Abstract:
A sensor of this comprises a receiving section that receives a signal that is sent from outside, a circuit whose impedance changes irreversibly in accordance with an environmental change, a measurement section that measures the impedance of the circuit in an event that a signal is received by the receiving section, and a sending section that sends data representing a measurement result of the measurement section.

Description:
BACKGROUND 
       [0001]    1. Technical Field 
         [0002]    The present invention relates to sensors. 
         [0003]    2. Related Art 
         [0004]    Sensors are known that acquire, from a distance, the temperature history of an environment in which goods are placed while those goods are being transported or stored. Such sensors are used to ascertain whether, for example, frozen food or the like has been kept in its frozen state until its arrival at a retail store or the consumer. 
         [0005]    A technology has been proposed, in which this type of sensor is combined with an IC tag, and data representing the temperature history measured with the sensor is stored in the IC. However, with this system, the data representing the temperature history is stored as electronic data, so that there is the risk that the data is tampered with. 
       SUMMARY 
       [0006]    According to an aspect of the invention, there is provided a sensor comprising a receiving section that receives a signal that is sent from outside, a circuit whose impedance changes irreversibly in accordance with an environmental change, a measurement section that measures the impedance of the circuit in an event that a signal is received by the receiving section, and a sending section that sends data representing a measurement result of the measurement section. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]    Exemplary embodiment(s) of the present invention will be described in detail based on the following figures, wherein: 
           [0008]      FIGS. 1A and 1B  show diagrams illustrating the configuration of a sensor  100 ; 
           [0009]      FIG. 2  is a diagram illustrating the configuration of a querying device  200 ; 
           [0010]      FIG. 3  is a diagram showing an example of a temperature judging table  2051 ; 
           [0011]      FIG. 4  is a diagram showing an example of a history table  2052 ; 
           [0012]      FIG. 5  is a diagram illustrating the operation flow of the sensor  100  and the querying device  200 ; 
           [0013]      FIGS. 6A ,  6 B,  6 C,  6 D and  6 E show diagrams illustrating modified examples; 
           [0014]      FIGS. 7A and 7B  show diagrams illustrating a sensor  300 ; and 
           [0015]      FIGS. 8A and 8B  show diagrams illustrating a sensor  400 . 
       
    
    
     DETAILED DESCRIPTION 
       [0016]    The following is an explanation of exemplary embodiments of the present invention, with reference to the accompanying drawings. 
       Configuration 
       [0017]      FIG. 1A  is a diagram showing the configuration of a sensor  100 . This sensor  100  is made of a substrate  1 , an IC (Integrated Circuit) tag  3  disposed on the substrate  1 , a capacitor  4  and an absorbing section  5 . 
         [0018]    Both electrodes of the capacitor  4  are connected to the IC tag  3 . The capacitor  4  is made of plate-shaped electrodes  41  and  42  that are arranged facing each other, and a lump of wax  43  that is interposed between the electrodes  41  and  42 . The lump of wax  43  acts as a dielectric. 
         [0019]    The absorbing section  5  is made of a material that absorbs liquid wax (such as paper), and is disposed adjacent to a location where the lump of wax  43  is not in contact with the electrodes  41  and  42 . When the temperature of the lump of wax  43  reaches the melting point of the wax, the lump of wax  43  melts and becomes liquid. The melted wax does not maintain the shape it had before melting, so that the wax flows out from between the electrodes  41  and  42 , as shown in  FIG. 1B . The absorbing section  5  absorbs the liquid wax that has flown out. Accordingly, the space between the electrodes  41  and  42  that was taken up by the lump of wax  43  is filled with air. The dielectric constant of air is smaller than the dielectric constant of wax, so that the static capacitance of the capacitor  4  decreases, and as a result, the impedance of the capacitor  4  increases. 
         [0020]    This increase of impedance is an irreversible change. This is because if the temperature drops below the melting point of the wax after the melted wax has been absorbed by the absorbing section  5 , the wax solidifies in a state in which it is held inside the absorbing section  5  and will not return to its original position. Thus, in the present application, “irreversible change” does not mean that the change of the state is under no circumstances irreversible, but rather that a change that has occurred due to an environmental change will not return to the original state or shape regardless of a shift in this environmental change, or will not return to the original state or shape unless an external force other than that due to the environmental change is applied. 
         [0021]    Furthermore, there is a certain range within which the amount of the impedance increases. That is to say, the longer the time for which the temperature has reached or exceeded the melting point of the wax, the greater the amount of wax that flows out. And the greater the amount of wax that flows out, the greater the proportion of space that is taken up by air between the electrodes  41  and  42 , and thus the greater the impedance. If the entire wax flows out, the impedance becomes greatest. Consequently, if the relation between the impedance of the capacitor  4  and the temperature is experimentally determined beforehand, then it is possible to judge, by measuring the impedance of the capacitor  4 , whether the lump of wax  43  has melted. 
         [0022]    The following is a description of the configuration of the IC tag  3 . 
         [0023]    The antenna  50  is made of a coil  51  and a capacitor  52  that is connected in parallel to the coil  51 . That is to say, the antenna  50  is configured as a resonance circuit. The capacitor  52  is made of a pair of electrodes and a dielectric interposed between the electrodes. The dielectric is for example a perovskite compound, and has the property that its dielectric constant changes depending on temperature. When the dielectric constant changes, the capacitance of the capacitor  52  changes. Since the antenna  50  is configured as a resonance circuit, when the capacitance of the capacitor  52  changes, also the resonance frequency changes. That is to say, the resonance circuit has the property that its resonance frequency changes depending on temperature. Consequently, if the relation between the resonance frequency of the resonance circuit and the temperature is experimentally determined beforehand, then it is possible to determine the temperature by measuring the resonance frequency of the antenna  50 . 
         [0024]    A power supply section  71  retrieves electric power that is induced into the coil  51  when the antenna  50  receives electromagnetic waves, and supplies this electric power to the various sections of the IC tag  3 . 
         [0025]    A clock generation section  72  generates a clock signal based on a carrier wave included in the received electromagnetic waves and supplies it to a control section  75 . 
         [0026]    A demodulation section  73  retrieves the data by demodulating the signal received with the antenna  50 . 
         [0027]    A modulation section  74  modulates signals based on the data representing the impedance of the capacitor  4 . 
         [0028]    The control section  75  receives the data from the demodulation section  73  and if the data represents a command, it performs processing according to that command. Examples of commands are a write command instructing that the received data is written into a storage section  76 , and a read command instructing that the data written into the storage section  76  is read out and sent. 
         [0029]    The storage section  76  is a non-volatile memory, from which data is not deleted even when there is no power supplied from the power supply section  71 . The storage section  76  stores an ID (identification code or identifier) for unambiguously identifying the IC tag  3 . Moreover, if data representing a judgment result by a melting judgment section  206  or a temperature judgment section  207  of a later-explained querying device  200  is sent from the querying device  200 , then the storage section  76  stores that data. 
         [0030]    Both electrodes of the capacitor  4  are connected to an impedance measurement section  77 . The impedance measurement section  77  measures the impedance of the capacitor  4  using electric power supplied from the power supply section  71 . 
         [0031]      FIG. 2  is a diagram showing the configuration of the querying device  200 . 
         [0032]    A control section  201  reads out a program for controlling the various sections of the querying device  200  from a storage section  205 . 
         [0033]    An antenna  202  sends and receives radio signals to/from the sensor  100 . 
         [0034]    A modulation section  203  generates signals for judging whether the wax has melted as well as for measuring the temperature and supplies these signals to the antenna  202 . 
         [0035]    A demodulation section  204  demodulates radio signals received with the antenna  202  and retrieves data. The retrieved data is data representing the impedance of the capacitor  4 . 
         [0036]    The storage section  205  stores a threshold value of the impedance of the capacitor  4 . As noted above, in the event that the lump of wax  43  has melted, the impedance of the capacitor  4  increases and takes on a value within a certain range. This range is determined for example experimentally and the lower limit of the determined range is taken as the threshold value. 
         [0037]    The storage section  205  also stores a temperature judgment table  2051 .  FIG. 3  is a diagram showing an example of the temperature judgment table  2051 . The temperature judgment table  2051  lists the relation between the resonance frequency of the antenna  50  and the temperature. 
         [0038]    The storage section  205  also stores a history table  2052 .  FIG. 4  is a diagram showing an example of the history table  2052 . In the history table  2052 , the date and time at which a temperature judgment and a melting judgment were carried out and the results of these judgments are stored in association with each other. 
         [0039]    If the impedance of the capacitor  4  has reached the threshold, the melting judgment section  206  judges that the lump of wax  43  has melted. 
         [0040]    The temperature judgment section  207  judges the temperature based on the strength of the signal received from the sensor  100  and the content of the temperature judgment table  2051 . 
         [0041]    The display section  208  is for example a liquid display panel and displays an image representing the judgment results of the melting judgment section  206  and the temperature judgment section  207 . 
         [0042]    A clock  209  keeps track of the time and date. 
         [0043]    An instruction receiving section  210  is for example a push button-type switch and sends a predetermined signal to the control section  201  in the event that the user has pushed this switch. When the control section  201  has received this signal, it lets the various sections of the querying device  200  perform the above-described processing. 
         [0044]    The following is a description of the operation of the sensor  100  and the querying device  200 . 
         [0045]      FIG. 5  is a diagram illustrating the operation flow of the sensor  100  and the querying device  200 . 
         [0046]    First, when at Step A 01  the instruction receiving section  210  is pushed down, the antenna  202  sends a signal for temperature measurement to the sensor  100 . More specifically, the modulation section  203  modulates a signal by gradually increasing its frequency from the lower limit to the upper limit within the range stored in the temperature judgment table  2051  and sends this signal via the antenna  202  to the sensor  100 . 
         [0047]    In Step B 01 , the sensor  100  receives this signal with the antenna  50  and sends a response signal in response to this signal via the antenna  50  to the querying device  200 . 
         [0048]    At Step A 02 , the temperature judgment section  207  of the querying device  200  determines the resonance frequency of the antenna  50  by measuring the strength of the received response signal. More specifically, the strength of the response signal drops sharply from a constant value prior to the resonance frequency of the antenna  50  and becomes lowest at the resonance frequency. Then, it increases sharply when the resonance frequency is exceeded and returns to the constant value. Consequently, by determining the frequency where the strength of the response signal has the lowest value, it is possible to determine the resonance frequency of the antenna  50 . Moreover, the temperature judgment section  207  determines the temperature corresponding to the determined resonance frequency from the temperature judgment table  2051 . 
         [0049]    At Step B 02 , the impedance measurement section  77  of the sensor  100  measures the impedance of the capacitor  4 . Then, the modulation section  74  supplies to the antenna  50  a signal representing the measurement result as well as the ID of the IC tag  3 , and the antenna  50  sends this signal to the querying device  200 . 
         [0050]    At Step A 03 , the melting judgment section  206  judges from this signal whether the lump of wax  43  has melted. More specifically, the antenna  202  receives the signal from the sensor  100 , the demodulation section  204  demodulates this signal, and the data representing the impedance of the capacitor  4  is retrieved. If the impedance represented by this data has reached the threshold, the melting judgment section  206  judges that the lump of wax  43  has melted. As explained above, in the event that the lump of wax  43  has melted, the impedance change of the capacitor  4  is irreversible. Since the melting judgment section  206  performs the melting judgment based on this impedance change, there is no risk that the data used for the melting judgment is tampered with. 
         [0051]    At Step A 04 , the display section  208  displays an image representing the temperature determined with the temperature judgment section  207  and the judgment result of the melting judgment section  206 . Moreover, this data is associated with the ID of the IC tag  3  and written into the history table  2052 . 
         [0052]    At Step A 05 , the temperature determined with the temperature judgment section  207 , the judgment result of the melting judgment section  206 , the current time obtained with the clock  209 , and data representing a write command are supplied to the modulation section  203 . The modulation section  203  modulates a signal based on this data, and sends the signal via the antenna  202  to the sensor  100 . 
         [0053]    At Step B 03 , the antenna  50  receives the signal sent from the querying device  200 , the demodulation section  73  demodulates the signal, and the obtained data is supplied to the control section  75 . If the data contains a write command, the control section  75  stores the temperature and the result of the melting judgment in the storage section  76 , similar to the history table  2052  of the querying device  200 . 
       MODIFIED EXAMPLES 
       [0054]    The foregoing description of the exemplary embodiments of the present invention has been provided for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Obviously, many modifications and variations will be apparent to practitioners skilled in the art. The exemplary embodiments were chosen and described in order to best explain the principles of the invention and its practical applications, thereby enabling others skilled in the art to understand the invention for various embodiments and with the various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the following claims and their equivalents. 
         [0055]    For example, embodiments in which the above-described exemplary embodiment is modified as explained below are also possible. 
       Modified Example 1 
       [0056]      FIG. 6A  is a diagram showing a modified example. In this modified example, a lump of salt  44  is used as the dielectric of the capacitor  4 . The lump of salt  44  can be, for example, calcium chloride. The lump of salt  44  is arranged between the electrodes  41  and  42  and the exposed surface of the lump of salt  44  is covered by a moisture-permeable film, through which for example water molecules in the air can pass through. Thus, in the event that the humidity around the sensor  100  reaches a predetermined humidity, the lump of salt  44  deliquesces and flows out from between the electrodes  41  and  42 . Accordingly, the impedance of the capacitor  4  changes as in the above-described exemplary embodiment, and does not return to its original value. Thus, it can be judged from the impedance of the capacitor  4  whether the humidity around the sensor  100  has reached a predetermined value. 
         [0057]    As shown in  FIG. 6B , it is also possible to place an insulator  47  between the lump of salt  44  and the electrodes  41  and  42 . In this case, the lump of salt  44  insulated by the insulator  47  functions in its entirety as the dielectric of the capacitor  4 . When the lump of salt  44  is insulated from the electrodes  41  and  42 , impedance of the capacitor  4  always takes a positive value even though the lump of salt  44  deliquesces and becomes a conductor at measurement timings, and it is possible to judge whether there were any critical changes of impedance of the capacitor  4  between the measurement timings on the basis of the measured values. 
       Modified Example 2 
       [0058]      FIG. 6C  is a diagram showing a modified example. In this modified example, a photo-curing resin  45 , which cures in the event that it is exposed to light of a predetermined wavelength, for example ultraviolet light or the like, is used as the dielectric of the capacitor  4 . This photo-curing resin  45  is arranged between the electrodes  41  and  42  and the exposed surface of the photo-curing resin  45  is covered by a transparent resin, for example. Thus, in the event that the sensor  100  is exposed to light, the photo-curing resin  45  is cured. Accordingly, the impedance of the capacitor  4  changes as in the above-described exemplary embodiment, and does not return to its original value. Thus, it can be judged from the impedance of the capacitor  4  whether the sensor  100  has been exposed to light of a predetermined wavelength. 
       Modified Example 3 
       [0059]      FIG. 6D  is a diagram showing a modified example. In this modified example, a reducing agent  46  such as metallic sodium is used as the dielectric of the capacitor  4 . Thus, in the event that the sensor  100  is placed in an oxygen atmosphere, the reducing agent  46  is oxidized. Accordingly, the impedance of the capacitor  4  changes as in the above-described exemplary embodiment, and does not return to its original value. Thus, it can be judged from the impedance of the capacitor  4  whether the sensor  100  has been placed in an oxygen atmosphere. 
         [0060]    As shown in  FIG. 6E , it is also possible to place an insulator  47  between the reducing agent  46  and the electrodes  41  and  42 . In this case, the reducing agent  46  insulated by the insulator  47  functions in its entirety as the dielectric of the capacitor  4 . When the reducing agent  46  is insulated from the electrodes  41  and  42 , impedance of the capacitor  4  always takes a positive value even though the reducing agent  46  is oxidized and becomes a conductor at measurement timings, and it is possible to judge whether there were any critical changes of impedance of the capacitor  4  between the measurement timings on the basis of the measured values. 
       Modified Example 4 
       [0061]      FIG. 7A  is a plan view showing a sensor  300 . In this modified example, a coil  6  is provided instead of the capacitor  4 , and both electrodes of the coil  6  are connected to an IC tag  3 . A holding section  61  is disposed next to the coil  6 . The holding section  61  is made of a frame  611  and latches  612 . The frame  611  is shaped like the Japanese character           (that is, like a rectangle missing its left side) when viewed in the top view and the latches  612  are provided at both ends of the character          . A ferromagnetic member  62 , for example made of iron, is formed to a size that is slightly smaller than the space inside the holding portion  61  viewed from above. The distance between the tips of the two latches  612  is smaller than the vertical width of the ferromagnetic member  62  in the drawing. Therefore, in a state in which no external force such as inertia acts on the ferromagnetic member  62 , the ferromagnetic member  62  is held inside the holding section  61  and will not escape out of the holding section  61 . 
         [0062]      FIG. 7B  is a drawing illustrating the movement of the ferromagnetic member  62 . In the event that an external force, such as inertia, acts on the ferromagnetic member  62  in the direction to the left in the figure and the size of this external force exceeds a predetermined value, the latches  612  are pushed apart by the ferromagnetic member  62 , the ferromagnetic member  62  is ejected out of the holding section  61  and enters the space inside the coil  6 . At a position on the other side of the coil  6  with respect to the holding section  61 , a stopper  63  stopping the movement of the ferromagnetic member  62  is provided. When the ferromagnetic member  62  is ejected completely out of the holding section, the latches  612  return to their original shape due to their elasticity. Accordingly, the distance between the tips of the two latches  612  is smaller than the vertical width of the ferromagnetic member  62 , so that even when an external force to the right in the figure acts on the ferromagnetic member  62 , the ferromagnetic member  62  will not enter the holding section  61 . That is to say, in the event that the ferromagnetic member  62  is completely ejected from the holding section  61 , the movement of the ferromagnetic member  62  is irreversible. 
         [0063]    Accordingly, in the event that the ferromagnetic member  62  is ejected from the holding section  61  and enters into the coil  6 , the impedance of the coil  6  changes and does not return to its original value. Thus, it can be judged from the impedance of the coil  6  whether an external force of a predetermined size has acted on the sensor  100 . 
       Modified Example 5 
       [0064]      FIG. 8A  is a plan view showing a sensor  400 . In this modified example, a switch  8  is provided instead of the capacitor  4 , and two contacts  81  and  82  of the switch  8  are connected to the IC tag  3 . A support section  84  is arranged next to a movable section  83  of the switch  8 . The supporting section  84  is for example a plate-shaped member. The movable section  83  and the supporting section  84  are joined together for example by a lump of wax  85  in a state in which the switch is open. In this state, the movable section  83  is elastically deformed, and the lump of wax  85  is held in a state in which the switch  8  is opened, countering the elastic force of the movable section  83 . The IC tag  3  has the function of testing conductivity of the circuit and can judge whether the switch  8  is open or not. 
         [0065]      FIG. 8B  is a diagram illustrating the open and closed states of the switch  8 . In the event that the temperature around the sensor  100  reaches the melting point of the wax, the lump of wax  85  melts, the movable section  83  is returned to its original shape due to its elasticity, and the switch  8  is closed. When the lump of wax  85  melts, the switch  8  does not return to its open state. Thus, by testing whether a current flows through the circuit, it can be judged whether the temperature around the sensor  100  has reached the melting point of the wax. 
         [0066]    Moreover, if a lump of salt having deliquescence is used instead of the lump of wax  85 , then by testing whether a current flows through the circuit, it can be judged whether the humidity around the sensor  100  has reached a predetermined value.