Patent Application
20080072677
a) and 1(b) are schematic diagrams of one embodiment of a bending mode accelerometer of the present invention.
Referring now to
A piezoelectric single crystal 22 positioned between the housing base portion 12 and the housing top portion 20. The piezoelectric single crystal 22 has a metal shim 24 that forms a unimorph bonded with a metal loaded electrical conductive epoxy 26, and forms an electrical connection at a top electroding surface 28 of the piezoelectric single crystal 22. The piezoelectric single crystal 22 includes a cantilevered free portion 28 that extends to the housing base portion 12. At least a portion of the top electroding surface 28 of the piezoelectric single crystal 22 provides for tuning of capacitance and provides an active electrical connection for the unimorph. At least a portion of a bottom surface 30 forms the electrical ground connection. The housing base metallization areas 16 and 18 are coupled by an electrical connector 32 to a piezoelectric single crystal electrical connection 34.
In one embodiment, the piezoelectric single crystal 22 is a compression mode (d31) relaxor single crystal. In another embodiment, the piezoelectric single crystal 22 is a piezoelectric crystal, poled along <110>. In another embodiment, the piezoelectric single crystal 22 is a PMN-PT or PZN-PT crystal, poled along <110> to optimize the highest (d31) piezoelectric output. In one embodiment, the piezoelectric single crystal 22 senses mechanical vibration in the 50 to 120 Hz range in a z-axial direction.
The base portion can be formed of a metallized ceramic. It will be appreciated that the accelerometer 10 can have different geometric configurations. In one embodiment, the accelerometer 10 is a rectangular prismatic structure.
An end portion of the cantilevered free portion is coupled with two high density metal masses. At least one of the two high density metal masses can be slotted to provide for insertion of the unimorph. The high density metal masses can be bonded to the unimorph with a metal filler loaded epoxy. In one embodiment, the high density metal mass is a metal with a density of at least 17000 kg/m3. The high density metal mass is selected to provide for an optimum mass loading-to-size ratio of the metal mass. This translates into utilizing as much space inside the housing allowing for a maximum of voltage/charge output of the unimorph sensing element of the accelerometer.
In one embodiment, the high density metal mass is tungsten. Other suitable metals include, but are not limited to, molybdenum, tantalum, hafnium, gold, platinum, ruthenium, iridium, palladium, renium, lanthanum metals, actinum metals., and the like.
In one embodiment, the accelerometer has a high voltage output. The high voltage output can be greater than 200 mV/g).
The accelerometer 10 can be included in a vibration measuring device that measures vibration. Suitable vibration measuring devices include but are not limited to, a cardiac rhythm management device, a cardiac monitoring device a neurostimulation device, a neurosignal generating device, interruption or blocking device, a clamp style ablation device, an internal catheter based ablation device, an external or internal measuring device for blunt force trauma to the body, a device for measuring external forces on the head mounted internally or externally, a body motion tilt sensing device, a device for measuring vibration, forces on, and movement of prosthetic limbs, and the like.
In one embodiment, the accelerometer 10 is configured to measure vibration at a frequency under resonance. In one embodiment, the accelerometer is configured to measure vibration in a range of 100 Hz to 2,500 Hz. When the vibration measuring device is a cardiac rhythm management device, the accelerometer 10 measures vibration of 20-200 Hz.
In one embodiment of a method of the present invention, the vibration measuring device is placed in a position to measure vibration at a selected site. Different types of sites can include, but are not limited to, the torso body cavity, the chest cavity inhabited by the heart, the back cavity inhabited by the spinal cord, the torso body cavities that are inhabited by organs that may require or be receptive to drug therapies, the ear canal, the external torso area, and external limb sites including the arms and legs, and the like. The accelerometer 10 measures vibration at the selected site.
The accelerometer 10 can be adhesively secured and electrically connected to pads on the ASIC substrate 11 by a conductive epoxy and structural epoxy between the pads and the metallization areas 16 and 18 . When acceleration is applied to the ASIC substrate 11 along a vector perpendicular thereto, the piezoelectric single crystal 22 flexes in response to the acceleration force.
The accelerometer 10 has a low capacitance. As can easily be appreciated, the piezoelectric single crystal 22 provides electrical charge in response to stressing of the piezoceramic portions thereof. In one embodiment, the accelerometer 10 has an internal capacitance of about 50 pF, with a charge sensitivity to acceleration along the principle is of 200 mV/G. This combination of charge sensitivity and low internal capacitance results in an electrical output from the accelerometer 10 which is easily accommodated by measurement circuitry external to the accelerometer 10.
In this example, the single crystal unimorph based accelerometer 10 is included in a cardiac rhythm management device. the piezoelectric single crystal 22 is a compression mode (d31) relaxor single crystal. The cardiac rhythm management device is designed to deliver an electrical signal to the heart muscle to regulate and control the heart beat rate. Certain inherent physical conditions, external conditions, and physical activities can cause the heart to beat at a rate lower than desired, as well as at a rate higher than desired. The single crystal unimorph based accelerometer 10 is capable of sensing the heart beat rate, and provides an electrical signal to the cardiac rhythm management device proportional to the heart beat rate. The cardiac rhythm management device uses this information to adjust its output to the heart muscle to correctly regulate or maintain the desire heart beat rate. Different cardiac rhythm management devices can also focus on regulation or control of the beat rate of specific chambers of the heat. Information from the single crystal unimorph based accelerometer 10 can also be used for these devices.
The single crystal unimorph based accelerometer 10 is mounted inside of a hermetically sealed enclosure of typically titanium material that houses the other components, battery, printed circuit boards, electrical lead connections, software storage devices, and operational logic devices that comprise a complete cardiac rhythm management device.
In this example, the single crystal unimorph based accelerometer 10 is used to measure vibration at a frequency under resonance. Vibrations can be measured in the range of 100 Hz up to 2,500 Hz.
The foregoing description of embodiments of the present invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Obviously, many modifications and variations will be apparent to practitioners skilled in this art. It is intended that the scope of the invention be defined by the following claims and their equivalents.
