Stabilization in a radar level gauge
A radar gauge adapted to sense fluid level in a tank and including a radar gauge circuit in which radar transmission and level sampling are controlled by a transmit frequency and a sample frequency respectively. A first frequency separation between first and second frequencies is controlled by a control input. The first and second frequencies can be divided to generate the transmit and sample frequencies, separated by a second frequency separation. At least one frequency difference is evaluated and the evaluation used to generate the control input, stabilizing the first frequency difference, and to correct the gauge output.
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Pulsed time-of-flight radar gauges are utilized for the measurement of level in process storage tanks. These gauges are mounted on the top of storage tanks, and transmit a pulse of microwave energy toward the surface of the product being stored in the tank. The gauge then receives energy which is reflected from the surface of the product, and calculates the level of the product based upon the time of flight of the pulse.
A PRIOR ART circuit 110 for creating the transmit and sample clocks is shown in FIG. 2. Circuits of this type are described in U.S. Pat. No. 5,563,605 McEwan. This circuit has the advantage that only one oscillator 112 is required, as the sample clock 114 is generated by continuously increasing phase delay in a variable delay generator 116 controlled by a delay control ramp 118. The phase delay circuit can be designed to be repeatable, therefore, errors due to changes in the difference frequency are reduced using this circuit design. However, this circuit has the disadvantage of having significant phase jitter or instability in the sample clock. This is a result of performance limitations of the high speed comparator required as part of the phase delay generator.
There is a need for a radar gauge circuit that is stabilized without the use of expensive, complex circuitry.
SUMMARY OF THE INVENTIONIn the present invention, a controller feeds back a control output to a clock source. The feedback stabilizes a first frequency separation between first and second clock frequencies generated by the clock source. A separation sensing circuit is coupled to the clock source and generates an evaluation output as a function of the first frequency separation. The evaluation output is coupled to the controller for controlling the control output. A radar gauge circuit receives the first or transmit frequency and the second or sample frequency and controls radar transmission and level sampling as a function of the transmit and sampling frequencies. The radar gauge circuit generates a level output that is stabilized and corrected as a function of the frequency separation.
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The circuit 100 in
Radar level gauge 14 includes an electronics housing 20, a housing to flange adapter 22, a process connecting flange 24 and an antenna 26. Radar level gauge 14 is mounted to a standoff pipe 28 which is fastened to the top of tank 10, around port 16. Tank flange 30 is fastened to standoff pipe 28. Process connecting flange 24 is secured with bolts to tank flange 30 and is sealed with a suitable gasket. Process connecting flange 24 supports both adapter 22 and electronics housing 20.
The electronics, comprised of the stack of printed circuit boards 42, provide microwave energy through a coaxial cable (coax) connection 46 which is coupled to a coaxial to rectangular waveguide adapter 48, positioned within electronics housing 20. The coax to rectangular waveguide adapter 48 is secured with screws to a raised boss 50 of housing to flange adapter 22. A waveguide aperture 52 extends through adapter 22 for transmitting the microwave energy to and from antenna 26. Adapter 22 includes a mounting plate 54, which is secured to a lower housing flange 56 of electronics housing 20 with bolts 58. Mounting plate 60 is secured to process mounting flange 24 with bolts 62. Antenna 26 is secured to a lower surface of process connecting flange 24 with bolts 64. Antenna 26 is of conventional design and includes a central aperture at an upper end that aligns with the waveguide aperture 52 in adapter 22 and an aperture 76 through flange 24. Other types of housing and assembly methods can be used for less demanding applications.
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The radar gauge circuit 124 includes a transmit pulse generator 182 and a sample pulse generator 190 controlled respectively by the transmit clock 126 and the sample clock 128. The output of the receive amplifier 198 is coupled to A/D converter 206 which converts the amplified signal to a digital form for use by microcontroller 208. Microcontroller 208 calculates the level and provides a level output on line 210 to a 4-20 mA output circuit 212. Output circuit 212 controls the 4-20 mA current energizing the radar level gauge to have an analog value representing the level. Microcontroller 208 utilizes memory 214 and also coupled to a digital I/O circuit 216 which provides two way digital communication over the 4-20 mA loop. The digital communication can be in the HART or Fieldbus format, or other known digital formats. Blocking capacitors 218 are interposed between digital I/O circuit 216 and the 4-20 mA loop to block the analog current from flowing through the digital I/O circuit. The radar gauge of
In the circuits of
A program to perform these processes can be loaded into controller 154 from a computer-readable medium having stored thereon a plurality of sequences of instructions for execution by a processor in a radar gauge adapted to sense fluid level in a tank.
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The sample polarity detector is connected as a latch that stores the polarity of the sample clock after the leading edge of the transmit clock toggles the Q/ output of the second difference frequency detector. The output of the transmit sample polarity detector is coupled to the microprocessor to indicates whether the sample clock has a lower or higher frequency that the transmit clock. The polarity detector resolves any ambiguity in the absolute value of the frequency difference.
The radar level gauge with stabilization has the advantage of low cost and low phase jitter, while improving overall performance.
The stabilization allows a low cost pulsed microwave radar measurement to be made with improved performance. The method involves measuring and correcting for the difference between the two critical clock frequencies required in this system, as opposed to trying to precisely generate or control these frequencies.
A timer in the microprocessor counts or times the outputs of the first and second difference frequency detectors. Based on these counts or times, the microprocessor calculates real time data representing the absolute value of the frequency difference between the transmit frequency and the sample frequency. The microprocessor then executes algorithm that adjusts the control voltage provided to the VCO to maintain the difference frequency in a desired range. The control algorithm in the microprocessor is adjusted so that it does not tightly control the frequency difference, but maintains only limited control within the desired range. The use of limited control rather than tight control of the frequency difference allows low power, low resolution components to be used in the frequency control. Oscillator drift is too fast for the low power, low resolution circuitry to control it, making frequency difference counts somewhat different during each measurement.
The timer is also used to precisely count the somewhat varying difference frequency during the exact time that the distance is being measured. The microprocessor then adjusts the distance calculation based on the actual count of the difference frequency. The timer can be a hardware timer, software implemented in a microprocessor, or a combination of both. In the microprocessor's algorithm or equation for calculating distance, the frequency difference term ΔF is a real time variable measured by the timer rather than a constant term or a term adjusted only infrequently for compensation.
The combination of limited control of the frequency difference with a precise count of the frequency difference enables the radar gauge to operate with lower noise due to phase jitter in combination with higher accuracy due to precise correction of distance measurement for variations in frequency during the measuring interval and the overall performance of the radar gauge is improved. High phase jitter on the sample clock leads to an unstable equivalent time measurement and instability at level output 132.
Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the scope of the invention.
Claims
1. A radar gauge adapted to sense fluid level in a tank, comprising:
- a radar gauge circuit adapted to receive a transmit frequency and a sample frequency controlling radar transmission and level sampling respectively, the radar gauge circuit generating a level output;
- a clock source generating first and second clock frequencies and having a control input setting a first frequency separation between the first and second clock frequencies;
- a separation sensing circuit coupled to the clock source and generating an evaluation output as a function of the first frequency separation;
- a controller receiving the evaluation output, the controller having a timer that measures the frequency separation and a control output feeding back to the control input that stabilizes the first separation as a function of timing the evaluation outputs, the controller further having a correction circuit that corrects the level output as a function of the first frequency separation;
- a divider circuit dividing the first and second clock frequencies and generating the transmit and the sample frequencies wherein the transmit and sample frequencies are separated from each other by a second frequency separation; and
- the separation sensing circuit further coupling to the divider circuit and generating a second evaluation output coupling to the controller as a function of the second frequency separation.
2. The radar gauge of claim 1 wherein the separation sensing circuit further comprises:
- a circuit sensing a polarity of the sample clock and generating a further evaluation output representative of the polarity.
3. The radar gauge of claim 1 wherein the clock source comprises a voltage controlled oscillator controlled by the control output and generating the second clock frequency.
4. The radar gauge of claim 3 wherein the controller comprises a digital-to-analog converter generating the control output.
5. The radar gauge of claim 1 wherein the controller includes a timer measuring time intervals of an evaluation output.
6. The radar gauge of claim 5 wherein the level output includes a current calculated distance that is a function of a current timer measurement.
7. The radar gauge of claim 1 wherein the controller includes a timer performing a timer measurement of a count an evaluation output during a time interval.
8. The radar gauge of claim 7 wherein the level output includes a current calculated distance that is a function of a current timer measurement.
9. The radar gauge of claim 1 wherein the radar gauge circuit includes a transmit pulse generator and a sample pulse generator controlled respectively by the transmit clock and the sample clock.
10. The radar gauge of claim 1 wherein the radar gauge is energized solely by a 4-20 mA analog current and includes a voltage regulator energized by the 4-20 mA analog current.
11. A method of stabilizing clock generation in a radar gauge adapted to sense fluid level in a tank, comprising:
- generating first and second clock frequencies separated from each other by a first frequency separation controlled by a control input;
- generating a first evaluation output as a function of the first frequency separation;
- generating a control output feeding back to the control input that stabilizes the first separation as a function of the evaluation output;
- generating a level output as a function of the stabilized first frequency separation, the level output corrected as a function of the first frequency separation;
- dividing the first and second clock frequencies to generate the transmit and sample frequencies separated from each other by a second frequency separation;
- generating a second evaluation output as a function of the second frequency separation;
- generating the control output as a further function of the second evaluation output; and
- correcting the level output as a function of the second evaluation output.
12. The method of claim 11, further comprising:
- sensing a polarity of the sample clock and generating a further evaluation output representative of the polarity.
13. The method of claim 11 further comprising:
- generating the second clock frequency in a voltage controlled oscillator wherein an oscillator control voltage is controlled by the control output.
14. The method of claim 13 further comprising:
- generating the oscillator control voltage in a digital-to-analog converter.
15. A radar gauge adapted to sense fluid level in a tank, comprising:
- means for receiving a transmit frequency and a sample frequency controlling radar transmission and level sampling respectively, and for generating a level output;
- means for generating first and second clock frequencies separated from each other by a first frequency separation, the clock source having a control input setting the first separation;
- means for dividing the first and second clock frequencies and for generating the transmit and sample clock frequencies separated from each other by a second frequency separation;
- means for sensing the first and second frequency separations and generating evaluation outputs as functions of the first and second frequency separations; and
- means for controlling a control output feeding back to the control input, stabilizing the first separation as a function of the evaluation outputs.
16. The radar gauge of claim 15, further comprising:
- means for sensing a polarity of the sample clock and generating a further evaluation output representative of the polarity.
17. A radar gauge adapted to sense fluid level in a tank, the gauge comprising:
- a radar gauge circuit adapted to receive a transmit frequency and a sample frequency controlling radar transmission and level sampling respectively, the radar gauge circuit generating a level output;
- a clock source generating first and second clock frequencies and having a control input setting a first frequency separation between the first and second clock frequencies;
- a separation sensing circuit coupled to the clock source and generating an evaluation output as a function of the first frequency separation;
- a controller receiving the evaluation output, the controller having a timer that measures the frequency separation and a control output feeding back to the control input that stabilizes the first separation as a function of timing the evaluation outputs, and controller further having a correction circuit that corrects the level output as a function of the first frequency separation;
- a circuit processing the first and second clock frequencies to generate third and fourth frequencies separated from each other by a second frequency separation; and
- wherein the separation circuit generates a second evaluation output as a function of the second frequency separation.
18. The gauge of claim 17, and further comprising a circuit sensing a polarity of the sample clock and generating a further evaluation output representative of the polarity.
19. The gauge of claim 17, wherein the further evaluation output is provided to the controller, which controller then uses the further evaluation output to generate, in part, the control input.
20. The gauge of claim 17, wherein the circuit sensing the polarity is embodied on a D-flip flop.
21. The gauge of claim 20, wherein the D flip flop is a 7474 clocked D-flip flop.
22. The gauge of claim 17, wherein the clock source further includes a voltage controlled oscillator (VCO) coupled to the control input.
23. The gauge of claim 17, wherein the separation sensing circuit is embodied on a D-flip flop.
24. The gauge of claim 23, wherein the D-flip flop is clocked 7474 D-flip flop.
25. The gauge of claim 17, and further comprising a divider circuit dividing the first and second clock frequencies and generating the transmit and sample frequencies wherein the transmit and sample frequencies are separated by a second frequency separation and wherein the first frequency separation is higher than the second frequency separation.
26. A method of stabilizing clock generation in a radar gauge adapted to sense fluid level in a tank, comprising:
- generating first and second clock frequencies separated from each other by a frequency separation controlled by a control input;
- generating a first evaluation output as a function of the frequency separation;
- generating a control output feeding back to the control input that stabilizes the separation as a function of the evaluation output;
- generating a level output as a function of the stabilized frequency separation, the level output corrected as a function of the frequency separation;
- generating an indication of the polarity of the sample clock;
- generating the control output as a further function of the evaluation output and the polarity indication; and
- correcting the level output as a function of the evaluation output.
27. A radar gauge adapted to sense fluid level in a tank, the gauge comprising:
- a radar gauge circuit adapted to receive a transmit frequency and a sample frequency controlling radar transmission and level sampling respectively, the radar gauge circuit generating a level output;
- an unstabilized clock generating a first clock frequency;
- a controllable oscillator generating a second clock frequency, the oscillator having a control input setting a first frequency separation between the first and second clock frequencies, the transmit and sample frequencies being related to the first and second clock frequencies;
- a separation sensing circuit coupled to unstabilized clock and the controllable oscillator, the sensing circuit generating a separation output as a function of the first frequency separation;
- a controller coupled to the radar gauge circuit and providing the level output; and
- wherein the separation output is operably coupled to the control input such that the first frequency separation is stabilized.
28. The gauge of claim 27, wherein the separation output is operably coupled to the control input through the controller.
29. The gauge of claim 28, and further comprising polarity sensing circuitry coupled to the sample clock and the controller, the polarity sensing circuitry being adapted to sense polarity of the sample clock, and provide a polarity output, and wherein the control input is based, at least in part, upon the first frequency separation and the polarity output.
30. The gauge of claim 29, wherein the controller is adapted to correct the level output as a function of the first frequency separation.
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Type: Grant
Filed: Oct 7, 2003
Date of Patent: Mar 4, 2008
Assignee: Rosemount Inc. (Eden Prairie, MN)
Inventor: John A. Kielb (Eden Prairie, MN)
Primary Examiner: John B Sotomayor
Attorney: Westman, Champlin & Kelly, P.A.
Application Number: 10/680,875
International Classification: G01S 13/08 (20060101);