MFPW radar level gauging with distance approximation

• US 8,730,093 B2
• Filed: 09/27/2011
• Issued: 05/20/2014
• Est. Priority Date: 09/27/2011
• Status: Active Grant

• Current Assignee: Rosemount Tank Radar AB (Emerson Electric Company)
• Original Assignee: Rosemount Tank Radar AB (Emerson Electric Company)
• Inventors: Edvardsson, Olov, Jirskog, Anders
• Primary Examiner(s): Sotomayor, John B
• Assistant Examiner(s): WINDRICH, MARCUS E
• Application Number:
  US13/246,265
• Publication Number:
  US 20130076560A1
• Time in Patent Office: 966 Days
• Field of Search: 342/118, 342/124
• US Class Current: 342/124
• 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

Level gauging including an approximation of a distance to the surface of a product kept in a tank. The approximation is determined by relating an amplitude of a first harmonic of an IF signal with an amplitude of a second harmonic of the IF signal.

Basically, each harmonics represents a given distance range. By determining the received power in two or more harmonics, and correlating them to each other, the distance may be estimated. Depending on modulation and other parameters the distance dependence may be very different, and may be selected to suit the application.

Claims

1. A method of using a radar level gauging system to determine a distance to a surface of a product kept in a tank, said method comprising:

forming a transmit signal as a pulse train of distinct carrier wave pulses having a duration greater than 1 microsecond and shorter than 100 milliseconds, each pulse being frequency modulated around a defined center frequency,

transmitting said transmit signal towards said surface,

receiving an electromagnetic return signal reflected at said surface,

mixing the return signal with the transmit signal in a first channel (I-channel) and 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 first selected harmonic of said modulation frequency,

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

calculating (step S5) by means of processing circuitry actual phase properties of each distinct pulse received in relation to each corresponding distinct pulse transmitted, based on said primary amplitude values and determining a distance measure based on said actual phase properties,

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

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

providing an approximation of said distance based on a relationship between said primary and said secondary amplitude values.

2. The method according to claim 1, wherein said relationship is a quotient between said primary and secondary amplitude values.

3. The method according to claim 1, wherein said approximation is used as input in a process to obtain a more accurate distance measure.

4. The method according to claim 1, wherein said approximation is used to verify a more accurate distance measure.

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

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

7. The method according to claim 6, wherein the frequency modulation is one of sinusoidal and triangular.

8. The method according to claim 6, wherein the modulation frequency is in the range 10 kHz-100 kHz.

9. The method according to claim 6, wherein the modulation introduces a variation of the carrier frequency in the range 1 MHz-100 MHz.

10. The method according to claim 1, further comprising accurately determining said distance by:

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

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

11. The method according to claim 10, 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.

12. The method according to claim 1, wherein the center frequency of each pulse is selected according to a frequency scheme within a predetermined frequency range.

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 12, wherein said predetermined frequency range is greater than 5% of an average center frequency.

15. The method according to claim 12, wherein said predetermined frequency range is greater than 10% of an average center frequency.

16. The method according to claim 1, wherein said pulse train has an average duty cycle of less than 50 percent.

17. The method according to claim 16, wherein said duty cycle is less than 5%.

18. A radar level gauging system, for determining a distance to a surface of a product kept in a tank, said system comprising:

a transceiver for transmitting electromagnetic transmit signals formed by a pulse train of distinct carrier wave pulses having a duration greater than 1 microsecond and shorter than 100 milliseconds, each pulse being frequency modulated around a defined center frequency, and receiving electromagnetic return signals reflected at said surface,

a set of RF mixers for mixing the return signal with the transmit signal in a first channel (I-channel) and mixing the return signal with a 90°

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

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

a first set of IF mixers for mixing the filtered signals of each channel with said first selected harmonic of said modulation frequency, to provide two primary amplitude values (I and Q),

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

a second set of IF mixers for mixing the filtered signals of each channel with said second selected harmonic of said modulation frequency, to provide two secondary amplitude values (I and Q), and

processing circuitry connected to receive outputs from said IF mixers and operable to;

calculate actual phase properties of each distinct pulse received in relation to each corresponding distinct pulse transmitted, based on said primary amplitude values and determining a distance measure based on said actual phase properties, and

providing an approximation of said distance based on a relationship between said primary and said secondary amplitude values.

19. The system according to claim 18, wherein said transceiver is configured to frequency modulate each carrier wave pulse with a random modulation.

20. The system according to claim 18, wherein said transceiver is configured to frequency modulate each carrier wave pulse with a periodic modulation having a modulation frequency.

21. The system according to claim 20, wherein the frequency modulation is one of sinusoidal and triangular.

22. The system according to claim 18, wherein said transceiver incldues a fed-back control loop providing a frequency accuracy deviation less than 1/1000.

23. The system according to claim 18, wherein the transceiver is configured to select the center frequency of each pulse according to a frequency scheme within a predetermined frequency range.

24. The system according to claim 23, wherein said transceiver is configured to said predetermined frequency range is greater than 10% of an average center frequency.

25. The system according to claim 18, wherein the transceiver is configured to control a duty cycle of said pulse train to be less than 50 percent.

26. The system according to claim 25, wherein the transceiver is configured to control a duty cycle of said pulse train to be less than 5 percent.