Apparatus and method utilizing magnetic field
Apparatus and method for harvesting energy from the environment and/or other external sources and converting it to useful electrical energy. The harvester does not contain a permanent magnet or other local field source but instead relies on the earth's magnetic field of another source of a magnetic field that is external to the sensing device. One advantage of these new harvesters is that they can be made smaller and lighter than energy harvesters that contain a magnet and/or an inertial mass.
1. An energy harvester without a local field source comprising:
a magnetic field sensing element including one or more layers of magnetostrictive material having a magnetization vector that responds to variations in an applied magnetic field by generating a stress, and one or more layers of electroactive material, mechanically bonded to the layer of magnetostrictive material, that responds to the stress by generating a voltage; and
a circuit coupled to the sensing element that converts the voltage to electrical power for immediate use or storage, wherein the sensing element either:
a) moves relative to a remote static external magnetic field, such that changes in orientation of the sensing element with respect to the external field generates the voltage; or
b) is stationary with respect to a remote changing external magnetic field, wherein the changing external field causes the sensing element to generate the voltage.
2. The energy harvester of claim 1 , wherein the electrical power comprises a voltage and current suitable for an intended application.
3. The energy harvester of claim 1 , wherein the magnetostrictive material layer has a magnetization vector that responds to variations in the magnetic field by rotating in a plane and wherein the electroactive material is poled in a direction substantially parallel to the plane in which the magnetization vector rotates.
4. The energy harvester of claim 1 , wherein the variations in the applied external field are in one or more of magnitude and direction of the field.
5. The energy harvester of claim 1 , wherein the sensing element includes electrodes for measuring the voltage generated and wherein the electrodes are configured such that the distance between the electrodes and cross sectional area between the electrodes are tailored to produce a desired electrical power.
6. The energy harvester of claim 1 , wherein the remote magnetic field is generated by one or more of an electrical transformer, motor, actuator, switch, electronic device, moving machinery or inductor.
7. The energy harvester of claim 6 , wherein the inductor is a wire or coil through which an alternating current is flowing, to produce the remote changing external magnetic field.
8. The energy harvester of claim 1 , wherein the changing external field or sensing element movement is at vibration or power transmission frequencies of no greater than 1 kHz.
9. The energy harvester of claim 1 , wherein the changing external field is at a resonance frequency in the range of that of the sensing element.
10. The energy harvester of claim 9 , wherein the changing external field is in a range of 20 to 50 kHz.
11. The energy harvester of claim 9 , wherein the external field frequency is equal to or close to the resonance frequency of the sensor, which varies roughly according to the equation
fr
≈
1
2
L
E
eff
ρ
eff
where L is a characteristic length of the sensor and E eff and p eff are the elastic modulus and mass density appropriate to describe the composite magnetostrictive/electroactive sensor properties.
12. The energy harvester of claim 1 , wherein the changing external field and sensing element are within a resonant frequency range.
13. The energy harvester of claim 12 , wherein the circuit is within the resonant frequency range.
14. The energy harvester of claim 9 , wherein the external changing field is outside a human or other animal body and the sensing element is inside the body.
15. A method of harvesting energy comprising:
providing a magnetic field sensing element including one or more layers of magnetostrictive material having a magnetization vector that responds to variations in an applied magnetic field by generating a stress, and one or more layers of an electroactive material, mechanically bonded to the layer of magnetostrictive material, that responds to the stress by generating a voltage;
wherein the voltage is generated by either:
moving the sensing element relative to a remote static external magnetic field, such that changes in orientation in the sensing element with respect to the external field generates the voltage; or
the sensing element is stationary with respect to a remote changing external magnetic field, and the changing external field causes the sensing elemenet to generate the voltage; and
converting the generated voltage to electrical power for immediate use or storage.