Patent Application
20130086988
This invention relates to pressure sensors. More specifically, this invention relates to a pressure sensor and method of measuring pressure by measuring position changes of an interface between a compressible and incompressible fluid.
Traditional silicon-based pressure sensors have reached maturity and are widely commercially available. They are generally limited to operation at temperatures of less than 200° C. due to technical limitations. Current technological improvements have yielded traditional sensors that can operate in temperatures of up to 500° C. and at pressures up to 50 MPa. Additionally, very few commercial units have been demonstrated to work in the body without foreign body response causing critical drift errors. There has also been no demonstration of micromachined pressure sensors that can sustain operation in long term caustic and radioactive environments such as those commonly found in the nuclear fuel cycle.
A typical interface-type sensor is shown in
PV=NκT where P=pressure, V=volume, N=number of particles in the gas, κ=Boltzmann's constant, and T=temperature. The distance that the liquid is driven in (or out) of the channel is directly related to the pressure and temperature of the environment by the gas laws.
The device is initially ‘primed” at a fixed gas pressure (Po), temperature (To) and has an initial volume (V0). A volume measurement is taken by one of several means (optically, capacitive, digitally, etc.) and the current pressure Pg is determined from the relative gas law equation (this equation can be modified to account for elements of Van der Waals equation if greater accuracy is required):
By measuring volume, the pressure in the channel can be calculated. Then, the pressure outside of the substrate can be determined.
These laws work well with large channels where only the ambient pressure is of concern.
The first deviation from operations governed by the previous equations is a pressure barrier that a meniscus creates inside the microchannel. There can be both advancing and receding meniscus' that sustain pressure drops, which creates hysteresis and measurement error. The capillary pressure (Pcap) created by a meniscus is given by a variant of the Young-Laplace equation:
where σ is the surface tension of the liquid and θ is the contact angle of the liquid on the surface. This yields a final measured pressure Pmeas that is related to the gas pressure Pg and the capillary pressure:
The meniscus effect can limit the minimum detectable resolution of an oscillating pressure because the meniscus changes from an advancing contact angle to a receding contact angle when pressure changes cause the liquid to change from moving forward in the channel to receding and vice versa. A worst case scenario for this pressure resolution would be the combination of the values of the capillary (microchannel) pressure for both advancing and receding contact angles.
What is needed is a pressure sensor for a microchannel substrate apparatus that measures and cancels out of the final pressure measurement the meniscus pressure.
The present invention is directed to a pressure sensor and a method of sensing pressure. In one embodiment of the present invention, a pressure sensor is disclosed. The sensor comprises a substrate and a fluid port coupled to the substrate. The sensor also includes a first microchannel within the substrate in fluid communication with the fluid port. The first microchannel includes a first compressible fluid, a first incompressible fluid, and at least one first meniscus (or fluid interface). The sensor further includes a second microchannel within the substrate in fluid communication with the fluid port. The second microchannel includes a second compressible fluid, a second incompressible fluid and at least one second meniscus. A pressure of at least one of the first meniscus and the second meniscus is determined.
In one embodiment, a cross-sectional dimension of the microchannels does not exceed 200 micrometers. In one embodiment, the microchannels differ in at least one of a cross-sectional area and length that allow for determination of meniscus pressure. As one example, the width of the first microchannel is approximately half its height and the width of the second microchannel is approximately twice its height.
In one embodiment, the sensor further includes at least one electrode. The at least one electrode can be implemented in a capacitive, inductive or resistive fashion to determine the displacement of a gas/liquid interface. The at least one electrode can be an analog electrode or a digital electrode. In one embodiment, the at least one electrode measures a volume of gas in each of the microchannels. In an alternative embodiment, the sensor can further comprise a non-electrical mechanism to measurement displacement of a gas/liquid interface. The non-electrical mechanism can be optical, acoustic or vibratory. In one embodiment, the sensor is embedded in a cerebrospinal fluid (CSF), a deep well, an automotive tire, or a component of another MEMS device.
The sensor can also include a plurality of microchannels within the substrate. Each of the plurality of microchannels is in fluid communication with the fluid port and wherein the plurality of microchannels differ in at least one of a cross-sectional area and channel length. In one embodiment, the plurality of microchannels comprise branching or bifurcating microchannels. Also, each microchannel has at least one meniscus at a gas/liquid interface, and a pressure of the at least one meniscus of the plurality of microchannels is determined.
In one embodiment, the sensor further comprises a temperature sensor in coordination with the pressure sensor to more accurately determine the pressure of at least one of the first and second meniscus. In one embodiment, the sensor is primed with any fluids at any pressure to operate in a variety of atmospheric conditions.
In another embodiment of the present invention, a method of sensing pressure is disclosed. The method comprises providing a substrate and coupling a fluid port to the substrate. The method further includes forming a first microchannel within the substrate, wherein the first microchannel is in fluid communication with the fluid port. The first microchannel includes a first compressible fluid, a first incompressible fluid and at least one first meniscus. The method also includes forming a second microchannel within the substrate, wherein the second microchannel is in fluid communication with the fluid port. The second microchannel includes a second compressible fluid, a second incompressible fluid and at least one second meniscus. The method also includes determining a pressure of at least one of the first meniscus and the second meniscus.
In another embodiment of the present invention, a pressure sensor is disclosed. The sensor comprises a substrate and a fluid port coupled to the substrate. The sensor also includes a channel within the substrate in fluid communication with the fluid port. The channel includes a compressible fluid, an incompressible fluid and at least one meniscus which forms a gas/liquid interface in the channel. The sensor also includes a device for measuring a displacement of the interface. The device can be an electrode or a non-electrical mechanism for measuring displacement of the interface. The non-electrical mechanism can be optical, acoustic or vibratory.
In one embodiment, the sensor further includes a plurality of channels within the substrate, wherein each of the plurality of channels is in fluid communication with the fluid port. Each of the plurality of channels includes a compressible fluid, an incompressible fluid and at least one meniscus which forms a gas/liquid interface in each of the plurality of channels. The meniscus moves due to pressure change, temperature change, or acceleration.
In another embodiment of the present invention, a method of sensing pressure is disclosed. The method comprises providing a substrate and coupling a fluid port to the substrate. The method also includes forming a channel within the substrate. The channel is in fluid communication with the fluid port and includes a compressible fluid, an incompressible fluid and at least one meniscus which forms a gas/liquid interface in the channel. The method further comprises measuring a displacement of the interface.
In one embodiment, the pressure sensor 400 includes an electrode (not shown). The electrode is implemented in a capacitive, inductive or resistive fashion to determine the displacement of a gas/liquid interface. The electrode can be an analog electrode or a digital electrode. The non-electrical mechanism, which measures displacement of the gas/liquid interface, can be optical, acoustic or vibratory.
In one embodiment, the pressure sensor 400 includes a plurality of microchannels within the substrate 450, wherein each of the plurality of microchannels is in fluid communication with the fluid port. The plurality of microchannels differ in either a cross-sectional area or channel length. The microchannels can include branching or bifurcating microchannels. Each of the plurality of microchannels has at least one meniscus at a gas/liquid interface. The pressure of the at least one meniscus is determined.
In one embodiment, the pressure sensor 400 further includes a temperature sensor in coordination with the pressure sensor 400 to more accurately determine the pressure of either the first or second meniscus. Also, the pressure sensor 400 can be primed with any fluids at any pressure in a variety of atmospheric conditions.
In one embodiment, the pressure sensor 500 includes a plurality of channels within the substrate 550. Each of the plurality of channels is in fluid communication with the fluid port and each of the plurality of channels includes a compressible fluid, an incompressible fluid and at least one meniscus which forms a gas/liquid interface in each of the plurality of channels. The plurality of channels can include branching or bifurcating channels. Further, the pressure sensor 500 can include a temperature sensor in coordination with pressure sensor 500 to more accurately determine the pressure of the meniscus 540.
A pressure sensor device, in accordance with one embodiment of the present invention, was fabricated with four microchannels made from silicon and glass. The device is shown in
A pressure sensor device having a single microchannel was made from silicon and glass. The device is similar in design to
Oil was introduced at 761 torr. Oil was used because it is dielectric in nature and unlikely to evaporate, making it a better choice for a proof of concept liquid than water or another aqueous solution.
The device was primed with oil and demonstrated displacement with pressure change. The meniscus moved into the microchannel with positive external pressure. And, as expected, the meniscus retreated with decreasing pressure. Average displacement was 18 μm/torr.
Deviations from theory were likely due to two causes: 1. Heating from the light source (temperature effects) and 2. Van der Waal's forces (can be accounted for).
All of the devices in the above experiments used a single entry port, although the invention is not limited in all embodiments to a single fluid port. In most cases, capillary forces were primed at atmosphere, but other testing was done with different “prime” pressures that provided different performance.
Geometric solutions demonstrated errors of approximately 1% in prototype pressure sensor devices using water.
A sensor was demonstrated that showed an average change of capacitance of −0.111 pF/torr, demonstrating electronic determination of the pressure.
The present invention has many potential market applications. These include, but are not limited to, the following:
The present invention has been described in terms of specific embodiments incorporating details to facilitate the understanding of the principles of construction and operation of the invention. As such, references herein to specific embodiments and details thereof are not intended to limit the scope of the claims appended hereto. It will be apparent to those skilled in the art that modifications can be made in the embodiments chosen for illustration without departing from the spirit and scope of the invention.
