|Publication number||USRE40128 E1|
|Application number||US 10/680,875|
|Publication date||Mar 4, 2008|
|Filing date||Oct 7, 2003|
|Priority date||Jul 2, 1999|
|Also published as||DE60019118D1, DE60019118T2, EP1192427A1, EP1192427B1, US6300897, WO2001002819A1|
|Publication number||10680875, 680875, US RE40128 E1, US RE40128E1, US-E1-RE40128, USRE40128 E1, USRE40128E1|
|Inventors||John A. Kielb|
|Original Assignee||Rosemount Inc.|
|Export Citation||BiBTeX, EndNote, RefMan|
|Patent Citations (18), Non-Patent Citations (2), Referenced by (8), Classifications (10), Legal Events (3)|
|External Links: USPTO, USPTO Assignment, Espacenet|
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.
In 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.
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.
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.
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.
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|Citing Patent||Filing date||Publication date||Applicant||Title|
|US7532155 *||Apr 10, 2006||May 12, 2009||Rosemount Tank Radar Ab||Radar level gauging system|
|US7619558 *||May 17, 2006||Nov 17, 2009||Vega Grieshaber Kg||Clock pulse control device of a microwave pulse radar|
|US7924217 *||Jun 3, 2008||Apr 12, 2011||Rosemount Tank Radar Ab||High sensitivity frequency modulated radar level gauge system|
|US8688279 *||Oct 21, 2009||Apr 1, 2014||Rosemount Tank Radar Ab||Energy storage at elevated voltage in a radar level gauge|
|US20060274871 *||May 17, 2006||Dec 7, 2006||Karl Griessbaum||Clock pulse control device of a microwave pulse radar|
|US20070236385 *||Apr 10, 2006||Oct 11, 2007||Mikael Kleman||Radar level gauging system|
|US20090315758 *||Dec 24, 2009||Anders Jirskog||High sensitivity frequency modulated radar level gauge system|
|US20110093129 *||Oct 21, 2009||Apr 21, 2011||Leif Nilsson||Energy storage at elevated voltage in a radar level gauge|
|U.S. Classification||342/124, 342/84, 342/87, 342/135, 342/82, 342/85|
|International Classification||G01S13/08, G01F23/284|
|Oct 23, 2008||FPAY||Fee payment|
Year of fee payment: 8
|Oct 28, 2008||CC||Certificate of correction|
|Mar 12, 2013||FPAY||Fee payment|
Year of fee payment: 12