Abstract:
The invention is intended to provide a technique regarding sensor nodes for impact detection to enable the intensities of impacts to be determined in a multi-value or analog mode and to reduce the power consumption of sensor nodes. The sensor node is provided with a shock detection sensor comprising a piezoelectric element unit which generates an electric charge corresponding to an external impact, a capacitor which rectifies and accumulates the electric charge so generated, and a voltage detector which operates on the accumulated power and externally outputs a signal when the accumulated voltage reaches a preset level; a stand-by control object section which is caused by the external signal to return from a stand-by state and to operate; and a power supply which feeds power to the stand-by control object section, wherein the operation of the stand-by control object section is triggered by the signal of impact detected by the piezoelectric element unit.

Description:
CLAIM OF PRIORITY 
   The present invention claims priority from Japanese application JP 2005-085441 filed on Mar. 24, 2005, the content of which is hereby incorporated by reference into this application. 
   BACKGROUND OF THE INVENTION 
   The present invention relates to a sensor node technique and more particularly to a sensor node for detecting impacts. 
   A sensor node senses information unevenly distributed in its environment at designated intervals of time, and transmits the sensed information values by wireless communication. On account of its intrinsic function to sense and transmit by wireless communication information unevenly distributed in its environment, a sensor node has to be able to operate on a battery for a long period. 
   Conventional devices for detecting impacts arising at irregular intervals of time for use in sensor nodes detect such impacts with a mechanical switch or a switch using electric power generated by electromagnetic inductance or a piezoelectric element, or through the measurement of variations in acceleration with an acceleration sensor which is kept operating all the time. 
   A conventional impact detecting sensor using a piezoelectric element utilizes the trend of the vibration, which accompanies the operation of the device, to increase when an abnormality arises, and detects a signal which is generated when the. vibration has surpassed a certain amplitude (see, for instance, Japanese Patent Application Laid-Open Nos. 8-145783 and 9-264778). 
   In another conventional sensor, when the acceleration working on the piezoelectric element is at or above a certain level, a voltage is applied to the gate of a MOS-FET to turn it on and detect a signal, and the duration of its being kept on can be adjusted with a resistor (see, for instance, Japanese Patent Application Laid-Open No. 10-260202). 
   SUMMARY OF THE INVENTION 
   Any such sensor node using a conventional impact detection involves a problem that, where a switch of the aforementioned type is used, only a two-value determination can be made, namely whether or not the sensed intensity of the impact has surpassed a threshold, but no multi-value or analog determination can be made. 
   Or where the aforementioned acceleration sensor is used, though analog determination is possible, measuring the acceleration by keeping the sensor in operation all the time involves another problem of greater power consumption, which makes it impossible to use the sensor node for a long continuous period. 
   An object of the present invention, therefore, is to provide a technique regarding sensor nodes for impact detection to enable the intensities of impacts to be determined in a multi-value or analog mode and to reduce the power consumption of sensor nodes. 
   In order to achieve the object stated above, according to the invention, an electric charge is generated by having a piezoelectric element distorted by an external impact, and a sensor node in a waiting state is returned to an active state, trigged by this charge. By measuring. the wattage corresponding to the generated charge with the sensor node, it is made possible to evaluate the intensity of the impact in a multi-valued or analog mode. 
   Since this enables power consumption by the sensor node in its waiting mode to be dramatically reduced, it is made possible to realize a sensor node for impact detection consuming very little power. 
   Typical examples of configuration of the sensor node for impact detection according to the invention will be summarized below. 
   (1) A configuration is characterized by being provided with a shock detection sensor comprising a piezoelectric element unit which generates an electric charge corresponding to an external impact, a capacitor which rectifies and accumulates the electric charge so generated,. and a voltage detector which operates on the accumulated power and externally outputs a signal when the accumulated voltage reaches a preset level; a stand-by control object section which is caused by the external signal to return from a stand-by state and to operate; and a power supply which feeds power to the stand-by control object section, wherein the operation of the stand-by control object section is triggered by the signal of impact detected by the piezoelectric element unit. 
   (2) A configuration is characterized by being provided with a shock detection sensor comprising a piezoelectric element unit which generates an electric charge corresponding to an external impact, a capacitor which rectifies and accumulates the electric charge so generated, and a voltage comparator which compares the accumulated voltage of the capacitor with a reference voltage and externally outputs a signal when the accumulated voltage has surpassed the reference voltage; a stand-by control object section which is caused by the external signal to return from a stand-by state and to operate; and a power supply which feeds power to the voltage comparator and the stand-by control object section, wherein the operation of the stand-by control object section is triggered by the signal of impact detected by the piezoelectric element unit. 
   (3) A configuration is characterized by being provided with a shock detection sensor comprising a piezoelectric element unit which generates an electric charge corresponding to an external impact, and a voltage comparator which compares the voltage so generated with a preset reference voltage and externally outputs a signal when the generated voltage has surpassed the reference voltage; a stand-by control object section which is caused by the external signal to return from a stand-by state and to operate; and a power supply which feeds power to the voltage comparator and the stand-by control object section, wherein the operation of the stand-by control object section is triggered by the signal of impact detected by the piezoelectric element unit. 
   (4) In a sensor node for impact detection of any of the configurations stated in (1) through (3), the shock detection sensor and the stand-by control object section are in a stand-by state until any impact is detected by the piezoelectric element unit. 
   (5) In a sensor node for impact detection of any of the configurations stated in (1) through (3), the stand-by control object section measures the intensity of an impact by measuring the time length of a signal outputted by the shock detection sensor. 
   (6) In a sensor node for impact detection of any of the configurations stated in (1) through (3), the piezoelectric element unit is provided with a planar piezoelectric element member, a plate which fixes one end of the piezoelectric element member and a mass installed at an end of a free end, which is the other end of the piezoelectric element member, and an impulse working on the plate deforms the piezoelectric element member to generate an electric charge. 
   (7) In a sensor node for impact detection of any of the configurations stated in (1) through (3), the stand-by control object section has means which, triggered by the signal of impact detected by the piezoelectric element unit, senses ambient information unevenly distributed in the environment, and processes the sensed information to perform wireless communication. 
   According to the invention, detection of the intensities of impacts in a multi-value or analog mode is realized, and further a technique to realize a sensor node for impact detection that can significantly reduce power consumption is achieved. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a block diagram illustrating the configuration of a sensor node for impact detection, which is a preferred embodiment of the present invention. 
       FIG. 2  is a block diagram illustrating the configuration of a sensor node for impact detection, which is another preferred embodiment of the invention. 
       FIGS. 3(   a ) and  3 ( b ) are profiles illustrating examples of piezoelectric element unit for use in the invention. 
       FIG. 4  shows one example of capacitor for use in the configurations shown in  FIG. 1  and  FIG. 2 . 
       FIG. 5  shows one example of shock detection sensor for use in the configuration shown in  FIG. 1 . 
       FIG. 6  shows one example of shock detection sensor for use in the configuration shown in  FIG. 2 . 
       FIG. 7  is a time chart also showing waveforms which occur when an impact is applied to the sensor node of  FIG. 1  or  FIG. 2 , wherein a capacitor is used. 
       FIG. 8  is a time chart also showing waveforms which occur when an impact is applied to the sensor node of  FIG. 2 , wherein neither a capacitor nor an impact detect circuit is used. 
       FIG. 9  shows another example of configuration of the sensor node for impact detection according to the invention. 
       FIG. 10  shows one example of configuration of a sensor network system for impact detection using sensor nodes of the type shown in  FIG. 1  or  FIG. 2 . 
       FIG. 11  shows one example of configuration of a power supply-free wireless communication node for impact detection using the sensor node shown in  FIG. 1  or  FIG. 2 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Preferred embodiments of the present invention will be described in detail below with reference to the accompanying drawings. 
   Embodiment 1 
     FIG. 1  is a block diagram illustrating the configuration of a sensor node for impact detection, which is a preferred embodiment of the invention. 
   A sensor node  101  comprises a shock detection sensor  102  configured of a piezoelectric element unit  105 , a capacitor  106  and a voltage detector  107 ; a stand-by control object section  103  configured of a wake up signal generator  108 , an impact detect circuit  113 , a microcomputer  109 , a radio frequency transceiver circuit  110 , an A/D converter  111 , a sensor  112  and other elements; and a power supply  104 . 
   Referring to  FIG. 1 , the shock detection sensor  102  requires no power supply, while the stand-by control object section  103  is supplied with power by the power supply  104 . Until any impact is detected, power supply is either off or in a waiting state for the radio frequency transceiver circuit  110 , the A/D converter  111  and the sensor  112 , in a waiting state for the microcomputer  109 , and in a state of awaiting a signal from the shock detection sensor  102  for the wake up signal generator  108  and the impact detect circuit  113 . These states constitute a stand-by state for the sensor node. As the microcomputer  109  consumes only about a few μW of power when in its waiting state while the wake up signal generator  108  and the impact detect circuit  113  also consume only a few μW or less even when operating, the sensor node can realize low power consumption in its stand-by state. 
   Next will be described how an impact is detected. 
   When an impact works on the sensor node  101 , the piezoelectric element in the piezoelectric element unit  105  is distorted to generate an electric charge. The charge generated by the piezoelectric element unit  105  is rectified and accumulated by the capacitor  106  to provide a charge corresponding to the external impact and, when accumulated to a certain voltage, is transmitted by the voltage detector  107  to the stand-by control object section  103  as the impact detection signal sensed by the shock detection sensor  102 . The signal transmitted by the shock detection sensor  102  is inputted into the wake up signal generator  108  and the impact detect circuit  113 , and the wake up signal from the wake up signal generator  108  causes a signal to be transmitted to the microcomputer  109 , which is thereby awakened from a stand-by state to an active state. The microcomputer  109  in the active state captures the signal from the impact detect circuit  113 , measures the intensity of the impact, and performs predetermined control. For the predetermined control, ambient information is sensed by the A/D converter  111  and the sensor  112 , the sensed information is processed by the microcomputer  109 , and communication is performed by the radio frequency transceiver circuit  110 . Upon completion of the predetermined processing, the sensor node  101  returns to the stand-by state. 
   Embodiment 2 
     FIG. 2  is a block diagram illustrating the configuration of a sensor node for impact detection, which is another preferred embodiment of the invention. 
   A sensor node  201  comprises a shock detection sensor  202  configured of a piezoelectric element unit  205 , a capacitor  206  and a voltage comparator  207 ; a stand-by control object section  203  configured of a wake up signal generator  208 , an impact detect circuit  213 , a microcomputer  209 , a radio frequency transceiver circuit  210 , an A/D converter  211 , a sensor  212  and other elements; and a power supply  204 . As will be described afterwards, the capacitor  206  in the shock detection sensor  202  may not be required depending on the method of shock detection. 
   In this embodiment, power is fed by the power supply  204  to the voltage comparator  207  and the stand-by control object section  203  in the shock detection sensor  202 . The piezoelectric element unit  205  and the capacitor  206  in the shock detection sensor  202  require no power supply. Until any impact is detected, power supply is either off or in a waiting state for the radio frequency transceiver circuit  210 , the A/D converter  211  and the sensor  212 , in a waiting state for the microcomputer  209 , and in a state of awaiting a signal from the shock detection sensor  202  for the wake up signal generator  208 , the impact detect circuit  213  and the voltage comparator  207 . These states constitute a stand-by state for the sensor node. As the microcomputer  209  consumes only about a few μW of power when in its waiting state while the wake up signal generator  208 , the impact detect circuit  113  and the voltage comparator  207  also consume only a few μW or less even when operating, the sensor node can realize low power consumption in its stand-by state. 
   Next will be described how an impact is detected. When an impact works on the sensor node  201 , thepiezoelectric element in the piezoelectric element unit  205  is distorted to generate an electric charge. The charge generated by the piezoelectric element unit  205  is rectified and accumulated by the capacitor  206  to provide a charge corresponding to the external impact and, when accumulated to a certain voltage, is transmitted by the voltage comparator  207  to the stand-by control object section  203  as the impact detection signal sensed by the shock detection sensor  202 . 
   Where the capacitor  206  is not used as referred to above, as the piezoelectric element unit  205  generates an AC voltage corresponding to the quantity of the generated charge, the AC voltage of the piezoelectric element unit  205  is inputted directly into the voltage comparator  207 ; when the voltage from the piezoelectric element unit  205  reaches a certain level, it is transmitted to the stand-by control object section  203  by the voltage comparator  207  as the impact detection signal sensed by the shock detection sensor  202 . The signal transmitted by the shock detection sensor  202  is inputted into the wake up signal generator  208  and the impact detect circuit  213 , and the wake up signal from the wake up signal generator  208  causes a signal to be transmitted to the microcomputer  209 , which is thereby awakened from a stand-by state to an active state. The microcomputer  209  in the active state captures the signal from the shock detection sensor  202 , measures the intensity of the impact, and performs predetermined control. For the predetermined control, ambient information is sensed by the A/D converter  211  and the sensor  212 , the sensed information is processed by the microcomputer  209 , and communication is performed by the radio frequency transceiver circuit  210 . Upon completion of the predetermined processing, the sensor node  201  returns to the stand-by state. 
     FIGS. 3(   a ) and  3 ( b ) are profiles illustrating examples of piezoelectric element unit for use in the invention. The piezoelectric element unit shown in  FIG. 3(   a ) is configured of a piezoelectric element  301 , a metal plate  302 , a mass  303 , a fixed plate  304 , a fixed face  305  and electrodes  306   a  and  306   b.    
     FIG. 3(   b ) shows a piezoelectric element unit shown in  FIG. 3A  with the mass  303  omitted. The piezoelectric element  301  is to be made of a material having a piezoelectric effect such as lead zirconate titanate ceramic, lead titanate ceramic or lead metaniobate ceramic. The metal plate  302  is to have a bimorph shape, sandwiched between the layers of the piezoelectric element  301 . The metal plate  302  is intended to increase the durability of the piezoelectric element  301  or increasing the distortion of the piezoelectric element  301  by external forces, but it can be dispensed with. The mass  303  should have the optimum size that is determined by the materials and sizes of the piezoelectric element  301  and the metal plate  302  and the intensity of the external impact to be measured. Depending on the intensity of the external impact, a shape which does not require the mass  303 , as shown in  FIG. 3(   b ) can be selected. When an impact works from outside, an electric charge corresponding to that external impact is generated between the electrodes  306   a  and  306   b . As will be described afterwards, this charge is used for measuring the intensity of the impact and the sensor node is returned from its stand-by state. 
     FIG. 4  shows one example of capacitor for use in the configurations shown in  FIG. 1  and  FIG. 2 . A capacitor  410  is configured of rectification diodes  402  through  405 , a charging capacitor  406 , a discharging resistance  407 , a Zener diode  408  for protection from breakdown voltage and output terminals  409   a  and  409   b . The piezoelectric element unit  401 , such as the one described with reference to  FIGS. 3A and 3B , is connected to rectification diodes. As described with reference to  FIG. 1  and  FIG. 2 , the piezoelectric element unit  401  performs rectification with the diodes  402  through  405  to generate electric charges corresponding to the external impact, and accumulates the charges in the charging capacitor  406 . The resistance  407 , as will be described afterwards, is a resistance for adjusting the duration of discharge, and the Zener diode  408  for protection from breakdown voltage is intended to prevent surpassing of the breakdown voltage of the charging capacitor  406  and the breakdown voltage of an additional circuit connected between the output terminals  409   a  and  409   b.    
     FIG. 5  shows one example of shock detection sensor for use in the configuration shown in  FIG. 1 . This shock detection sensor is configured of a piezoelectric element unit  500 , a capacitor  501 , a voltage detector  502 , terminals  503   a  and  503   b  for connecting the capacitor  501  and the voltage detector  502 , and terminals  504   a  and  504   b  for transmitting signals to the stand-by control object section shown in  FIG. 1 . The voltage detector  502  outputs from the terminals  504   a  and  504   b  a voltage (equal to either the input voltage or to the detect voltage) when the voltage of the capacitor  501  has surpassed the detect voltage. 
   Since the current required for operating the voltage detector  502  is only about a few μW, the size of piezoelectric element unit of the type described with reference to  FIG. 4  (denoted by  401  in  FIG. 4 ) and its capacitor (denoted by  406  in  FIG. 4 ) are designed to optimally match the operating power of the voltage detector  502 . The shock detection sensor in this embodiment, as is evident from the foregoing description, consumes no power when standing by. Nor does it require any power supply when in operation because it relies on power generated by the piezoelectric element unit. 
     FIG. 6  shows one example of shock detection sensor for use in the configuration shown in  FIG. 2 . It is configured of a piezoelectric element unit  600 , a capacitor  601 , a voltage comparator  602 , terminals  603   a  and  603   b  for connecting the output terminals of the piezoelectric element unit shown in  FIG. 3  ( 306   a  and  306   b ) or the output terminals of the capacitor  410  ( 409   a  and  409   b ) to the voltage comparator  602 , terminals  604   a  and  604   b  for transmitting signals to the stand-by control object section shown in  FIG. 1 , a reference voltage generator  607 , a power supply  605  and another power supply  606  (corresponding to the power supply  204  shown in  FIG. 2 ). The voltage comparator  602  outputs from the terminals  604   a  and  604   b  a certain voltage when the voltage of the reference voltage generator  607  has surpassed the reference voltage. 
   Further, as described with reference to the embodiment shown in  FIG. 2 , as the piezoelectric element unit  600  generates an AC voltage when no capacitor is used, the AC voltage from the piezoelectric element unit  600  is directly inputted into the voltage comparator  602 , and when the voltage of the piezoelectric element unit  600  reaches the reference voltage, the voltage comparator  602  outputs a certain voltage from the terminals  604   a  and  604   b.    
   Since power is externally fed to the voltage comparator  602 , it is sufficient for a voltage for impact detection to be supplied from the capacitor  601  or, where no capacitor is used, from the piezoelectric element unit  600 . For this reason, the size of piezoelectric element of the piezoelectric element unit and the capacitor shown in  FIG. 4  (denoted by  406  in  FIG. 4 ) can be smaller than in the case shown in  FIG. 5 . 
     FIG. 7  is a time chart also showing waveforms which occur where the capacitor  206  is used in the sensor node of  FIG. 1  or  FIG. 2  and an impact is applied to the sensor node. In the following description and drawings, parenthesized numerals or phrases refer to a case in which the capacitor  206  is used in the sensor node of  FIG. 2 . 
   When an impulse-form impact  701  is applied to the sensor node  101  ( 201 ) of  FIG. 1 , the output signal of the piezoelectric element  114  ( 214 ) supplied from the piezoelectric element unit  105  ( 205 ) manifests a repetitive attenuating waveform denoted by  702 . Here, t o  is uniquely determined by the size of the piezoelectric element unit in  FIGS. 3A and 3B  and  FIG. 9  to be referred to below. The waveform of  702  is converted by the capacitor  106  ( 206 ) into a waveform denoted by  703  as the output signal of the capacitor  115  ( 215 ). The waveform denoted by  703  is converted by the voltage detector  107  (the voltage comparator  207 ) into the output signal of the voltage detector  116  (the output signal of the voltage comparator  216 ) having the waveform denoted by  704 . Here the waveform of  704  is generated from the waveform of  703  by using a detection voltage (comparison voltage) V ref . The t d  of  704  can be determined by appropriately selecting the size of the piezoelectric element unit  105  ( 205 ), the resistance of the capacitor  106  ( 206 ) and the charge capacity according to the intensity of the impact to be detected. By measuring the length of this t d , the intensity of vibration can be detected in an analog value. 
   When the output signal of the voltage detector  116  ( 216 ) has surpassed V ref , which is the detection voltage (comparison voltage), the wake up signal  117  ( 217 ) is transmitted by the wake up signal generator to the microcomputer  109  ( 209 ) according to the part of the time chart denoted by  705 . This wake up signal brings the microcomputer  109  ( 209 ) into an awaken state. Also, as described earlier, the output signal of the voltage detector  116  ( 216 ) and the impact detect circuit  113  ( 213 ) give an impact level signal  119  ( 219 ) of the form denoted by  707  in the time chart. As an example of this impact detect circuit  113  ( 213 ), a counter circuit can be used. In this case, as long as the voltage detect signal is on, the clocks of the stand-by control object section  103  ( 203 ) are inputted into the counter, which counts the pulses generated during the period of time t d  to detect the intensity of the impact. 
   The microcomputer  109  ( 209 ) captures the intensity of the impact from the impact detect circuit  113  ( 213 ); after completing other predetermined steps of processing, it transmits reset signals  118  and  120  ( 218  and  220 ) in accordance with the  706  part of the time chart and, after resetting the wake up signal generator  108  ( 208 ) and the impact detect circuit  113  ( 213 ), enters into a stand-by state. A time chart regarding the operating state of the microcomputer  109  ( 209 ) is shown as denoted by  708 . 
   The method for analog detection of the impact intensity charted in  FIG. 7  is one example, and any other appropriate method can be used as well. 
     FIG. 8  is a time chart also showing waveforms which occur when an impact is applied to the sensor node of  FIG. 2 , wherein neither the capacitor  206  nor the impact detect circuit  213  is used. 
   When an impulse-form impact  801  denoted by  801  is applied to the sensor node of  FIG. 2 , the output signal of the piezoelectric element  214  supplied from the piezoelectric element unit  205  of  FIG. 2  manifests a repetitive attenuating waveform denoted by  802 . Here, t o  is uniquely determined by the size of the piezoelectric element unit in  FIGS. 3A and 3B  and  FIG. 9  to be referred to below. The waveform of  802  takes on a waveform denoted by  803 , and the output signal of the voltage comparator  216  takes on the pulse waveform denoted by  804 . Here the waveform of  804  is generated from the waveform of  802  by using a comparison voltage V ref . When the output signal of the voltage comparator  216  has surpassed V ref , which is the comparison voltage, the wake up signal  217  is transmitted by the wake up signal generator to the microcomputer  209  according to the part of the time chart denoted by  805 . This wake up signal brings the microcomputer  209  into an awaken state. 
   Further, the output signal of the voltage comparator  216  is directly inputted into the microcomputer  209  as described above, and the microcomputer  209  counts the number of pulses to detect the intensity of the impact. The judgment by the microcomputer  209  that the output signal of the voltage comparator  216  has ended is based on the lapse of a period of time not less than a certain length (t&gt;t o ) after the wave form  804  ceases to manifest any pulse as denoted by  806  of the time chart. The microcomputer  209 , upon completing the detection of the intensity of the impact and other predetermined steps of processing, transmits a reset signal  220  and, after resetting the wake up signal generator  208 , enters into a stand-by state. A time chart regarding the operating state of the microcomputer  209  is shown as denoted by  807 . 
   The method for analog detection of the impact intensity charted in  FIG. 8  is one example, and any other appropriate method can be used as well. 
     FIG. 9  shows another example of configuration of the sensor node for impact detection according to the invention. The sensor node for impact detection of this example is configured of a piezoelectric element  901 , a metal plate  902 , a mass  903 , a fixed plate  904 , a fixed face  905  and electrodes  906   a  and  906   b . The shape of the mass  903  differs from its counterpart in the configuration of the piezoelectric element unit shown in  FIG. 3A . 
   Regarding the mass, it was stated with reference to the example shown in  FIG. 3A  that its size should be appropriate relative to the external vibration and the size of the piezoelectric element. However, depending on conditions, the mass  903  may be so large as to swell beyond the size of the piezoelectric element  901  at the other end than the fixed plate  904 , or may extend in the direction of the fixed plate  904  and the fixed face  905 , resulting in an increased size of the case to protect the piezoelectric element unit. In this embodiment, the protective case for the piezoelectric element unit can be made smaller by folding back the mass  903  toward the fixed plate  904 . 
   Embodiment 3 
     FIG. 10  shows one example of configuration of a sensor network system for impact detection using sensor nodes of the type shown in  FIG. 1  or  FIG. 2 . This embodiment is configured of sensor nodes  1004  through  1006 , base stations  1007  through  1009  for wireless reception of various items of information from the sensor nodes, a network  1010  to which the base stations are connected, a system control device  1011  for receiving information fromthe base stations via the network  1010  and processing the information into desired data, and a control information database  1012  for storing the data processed by the system control device  1011 . In this embodiment, each of the sensor nodes  1004  through  1006  is installed on the object of intended detection, such as a door  1001 , a cargo  1002  or a lid  1003 . These objects of intended detection are mere examples, and many other items can be objects of detection. 
   Each of the sensor nodes  1004  through  1006 , when detecting any external impact, actuates itself to perform predetermined control, and transmits the intensity of the impact, the time of detection of the impact, the value measured by the sensor and information to identify the sensor among other items of information. 
   Each of the base stations  1007  through  1009 , when receiving from a sensor node information on the items referred to above, adds supplementary information including the time of receiving a wireless packet and information to identify the base station having received the wireless packet to the intensity of the impact, the time of measurement, the value measured by the sensor, information to identify the sensor and so forth, and transmits these items of information to the network  1010 . These items of information are processed by the system control device  1011 , and stored into the control information database  1112 . The sensor nodes may either operate only when any impact is detected, or operate intermittently in normal times to sense various items of information and perform node operation, not intermittent, when there is any external impact. 
   Embodiment 4 
     FIG. 11  shows one example of configuration of a power supply-free wireless communication node for impact detection using the sensor node shown in  FIG. 1  or  FIG. 2 . This embodiment comprises a wireless communication node for impact detection  1101  configured of a piezoelectric element unit  1102 , a capacitor  1103  and a radio frequency transceiver circuit  1104 , a base station  1105  for wireless reception of various items of information from the wireless communication node for impact detection, and a display device  1106  for receiving the information from the base station and display its state. Since the radio frequency transceiver circuit operates only on the wattage generated by the piezoelectric element system  1102  in response to an external impact, here is realized a wireless communication node for impact detection requiring no power supply. 
   As hitherto described in detail, according to the present invention, it is made possible to save power consumption by measuring the intensity of an external impact in a multi-valued or analog mode according to the level of electric power generated by the distortion of a piezoelectric element by the impact and actuate a sensor node in a waiting mode by the generated power.