Data processing: measuring – calibrating – or testing – Measurement system – Measured signal processing
Reexamination Certificate
1998-05-26
2001-04-24
Assouad, Patrick (Department: 2837)
Data processing: measuring, calibrating, or testing
Measurement system
Measured signal processing
C702S042000, C073S001150
Reexamination Certificate
active
06223138
ABSTRACT:
PRIORITY CLAIM
This application is based on and claims the priority under 35 U.S.C. §119 of German Patent Application 197 22 077.0, filed on May 27, 1997.
FIELD OF THE INVENTION
The invention relates to a method and apparatus for measuring the output of a sensor or pick-up transducer in a bridge circuit using a carrier frequency.
BACKGROUND INFORMATION
Various methods are known for measuring or evaluating the measurement output of foil strain gage transducers arranged in a bridge circuit. However, difficulties arise in such measuring methods because the strain gage transducer bridge circuit typically outputs a measurement voltage of only a few millivolts, even under maximum load, whereby this small measurement voltage must be amplified in corresponding measuring amplifiers for further processing, evaluation and transmission. In order that these analog measurement values can be transmitted and evaluated in microprocessor circuits with the lowest possible distortion or interference, the analog output signals are usually converted into digital measurement values, which are then provided to a microprocessor for further processing.
Such a typical measuring method for use in connection with a carrier frequency measuring amplifier, with transducers arranged in a bridge circuit, is described in M. Kochsiek, “Handbuch des Waegens” (Hand Book of Weighing), 2nd Edition, Braunschweig, Germany, 1989, page 551. The circuit and method according to this publication include an oscillator that generates an alternating voltage having a carrier frequency, which in turn is modulated with a measurement value by the transducer circuit. The resulting amplitude modulated carrier is then amplified in an input amplifier. This input amplifier is selective and transmits only frequencies in the range of the carrier frequency. The amplified carrier voltage is then demodulated, the remaining signal components of the carrier are filtered out, and the resulting measurement signal is finally provided to an output amplifier, which outputs the amplified measurement signal.
In order to allow further measured value transmission and processing in a microprocessor system, the above described analog output signal would have to be converted to a corresponding digital signal in an analog-to-digital converter. The demodulation carried out before the analog-to-digital conversion is typically carried out by means of a phase controlled rectifier, which always necessarily involves a loss or degradation of information, which in turn makes it difficult to achieve an exact calibration and evaluation of the measurement results.
German Patent Laying-Open Publication 4,417,228 (Altwein) also describes a carrier frequency measuring method with measuring transducers in a bridge circuit. According to this reference, before transmission of the measured value, the measurement signal is modulated by means of a modulation amplifier, amplified, and then again demodulated by means of a phase controlled bridge rectifier. However, in this process it must first be ensured before carrying out the demodulation, that the carrier frequency applied to the bridge is exactly in phase with the measurement voltage. For this reason, any existing phase differences are compensated or tuned-out by means of a manual phase compensation, upon beginning the signal processing. Since in practice new phase shifts arise over time, and such phase shifts lead to measurement inaccuracies, it is necessary to repeat the above-mentioned manual phase compensation measures at certain specified time intervals.
SUMMARY OF THE INVENTION
In view of the above it is the aim of the invention to provide a carrier frequency measuring method of the above described general type, which is improved in such a manner as to increase the measurement accuracy and to ensure that the measurement accuracy remains constant over time. The invention further aims to avoid or overcome the other disadvantages of the prior art, and to achieve additional advantages, as apparent form the present description. The invention also aims to provide an apparatus circuit arrangement for carrying out the method and achieving the previously mentioned objects.
The above objects have been achieved in a measuring method according to the invention, for measuring the output of transducers in a bridge circuit. An alternating voltage is applied as a carrier voltage to the measuring bridge, and the measuring bridge modulates the carrier with a measurement value to produce a modulated measurement voltage. This modulated measurement voltage, which accordingly contains or comprises the measurement value, is then further processed for determining or evaluating the measurement value. Particularly, the modulated measurement voltage is directly provided to an analog-to-digital converter which operates at a sampling rate that is at least twice the carrier frequency. Then, the digitalized measurement voltage is provided to a digital signal processor, which carries out a Fourier transform via hardware and/or software so as to calculate or determine the magnitude and the phase of a resultant measurement signal from the digitalized measurement voltage.
The above objects have further been achieved by a circuit arrangement according to the invention, especially for carrying out the above method. The circuit arrangement includes a signal generator for generating an alternating voltage as a carrier, which is connected to the input of the transducer bridge circuit. The output of the transducer bridge circuit is connected through an amplifier circuit directly to an analog-to-digital converter, which in turn is further connected to a digital signal processor. According to particular preferred features of the invention, the sampling rate of the analog-to-digital converter is a whole number multiple of the carrier frequency. Furthermore, a digital reference signal may be formed by analog-to-digital conversion from the analog input voltage of the bridge circuit, whereby this digital reference signal may be taken into account for calculating the magnitude and the phase of the measurement voltage. The circuit arrangement may be embodied according to the four-wire connection technology or the six-wire connection technology for the connection between the bridge circuit and the measuring circuit. The amplifier circuit, the converter circuits, and the digital signal processor may all be integrated in a transducer housing.
A major advantage of the invention is that it does not require demodulation of the modulated signal, because of the employed direct digitalization. For this reason, it is simultaneously possible to avoid the need for costly and complicated narrow band filters that are typically required after the demodulation in the prior art. Moreover, a necessary taring or zero point compensation can be carried out by a complex analysis, i.e. using complex values, by means of a microprocessor using the digitalized measurement values, whereby the measurement accuracy can be maintained.
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patent: 5088330 (1992-02-01), Talmadge
patent: 5668480 (1997-09-01), Nintzel
patent: 5777235 (1998-07-01), Altwein
patent: 5821417 (1998-10-01), Naruo et al.
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patent: 4417228A1 (1995-11-01), None
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“Handbuch des Wägens” (Hand Book of Weighing), 2nd Edition; Braunschweig, Germany, 1989, p. 551, M. Kochsiek.
“Für hochgenaues Messen”, by Reinhard Bertermann, 312 Elektrotechnik 68, Nov. 25, 1986, No. 20, Würzburg, W. Germany; pp. 551.
Pitz Juergen
Raffius Gerhard
Sonnenschein Roland
Assouad Patrick
Carl Schenck AG
Fasse W. F.
Fasse W. G.
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