Pressure sensor with differential capacitive output
A MEMS pressure sensor device is provided that can provide both a linear output with regard to external pressure, and a differential capacitance output so as to improve the signal amplitude level. These benefits are provided through use of a rotating proof mass that generates capacitive output from electrodes configured at both ends of the rotating proof mass. Sensor output can then be generated using a difference between the capacitances generated from the ends of the rotating proof mass. An additional benefit of such a configuration is that the differential capacitance output changes in a more linear fashion with respect to external pressure changes than does a capacitive output from traditional MEMS pressure sensors.
1. A micro-electromechanical system (MEMS) pressure sensor comprising:
a rotating proof mass, wherein the rotating proof mass comprises
a moveable element adapted for motion relative to a rotational axis offset between first and second ends thereof to form a first section between the rotational axis and the first end and a second section between the rotational axis and the second end,
the first section comprising an extended portion spaced away from the rotational axis, and
the second section comprising an extended portion spaced away from the rotational axis at a length approximately equal to a length of the extended portion of the first section, such that the rotational axis is at a center of mass of the moveable element;
a diaphragm configured to deform in response to a first fluid pressure external to a package comprising the diaphragm and the rotating proof mass; and
a linkage configured to couple a surface of the diaphragm internal to the package to a point along the first section of the rotating proof mass, wherein
the rotating proof mass is configured to rotate in response to deformation of the diaphragm.
2. The MEMS pressure sensor of claim 1 wherein the second section of the moveable element further comprises:
a counterweight configured to offset weight of the diaphragm and linkage so as to maintain the center of mass of the coupled moveable element, linkage and diaphragm at the rotational axis.
3. The MEMS pressure sensor of claim 1 further comprising:
the rotating proof mass further comprising a first major surface and a second major surface;
a first moveable element electrode placed on the first major surface of the first section of the moveable element at a first distance from the rotational axis;
a second moveable element electrode placed on the first major surface of the second section of the moveable element at a second distance from the rotational axis;
a first fixed electrode placed on a fixed surface of the package in a location opposing the first moveable element electrode;
a second fixed electrode placed on the fixed surface of the package in a location opposing the second moveable element electrode, wherein
the first moveable element electrode is electrically isolated from the second moveable element electrode,
the first fixed electrode is electrically isolated from the second fixed electrode,
the first moveable element electrode and the first fixed electrode form a first variable capacitor, and
the second moveable element electrode and the second fixed electrode form a second variable capacitor.
4. The MEMS pressure sensor of claim 3 further comprising:
a processor, coupled to the first and second variable capacitors, and configured to measure a difference between a first capacitance of the first variable capacitor and a second capacitance of the second variable capacitor, wherein the first and second capacitances are responsive to the first fluid pressure external to the package as applied to the diaphragm.
5. The MEMS pressure sensor of claim 1 further comprising:
a second rotating proof mass, wherein the second rotating proof mass comprises
a second moveable element adapted for motion relative to a second rotational axis offset between third and fourth ends thereof to form a third section between the rotational axis and the third end and a fourth section between the rotational axis and the fourth end,
the third section comprising an extended portion spaced away from the second rotational axis, and
the fourth section comprising an extended portion spaced away from the second rotational axis at a length approximately equal to a length of the extended portion of the third section, such that the second rotational axis is at a center of mass of the second moveable element.
6. The MEMS pressure sensor of claim 5 further comprising:
a second linkage configured to couple the surface of the diaphragm internal to the package to a point along the third section of the second rotating proof mass, wherein
the second rotating proof mass is configured to rotate in response to deformation of the diaphragm, and
a rotation of the second rotating proof mass is in an opposite rotational direction to that of the rotating proof mass.
7. The MEMS pressure sensor of claim 5 further comprising:
a second diaphragm configured to deform in response to a second fluid pressure external to the package, wherein
the second fluid pressure is associated with a second fluid source distinct from a first fluid source associated with the first fluid pressure; and
a second linkage configured to couple a surface of the second diaphragm internal to the package to a point along the third section of the second rotating proof mass,
wherein,
the second rotating proof mass is configured to rotate in response to deformation of the second diaphragm.
8. The MEMS pressure sensor of claim 7 wherein a rotation of the second rotating proof mass is in an opposite rotational direction to that of the rotating proof mass in response to an increase in pressure experienced by the second diaphragm.
9. The MEMS pressure sensor of claim 7 further comprising:
the rotating proof mass further comprising a first major surface and a second major surface;
a first moveable element electrode placed on the first major surface of the first section of the moveable element at a first distance from the rotational axis;
a second moveable element electrode placed on the first major surface of the second section of the moveable element at a second distance from the rotational axis;
the second rotating proof mass further comprising a third major surface and a fourth major surface;
a third moveable element electrode placed on the third major surface of the third section of the moveable element at a third distance from the rotational axis;
a fourth moveable element electrode placed on the third major surface of the fourth section of the moveable element at a fourth distance from the rotational axis;
a first fixed electrode placed on a fixed surface of the package in a location opposing the first moveable element electrode and the fourth moveable element electrode;
a second fixed electrode placed on the fixed surface of the package in a location opposing the second moveable element electrode and the third moveable element electrode, wherein
the first moveable element electrode is electrically isolated from the second moveable element electrode,
the third moveable element electrode is electrically isolated from the fourth moveable element electrode,
the first fixed electrode is electrically isolated from the second fixed electrode,
the first moveable element electrode and the first fixed electrode form a first variable capacitor,
the second moveable element electrode and the second fixed electrode form a second variable capacitor,
the third moveable element electrode and the second fixed electrode form a third variable capacitor, and
the fourth moveable element electrode and the first fixed electrode form a fourth variable capacitor.
10. The MEMS pressure sensor of claim 9 further comprising:
a processor, coupled to the first, second, third, and fourth variable capacitors, and configured to
measure a difference between a first capacitance of the first variable capacitor and a second capacitance of the second variable capacitor, and
measure a difference between a third capacitance of the third variable capacitor and a fourth capacitance of the fourth variable capacitor, wherein
the first and second capacitances are responsive to the first fluid pressure external to the package as applied to the diaphragm, and
the third and fourth capacitances are responsive to the second fluid pressure external to the package as applied to the second diaphragm.
11. The MEMS pressure sensor of claim 10 wherein the processor is further configured to determine a difference between the first fluid pressure and the second fluid pressure using the first, second, third, and fourth capacitances.
12. A tire pressure monitoring system comprising:
a wheel module, configured to be mounted in a vehicle tire, comprising
a process controller configured to process signals from one or more sensors,
a micro-electromechanical system (MEMS) pressure sensor coupled to the process controller, the MEMS pressure sensor comprising
a rotating proof mass, wherein the rotating proof mass comprises
a moveable element adapted for motion relative to a rotational axis offset between first and second ends thereof to form a first section between the rotational axis and the first end and a second section between the rotational axis and the second end,
the first section comprising an extended portion spaced away from the rotational axis, and
the second section comprising an extended portion spaced away from the rotational axis at a length approximately equal to a length of the extended portion of the first section, such that the rotational axis is at a center of mass of the moveable element,
a diaphragm configured to deform in response to a gas pressure of the vehicle tire, and
a linkage configured to couple a surface of the diaphragm internal to the package to a point along the first section of the rotating proof mass, wherein
the rotating proof mass is configured to rotate in response to deformation of the diaphragm, and
a radio-frequency (RF) transmitter, coupled to the process controller, and configured to transmit sensor information provided by the process controller.
13. The tire pressure monitoring system of claim 12 wherein the MEMS pressure sensor further comprises:
the rotating proof mass further comprising a first major surface and a second major surface;
a first moveable element electrode placed on the first major surface of the first section of the moveable element at a first distance from the rotational axis;
a second moveable element electrode placed on the first major surface of the second section of the moveable element at a second distance from the rotational axis;
a first fixed electrode placed on a fixed surface of the package in a location opposing the first moveable element electrode;
a second fixed electrode placed on the fixed surface of the package in a location opposing the second moveable element electrode, wherein
the first moveable element electrode is electrically isolated from the second moveable element electrode,
the first fixed electrode is electrically isolated from the second fixed electrode,
the first moveable element electrode and the first fixed electrode form a first variable capacitor, and
the second moveable element electrode and the second fixed electrode form a second variable capacitor; and
the process controller is further configured to measure a difference between a first capacitance of the first variable capacitor and a second capacitance of the second variable capacitor, wherein the first and second capacitances are responsive to the gas pressure of the vehicle tire as applied to the diaphragm.
14. The tire pressure monitoring system of claim 13 wherein the MEMS pressure sensor is insensitive to acceleration forces caused by rotation of the vehicle tire.