Data processing: measuring – calibrating – or testing – Calibration or correction system – Temperature
Reexamination Certificate
2001-12-21
2003-11-18
Barlow, John (Department: 2857)
Data processing: measuring, calibrating, or testing
Calibration or correction system
Temperature
C073S708000, C073S023200, C324S130000, C324S704000, C324S115000, C374S183000, C323S367000
Reexamination Certificate
active
06651020
ABSTRACT:
TECHNICAL FIELD
The present invention relates to measuring and recording devices and techniques for compensating electronic measurement systems for the effects of electronic component drift over time and temperature. By way of example but without limitation, one embodiment of the present invention relates to temperature measuring and recording devices and techniques which perform high resolution temperature difference measurements, on the order of micro-degrees centigrade.
The method and apparatus of the present invention accurately resolve extremely small differences in electrical signals, in a very low cost, highly portable apparatus that can be battery operated. In an exemplary embodiment, the method and apparatus of the present invention are directed to the measurement of temperature differences, on the order of micro-degrees centigrade, by utilizing predictable behavior in the relative time drift of thermal offset curves, for various circuit elements, including a difference signal means optionally having amplification for providing an amplified difference signal, an ambient temperature amplification means, and an analog to digital converter means. In an initial calibration mode, preferably performed at the time of manufacture, the exemplary embodiment records several thermal offset curves, stored in memory, which correlate ambient temperature measurements to offset measurements acquired from the ambient temperature amplification means and the difference signal means, with both of said means connected to a measurement bridge, comprising two thermistors and two resistors, for measuring ambient temperature and temperature differences (via nodes of the measurement bridge). Thermal offset curves recorded in the initial calibration mode, correlating ambient temperature measurements to measurements from the difference signal means, include one curve recorded with both inputs of the difference signal means held at equal potential and another curve recorded with both thermistors of the measurement bridge held at the same temperature, over a given ambient temperature range. Another thermal offset curve, preferably recorded at the time of manufacture, correlates measured ambient temperature from the ambient temperature amplification means to measurements from the ambient temperature amplification means, with inputs to said ambient temperature amplification means shorted together or, alternatively, shorted together and connected to one or more reference signals (such as system ground), which in the exemplary embodiment are preferably voltages from a reference resistance bridge, preferably comprising substantially time stable (not necessarily temperature stable) resistors. The method and apparatus of the present invention require few components, and no precision active or passive components, resulting in low power consumption, and low cost. The method and apparatus of the present invention overcome time and temperature component drift, by utilizing the fact that the thermal offset curves, acquired in the initial calibration mode (preferably at the time of manufacture), drift with time in a predominantly linear fashion relative to one another. Consequently, during normal operation, these offset curves representing temperature drift behavior, among electrical components, can be updated for time drift, at a single, current arbitrary ambient temperature, the measurements for which can be obtained quickly and applied as a time drift correction to thermal offset curves, without interrupting normal system operation. Additionally, the present invention dynamically tracks cumulative system errors associated with the method of the present invention, in order to dynamically calculate optimal system resolution, based upon current operating conditions (rather than based upon more general component drift specifications).
BACKGROUND OF THE INVENTION
Various electronic systems exist for measuring extremely small differences in sensor measurements, such as temperature, for use in biological and physical analyses. It is known in the art that active and passive electronic components in such systems are subject to time and temperature drift, and that under normal operating conditions, the amplitude of time and temperature component drift is typically much greater than the amplitude of other inaccuracies generated by system components, such as amplifier noise voltage, noise current, and resistor noise. Consequently, component time and temperature drift are significant limiting factors to high resolution measurements, such as temperature difference measurements. To address the problem of component drift in electronic measurement systems generally, various approaches to compensate for drift have been devised.
For example, U.S. Pat. Nos. 5,253,532 (Kamens); 5,042,307 (Kato); 4,611,163 (Madeley); and 3,831,042 (La Claire) disclose electronic measurement systems (principally directed to pressure sensing, in the preferred embodiments) which include additional hardware components that change their electrical resistance, or other electrical parameters, with ambient temperature, in such a way as to compensate for thermal drift in measurement systems to which they are electrically connected. While such hardware compensation systems provide some compensation for thermal drift inaccuracies, they do not compensate for component drift over time, particularly the drift of sensors, such as thermistors. This would be sufficient to preclude temperature difference measurements, with resolution on the order of micro-degrees centigrade, if these techniques were applied to that purpose. Additionally, such hardware based compensation techniques do not readily compensate for component drift, resulting from the combined time drift characteristics of multiple system components, located at different parts of the system, with different thermal drift characteristics, and subject to non-uniform aging. In any case, the ability of the above hardware based compensation systems and techniques to compensate for system thermal drift are limited by the extent to which the particular technique tracks with thermal drift of the overall system, over time and temperature. Consequently, such techniques would not provide sufficient compensation for component time and temperature drift to permit differential temperature measurements, with resolution on the order of micro-degrees centigrade, if these techniques were applied to that purpose.
Other hardware compensation techniques, such as disclosed in U.S. Pat. Nos. 5,616,846 (Kwasnik); 5,171,091 (Kruger et al.); and 5,132,609 (Nguyen), require a time and temperature stable reference signal, and U.S. Pat. No. 5,351,010 (Leopold et al.) requires the use of precision analog amplification hardware and costly time and temperature stable resistors. The required precision analog components in these systems results in increased cost, complexity, and power consumption. Moreover, these systems do not compensate for time drift of passive components, such as thermistors, which would be sufficient to preclude temperature difference measurements, with resolution on the order of micro-degrees centigrade, if these systems were applied to that purpose.
Additionally, U.S. Pat. Nos. 5,162,725 (Hodson et al.); 5,065,613 (Lehnert); 4,958,936 (Sakamoto et al.); and 4,464,725 (Briefer) describe electronic measurement systems which compensate for thermal drift, and other system inaccuracies, by utilizing a computer, and memory for storing known temperature behavior of a measurement system, at various calibration temperatures. That is, system inaccuracies due to temperature drift are recorded at specific calibration temperatures. This stored temperature behavior is then used to interpolate system inaccuracies due to thermal component drift at operational temperatures within the calibration range. This has been accomplished by using mathematical formulae to model thermal offset curves (e.g., using a parabolic interpolation, such as the LaGrange method, to plot offset curves, based upon discrete offset measurements, at discre
Bhat Aditya S
Milks, III William C.
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