Method and systems for filtering unwanted noise in a material metering machine

• US 6,774,822 B1
• Filed: 01/09/2003
• Issued: 08/10/2004
• Est. Priority Date: 01/09/2003
• Status: Active Grant

• Current Assignee: Process Control Corporation
• Original Assignee: Process Control Corporation
• Inventors: Thomson, Malcolm G.
• Primary Examiner(s): Jeanglaude, Jean
• Application Number:
  US10/340,310
• Time in Patent Office: 579 Days
• Field of Search: 341/155, 341/144, 341/61, 341/143, 375/346, 708/322
• US Class Current: 341/61
• CPC Class Codes: H03H 17/025 Notch filters
• CPC Class Codes: H03H 17/0664 where the output-delivery frequency is lower than the input sampling frequency, i.e. decimation

Abstract

Methods and systems for filtering an analog signal sampled at a very high frequency and outputting a digital signal that has a very low sampling frequency to drive a material metering machine. The high frequency digital input signal is input to a first decimation element, which filters out the noise in the signal introduced by an analog-to-digital (A/D) converter and reduces the sampling frequency of the digital signal to a lower sampling frequency of 1200 hertz. The reduced rate digital signal is input into a second decimation element that contains several decimation filters, which reject the 60 hertz line noise and its harmonics while simultaneously reducing the sampling frequency of the digital signal to 10 hertz. The output of the second decimation element is then passed to a bank of selectable filters with sub-hertz cutoff frequencies to remove the machine noise from the material metering machine.

Claims

1. An apparatus for receiving a digital signal having an initial sampling rate and converting the digital signal to an output digital signal having a reduced sampling rate, comprising:

a decimation element having a decimation ratio and a filter length operable for;

filtering out line noise at 50 Hz and 60 Hz frequencies generated by a line voltage from the digital signal; and

decimating the initial sampling frequency of the digital signal to the reduced sampling rate of the output digital signal,

wherein the reduced sampling frequency is equal to the initial sampling frequency divided by the fixed decimation ratio.

2. The apparatus of claim 1, wherein the initial sampling rate is 1200 Hz.

3. The apparatus of claim 2, wherein the reduced sampling rate is 10 Hz.

4. The apparatus of claim 3, wherein the decimation element comprises:

a first filter having a decimation ratio of 2 and a length of 20 for receiving the digital signal at the initial sampling frequency of 1200 Hz and outputting a digital signal having a sampling frequency of 600 Hz, such that the frequencies at integer multiples of 10 Hz are filtered out;

a second filter having a decimation ratio of 10 and a length of 20 operable for;

receiving the digital signal from the first filter; and

outputting a digital signal having a sampling frequency of 60 Hz such that the frequencies at integer multiples of 30 Hz are filtered out; and

a third filter having a decimation ratio of 6 and a length of 10 and operable for

receiving the digital signal from the second filter;

generating the output digital signal at the reduced sampling frequency of 10 Hz such that the frequencies at integer multiples of 10 Hz are filtered out.

5. The apparatus of claim 4, further comprising a first decimation element prior to the decimation element, comprising:

at least one decimation element having a fixed decimation element operable for;

receiving the digital signal from an analog-to-digital (A/D) converter;

filtering the noise from digital signal generated by the A/D converter; and

decimating the sampling frequency of the digital signal to the initial sampling frequency of 1200 Hz.

6. The apparatus of claim 1, further comprising:

an analog-to-digital (A/D) converter operable for receiving an analog data signal;

converting the analog signal into a digital signal at the initial sampling rate; and

inputting the digital signal to the decimation element.

7. The apparatus of claim 6, wherein the sampling frequency of the digital signal from the A/D converter falls within an operating range associated with the A/D converter.

8. The apparatus of claim 7, wherein the sampling frequency of the digital signal from the A/D converter unit is about 307.2 kHz.

9. The apparatus of claim 1, further comprising:

a selectable filter bank comprising a plurality of selectable filters,

wherein the plurality of selectable filters are operable for removing machine noise from a material metering machine connected to the decimation element; and

a selectable filter switch for selecting at least one of the plurality of selectable filters,

wherein the selectable filter switch is selected based on operating criteria associated with the material metering machine through a closed feed back loop.

10. The apparatus of claim 9, wherein the plurality of selectable filters comprises SINC filters having sub-hertz 3-dB cutoff frequencies.

11. The apparatus of claim 9, wherein the plurality of selectable filters comprises raised cosine filters, each raised cosine filter having a sub-hertz 3-dB cutoff frequency.

12. The apparatus of claim 9, wherein the plurality of selectable filters comprise a combination of SINC filters and raised cosine filters, each filter comprising a sub-hertz 3-dB cutoff frequency.

13. A method for receiving a digital signal having an initial sampling frequency and converting a digital signal to an output digital signal having a reduced sampling frequency, comprising:

filtering out line noise at 50 Hz and 60 Hz introduced by a line voltage from the digital signal; and

decimating the initial sampling frequency of the digital signal to the reduced sampling frequency of output digital signal,

wherein the reduced sampling frequency is equal to the initial sampling frequency divided by a fixed decimation ratio.

14. The method of claim 13, wherein the initial sampling frequency is 1200 Hz.

15. The method of claim 14, wherein the reduced sampling frequency is 10 Hz.

16. The method of claim 15, wherein decimating the signal comprises:

receiving the digital signal at the initial sampling frequency of 1200 Hz at a first filter having a length of 20 and outputting a digital signal having a sampling frequency of 600 Hz, thereby filtering out frequencies at integer multiples of 60Hz;

passing the digital signal from the first filter through a second filter having a length of 20 operable for;

outputting a digital signal having a sampling frequency of 60 Hz, thereby filtering frequencies at integer multiples of 30 Hz; and

passing the digital signal from the second filter through a third filter having a length of 6 and operable for;

generating the output digital signal at the reduced sampling frequency of 10 Hz; and

filtering frequencies at integer multiples of 10 Hz.

17. The method of claim 13, further comprising:

receiving an analog data signal from a load cell associated with a material metering machine; and

converting the analog signal into the digital signal with the initial sampling frequency.

18. The method of claim 17, further comprising:

receiving the digital signal from an analog-to-digital converter;

filtering the noise from digital signal generated by the A/D converter; and

decimating the sampling frequency of the digital signal to the initial sampling frequency of 1200 Hz.

19. The method of claim 18, wherein the sampling frequency of the digital signal from the A/D converter falls within an operating range associated with the A/D converter.

20. The method of claim 19, wherein the sampling frequency of the digital signal from the A/D converter is about 307.2 kHz.

21. The method of claim 18, further comprising:

selecting at least one selectable filter from a selectable filter bank having a plurality of selectable filters, the plurality of selectable filters being operable for removing machine noise from the material metering machine based on operating criteria associated with the material metering machine.

22. The method of claim 21, wherein the plurality of selectable filters are SINC filters having a sub-hertz 3-dB cutoff frequencies.

23. The method of claim 21, wherein the plurality of selectable filters comprise raised cosine filters, each raised cosine filter having a sub-hertz 3-dB cutoff frequency.

24. The method of claim 21, wherein the plurality of selectable filters comprise a combination of SINC filters and raised cosine filters, each filter having a sub-hertz 3-dB cutoff frequency.

25. A system for measuring flow rate in a material metering machine, comprising:

a weigh board for measuring the weight of the material in the material metering machine at a given time and converting the weight to an analog signal;

an analog front end operable to;

receive the analog signal from the weigh board; and

convert the analog signal to a digital signal having a first sampling frequency;

a digital signal processing (DSP) unit, comprising;

a first decimation element operable to;

decimate the sampling frequency of the digital signal from the first sampling frequency a second sampling frequency; and

filter out noise introduced by the conversion of the analog signal to a digital signal;

a second decimation element operable to;

decimate the second sampling frequency of the received signal by a second decimation ratio to a third sampling frequency; and

filtering out line noise injected into the signal by a line voltage; and

a selectable filter bank comprising a plurality of selectable filters, wherein each selectable filter has a sub-hertz 3-dB cutoff frequency to filter out noise from the material metering machine.

26. The system of claim 25, wherein the DSP unit further comprising a selectable filter switch for selecting at least one of the plurality of selectable filters based on operating criteria associated with the material metering machine through a closed feed back loop.

27. The system of claim 25, wherein the first decimation element comprises:

a first decimation filter comprising a filter length of 16 to decimate the first sampling frequency of the digital signal from the analog front end to an intermediate frequency; and

a second decimation filter comprising a filter length of 16 to decimate the intermediate sampling frequency to the second sampling frequency.

28. The system of claim 27, wherein the second decimation element comprises:

a first decimation filter having a first filter length to decimate first sampling frequency of the signal received from the first decimation element to a first intermediate sampling frequency while placing a null at every multiple of about 60 hertz to filter out line noise;

a second decimation filter having second filter length to decimate the first intermediate sampling frequency to a second intermediate sampling frequency while placing a null at integer multiples of about 30 hertz to filter out the line noise at 60 hertz;

a third decimation filter comprising a third filter length to decimate the second intermediate sampling frequency to the third sampling frequency while placing a null at integer multiples of about 10 hertz to filter out line noise associated with 60 hertz frequency and 50 hertz.

29. The system of claim 28, wherein the first sampling frequency is about 307.2 kilohertz.

30. The system of claim 28, wherein the second sampling frequency is about 1200 hertz and the third sampling frequency is about 10 hertz.

31. The system of claim 30, wherein the first filter length is about 20, the second filter length is about 20, and the third filter length is about 6.

32. The system of claim 28, wherein the decimation filters are finite impulse response (FIR) moving average filters.

33. The system of claim 28, wherein the plurality of selectable filters comprise SINC filters, each SINC filter having a sub-hertz 3-dB cutoff frequency.

34. The system of claim 28, wherein the plurality of selectable filters comprise raised cosine filters, each raised cosine filter having at sub-hertz 3-dB cutoff frequency.

35. The system of claim 28, wherein the plurality of selectable filters comprise a combination of SINC filters and raised cosine filters, each filter having a sub-hertz 3-dB cutoff frequency.

36. A selectable filter bank for use with a material metering machine to measure the flow rate of a material, the selectable filter bank receiving an input signal from a decimation element, the selectable filter bank comprising:

a plurality of selectable filters, wherein each selectable filter has a sub-hertz 3-dB cutoff frequency for filtering out the machine noise injected by the material metering machine.

37. The system of claim 36, wherein the plurality of selectable filters comprises SINC filters, each SINC filter having a sub-hertz 3-dB cutoff frequency.

38. The system of claim 36, wherein the plurality of selectable filters comprises raised cosine filters, each raised cosine filter having at sub-hertz 3-dB cutoff frequency.

39. The apparatus of claim 36, wherein the plurality of selectable filters comprises a combination of SINC filters and raised cosine filters, each filter having a sub-hertz 3-dB cutoff frequency.