Electricity: measuring and testing – Measuring – testing – or sensing electricity – per se – Magnetic saturation
Reexamination Certificate
1999-07-30
2001-04-17
Metjahic, Safet (Department: 2858)
Electricity: measuring and testing
Measuring, testing, or sensing electricity, per se
Magnetic saturation
Reexamination Certificate
active
06218825
ABSTRACT:
The invention relates to a current sensor with a magnetic core on which, in addition to a primary winding in which the current to be measured flows, at least one secondary winding is wound, into which winding an alternating current that is generated by a generator circuit is fed, which current saturates the magnetic core in alternating fashion in at least one direction.
German reference 42 29 948 A1 describes a current sensor in which a softly magnetic core is provided with primary and secondary windings. In series with the secondary winding, an additional current source is connected which sends a magnetizing current through the secondary winding, which current controls the softly magnetic core into positive, or respectively, negative saturation in alternating fashion. The softly magnetic core has an essentially rectangular magnetization characteristic, so that a respectively constant current flows between two saturation states during the reversal of magnetization, since, as a result of the nearly perpendicular curve of the magnetization characteristic, the inductive resistance tends to infinity. This current differs from an average value in the positive, or respectively, negative half-wave by a value which is defined by the hysteresis of the magnetization curve. By the formation of average values of the constant current flowing during the reversal of magnetization given two consecutive half-waves with different polarities, the influence of the hysteresis in the magnetization loop can be compensated, so that the measuring of the current in the secondary winding flowing during the magnetic reversal time yields a current that is directly proportional to the current in the primary winding, which is to be measured. To achieve a certain level of precision, however, a relatively high wiring outlay is necessary.
SUMMARY OF THE INVENTION
It is thus the object of the invention to provide a current sensor which offers a lower outlay given the same degree of precision, or respectively, offers a higher precision given the same outlay.
The object is inventively achieved by a current sensor of the abovementioned type in that the generator circuit for generating the alternating current is free-running. The polarity of the alternating current that is generated by the generator circuit is a function of the current flowing in the secondary winding and at least one threshold value for this current change. The non-linearity of the characteristic of the magnetic core material is electronically exploited in that switching threshold values are defined which electronically change the polarity of the alternating current. The loop shape, the value of the saturation, and to a certain extent the hysteresis of the magnetic core material, also, are only of secondary importance.
In a preferred embodiment of an inventive current sensor, it is provided that the generator circuit comprises a serial circuit of two inverting amplifiers (e.g. inverters, logic gates, inverting switching amplifiers, comparators, etc.), whereby the secondary winding is connected between input and output of one amplifier, and the input of this one amplifier is additionally coupled with the output of the other amplifier via a first resistor. A pulsewidth-modulated output signal stands ready at the outputs of both amplifiers and can be respectively tapped there. In this embodiment, the secondary winding is driven with a rectangular voltage which is changed over whenever the current through the secondary winding exceeds a defined value. The characteristic is offset according to the size of the current to be measured. Consequently, positive and negative half-waves are no longer the same size. However the pulsewidth ratio of the output signal varies, which can be evaluated as a signal mean value.
In another preferred embodiment, the generator circuit is constructed such that a second resistor is connected between a first node point and a first terminal of the secondary winding, and a third resistor is connected between a second node point and a second terminal of the secondary winding. The second terminal of the secondary winding is connected to an input of a first NAND gate directly and to an input of an NOR gate given the insertion of a first inverter. A second input of the first NAND gate and a second input of the NOR gate are connected to the second node point. Furthermore, an input of a second NAND gate is connected to the first terminal of the first secondary winding, and its second input is connected to the output of the first NAND gate. Finally, the outputs of the NOR gate and the first NAND gate are connected across the inputs of an AND gate whose output that carries the pulsemodulated output signal is connected to the second node point directly and to the first node point given the insertion of a second inverter. In this embodiment, only one of the two switching edges is exploited, thereby advantageously avoiding a large offset, which arises by virtue of the different switching levels at positive and negative switching edges.
It can also be provided that a freewheeling diode is respectively provided between the first node point and the first terminal of the secondary winding and between the second node point and the second terminal of the secondary winding.
In a development of the invention, a capacitor is respectively connected in series to first and second resistors. The secondary winding is thus driven with an approximately saw-toothed voltage instead of a rectangular voltage, thereby increasing the sensitivity of the sensor characteristic.
A low pass is preferably connected to the second input of the first NAND gate. Furthermore, the first input of the second NAND gate can be coupled with the first terminal of the secondary winding via a fourth resistor, with the first node point via a fifth resistor, and with a supply potential via a clamping diode. Accordingly, the input of the first inverter and the first input of the first NAND gate can be coupled with the second terminal of the secondary winding via a sixth resistor, with the second node point via a seventh resistor, and with the supply potential via another clamping diode.
It can also be provided that the magnetic core additionally comprises a compensation winding which is driven, by an evaluation circuit, with a compensation current which is derived on its part from a signal of the secondary winding. The evaluation circuit preferably contains a bridge amplification circuit which is driven by a pulsewidth-modulated signal. Due to the bridge amplification circuit, an operation is guaranteed given small unipolar supply voltages, and a very slight power loss arises due to the cycled operation of the bridge amplification circuit by means of the pulsewidth-modulated signal.
Finally, in an embodiment which involves very little outlay, the secondary winding is connected in series to a resistor situated between the input and output of a Schmitt trigger.
In the inventive current sensor, given driving with a rectangular current, it is also possible to evaluate the arising voltage impulses or the current during the voltage maximum at the coils.
REFERENCES:
patent: 5568047 (1996-10-01), Staver et al.
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patent: 42 29 948 A1 (1994-03-01), None
patent: 295 20 066 U1 (1996-02-01), None
Patent Abstracts of Japan, vol. 10, No. 19 (p-423) dated Jan. 24, 1986 & JP 60-173475 dated Sep. 6, 1985, Mishima Kosan Co Ltd., S Kimisuke, Self-Oscillation Type Current Sensor, single page.
Patent Abstract of Japan, vol 10, No. 52, (p-432) dated Feb. 28, 1986 & JP 60-196678 dated Oct. 5, 1985, Mishima Kosan Co Ltd., S Kimisuke, Differential Self-Exciting Bridge Type Current Sensor, single page.
Patent Abstracts of Japan, vol. 10, No. 19 (P-423) dated Jan. 24, 1986 & JP 60-173476 dated Sep. 6, 1985, Mishima Kosan Co Ltd., S Kimisuke, Bridge Type Current Sensor, pp. 469-475.
Kerveros J
Metjahic Safet
Schiff & Hardin & Waite
Vacuumschmelze GmbH
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