Reduced noise sensitivity in magnetic flowmeter
A magnetic flowmeter for measuring flow of a process fluid includes a coil configured to apply a magnetic field to the process fluid. Electrodes are arranged to sense a fluid voltage signal related to the applied magnetic field and fluid flow. An analog to digital converter provides a digital output comprising a plurality of digital samples related to the sensed fluid voltage signal. Computation circuitry receives the digitized samples and responsively provides an output related to flow.
1. A magnetic flowmeter for measuring flow of a process fluid, comprising:
a coil configured to apply a magnetic field to the process fluid;
a coil driver configured to apply a coil drive signal to the coil;
electrodes arranged to sense a fluid voltage signal related to the applied magnetic field and fluid flow;
an analog to digital converter having a digital output comprising a plurality of digital samples related to the sensed fluid voltage signal;
digital computation circuitry configured to receive the digitized samples, filter the samples in accordance with a non-linear filter implemented in software, and responsively provide an output related to flow;
wherein the digital computation circuitry inverts at least a portion of the digitized sensed fluid signal; and
the portion comprises a trailing portion of a half cycle of the fluid voltage signal.
2. The apparatus of claim 1 wherein the computation circuitry comprises software implemented in a microprocessor system.
3. The apparatus of claim 1 wherein the non-linear filter comprises a median filter.
4. The apparatus of claim 1 wherein the portion comprises 20% of a half period of the fluid voltage.
5. The apparatus of claim 1 wherein the inverting is synchronized with the coil drive signal.
6. The apparatus of claim 1 wherein the computation circuitry calculates an average of a portion of the digital samples.
7. The apparatus of claim 6 wherein the average is taken over a period of the coil drive signal.
8. The apparatus of claim 1 including a low pass filter configured to filter the digital samples.
9. A magnetic flowmeter for measuring flow of a process fluid, comprising:
a coil configured to apply a magnetic field to the process fluid;
a coil driver configured to apply a coil drive signal to the coil;
electrodes arranged to sense a fluid voltage signal related to the applied magnetic field and fluid flow;
an analog to digital converter having a digital output comprising a plurality of digital samples related to the sensed fluid voltage signal;
computation circuitry configured to receive the digitized samples and provide an output related to flow by inverting using software at least a portion of the digitized sensed fluid voltage signal; and
the portion comprises a trailing portion of a half cycle of the fluid voltage signal.
10. The apparatus of claim 9 wherein the computation circuitry is configured to filter the samples in accordance with a non-linear filter.
11. The apparatus of claim 9 wherein the computation circuitry comprises software implemented in a microprocessor system.
12. The apparatus of claim 9 wherein the non-linear filter comprises a median filter.
13. The apparatus of claim 9 wherein the portion comprises 20% of a half period of the fluid flow signal.
14. The apparatus of claim 9 wherein the inverting is synchronized with the coil drive signal.
15. The apparatus of claim 9 wherein the computation circuitry calculates an average of a portion of the digital samples.
16. A method for measuring flow in a process fluid, comprising:
applying a coil drive signal to a coil proximate the process fluid and thereby applying a magnetic field to the process fluid;
sensing a fluid voltage signal in the process fluid related to the applied magnetic field and to fluid flow;
digitizing samples of the voltage signal;
digitally filtering samples of the digitized voltage signal using a non-linear filter implemented in software;
including inverting at least a portion of the digitized sensed fluid flow signal;
the portion comprising a trailing portion of a half cycle of the fluid voltage signal; and outputting flow based upon the filtered samples.
17. The method of claim 16 wherein the non-linear filter comprises a median filter.