METHOD AND APPARATUS FOR THE MEASUREMENT OF ELECTRICAL CURRENT BY MEANS OF A SELF-COMPENSATING CONFIGURATION OF MAGNETIC FIELD SENSORS

• US 20170276707A1
• Filed: 02/14/2017
• Published: 09/28/2017
• Est. Priority Date: 02/15/2016
• Status: Active Grant

• Current Assignee: C-Sigma SRL
• Original Assignee: C-Sigma SRL
• Inventors: Ghislanzoni, Luca
• Granted Patent: US 10,161,969 B2
• Time in Patent Office: Days
• CPC Class Codes: G01R 15/183 using transformers with a magnetic core
• CPC Class Codes: G01R 15/202 using Hall-effect devices Hall elements in arrangements for measuring electrical power G01R21/08
• CPC Class Codes: G01R 15/205 using magneto-resistance devices, e.g. field plates
• CPC Class Codes: G01R 19/32 Compensating for temperature change
• CPC Class Codes: H01F 27/24 Magnetic cores
• CPC Class Codes: H01F 27/28 Coils; Windings; Conductive connections

Abstract

Two magnetic field sensors, ratiometric with respect to their common supply and featuring matched thermal coefficients, are inserted in the two airgaps of a magnetic circuit arranged so that said airgaps appear in series with respect to the magnetic flux generated by the current to be measured, while appearing in parallel with respect to the reference flux generated by a stable permanent magnet. The output signal of one of the sensors is thus proportional to the sum of said fluxes, the other to their difference. Adding and subtracting said signals produces two outputs, one proportional solely to the current to be measured, and the other solely to the reference flux. A feedback loop acts on the common supply of the two sensors in order to hold constant the output proportional to the reference flux, thus producing the effect that drifts with temperature of the magnetic sensitivities are intrinsically compensated for.

Claims

1. A method of measuring electrical current by computing a function of the output signals of two magnetic field sensors (H1, H2) placed at the two airgaps of a magnetic circuit, both sensors belonging to the type ratiometric with respect to the bias current or both belonging to the type ratiometric with respect to the supply voltage and selected to feature matched temperature coefficients of the respective magnetic field sensitivity;

wherein a magnetic flux is established in said magnetic circuit by a first winding carrying the current to be measured (I_M), and thus resulting, at both magnetic field sensors locations, in a first magnetic field component (B_M) proportional to said current;

said method further comprising the step of establishing at said two airgap locations a second magnetic field component (B_R) possessing a constant reference value independent from the value of the current to be measured, and in such a way that said second magnetic field component adds to said first component at one of said two locations, while subtracting from it at the other location;

wherein said reference field component can be established by means of at least one permanent magnet or by means of at least one second winding carrying a current of a constant reference value;

said method further comprising the step of processing through and adder circuit and a subtractor circuit the signals generated by said two magnetic field sensors, so as to produce an output sum signal and an output difference signal, and thus resulting in one of said output signals being proportional to the reference field component solely and the other output signal being proportional solely to the magnetic field component established by the current to be measured;

whereby the output signal proportional to the reference field component is then used in a feedback loop arranged to control the common biasing current, in case the magnetic field sensors belong to the type ratiometric with respect to the biasing current, or the common supply voltage, in case the magnetic field sensors belong to the type ratiometric with respect to the supply voltage, of said magnetic field sensors in such a way that said output signal is held at a constant reference voltage value, consequently resulting in an automatic compensation of the impact of temperature drifts of the magnetic sensitivities on the accuracy of the output signal proportional to the current to be measured.

2. An apparatus for implementing the method as claimed in claim 1, comprising two identical magnetic circuits featuring one airgap each, the first winding carrying the current to be measured (I_M) having the function to establish at both of said airgaps identical values of said first magnetic field component (B_M) proportional to said current, and further comprising two identical auxiliary windings, each winding coupling to just one of said magnetic circuits, serially connected in order to carry the same constant reference current whose function is to establish in both of said airgaps the reference magnetic field component (B_R), wherein said reference current flows through one of said auxiliary windings in the direction resulting in said reference field component adding to the first magnetic field component at the corresponding airgap, while flowing in the opposite direction through the other winding and thus resulting in the reference field component subtracting from the first magnetic field component at the other airgap;

whereby said two magnetic field sensors (H1, H2) are placed at said two airgaps, and their common supply and their output signals are then processed according to the method.

3. An apparatus for implementing the method as claimed in claim 1, comprising a magnetic circuit so arranged that the two airgaps appears in series with respect to the magnetic flux generated by the first winding carrying the current to be measured (I_M) and featuring a second winding wound around a parallel branch of said magnetic circuit, and wherein said parallel branch is positioned in such a way that said two airgaps appear in parallel with respect to the magnetic flux generated by said second winding, while the net magnetic flux fraction generated by the first winding and which closes through said parallel branch is null;

wherein said second winding carries a constant reference current having the function to establish in both of said airgaps a reference magnetic field component (B_R) which adds to the first magnetic field component at one of the airgaps, while subtracting from it at the other airgap;

whereby said two magnetic field sensors (H1, H2) are placed at said two airgaps, and their common supply and their output signals are then processed according to the method.

4. An apparatus for implementing the method as claimed in claim 1, comprising a magnetic circuit so arranged that the two airgaps appears in series with respect to the magnetic flux generated by the winding carrying the current to be measured (I_M), wherein said magnetic circuit features at least one parallel magnetic circuit branch excited by at least one permanent magnet and positioned in such a way that the two airgaps both appear in parallel with respect to the magnetic flux generated by said permanent magnet(s), so that one of the airgaps receives one half of said flux while the other airgap receives the other half;

wherein said magnetic flux generated by said permanent magnet(s) has the function to establish in both of said airgaps a reference magnetic field component (B_R) which adds to the first magnetic field component at one of the airgaps, while subtracting from it at the other airgap;

whereby said two magnetic field sensors (H1, H2) are placed at said two airgaps and their common supply and their output signals are then processed according to the method.