Patent Publication Number: US-2011074225-A1

Title: Reduced power consumption sensor device and illumination system comprising such a sensor device

Description:
FIELD OF THE INVENTION 
     The invention relates to the field of wireless sensor devices, to a method of operating such devices and to an illumination system comprising one or more of such wireless sensor device. More specifically, the invention relates to a wireless sensor device comprising a motion sensor, a signal processor and an on-board power supply capable of reducing power consumption. 
     BACKGROUND OF THE INVENTION 
     In recent years, attention has increased for energy saving measures in the field of illumination of areas, such as rooms. As an example, control systems have been developed that activate light sources only when the presence of human beings is detected, using occupancy sensors (e.g. pyroelectric infrared sensors), and to adapt the level of artificial light from the light sources depending on the level of daylight in the area, using light level sensors. Occupancy sensors and light sensors may advantageously be used in combination. 
     The sensor devices typically need to be installed at particular locations in the area in order to fulfill the sensing function. Installation of such sensor devices is greatly facilitated by providing wireless sensor devices. Such sensor devices are capable of wireless transmission of the sensing data, or derivatives thereof, to the light sources (or a central controller thereof). 
     For true wireless sensor devices, not only the sensing data should be transmitted wirelessly, but also energizing the sensor devices should not require wire connections. A battery or other energy storage means may be used to accomplish such a wireless sensor device. For such sensor devices, power consumption is critical, o.a. for the lengthening the operating lifetime of the energy storage means. 
     US 2003/0047671 discloses an object detecting device utilizing a pyroelectric sensor providing a sensing current, a voltage amplifier providing an amplified voltage proportional to the sensing current, and a level monitor providing a detection signal when the amplified voltage exceeds a detection threshold. The device has a standby mode where the voltage amplifier receives a limited source current to provide the voltage of low amplification, and the level monitor provides a wake-up signal when the low amplified voltage exceeds a preliminary threshold lower than the detection threshold. In response to the wake-up signal, the device is switched to an operation mode where the amplifier receives a rated source current to provide a voltage of high amplification for comparison with the detection threshold. 
     There is a need in the art for a wireless sensor device configured for further reduction of the power consumption. 
     SUMMARY OF THE INVENTION 
     A wireless sensor device configured for sensing motion of an object in an area is proposed. The device comprises a motion sensor configured for generating a sensing signal when sensing motion in the area and a signal processor configured for processing the sensing signal. A power supply is provided for providing power to the motion sensor. Furthermore, the wireless sensor device comprises a controller configured for distinguishing, from the sensing signal, large object movements and small object movements of the object in the area. The controller may possibly use the signal processor for that purpose. The wireless sensor device is configured to perform one or more power consumption reduction operations upon detection of large object movement. The power consumption reduction operations include temporarily disconnecting the motion sensor from the power supply and decreasing a duty cycle of the signal processor. 
     An illumination system comprising one or more light sources and at least one such wireless sensor device is also proposed. 
     Furthermore, a method of operating a wireless sensor device configured for sensing motion of an object in an area is proposed. The device comprises a motion sensor configured for generating a sensing signal when sensing motion in the area and a signal processor configured for processing the sensing signal. A power supply is provided for providing power to the motion sensor. Large and small object movements of the object in the area may be distinguished from the sensing signal (or derivatives thereof). One or more power consumption reduction operations can be performed in response to large object movement. The power consumption reduction operations include temporarily disconnecting the motion sensor from the power supply and decreasing a duty cycle of the signal processor. 
     Large object movement typically involves motion of the complete object (e.g. someone entering a room), while small object movement involves motion of only a part of the object (e.g. someone picking up a telephone). 
     Power may be interrupted by switching off the power supply or by disconnecting the power supply from the motion sensor or other component. 
     In the prior art, the amplifier is kept to be less-power consuming in the absence of the wake-up signal, reducing a power requirement. The amplifier, however, only consumes a relatively small amount of power. The invention is based on the insight that the motion sensor (e.g. a pyroelectric infrared motion sensor) and the signal processor are the components mainly responsible for the energy consumption of the wireless sensor device. The defined power consumption reduction operations specifically target these components, thereby prolonging the life time of the power supply. 
     For some applications, such as lighting applications, a wireless sensor device switches on the light when someone enters a room (i.e. a large object movement). If no further movement is detected, however, the light is not switched off immediately but only after a predetermined period of time to avoid continuous switching between an on-state and an off-state of the light source. Since motion sensing is not required in this period for controlling the light sources, the motion sensor can be switched off temporarily, thereby saving energy. 
     A lower duty cycle can be used in the processor for processing a signal indicative of large object movement than for processing a small object movement signal. A lower duty cycle generally reflects lower power consumption. The duty cycle of the signal processor may also be adapted in response to large object movement. As an example, if someone has left the room, resulting in large object movement, generally large object movement (viz. someone entering the room again) can be expected before small object movement and, accordingly the duty cycle of the processor can be lowered. 
     The embodiment of claim  2  provides the advantage of switching on the motion sensor after a predetermined period in order to enable motion detection again. The predetermined period is less than 10 minutes, preferably less than 5 minutes to achieve an optimal balance between energy consumption reduction and sensing functionality of the wireless sensor device. 
     The embodiments of claims  3  and  4  provide for reduced energy consumption in the period after reconnection of the power supply to the motion sensor. 
     Generally, the sensing signals of the motion sensor may be amplified before being processed further. This is particularly true for sensing signals resulting from small object movements. The embodiment of claim  6  provides the advantage that the amplifier is powered only after having detected large object movement, thereby saving energy for the wireless sensor device in the period prior to the large object movement. 
     Pyroelectric infrared (PIR) motion sensors, as defined in claim  7 , are suitable for motion detection. Such PIR sensors provide sensing signals indicative of large object movement and small object movements on the basis of temperature differences between the object and the background. 
     The embodiment of claim  8  is advantageous in that the on-board power supply avoids the need for installing power wires and provides the freedom of installing the wireless sensor device at the desired location. 
     The embodiment of claim  9  provides the advantage of further prolonging the life time of the power supply by enabling energy harvesting from the ambient light and light emitted from available light sources. 
     Hereinafter, embodiments of the invention will be described in further detail. It should be appreciated, however, that these embodiments may not be construed as limiting the scope of protection for the present invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the drawings: 
         FIG. 1  is a diagrammatic view of a room comprising a wireless sensor device and an illumination system according to an embodiment of the invention; 
         FIG. 2  is a schematic illustration of the wireless sensor device of  FIG. 1 ; and 
         FIGS. 3A-3B  provide flow charts of methods of operating the wireless sensor device of  FIG. 2 . 
     
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows illumination system  1  provided in an area (such as a room) occupied by a person P. The illumination system  1  comprises a wireless sensor device  2  and at least one armature  3  having one or more light sources (not shown). The area may e.g. be an office environment or a domestic environment. Alternatively, a central controller  4  may be present in the room to receive the signals from the wireless sensor device  2  and to provide operation commands for the armature or armatures  3  in the room. 
     In the present embodiment, the wireless sensor device  2  controls the operation of the armature  3  in response to triggers sensed by the sensing device  2 . Such triggers may include the entrance of the person P into a room or a change of the ambient light level in the room. The type of trigger that may be detected depend on the sensors comprised in the wireless sensor device  2 . Examples of control of the operation of the armature  3  include switching on/off one or more light sources of the armature, adapting the color and/or brightness of the light emitted by the light sources, redirecting the light of the light sources etc. 
       FIG. 2  is a schematic illustration of the wireless sensor device of  FIG. 1 . The wireless sensor device  2  comprises a motion sensor  10 , e.g. a pyro-electric infrared (PIR) sensor. Such a PIR sensor  10  provides sensing signals indicative of large object movement and small object movements on the basis of temperature differences between the object and the background. In the example of  FIG. 1 , movement of the person P may be detected. 
     The sensing signals of the PIR sensor  10  are sensing currents. I/V converter  11  converts the signals from the PIR sensor  10  into voltage signals that are subsequently amplified by amplifier  12  to obtain an amplified sensing signal. A signal processor  13  receives the amplified sensing signal for further processing. The signal processor processes the signals with a variable duty cycle, as will be explained below in further detail. 
     The wireless sensor device has an on-board power supply  14  for providing power to PIR sensor  10 , I/V converter  11 , amplifier  12  and signal processor  13 . The power supply  14  preferably comprises a battery. Energy can be harvested to charge the battery by means of a photo electric module  15 , e.g. a solar cell. Photo electric module  15  may also comprise a light sensor used for both sensing and energy harvesting purposes. 
     The wireless sensor device  2  contains a controller  16 . Controller  16  is configured for controlling the power supply to the PIR sensor  10  and amplifier  12 . Moreover, controller  16  may control the duty cycle of signal processor  13 . By these means, controller  16  is configured for performing power consumption reduction operations by controlling operation of the PIR sensor  10  and the amplifier  12  and/or the duty cycle of the signal processor  13  as will be explained in further detail with reference to  FIGS. 3A and 3B . 
     Finally, wireless sensor device  2  may comprise a command transmitter  17  configured for transmitting operating commands to the armature  3  of the illumination system  1  ( FIG. 1 ). 
     In  FIG. 3A , a flow chart of performing a power consumption reduction operation is depicted wherein power supply for the PIR sensor is temporarily interrupted by controller  16 . 
     In step  20 , PIR sensor  10  is powered by power supply  14 . Person P enters a room which is detected by PIR sensor  10  in step  21  as a large movement using signal processor  13 . Amplifier  12  may be off and signal processor  13  may be run at a low duty cycle (see  FIG. 3B ). 
     In step  22 , controller  16  is informed of the large object movement and disconnects the power supply  14  from the PIR sensor  10  for a predetermined period P1. P1 may e.g. be 5 minutes. Of course, during this period P1, the PIR sensor  10  will not be able to detect object movements in the room. However, this is also not necessary since for lighting applications, typically, light remains switched on until some time (e.g. 10-15 minutes) after detecting movement in the room. 
     After expiry of period P1, controller  16  controls feeding power from power supply  14  to PIR sensor  10  again in step  23 . It is noted that PIR sensors generally require considerable time (e.g. 30 seconds-1 minute) before stabilization in order to provide a reliable sensing signal. 
     After the stabilization time, PIR sensor  10  remains powered during a period P2 to detect object movement in the room (step  24 ). Amplifier  12  may also be powered and the duty cycle of signal processor  13  may be increased in order to reliably detect small object movements in the room. The latter operation is schematically depicted in the last block of  FIG. 3B . 
     If movement is detected during period P2, controller  16  may again interrupt power feeding from power supply  14  to PIR sensor  10  for a period P1. 
     If no movement is detected during period P2, controller  16  may operate command transmitter  17  to instruct armature  3  to switch off the light (step  25 ). Power remains to be supplied from power supply  14  to PIR sensor  10  for detecting object movement. 
     The duty cycle of the signal processor may be decreased by a variety of triggers, such as the expiry of a time period wherein the PIR sensor  10  did not register object motion in the room.