Abstract:
A system includes a structure and a circuit. The circuit is mounted to the structure. The circuit includes a sensor, a non-volatile memory, and a voltage sensitive switch. The memory and the voltage sensitive switch are connected for recording an event sensed by said sensor. The recording only uses power derived from the sensor. One embodiment of the circuit includes a processor connected for receiving a signal derived from the sensor. In this embodiment the non-volatile memory is connected to the processor for receiving and storing data derived from the signal. In one embodiment a first energy storage device is connected to receive energy from the sensor. The voltage sensitive switch is connected for releasing energy from the first energy storage device when energy stored in the first energy storage device exceeds a threshold. The processor and the non-volatile memory are connected for receiving power from the released energy.

Description:
RELATED APPLICATIONS AND PRIORITY 
     This application claims priority of Provisional Patent Application 60/753,481, filed Dec. 22, 2005, incorporated herein by reference. 
     This application is related to the following commonly assigned patent applications: 
     “Structural Damage Detection and Analysis System,” U.S. patent application Ser. No. 11/585,059 to M. Hamel, et al, filed Oct. 23, 2006, (“the &#39;059 application”). 
     “Remotely Powered and Remotely Interrogated Wireless Digital Sensor Telemetry System,” U.S. patent application Ser. No. 10/668,827 to M. Hamel, filed Sep. 23, 2003 (“the &#39;827 application”). 
     “Energy Harvesting for Wireless Sensor Operation and Data Transmission,” U.S. Pat. No. 7,081,693 to M. Hamel et al., filed Mar. 5, 2003 (“the &#39;693 patent”). 
     “Shaft Mounted Energy Harvesting for Wireless Sensor Operation and Data Transmission,” U.S. patent application Ser. No. 10/769,642 to S. W. Arms et al., filed Jan. 31, 2004 (“the &#39;642 application”). 
     “Robotic system for powering and interrogating sensors,” U.S. patent application Ser. No. 10/379,224 to S. W. Arms et al, filed Mar. 5, 2003 (“the &#39;224 application”). 
     “Energy Harvesting, Wireless Structural Health Monitoring System,” U.S. patent application Ser. No. 11/518,777, to S. W. Arms et al, filed Sep. 11, 2006 (“the &#39;777 application”). 
     All of the above listed patents and patent applications are incorporated herein by reference. 
    
    
     FIELD 
     This patent application generally relates to a system for using energy generated by a sensor to power logging data from the sensor. 
     BACKGROUND 
     Several types of sensors generate a voltage or current when excited by an event, including piezoelectric sensors, thermocouples, pressure sensors, and displacement sensors. For example, when a piezoelectric sensor is flexed it generates a voltage that depends on the magnitude of flexing. The voltage generated can be supplied to a circuit that may record data regarding bending, tensile, and compression events sensed by the piezoelectric sensor, as described in the &#39;693 patent and the &#39;059 patent application. Circuits for non-volatile recording of data from an event have needed a source of power, such as may be supplied by a wired source of power or by an energy storage device, such as a capacitor or battery. The energy storage device could be recharged by energy harvesting. For example, piezoelectric devices have been used to harvest energy from a structure subject to strain or vibration, as described in the &#39;693 patent. This energy could then be stored and used to power the circuit for logging data from sensors, processing the data, and transmitting the data. 
     A circuit that records an event sensed by a sensor using only energy provided by that event was described in the commonly assigned &#39;059 patent application. In this regard, the &#39;059 application describes a system for electronically recording an event that provides mechanical energy to a structure. The system includes the structure and an event sensing and recording node. The event sensing and recording node is mounted on the structure and includes a sensor and a first electronic memory. The sensor includes a device for converting the mechanical energy into an electrical signal. The first electronic memory uses energy derived from the electrical signal for electronically recording the event. All energy for sensing the event and recording the event in the first electronic memory is derived from the mechanical energy. A circuit was also described in that the &#39;059 application that allows for measuring the magnitude of that event. That patent application also provided several uses for the circuit. 
     As the &#39;059 application noted, “advantageously, the system uses little energy for recording the event and it can harvest that energy from the event itself.” The &#39;059 application also noted that the “event logging circuit [of the &#39;059 application] is self-powered because electricity generated by one of the piezoelectric sensors of [the] array . . . [of the &#39;059 application], as it senses an event, is the electricity used for logging that event in a memory location of [the] event logging circuit . . . . While another source of power may be needed for circuits that read that memory or that take further action based on data in that memory, the event logging circuit itself is self-powered since the event it is detecting is the sole source of energy for operation of the event logging circuit to log the event in its memory. The . . . inventors [of the &#39;059 application] have also found a way to arrange these circuits to provide a self-powered recording indicating the magnitude of the event.” 
     The present application provides an alternate scheme for using energy from an event sensed by a sensor to power a circuit for recording that event and for measuring its magnitude. 
     SUMMARY 
     One aspect of the present patent application is a system that includes a structure and a circuit. The circuit is mounted to the structure. The circuit includes a sensor, a non-volatile memory, and a voltage sensitive switch. The memory and the voltage sensitive switch are connected for recording an event sensed by said sensor. The recording only uses power derived from the sensor. 
     Another aspect is a system that includes a structure and a circuit. The circuit is mounted to the structure. The circuit includes a sensor, a processor, and a non-volatile memory. The processor and the memory are connected for measuring and recording an event sensed by the sensor. The measuring and recording uses only power derived from the sensor. 
     Another aspect is a system that includes a structure and a circuit. The circuit is mounted to the structure. The circuit includes a sensor, a voltage sensitive switch, a logic circuit, and a memory. The logic circuit and the memory are connected for recording an event sensed by the sensor. The recording uses only power derived from the sensor. 
     Another aspect is a system that includes a structure and a circuit. The circuit is mounted to the structure. The circuit includes a sensor, a voltage sensitive switch, a processor and a memory. The processor and the memory are connected for recording an event sensed by the sensor. The recording uses only power derived from the sensor. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing will be apparent from the following detailed description as illustrated in the accompanying drawings, in which: 
         FIG. 1   a  is a schematic diagram of an energy harvesting sensor measuring circuit that uses energy derived from a sensor to power electronic devices for measuring and logging the data in a non-volatile memory; 
         FIG. 1   b  is a schematic diagram of an inductive scheme for powering the electronic devices of  FIG. 1   a  using switched reactance for reading data stored in memory; 
         FIGS. 2   a - 2   d  are timing diagrams for the circuit of  FIG. 1   a  showing energy and voltages at various points in the circuit as time progresses in response to an event sensed by a piezoelectric sensor; 
         FIGS. 3   a - 3   d  are timing diagrams for the circuit of  FIG. 1   a  showing energy and voltages at various points in the circuit as time progresses in response to events having different magnitudes and producing different magnitude responses in the piezoelectric sensor; 
         FIG. 4   a  is a block diagram of an alternate embodiment of an energy harvesting sensor circuit that uses energy derived from a sensor to power discrete electronic devices for counting the number of events in a non-volatile memory; 
         FIG. 4   b  are timing diagrams for the circuit of  FIG. 4   a  showing voltages at various points in the sequencer circuit as time progresses in response to an event that provides sufficient energy to power on the voltage regulator; 
         FIG. 5   a  is a block diagram of an alternate embodiment of a bin counter connected to an energy harvesting sensor circuit that uses energy derived from a sensor to power discrete electronic devices for counting the number of events at a number of different energy levels and storing the data in non-volatile memory locations; and 
         FIG. 5   b  is a logic table of the output of the circuit of  FIG. 5   a.    
     
    
    
     DETAILED DESCRIPTION 
     The present applicants found that the voltage provided by a piezoelectric sensor provides a tiny amount of power that can be recorded and measured by a circuit and that can also be scavenged to power the measuring and recording circuit. Thus, the power generated by the sensor is used both as signal to and power source for an electronic circuit that records the event. 
     In one embodiment, a power conditioning circuit and a digital delay circuit provide both power and signal to a low power non-volatile ferro-magnetic memory to record an event and to a counter to record the number of such events using only the power from the event. In another embodiment, a power conditioning circuit provides power for operating a low power microcontroller unit and a low power non-volatile ferro-magnetic memory for both measuring the magnitude of an event and record the event using only the power from the event. 
     In either case data can be retrieved at a later time using power from another source, such as a wired connection or a switched reactance reader, as described in the &#39;827 application. The wired connection could be a USB interface. 
     Power generated from an event by energy harvesting piezoelectric sensor  20  is rectified by diode bridge  22  including diodes D 5  and D 6  and stored in DC power reservoir capacitor  24  (capacitor C 9 ), as shown in the schematic diagram in  FIG. 1   a . For sensing the magnitude of the event, the rectified voltage stored in reservoir capacitor  24  is first divided by voltage divider  26 , including resistors R 8  and R 9 , to reduce the potentially high voltage signal across reservoir capacitor  24  to a safe range that can be provided to the analog to digital converter (ADC) input of low power microcontroller  36  (U 2 ) part number C8051F300 available from Silicon Labs, Austin, Tex. Further protection of microcontroller  36  is provided by diode clamps  38 ,  40  in diode D 3  that limit the range of voltages that can be supplied to the ADC input of microcontroller  36 . Microcontroller  36  will provide signal conditioning, including analog to digital conversion, and supply data based on the voltage applied to its ADC input as a digital signal along with time from real time clock  42  (U 1 ) part number STM41T62 from STMicroelectronics, Carrollton, Tex., to non-volatile memory  44 , such as ferroelectric random access memory (FRAM) (U 3 ) FM24CL64-DG from Ramtron Corp., Colorado Springs, Colo. Electrically erasable programmable read only memory (EEPROM) or another memory device can also be used. FRAM has advantage because of its faster write time, lower power consumption, and the fact that it can be written to many more times than EEPROM. 
     For providing power to microcontroller  36  and memory  44 , the rectified voltage stored in reservoir capacitor  24  is also applied to very low power energy harvesting circuit  50  that prevents power from being applied to microprocessor  36  unless sufficient charge has accumulated and that provides power to microprocessor  36  at a high enough rate to avoid damage to microprocessor  36 . Energy harvesting circuit  50  includes voltage divider  56 , pass transistor  52  (Q 2 ), and switching transistor  54  (Q 3 ). Voltage divider  56  includes resistors R 10  and R 14 . In voltage divider  56 , resistor R 10  is provided with a resistance large enough to make leakage negligible when switching transistor  54  turns on. Pass transistor  52  may be a MOS PFET and switching transistor  54  may be a Darlington pair. 
     A single energetic event can provide sufficient charge to reservoir capacitor  24  to turn on energy harvesting circuit  50 . Energy harvesting circuit  50  waits for a signal from piezoelectric sensor  20  and diode bridge  22  that provides a potential across reservoir capacitor  24  that reaches a preset voltage to provide sufficient current to base  62  to turn on switching transistor  54 . Turned on switching transistor  54  lowers the voltage on gate  64  of PFET pass transistor  52  which then turns on, allowing charge in reservoir capacitor  24  to dump into regulator filter capacitor  70 . Energy harvesting circuit  50  includes switching transistor  54  and pass transistor  52  to prevent charge from being transferred to regulator filter capacitor  70  until sufficient energy has been provided by an event detected by piezoelectric sensor  20  to rapidly turn on and operate low power microcontroller  36  and FRAM  44  for a sufficient time to record the data about the event. 
     Once charge has been transferred to regulator filter capacitor  70 , voltage V raw  across regulator filter capacitor  70  should be sufficient to turn on voltage regulator  72  (U 4 ) and provide regulated voltage output V reg  that is used to power microcontroller  36  and memory  44 . In this case V reg  is 3.3 volts. Output of voltage regulator  72  is provided through diode clamp  74  (D 4 ). Diode clamp  74  is a blocking diode, preventing reverse current from flowing into voltage regulator  72 , for example from clamp  38 , if there is a large voltage spike from piezoelectric  20 . Zener diode  76  (D 7 ) limits the voltage that is provided to regulator  72  to keep it from being damaged if voltage V raw  across regulator filter capacitor  70  gets too high. Resistor R 13  is a very small resistor, dissipating little power but it provides a way to measure the current to voltage regulator  72  during testing. It can be removed after testing of the circuit is complete. 
     Output of voltage regulator  72  at diode clamp  74  provides a regulated power for other components, including microprocessor  36  and FRAM  44 . 
     Voltage regulator  72  may continue to provide regulated voltage output V reg  until voltage V raw  across regulator filter capacitor  70  has been reduced below the threshold voltage needed to operate voltage regulator  72 . Resistor R 12  provides feedback to the base of switching transistor  54 . With switching transistor  54  off, V raw  is low, 0V, and charge on C 10  is also zero. In this case resistor R 12  is in parallel with resistor R 14  in voltage divider  56 . When switching transistor  54  is on, resistor R 12  is now connected to V sr , V raw , and is in parallel with resistor R 10 . Switching transistor  54  turning on effectively changes the role of resistor R 12 . The parallel resistors R 10  and R 12  have lower equivalent resistance than just R 10 , which brings base voltage of switching transistor  54  higher. This holds switching transistor  54  on, keeping switch  52  open after it would have closed without resistor R 12 . Thus, resistor R 12  provides hysteresis, keeping switch  52  on longer. This allows the initial turn on voltage of switching transistor  54  to be higher than the voltage needed to keep it on, allowing switch  52  to stay on as V sr  falls from its 15 volt turn on until it is less than about 5 volts. 
     Voltage V raw  on regulator filter capacitor  70  may remain high enough to allow voltage regulator  72  to continue to provide a regulated voltage after voltage on reservoir capacitor  24  has diminished to the point that pass transistor  52  and switching transistor  54  have turned off. 
     In one example of operation of energy harvesting circuit  50 , charge Q sense  is provided over a period of time by an event sensed by piezoelectric sensor  20 , as shown by  FIG. 2   a . Voltage V sr  measured across reservoir capacitor  24  increases until voltage provided by voltage divider  56  is sufficient—in this case 16 volts—to turn on pass transistor  52  and switching transistor  54 , quickly transferring part of the charge on reservoir capacitor  24  to regulator filter capacitor  70 , rapidly lowering voltage V sr  across reservoir capacitor  24 , as shown in  FIG. 2   c , and rapidly increasing voltage V raw  across regulator filter capacitor  70  to the same value—in this case 5 volts, as shown in  FIG. 2   c . Equalized voltage across both capacitors  24  and  70  then continues to decline as power is used and leaks away as also shown in  FIG. 2   b  and  FIG. 2   c . After pass transistor  52  opens voltage regulator  72  may continue to draw current from capacitor  70  as further described herein below. As long as voltage across capacitor  70 , V raw , provided as an input to voltage regulator  72 , is sufficient, output voltage V reg  of voltage regulator  72  remains at a constant value, in this case 3.3 volts. During this period of time microprocessor  36  and non-volatile memory  44  remain on, processing data and storing data. 
     The magnitude of the event is measured as shown in  FIGS. 3   a - 3   d . As the magnitude of the charge Q sense  generated by the event decreases from Q 1  to Q 2  to Q 3 , as shown in  FIG. 3   a , voltage supplied to ADC input of microprocessor  36  varies but microprocessor  36  remains off until regulated voltage V reg  begins to be supplied by voltage regulator  72 . No detection or measurement is made until microprocessor  36  powers up. Once pass transistor  52  and switching transistor  54  turn on and voltage regulator  72  begins to provide regulated voltage V reg , the different voltages Q 1 , Q 2 , Q 3  that may be provided along line Vhmon by different events to ADC input of microprocessor  36  can be measured, as shown in  FIG. 3   b . The magnitude of V raw  and the length of time voltage regulator supplies regulated voltage V reg  varies with the magnitude of the pulse, as shown in  FIGS. 3   c  and  3   d.    
     In the circuit of  FIG. 1   a , RV 1  is a voltage suppressing varistor RV 1  provided to prevent an overload or an electrostatic discharge from damaging such circuit elements as diolde bridge  22 , capacitor C 9 , and MOSFET Q 2 . Varistor RV 1  provides a variable resistance that automatically decreases as the magnitude of the voltage across RV 1  increases above a threshold. A voltage suppressing varistor is selected to provide a this resistance lowering with a threshold voltage that is higher than can be provided by piezoelectric  20  and lower than a voltage that would damage the circuit elements. 
     Capacitors C 11 , C 8 , C 5 , C 6  and C 1  are all noise filtering capacitors to prevent high frequency noise from turning on such devices as switching transistor  54 , processor  36 , clock  42 , and non-volatile memory  44 . 
     Real time clock  42  has its own source of power, such as a button battery  78 , so it can provide time regardless of the energy available from energy harvesting sensor  20 . Regulated voltage from regulator  72  is supplied for communications between real time clock  42 , non-volatile memory  44 , and processor  36  on a shared serial bus, including SCL and SDA signals, as further described herein below. 
     A status LED D 2  with current limiting resistor R 7  can be used to indicate processor activity showing that an event occurred. Connector JP 1  can be provided for contacting pins of microprocessor  36  for programming. R 1 , R 2 , and R 3  are all pull up resistors that are specified by the manufacturer for operation of processor  36 , real time clock  42 , and non-volatile memory  44  to prevent an undefined state. 
     In order to read the data recorded in the memory  44  an alternate source of power may be used to power microcontroller  36  memory  44 . V ext  on pin  1  of diode  79  may be any external DC power source, such as that available through a USB connector. V ext  is supplied through diode  79  to V raw  that provides power from this external source to capacitor  70  and voltage regulator  72 . Schottky barrier diode  79  protects the external source from any high voltage that may otherwise appear at V raw . 
     The alternate source of power can also be inductive power source  80 , as shown in  FIG. 1   b . In this circuit, an external AC magnetic field is applied to coil  82 . The AC magnetic field may be supplied by a coil of a reader (not shown) excited by an AC source and brought into close proximity with coil  82 . This AC magnetic field generates an AC voltage in coil  82 . This AC voltage is rectified by diode  84  and supplied to the common voltage regulator input V raw  that provides power from this inductive source to capacitor  70  and voltage regulator  72 . 
     The AC magnetic field can also be generated by a magnet moving relative to coil  82 . Such movement may be provided by a vibrating or rotating body in the vicinity of coil  82 . This vibration could be produced by an impact event. 
     Thus capacitor  70  and voltage regulator  72  have inputs coming from three possible sources of electricity, piezo  20 , V ext , and an external AC magnetic field. Diodes  74 ,  79 , and  84  provide a diode power multiplexer function enabling these three sources. 
     Reading the data can be accomplished using switched reactance circuit  90  including capacitors C 2 , C 3 , and C 4  and transistor switch  94  that operates under control of processor  36  through line TXD seen in both  FIG. 1   a  and  FIG. 1   b . Processor sends data from memory  44  along line TXD to control operation of transistor switch  94  that modulates reactance of tank circuit  96  including coil  82  and capacitors C 4 , C 2 , and C 3 , as shown in  FIG. 1   b , and thus modules a carrier signal radiated by and detected by the reader. 
     Microcontroller  36  can include many analog and digital functions, and its functionality to energy ratio is usually much higher than can be obtained with discrete or programmable gate array circuits. Although most small 8-bit microcontrollers take very little power to run, the power needed is usually higher than can be obtained directly from most voltage and current generating sensors. The power harvesting circuit described herein above allows enough charge to be collected at high voltage to power almost any microcontroller for the very short time needed to record the signal generated by a sensor. 
     Two factors that regulate how much the microcontroller can do in the short amount of “on” time provided by the harvesting circuit are the startup time and the energy per instruction cycle. The quicker the startup and the lower the energy per instruction cycle, the less energy required to record data from a sensor, allowing smaller and higher sensitivity sensors to be used. 
     Microcontroller  36  may be a small 8 bit microcontroller, such as part number C8051F300 described herein above, with an internal RC oscillator and very low charge per instruction cycle, on the order of 0.27 nA-Sec/instruction. Although RC oscillators usually take more energy than crystal oscillators, they start up almost instantaneously and the advantage in startup time for a microcontroller with an RC oscillator far outweighs the penalty in higher continuous current draw compared to a microcontroller with a crystal oscillator. 
     In addition to FRAM, various non-volatile memories can be used for recording data. EEPROM, Flash, NVRAM, and low power static RAM with battery backup could be used. 
     A bidirectional, multimaster, synchronous, shared serial bus, such as an I2C interface as defined by the Philips Corporation, including the bidirectional signals SDA and SCL, is used in this circuit to communicate data and clocking between microprocessor  36 , real time clock  42 , and non-volatile memory  44  because the I2C interface allows for access from external processors, such as a personal computer, for reading and writing data and timing, if desired. 
     As described herein above, coil  82  and switched reactance circuit can be used both to provide power for reading data logged in memory  44  and to provide a path for transmitting that data from memory  44  to an external reader. Alternatively, because memory  44  has an I2C interface it could also be read by a second externally powered processor such as a personal computer with an I2C interface card. 
     One alternate embodiment uses digital delay line  110  rather than a microprocessor, as shown in  FIGS. 4   a - 4   b  and  5   a - 5   b . Discrete sensor, energy harvester, and data recording circuit  112  includes power conditioning circuit  114 , sequencer  116  and memory incrementor  118 . Power conditioning circuit  114  includes bridge rectifier and energy harvesting circuit  120  and voltage regulator with power on reset  122 . Memory incrementor  118  includes a fixed address in non-volatile memory  124  and counter  126   a ,  126   b . Power conditioning circuit  114  may be similar to energy harvesting energy harvesting circuit  50  of  FIG. 1   a . Digital delay line  110  can include logic elements, such as flip flops. Digital delay line  110  can also include a programmable digital delay line, available from companies such as Maxim Integrated Products, Inc., Sunnyvale, Calif. 
     Voltage regulator with power on reset  122  provides two outputs, Vcc and power good (PG). The PG output is high when the regulated voltage is correct and is not lower than expected. Digital delay line  110  includes enabling input pin  130  that receives signal from PG. Digital delay line  110  also has four outputs, A, B, C, and D that are used by non-volatile memory  124  and by counter  126   a ,  126   b . The rising or falling of one output causes the next to rise, and that sequencing gives the measured and desired delay for clocking specifications of non-volatile memory  124  and counters  126   a ,  126   b , as shown in  FIG. 4   b.    
     If input signal  128  from the piezoelectric to power conditioning circuit  114  is of sufficient magnitude PG output of voltage regulator with power on reset  122  will go high. Digital delay line  110  provides a rising edge for output C in response to receiving this high PG signal at pin  130 . Output C is connected to chip enable pin CE of non-volatile memory  124 , enabling reading and writing of data to and from memory  124  to counter  126   a ,  126   b  in the following sequence. 
     In addition to enabling non-volatile memory  124 , the rising edge of output C also provides a rising edge on output B that reads the number stored in a specified memory location of non-volatile memory  124  and writes that data to counter  126   a ,  126   b.    
     A falling edge of output B causes a high voltage on output D that increments the number now in counter  126   a ,  126   b  by one. 
     A falling edge of output D causes a high voltage on output A that reads the new number in counter  126   a ,  126   b  and writes the new number in counter  126   a ,  126   b  to the fixed address in non-volatile memory  124 . 
     Thus, events that provide sufficient power to bridge rectifier and energy harvesting circuit  120  to provide adequate voltage from voltage regulator with power on reset  122  are counted by discrete sensor, energy harvester, and data recording circuit  112  without the need for a microprocessor and using substantially less energy than would normally be needed to power a microprocessor. Events that do not meet the threshold energy to provide the correct voltage from voltage regulator with power on reset  122  are ignored by discrete sensor, energy harvester, and data recording circuit  112 . 
     Bin counter  140  can be used to provide magnitude of such events can be accomplished with laddered resistors  142  and comparators  144   a ,  144   b , as shown in an illustrative example in  FIGS. 5   a - 5   b . Input signal  146  from a piezoelectric and from a bridge rectifier that, for example, may be in the range from 0 to 80 volts is divided in 20 fold voltage divider  142  to provide a voltage to one leg of comparators  144   a ,  144   b  in the 0 to 4 volt range, as shown in  FIG. 5   a . Voltage divider  142  includes resistors R 16  and R 17 . 
     Regulated output from voltage regulator  122  in power conditioning circuit  114  is provided to the other leg of comparators  144   a ,  144   b  through another voltage divider  148  to provide comparison reference voltages of 1 V and 2 V. Voltage divider  148  includes resistors R 18 , R 19  and R 20 . As long as power conditioning circuit  114  is providing its designed regulated voltage output reference voltages provided to comparators  144   a ,  144   b  are correct. 
     Outputs of comparators  144   a ,  144   b  provide the addresses of memory locations in non-volatile memory  124  in memory incrementor  118  according to the magnitude of the voltage determined by comparators  144   a ,  144   b , as shown by the logic table of  FIG. 5   b . A number stored in a memory location for a specified value of voltage is incremented by an event having that value of voltage by operation of sequencer  116 , memory incrementor  118  and address selector  164 , as described for the counter of  FIG. 4   a.    
     A measure of the magnitude of the event is determined from the logic table of  FIG. 5   b . If input signal  146  provides a divided voltage to comparators  144   a ,  144   b  that is greater than the 1 V reference A is high. If the divided voltage signal is greater than the 2 V reference B is also high. According to the logic table, if neither A nor B is high, input signal  146  provided a divided voltage of less than 1 volt. If A is high and B is low, input signal  146  provided a divided voltage that was greater than 1 volt and less than 2 volts. If A is high and B is high, input signal  146  provided a divided voltage of between 2 and 4 volts. 
     Bin counter  140  provides a specified memory address for each possible voltage level. Bin counter  140  accumulates a count of the number of input signals  146  that had a particular voltage level in the memory location having that address. The number stored in in a memory addresses for a specified voltage level is incremented according to the results of the logic table for each subsequent input signal  146  to provide the count. A histogram of data from all the input signals  146  is obtained by looking at the contents of all the used memory addresses. 
     The actual voltages provided by input signals  146  themselves, in the example illustrated in  FIG. 5   a  would be 20 times larger than the voltages seen by comparators  144   a ,  144   b  which are connected to voltage divider  148 . Thus, the magnitude of each input signal  146  of a series of input signals  146  can be measured and recorded in non-volatile memory using only discrete logic elements and using only the energy provided by the input signals  146  themselves. 
     The sensor of the present patent application can be used to detect impacts to structures and vehicles. One example is for tracking damage to helicopter components, such as a landing gear. Impact of a landing may be recorded and its magnitude measured using the energy generated in a piezoelectric sensor from the impact itself using any of the technique provided of the present patent application. 
     In addition to energy from a single event, such as a gun shot or a hard landing, energy from cyclic strains above a threshold within a component, such as may be present in a helicopter&#39;s rotating components when the helicopter is conducting a maneuver, may also be converted into electrical power for both logging data and for powering the circuit for logging the data. 
     Stored energy obtained from the energy harvesting sensor can also be used to provide power for another sensor for reading data from a sensor with a sensing node. For example, while processor  36  is operating it can read temperature data. 
     Under processor control, once data from an event has been recorded in memory, the processor can direct that any additional energy stored in capacitors  24  and  70  be transferred to another energy storage device, such as a battery or super capacitor, as described in the &#39;693 patent. This energy can then be used to power a wireless transmitter, as also described in the &#39;693 patent. 
     Uses described in the &#39;059 patent application, including monitoring impact events, such as from flying objects on a protective skin or tile of an aircraft, firing of a gun, impacts on landing gear and suspension systems, impacts on a racquet, and the opening and closing of doors and protective enclosures, also apply to embodiments of the present patent application. Embodiments of the present patent application can also be integrated with circuits of the &#39;059 patent application, including  FIGS. 11   a ,  11   b , and  12   a - 12   e , adding function, such as additional energy harvesting from vibration, solar, and external electromagnetic field to charge a rechargeable battery or capacitor. 
     An array of piezoelectric impact sensors can be deployed, any one of which can supply power for operating CPU  24  in  FIG. 5   d  of the &#39;059 patent application. 
     While the disclosed methods and systems have been shown and described in connection with illustrated embodiments, various changes may be made therein without departing from the spirit and scope of the invention as defined in the appended claims.