodiment of a method according to some inventive principles of this patent disclosure. In the embodiment of  FIG. 6 , the ultrasound driver is left on for a short period of time, e.g. a few seconds, immediately after the lights are turned off. This enables the ultrasound sensor to continue detecting reflected energy that would indicate small movements, for example, an occupant waving his or her arms to turn the lights back on after a false determination of an unoccupied state. 
     Referring to  FIG. 6 , the PIR signal indicates a large movement at time t 0 , thus the lights are turned on and OCC TIMER and U/S DLY are reset. At time t 1 , the U/S DLY reaches zero, and the ultrasound driver is energized. No large or small movements are detected before OCC TIMER reaches zero, so the lights are turned off at time t 2 . The ultrasound driver, however, remains energized as shown in the shaded area long enough to detect a small motion at time t 3 . Therefore, the lights are turned back on, and OCC TIMER and U/S DLY are reset. At time t 4 , the U/S DLY reaches zero, and the ultrasound driver is energized. Another small motion is detected at t 5 , and therefore, OCC TIMER and U/S DLY are reset. At time t 6 , U/S DLY reaches zero, and the ultrasound driver is energized. No large or small movements are detected before OCC TIMER reaches zero, so the lights are turned off at time t 7 . The ultrasound driver remains energized again as shown in the shaded area. This time, however, no small motion is detected during the predetermined short time period, and therefore, the lights remain off until another large motion is detected by the PIR sensor. 
       FIG. 7  illustrates another embodiment of a method according to some inventive principles of this patent disclosure. In the embodiment of  FIG. 7 , the ultrasound driver is periodically de-energized to enable the ultrasonic sensor to detect the presence of ultrasound energy that was not emitted by the ultrasound driver. The presence of ultrasound energy that was not emitted by the ultrasound driver may indicate the presence of other devices that utilize ultrasound technology, for example, an electronic whiteboard, ultrasonic medical equipment, etc. In one example embodiment, the driver may be energized for one minute, then de-energized for one second to listen for other sources of ultrasound. Other time periods may be used in accordance with the inventive principles of this patent disclosure. 
     In  FIG. 7 , the symbol ( ) in the U/S DET line indicates when the signal from the ultrasound sensor is checked to see if it has detected energy that was not emitted by the ultrasound driver. The symbol (↓) in the U/S DET line indicates energy that was not emitted by the ultrasound driver has been detected. 
     Referring to  FIG. 7 , the PIR signal indicates a large movement at time t 0 , thus the lights are turned on, OCC TIMER is reset, and the ultrasound driver is energized for a predetermined period of time. This may be associated, for example, with an occupant entering a room. After the driver is de-energized, the U/S DET signal is checked at t 1  to determine if there are any other ultrasound sources in the space. No other sources are detected, so the ultrasound driver is energized again at t 2 . A small movement is detected at t 3 , and OCC TIMER is reset. This may be associated, for example, with the occupant working at a desk or conference table. No other ultrasound sources are detected at two additional de-energize and sense cycles at times t 4  and t 6 . A large movement is detected by the PIR sensor at t 8 , and therefore, OCC TIMER is reset. This large movement may be associated, for example, with a second occupant entering the room. At time t 9 , the U/S DET signal is checked to determine if there are any other ultrasound sources in the space. This time, the U/S DET signal indicates the presence of another source of ultrasound energy in the space. This may be associated, for example, with the first and second occupants using an electronic white board. Therefore, the ultrasound driver is maintained in a de-energized state for the remainder of the time during which the space is determined to be occupied. At time t 10 , another large movement is detected, and OCC TIMER is reset. This may be associated, for example, with both occupants leaving the room. OCC TIMER reaches zero at time t 11 , as which time the lights are turned off, and the system waits to detect another large movement. 
     Thus, the embodiment of  FIG. 7  may reduce or eliminate potential interference between the occupancy sensor and other apparatus that utilizes ultrasound energy. 
       FIG. 8  illustrates another embodiment of a method according to some inventive principles of this patent disclosure. In the embodiment of  FIG. 8 , the system checks for the presence of other sources of ultrasound energy in the monitored space before the ultrasound drive is ever energized. 
     Referring to  FIG. 8 , the PIR signal indicates a large movement at time t 0 , thus the lights are turned on and OCC TIMER and U/S DLY are reset. Before U/S DLY reaches zero, the U/S DET signal is checked at time t 1  to determine the presence of ultrasound energy that was not emitted by the ultrasound driver. In this example, none is detected at t 1 , so the ultrasound driver is energized at time t 2  when U/S DLY reaches zero. The U/S DET signal indicates a small movement at time t 3 , and therefore, OCC TIMER and U/S DLY are reset. Before U/S DLY reaches zero again, the U/S DET signal is checked again at time t 4  to determine the presence of other sources of ultrasound energy in the monitored space. This time, ultrasound energy that was not emitted by the ultrasound driver is detected, indicating the possible presence of potentially interfering energy. Therefore, the ultrasound driver is disabled for the remainder of the occupied period, or any other suitable length of time. 
     From  FIG. 8  it is apparent that this embodiment may provide even better protection from harmful interference, more power savings, etc. 
       FIG. 9  illustrates another embodiment of a method according to some inventive principles of this patent disclosure. The embodiment of  FIG. 9  is an ultrasound-only implementation. Therefore, the ultrasound driver is energized during all, or any suitable portion of a period during which the space is determined to be unoccupied. In one possible embodiment, the driver may remain energized during the entire unoccupied period. In other possible embodiment, the driver may be alternately de-energized, then energized for short periods of time to check for movement indicating the presence of an occupant. 
     Referring to  FIG. 9 , the ultrasound driver is energized prior to t 0  at which time a motion is indicated by the U/S DET signal. Therefore, the lights are turned on, and OCC TIMER and U/S DLY are reset. At time t 1 , U/S DLY reaches zero, and the ultrasound driver is energized. Motion is detected at t 2 , at which time OCC TIMER and U/S DLY are reset. The ultrasound driver is energized again at t 3  and motion is detected at t 4 , thus, OCC TIMER and U/S DLY are reset again. At time t 5 , U/S DLY reaches zero and the ultrasound driver is energized. This time, no motion is detected before OCC TIMER reaches zero at time t 6 , and therefore the lights are turned off. In this example, the ultrasound driver remains continuously energized after the lights are turned off, but as discussed above, the ultrasound driver may be periodically energized and de-energized during an unoccupied period. 
     From  FIG. 9  it is apparent that, even in an ultrasound-only embodiment, some of the inventive principles of this patent disclosure may enable a reduction in power consumption. 
       FIG. 10  illustrates another embodiment of a method according to some inventive principles of this patent disclosure. The embodiment of  FIG. 10  is another ultrasound-only implementation. In this embodiment, however, the system checks for the presence of other sources of ultrasound energy in the monitored space prior to energizing the ultrasound driver during an occupied period. This embodiment also energizes the driver for a short time period immediately after the lights are turned off to detect small motions that may signify a false determination that the space is unoccupied. Although this short time period may be merged with the normal energizing of the driver during an unoccupied time in an ultrasound-only embodiment, there may be differences in the detection threshold. For example, the system may have a low threshold (high sensitivity) to look for a small movement during the short time period immediately after turning the lights off, but thereafter, a higher threshold may be require to indicate a large movement needed to turn the lights back on. 
     Referring to  FIG. 10 , the ultrasound driver is energized prior to t 0  at which time a motion is indicated by the U/S DET signal. Therefore, the lights are turned on, and OCC TIMER and U/S DLY are reset. Before U/S DLY reaches zero, however, the U/S DET signal is checked at time t 1  to determine the presence of ultrasound energy that was not emitted by the ultrasound driver. In this example, none is detected at t 1 , so the ultrasound driver is energized at time t 2  when U/S DLY reaches zero. The U/S DET signal indicates movement at time t 3 , and therefore, OCC TIMER and U/S DLY are reset. Before U/S DLY reaches zero again, the U/S DET signal is checked again at time t 4  to determine the presence of other sources of ultrasound energy in the monitored space. This time, ultrasound energy that was not emitted by the ultrasound driver is detected, indicating the possible presence of potentially interfering energy. Therefore, the ultrasound driver is de-energized for the remainder of the occupied period, or any other suitable length of time. In this example, the ultrasound driver is de-energized until OCC TIMER reaches zero at t 6 , at which time the lights are turned off, but the ultrasound driver is energized to enable the ultrasound sensor to detect small movements which would indicate a false determination of an unoccupied condition. Such a small movement is detected at time t 7 , and therefore, the lights are turned back on, and OCC TIMER and U/S DLY are reset. The presence of other sources of ultrasound energy are again detected at time t 8 , so the driver remains de-energized until OCC TIMER reaches zero and the lights are turned off at time t 10 . The ultrasound driver is energized again for a short false-unoccupied detection period. No small movement is detected during the short time period, and therefore, at time t 11 , the occupancy sensor enters the normal unoccupied mode. 
     The inventive principles of this patent disclosure have been described above with reference to some specific example embodiments, but these embodiments can be modified in arrangement and detail without departing from the inventive concepts. Such changes and modifications are considered to fall within the scope of the following claims.