Pulse or digital communications – Systems using alternating or pulsating current
Reexamination Certificate
1999-04-20
2001-09-04
Chin, Stephen (Department: 2634)
Pulse or digital communications
Systems using alternating or pulsating current
C375S268000, C375S316000, C375S320000, C329S361000, C329S362000
Reexamination Certificate
active
06285719
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a method and a system for an improved processing of measuring signals from a sensing transducer, particularly for application to sensing transducers utilizing an excitation AC signal which will be modulated by the sensor.
BACKGROUND
Demodulation of an amplitude-modulated sensor signal from, for instance transducers like a Pressductor© or a LVDT (Linear Variable Differential Transformer), is normally realized using analog techniques and implemented by phase sensitive rectification followed by low-pass filtering. The low-pass filtering is necessary for removing frequency components of higher order (at the excitation frequency and around its overtones) which are introduced by the phase sensitive rectification. In most applications today there is a demand that the signal processing should deliver a digital signal output and be able to communicate with a parent control computer. In many cases the sensor is part of a regulation system.
Such a regulation system is normally accomplished by including a microprocessor in the transducer electronic circuitry. Instead of having the usual analogue demodulation there are many advantages in also introducing digital techniques within this processing stage. The electronic device will for instance contain less components, be more easy to miniaturize and therefore less costly to produce. Besides, the same hardware may be used for several excitation frequencies as the necessary low-pass filtering then is made by means of software control
The trivial realization of the analogue solution by means of digital techniques according to the state of the art consists of sampling the transducer signal with a high sampling frequency, rectifying the signal in a phase sensitive way and then achieving low-pass filtering by using a digital filter. Due to the high sampling frequency this method first of all demands a high calculation capacity which must be implemented with a fast digital signal processor (DSP).
A phase sensitive rectification of the periodic quantity to be measured will normally be realized utilizing a phase-locked sampling of the periodic quantity. An example of such a sampling solution is for instance disclosed in U.S. Pat. No. 4,646,004 by the assignee of the present invention.
Another U.S. Pat. No. 5,055,843 discloses a sigma delta modulator with distributed pre-filtering and feedback. An additional filter used allows control of the quantization noise transfer function profile independent of the forward signal transfer function. However, this technical solution involves quite a complicated circuitry for the reduction of the quantization noise and does still not practically solve the technical problems discussed above.
Consequently, there is still a call for simplifying the processing of sensor signals from AC driven transducers to ensure a simple and low cost build-up as well as presenting a reliable operation offering a high measurement resolution. A technique considering an improved method as described below presents a new inventive solution to the problem.
SUMMARY
The present invention relates to an arrangement for a phase sensitive rectification of the sensor signal using a method, which demands a very moderate calculation capacity to thereby facilitate the use of ordinary microprocessors. In addition the method makes it possible to emulate the behavior of older analog signal processing systems, which can therefore be replaced without the need to replace or recalibrate the sensors. The present method and system utilize the fact that the sampling is synchronized to the excitation frequency, f, of the sensor and the phase sensitive rectification is obtained by first sampling the sensor signal at a high sampling frequency, nf. The signal, sampled by means of an A/D-converter controlled by the sampling frequency nf, then is averaged over half a period of the excitation signal frequency, whereby the starting point for the averaging is decided by a synchronizing signal corresponding to the time of commutation. This is equivalent to filtering the sampled signal by a decimating filter with notches at even multiples of the excitation frequency. The rectification may then be performed with a number sequence sampled by the optimal sampling frequency 2·f (twice the excitation frequency) instead of the sampling frequency n·f used for obtaining high resolution. The output then is further filtered by means of a digital filter for achieving a desired output voltage frequency response. The signal output will be easily available for further processing. However, the output still offers high resolution measurements in spite of only demanding moderate speed microprocessor capacity for the further processing, as well as a low resolution A/ D-converter.
According to one further embodiment of the method the average of the decimated signal before the phase sensitive rectification is computed by a suitable averaging procedure. This averaging procedure may involve some sort of digital low-pass filtering with a long time constant corresponding to the time constant of changes in an offset voltage. The average is the subtracted from the decimated signal in order to eliminate the influence of a possible offset.
According to still a further embodiment the method uses a digital notch filter applied to the decimated signal after the rectification. Since the ripple due to demodulated DC offsets appears exactly at the excitation frequency, a running average of two values completely eliminates the offset. The method according to the present invention is set forth by the independent claim
1
and the dependent claim
2
-
10
. Further a system according to the present invention is set forth by the independent claim
11
and further embodiments of the system are set forth by the dependent claims
12
-
20
.
REFERENCES:
patent: 3793592 (1974-02-01), Matonak et al.
patent: 3795862 (1974-03-01), Fletcher et al.
patent: 4430620 (1984-02-01), Fisher et al.
patent: 4646004 (1987-02-01), Brandt et al.
patent: 4999831 (1991-03-01), Grace
patent: 5036289 (1991-07-01), Duran
patent: 5055843 (1991-10-01), Ferguson, Jr. et al.
patent: 5477473 (1995-12-01), Mandl et al.
patent: 5742132 (1998-04-01), Huber et al.
patent: 5940447 (1999-08-01), Connell et al.
The Communications Handbook, Boca Raton, FL, CRC Press, 1997. pp. 3-5. TK5101.C6583 1996.
ABB AB
Chin Stephen
Dykema Gossett PLLC
Rupert Paul N.
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