Electricity: measuring and testing – Impedance – admittance or other quantities representative of... – Lumped type parameters
Reexamination Certificate
1999-04-21
2001-07-10
Metjahic, Safet (Department: 2858)
Electricity: measuring and testing
Impedance, admittance or other quantities representative of...
Lumped type parameters
C324S118000
Reexamination Certificate
active
06259259
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This application relates to the field of measuring a variable admittance and more particularly to the field of measuring a level in vessels with a capacitance sensor.
2. Description of Related Art
Capacitive level sensors employed in practice have a length of between several centimeters and approximately 30 meters. The dielectric constant of the products to be measured varies between approximately 1.5 and 100, and the products may also be electrically conducting. This produces a difference in the capacitance between a covered portion of the sensor and an uncovered portion of the sensor of between approximately 6 pF and 3,000 pF. The capacitance and admittance measurement apparatus which processes the sensor results are therefore preferably adjustable over a variable capacitance range (Turn-Down) of approximately 1:500.
Conventional devices use a combination of two different arrangements. According to one arrangement, the measurement result is amplified with a variable amplification and/or expanded (either using analogue amplifier stages or digitally using a microprocessor). According to another arrangement, the capacitance measurement range of the admittance measuring device is switched.
German patent DE 42 35 243 C1 by the Applicant discloses a method for switching the capacitance measurement range. The supply voltage of the sensor and thereby the sensitivity of the capacitance measurement is herein adjusted, for example, with a ratio of 1:5:25 by a three-stage switch which can be used to connect the sensor to three different winding tabs of the oscillator transmitter.
This conventional approach is disadvantageous in that the switching device is part of an extrinsic circuit operating at a different potential. Moreover, a bipolar high frequency alternating current has to be switched, wherein the turn-on resistance of the circuit element should be kept below 1 ohm to eliminate phase shifts in the measurement circuit. The self-capacitance of the switching element should also be less than a few pF to minimize adverse effects on the measured value.
As a result, a mechanical switch should be used as a switching element. This conventional approach does not enable microprocessor-controlled electronic switching of the capacitance measurement range.
It is therefore desirable to improve the conventional method for switching the measurement range of admittance level sensors so as to enable automatic electronic switching of the measurement range, while at least maintaining the switching ratio attainable with a mechanical switch. It is also desirable to provide an apparatus for automatically switching the measurement range.
SUMMARY OF THE INVENTION
In general, according to one aspect of the invention, a synchronous demodulator is employed for rectifying a sensor current, wherein a rectifying factor of the synchronous demodulator can be controlled by alternately switching a timing or clock pulse signal between at least two different states. Each of the two states corresponds to a different rectifying factor. The resulting total rectifying factor of the demodulator can be freely selected via the pulse duty cycle between the two states.
The invention thus advantageously provides electronic switching of the measurement range which is increasingly common with intelligent smart transmitters employed in industrial level measurement applications.
The method and the apparatus of an intelligent admittance level sensor according to the invention provides the following advantages to the user:
automatically selecting a suitable measurement range when the sensor is set up in the vessel, thereby preventing users from entering erroneous set points;
eliminating the possibility of having to refill the container for calibration purposes because the measurement range was erroneously preselected;
enabling remotely setting the level transmitter through a hand-held terminal which can be clamped to the sensor line at any desired location, to remotely set all parameters of the sensors using an appropriate communication protocol (e.g., HART);
providing pressure-resistant, encapsulated sensor housings which may no longer be opened during the sensor operation for adjusting the settings; and
providing the sensor with a field bus system (e.g. Profibus) to connect different sensors in a digital network with a process control located in a central control center via a common bus, wherein all sensor settings can be adjusted by the control center via the bus system.
Further features and advantages of the present invention will be apparent from the following description of preferred embodiments and from the claims.
REFERENCES:
patent: 4242631 (1980-12-01), Hall
patent: 4235243C2 (1994-04-01), None
patent: 4244739C2 (1994-04-01), None
Raffalt Felix
Woehrle Siegbert
Kerveros J.
Metjahic Safet
Ropes & Gray
Vega Grieshaber KG
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