SE544639C2 - Method and system for determining the presence of a person to wake up a device - Google Patents

Method and system for determining the presence of a person to wake up a device

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Publication number
SE544639C2
SE544639C2 SE2050418A SE2050418A SE544639C2 SE 544639 C2 SE544639 C2 SE 544639C2 SE 2050418 A SE2050418 A SE 2050418A SE 2050418 A SE2050418 A SE 2050418A SE 544639 C2 SE544639 C2 SE 544639C2
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SE
Sweden
Prior art keywords
signal
mode
threshold
smoothed
sensor
Prior art date
Application number
SE2050418A
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Swedish (sv)
Other versions
SE2050418A1 (en
Inventor
Niklas Kvist
Original Assignee
Jondetech Sensors Ab Publ
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Filing date
Publication date
Application filed by Jondetech Sensors Ab Publ filed Critical Jondetech Sensors Ab Publ
Priority to SE2050418A priority Critical patent/SE544639C2/en
Priority to CN202180038089.9A priority patent/CN115698657A/en
Priority to PCT/EP2021/059505 priority patent/WO2021209420A1/en
Publication of SE2050418A1 publication Critical patent/SE2050418A1/en
Publication of SE544639C2 publication Critical patent/SE544639C2/en

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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
    • G01J5/00—Radiation pyrometry, e.g. infrared or optical thermometry
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
    • G01J5/00—Radiation pyrometry, e.g. infrared or optical thermometry
    • G01J5/0022—Radiation pyrometry, e.g. infrared or optical thermometry for sensing the radiation of moving bodies
    • G01J5/0025—Living bodies
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
    • G01J5/00—Radiation pyrometry, e.g. infrared or optical thermometry
    • G01J5/02—Constructional details
    • G01J5/0265—Handheld, portable
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
    • G01J5/00—Radiation pyrometry, e.g. infrared or optical thermometry
    • G01J5/02—Constructional details
    • G01J5/06—Arrangements for eliminating effects of disturbing radiation; Arrangements for compensating changes in sensitivity
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
    • G01J5/00—Radiation pyrometry, e.g. infrared or optical thermometry
    • G01J5/10—Radiation pyrometry, e.g. infrared or optical thermometry using electric radiation detectors
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
    • G01J5/00—Radiation pyrometry, e.g. infrared or optical thermometry
    • G01J5/10—Radiation pyrometry, e.g. infrared or optical thermometry using electric radiation detectors
    • G01J5/12—Radiation pyrometry, e.g. infrared or optical thermometry using electric radiation detectors using thermoelectric elements, e.g. thermocouples
    • G—PHYSICS
    • G06—COMPUTING; CALCULATING OR COUNTING
    • G06F—ELECTRIC DIGITAL DATA PROCESSING
    • G06F1/00—Details not covered by groups G06F3/00 - G06F13/00 and G06F21/00
    • G06F1/26—Power supply means, e.g. regulation thereof
    • G—PHYSICS
    • G06—COMPUTING; CALCULATING OR COUNTING
    • G06F—ELECTRIC DIGITAL DATA PROCESSING
    • G06F1/00—Details not covered by groups G06F3/00 - G06F13/00 and G06F21/00
    • G06F1/26—Power supply means, e.g. regulation thereof
    • G06F1/32—Means for saving power
    • G—PHYSICS
    • G06—COMPUTING; CALCULATING OR COUNTING
    • G06F—ELECTRIC DIGITAL DATA PROCESSING
    • G06F1/00—Details not covered by groups G06F3/00 - G06F13/00 and G06F21/00
    • G06F1/26—Power supply means, e.g. regulation thereof
    • G06F1/32—Means for saving power
    • G06F1/3203—Power management, i.e. event-based initiation of a power-saving mode
    • G06F1/3234—Power saving characterised by the action undertaken
    • G06F1/3287—Power saving characterised by the action undertaken by switching off individual functional units in the computer system
    • G—PHYSICS
    • G08—SIGNALLING
    • G08B—SIGNALLING OR CALLING SYSTEMS; ORDER TELEGRAPHS; ALARM SYSTEMS
    • G08B13/00—Burglar, theft or intruder alarms
    • G08B13/18—Actuation by interference with heat, light, or radiation of shorter wavelength; Actuation by intruding sources of heat, light, or radiation of shorter wavelength
    • G08B13/189—Actuation by interference with heat, light, or radiation of shorter wavelength; Actuation by intruding sources of heat, light, or radiation of shorter wavelength using passive radiation detection systems
    • G08B13/19—Actuation by interference with heat, light, or radiation of shorter wavelength; Actuation by intruding sources of heat, light, or radiation of shorter wavelength using passive radiation detection systems using infrared-radiation detection systems
    • G—PHYSICS
    • G08—SIGNALLING
    • G08B—SIGNALLING OR CALLING SYSTEMS; ORDER TELEGRAPHS; ALARM SYSTEMS
    • G08B29/00—Checking or monitoring of signalling or alarm systems; Prevention or correction of operating errors, e.g. preventing unauthorised operation
    • G08B29/18—Prevention or correction of operating errors
    • G08B29/185—Signal analysis techniques for reducing or preventing false alarms or for enhancing the reliability of the system
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J50/00—Circuit arrangements or systems for wireless supply or distribution of electric power
    • H02J50/60—Circuit arrangements or systems for wireless supply or distribution of electric power responsive to the presence of foreign objects, e.g. detection of living beings
    • H—ELECTRICITY
    • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
    • H05B—ELECTRIC HEATING; ELECTRIC LIGHT SOURCES NOT OTHERWISE PROVIDED FOR; CIRCUIT ARRANGEMENTS FOR ELECTRIC LIGHT SOURCES, IN GENERAL
    • H05B47/00—Circuit arrangements for operating light sources in general, i.e. where the type of light source is not relevant
    • H05B47/10—Controlling the light source
    • H05B47/105—Controlling the light source in response to determined parameters
    • H05B47/115—Controlling the light source in response to determined parameters by determining the presence or movement of objects or living beings
    • H05B47/13—Controlling the light source in response to determined parameters by determining the presence or movement of objects or living beings by using passive infrared detectors
    • G—PHYSICS
    • G06—COMPUTING; CALCULATING OR COUNTING
    • G06F—ELECTRIC DIGITAL DATA PROCESSING
    • G06F1/00—Details not covered by groups G06F3/00 - G06F13/00 and G06F21/00
    • G06F1/26—Power supply means, e.g. regulation thereof
    • G06F1/32—Means for saving power
    • G06F1/3203—Power management, i.e. event-based initiation of a power-saving mode
    • G06F1/3206—Monitoring of events, devices or parameters that trigger a change in power modality
    • G06F1/3231—Monitoring the presence, absence or movement of users
    • G—PHYSICS
    • G08—SIGNALLING
    • G08B—SIGNALLING OR CALLING SYSTEMS; ORDER TELEGRAPHS; ALARM SYSTEMS
    • G08B13/00—Burglar, theft or intruder alarms
    • G08B13/18—Actuation by interference with heat, light, or radiation of shorter wavelength; Actuation by intruding sources of heat, light, or radiation of shorter wavelength
    • H—ELECTRICITY
    • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
    • H05B—ELECTRIC HEATING; ELECTRIC LIGHT SOURCES NOT OTHERWISE PROVIDED FOR; CIRCUIT ARRANGEMENTS FOR ELECTRIC LIGHT SOURCES, IN GENERAL
    • H05B47/00—Circuit arrangements for operating light sources in general, i.e. where the type of light source is not relevant
    • H05B47/10—Controlling the light source
    • H05B47/105—Controlling the light source in response to determined parameters
    • H05B47/115—Controlling the light source in response to determined parameters by determining the presence or movement of objects or living beings
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02B—CLIMATE CHANGE MITIGATION TECHNOLOGIES RELATED TO BUILDINGS, e.g. HOUSING, HOUSE APPLIANCES OR RELATED END-USER APPLICATIONS
    • Y02B20/00—Energy efficient lighting technologies, e.g. halogen lamps or gas discharge lamps
    • Y02B20/40—Control techniques providing energy savings, e.g. smart controller or presence detection
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02D—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN INFORMATION AND COMMUNICATION TECHNOLOGIES [ICT], I.E. INFORMATION AND COMMUNICATION TECHNOLOGIES AIMING AT THE REDUCTION OF THEIR OWN ENERGY USE
    • Y02D10/00—Energy efficient computing, e.g. low power processors, power management or thermal management

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  • Physics & Mathematics (AREA)
  • General Physics & Mathematics (AREA)
  • Engineering & Computer Science (AREA)
  • Theoretical Computer Science (AREA)
  • Spectroscopy & Molecular Physics (AREA)
  • General Engineering & Computer Science (AREA)
  • Power Engineering (AREA)
  • Computer Networks & Wireless Communication (AREA)
  • Computer Security & Cryptography (AREA)
  • Computer Hardware Design (AREA)
  • Computing Systems (AREA)
  • Geophysics And Detection Of Objects (AREA)
  • Radar Systems Or Details Thereof (AREA)

Abstract

A method for determining the presence of a person comprisingreceiving an IR sensor signal (50) and using the IR sensor signal (50) to determine a smoothed IR signal (51),in a first mode, determine a threshold (52, 53) as an offset from the smoothed IR signal (51),where presence determination is carried out in the first mode by comparing the IR sensor signal (50) to the threshold (52, 53), andwhere presence determination is carried out in a second mode by comparing the IR sensor signal (50) to the smoothed IR signal (51),where switching from the first mode to the second mode is triggered when the IR sensor signal (50) passes the threshold value (52, 53) andswitching from the second mode to the first mode is triggered when the variability (54) of the IR sensor signal (50) is below a variability threshold (55).

Description

METHOD AND SYSTEl\/I FOR DETERl\/IINING THE PRESENCE OF A PERSON TO WAKE UP ADEVICE Field of the invention This invention relates to systems and methods for detecting the presence of a person using infrared radiation, and in particular to wake up a device.
Background Presence detection is the ability of devices or systems to detect if a person is present. It isknown to use IR sensors for contactless presence detection by detecting body heat. Pres-ence detection may be used, for example, to adjust the operation of a device, for exampleswitching a device on or off. Examples of devices that use contactless presence detectioninclude computers that go to power save mode if a person is not present, heating /air-con-ditioning systems that go to power save mode and illumination devices such as lamps, that are automatically switched on when a person is present.
Thermopiles is an IR sensor that can deliver output as absolute temperature and not only atemperature change. However, when using thermopiles for presence detection it hasturned out that thermopiles are difficult to calibrate with respect to the background IR ra-diation in the room, in particular in ”noisy” environments where the room temperature fluctuates.
US20150185806 describes presence detection with a thermophile using a predeterminedthreshold.
Summary of invention In a first aspect of the invention there is provided a method for determining the presenceof a person comprising receiving an IR sensor signal and using the IR sensor signal to determine a smoothed IR sig-nal, in a first mode, determine a threshold as an offset from the smoothed IR signal, where presence determination is carried out in the first mode by comparing the IR sensorsignal to the threshold, and where presence determination is carried out in a second mode by comparing the IR sensorsignal to the smoothed IR signal, where switching from the first mode to the second mode is triggered when the IR sensorsignal passes the threshold value and switching from the second mode to the first mode is triggered when the variability of the IR sensor signal is below a variability threshold.
The two different modes of detection avoid triggering change of state due to fluctuatingbackground temperature while providing high sensitivity when someone is approaching or moving away from the IR sensor.
It is preferred that the smoothed IR signal is adjusted to the IR sensor signal over timewith a first rate in the first mode and adjusted to the IR sensor signal over time with a sec-ond rate in a second mode were the first rate is lower than the second rate. This increases the sensitivity in the second mode.
Presence determination may be carried out in the first mode by comparing the IR sensorsignal to the threshold, such that a transition from the non-present state to the presentstate occurs when the IR sensor signal passes the threshold and where presence detectionin the second mode occurs by comparing the IR sensor signal to the smoothed IR signalsuch that a transition from the present state to the non-present state occurs when the IR sensor signal passes the smoothed signal.
In the first mode an upper threshold may be determined as the smoothed IR signal plus anoffset value and a lower threshold is determined as the smoothed IR signal minus an off- set value and where, in the first mode, the upper threshold is used to determine transitionfrom the non-present state to the present state, and the lower threshold is used to deter- mine transition from the present state to the non-present state. in the second mode, transition from the non-present state to the present state and transi-tion from the non-present state to the present state may occur when the IR sensor signal passes the smoothed IR signal.
Upon initiation ofthe method the present or the non-present state may be assumed. This results in a self-learning system.
The IR sensors is preferably a thermopile. A thermopile has the advantage that it responds quickly to temperature changes and reports absolute temperature values.
The state (present or non-present) is preferably updated repeatedly, preferably with a predetermined time interval which preferably is at least every second.
In a second aspect of the invention there is provided a system being configured to carry out the method according to the first aspect of the invention.
In a third aspect ofthe invention there is provided a device comprising a system accordingto the second aspect of the invention or being configured to receive information from asystem according to the second aspect of the invention, the device being configured to, when receiving information from the system that no per-son is present to put the device in power save mode, or, to, when receiving information from the system that a person is present, to wake up the device from power save mode.
Drawings The accompanying drawings form a part of the specification and schematically illustratepreferred embodiments of the invention and serve to illustrate the principles of the inven- tion.
Fig. 1 is a schematic drawing of a system.
Fig. 2 is schematic drawing of a memory.
Fig. 3 is a schematic drawing of a system and a person.
Fig. 4 is a schematic drawing of a system comprising a display, and a person.Fig. 5 is a flow chart that shows a method.
Fig. 6 are two graphs.
Fig. 7 is a diagram.
Detailed description Fig. 1 shows one embodiment of a presence determination system 1 comprising an IR sen-sor 2 which preferably is an IR sensor that can measure an absolute temperature value,preferably a thermopile 2. A useful thermopile is shown in WO20040968256 but other typesof thermopiles can be used also. However, any suitable type of IR sensor can be used, forexample a bolometer. The thermopile 2 is able to detect IR radiation and to provide voltage,to subsystem 3. Subsystem 3 may be implemented in hardware or software or com binationsthereof and Fig. 1 shows an embodiment implemented in hardware and software. Subsys-tem 3 comprises input interface 4 that receives voltage from the IR sensor 2. Subsystemfurther comprises processor 5, memory 6 and output interfaceThe system 1 may also include one or more of a signal filter, an amplifier, an A/D converterand similar devices known in the art of signal processing, and in particular for processingthe voltage from the IR sensor 2. System 1 is powered by a power source. Subsystem 3 may be mounted into the same device as IR sensor 2 or may be separate from IR sensor 2. In one embodiment, all steps in the methods described herein are carried out by the same proces- sorWith reference to Fig. 2 memory 6 is able to store data such as IR sensor data 8 and thresh-old data 9, present state data 12, mode data 20 and smoothed IR signal data 21. I\/Iemory 6has threshold determination logic 10 and presence determination logic 13. I\/Iemory 6 also has signal processing moduleThe system 1 is typically able to output information about at least two states: ”person pre-sent” and ”person not present", or alternatively information about transit from the ”pre-sent” to the ”non-present state", and back again. The current state (present/non-present) of system 1 is stored as present state data 12 in memoryThe system 1 is furthermore able two switch between a first mode and a second mode, andback again as described in more detail below. This is also carried out by presence determi-nation logic 13 as is described in more detail below. Hence presence determination logic 13is able to compare smoothed IR signal 51 to thresholds 52, 53, and compare the IR sensorsignal 50 to the smoothed IR signal 51. Presence determination logic 13 is also able to de-termine the variability 54 ofthe IR sensor signal 50 and compare the variability 54 to a var-iability threshold 55. Presence determination logic 13 is able to output the state to outputinterface 7 and to output the mode to signal processing module 11 (signal processing mod- ule 11 uses the state to know how to determine smoothed IR signal 51).
System 1 is able to store information about which mode the system is as mode dataWith reference to Figs. 3-4, the IR sensor 2 has a field of view 15 where IR sensor 2 detectsIR radiation and where the system 1 determines ifa person 16 is present. Person 16 typicallyradiates IR radiation that is stronger than the background IR radiation (which is caused bythe ambient room temperature, for example). Hence the IR radiation detected in the fieldof view 15 can be used to determine if a person 16 is present in the field of view 15 or not.
In some embodiments system 1 may have a plurality ofthermopiles 2 each with a different field of view 15. A plurality of IR sensors 2 may use the same subsystem 3, and subsystem3 then may provide data storage, signal processing and threshold determination for each of the IR sensorsSystem 1 is able to provide information to a second device 17 about if a person is presentor not, with the use of output interface 7. Output interface 7 may be any suitable interfacewith which system 1 is able to provide data to second device 17, and output interface 7 maybe implemented in hardware and/or software. The second device 17 may be a computersystem. Information provided from system 1 to second device 17 can be information aboutstate (present/non-present) or information about state transition from one state to the other.
The second device 17 may be able to use the output from system 1 in various manners. Thepresent and the non-present state may be used to switch on or off a second device 17 asthe case may be. Second device 17 may be any type of device that may benefit from con-tactless presence detection, such as for example a personal computer such as a laptop, aheating system, air condition system or ventilation system, such as to make sure that suchsystems are shut down or put in power save mode when a person is not present during aset time period, or similar. Hence, the present or non-present state may trigger a timer. Thesecond device 17 may also be an appliance providing light such as lamps indoors or out-doors. The second device 17 may also be switched on by the present state, such that a com-puter, an information or advertising display, a lamp, an access control device, or a ventila-tion system is switched on when a person is present. Second device 17 may also be an alarmdevice, such as an intruder alarm. ln general, all of these devices may benefit from beingswitched on become activated, or switched off, being put in power save mode or being woken up from power save mode by the methods and system herein.
For example, the output from system 1 may be used to put a second device 17 in powersave mode and/or shut down a display 18. Thus, when a person is not present the second device 17 may be put in power save mode, possibly after a non-present state has been detected for a certain minimum time. The ”present state” provided by system 1 may be used to wake up the second deviceIt is to be noted that parts of subsystem 3, such as for example the processor 5 and/ormemory 6, may be a part of the second device 17, in particular when second device 17comprises or consists of a computer. Then the software described herein may installed onthe hard drive of the computer and used by the CPU of the computer. In general, parts ofsystem 1 may be integrated with parts of second device 17, such that processor 5 ormemory 6 can be a part of second device 17. The whole system 1 may also be fully inte-grated in second device 17, as a subpart thereof implemented in a suitable combination of hardware and software.
Namely, system 1 may be mounted into a second device 17 or may be separate from seconddevice 17. In one embodiment (shown in Fig. 4) the IR sensor 2 is mounted next to a display18 for a computer, which may be the display 18 of a laptop computer, a tablet computer, asmartphone (such as an iPhone or an Android phone) or a free-standing display 18 for astationary computer. The field of view 15 is preferably directed towards the intended posi- tion of the person 16 in front ofthe displayIR sensor voltage is provided from the IR sensor 2 to the subsystem 3. Subsystem 3 analysesthe sensor voltage and provides present/non-present output to a second device 17. System1 may sample heat data in the field of view 15 and using any suitable sampling interval.Preferably the sampling frequency is from once every 5 seconds to 100 times/second. Sam-pling of IR sensor voltage to obtain IR sensor signal 50 is carried out by signal processingmodule 11 and is stored as IR sensor data 8. Typically, the thermopile delivers its output asan output voltage. Preferably all IR sensor data herein (thresholds, smoothed signals, etc)are - or can be converted to - absolute temperature values, hence not relative temperature values. Hence, preferably a thermopile is used as the IR sensorSignal processing module 11 is able to produce a smoothed IR signal 51. The smoothed IR signal 51 may be a moving average of the IR sensor signal 50. Smoothed IR signal 51 is IR sensor data 8 that has been treated with a (prefera bly digital) smoothing method to removeextreme values in order to show trends, and may be a moving average ofthe IR sensor signal50. Examples of useful smoothing methods include moving average or median values. In some embodiments, outliers in the IR sensor data 8 are removed.
A method for determining that a person is present or absent will now be described withreference to Figs. 5-7. It is understood that the system 1 is arranged to perform such amethod. Below it is mostly described how the system 1 is initially in the non-present stateand in the first mode, but switching from the first mode to the second mode and from thesecond mode to the first mode is done in the same principles. Also switching from the pre-sent state to the non-present state and switching from the present state to the non-present state is done using the same principles as set forth below.
It is to be noted that the method should be implemented to keep the various states and modes updated in real time or close to real time.
In step 100 IR sensor 2 provides voltage to the subsystem 3 to produce IR sensor signal 50,and an example of how the IR sensor signal varies over time is shown in Fig. 6. The signal processing module 11 uses the IR sensor signal 50 to determine a smoothed IR signalThe smoothed IR signal 51 represents historic IR sensor data 8 that has been treated with a(preferably digital) smoothing method to remove extreme values in order to show trends.Examples of useful smoothing methods include moving average or median values. In some embodiments, outliers in the data are removed.
Signal processing module 11 may for example use a moving average ofthe IR sensors signal50 to produce the smoothed IR signal 51. The smoothed IR signal 51 may closely follow theIR sensor signal 50 for example by using a moving average across a short time period (sec-ond mode) or may be follow the IR sensor signal 50 more slowly, by using a moving averageacross a longer time (first mode).Hence, it is preferred that, in the first mode the smoothed IR signal 51 is adjusted slowly and in the second mode the smoothed IR signal 51 is adjusted to the IR sensor signal 50 faster. For example, when a moving average is used, a movingaverage for a longer time period is determined in the first mode than in the second mode.The time period may be longer with a factor of from 3- 100, more preferably from 3 to 20in the first mode compared to the second mode. For example, a moving average may bedetermined for a period of from 1 to 20 seconds in the first mode, but from 0.5 to 0.1 sec- onds, for example 0.25 seconds in the second mode.
A moving average is a useful way to determine the smoothed IR signal, however there are other useful statistical methods that can be used.
In step 101 first and second thresholds 52, 53 are determined by threshold determinationlogic 10. Threshold determination logic 10 uses the smoothed signal 51 to determine thefirst and second thresholds 52, 53. The threshold values 52, 53 are determined as the smoothed IR signal 51 +/- an offset.
First Threshold = Smoothed IR signa|+ offset.
Second threshold = Smoothed IR signal- offset.
The offset may be predetermined. The offset may be determined for the particular applica-tion and the IR sensor 2 that is used. The offset may or may not be the same for upperthreshold 52 and lower threshold 53, but in a preferred embodiment the same offset is usedfor upper threshold 52 and lower threshold 53. The offset is chosen dependant on config-uration ofthe IR sensor and choice of amplifier and A/D converter, and may be determinedusing the particular configuration of system 1. As a rule of thumb, the offset may be around10%-70% of the difference between a typical difference of the measured signal strength in the present state and in the non-present state.
IR sensor signal 50 and the smoothed IR signal 51 may fluctuate with ventilation, sunshine,opening a window, etc in the non-present state. The offset should be selected to avoid er-roneous triggering from the first mode to the second mode and false triggering of pres- ence/non -presence. The determined thresholds 52, 53 are continuously updated (see below) and stored as threshold data 9 in memory 6. Steps 100 and 101 may be carried outrepeatedly, thereby dynamically changing the smoothed IR signal 51 and the thresholds 52,In the first mode, it is repeatedly checked, in step 102, if the IR sensor signal 50 passes aboveor below the first or second thresholds 52, 53. This can be done by using a IR sensor valuethat is determined after the determination of the threshold, preferably immediately after determining the threshold.
Ifthe IR sensor signal 50 is higher than the upper threshold 52 it is determined that a personis present. It may be enough with one single measurement that is higher than the threshold,but it may also be required that the threshold is passed for a minimum duration, such ascontinuously or repeatedly during. Determination of presence is preferably done with the same frequency as updating of the thresholds 52,Ifthe system is in the non-present state and the IR sensor signal 50 passes above the upperthreshold 52, two things happen: 1) the mode changes from the first mode to the secondmode and 2) the state changes from non-present to present. If the lower threshold 53 ispassed nothing happens because the system 1 is already in the non-present state. However,the mode will change to the second mode. The change in state is stored as present statedata 12. The change in state may be output using output interface 7. The change in modeis stored as mode data 20 in memory 6. The change in mode does not need to be output form systemIn a similar fashion, when the system 1 is in the present state, it will change to the non- present state ifthe lower threshold 53 is passed, and will also change to the second mode.
Fig. 6 shows how an IR sensor signal 50 changes over time where initially there is no person16 in the field of view 15, and the system 1 is initially in the non-present state, first mode,and a person 16 then enters the field of view 15, stays in the field of view 15 and then leaves the field of view 15. Looking at Fig. 6, the system 1 is initially in the non-present state, firstmode. In this particular example there is no person 16 in the field of view 15 and the system1 is detecting background temperature only. However, a person 16 enters the field of view15 at 29 seconds which makes the IR radiation in the field of view 15 to increase sharply. At29 seconds, the IR sensor signal 50 therefore passes the upper threshold 52. This triggerchange in state from non-present to present and trigger change in mode from the first modeto the second mode. Hence from time point 29 seconds to time point 38 the smoothed IRsignal 51 is adjusted faster to the IR sensor signal 50, as can be seen in how fast the smoothed IR signal 51 rises after time point 29 seconds.
In the second mode, change from the non-present state to the present state (and back) isnot done using thresholds 52, 53 but instead by comparing the IR sensor signal 50 to thesmoothed IR signal 51. Hence, when the presence determination logic 13 detects a sensorsignal value 50 that is lower than the smoothed IR signal 51, the system changes state frompresent to non-present (if not already in the non-present state) and if the presence deter-mination logic 13 detects a value from IR sensor signal 50 that is higher than the smoothedIR signal, the system changes from the non-present state to the present state (if not already in the present state).
I\/Ioreover, in the second mode, the variability ofthe IR sensor signal 50 is determined. Thelower graph in Fig. 6 shows how the variability 54 of the IR sensor signal 50 varies over timefor the IR sensor signal 50 in the upper graph. The lower graph of Fig. 6 also shows thevariability threshold 55. The variability threshold 55, is selected based on the configurationof the system 1. The variability threshold 55 is preferably predetermined. The variabilitythreshold 55 may be, for example, from 5% to 40% ofthe typical difference of the measuredsignal strength in the present state and in the non-present state. The variability threshold 55 may be stored in the presence determination logicHence, in step 103 the variability of the IR data 50 is determined. This determination is per-formed by the signal processing module 11. Any useful dispersion parameter or variabilitymeasure can be used and applied to IR signal 50. For example, the standard deviation, the absolute variation, the mean absolute deviation or the variance of the IR sensor signalmay be used. The variability 54 may be determined as the difference between the highestvalue and the lowest value for the first time period. In one embodiment, the variabilityof the smoothed signal 51 is determined instead of variability 54 of the IR sensor signalThe variability 54 is determined for the IR sensor signal 50 over a predetermined time periodwhich may be from 3 to 15 cycles. Thus, in one embodiment the standard deviation of IRsensor signal 50 is used to determine variability 54 of the IR sensor signal 50. In certainembodiment the variability of the smoothed IR signal 51 is determined and used as the va riabilityHigher background noise of the IR signal 50 will result in higher variability 54 and hence higher standard deviation across the measured time interval.
When the variability 54 is high, the system 1 stays in the second mode. But when the varia-bility falls below the variabilitythreshold 55, the mode of system 1 changes from the secondmode to the first mode in step 104. This happens at 38 seconds in Fig. 6, where it can beseen how the IR sensor signal 50 stabilizes from about 31 seconds, triggering the first modeat time 38. There may be a delay, such that the variability 54 needs to be below the varia-bility threshold 55 for a certain time, for example from 3 to 15 cycles in order to trigger transition to the first mode.
In Fig. 6, from time point 29 to time point 38 the system is in the present state, secondmode, and the state could revert back to the non-present state if (step 105) the IR sensorsignal 50 would become lower than the smoothed IR signal 51 (i.e. be below smoothed IRsignal 51 at some time point). However, that does not occur in the example of Fig. 6. Instead,in Fig. 6, from time point 38 the system 1 which is in the present state, enters the first modebecause the IR sensor signal 50 has stabilized sufficiently for the variability 54 to fall below the threshold 55 (step 104).
At time point 54, the IR sensor signal 50 passes below the lower threshold 53 because the person 16 is leaving the field of view 15. This triggers the non-present state and the secondmode (step 106). Hence, the smoothed IR signal 51 now begins to adjust to the IR sensor signal 50 fast. At the same time, the variability 54 of the IR sensor signal 50 is being detected and when the variability 54 falls below the variability threshold 55, the system reverts to the first mode at time pointHence the system may have four different combinations of states and modes as seen in Table 1.State/mode Time pe-riod inFig. 6(sec-onds)1 Non present in First mode. Thresh- Presence is triggered by upper thresh- 1-29olds 52, 53 are determined. old 52, second mode is triggered by up-Smoothed IR signal 51 is adjusted per and lower threshold 52, 53.slowly.2 Present in Second mode. Smoothed Non-presence is triggered by IR signal 29-38IR signal 51 is adjusted fast. Variabil- 50 crossing smoothed IR signal 51ity 54 of signal 50 is determined. (downwards). First mode is triggered bylow variability 54 of signal 50.3 Present in First mode. Thresholds Non-presence is triggered by lower 38-5452, 53 are determined. Smoothed IR threshold 53, second mode is triggeredsignal 51 is adjusted slowly. by upper and lower threshold 52, 53.4 Non-present in Second mode. Presence is triggered by IR signal cross- 54-Smoothed IR signal 51 is adjustedfast. Variability 54 of signal 50 is de-termined. ing smoothed IR signal (upwards). Firstmode is triggered by low variability 54of signalTableFig. 7 shows the four possible combinations of modes and states and the possible transitions between them. The present state and mode are stored as present state data 12 and mode data 20 in the memory 6 of system 1. Presence determination logic 13 determines the state (present, non-present) and the mode (first, second).The system 1 repeatedly samples the IR sensor signal 50 and determines the state and themode. Voltage is provided from sensor 2 to subsystem 3, IR sensor signal 50 and smoothedIR signal 51 are determined by signal processing module 11, which provides these to pres-ence determination logic 13. Signal processing module 11 also provides the smoothed IR signal 51 to threshold determination logicThe IR sensor signal 50 is used by presence determination logic 13, which has access tothresholds 52,53 in the form of threshold data 9 in memory 6, and access to the smoothed IR signal 52, to determine presence.
The state (present or non-present) is updated repeatedly, with a predetermined frequency.Hence, the smoothed IR signal 51, the thresholds 52, 53 and the variability 54, and the stateand mode is preferably updated repeatedly, with a predetermined frequency. In each cycle,the smoothed IR signal 51, the thresholds 53, 53 and the variability and the mode and statemay be determined. The frequency may be at least once every 5 seconds, more preferablyat least every 1 seconds more preferably at least 2 times per second, and most preferablyat least ten times per second. Update may occur faster in the second mode than in the first mode.
The thresholds 52, 53 may be used by presence determination logic 13 in the cycle imme-diately following the cycle where they are determined by threshold determination logic 10.In a similarfashion, the value ofthe smoothed IR signal 51 may be compared to the IR sensor signal 50 determined in the cycle immediately following.
Parameters not needed in a certain mode or state may be left non-updated in that mode orstate. In the first state, the variability 54 of the IR sensor signal 50 does not need to bedetermined. Thresholds 52, 53 does not need to be determined in the second state. In thefirst state it does not need to be checked weather the IR signal 50 crosses the smoothed IRsignal 51. In the second mode it does not need to be checked whether the IR signalcrosses the thresholds 52, 53 (which are not needed to begin with).
When the system 1 is starting up, system 1 may in one embodiment, be predetermined tobe in one state selected from the present and the non-present state. The system 1 may then change state depending on ifthe upper threshold 52 or the lower threshold 53 is crossed. ln the following it is described that it is assumed that the system is in present state at start-up. lf the assumption is correct the lower threshold 53 will be crossed by IR sensor signal50 when the person 16 leaves the field of view 15 and the system 1 will go to the non- pFeSeHt State. lf, however, a person was not present in the field of view 15 at start-up, the initial assump-tion, that a person 16 is present, is erroneous. However, the system will self-calibrate asfollows. lf a person 16 subsequently comes int o the field of view 15, the upper threshold52 will be passed, in a situation similar to the one at 29 seconds in Fig. 6. The system willthen remain in the present state. The IR signal 50 will eventually stabilize at a new higherlevel (similar to the situation at 38 seconds in Fig. 6) and new thresholds 52, 53 will be de-termined at this higher level. The system is now correctly calibrated, because it will go tothe non-present state when the lower threshold 53 is passed (as at 54 seconds in Fig. 6).
Hence the system is self-calibrating.
While the invention has been described with reference to specific exemplary embodiments,the description is in general only intended to illustrate the inventive concept and should not be taken as limiting the scope ofthe invention. The scope is generally defined by the claims.

Claims (9)

CLAIMS:
1. A method for determining the presence of a person (16) comprising receiving an IR sensor signal (50) and using the IR sensor signal (50) to determine asmoothed IR signal (51), in a first mode, determine a threshold (52, 53) as an offset from the smoothed IRsignal (51), where presence determination is carried out in the first mode by comparing the IRsensor signal (50) to the threshold (52, 53), where presence determination is carried out in a second mode by comparing theIR sensor signal (50) to the smoothed IR signal (51), where switching from the first mode to the second mode is triggered when the IRsensor signal (50) passes the threshold value (52, 53) and switching from the second mode to the first mode is triggered when the variability (54) of the IR sensor signal (50) is below a variability threshold (55).
2. The method of claim 1 where the smoothed IR signal (51) is adjusted to the IR sen-sor signal (50) over time with a first rate in the first mode and adjusted to the IRsensor signal (50) over time with a second rate in a second mode where the first rate is lower than the second rate.
3. The method of claim 1 or 2 where presence determination is carried out in the firstmode by comparing the IR sensor signal (50) to the threshold (52, 53), such that atransition from the non-present state to the present state occurs when the IR sen-sor signal (50) passes the threshold (52, 53) and where presence detection in thesecond mode occurs by comparing the IR sensor signal (50) to the smoothed IR sig-nal (51) such that a transition from the present state to the non-present state oc- curs when the IR sensor signal (50) passes the smoothed signal (51).4. The method of any one of claims 1 to 3 where, in the first mode, an upper thresh-old (52) is determined as the smoothed IR signal (51) plus an offset value and alower threshold (53) is determined as the smoothed IR signal (51) minus an offsetvalue and where, in the first mode, the upper threshold (52) is used to determinetransition from the non-present state to the present state, and the lower threshold(53) is used to determine transition from the present state to the non-present
4. State.
5. The method of any one of claims 1 to 4 where, in the second mode, transition fromthe non-present state to the present state and transition from the non-presentstate to the present state occurs when the IR sensor signal (50) passes the smoothed IR signal (51).
6. The method of any one of claims 4 to 5 where upon initiation of the method, thesystem is predetermined to be is either in the present state or the non-present State.
7. The method of any one of claim 1 to 6 where the IR sensor (2) is a thermopile.
8. The method of any one of claims 1 to 7 where the state is updated at least every second.
9. A system (1) comprising an IR sensor (2) where the system (1) is configured to de-termine if a person (16) is present in a field of view (15) of the IR sensor (2), thesystem (1) further comprising a signal processing module (11), a threshold deter-mination logic (10) and presence determination logic (13), the signal processingmodule (11) being configured to receive voltage from the IR sensor (2) and to de-termine an IR signal (50) and a smoothed IR signal (51) and further configured todetermine the variability (54) of the IR sensor signal (50), where the threshold de- termination logic (10) is configured to determine a threshold (52, 53) as an offsetfrom the smoothed IR signal (51), and where the presence determination logic (13)is configured to determine presence in a first mode by comparing the IR sensor signal (50) to the threshold (52, 53), and configured to determine the presence in a second mode by comparing the IRsensor signal (50) to the smoothed IR signal (51), and where switching from thefirst mode to the second mode is triggered when the IR sensor signal (50) passesthe threshold value (52, 53) and where switching from the second mode to the first mode is triggered when the variability (54) of the IR sensor signal (50) is below a variability threshold (55). A device comprising a system (1) according to claim 9 or configured to receive in-formation from a system (1) according to claim 9, the device being configured to, when receiving information from the system (1)that no person (16) is present to put the device in power save mode, or, to, whenreceiving information from the system (1) that a person (16) is present, to wake up the device from power save mode.
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