Abstract:
A power system includes an energy harvesting device, a battery coupled to the energy harvesting device, and a circuit coupled to the energy harvesting device and the battery. The circuit is adapted to deliver power to a load by providing power generated by the energy harvesting device to the load without delivering excess power to the battery and to supplement the power generated by the energy harvesting device with power from the battery if the power generated by the energy harvesting device is insufficient to fully power the load. A method of operating the power system is also provided.

Description:
[0001]     This invention was made with Government support under contract number DE-FC36-04GO14001 awarded by Department Of Energy. The Government has certain rights in the invention. 
     
    
     BACKGROUND  
       [0002]     The invention relates generally to the field of energy harvesting, and more particularly, to energy harvesting circuits for reducing battery drain.  
         [0003]     Energy harvesting is a process for recovering power that is otherwise dissipated or lost in a system. For example, energy harvesting may be used to obtain energy from solar activity, wind, thermal sources, wave action, water currents, and the like. Similarly, energy may be harvested from other sources, such as motor vibrations, pressure changes in the soles of shoes, and the like. In many systems, harvested energy may be used in conjunction with battery power to provide power to a load, such as a sensor or the like. Harvested energy may be used to power the load under normal conditions, with power from the battery being used as a supplement during periods when harvested energy is insufficient to fully power the load. Such systems may extend the useful lifetime of the battery.  
         [0004]     Some systems utilize excess harvested energy to recharge the battery in an attempt to further maximize battery life. However, this requires a rechargeable battery for functioning. One drawback with rechargeable batteries is that they have low useful battery life compared to non-rechargeable batteries if there is insufficient harvested energy to recharge the battery.  
         [0005]     Other systems utilize a harvesting energy source along with non-rechargeable batteries. In such systems, non-rechargeable batteries are primarily used to prolong the continuous delivery of power to the system. These systems may accidentally charge the battery or deliver an accidental charging current to the battery. Because, the life of non-rechargeable batteries may be affected if they are charged or if they receive a charging current, such systems are not effective for long-life applications. An improved circuit for utilizing harvested energy to power a load in conjunction with a battery is desirable.  
       SUMMARY  
       [0006]     In accordance with one aspect of the present technique, a power system is provided. The power system includes an energy harvesting device, a battery coupled to the energy harvesting device, and a circuit coupled to the energy harvesting device and the battery. The circuit controls delivery of power to a load by (i) providing power generated by the energy harvesting device to the load without delivering excess power to the battery, and (ii) supplementing the power generated by the energy harvesting device with power from the battery if the power generated by the energy harvesting device is insufficient to fully power the load. A method of operating the power system is also provided.  
         [0007]     These and other advantages and features will be more readily understood from the following detailed description of preferred embodiments of the invention that is provided in connection with the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]      FIG. 1  is a block diagram of an exemplary power system in accordance with aspects of the present technique.  
         [0009]      FIG. 2  is a block diagram of a motor driven system in accordance with aspects of the present technique.  
         [0010]      FIG. 3  is a schematic diagram of a power system in accordance with aspects of the present technique.  
         [0011]      FIG. 4  is a schematic diagram of an alternative embodiment of a power system in accordance with aspects of the present technique.  
         [0012]      FIG. 5  is a schematic diagram of another alternative embodiment of a power system in accordance with aspects of the present technique. 
     
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS  
       [0013]     In subsequent paragraphs, various circuits, systems, and methods for implementation of different aspects of the power system will be described in greater detail.  FIG. 1  is a block diagram of an exemplary power system  10  in accordance with aspects of the present technique. The power system  10  comprises an energy harvesting device  12  that provides power to a load  14 . The energy harvesting device  12  may be a piezoelectric transducer or a generation device that converts various types of mechanical vibrations or disturbances into electrical power. For example, vibrations from pumps, turbines, engines, bridges when vehicles travel across, and the like may be utilized depending on specific applications. In alternative implementations, an acoustic transducer or a transducer that converts light energy into electrical energy may be employed to generate electrical power. In another implementation, a thermal transducer designed to detect various degrees of thermal gradients may be utilized. The detected thermal gradient may be converted into electrical energy and may be utilized to power the load  14 . Similarly, other transducers that can provide electrical energy from any other form of energy may also be utilized.  
         [0014]     A rectifier  16  converts varying or alternating current (ac) provided by the energy harvesting device  12  into a direct current (dc) signal. The specific configuration details of the rectifier  14  are matters of design choice and should not be considered limitations to the scope of the present technique. By way of example and not limitation, half-wave, full-wave, or voltage doubling rectifiers may be used as well as voltage multiplying circuits in general. Examples of voltage multiplying circuits include Cockroff and Walton voltage multiplying circuits. The rectified power output of the rectifier  16  is provided to the load  14 . A battery  18  supplements the power provided by the energy harvesting device  12 , such that if the amount of power required by the load  14  is not provided by the energy harvesting device  12 , the battery  18  provides the load  14  with the deficient power. One example of a battery that may be utilized is a Lithium-ion non-rechargeable battery. Furthermore, the power system  10  may be designed to avoid any accidental charging of the battery  18 .  
         [0015]     Referring generally to  FIG. 2 , a diagrammatical view of the power system  10  implemented in a motorized system  20  is shown. As illustrated, a motor  22  drives a motor-driven-system  24 , and the power system  10  is coupled to the motor  22  for monitoring. Vibrations, for example, bearing vibrations, generated by the motor  22  are converted into electrical power by the energy harvesting device  12  in the power system  10 . As had been previously discussed with respect to  FIG. 1 , the load  14 , which is a wireless sensor in this exemplary embodiment, is powered by the harvested power generated by the energy harvesting device  12 . However, when the power generated by the vibrations is not enough to power the wireless sensor  14 , the battery  18  provides the deficient power.  
         [0016]     The wireless sensor  14  may provide a signal indicative of the status of the motor  22 . For example, the amount of vibrations generated by the motor  22  increases with aging of the motor  22 . This change in vibrations may be detected and transmitted to a remote location by the wireless sensor  14 . Alternatively, the amount of vibrations may be detected and transmitted to the remote location for further processing, such as maintenance of motor statistics, periodic maintenance checks, current motor use statistics, and the like. When the vibrations increase in the motor  22 , the harvested power is higher and correspondingly lesser amount of power is drawn from the battery at a stage when the wireless sensor  14  requires the maximum power.  
         [0017]     The embodiment described hereinabove is just one of the many implementations in which embodiments of the present technique may be employed. However, the system may be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements. For example, the power system  10  may be incorporated in the tire of a vehicle, wherein a low power wireless sensor may be designed to transmit a plurality of data, including air pressure within the tire, temperature of the tire, and the like. Similarly, the power system  10  may be incorporated in a pedometer, railroads, ductwork in buildings, household appliances that may serve as vibration sources, for providing data representative of one or more parameters of the respective equipment.  
         [0018]      FIG. 3  is a schematic diagram of one implementation of the power system  10  in accordance with aspects of the present technique. A vibration source  12  is coupled with a piezoelectric beam  26  that is mechanically tuned, to the expected vibration frequency, with a tuning mass  28 . The choice of material and other specifications for the tuning mass  28  may depend on the application and the expected vibration frequency. Alternatively, the length or mass distribution of the piezoelectric beam  26 , or both, may be altered to tune the piezoelectric beam  26 . The piezoelectric beam  26  generates a varying or ac voltage when vibrations are present. Rectifier  16  transforms ac voltage into dc voltage. An optional zener diode  30  clamps the output voltage level of the rectifier  16  to a desired level. A filter capacitor  32  may be used to smooth or filter variations in the output voltage of the rectifier  16 . The voltage across the filter capacitor  32  is directly fed to the load  14 . Battery  18  provides power to the load  14  if the voltage across the filter capacitor  32  falls below the battery voltage minus the forward voltage drop of diode  34 . This happens when the harvested energy is not sufficient to power the load  14 . Diode  34  comes into conduction when the battery  18  is supplying power to the load  14 . When energy harvesting device can supply the entire load power, battery  18  does not supply power to the load  14 , during which period, diode  34  prevents accidental charging of battery  18 . In one embodiment, a Schottky diode  34  is used, which provides a low voltage drop of about 0.3 volts across itself when in conduction.  
         [0019]     When vibrations generated are not high enough to supply all of the load energy, diode  34  comes into conduction, and, the piezoelectric source  12  will naturally “ring up” (build up at resonance) in voltage to the battery voltage minus the voltage drop of diode  34 . Thus, a portion of the load energy will be supplied from the vibrations. Note that even with relatively low vibrations the output voltage of piezoelectric source  12  will ring up to the battery voltage (minus the voltage drop of diode  34 ). This is because the piezoelectric source  12  driven at resonance has a relatively high quality factor (Q). If no energy is drawn from the piezoelectric source  12 , the output voltage can ring up to relatively high values. Thus, the voltage will ring up until some energy is drawn at the output voltage determined by the battery (minus the voltage drop of diode  34 ). When no vibrations are present, the battery supplies all of the load power.  
         [0020]      FIG. 4  is a schematic diagram of an alternative embodiment of a power system  10  in accordance with aspects of the present technique. As described with respect to  FIG. 3 , the harvested power is rectified, clamped, and filtered by the rectifier  16 , zener diode  30 , and filter capacitor  32 , respectively. The voltage across the filter capacitor  32  is utilized to power a comparator  36 . The positive and negative inputs for the comparator  36  are provided from nodes  38  and  40 , respectively. Node  38  is at the ground potential, while node  40  is electrically coupled to the negative terminal of battery  18 . A MOSFET active diode combination  42  is driven by the output of the comparator  36 . In one embodiment, an n-channel enhancement MOSFET  44  is used in the MOSFET active diode combination  42 , while the parasitic diode  46  of the MOSFET obviates the need for a separate discrete diode. However, a Schottky diode may be electrically coupled in parallel to the MOSFET active diode combination  42 . The drain of MOSFET  44  is connected to node  40  at the negative terminal of the battery  18 , while the substrate and the source terminals of MOSFET  44  are tied with the ground at node  38 .  
         [0021]     A MOSFET of desirable conduction or on resistance may be chosen for producing a voltage drop across itself, which facilitates switching of the comparator  36 . However, if a MOSFET  44  is chosen that does not have sufficient resistance, which is required to switch the states of the comparator  36 , then an optional resistance element may be introduced between node  40  and the drain of the MOSFET  44 . This resistance element will then provide the required voltage drop for switching the comparator  36 . When the voltage across the filter capacitor  32  is higher than the voltage across the battery  18  and the MOSFET active diode combination  42  in series, the battery may be subject to charging, so that current will flow in the direction from node  40  to node  38  through the MOSFET active diode combination  42 . However, this will cause a drop across the MOSFET  44 , which renders node  40  at a positive potential with respect to node  38 . However, in the present configuration if node  40  is at a higher potential than node  38 , the output of comparator  36  becomes low. This low output of the comparator  36  switches the MOSFET  44  into an off state, preventing charging of the battery  18 .  
         [0022]     Conversely, when node  40  is at a lower potential compared to node  38 , the output of comparator  36  is positive, which causes MOSFET  44  to come into conduction and the battery  18  provides power to the load  14 . This happens when the voltage across the filter capacitor  32  is not sufficient to keep the battery from supplying power to the load. Therefore, the MOSFET  44  and diode  46  pair prevents accidental charging of the battery  18  but switches to provide battery power when needed. Furthermore, the MOSFET active diode combination  42  provides a low power consumption of less than about five microwatts when in conduction and supplying tens or hundreds of microwatts.  
         [0023]      FIG. 5  is a schematic diagram of another alternative embodiment of the power system  10 , which utilizes an integrated circuit  48 . As described with respect to  FIG. 3  and  FIG. 4 , the harvested power is rectified, clamped, and filtered by the rectifier  16 , zener diode  30 , and filter capacitor  32 , respectively. An integrated circuit microcontroller  50  may be utilized for switching MOSFET  44 , which is a p-channel enhancement MOSFET with its parasitic diode  46 , in this exemplary embodiment. One example of an integrated circuit microcontroller that may be utilized is Low Loss PowerPath™ Controller (LTC4412) that is commercially available from Linear Technology Corporate of Irvine, Calif. The power inputs for the microcontroller  50  are provided at the V in  and ground (GND) pins. The GATE pin of microcontroller  50  drives the MOSFET  44 . When the voltage level at the SENSE pin is higher than the voltage at the V in  pin, the microcontroller  50  will pull up the GATE voltage, thus preventing MOSFET  44  from coming into conduction. The load is therefore supplied by the harvested power. In other words, when the harvested power is enough to supply the load fully, battery utilization is minimized. However, once the voltage difference between V in  and SENSE pins is higher than about 20 mV, the GATE pin of the microcontroller  50  is pulled down, thus bringing MOSFET  44  into conduction. This causes the battery to supply the deficient power to the load.  
         [0024]     While the invention has been described in detail in connection with only a limited number of embodiments, it should be readily understood that the invention is not limited to such disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. Additionally, while various embodiments of the invention have been described, it is to be understood that aspects of the invention may include only some of the described embodiments. Accordingly, the invention is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims.