Electricity: measuring and testing – Impedance – admittance or other quantities representative of... – Using a particular bridge circuit
Reexamination Certificate
1998-12-01
2001-03-20
Metjahic, Safet (Department: 2858)
Electricity: measuring and testing
Impedance, admittance or other quantities representative of...
Using a particular bridge circuit
C324S651000
Reexamination Certificate
active
06204673
ABSTRACT:
TECHNICAL FIELD
The invention relates to impedance measuring, and, more particularly, to method and apparatus for measuring impedance using a bridge circuit with digital signal processing and, even more particularly, to method and apparatus for obtaining accurate relationships between signals used in ratio transformer capacitance bridge circuits.
CROSS REFERENCE TO RELATED PATENT
Reference is made to U.S. Pat. No. 4,772,844, the entire disclosure of which hereby is incorporated by reference.
BACKGROUND OF THE INVENTION
This invention relates to the measurement of electrical impedance, and in particular to the measurement of the loss and the very precise measurement of the capacitance of an unknown impedance where “loss” is used as a collective term to mean resistance, conductance, dissipation factor or any other term used to describe the real component of impedance. The technical literature is replete with numerous examples of impedance bridges of all kinds. These loss terms If are used equivalently and interchangeably below. Bridges have been in a state of continuous development and improvement for more than a century. Improvements have taken almost every conceivable form, in efforts to achieve higher accuracy, lower cost, better reliability, higher speed, wider range, etc. Some high performance bridges have been automated with the incorporation of microprocessors or related devices to allow these bridges not only to correct for various measurement errors, but to report their measurement results on sophisticated local displays or remotely via several different kinds of communication channels. Sufficient programming control is often provided to allow for sustained unattended operation.
In spite of the considerable attention given to impedance bridges in general, not all areas of bridge development have benefitted from new ideas, particularly in the application of microprocessors and digital signal processors (sometimes referred to below as DSP). Ratio transformer bridges have been used for high precision measurements of capacitance and loss. Several examples of ratio transformer bridges include those disclosed in U.S. Pat. No. 4,772,844. U.S. Pat. No. 4,772,844 describes a method for producing very accurate magnitude and phase relationships between two sinusoidal signals generated by two ratio transformers. Such signals are useful in a particular kind of ratio-transformer capacitance bridge where instead of balancing the resistive component of the unknown impedance against a reference resistor, it is instead, balanced against a reference capacitor driven by a 90° phase-shifted signal.
SUMMARY OF THE INVENTION
A new method described herein is an improvement to the method described in U.S. Pat. No. 4,772,844. The new method has the major advantages over the old of being inexpensive and reliable to implement over a wide range of frequencies. It has a further major advantage over the old since it can continuously correct the relationships between two sinusoidal signals in real time.
An aspect of the invention is to produce from independent ratio transformers two sinusoidal signals whose magnitudes and relative phase are accurately related. It is a straightforward problem to program a digital signal processor (sometimes referred to below as a DSP) to produce very accurate sinusoidal waveforms as a function of time and to use these waveforms to drive digital-to-analog converters (sometimes referred to below as D/A or in the plural as D/A's) which will produce signals that can drive one or more ratio transformers. The difficulty with this is that the conversion and driving circuits will introduce gain and phase errors so that the analog signals coming out of the ratio transformers deviate unacceptably from the digital signals produced within the DSP. What is worse is that these errors will change over time as a result of changes in temperature, changes in components, and changes in the loading of the ratio transformers. It is an aspect of the invention to correct these errors in real time.
The new method uses a DSP fast enough to handle the desired frequencies. It also uses analog-to-digital converters (sometimes are referred to below as A/D or in the plural as A/D's) and D/A's with adequate resolution and employs a technique to increase precision.
In U.S. Pat. No. 4,772,844 it is an object to measure impedance, and particularly capacitance, to extremely high precision at an improved speed and ease of use. It is an aspect of the present invention to measure such impedance at multiple frequencies. It also is desirable to measure such impedance using DSP technology for signal generation, control purposes, and the like.
In U.S. Pat. No. 4,772,844 is disclosed a ratio transformer capacitance bridge useful at herein, including solid state ratio transformer driven, multiplying digital to analog converters (hereinafter referred to as SSRTMDACs) which can help measure capacitance and loss rapidly, with low cost and good reliability.
Various objectives of U.S. Pat. No. 4,772,844 also are applicable to respective features of the present invention.
It would be desirable to be able to measure loss at more than one frequency and, especially, to measure loss at multiple frequencies (sometimes referred to below as multi-frequencies), for example at discrete frequencies and/or over a continuous frequency range, e.g., over a continuous range or spectrum of frequencies. It would be desirable to measure loss using techniques of U.S. Pat. No. 4,772,844, for example. It also would be desirable to make such measurements using signals in a ratio transformer type bridge circuit wherein the relationship, such as the amplitude and phase relation, between plural respective signals output by the ratio transformers can be accurately controlled and/or corrected to maintain a desired relationship. In an exemplary embodiment described in detail below, two ratio transformers are used, and the relationship between the output signals produced thereby is equal amplitude and 90° phase shift; however, it will be appreciated that the objectives and features of the invention include maintaining different relationships between such signals.
Another aspect of the invention relates to the use of DSP technology to develop signals such as sinusoidal signals for driving ratio transformers in a ratio transformer capacitance bridge circuit and, based on appropriate feedback, to control those sinusoidal signals to maintain a desired relationship between the outputs produced by the ratio transformers.
Another aspect of the invention relates to the use of commutation technique to average feedback signals representing outputs from respective ratio transformers enabling the use of relatively inexpensive and low resolution components, such as D/A's and A/D's in the driving and feedback circuits of the ratio transformer capacitance bridge while maintaining high precision, for example, including resolution, accuracy and stability. For example, precision may include resolution, accuracy and stability. One can put a specific number or value on resolution, accuracy; and stability; resolution, accuracy and stability may be components of the term or concept of precision.
Another aspect of the invention is to use conventional, relatively economical parts to generate accurately plural sinusoidal signals that have a desired amplitude and phase relationship, such as, for example, two sinusoidal signals that are of equal amplitude and 90° out of phase.
Another aspect of the invention is to generate plural signals having a desired amplitude and phase relationship, such as, for example, two signals that are of equal amplitude and out of phase by 90° or some other desired relationship, with high precision using DSP and A/D and D/A techniques.
Another aspect is to measure loss, such as, for example, dissipation factor, of an unknown capacitance with high precision, for example, with resolution at least to 0.01%, more preferably so the resolution is one part in 100,000, even more preferably so that accuracy is to one part in 30
Andeen Carl G.
Flocke Stephen E.
Hagerling Carl W.
Andeen-Hagerling, Inc.
Kerveros J.
Metjahic Safet
Renner , Otto, Boisselle & Sklar, LLP
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