Radar level gauging using frequency modulated pulsed wave

• US 9,513,153 B2
• Filed: 12/29/2011
• Issued: 12/06/2016
• Est. Priority Date: 12/30/2010
• Status: Active Grant

• Current Assignee: Rosemount Tank Radar AB (Emerson Electric Company)
• Original Assignee: Rosemount Tank Radar AB (Emerson Electric Company)
• Inventors: Jirskog, Anders, Edvardsson, Olov, Westerling, Jan
• Primary Examiner(s): McGue, Frank J
• Application Number:
  US13/976,567
• Publication Number:
  US 20140085132A1
• Time in Patent Office: 1,804 Days
• Field of Search: 342/124
• US Class Current: 1/1
• CPC Class Codes: G01F 23/284 Electromagnetic waves
• CPC Class Codes: G01S 13/26 wherein the transmitted pulses use a frequency- or phase-modulated carrier wave
• CPC Class Codes: G01S 13/88 Radar or analogous systems specially adapted for specific applications electromagnetic prospecting or detecting of objects, e.g. near-field detection, G01V3/00

Abstract

A method for measurement of a distance to a surface of a product kept in a tank method comprises transmitting a pulse train of distinct carrier wave pulses having a duration greater than 1 microsecond and shorter than 100 milliseconds, the pulse train has an average duty cycle of less than 50 percent, each pulse has a defined center frequency, selected according to a frequency scheme within a predetermined frequency range, greater than 5% of an average center frequency. The method further comprises correlating actual phase properties of received pulses with expected phase properties to provide an updated estimation of the distance.

The present invention is based on transmitting a set of carrier wave pulses, each having a distinct frequency selected within a frequency range. The method is therefore referred to as a Frequency Modulated Pulsed Wave (FMPW).

Claims

1. A method of providing low power Multiple Frequency Pulsed Wave measurement of a distance to a surface of a product kept in a tank, said method comprising:

transmitting low power electromagnetic transmit signals towards said surface,

receiving electromagnetic return signals reflected at said surface,

determining said distance based on an initially estimated distance and a relationship between said transmit signals and said return signals,

said transmit signals being formed as a pulse train of distinct carrier wave pulses having a duration greater than one microsecond and shorter than 100 milliseconds,

said pulse train having an average duty cycle of less than 50 percent,

each pulse having a defined center frequency, selected according to a frequency scheme within a predetermined frequency range,

said predetermined frequency range being greater than 5% of an average center frequency,

determining actual phase properties of each distinct pulse received in relation to each corresponding distinct pulse transmitted,

determining, based on said initially estimated distance, expected phase properties of each received distinct pulse in relation to each corresponding transmitted distinct pulse,

correlating said actual phase properties with said expected phase properties to provide an updated estimation of said distance, and

using a Frequency Modulated Continuous Wave (FMCW) frequency sweep distance measurement for at least one of verifying said updated estimation of said distance, correcting said updated estimation of said distance, and providing a new, initially estimated distance.

2. The method according to claim 1, wherein said initially estimated distance is acquired by an initial level measurement.

3. The method according to claim 1, wherein said correlating includes:

determining a distance offset of said distance based on said actual phase properties and said expected phase properties, and

determining said updated estimation of said distance based on said initially estimated distance and said distance offset.

4. The method according to claim 1, wherein said correlating includes determining a relationship between actual phase and carrier wave frequency.

5. The method according to claim 1, further comprising:

homodyne mixing the return signal with the transmit signal in a first channel (I-channel) and homodyne mixing the return signal with a 90°

phase shifted transmit signal in a second channel (Q-channel), to provide two amplitude values, and

calculating said phase properties of each distinct received pulse based on said amplitude values.

6. The method according to claim 1, further comprising:

determining a variance of said updated estimation, and

updating said frequency scheme to include a greater number of frequencies if said variance exceeds a predetermined variance threshold.

7. The method according to claim 1, further comprising frequency modulating each carrier wave pulse with a modulation frequency.

8. The method according to claim 7, further comprising:

homodyne mixing the return signal with the transmit signal in a first channel (I-channel) and homodyne mixing the return signal with a 90°

phase shifted transmit signal in a second channel (Q-channel), to provide two IF (intermediate frequency) signals,

filtering said IF signals to provide a filtered signal corresponding to a selected harmonic of said modulation frequency,

mixing the filtered signals of each channel with said selected harmonic of said modulation frequency, to provide two amplitude values (I and Q), and

calculating said phase properties of each distinct received pulse based on said amplitude values.

9. The method according to claim 7, wherein the frequency modulation is one of sinusoidal and triangular.

10. The method according to claim 7, wherein the modulation frequency is in the range 10 kHz-100 kHz.

11. The method according to claim 7, wherein the modulation introduces a variation of the carrier frequency in the range 1 MHz-100 MHz.

12. The method according to claim 1, wherein said frequency scheme is determined by:

defining N subsets of frequencies within the predetermined frequency range and randomly selecting one frequency from each subset.

13. The method according to claim 1, wherein each carrier wave frequency is controlled in a fed-back control loop providing a frequency accuracy deviation less than 1/1000.

14. The method according to claim 1, wherein said predetermined frequency range is greater than 10% of an average center frequency.

15. The method according to claim 1, wherein said duty cycle is less than 5%.

16. The method according to claim 1, wherein said updated estimation of said distance is provided on average more often than said Frequency Modulated Continuous Wave (FMCW) frequency sweep distance measurement.

17. The method according to claim 16, wherein said updated estimation of said distance is provided on average at a ratio to said Frequency Modulated Continuous Wave (FMCW) frequency sweep distance measurement greater than a ratio selected from a group consisting of:

10;

1, 50;

1, 100;

1, 500;

1, and 1000;

1.

18. The method according to claim 1, wherein each of said Frequency Modulated Continuous Wave (FMCW) frequency sweep distance measurements is inserted at a time determined by:

a. A predetermined time interval or intervals following the determination of the initially estimated distance;

b. A predetermined time interval or intervals following a preceding FMCW frequency sweep;

c. When an updated estimation of the distance has changed in relation to an initially estimated distance by a predetermined value or values;

d. When an updated estimation of the distance has changed in relation to a distance determined based on a preceding FMCW frequency sweep by a predetermined value or values;

e. A time interval determined by a user selecting a configuration of the radar level gauge directly (by selecting a time interval scheme) or indirectly (by selecting a typical measurement situation, such as slow change, rapid change, storage tank, etc, which gauge circuitry translates to a time interval scheme); and

/or

f. A time interval determined by a diagnostic function of the radar level gauge producing a tank situation as perceived by the radar level gauge.

19. A Multiple Frequency Pulsed Wave radar level gauging system, for detecting a distance to a surface of a product in a tank, said system comprising:

a transceiver for transmitting low power electromagnetic transmit signals and receiving electromagnetic return signals reflected at said surface,

processing circuitry for determining said distance based on an initially estimated distance and a relationship between said transmit signals and said return signals,

said transceiver being arranged to form said transmit signals as a pulse train of distinct carrier wave pulses having a duration greater than one microseconds and shorter than 100 milliseconds, said pulse train having an average duty cycle of less than 50 percent, each pulse having a defined center frequency, selected according to a frequency scheme within a predetermined frequency range, said predetermined frequency range being greater than 5% of an average center frequency,

said processing circuitry being arranged to;

determine actual phase properties of each distinct pulse received in relation to each corresponding distinct pulse transmitted,

determine, based on said initially estimated distance, expected phase properties of each received distinct pulse in relation to each corresponding transmitted distinct pulse,

correlate said actual phase properties with said expected phase properties to provide an updated estimation of said distance, and

use a Frequency Modulated Continuous Wave (FMCW) frequency sweep distance measurement for at least one of verifying said updated estimation of said distance, correcting said updated estimation of said distance, and providing a new, initially estimated distance.

20. The system according to claim 19, further comprising:

a first mixer for homodyne mixing the return signal with the transmit signal in a first channel (I-channel) to provide a first amplitude value, and

a second mixer for homodyne mixing the return signal with a 90°

phase shifted transmit signal in a second channel (Q-channel) to provide a second amplitude value,

wherein said processing circuitry is further arranged to calculate said phase properties of each distinct received pulse based on said first and second amplitude values.

21. The system according to claim 19, further comprising a modulator for frequency modulating each carrier wave pulse with a modulation frequency.

22. The system according to claim 21, further comprising:

a first mixer for homodyne mixing the return signal with the transmit signal in a first channel (I-channel) to provide a first IF (intermediate frequency) signal, and

a second mixer for homodyne mixing the return signal with a 90°

phase shifted transmit signal in a second channel (Q-channel) to provide a second IF (intermediate frequency) signal,

filters for filtering said first and second IF signals to provide a filtered signal corresponding to a selected harmonic of said modulation frequency, and

mixers for mixing the filtered signal of each channel with said selected harmonic of said modulation frequency, to provide two amplitude values (I and Q), and

wherein said processing circuitry is further arranged to calculate said phase properties of each distinct received pulse based on said amplitude values.

23. The system according to claim 21, wherein the modulator is arranged to provide one of sinusoidal and triangular frequency modulation.

24. The system according to claim 21, wherein the modulator is arranged to provide a modulation frequency in the range 10 kHz-100 kHz.

25. The system according to claim 21, wherein the modulator is arranged to introduce a variation of the carrier frequency in the range 1 MHz-100 MHz.

26. The system according to any claim 19, further comprising a fed-back control loop for controlling each carrier wave frequency to provide a frequency accuracy deviation less than 1/1000.

27. The system according to claim 19, wherein said predetermined frequency range is greater than 10% of an average center frequency.

28. The system according to claim 19, wherein said duty cycle is less than 5%.

29. The system according to claim 19, wherein said updated estimation of said distance is provided on average more often than said Frequency Modulated Continuous Wave (FMCW) frequency sweep distance measurement.

30. The system according to claim 19, wherein said updated estimation of said distance is provided on average at a ratio to said Frequency Modulated Continuous Wave (FMCW) frequency sweep distance measurement greater than a ratio selected from a group consisting of:

10;

1, 50;

1, 100;

1, 500;

1, and 1000;

1.

31. The system according to any claim 19, wherein each of said Frequency Modulated Continuous Wave (FMCW) frequency sweep distance measurements is inserted at a time determined by:

a. A predetermined time interval or intervals following the determination of the initially estimated distance;

b. A predetermined time interval or intervals following a preceding FMCW frequency sweep;

c. When an updated estimation of the distance has changed in relation to an initially estimated distance by a predetermined value or values;

d. When an updated estimation of the distance has changed in relation to a distance determined based on a preceding FMCW frequency sweep by a predetermined value or values;

e. A time interval determined by a user selecting a configuration of the radar level gauge directly (by selecting a time interval scheme) or indirectly (by selecting a typical measurement situation, such as slow change, rapid change, storage tank, etc, which gauge circuitry translates to a time interval scheme); and

/or

f. A time interval determined by a diagnostic function of the radar level gauge producing a tank situation as perceived by the radar level gauge.