Data processing: measuring – calibrating – or testing – Calibration or correction system – Sensor or transducer
Reexamination Certificate
1998-04-03
2001-11-20
Hoff, Marc S. (Department: 2857)
Data processing: measuring, calibrating, or testing
Calibration or correction system
Sensor or transducer
C702S090000
Reexamination Certificate
active
06321171
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to correcting measurements conducted through a probe or transducer accessory to an electrical measurement instrument, and more particularly to an improved method for correcting for offset, gain, and linearity of such instrument accessories.
CROSS-REFERENCE TO RELATED APPLICATIONS
[Not applicable]
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT
[Not applicable]
BACKGROUND OF THE INVENTION
It has long been known to correct for probe, or transducer, offset and gain errors with linear equations of the form: S=As+B, where “S” is the corrected signal, “s” is the uncorrected signal, and A and B are correction coefficients. It has also been known to correct for offset, gain, and linearity errors with a quadratic equation of the form: S=Cs
2
+Ds+E, where again “S” is the corrected signal, “s” is the raw data signal, and C, D, and E are correction coefficients. It has even been known to correct for offset, gain, linearity, and higher order non-linearities by the use of third order equations: S=Fs
3
+Gs
2
+Hs+I, where again “S” is the corrected signal, “s” is the uncorrected signal, and F, G, H, and I are the correction coefficients.
Such first, second, and third order correction equations, and the coefficients used with them, are fairly successful at reducing overall error in the corrected signal “S”, and increasingly so as additional terms of higher order are employed. They are, however, less effective at dealing with “near-zero” errors, particularly when such errors are expressed as a percentage of the measured value. As the measured value approaches zero, any error that remains becomes much larger by comparison with what is being measured.
For example, if the goal is to measure 150 amperes, and the results are subject to an error of +/−1.0 A, the percentage error is only 0.67%. However, if the goal is to measure 50 amps, and the results are subject to an error of +/−0.5 A, the percentage error increases to 1.0%, even though the absolute error is only half as much as it was at 150 A. Further illustrating this problem, if the goal is to measure only 5.0 A, and the absolute error decreases to 0.2 A, the percentage error nonetheless rises to 4%. Thus, it becomes apparent that offset errors, or apparent offset errors, overtake gain and linearity as the source of percentage error as the quantity being measured approaches zero. By “apparent offset” errors, we include those which arise from the “best fit” process by which the calibration coefficients are derived from the errors in uncorrected data.
Typically, electronic measurement instrument vendors must include two components in accuracy specifications, a constant and a percentage of the measurement result. This dictates that guaranteed accuracy is subject to plus or minus errors that are the greater of a constant and a percentage. What is desired is a way to reduce the “near-zero” or “apparent offset” contribution to residual errors after a correction computation has been performed.
BRIEF SUMMARY OF THE INVENTION
In accordance with the present invention, a method for correcting errors in the offset, gain, and linearity of an accessory transducer for an electronic measurement instrument utilizes an error correction equation wherein at least one of the terms has an exponent of less than one. The correction equation has the general form S=Cs2+Bs+A+b|s|
x
, where x, the exponent of s in the any terms to the right of the offset constant, A, is less than one, i.e., 0<x<1. In a simple, and therefore preferred, embodiment x=0.5.
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Dally et al, Instrumentation for Engineering Measurements. John Wiley & Sons, Inc., pub. 1993 pp 14-20.
Bucher William K.
Griffith Boulden G.
Hoff Marc S.
Miller Craig Steven
Tektronix Inc.
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