Radar level gauging with detection of moving surface

• US 8,854,253 B2
• Filed: 09/27/2011
• Issued: 10/07/2014
• 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
• Primary Examiner(s): BYTHROW, PETER M
• Application Number:
  US13/246,261
• Publication Number:
  US 20130076559A1
• Time in Patent Office: 1,106 Days
• Field of Search: 342/118, 342/120, 342/124, 342127-144
• US Class Current: 342/124
• CPC Class Codes: G01F 23/284 Electromagnetic waves
• CPC Class Codes: G01S 13/36 with phase comparison between the received signal and the contemporaneously transmitted signal

Abstract

A radar level gauging principle including transmitting at least two time separated carrier wave pulses having equal carrier frequencies, determining a change in phase shift associated with two pulses in the transmit signals having equal frequencies, comparing the change with a threshold value, and depending on a result of the comparing step, determining the distance based on a relationship between the transmit signals and the return signals.

By comparing the actual phase resulting from two substantially identical pulses transmitted a certain time apart, an indication of a change of filling level can be obtained. This indication can then be used to initiate a full measurement cycle.

Claims

1. A method of determining a distance to a surface of a product kept in a tank, said method comprising the steps of:

transmitting electromagnetic transmit signals towards said surface,

receiving electromagnetic return signals reflected at said surface,

said transmit signals comprising at least two time separated carrier wave pulses having equal carrier frequencies, each pulse having a duration greater than 1 microsecond and shorter than 100 milliseconds,

determining actual phase properties of each pulse in the transmit signals in relation to each corresponding pulse in the return signals, said phase properties including a phase shift,

determining a change in phase shift associated with two pulses in the transmit signals having equal frequencies,

comparing said change with a threshold value, and

depending on a result of said comparing step, determining said distance based on a relationship between said transmit signals and said return signals.

2. The method according to claim 1, said method comprising operating in a first operating mode and a second operating mode, wherein said distance is determined in said first operating mode, wherein said phase shift change is determined in said second mode, and wherein said first operating mode is selected depending on a result of said comparing step.

3. The method according to claim 2, wherein an average transmitted power is lower in said second operating mode than in said first operating mode.

4. The method according to claim 2, said method comprising

transmitting, in said second operating mode, a plurality of time separated groups of pulses each including at least two pulses having different frequency, at least one pulse in each group of pulses having an equal frequency as a pulse in a preceding group of pulses, and

determining a plurality of phase shift changes relating to different pairs of pulses having equal frequency.

5. The method according to claim 4, at least one of the pulses in each group of pulses having a frequency which was not included in a preceding group of pulses.

6. The method according to claim 2, said method comprising

transmitting, in said first operating mode, a pulse train of distinct carrier wave pulses, said pulse train having an average duty cycle of less than 50 percent, each pulse having a duration greater than 1 microsecond and shorter than 100 milliseconds and 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, and

determining said distance by:

calculating, 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.

7. The method according to claim 2, said method comprising, in said first operating mode:

transmitting a train of short pulses, and

determining said distance based on the time of flight for each pulse.

8. The method according to claim 2, said method comprising, in said first operating mode:

transmitting a frequency sweep over a range of frequencies, and

determining said distance based on a relationship between the transmitted sweep and the reflected sweep.

9. The method according to claim 1, wherein said pulses having equal frequencies are time separated by at least 5 seconds.

10. The method according to claim 1, further comprising determining a plurality of phase shift changes before determining said distance.

11. 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.

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

a transceiver for transmitting electromagnetic transmit signals and receiving electromagnetic return signals reflected at said surface, said transceiver being arranged to transmit signals comprising at least two time separated carrier wave pulses having equal frequencies, each pulse having a duration greater than 1 microsecond and shorter than 100 milliseconds,

processing circuitry configured to;

determine actual phase properties of each pulse in the transmit signals in relation to each corresponding pulse in the return signals, said phase properties including a phase shift,

determine a change in phase shift associated with two pulses in the transmit signals having equal frequencies,

compare said change with a threshold value, and

depending on a result of said comparing step, determine said distance based on a relationship between said transmit signals and said return signals.

13. The system according to claim 12, wherein said transceiver circuitry and said processing circuitry are adapted to operate in a first operating mode and a second operating mode, wherein said distance is determined in said first operating mode, wherein said phase shift change is determined in said second mode, and

wherein said processing circuitry is configured to select said first operating mode depending on a result of said comparing step.

14. The system according to claim 12, wherein said transceiver circuitry is adapted to transmit a lower average transmitted power in said second operating mode than in said first operating mode.

15. The system according to claim 13, wherein said transceiver circuitry in said first operating mode is adapted to transmit a pulse train of distinct carrier wave pulses, said pulse train having an average duty cycle of less than 50 percent, each pulse having a duration greater than 1 microsecond and shorter than 100 milliseconds and 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, and

wherein said processing circuitry in said first mode is adapted to determine said distance by:

calculating, 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.

16. The system according to claim 13, wherein said transceiver circuitry in said first operating mode is adapted to transmit a train of short pulses, and wherein said processing circuitry is adapted to determine said distance based on the time of flight for each pulse.

17. The system according to claim 13, wherein said transceiver circuitry in said first operating mode is adapted to transmit a frequency sweep over a range of frequencies, and wherein said processing circuitry is adapted to determine said distance based on a relationship between the transmitted sweep and the reflected sweep.

18. The system according to claim 12, further comprising a fed-back control loop for controlling each carrier wave frequency to provide a frequency accuracy deviation less than 1/1000.

19. The system according to claim 12, wherein a time separation between pulses having equal frequencies is at least 5 seconds.