System, method, and sensors for sensing physical properties

Details System, method, and sensors for sensing physical properties System, method, and sensors for sensing physical properties

1999-12-06

2001-08-21

Lee, Benjamin C. (Department: 2632)

Communications: electrical

Continuously variable indicating

Condition responsive

C340S870170, C340S870310, C340S870280, C340S447000, C340S449000, C340S451000, C340S584000, C340S665000, C340S521000, C340S010100, C374S120000, C374S183000, C374S184000, C422S082020, C331S066000, C324S655000, C073S774000

Reexamination Certificate

active

06278379

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to the field of physical sensors, and, more particularly, to sensors for wirelessly sensing pressure, temperature and other physical properties in a specific environment.
BACKGROUND OF THE INVENTION
Sensing technology is currently employed in a number of different environments. Specifically, sensors employed to determine the pressure or temperature of a medium are used in a wide variety of applications. Most such applications involve the use of temperature and pressure sensors in environments of low temperature or in environments of high temperature which require adequate cooling measures or the use of high temperature materials in the construction of such sensors. In such applications, for example, micro-machining techniques exist by which pressure sensors are constructed using silicon as a substrate.
An example of a micromachined silicon pressure sensor design is a capacitive pressure sensor. This sensor uses a parallel plate capacitor and a flexible silicon diaphragm. Two silicon wafers are bulk machined to create cavities in the silicon. One silicon wafer is bulk micromachined to create a deep cavity and subsequently a thin membrane. Metal layers are deposited onto appropriate boundaries of the cavities creating the conductors of the parallel plate capacitor. The wafers are bonded so that the metal conductors are facing each other and a capacitor is formed. The capacitor is electrically connected to a silicon circuit on the substrate that in turn is connected to external electronic devices via wire leads. As pressure of the medium in which the sensor is placed increases, the diaphragm deflects and the distance between the plates of the capacitor decreases, causing an increase in the capacitance. The silicon circuit reads the change in capacitance, and a resultant voltage is output via the wire leads.
Micromachined sensors such as the example given above suffer problems when exposed to certain environmental conditions. In high temperature applications, the silicon sensor and similar sensors do not operate reliably or cease to function completely due to the heat. For example, silicon begins to plastically deform at approximately 800° C. and melts at approximately 1400° C. The pressure readout due to the deflection of the flexible silicon diaphragm is compromised by the plastic deformation of silicon causing permanent measurement error. Many other sensor materials have even lower melting points that limit the operating temperature of the environment. In addition, different environments may include corrosive elements in which silicon or other similar materials may not survive.
Another problem with micromachined silicon sensors and similar sensor technology is that circuitry, electrical connections, and wire leads through which temperature, pressure, or other physical information is obtained can not withstand high temperature applications or corrosive environments. For example, silicon circuitry does not function at temperatures greater than 300° C. and high temperature solders, conductive adhesives, and wiring schemes are difficult to implement.
In addition, in the case where a temperature, pressure, or other physical reading of an environment is measured from a sensor mounted to a mobile structure such as a turbine blade or other moveable apparatus, chamber or vessel, the wire leads connected to traditional sensors may interfere with the operation of the particular mobile structure. Such would also be the case of mobile vessels in which interior pressure sensing is desired.
SUMMARY OF THE INVENTION
In accordance with a first embodiment of the present invention, there is provided a sensor for determining the pressure of a specific environment. The pressure sensor features an inductive-capacitive (LC) resonant circuit with a variable capacitor. The capacitance varies with the pressure of the environment in which the capacitor is placed. Consequently, the resonant frequency of the LC circuit of the pressure sensor varies depending on the pressure of the environment. The pressure sensor is made of completely passive components having no active circuitry or power sources such as batteries. The pressure sensor is completely self-contained having no leads to connect to an external circuit or power source.
In accordance with a second embodiment of the present invention, there is provided a sensor for determining the temperature of a specific environment. The temperature sensor features an inductive-capacitive (LC) resonant circuit with a variable capacitor. The capacitance varies with the temperature of the environment in which the capacitor is placed, the capacitor having a dielectric with a permittivity that varies with varying temperature. Consequently, the resonant frequency of the LC circuit of the pressure sensor varies depending on the temperature of the environment. The temperature sensor is made of completely passive components having no active circuitry or power sources such as batteries. Also, the temperature sensor is completely self-contained having no leads to connect to an external circuit or power source.
In accordance with a third embodiment of the present invention, there is provided a combination pressure and temperature sensor for determining both the pressure and temperature of a specific environment. The combination sensor features a first inductive- capacitive (LC) resonant circuit similar to that of the first embodiment, and a second LC circuit similar to that of the second embodiment. The temperature sensing portion of the combination sensor provides an independent source of temperature information that may be employed in real time calibration of the pressure sensor. The combination sensor is also constructed of completely passive components having no active circuitry or power sources such as batteries. Also, the combination sensor is completely self-contained having no leads to connect to an external circuit or power source.
In accordance with a fourth embodiment of the present invention, there is provided a sensor having a resistance that is variable with a specific property or physical condition of a specific environment. The variable resistance sensor features a resistive-inductive- capacitive (RLC) resonant circuit with a variable resistance. The resistance may vary with the temperature, chemical makeup of the environment including chemical species, or other physical condition of the environment to which the variable resistance is exposed. Consequently, the bandwidth of the RLC circuit of the variable resistance sensor varies depending on the value of the variable resistance which depends on a specific physical condition of the environment. The variable resistance sensor is made of completely passive components having no active circuitry or power sources such as batteries and is completely self-contained having no leads to an external circuit or power source.
In accordance with a fifth embodiment of the present invention, there is provided a variable resistance and pressure sensor that is a combination of the first and fourth embodiments. In the fifth embodiment, the inclusion of a variable resistance in the LC circuit of the first embodiment allows the determination of both the pressure from the resonant frequency of the resulting RLC circuit due to the variable capacitance, and the temperature or other physical condition from the bandwidth of the RLC circuit due to the variable resistance.
The sensors of the present invention are used in conjunction with several different excitation systems, resulting in a system and method for determining the pressure, temperature, or other physical condition. Accordingly, each of the above described sensors is electromagnetically coupled to a transmitting antenna. Consequently, a current is induced in each of the sensors that oscillates at the resonant frequency of the sensor in question. This oscillation causes a change in the frequency spectrum of the transmitted signal. From this change, the bandwidth and resonant frequency of the particular sensor may be determined,

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