Abstract:
A circuit passively discharges energy from a piezoelectric device and stores the energy in a power storage element. The circuit has two parallel flow paths each including a diode set and an inductor electrically connected to opposite sides of a piezoelectric device. A power storage element is connected to both inductors. The diode sets are alternately forward biased, and energy from the piezoelectric device discharges through the inductor(s). A portion of the energy is stored in the inductor(s) and the remaining portion is stored in the power storage element. The benefits achieved include passive switching between the parallel circuit paths through diodes in place of traditional switches, and the additional energy stored by the inductors which is also transferred to the power storage element. Passive switching conserves additional energy.

Description:
FIELD OF THE INVENTION 
     The present invention relates in general to piezo device energy harvesting and more specifically to a method and apparatus to discharge and store electrical energy from piezoelectric devices. 
     BACKGROUND OF THE INVENTION 
     Piezoelectric devices are known which can be used as a power source and are particularly useful for remote applications where other power sources such as batteries or generators are impractical. In common applications, the energy of the piezoelectric device is harvested and stored in an energy storage element. Piezoelectric devices internally produce energy as the device vibrates. The energy available from a piezoelectric device increases as the square of the voltage. The relationship is that the energy (E) stored in a device equals ½ CV 2 , C being the capacitance of the device and V being the voltage. Although the energy of the device increases as a squared function, the charge available goes up linearly. 
     Common circuits to discharge piezoelectric devices use capacitors as the power storage element. The disadvantage of discharging a piezoelectric device directly to a capacitor is that the energy generated by the piezoelectric device is not fully utilized. To improve the transfer and storage of a piezoelectric device&#39;s energy, inductors (e.g., coils) are disposed in the flow path to the energy storage element (e.g., the capacitor). The inductors store a portion of the energy discharged from the piezoelectric device and this energy can be harvested once the piezoelectric charge dissipates. 
     Piezoelectric devices generally produce energy at high frequency and low amperage. The intent of the energy storage elements, therefore, is to store this energy in a device capable of providing the higher amperage necessary to operate other devices (e.g., sensor systems and remote devices) which are an increasing area of use for piezoelectric devices. Common piezoelectric device energy harvesting circuits include diodes and inductors arranged in parallel circuit paths across the terminals of the piezoelectric device. Switches are commonly disposed in each parallel circuit path which are timed to match the driving frequency inducing piezoelectric device energy generation. The disadvantage of using switches is that each switch and its controlling circuitry dissipates a portion of the already low charge of the piezoelectric device before it reaches the energy storage element. 
     It is therefore desirable to provide a circuit for discharging piezoelectric devices which eliminates the switches such that the energy normally lost to operate the switches is retained and saved by the energy storage element. It is also desirable to provide a passive piezoelectric energy harvesting circuit to reduce circuit cost and decrease circuit complexity. 
     SUMMARY OF THE INVENTION 
     In a preferred embodiment of the present invention, a circuit passively discharges energy from a piezoelectric device and stores the energy in a power storage element. The circuit has two flow paths arranged in parallel about the terminals of the piezoelectric device. Each flow path includes a plurality of diodes and an inductor electrically connected to one terminal of the piezoelectric device. A power storage element is connected at a discharge end of both inductors. Selected diodes are alternately forward biased, and an energy from the piezoelectric device discharges through the inductor(s). A portion of the energy is stored in the inductor(s) as current flows through the inductor(s) and the remaining portion charges the power storage element. 
     In an exemplary operating condition, the voltage at a first terminal of the piezoelectric device is positive. A positive voltage across the first inductor forward biases select diodes to close inducing current flow across the first inductor into the energy storage element. The positive voltage at the first terminal of the piezoelectric device decreases as current flows across the first inductor into the power storage element. As the voltage across the first inductor changes to a negative voltage, the current with respect to time (dI/dT) decreases across the first inductor and selected diodes in this flow path are re-biased which opens the flow path to the first terminal of the piezoelectric device and closes a flow path between the first inductor and the power storage element. The closed flow path discharges the energy temporarily stored in the first inductor to the power storage element. 
     As the voltage at the first terminal of the piezoelectric device changes to a negative voltage and the voltage at the second terminal of the piezoelectric device changes to a positive voltage, a positive voltage across the second inductor forward biases select diodes to close a flow path inducing current flow across the second inductor into the energy storage element. The positive voltage at the second terminal of the piezoelectric device gradually decreases as current flows across the second inductor into the power storage element. As the voltage across the second inductor changes to a negative voltage, the current with respect to time (dI/dT) decreases across the second inductor and selected diodes in this flow path are re-biased which opens the flow path to the second terminal of the piezoelectric device and closes a flow path between the second inductor and the power storage element. This closed flow path discharges the energy temporarily stored in the second inductor to the power storage element. 
     In a preferred embodiment of the present invention, a voltage regulator is included adjacent to the power storage element. The diodes of the present invention are preferably nano-second time response diodes to follow the operating frequency of the piezoelectric device. The diodes of the present invention provide a passive switching operation, advantageously using the switching capability and low power consumption of diodes to reduce the power loss of the circuit and maximize the energy discharged from the piezoelectric device and stored in the power storage element. 
     In a preferred embodiment, six diodes are used in the circuit of the present invention. Each of the six diodes are forward or reverse biased to open or close a flow path between the piezoelectric device, each of the inductors, and the energy storage element. 
     Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein: 
         FIG. 1  is a schematic diagram of a preferred embodiment of the present invention, having a piezoelectric element, a pair of inductors, a plurality of diodes, and an energy storage element; 
         FIG. 2  is a simplified view of the diagram of  FIG. 1  showing a current flow path from a first terminal of the piezoelectric element through a first inductor and discharging into the energy storage element; 
         FIG. 3  is a simplified view of the diagram of  FIG. 1  showing a current flow path to discharge the temporarily stored energy in the first inductor to the energy storage element, following a diode bias change in the circuit; 
         FIG. 4  is a diagram similar to  FIG. 2  showing a current flow path to discharge the energy from the second terminal of the piezoelectric device through the second inductor into the energy storage element; and 
         FIG. 5  is a diagram similar to  FIG. 3  showing a current flow path to discharge the temporarily stored energy in the second inductor to the energy storage element, following a diode bias change in the circuit. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following description of the preferred embodiment(s) is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses. 
     Referring to  FIG. 1 , a passive circuit  10  in accordance with a preferred embodiment of the present invention is shown. The circuit  10  is used for discharging the energy stored in a piezoelectric device  12  to an energy storage element  14 . The passive circuit  10  includes a first inductor  16  and a second inductor  18 . The passive circuit  10  also includes a plurality of diodes, and in the embodiment shown six diodes: a first diode  20 , a second diode  22 , a third diode  24 , a fourth diode  26 , a fifth diode  28  and a sixth diode  30 . 
     The diodes are disposed in the passive circuit  10  such that a cathode terminal of the first diode  20  is connected to a first terminal  32  of device  12  via a connector  34 . An anode terminal of the second diode  22  is also connected to the connector  34  and thereby to the first terminal  32 . The cathode terminal of the second diode  22  is connected to a first terminal  36  of the first inductor  16  via a connector  38 . A second terminal  39  of the first inductor  16  is connected to the positive terminal of the energy storage element  14  via a first inductor discharge path  40 . An anode terminal of the first diode  20  is connected to the negative terminal of the energy storage element  14  via a common connector  42 . A cathode terminal of the third diode  24  is connected to the connector  38  between the second diode  22  and the first terminal  36  of the first inductor  16 . An anode terminal of the third diode  24  is connected to the common connector  42 . 
     In a parallel circuit path, a cathode terminal of the fourth diode  26  is connected to a second terminal  44  of the piezoelectric device  12  via a connector  46 . An anode terminal of the fifth diode  28  is also connected to the connector  46  and thereby to the second terminal  44 . A cathode terminal of the fifth diode  28  is connected to a first terminal  47  of the second inductor  18  via a connector  48 . A second terminal  50  of the second inductor  18  is connected to the positive terminal of the energy storage element  14  via a second inductor discharge path  52  and a connector  54  respectively. The connector  54  and the first inductor discharge path  40  form a common connection between the first inductor  16 , the second inductor  18  and the positive terminal of the energy storage element  14 . The second inductor discharge path  52  is also in parallel connected to the common connector  42  and thereby to the negative terminal of the energy storage element  14 . 
     An anode terminal of the fourth diode  26  is connected to the common connector  42  and thereby to the negative terminal of the energy storage element  14 . The sixth diode  30  has a cathode terminal connected to the connector  48  between the fifth diode  28  cathode terminal and the first terminal  47  of the second inductor  18 . An anode terminal of the sixth diode  30  is connected to the common connector  42  and thereby to the negative terminal of the energy storage element  14 . In another preferred embodiment of the present invention, a voltage regulator is disposed in the passive circuit  10  adjacent to the energy storage element  14 . 
     Referring now to  FIG. 2 , an exemplary flow path for discharging energy of the piezoelectric device  12  from the first terminal  32  to the energy storage element  14  is shown. A positive voltage is shown at the first terminal  32  and a negative voltage is shown at the second terminal  44 . A positive voltage across the first inductor  16  forward biases the second diode  22  and the fourth diode  26 . The first diode  20 , the third diode  24 , the fifth diode  28 , and the sixth diode  30  are reverse biased. A flow path is therefore closed between the first terminal  32  and the energy storage element  14  as follows: current flows from the first terminal  32  via the forward biased second diode  22  into the first inductor  16 , temporarily charging the first inductor  16  using a first portion of the energy of the piezoelectric device  12 . From the first inductor  16 , current flows into the positive terminal of the energy storage element  14 , storing a second portion of the energy of the piezoelectric device  12  in the energy storage element  14 . The circuit path is completed from the negative terminal of the energy storage element  14  to the piezo second terminal  44  via the forward biased fourth diode  26 . This current flow path is represented by the flow arrows A as shown. 
     Referring to  FIG. 3 , the positively charged first terminal  32  gradually decreases in voltage as current flows to the first inductor  16  and the energy storage element  14  until the voltage of the energy storage element  14  equals or exceeds the voltage at the first terminal  32 . A negative voltage across the first inductor  16  causes a reverse bias of both the second diode  22  and the fourth diode  26 , and a forward bias of the third diode  24 . This closes a flow path represented by the flow arrows B. The energy temporarily stored in the first inductor  16  is discharged by a current path from the first inductor  16  to the positive terminal of the energy storage element  14  and from the negative terminal of the energy storage element  14  through the now forward biased third diode  24 . Current from the first inductor  16  continues to flow to the energy storage element  14  until the voltage of the energy storage element  14  equals or exceeds the voltage across the first inductor  16 . Since the current flow from the first inductor  16  has a negatively changing dI/dT, the current flow rapidly decays to zero. In the flow path represented in  FIG. 3 , each of the first diode  20 , the second diode  22 , the fourth diode  26 , the fifth diode  28 , and the sixth diode  30  are reverse biased. 
     Referring to  FIG. 4 , an exemplary flow path for discharging energy of the piezoelectric device  12  from the second terminal  44  to the energy storage element  14  is shown. A positive voltage is shown at the second terminal  44  and a negative voltage is shown at the first terminal  32 . A positive voltage across the second inductor  18  forward biases the fifth diode  28  and the first diode  20 . The second diode  22 , the third diode  24 , the fourth diode  26 , and the sixth diode  30  are reverse biased. A flow path is therefore closed between the second terminal  44  and the energy storage element  14  as follows: current flows from the second terminal  44  via the forward biased fifth diode  28  into the second inductor  18 , temporarily charging the second inductor  18  using a first portion of the energy of the piezoelectric device  12 . From the second inductor  18 , current flows into the positive terminal of the energy storage element  14 , storing a second portion of the energy of the piezoelectric device  12  in the energy storage element  14 . The circuit path is completed from the negative terminal of the energy storage element  14  to the first terminal  32  via the forward biased first diode  20 . This current flow path is represented by the flow arrows C. 
     Referring to  FIG. 5 , the positively charged second terminal  44  gradually decreases in voltage as current flows to the second inductor  18  and the energy storage element  14  until the voltage of the energy storage element  14  equals or exceeds the voltage at the second terminal  44 . A negative voltage across the second inductor  18  causes a reverse bias of both the fifth diode  28  and the first diode  20 , and a forward bias of the sixth diode  30 . This closes a flow path represented by the flow arrows D. The energy temporarily stored in the second inductor  18  is discharged by a current path from the second inductor  18  to the positive terminal of the energy storage element  14  and from the negative terminal of the energy storage element  14  through the now forward biased sixth diode  30 . Current from the second inductor  18  continues to flow to the energy storage element  14  until the voltage of the energy storage element  14  equals or exceeds the voltage across the second inductor  18 . Since the current flow from the second inductor  18  has a negatively changing dI/dT, the current flow rapidly decays to zero. In the flow path represented in  FIG. 5 , each of the first diode  20 , the second diode  22 , the third diode  24 , the fourth diode  26 , and the fifth diode  28  are reverse biased. 
     The passive circuit  10  of the present invention can be used for any operating frequency of the piezoelectric device  12 , however at frequencies of operation below approximately 1 kHz, the inductance of the first and second inductors  16  and  18  respectively, will require inductors sized in the multiple farad range. The practicality of employing inductors of this size will determine the operating frequency cutoff the circuit designer selects to use a circuit of the present invention. At frequencies above approximately 10 kHz, the inductance of the first and second inductors  16  and  18 , respectively, is reduced to approximately 0.02 farads or less. 
     The diodes of the present invention preferably comprise nano-second time response diodes and are preferably of the Schottky barrier diode technology. The diodes are selected to support the operation frequency of the piezoelectric device  12 . As the frequency of the piezoelectric device  12  increases, the operating speed of the diodes increases. 
     The passive circuit  10  of the present invention provides several advantages. By using diodes in place of switches commonly used for the application of discharging piezoelectric devices, a passive circuit is created. Energy loss associated with operation of the switches is reduced through the use of the diodes  20 - 30 . This energy is therefore retained and saved by the energy storage element. The circuit  10  of the present invention can be used over the frequency range of a piezoelectric device, having practical limits only depending on the size of the inductors and the capacitor used for the energy storage element. 
     A capacitor is commonly used as an energy storage element in discharging energy from a piezoelectric device for storage. The present invention is not limited to a capacitor for energy storage. Other storage devices can be used, including batteries. The passive circuit of the present invention can also be used with a variety of piezoelectric devices. 
     The description of the invention is merely exemplary in nature and, thus, variations that do not depart from the gist of the invention are intended to be within the scope of the invention. Such variations are not to be regarded as a departure from the spirit and scope of the invention.