Planar coil device, method and system for sensing changing currents in a planar conductor path

[0002] Conventional devices consist of a current-carrying conductor around which a coil is arranged. An alternating current flowing in the conductor induces in the coil a voltage which is proportional to the change over time in the current. Suitable analysis of the induced voltage yields information concerning the current flowing in the conductor. Devices which perform this type of current measurement are available commercially as complete components, and can easily be integrated into electronic circuits. All that is needed to be provided for the purpose is one connection for the current to be measured, as well as a further connection to lead the measured signal out. Unsatisfactory aspects of such conventional current measurement components are their comparatively large space requirement which limits the miniaturization of circuits, and their high price due to their embodiment as a sealed subassembly.

[0003] One of the objects of the present invention is to provide a current measurement device which requires as little installation space as possible and is also economical.

[0004] According to the present invention, the sensing device is configured in planar fashion on the surfaces of a circuit board, and requires no installation depth. Production of the actual measurement arrangement is accomplished by configuring a conductive path structure, and is thus extraordinarily economical. One of the advantages of the sensing device according to the present invention is that additional actions for galvanic separation of the current-carrying conductor and the measurement arrangement are not necessary. Manufacture is thereby made simpler and more flexible. The configuration as a conductive path structure also offers the advantages of conductive paths in terms of thermal insensitivity, contact reliability, insensitivity to mechanical damage, and good reproducibility of the mechanical arrangement.

[0005] FIG. 1 shows a basic sensing principle for a sensing device according to the present invention.

[0006] FIG. 2 shows a cross section through the sensing device.

[0007] FIG. 3 shows a schematic diagram of a monitoring circuit.

[0008] FIG. 4 shows an arrangement utilizing the sensing device in a welding system.

[0009] FIG. 1 illustrates the physical principle underlying the sensing device according to the present invention. Alternating currents I_Z1, I_Z2, which carry along with them respective magnetic fields Φ_Z1, Φ_Z2surrounding conductors 11, 12, flow in opposite directions through the respective conductors. A conductor loop 10 is located beneath conductors 11, 12 in such a way that magnetic fields Φ_Z1, Φ_Z2penetrate conductor loop 10. A change in magnetic fields Φ_Z1, Φ_Z2resulting from a change in currents I_Z1, I_Z2induces in conductor loop 10 a voltage U_Iwhich is an indication of the alternating current flowing in conductors 11, 12.

[0010] FIG. 2 shows a cross section through a sensing device utilizing the principle described above with respect to FIG. 1. Two conductive paths (e.g., conductors) 11 and 12 are applied at a spacing D1 from one another onto a first surface 17, designated here as the upper side, of a circuit board 13. Applied onto the opposite surface 18, designated here as the lower side, of circuit board 13, is a conductor loop 10. In order to magnify the measured signal, it is wound several times around itself as indicated by turns 101, 102. Outside diameter D2 of conductor loops 10, 101, 102 is matched to spacing D1 of conductive paths 11, 12 on upper side 17 of circuit board 13. It is selected, as a function of spacing D1, in such a way that the turns of conductor loops 10, 101, 102 are reliably penetrated by the magnetic field surrounding conductive paths 11, 12.

[0011] Voltage U_iinduced in conductor loop 10 is, as shown in FIG. 3 which reproduces a conductor loop 10 in a plan view, conveyed for amplification to an amplifier 14. The amplified measured signal U_iis conveyed to first input 151 of a comparator 15. Connected to the second input 152 thereof is a reference voltage U_refwhich corresponds to a maximum value for the current changes dI_Z1/dt, dI_Z2/dt occurring in conductors 11, 12. There is thus present at output 16 a signal which provides information as to whether the current changes dI_Z1/dt, dI_Z2/dt occurring in conductors 11, 12 lie within a predefined normal range or exceed a limit value.

[0012] FIG. 4 shows an application of the sensing device described above in a welding system operating according to the frequency converter principle. It shows at its input side a rectifier unit 20 for transferring a three-phase input alternating current L1, L2, L3 into a direct-current link circuit constituted by the two conductors 11, 12. The direct-current voltage present in link circuit (e.g., two conductors) 11, 12 is converted in the downstream inverter 21 back into a high-frequency alternating-current voltage which is then conveyed to a welding transformer unit 22, from which it is applied via welding electrodes 23 onto a workpiece 1 being processed. Four power semiconductor switches 211 through 214, each with associated drivers 215 through 218, are the core of inverter 21. To control power semiconductor switches 211 through 214, drivers 215 through 218 are connected to a welding controller 24. Also connected to welding controller 24 is an inductive conductor loop 10, as described with respect to FIGS. 2 and 3. Conductor loop 10 is located on the upper side of a circuit board 25 on whose lower side, opposite conductor loop 10, conductors 11, 12 forming the link circuit are guided. A circuit as shown in FIG. 3 is located inside welding controller 24. In this application, the purpose of sensing device 10, 11, 12, 24, 25 is to detect the occurrence of overcurrents and, if applicable, bring about deactivation of power semiconductor switches 211 through 214. The overcurrents detected are, in particular, those caused by short circuits in the welding circuit.

[0013] The overcurrent monitoring system illustrated in FIG. 4 can be transferred to many other circuits. It is also suitable, in particular, for frequency converter-driven drive arrangements, to detect therein, in timely fashion, overcurrents that may be caused by link circuit short circuits or short circuits in the motor winding or the motor supply lead.

[0014] Induction loop 10 supplies a signal of the form dI/dt that is proportional to the change over time in the measured current. If what is desired is to know the value of the current itself, this can easily be implemented by incorporating an integration member into the circuit.