Temperature compensated transmission line based liquid level sensing apparatus and method
First Claim
1. A sensor for measuring liquid level within a container, said liquid having a dielectric constant, the sensor comprising:
- a. a voltage signal generator which provides a carrier frequency;
b. a dual line transmission line probe of at least one segment, the dual line transmission probe having a length that is submerged in the liquid with one input line coupled to a fixed potential, and another input line coupled to the signal generator throughc. a complex impedance network; and
d. an AM demodulator coupled to the complex impedance network;
the AM demodulator providing an output voltage signal correlated to the liquid'"'"'s dielectric constant and the length of the dual transmission line that is submerged in the liquid;
wherein the said AM demodulator circuit is comprised of a temperature compensated peak detector circuit further comprising two simple peak detector circuits, each simple peak detector circuit still further comprising a diode that is thermally coupled to the diode of the other simple peak detector circuit and with an input of one simple peak detector coupled to the said transmission detector circuit coupled to the said signal generator through a second complex impedance network, and with outputs of both peak detectors coupled to inputs of a differential amplifier which subtracts the signals resulting in a the removal of common mode temperature fluctuations and an amplified result.
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Abstract
A sensor having a transmission line acting as a probe and an electronic circuit measures liquid levels. The sensor circuit uses a periodic signal generator to produce a carrier wave, which stimulates the transmission line, through a complex network coupling element. The complex impedance network forms a voltage divider with the transmission line, wherein the output of the voltage divider, is demodulated with an AM demodulator, such as a thermally compensated peak detector. This demodulated signal is related to the dielectric constant, and thus the liquid level of the material surrounding the partially submerged transmission line probe. The liquid level and the demodulated signal may be offset, scaled and linearized with a microcontroller containing signal processing routines, linearizing equations, stored calibration constants and look up tables.
32 Citations
9 Claims
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1. A sensor for measuring liquid level within a container, said liquid having a dielectric constant, the sensor comprising:
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a. a voltage signal generator which provides a carrier frequency; b. a dual line transmission line probe of at least one segment, the dual line transmission probe having a length that is submerged in the liquid with one input line coupled to a fixed potential, and another input line coupled to the signal generator through c. a complex impedance network; and d. an AM demodulator coupled to the complex impedance network; the AM demodulator providing an output voltage signal correlated to the liquid'"'"'s dielectric constant and the length of the dual transmission line that is submerged in the liquid;
wherein the said AM demodulator circuit is comprised of a temperature compensated peak detector circuit further comprising two simple peak detector circuits, each simple peak detector circuit still further comprising a diode that is thermally coupled to the diode of the other simple peak detector circuit and with an input of one simple peak detector coupled to the said transmission detector circuit coupled to the said signal generator through a second complex impedance network, and with outputs of both peak detectors coupled to inputs of a differential amplifier which subtracts the signals resulting in a the removal of common mode temperature fluctuations and an amplified result. - View Dependent Claims (2, 3, 4, 5, 6)
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7. A method of measuring liquid level in a container, comprising the steps of:
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a. providing a transmission line probe of at least one segment and partially submerging the transmission line probe into a liquid medium having a dielectric constant; b. providing a periodic voltage signal to an input of the transmission line probe through a complex impedance network; c. providing an AM demodulator coupled to the input of the transmission line probe; and
,d. the demodulator providing a first output signal correlated to the dielectric constant of the liquid medium and a length of the transmission line probe wherein a signal from the probe is demodulated and thermally compensated by;
e. providing a primary peak detector coupled to the said complex impedance network;
f. providing a secondary complex impedance network coupled to the periodic voltage signal;
g. providing a secondary peak detector coupled to the said secondary complex impedance network, the secondary peak detector providing a second output signal;
h. thermally coupling two diodes of the primary and secondary peak detectors;
i. subtracting the second signal from the first signal to remove common mode thermal fluctuations. - View Dependent Claims (8, 9)
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Specification