Patent Publication Number: US-2020287479-A1

Title: Self-powering wireless device and method

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of priority from U.S. Provisional Application No. 62/814,191 filed Mar. 5, 2019 and entitled SELF-POWERING WIRELESS DEVICE AND METHOD, which is incorporated herein by reference in its entirety. 
    
    
     TECHNICAL FIELD 
     This disclosure relates to self-powering wireless devices and related methods, including cold-start circuitry and methods for self-powering wireless devices. 
     BACKGROUND 
     Wireless sensors and sensor networks are promising for many applications, ranging from transforming ordinary buildings into smart buildings to monitoring multiple physical parameters at industrial plants and/or in challenging terrains and environments. These technologies have been attracting a tremendous amount of research effort due to the increasing growing and maturity of wireless standards, such as Bluetooth and ZigBee for the wireless sensor network. A central technical challenge confronting such sensors and sensor networks is that most, if not all of today&#39;s existing technologies rely on battery-powered devices. Battery replacement is inevitable on a yearly basis, if not quarterly or monthly, thus giving rise to excess field services and maintenance costs. This challenge is especially acute in a number of applications, where long-term, continuous (non-stop), and real-time sensing or monitoring are needed. In such cases, it is desirable to have self-powering sensor nodes that do not require batteries and only need minimal or zero maintenance beyond their installation. 
     SUMMARY 
     In an example, an energy harvesting system includes a support apparatus. A piezoelectric element is configured as a plate supported at its periphery by the support apparatus to enable a central portion of the piezoelectric element to move along an axis that is orthogonal to a contact surface of the plate. A body having a mass is configured to move in a direction that is substantially parallel to the axis of the plate and apply force to deform the contact surface of the plate, such that electrical energy is generated by the piezoelectric element based on the applied force. 
     In another example, a self-powering wireless device includes an energy harvester module configured to convert ambient energy into an electrical signal. A wireless communication unit is configured to wirelessly transmit data within a wireless network. A power circuit includes a power converter circuit configured to convert the electrical signal to a supply voltage at an output thereof. The power circuit also includes an energy storage device coupled to the output of the power converter circuit to store electrical energy in response to the supply voltage. A battery monitor is configured to monitor the supply voltage, the battery monitor configured disable shutdown of a power management circuit during a start-up phase of the self-powering wireless device so as to provide sufficient time to allow the wireless communications unit to receive electrical power join a wireless network during the start-up phase. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  depicts an example of an energy harvesting system implemented in conjunction with a self-powering wireless sensor. 
         FIG. 2  depicts an example of an energy harvesting apparatus. 
         FIG. 3  depicts another view of the energy harvesting apparatus of  FIG. 2 . 
         FIG. 4  depicts an example of a self-powering wireless sensor. 
         FIG. 5  depicts examples from different views for a self-powering wireless sensor system. 
         FIG. 6  includes  FIGS. 6A, 6B and 6C  to depict respective graphs of different operating parameters for a self-powering sensor system. 
         FIG. 7  is a block diagram of a power circuit for an energy harvesting apparatus. 
         FIG. 8  depicts an example of a battery monitor for controlling cold start of a self-powering wireless apparatus. 
         FIG. 9  is a graph of voltage versus time and acceleration versus time for an example energy harvesting apparatus. 
     
    
    
     DETAILED DESCRIPTION 
     This disclosure relates to self-powering wireless devices and related methods. More particularly, this disclosure relates to an energy harvesting apparatus and a wireless sensor node that includes such harvesting apparatus. This disclosure also relates to cold-start circuitry and methods for cold-starting self-powering wireless devices. 
     As disclosed herein, the approach disclosed herein may include individually or in combination (i) self-powering via vibration energy harvesting, (ii) high-efficiency power management circuit, and (iii) low power wireless communication. 
     As an example, sensor nodes use piezoelectric (PZE) material based mechanical resonators to harvest the vibrational energy of the equipment surfaces. Mechanical vibrations are prevalent in industrial buildings. The equipment, including motor and pump, remains on for hours or all day. Such vibration energy is usually wasted. The typical frequency range of industrial instruments is from 6 to 80 Hz, matching the resonance frequency mm- to cm-sized mechanical resonators. 
     As a further example, the amplitude of voltage generated from the PZE harvester depends on the intensity of the vibration source. However, to power an electronic device like sensors or a radio frequency (RF) transmitter, it needs a constant voltage, such as 3.3 V or 3 V. The power management circuit can convert the voltage from the PZE harvester and boost it to a fixed value, like 3.3 V, with high efficiency. The power management circuit further exhibits low power consumption, which conserves the energy harvested by the PZE for delivery to the functioning electronics. 
     As yet a further example, the sensor node also is configured to implement ultralow-power wireless communication, such as according to a ZigBee or other low power protocol. ZigBee protocol uses 2.4 GHz bandwidth and is effective for low power communication. The benefit of using existing communication protocol is the ease of establishing a wireless network and connecting to such network by the sensor node. Other protocols may be used in other examples. 
       FIG. 1  depicts an example of a self-powering wireless sensor system  10 . The system  10  includes an energy harvesting apparatus that is configured to generate electrical energy that is provided to associated circuitry  14 . Thus, the associated circuitry  14  is self-powered by electric energy produced by the energy harvesting apparatus  12 . 
     In the example of  FIG. 1  (as well as  FIGS. 2 and 3 ). The energy harvesting apparatus  12  includes a support structure (e.g., frame)  16  that supports a piezoelectric element  18  along a periphery thereof. For example, the piezoelectric element is configured as a pliant plate of a piezoelectric material. The support  16  contacts a periphery of an orthogonal surface to enable a central portion of the piezoelectric element  18  to move with respect to the support generally along a central axis  20 , which is orthogonal to a contact surface  22  of the piezoelectric element  18 . 
     As an example, the piezoelectric element  18  includes a disc-shaped plate that includes the contact surface  22  and an opposing side surface  24 . For example, the support apparatus  16  includes a recess extending from the support surface to provide a space (e.g., a void) in the support apparatus into which the central portion of the piezoelectric element can move. The spatial region of the recess also has a central axis that is aligned substantially coaxially with the axis  20 . As an example, the piezoelectric element  18  can be a multi-layer structure such as including a layer of piezoelectric material (e.g., lead zirconate titanate (PZT)) that is mounted on a substrate material layer such as brass. Other piezoelectric materials may be used in other examples (e.g., ZnO 2 , AlN, PDVF or the like). As used herein, the term “substantially” is used to indicate that some amount of variation (e.g., approximately +/−5% or less) from the intended relationship may occur, such as to allow some manufacturing tolerances or other deviation from the intended relationship. 
     The energy harvesting apparatus  12  also includes a body (e.g., a proof mass)  30  configured to move in a direction that is substantially parallel to the axis  20  and to contact the piezoelectric element  18  for harvesting vibration energy as electrical energy. The body  30  is configured to move in response to vibration of the harvesting apparatus  12 . Movement of the body causes the body to apply force to the contact surface  22  of the piezoelectric element  18  to deform the plate, such that electrical energy is generated by the piezoelectric element based on the applied force (e.g., converting vibration energy to electrical energy). For example, the applied force results from vibration of the harvesting apparatus deforms and strains the piezoelectric element to generate the electrical energy. 
     A body support  32  is configured to constrain movement of the body  30  in the direction demonstrated at  34 , which is substantially parallel to the axis  20 . Each of the plate support  16  and the body support  32  can be fixed with respect to each other. For example, a housing, schematically demonstrated at  40 , can include the plate support  16 , the body support  32  as well as contain the piezoelectric element  18  and the body  30  within such housing. The body support  32  and plate support  16  further can be fixed with respect to the housing  40 , such as forming or attached to opposing walls of the housing. 
     As shown in the examples of  FIGS. 1-3 , the body support  32  includes an elongated protrusion  36  that extends from an interior side of the body support  32 . For example, the protruding support  36  extends coaxial with respect to the axis  20 . The body  30  includes a central slot  42  extending from a proximal end  44  of the body into the body, and may extend completely through the body in some examples. The slot  42  is configured to slidably receive the protruding support  36  so as to constrain movement of the body  30  within the housing in the direction  34 . For example, by mounting the housing  40  to a vibrating structure, such as a motor, generator or other plant equipment, the vibration of the structure to which the housing is attached are communicated through the housing to cause the body  30  to move along the support  36  in the direction  34  and repeatedly deform the piezoelectric element  18  over time and thereby generate electrical energy. As an example, housing  40  can be affixed to a vibrating structure by magnet adhesive or other means of attachment (e.g., weld, solder, straps, etc.). 
     The mass of the body  30  is configured to set a resonance frequency of the energy harvesting apparatus  12 . For example, the mass is configured to provide a resonance frequency that is less than  120  hertz, such as ranging from about  30  to about  60  hertz, corresponding to the resonance vibrational frequency of the plant equipment to which the housing is attached. In an example, the housing  40  provides an enclosure and such enclosure includes means to access an interior of the housing, such as for accessing and replacing the body  30  with a different body  30  having a different mass. In this way, a body  30  can be selected from a plurality of different interchangeable bodies having different masses. Each mass thus can provide a different corresponding resonance frequency that can be matched to the equipment to which the housing is to be attached to increase energy harvesting capacity. This is in contrast to cantilever of other designs for energy harvesters that have fixed proof masses. 
     In the example of  FIG. 1 , the system  10  also includes an energy harvest circuit  60  that is electrically coupled to receive electrical energy generated by the piezoelectric element  18 . The energy harvest circuit  60  is configured to convert the electrical energy from the piezoelectric element  18  (e.g., alternating current) into a corresponding voltage that can be supplied to one or more storage devices  62 . For example, the energy harvest circuit  60  includes an AC-DC power converter (e.g., a rectifier and buck boost converter) configured to supply DC voltage that is stored in the one or more energy storage devices  62 . The energy storage device  62  thus provides power to the system  10 . 
     For example, the system  10  includes a power management circuit  64  configured to control the managed consumption of electrical energy stored in the device  62 . For example, the power management circuit  64  is implemented as an integrated circuit chip, such as an application specific integrated circuit (ASIC) configured to perform power management functions as well as control other functions of the system  10 . 
     In the example of  FIG. 1 , the power management circuit  64  includes a wireless communication unit  66  that is configured to send a communication signal based on a sensor signal. The power management circuit  64  also includes one or more sensors  68  configured to detect a sensor condition and provide a corresponding sensor signal, which can be converted to a digital value. For example, the one or more sensors  68  may include a temperature sensor, pressure sensor, humidity sensor, light sensor, occupancy sensor or sound sensor. Other sensors may also be utilized in other examples, such may vary according to application requirements. The power management circuit  64  can control the wireless communication unit  66  to send a communication signal based on the sensor signal. For example, the sensor signal can be sampled over time and stored in memory (not shown) as digital sensor data. The wireless communication unit  66  can provide an RF signal to a corresponding antenna to wirelessly transmit the sensor data through a wireless network to a remote location, which may be connected in the wireless network, directly or indirectly. 
     As one example, the sensor  68  may include an acceleration sensor (e.g. accelerometer) configured to measure acceleration data, delivers the data to the wireless communication unit through a bus for wireless transmission. For accurate measurement, traditional vibration sensor has a high sample rate to collect many acceleration data and then do the computation on the device  14 . However, a wireless transmission rate of about 1 data per 2 minutes may not provide meaningful data because the acceleration reading could be the positive peak or negative peak or any place in the sine wave. In an example, the sensor  68  may include a multi-level buffer. For the example of a sample rate of about 400 Hz, the sensor  68  may be configured to collect 8×32 data points in the buffer. The system can be configured to perform a root mean square (RMS) calculation to get the sensor RMS reading. The RMS reading can be stored in memory and be transmitted wirelessly through the wireless communication unit  66 . 
     As a further example, the RMS equation is shown below, which may be implemented on data acquired in variety of approaches. A first example approach is to take 32 acceleration data 8 times in a row so there are 256 acceleration data in total and then do RMS. A second way is to compute RMS on the 32 acceleration data and repeat this eight times and find the max value among these g&#39;RMS values. A third method calculates RMS on the 32 acceleration data and repeat for eight times, then find the average value among these g&#39;RMS values. 
     
       
         
           
             
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     By way of example, the wireless communication unit  66  includes RF circuitry configured to wirelessly transmit sensor data to a remote device. The wireless communication unit  66  may transmit the sensor data periodically or intermittently. In some examples, the transmission may be triggered in response to detecting a given event (e.g., by the sensor). As a further example, the wireless communication unit  66  may be configured to implement a wireless protocol, such as ZigBee, Z-wave, LoRaWAN, NarrowBand IOT, LTE-M and IEEE 802.15.4 to name a few. The wireless protocol may be implemented to reduce the power consumption. 
     In some examples, the wireless communication unit  66  may be configured as a transceiver to provide bi-directional communication in the wireless network, such as to enable programming operation and/or control of the power management circuit  64 , including the sensor  68  and/or the wireless communication unit  66 . The time interval for data communications can be programmed to vary as a function of the energy storage device, which may be set to default parameter or be user-programmable. For the example, where the sensor  68  is a temperature sensor, a periodic transmission rate of approximately 15 minutes per data transmission may be used to send the on-chip temperature data through the network. Other transmission intervals may be used in other examples. 
     In some examples, the power management circuit  64  also includes a cold start module  70  that is configured to supply power to the wireless communication unit and to disable shut down of a power management circuit during a startup phase of the system  10 . This is to allow sufficient time for the wireless communication unit  66  to join a corresponding wireless network, such as disclosed herein. In this way, the system  10  can provide a self-powering sensor system  10  is capable of operating from cold start and connecting with a wireless network without shutting down prematurely. For example, when the supply voltage value is low (e.g., it is difficult to drive the wireless sensor node), the cold start module  70  will shut down (deactivate) a regulator of the power management circuit  64  that supplies the sensor and wireless communication unit. This allows the energy harvester apparatus  12  to charge the storage device  62 . When enough energy is harvested (e.g., after a sufficient time interval), the cold start module  70  will enable (e.g., activate) the regulator to supply power to the wireless communication unit  66 . During start up, the wireless communication unit  66  can implement a joining process to join the wireless network (e.g., including scanning channels, listening for beacons, and implement security, if any, etc.) and register the system to operate as a wireless sensor node in the network. Further examples of cold start circuitry are disclosed herein with respect to  FIGS. 8 and 9 . 
       FIGS. 2-3  depict another example of an energy harvesting apparatus  80  that includes a housing  82  (e.g., corresponding to housing  40 ). The housing  82  is demonstrated as rectangular prism although other configurations of houses may be used in other examples. The housing includes or contains a plate support portion  84  that is configured to support a circumferential edge of the piezoelectric plate  86 , such that a central piezoelectric active portion  88  of the plate  86  can deform in a direction that is orthogonal to a surface of the plate. For example, a retaining shoulder  92  of the support  84  engage a circumferential edge of the plate  86  to support the central active portion  88  above the recess  90 . The plate  86  may be fixed to the support  84 . 
     An elongated cylindrical body (e.g., corresponding to the body  30 )  98  can include a central aperture or slot to receive a protruding member  96  that extends longitudinally from an opposite side of the housing  82 . The protruding member  96  thus can be received within the central aperture body  98  to constrain the direction of movement of the body  98  along an axis that is substantially orthogonal to the plate surface. The apparatus  80  also includes associated circuitry  100  that is contained within the housing  82  such as can be attached fixedly or flexibly to a wall of the housing. The circuitry  100  can include the circuitry  14  of  FIG. 1 . 
       FIG. 4  depicts a schematic illustration of a self-powering sensing system  120 . In the example of  FIG. 4 , a sensor  122  is demonstrated as being external the sensor package, such as attached by a spring-loaded connector. It is understood that the location and construction of the sensor  122  can vary depending upon the condition being sensed. Such a flexible connector allows the temperature sensor (e.g., breakout board) to move up and down freely. In the example of  FIG. 4 , the external sensor  122  is demonstrated as being a temperature sensor. The system  120  also includes a magnet  124  that can be utilized to attach a package housing of the system  120  to a corresponding metal structure. Thus, while the package is attached to the equipment, the magnet will hold the sensor package to the equipment surface, and the spring loaded pin will make the temperature sensor contact the equipment surface firmly. The direct contact between the temperature sensor and equipment surface make the temperature reading very accurate to reflect the equipment operating condition. Additionally, the magnet may be configured so that, when attached to the object, the long axis of the body mass is aligned parallel to the direction of vibration to facilitate energy harvesting. 
       FIG. 5  depicts an example of another self-powering sensor device such that can be implemented according to the examples of  FIGS. 1-3 . In the example of  FIG. 5 , an antenna extends outwardly from a surface of the package housing, for example from a side that is opposite the surface to which a magnet is attached. The housing may be formed of a metal or other rigid material to facilitate transfer vibrational energy from an object to which the device is attached to the energy harvesting apparatus (e.g., apparatus  12 ,  80 ) that is inside the housing. The material use for the housing may be selected according to the environment where the device is intended to be deployed. 
       FIG. 6  depicts plots demonstrating operation of a disc-structured PZE harvester such as disclosed herein. For example,  FIG. 6( a )  demonstrates frequency responses with different bodies (e.g., different weight proof masses). As mentioned, the weight of proof mass can be adjusted to change resonance frequency. For example, the plot of  FIG. 6( a )  shows gain versus measurement for two different example proof masses (e.g., 28.4 g and 56.4 g) showing the resonance frequency being adjusted from about 30 Hz to about 60 Hz, which cover most of the equipment in the power plant. Additionally, changing proof mass is simple since, in some examples, the proof mass is not attached to the disc. 
       FIG. 6( b )  demonstrates an example of PZE harvester output voltage as a function of frequency for an example disc-shaped harvester (e.g., as shown in  FIGS. 1-3 ) for a 28.4 g proof mass under 0.05 g excitation acceleration.  FIG. 6( c )  is a plot of electrical power versus voltage demonstrating a comparison of output power performance between a disc-structured PZE harvester (e.g.,  FIGS. 1-3 ) and a cantilever-shaped PZE harvester under 0.05 g acceleration.  FIG. 6( c )  shows the disc-structure PZE harvester exhibits a boost in output power over the cantilever-shaped PZE harvester. 
       FIG. 7  is a block diagram of a power circuit for an energy harvesting apparatus. The circuit  200  includes a power converter  202  that is coupled to receive an electrical signal (e.g., and AC signal) from an output of the energy harvesting apparatus. For example, the power converter  202  includes a rectifier to convert the AC electrical signal from the energy harvesting apparatus to a DC signal. The power converter  202  may also include a boost convert to boost the rectified voltage signal to a desired level for charging an energy storage device  204  to provide a supply voltage, corresponding to VBATT. The battery voltage VBATT is supplied to a cold start module  206 . The energy storage device  204  may be a supercapacitor or rechargeable battery (e.g., polymer Li-ion rechargeable battery), for example. 
     The cold start module  206  is configured to supply power to a sensor node  212  based on the voltage VBATT. The sensor node  212  includes a wireless communication unit (e.g., unit  66 )  214  and a sensor (e.g., sensor  68 )  216 . During a start-up phase of the system, the cold start module  206  operates to disable shutdown of the power circuit  200  (e.g., by forcing it to remain enabled) for a sufficient time interval so that the sensor node  212  receives power. The time interval can be set to allow the wireless communications unit  214  to activate and join a wireless network, such as disclosed herein. 
     For example, the cold start module includes a regulator, such as a low dropout (LDO) regulator  208  and a battery monitor  210 . The LDO  208  is configured to supply a stable DC voltage for electronics of the sensor node  212  to operate efficiently. The battery monitor  210  is configured to monitor the voltage level of VBATT, which is based on the energy harvested from the PZE harvester, and to control the operating state of the LDO. In response to the battery monitor detecting that enough energy is harvested (e.g., based on VBATT exceeding a given threshold voltage), the battery monitor  210  will activate (enable) the LDO to provide power to the sensor node. When the voltage value is low (e.g., detected by monitor to be below a threshold voltage), such as when it is hard to drive the sensor node, the battery monitor  210  is configured to deactivate the LDO to stop supplying the output voltage to sensor node. This allows the energy harvester to just charge the storage element without consuming power by the sensor node. However, during an initial time interval after having met the given threshold and activating the LDO, the battery monitor  210  is configured to disable the shutdown of the LDO temporarily for a duration. As mentioned, this duration allows the wireless communications unit  214  to join a wireless network. After such duration, the low-voltage shutdown function of the battery monitor can be enabled. 
       FIG. 8  depicts an example of a battery monitor for controlling cold start of a self-powering wireless apparatus. The battery monitor includes a resistor network of resistors R 1 , R 2 , R 3 , R 4  connected between VBATT and ground. Nodes between resistors are tapped to provide respective low and high thresholds, which are provided to non-inverting inputs of comparators. A reference voltage is applied to the inverting inputs of the comparators. Each of the comparators provides a corresponding output to respective inputs of an output circuit (e.g., to R S inputs of a flip flop). The output circuit provides an enable (EN) signal at an output, which is connected to activate or deactivate the regulator (e.g., LDO  208 ). A timer circuit, such as an RC network (R 5  and C 1 ), is coupled between the output of the flip flop and ground. A node between R 5  and C 1  is also coupled to control a switch S 1 , which is coupled between R 4  and ground. The timer circuit thus control activation of the switch when the enable signal changes from high to low. For example, the RC network delays activation of the switch at such transition for a time interval corresponding to the RC time constant. Thus, the time constant can be set to provide a time interval sufficient to allow the wireless communications unit to receive electrical power join a wireless network during the start-up phase. After the switch is activated, the low voltage detection is enabled, such that if VBATT falls below the low threshold, the output circuit resets to provide a low output to disable the LDO. 
       FIG. 9  is a graph  300  of voltage versus time and acceleration versus time for an example energy harvesting apparatus implementing the cold start function. VBATT is the voltage across the storage element, such as having an initial voltage is around 3 V. As mentioned, the battery monitor (e.g., monitor  210 ) is configured to turn on the wireless sensor by controlling the regulator (e.g., LDO  208 ) when VBATT exceeds a high voltage threshold (e.g., VBATT&gt;3.45 V) and shut the wireless sensor down when VBATT is less than a low threshold (e.g., VBATT&lt;3.1 V). The line  304  represents the output from an accelerometer to monitor the vibration condition of the vibration generator. When the vibration generator begins to vibrate (zone  306 ), the PZE harvester will charge the storage element and therefore VBATT  302  is increasing. When the VBATT exceeds the high threshold (e.g., 3.45 V), the LDO begins to supply the wireless sensor node (e.g., node  212 ) with a stable voltage of about 2.6 V, demonstrated at  308 . In an intermediate zone  310 , the vibration amplitude of the vibration generator is adjusted, such that a 0.07 g vibration amplitude is needed for this wireless sensor node to operate continuously. For example, it is expected that most of the equipment in the power plant can provide 0.05 to 0.15 g vibration. The zone  312  shows an example of when the vibration generator is turned off (e.g., no vibration). Thus in zone  312 , while no (or little) energy is being harvested, the wireless sensor node consistently drains energy from the storage element and eventually the circuit will turn off the LDO, which shuts down the wireless sensor (zone  314 ). The cold start function implemented by the battery monitor improves operation for the wireless sensor node because the vibration from equipment may vary and even stop when the power plant is under a scheduled outage. Accordingly, a wireless self-powering node, as disclosed herein, is able to cold start itself when the vibration is reassumed. 
     What have been described above are examples. It is, of course, not possible to describe every conceivable combination of components or method, as many further combinations and permutations are possible. Accordingly, the disclosure is intended to embrace all such alterations, modifications, and variations that fall within the scope of this application, including the appended claims. As used herein, the term “includes” means includes but not limited to, the term “including” means including but not limited to. The term “based on” means based at least in part on. Additionally, where the disclosure or claims recite “a,” “an,” “a first,” or “another” element, or the equivalent thereof, it should be interpreted to include one or more than one such element, neither requiring nor excluding two or more such elements.