Measuring and testing – Liquid level or depth gauge
Reexamination Certificate
1999-12-30
2003-11-11
Williams, Herzon (Department: 2856)
Measuring and testing
Liquid level or depth gauge
C073S30400R, C073S29000R
Reexamination Certificate
active
06644114
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to pulsed electromagnetic sensors, and more particularly to fluid and material level sensors using pulse width modulated (PWM) time-domain reflectometry (TDR). These sensors can be used for (but are not limited to) determining or controlling the material level in a tank, vat, irrigation ditch, silo, pile, or conveyor.
2. Description of Related Art
Ever since time domain reflectometry was first used in liquid level gages, as seen in U.S. Pat. No. 3,703,829, Liquid Quantity Gaging System, to Dougherty, TDR-based level sensors have grown in popularity. The TDR technique involves connecting a time-domain reflectometer to a transmission line (or probe) immersed in a liquid. The TDR measures the round-trip delay of a pulse propagating from the TDR to the liquid surface and reflecting back to the TDR. The propagation delay is independent of air temperature, pressure and humidity, and the precise location of the pulse reflection depends only on the location of liquid surface and is independent of the liquid's dielectric constant.
The probe can be a hollow coaxial structure (except for the normal center conductor) which fills with the liquid so a TDR reflection occurs at the liquid surface within. The probe can be simplified to a single-wire transmission line, as disclosed in U.S. Pat. No. 3,995,212, “Apparatus and Method for Sensing a Liquid with a Single Wire Transmission Line” to Ross.
The disadvantages to a single-wire transmission line are (1) high line impedance, making reflection-free matching to common 50-ohm interconnect cable impossible, (2) low reflection amplitude, making ringing and other aberrations more dominant, (3) sensitivity to nearby objects such as tank walls, (4) susceptibility to RF interference (no shielding), and (5) inability to form a PWM signal directly on the line, thereby requiring more complicated and less accurate pulse detectors.
Coaxial probes, while more complicated than single wire probes, have their advantages: (1) they produce strong reflection amplitudes, which is an advantage with low dielectric constant materials, (2) they exhibit stilling action, wherein sloshing is less pronounced inside the coaxial probe so steadier measurements can be obtained, and (3) they have natural shielding against the effects of nearby walls and radio frequency interference. The primary drawbacks to coaxial probes are (1) potential to become clogged, (2) greater mechanical complexity relative to a simple wire, (3) difficulty in segmenting or coiling long lengths for shipping, and (4) difficulty in cutting a long length to a custom length at the customer's location to suit specific tank dimensions. In spite of these limitations, the advantages to the coaxial probe outweigh the disadvantages in many applications, particularly where the probe length is on the order of a meter or less.
The simplicity of the TDR probe and the constancy of the speed of light (at which the TDR pulses travel), make TDR an ideal level sensing technology. Unfortunately, TDR can be expensive and of limited accuracy (0.1-1%). In order to simplify the TDR apparatus while improving its accuracy, TDR pulse processing and timing must be advanced beyond the present state.
SUMMARY OF THE INVENTION
The concept behind the present invention is (1) to set up the TDR pulses on the transmission line to directly form a PWM pulse, perhaps on a millivolt scale, (2) to detect the transmission-line PWM pulse and convert it to a logic-level PWM pulse, and (3) to process the logic-level PWM pulse by using it to gate a range counter to obtain a level indication, or by averaging it into a voltage or current proportional to level. In short, the present invention passes a transmission-line PWM pulse directly—and accurately—to the output as a digital PWM pulse without altering the PWM pulse duty cycle. This “direct-PWM” process can be implemented in a realtime mode, where the digital output PWM pulse has a realtime scale, or in an expanded-time mode, where the digital output PWM pulse has been expanded in time by a large factor to facilitate high accuracy detection and range counting.
In the realtime mode, the transmission-line PWM signal is threshold detected to form a realtime digital PWM signal having an amplitude set by the power supply voltage V
s
, a PWM width set by the transmission-line PWM signal, and a period T
clk
set by the TDR clock. The realtime digital PWM signal can be averaged with a simple low pass filter to obtain a voltage V
ave
that is linearly proportional to the height of the liquid. V
ave
is a ratiometric voltage, V
ave
/V
s
=PWM/T
clk
. V
s
can be set to high precision, or preferably, it can be used as a reference to a ratiometric A/D converter so its amplitude does not matter. The clock period, T
clk
can be set to high accuracy with a quartz crystal oscillator. Thus, the realtime mode can be precise, in principle. In practice, propagation delay variations and other shifts in the realtime threshold detector limit accuracy. Yet, the extreme simplicity of the realtime PWM TDR apparatus is hard to ignore. It is the best mode for low cost and moderate accuracy (a few percent) tank gages or for fixed fill/overfill switches.
In the expanded-time mode, the transmission-line PWM signal is sampled to produce an expanded-time (ET) PWM signal that is a replica of the transmission-line PWM signal, except on a vastly expanded time scale. The expanded-time technique employs a stroboscopic sampling effect to convert nanosecond PWM signals to millisecond PWM signals for more accurate processing.
The expanded-time mode can be implemented in either of two ways: (1) using a single clock signal that is split into a transmit clock signal and a sampling (or receive) clock signal with a swept phase-difference between them, wherein the receive clock signal operates a sampler to produce a time-expanded PWM signal; and (2) using separate transmit and sampling (or receive) clock oscillators with a floating, i.e., unregulated, offset frequency between them to provide a steady phase slip.
The scale-factor accuracy of the two-oscillator technique is set by the accuracy of a crystal-controlled transmit clock, or ~0.003% without calibration during manufacture. With either expansion process, the sampled PWM signal is threshold-detected to produce an expanded-time digital PWM signal.
The present invention can be used as an electronic dipstick for innumerable applications in material level sensing. In combination with a valve, it can be used to control or automatically regulate the level in a tank, for example. As a linear displacement transducer, where the coaxial probe is configured as a piston and cylinder assembly, vehicle height can be sensed or pneumatic/hydraulic cylinder displacement can be measured for safety or automatic control.
REFERENCES:
patent: 3695107 (1972-10-01), Hertz et al.
patent: 3703829 (1972-11-01), Dougherty
patent: 3832900 (1974-09-01), Ross
patent: 3874237 (1975-04-01), Zwarts
patent: 4196385 (1980-04-01), Vestergaard et al.
patent: 4287472 (1981-09-01), Pan et al.
patent: 4359902 (1982-11-01), Lawless
patent: 4425793 (1984-01-01), Turton et al.
patent: 4495807 (1985-01-01), Field et al.
patent: 4503384 (1985-03-01), Nagy et al.
patent: 4543823 (1985-10-01), Nagy et al.
patent: 4544880 (1985-10-01), Nagy et al.
patent: 4766386 (1988-08-01), Oliver et al.
patent: 4786857 (1988-11-01), Mohr et al.
patent: 4864547 (1989-09-01), Krsna
patent: 4866371 (1989-09-01), De
patent: 5335545 (1994-08-01), Leszczynski
patent: 5400651 (1995-03-01), Welch
patent: 5457990 (1995-10-01), Oswald et al.
patent: 5483226 (1996-01-01), Menut
patent: 5554936 (1996-09-01), Mohr
patent: 5565774 (1996-10-01), Dudding et al.
patent: 5609059 (1997-03-01), McEwan
patent: 5610611 (1997-03-01), McEwan
patent: 5656774 (1997-08-01), Nelson et al.
patent: 5717337 (1998-02-01), Kelly
patent: 5723979 (1998-03-01), Mohr
patent: 5726578 (1998-03-01), Hook
patent: 5784377 (1998-07-01), Baydar et al.
patent: 5811677 (1998-09-01), Cournan
Frank Rodney
McEwan Technologies, LLC
Weide & Miller Ltd.
Williams Herzon
LandOfFree
Direct PWM reflectometer does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Direct PWM reflectometer, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Direct PWM reflectometer will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3166061