WO2002033424A2 - Arrangement for potential-free measurement of high currents - Google Patents

Abstract

The invention relates to an arrangement comprising a current sensor, containing a bypass-branch (3) which is electrically connected in a parallel to a defined section of the current conductor (1) and a current probe (5) for measuring the current in the bypass-branch (3), a fixed dividing ratio for the current passing therethrough existing between the bypass-branch (3) and the bridged section of the current conductor (1). The dividing ratio and the electric resistance of the bypass-branch (3) are relatively high in comparison to the resistance of the bridged section of the current conductor (1), whereby the contact resistances (Rk) between the current conductor (1) and the bypass-branch (3) are negligible in relation to the resistance (RBY) of the bypass-branch (3).

Description

description

Arrangement for the potential-free measurement of high currents

The present invention relates to an arrangement for the potential-free measurement of high currents.

The measurement of high currents is particularly important in the automotive sector, for example when recording the on-board electrical system current or battery current for safety purposes and simple control functions. These are currents of up to 400 A, whereby the accuracy requirements are moderate.

Furthermore, a measurement of high currents when recording the battery current is important for battery energy management systems. These are currents of up to 400 A, and the accuracy requirements are high.

When detecting the phase current in a converter-operated electrical machine such as a starter generator or drive motor of hybrid or electric vehicles, currents of up to 1300 A are processed, the accuracy requirements usually being between the first two.

In addition to low manufacturing costs, such a current sensor also requires a compact design because there is little installation space available for its installation.

So far, the following current sensors have been used:

Shunt: A resistor is inserted into a circuit, at which the voltage drop is measured. This resistance can also simply be a piece of the existing conductor. The disadvantages associated with this technology are, on the one hand, the dependence of the measurement result on the conductor temperature. On the other hand, in the interest of accuracy, either extremely low voltages must be measured or a relatively large resistance must be selected. However, high resistances lead to heating and thus to power loss. In addition, a shunt cannot be used in many areas because it cannot tolerate short-term, significantly higher currents than the maximum current to be measured, as is the case with car batteries, for example, where currents of up to 1500 A occur when starting.

Toroidal current sensor: The conductor is led through the opening of a magnetic toroid. The core is magnetized by the magnetic field of the current. The magnetization in a slot in the core is measured by a magnetically sensitive element (e.g. Hall IC). The signal from this probe is either used directly as an output signal (direct imaging sensor) or fed via an amplifier to a compensation coil on the toroid, which compensates for the magnetization of the core to zero (compensation sensor). The second variant has the higher accuracy and the far better linearity. The assembly process is extremely unfavorable because the conductor has to be led through the opening of the sensor. In addition, toroidal current sensors are comparatively expensive due to their size.

Magnetic field probe: Magnetic field sensitive probes (eg Hall sensors or magnetoresistive sensors) are attached to the conductor, through which the current is measured via the magnetic field of the conductor. However, it is disadvantageous that the measured value depends on the distance from the probe to the conductor, so that the mechanical tolerances of the arrangement are clearly included in the accuracy. In addition, the magnetic field probes, so usually a structurally ^¬-ended and costly shielding aufwen required also sensitive to external (interference) fields. Such sensors are also used as gradient sensors in which the difference in magnetic fields is measured at two locations becomes. This partially eliminates the disadvantage of sensitivity to interference.

Bypass sensor: The current path is divided and the smaller part is measured with a sufficiently accurate toroidal sensor. This sensor is then smaller and less expensive because it is designed for lower currents. A disadvantage of the bypass sensors currently used is the difficulty in keeping the distribution factor constant over a longer period of time and when the temperature changes, since the contact resistances at the contact points between the two conductors are subject to corrosion influences and mechanical stresses. In addition, both paths are difficult to maintain at the same temperature, so that the division ratio becomes temperature-dependent.

Although useful results can be achieved with the sensors used for measuring high currents according to the prior art, these are complex, cost-intensive, and in some cases not reliable and thus overall unsatisfactory.

The object of the present invention is therefore to provide an arrangement for measuring high currents, which rather at least partially avoids the disadvantages listed above. It is a particular object of the present invention to provide a reliable but inexpensive current sensor which has a measurement accuracy which is at least equal to that of the conventional current sensors.

The object is achieved by an arrangement with the features of independent claim 1. Refinements and developments of the inventive concept are the subject of dependent patent claims.

The arrangement according to the invention for the potential-free measurement of high currents in a current conductor comprises a current sensor, which has a bypass conductor, via which a small but defined part of the current is conducted in parallel to a specific section of the current conductor, and a current probe, which measures the current in the bypass branch, from which the total current can be determined using the known division ratio. There is such a high division ratio between the bypass branch and the current conductor that the contact resistances at the contact points between the two conductors cannot significantly influence the division ratio.

This is based on the following consideration: With normal division ratios of 1: 1 to 5: 1, based on the ratio of the current in the main conductor to the current in the bypass conductor, the main conductor and the bypass conductor have comparable cross sections and without the use of expensive measures Contact resistances at the contact points should be of the same order of magnitude as the resistance of the bypass conductor. As already mentioned above, these contact resistances are difficult to keep constant. They influence the division ratio, which makes an exact measurement impossible. If, however, the cross-section of the bypass conductor is reduced so much in comparison to the main conductor that the division ratio is significantly greater than 10: 1, preferably greater than 50: 1 to 1000: 1, the contact resistance becomes negligible and the division ratio is only reduced to Cross-section, material and length of the bypass conductor determined in comparison to the bridged section of the main conductor.

The application of the bypass principle to a short section of an electrical conductor in connection with a high division ratio is the subject of independent claim 1.

Accordingly, the sensor is designed with a bypass branch and a current probe, which essentially does not introduce any additional resistance into the bypass branch. It is designed according to the bypass principle with a high division ratio, preferably a division ratio of 50: 1 to 1000: 1st The division ratio can, for example, be designed such that the maximum current to be measured in the bypass branch is below 20 A, preferably in the range of 1 and 2 A. The actual measurement of the current does not take place on or on the conductor, but in the bypass branch. A current probe is selected for the actual measurement, which has good accuracy (for example in the range of 20 A) and in particular does not introduce any additional resistance into the bypass branch.

The current probe, which measures the current in the bypass branch, must not, on the one hand, introduce any additional resistance into the bypass branch and, on the other hand, must be suitable for measuring small currents, that is to say in particular have a high resolution and a low offset. The low offset (current) results in particular from the use of a closed core and is less than 50 A, preferably less than 20 mA and in particular 10 mA. Direct imaging low-current sensors are suitable for this purpose, which have a small toroidal core made of amorphous or nanocrystalline metal, which is wound with a coil of copper wire. Such probes are characterized by a particularly high impedance ratio (greater than 4: 1, for example 10: 1) between the saturated and the unsaturated state of the magnetic core. The bypass conductor is passed through the inner hole of the wound core.

This probe is preferably operated by evaluation electronics, which reverses the polarity of the current in the toroidal core coil as soon as it exceeds a certain limit value. As a result, the probe core is constantly magnetized at high frequency until saturation. The magnetic reversal time differs depending on the direction of the current, the difference being proportional to the current to be measured. In the case of a freely oscillating arrangement, a PWM square-wave signal (PWM = pulse width modulated) is automatically generated, the duty cycle of which is determined by the current to be measured. The application of this probe principle to the arrangement according to independent claim 1 is an advantageous development of the inventive concept and the subject of dependent claims 9 to 12.

Further advantageous developments of the inventive concept relate to the mechanical structure of the sensor and are the subject of dependent claims 2 to 8.

Current conductors for high currents are often flat copper brackets.

In order to achieve a compact and inexpensive arrangement for measuring the current in such a bracket, the current sensor with bypass conductor and current probe and optionally also the evaluation electronics is implemented on a small circuit board or another suitable carrier. This carrier can be firmly connected to the conductor.

The current sensor is preferably thermally coupled to the current conductor, so that different temperatures of the current conductor and current sensor are avoided and the division ratio remains constant.

Furthermore, the bypass branch can be designed as a wire over at least part of its length and this wire can be at least partially wound up into a coil. This ensures that the influence of the skin effect that is noticeable at high frequencies is largely compensated for.

The current sensor is preferably attached to the current conductor via the printed circuit board by means of clips or screws.

One or both outer sides of the circuit board can be metallized all around. The metallization on the underside can then be used as a contact area for the connection between the printed circuit board and the current bracket and should advantageously be designed to be sufficiently large. The me- The metalization on the underside can also be connected to the conductor tracks on the top side by means of a via, so that metallization of the circuit board around the ends is not necessary.

It is preferably provided that a toroidal core current probe is provided as the current probe, in which the sensing element is a wound toroidal core made of amorphous or nanocrystalline metal.

For the purpose of further shielding against interference, a cover, in particular a cylinder made of magnetically conductive material, preferably a highly permeable nickel-iron alloy, can be arranged around the current probe and shields the current probe from the outside. This is particularly advantageous since the main conductor generates a high magnet eld at the location of the current probe due to the high current and the short distance from the current probe. The direction of this field is in the insensitive direction of the current probe, but because of its strength, it could still have a negative effect on the measurement without the shielding measure mentioned.

Finally, the current probe can have evaluation electronics, which are preferably in the form of an integrated circuit (e.g.

ASIC) is executed and mounted on the guide plate and contains, for example, all essential elements except the sensor.

The invention is based on the in the figures of the

Drawing illustrated embodiments explained in more detail. It shows :

FIG. 1 shows a circuit diagram for a bypass solution,

FIG. 2 shows a preferred construction of a current probe with connected evaluation electronics, 3 shows the basic circuit diagram of this evaluation electronics and

FIG. 4 shows a preferred embodiment of a current sensor in a schematic representation.

In the preferred embodiments according to the invention described below, identical or similar elements are provided with the same reference symbols.

A bypass solution is described in the circuit diagram according to FIG. 1. R- _j _ is the resistance of the conductor section over which the bypass branch (resistance Rgy) is placed. R ^ are contact resistances. I-ι_ and I _By denote currents through the conductor section or the bypass branch. The primary side of the sensor has zero resistance (toroid inserted over the conductor). The division ratio is calculated as follows:

II ₌ 2 R _k + R _py

^I By ^R l

The general problem with bypass solutions is the contact resistance at the transition point between the two conductors. These contact resistances can change due to mechanical stress or corrosion and thereby have a lasting effect on the division ratio. Self-heating when passing a high current also makes a significant contribution to this.

A high division ratio results when the resistance of the bypass branch R _{By is} large compared to the resistance of the bridged conductor section. The extent to which the contact resistances can be minimized without great effort is limited by the size of the main conductor (for example the size of the possible contact surface of the relevant metallization on the underside of the circuit board). With high division ratio it is therefore possible to keep easily the contact resistances R _κ so small that they are vernach ^¬ lässigbar against the resistance of the bypass conductor Rg _y in general. At least variations in the contact resistances then only have an insignificant influence on the division ratio

With a division ratio of 500: 1 and a maximum current to be measured of 1000 A, according to a preferred embodiment, only a maximum of 2 A flows in the bypass path. With a current bracket of 15 x 2 mm ² cross section as a conductor, the bypass can - Branch are formed by a wire with a diameter of 0.28 mm, which has the same length as the bridged conductor piece. The current probe uss accordingly has a high measuring accuracy at currents from 10 mA to 2 A.

The bypass branch is in good thermal contact with the main conductor due to the arrangement of the bypass branch on or in a printed circuit board, which in turn is fastened directly on the current conductor. Different temperatures of the two conductors are avoided and the division ratio remains constant.

If the contact to the current clamp is made appropriately, the contact resistances are negligible. Because of the low current through the bypass branch or due to the significantly larger cross section of the contact points compared to the bypass conductor, no additional heating of the contact points occurs.

A preferred current probe is shown in FIG. 2 together with the

Evaluation electronics shown. It is a current probe for the potential-free measurement of small currents, whereby the sensing element consists of a wound toroid. Exemplary dimensions are 010 mm x 04 mm x 4 mm. DE 197 05 770 and DE 198 44 729 are also simple

Current probes for potential-free measurement of small currents wrote, which are suitable for the current sensors according to the present invention.

The current probe is suitable for applications with moderate accuracy requirements for currents up to about 20 A. to

Measuring v'oii currents in the range up to about 2 A, it has very good accuracy and is superior to the otherwise highly accurate toroidal compensation sensors in this measuring range.

In the exemplary embodiment according to FIG. 3 relating to the electronic evaluation of the probe signal, the sensor T is shown with the circuit symbol of a transformer (equivalent circuit diagram). The sensor T is driven on the primary side with a current I to be measured. This is generally the current through the conductor inserted through the inner hole of the wound toroid, alternatively it may be a few windings around the toroid instead of this conductor. The secondary winding designates the actual toroidal winding of the current probe with inductance L.

A comparator K evaluates a current i through the secondary winding by comparing the absolute value of the voltage drop across the upstream resistor R with a predetermined value. Depending on the comparison result, a switching device S is controlled by the comparator K, which reverses the polarity of the direct voltage of the amount U at the series circuit of a resistor R and the inductor L as soon as the absolute amount of the current i through the resistor R and inductor L exceeds a certain value. A square-wave signal is thus generated on the series circuit of resistor R and inductance L, which serves as output signal U _ou t of the current sensor and whose pulse duty factor Q behaves as follows:

Q = (T.-T.) / (T ₊ + T.) = (R'I / UN) where T ₊ and T_ represent the pulse durations of the signal levels "High" and "Low" and N stands for the number of turns of the secondary winding of the current sensor (probe winding).

The current in the toroidal coil of sensor L is therefore reversed as soon as it exceeds a certain limit value. As a result, the probe core is constantly magnetized at high frequency until saturation. The remagnetization time differs depending on the direction of the current, the difference being proportional to the current to be measured. In the case of a freely oscillating arrangement, a PWM square-wave signal (PWM = pulse width modulated) is therefore automatically generated, the pulse duty factor of which is determined by the current to be measured.

A particularly advantageous development of the invention is shown schematically in FIG.

The current conductor 1 is a copper bracket. The current sensor 1 is implemented on a small circuit board 2 with dimensions of 15 mm x 20 mm. The narrower sides of the circuit board 2 are metallized all around. The circuit board 2 is pressed onto the copper bracket by suitable clamps or by screws or by other suitable methods such that there is good contact from the bracket 1 to the metallized end surfaces of the circuit board 2.

The bypass branch is designed as a conductor track 3 on the printed circuit board 2. The current probe, according to this preferred embodiment, comprises a wound toroidal core made of amorphous or nanocrystalline metal, the dimensions of which, including the winding, are approximately 4 mm inner diameter x 10 mm outer diameter x 4 mm height. The toroidal core of the current probe 5 is placed flat on the circuit board 2. The first part of the conductor track 3 leads under the winding of the current probe 5 through its inner hole. There it is electrically connected to a U-shaped wire bracket 4, which through the inner hole 5 over the edge thereof to the first part of the lead Terbahn 1 opposite side of the circuit board 2 leads. From there, the bypass branch closes via the second part of the conductor track 1 and the second metallized end face of the circuit board 2.

The bypass branch can also be designed as a wire over part of its length. In order to compensate for the influence of the skin effect at high frequencies, the wire is wound over a certain length into a coil according to a preferred embodiment. In addition to the resistance, it also has a certain inductance, which can compensate for the temporally leading behavior of the bypass branch within certain limits.

In order to prevent the current probe 5 from being influenced by the strong field of the main conductor 1, a cover, preferably a cylinder, made of a magnetically conductive material, which covers the cross section of the current probe to the outside, is arranged around the current probe 5. For example, said cylinder is designed about 1 to 2 mm higher than the current probe.

The evaluation electronics 7 for the current probe are also mounted on the printed circuit board 2. This can preferably be rationally integrated on an ASIC (with RC circuitry) (the placement area is only 15x25 mm ^2, for example, even with a discrete structure). Outputs are a PWM signal (0 / 5V), the pulse width ratio of which depends on the current to be measured, and an overcurrent output that is below a certain limit value at potential "Low" and above this limit value at potential "High".

The PWM output can be counted in a microprocessor (no A / D converter is required) or integrated with the help of a simple RC element. The output voltage is then linearly dependent on the current. For example U _out = U / 2 ± 1.5V if the supply voltage U = 5V is. Such a signal is compatible with many other sensor principles.

Only connections for ground, supply voltage (e.g. 5 volts) and output voltage lead to the PCB.

Due to the compact and flat arrangement on the printed circuit board 2, the bypass branch 3 is in good thermal contact with the current conductor 1, so that different temperatures of the two conductors are avoided and the division ratio remains constant.

According to a development of the present invention, an arrangement of the bypass branch 3 on the underside of the

Printed circuit board with very thin insulation film to the main conductor. According to a further preferred development, the bypass branch 3 is laser-adjusted for the exact setting of the division ratio.

Claims

claims 1. Arrangement for the potential-free measurement of high currents in a current conductor with a current sensor, the bypass branch (3) electrically parallel to a certain section of the current conductor (1) and a current probe (5) for measuring the current in the bypass branch (3 ), whereby between the bypass branch (3) and the bridged section of the current conductor (1) there is a fixed division ratio of the currents flowing through it, characterized in that the current sensor has a small offset and the division ratio and thus the electrical resistance of the bypass branch (3) with respect to the resistance of the bridged section of the current conductor (1) is so high that the contact resistances (R _κ ) between current conductor (1) and bypass branch (3) with respect to the resistance (R _x ) of the bypass branch (3) are negligible. 2. Arrangement according to claim 1, characterized in that the current sensor has a suitable carrier (2) on which the bypass branch (3) and the current probe (5) are arranged and this carrier is arranged on the current conductor (l) and is mechanically firmly connected to this. 3. Arrangement according to claim 1 or 2, characterized in that the current sensor is thermally coupled to the current conductor (1). 4. Arrangement according to one of claims 1 to 3, characterized in that the division ratio is between 50: 1 and 1000: 1 see. 5. Arrangement according to one of claims 1 to 4, characterized in that the carrier (2) is a printed circuit board and the bypass branch (3) except for a wire bracket (4) is designed as a conductor track on this circuit board 6. Arrangement according to one of claims 1 to 5, characterized in that the bypass branch (3) is designed at least over part of its length as a wire or is at least partially wound into a coil. 7. Arrangement according to one of claims 1 to 6, characterized in that in the carrier (2) at least one side is metallized all around (2 ') and that this metallization as a contact surface for the transition between the bypass branch (3) and the current conductor ( 1) is provided. 8. Arrangement according to one of claims 1 to 7, characterized in that the current probe (5) has a ring core which is provided as a sensing element. 9. Arrangement according to one of claims 1 to 8, characterized in that a ring band core made of amorphous or nanocrystalline metal is provided as the ring core. 10. Arrangement according to one of claims 1 to 9, characterized in that the current probe (5) has evaluation electronics (7) and the current probe is followed by a resistor (R), whereby the evaluation electronics (7) on the series circuit from the Inductance of the current probe (L) and the resistor (R) generates a periodic square wave voltage, the duty cycle of which is a measure of the current in the bypass branch (3). 11. The arrangement according to claim 10, characterized in that the square wave voltage is integrated so that a DC voltage is formed, which is a measure of the current in the bypass branch (3). 12. Arrangement according to one of claims 1 to 11, characterized in that a cover (6), in particular a cylinder, made of a magnetically conductive material is arranged around the current probe, which shields the cross section of the current probe from the outside. 13. Arrangement according to one of claims 1 to 11, characterized in that the current sensor from a high impedance ratio between the saturated and the unsaturated state of the magnetic core. 14. Arrangement according to claim 13, characterized in that a direct imaging low current sensor is provided as the current sensor, which has a small toroidal core made of amorphous or nanocrystalline metal, which in turn is wrapped with a coil of copper wire, the bypass conductor being through the inner hole of the wound toroidal core is passed.