Data processing: measuring – calibrating – or testing – Measurement system – Temperature measuring system
Utility Patent
1997-12-31
2001-01-02
Shah, Kamini (Department: 2857)
Data processing: measuring, calibrating, or testing
Measurement system
Temperature measuring system
C702S050000, C073S061760, C374S137000, C374S043000
Utility Patent
active
06169965
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to the determination of fluid properties and, more particularly, to the determination of the thermal conductivity, thermal diffusivity, specific heat and velocity of a fluid of interest.
2. Description of the Prior Art
A number of approaches have been devised to measure the thermal conductivity, thermal diffusivity, specific heat and fluid velocity of a fluid of interest. Typically, these and other properties are detected through the use of various types of thermal sensors including resistive sensors with thermally isolated drive and sensing elements located on unsupported thin-film bridge or membrane microstructures.
One approach for determining thermal conductivity is described in U.S. Pat. No. 4,735,082 in which a Wheatstone bridge circuit with a heated element in one leg of the bridge is placed or positioned in a cavity and in contact with the sample fluid of interest. The heated element is used to transfer a series of amounts of thermal energy into the fluid of interest at various levels by periodically varying the input voltage to the heater element which, are, in turn, detected at a sensor in another leg as voltage difference signal across the bridge. Integration of the changes of the value of the successive stream of signals yields a signal indicative of the heat dissipation through the fluid, and thus, the thermal conductivity of the fluid.
Further to the measurement of thermally induced changes in electrical resistance, as will be discussed in greater detail below, especially with reference to prior art FIGS.
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, very small and very accurate “microbridge” semiconductor chip sensors have been described in which such microelements are used as heaters and sensors. Such sensors might include, for example, a pair of thin film sensor elements around a thin film heater element for measuring flow rates. Semiconductor chip sensors of the class described are treated in a more detailed manner in one or more of patents such as U.S. Pat. No. 4,478,076, U.S. Pat. No. 4,478,077, U.S. Pat. No. 4,501,144, U.S. Pat. No. 4,651,564, and U.S. Pat. No. 4,683,159, all of common assignee with the present invention.
Another approach for measuring the thermal conductivity, thermal diffusivity and specific heat of a fluid is disclosed in U.S. Pat. No. 4,944,035 to Aagard et al. Aagard et al. discloses using a microbridge structure that has a heater film and at least one spaced sensor films. A pulse of electrical energy is applied to the heater at a level and duration such that both a transient change and a substantially steady-state temperature occur at the sensor. The thermal conductivity of the fluid of interest is determined based upon a known relation between the sensor output and the thermal conductivity at steady-state sensor temperatures. The specific heat and thermal diffusivity of the fluid of interest are determined based on a known relation among the thermal conductivity, the rate of change of the sensor output during a transient temperature change in the sensor, and the thermal diffusivity and specific heat.
A typical approach for determining the velocity of a fluid of interest is to determine the time required for a thermal wave to travel from a source heater element to a destination sensor element. By knowing the distance between the heater element and the sensor element, as well as the contribution of thermal diffusivity, the velocity of the fluid can be calculated. This approach is suggested in U.S. Pat. No. 4,576,050 to Lambert. Lambert energizes a heater strip with an oscillating heater input signal to emit thermal waves in the fluid. The thermal waves propagate through the fluid at a rate that is dependent on the fluid velocity that flows perpendicular to the heater strip. A thermo-electric detector, spaced from one or both side of the heater, senses the thermal wave and provides a corresponding detector output signal. The velocity of the fluid is determined, at least to first order, from the time difference between the heater input signal and the detector output signal.
A limitation of many of the above prior art approaches is that a substantial amount of support hardware and/or software are required. For example, in many of the prior art approaches, a number of frequency generators are used to provide a frequency input signal to the heater element. Frequency generators can be relatively expensive, both in terms of hardware and power. Likewise, many of the prior art approaches require one or more high frequency timers to measure the time or phase lag between the heater input signal and a corresponding temperature disturbance in the fluid. Like fixed frequency generators, high frequency timers can be relatively expensive, both in terms of hardware and power. Finally, many of the prior art approaches are prone to errors caused by resistive element drifts.
SUMMARY OF THE INVENTION
The present invention overcomes many of the disadvantages associated with the prior art by providing a fluid sensor that uses a common frequency generator for the heater and/or sensor elements. The frequency generator may sequentially and/or simultaneously provide input signals to selected heater and sensor elements. More than one frequency component may be applied by the common frequency generator to more efficiently obtain time and/or phase lags at various frequencies. Further, it is contemplated that an FFT algorithm may be used to separate the frequency components and/or determine the phase lags of selected input and output signals. From the phase lags, selected fluid properties can be determined as more fully described below. Because the present invention contemplates using phase lags or frequencies to determined the fluid properties, whereby the variability of the involved microheater resistive elements only have a second order influence, the thermal properties of the fluid may be determined more accurately than in many prior art approaches.
In a first illustrative embodiment of the present invention, a frequency generator provides a time-varying input signal to one or both of a heater element and a sensor element. The heater element and the sensor elements are preferably provided in one leg of a corresponding Wheatstone bridge circuit. A heater output signal and a sensor output signal indicate the resistance, and thus the temperature, of the heater element and sensor element, respectively.
Because the heater element is closely coupled to the fluid of interest, the thermal conductivity “k” of the fluid directly affects the time variable temperature response of the heater element. Further, the thermal conductivity of the fluid is typically dependent on the pressure and/or temperature of the fluid. Thus, it has been found that the thermal conductivity, pressure and/or temperature of the fluid of interest can be determined by examining a variable phase lag or time lag between the time-varying input signal provided to the heater element and a subsequent transient temperature response of the heater element when measured with substantially zero fluid flow.
To determine the desired phase lags, the present invention contemplates providing a processor implementing an FFT algorithm. The term processor as used herein includes any hardware or software implementation. The processor may, for example, be used to determining the phase lag between the time-varying input signal and the heater output signal during the transient elevated temperature condition. The processor may receive both the time-varying input signal provided by the frequency generator and the heater output signal. Using an FFT algorithm and/or cross-correlation method, the phase lag between the time-varying input signal and the heater output signal may be determined. From the phase lag, the temperature, pressure and/or thermal conductivity of the fluid of interest can be calculated.
To determine other fluid properties such as thermal diffusivity, specific heat and/or fluid velocity, both the heater and sensor element may be used. In one illustra
Bonne Ulrich
Kubisiak David
Honeywell International , Inc.
MacKinnon Ian D.
Shah Kamini
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