Communications: directive radio wave systems and devices (e.g. – Determining distance – Material level within container
Reexamination Certificate
2000-11-28
2003-03-18
Tarcza, Thomas H. (Department: 3662)
Communications: directive radio wave systems and devices (e.g.,
Determining distance
Material level within container
C342S118000, C342S134000
Reexamination Certificate
active
06535161
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to rangefinders, and more particularly to pulse-echo radar rangefinders that are powered by an industrial two-wire current signaling loop.
2. Description of Related Art
Control instrumentation for industrial processes traditionally use a two-wire or a four-wire interface between a sensor (called a “transmitter”) and a controller or data processor. The four-wire arrangement uses two wires for power, and two other wires for signaling using a current-loop format. Carrier-based digital modulation may also be impressed on the two-wire current loop, such as the HART® Protocol, for communication and control.
The two-wire interface uses only two wires for both conveying power to the transmitter and conveying proportional analog data from the transmitter. The proportional analog signal most often conforms to a 4-20 mA standard that specifies 4 mA to power the transmitter and 0-16 mA to indicate an analog value. The two-wire loop is preferred due to its lower cost, its lower explosion hazard and a large installed base of two-wire links at industrial sites. However, two-wire operation poses severe power constraints on the transmitter: a few tens of milliwatts.
Loop-powered transmitters date back to at least 1977. For example, U.S. Pat. No. 4,016,763 to Grindheim, 1977, discloses a resistance bridge in a loop-powered circuit. U.S. Pat. No. 4,242,665 to Mate, 1980, discloses a two-wire circuit that achieves low average power using a high power sensor operated with duty-cycled power. An ultrasonic rangefinder operating on loop power was presented in “A Two-Wire Ultrasonic Level Meter with Piezoelectric Polymer-Film Sensor” by Owada et al, Proceedings of the ISA/88 International Conference and Exhibit, Vol. 43, Part 3, 1988. Thus, by 1988 the details of loop-powered pulse-echo ultrasonic rangefinders were published. However, pioneering work on loop-powered pulse-echo radar rangefinders did not commence until the 1990's.
A motion sensor using a high power radar that achieves low average power by using duty-cycled power was disclosed in U.S. Pat. No. 4,131,889 to Gray, 1978. While Gray did not power his radar from an industrial loop, it would have been evident to do so by 1978 in view of Grindheim and 1980 in view of Mate. Nonetheless, the resulting loop-powered radar based on the Gray patent would not be capable of measuring range, or more particularly, tank levels since Gray's radar only detected motion.
FIG. 1
a
schematically depicts the first known low power radar to operate on a two-wire loop. It was prototyped in 1993 and disclosed in U.S. Pat. No. 5,465,094 to McEwan, 1995. Although the loop was scaled for automotive use and signaled a discrete current level, it would have been apparent to a practitioner in 1993 to adapt it to the 4-20 mA industrial standard. In operation, a low current is received over two wires
20
and stored in a power store element
14
such as a capacitor, which provides power to voltage regulator
12
, which powers radar
10
. A low power radar
10
(also known as micropower impulse radar, or MIR) detects motion and responsively operates a shorting switch
16
to increase the current on the 2-wire interface
20
. During the time switch
16
is closed, the voltage on wire pair
20
drops to zero, so radar
10
operates on stored power from power store element
14
.
In early 1994 a low power MIR impulse radar rangefinder was prototyped, forming the basis for U.S. Pat. Nos. 5,774,091; 5,757,320; and 5,805,110, all to McEwan, 1998. While not specifically set up for loop operation, a practitioner could readily have adapted the impulse radar for 4-20 mA two-wire loop operation in early 1994.
FIG. 1
b
schematically depicts a loop-powered impulse radar rangefinder as disclosed in U.S. Pat. No. 5,672,975 to Kielb et al, 1997, assigned to Rosemount, Inc. A two-wire interface
20
provides power to voltage regulator
12
, which in turn powers impulse radar
11
. A measurement circuit
22
initiates transmissions and starts a range measurement timer. Impulse radar
11
ends the range measurement upon receipt of an echo. The measurement circuit
22
provides an output to a 0-16 mA proportional analog current source
18
to signal the measured range across two-wire loop
20
. The entire apparatus must draw 4 mA from two-wire loop
20
so the total current spans 4-20 mA.
FIG. 1
c
schematically depicts a loop-powered impulse radar rangefinder disclosed in U.S. Pat. No. 6,014,100 to Fehrenbach et al, 2000, assigned to Vega Grieshaber, AG. A high power radar
13
is operated with duty-cycled power to achieve low average power. Power store element
14
provides high current surges to high power radar
13
and averages the high power surges with inactive periods drawing little or no power so the current draw from regulator
12
is low. This duty-cycled power technique appears to be similar to that described by the Gray '889 patent in 1978. The advantage to using a high power radar design is that the analog circuitry can operate with lower impedances for better moisture immunity and stability, lower cost, and less complexity. Further, high frequency transistors require about 10 mA bias current, which alone could exceed the available power. Aside from duty-cycled power, high power radar
13
appears to be similar in operation to low power radar
11
, as stated by Fehrenbach et al, “signal generation and processing during and after measurements are as described, for instance, in U.S. Pat. No. 5,672,975.” Having provided no other technical details on radar
13
, it can only be assumed that it is an impulse radar having similar timing to that of impulse radar
11
.
The prior loop-powered rangefinding radars, as depicted in
FIGS. 1
b
and
1
c
, are impulse radars. Step generator
76
in
FIG. 2
of the '975 patent indicates its impulse nature. The output of step generator
76
is differentiated into an impulse by antenna
18
—all antennas, including antenna
18
in the '975 patent, differentiate a step input into a radiated impulse. Microwave circulators, such as circulator
78
in the '975 patent, pass an ultrawideband spectrum and offer essentially no bandlimiting action, so antenna
18
defines the emission spectrum. Thus, radars
11
,
13
are damped wave devices, and most likely radiate over a broad spectral region, such as 1-5 GHz, or with a resonant horn antenna, perhaps 4-8 GHz. Radars
11
,
13
pose a serious regulatory limitation: damped wave emitters have been prohibited in the U.S. and internationally since 1934. An impulse radar spectrum crosses numerous restricted bands, particularly those used by GPS equipment and aviation safety radar. Impulse radars
11
,
13
cannot receive FCC equipment authorization under current regulations and therefore have little or no commercial value.
The FCC strictly prohibits intentional radiation in the restricted bands, no matter how weak. Accordingly, adding a filter to the output of an impulse radar to limit spectral radiation in the restricted bands may be viewed in the same light as adding an attenuator to the output—it does not change the intent of the emissions. Similarly, operating an impulse radar in a tank may be viewed as adding an attenuator to the output of a radar having intentional radiation in the restricted bands.
FCC prohibitions notwithstanding, the impulse radar described in the '975 patent (and by incorporation, the '100 patent) appears to have at least four deficiencies which would block practical implementation. First, the '975 specification cites the receive clock frequency f
2
=f
1
+&Dgr;f, where f
1
is the transmit clock and f
2
is the receive clock, &Dgr;f being a 10-40 Hz offset. As is well known in this type of slipped-phase clock system, the frequency relation should be f
2
=f
1
−&Dgr;f. The effect of this error is to make the sampled equivalent time output of receiver
70
appear to run backwards, so an echo appears before a pulse is transmitted. There
Andrea Brian
McEwan Technologies, LLC
Tarcza Thomas H.
Weide & Miller Ltd.
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