Endress+hauser Gmbh+co.

Endress U. Hauser Gmbh U. Co.

Gustav Wetzel

Werner Thoren

Jurgen Meier

Frank Seibert

A capacitive proximity or limit switch comprises a capacitive probe having a sensor electrode and a guard electrode. The sensor electrode is connected to an oscillator formed by a feedback system of two operational amplifiers, of which one operational amplifier is configured as a differential amplifier, the other operational amplifier as a voltage follower. The probe electrode is connected to the inverting input of the differential amplifier, in the feedback circuit of which a capacitor is provided, as a result of which the AC gain is determined. An adjustable portion of the output voltage of the differential amplifier is applied to the non-inverting input of the voltage follower, the output of which is connected to the non-inverting input of the differential amplifier to which the guard electrode is connected. As a result of this, the start and stop of oscillation of the oscillator is determined by variations of the capacitance of the sensor electrode, and the guard electrode is always maintained precisely at the same AC potential as the sensor electrode.

Capacitive switch actuated by changes in a sensor capacitance

The capacitive level limit switch used for monitoring the level of a material in a container has a capacitive probe fitted at the height of the level to be monitored so that it is covered by the material when the level has attained this height. The capacitance of the probe is compared to a threshold value determining the switching point, for establishing whether the probe is covered by material or not. For setting the switching point, the capacitance of the probe measured in one condition of the probe is stored as a first reference value and a provisional second reference value is calculated from the first reference value by addition or subtraction of a predetermined amount and is also stored. From the stored reference values the threshold value is calculated according to a prescribed function so that it lies between the two reference values. If it is determined in operation that the measured capacitance value deviates by a certain amount from the second reference value, this measured capacitance value is stored as a new second reference value, and from the reference values now stored a new threshold value is calculated according to the prescribed function.

Method of setting the switching point of a capacitive level limit switch

A transmitting station is connected to a receiving station by a two-wire line via which the supply current required for operating the transmitting station is transferred from the receiving station to the transmitting station. The transmitting station comprises a supply circuit including a voltage regulator for maintaining the operating voltage of the transmitting station constant and an energy store for compensating current and/or voltage fluctuations. Via the two-wire line a signal current, variable between two limit values, is further transferred from the transmitting station to the receiving station. This signal current consists of the supply current for the transmitting station and a supplementary current determined by the transmitting station. For transfer of the communication information between the transmitting station and the receiving station, a symmetrical communication current is superimposed on the signal current. The communication current is superimposed on the current flowing from the two-wire line into the supply circuit of the transmitting station.

Arrangement for signal transmission between a transmitting station and a

The circuit arrangement is used for capacitance or admittance measurement by the principle of apparent current measurement in that to the capacitance or admittance to be measured an AC voltage of fixed magnitude and fixed frequency is applied and the AC current flowing through the capacitance or admittance is used as a measure of the measured value. The test object whose capacitance or admittance is to be measured lies in the emitter circuit of a transistor connected as voltage follower, to the base electrode of which the AC voltage is applied. The emitter terminal of the transistor connected to the test object is connected to the corresponding pole of the operating voltage source via an adjustable current source. The collector connection of the transistor is connected via an impedance of high resistance for alternating current to the other pole of the operating voltage source. Connected to the collector is a rectifier circuit which converts the alternating current applied thereto to a measuring DC voltage proportional thereto. The direct current flowing via the adjustable current source is set in dependence upon the measuring DC voltage such that it is always slightly greater than the peak value of the alternating current flowing through the test object. Instead of a transistor, another amplifier element with controllable current path may also be used, for example a field-effect transistor. The impedance of high resistance to alternating current is preferably a second adjustable current source which is so constructed that it is controlled automatically to synchronize with the first adjustable current source.

Circuit arrangement for capacitance or admittance measurement

A plastic scintillation detector (14) generates on the basis of ionizing radiation coming from a radiation source (16) light flashes which are converted by a photoelectric transducer (20), for example a photoelectron multiplier, to electrical output pulses. An evaluation circuit (22) counts the output pulses as a measure of the radiation intensity. For automatic stabilization of the scintillation detector a light source (24) is controlled by a driving circuit (26) in such a manner that it transmits light pulses whose intensity is modulated in accordance with a predetermined periodic function, for example a sawtooth function. The modulated light pulses are also intercepted by the photoelectric transducer (20) and converted by the latter into electrical reference pulses. A monitoring circuit (28) determines in each modulation period the number of reference pulses whose amplitude exceeds a predetermined reference threshold value lying in the modulation interval. The pulse number determined is used to stabilize the scintillation detector, either by controlling the amplification in the sense of maintaining a predetermined pulse number or by corresponding correction of the measurement result furnished by the evaluation circuit.

Method and arrangement for the automatic stabilization of a

An ultrasonic filling position limit switch having disposed in the container at the full level thereof, at least one ultrasonic transducer for sending out ultrasonic pulses and for receiving echo pulses reflected off the opposite container wall. An amplifier of the electric output signals of the transducer is connected to a threshold limit switch which provides an output signal when the amplifier output signal exceeds a predetermined threshold value. The amplifier output is fed back to an amplifier control input by means of an amplifier control circuit which seeks to maintain the amplifier output signal at an adjustable constant nominal value. A control device blocks the control circuit for the duration of time in each sending period required for the sending out of the ultrasonic sending pulses and for receiving the echo pulses.

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Lorrach, Germany

Gustav Wetzel