Magnetic current sensor calibration system

A magnetic current sensor calibration system includes a plurality of sensors and a substrate. The substrate has a first surface and a second surface, and the sensors are mounted on the first surface. The substrate includes a bipolar calibration conductor and a unipolar calibration conductor. The bipolar calibration conductor is spaced apart from the plurality of sensors and is disposed between the first and second surfaces. The unipolar calibration conductor is spaced apart from the plurality of sensors and the bipolar calibration conductor, and is disposed between the first and second surfaces.

TECHNICAL FIELD

The present invention generally relates to current sensors, and more particularly relates to systems and methods for calibrating magnetic current sensors.

BACKGROUND

Current sensors are used in myriad systems to monitor the magnitude of electrical current being supplied to or drawn by various electrical loads. Numerous techniques have been developed for sensing electrical current. One particular technique, which is used to measure relatively large current magnitudes, involves measuring the magnetic field that is generated when electrical current flows in a conductor.

Regardless of the technique that is employed, many current sensors that are used to measure relatively large current magnitudes operate in challenging environments. For example, these current sensors may be exposed to relatively large temperature variations, relatively high and/or low temperatures, and to vibration. It is desirable, in most instances, that these current sensors exhibit robust performance, such as very low offset and stable gain, in these challenging environments. It is additionally desirable that these current sensors provide health monitoring capability, and are further configured to issue an alert in the unlikely event of a malfunction. Unfortunately, many relatively high-accuracy, robust current sensors can be costly, and many rely on relatively cumbersome calibration procedures.

Hence, there is a need for a robust, accurate current sensor that can sense relatively large current magnitudes, provides health monitoring capability, and does not rely on a cumbersome calibration procedure. The present invention addresses at least these needs.

BRIEF SUMMARY

In one embodiment, a magnetic current sensor calibration system includes a plurality of sensors and a substrate. The substrate has a first surface and a second surface, and the sensors are mounted on the first surface. The substrate includes a bipolar calibration conductor and a unipolar calibration conductor. The bipolar calibration conductor is spaced apart from the plurality of sensors and is disposed between the first and second surfaces. The unipolar calibration conductor is spaced apart from the bipolar calibration conductor, and is disposed between the first and second surfaces.

In another embodiment, a magnetic current sensor calibration system includes a plurality of sensors, a substrate, and a controller. The substrate has a first surface and a second surface, and the sensors are mounted on the first surface. The substrate includes a bipolar calibration conductor and a unipolar calibration conductor. The bipolar calibration conductor is spaced apart from the plurality of sensors and is disposed between the first and second surfaces. The unipolar calibration conductor is spaced apart from the bipolar calibration conductor, and is disposed between the first and second surfaces. The controller is electrically coupled to the bipolar calibration conductor, to the unipolar calibration conductor, and to the plurality of sensors. The controller is configured to energize the bipolar calibration conductor and the unipolar calibration conductor, adjust a gain and an offset of each of the plurality of sensors, and measure outputs of each of the plurality of sensors and differential outputs of sensor pairs.

In yet another embodiment, a magnetic current sensor calibration system includes a plurality of sensors, a substrate, a first bipolar calibration current output conductor, a second bipolar calibration current output conductor, a bipolar calibration current input conductor, a unipolar calibration current output conductor, and a unipolar calibration current input conductor. The substrate has a first surface and a second surface, and the sensors are mounted on the first surface. The substrate includes a bipolar calibration conductor and a unipolar calibration conductor. The first bipolar calibration current output conductor is electrically connected to the bipolar calibration conductor. The second bipolar calibration current output conductor is spaced apart from the first bipolar calibration current output conductor and is electrically connected to the bipolar calibration conductor. The bipolar calibration current input conductor is electrically connected to the bipolar calibration conductor and is disposed between, and is spaced equidistant from, the first and second bipolar calibration current input conductors. The unipolar calibration current output conductor is electrically connected to the unipolar calibration conductor. The unipolar calibration current input conductor is electrically connected to the unipolar calibration conductor and is spaced apart from the unipolar calibration current input conductor. The bipolar calibration conductor is spaced apart from the plurality of sensors and is disposed between the first and second surfaces. The unipolar calibration conductor is spaced apart from the plurality of sensors and the bipolar calibration conductor, and is disposed between the first and second surfaces. The bipolar calibration conductor is configured such that, upon being electrically energized, a first half of the sensors are exposed to a first magnetic field, and a second half of the sensors are exposed to a second magnetic field, the first magnetic field having a first magnitude and a first direction, the second magnetic field having the first magnitude and a second direction, the second direction opposite the first direction. The unipolar calibration conductor is configured such that, upon being electrically energized, all of the sensors are exposed to a third magnetic field having a third magnitude and one of the first direction or the second direction.

Furthermore, other desirable features and characteristics of the magnetic current sensor calibration system will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the preceding background.

DETAILED DESCRIPTION

Referring now toFIGS. 1-3, side, top, and cross section schematic views, respectively, of a current sensor system100are depicted. The depicted system includes a conductor102and a current sensor104. The conductor102, which may be, for example, a bus bar made of copper, aluminum, or any one of numerous other electrical conductors, is configured to carry relatively high electrical currents. The conductor102has a top surface106, a bottom surface108, and an opening110formed therein that extends between the top and bottom surfaces106,108. Preferably, the opening110is disposed substantially in the middle of the width of the conductor102.

The current sensor104is disposed within the opening110and includes a mount112and a plurality of sensors114. The mount112may be variously configured and implemented, but in the depicted embodiment includes at least a conductor mount portion116and a sensor mount portion118. The conductor mount portion116, which may be variously configured, is used to secure the current sensor104within the opening110. The sensor mount portion118, which extends perpendicularly from the conductor mount portion116, has the plurality of sensors114mounted thereon. Preferably, the sensors114are mounted on the sensor mount portion118such that the sensors114are disposed substantially halfway between the top and bottom surfaces106,108of the conductor102, and substantially in the middle of the opening110. The sensors114are preferably implemented using magnetic sensors, such as anisotropic magneto-resistive (AMR) sensors, and thus sense the magnetic field that is generated when current flows in the conductor102. Preferably, the magnetic field at the center of the opening, where the sensors114are disposed, is sufficiently small, even when relatively large current is flowing in the conductor102. This allows very sensitive magnetic-field sensors114to be used.

Turning now toFIGS. 4 and 5, more detailed, but simplified representations of a portion of the sensor mount portion118are depicted. The sensor mount portion118is preferably formed of a multilayer substrate402, such as a multilayer circuit board. The substrate402includes a first surface404, a second surface406, a bipolar calibration conductor408, and a unipolar calibration conductor412. In the depicted embodiment, there are four independent sensors114, all of which are mounted on the first surface404. It will be appreciated that more or less than this number of sensors114could be used; however, the number of sensors114is preferably an even number. This is because a first half of the sensors114(e.g.,114-1,114-3) are mounted on one side of an axis of symmetry413of the bipolar and unipolar calibration conductors408,412, and a second half of the sensors114(e.g.,114-2,114-4) are mounted on the other side of the axis of symmetry413.

The bipolar calibration conductor408is spaced apart from the plurality of sensors114and is disposed between the first and second surfaces404,406of the substrate402. The unipolar calibration conductor412is spaced apart from the bipolar calibration conductor408, also between the first and second surfaces404,406of the substrate402. Although the bipolar calibration conductor408is depicted as being disposed closer to the first surface404than to the second surface406, and the unipolar calibration conductor412is depicted as being disposed closer to the second surface406than to the first surface404, this is merely an example of one embodiment. Indeed, in other embodiments, the locations of the bipolar calibration conductor408and the unipolar calibration conductor412could be switched.

AsFIGS. 4, 6, and 7also depict, the calibration conductors408,412have different sizes. In the depicted embodiment, the bipolar calibration conductor408has a first area (A1), and the unipolar calibration conductor412has a second area (A2) that is larger than the first area. It will be appreciated, however, that this too is merely an example of one embodiment, and in other embodiments the areas (A1, A2) could be equal, or the first area (A1) could be larger than the second area (A2).

AsFIGS. 4-7also depict, the sensor mount portion118additionally includes a plurality of calibration current conductors, each of which is formed in the substrate402. These conductors include a first bipolar calibration current output conductor414, a second bipolar calibration current output conductor416, a bipolar calibration current input conductor418, a unipolar calibration current input conductor422, and a unipolar calibration current output conductor424. The first and second bipolar calibration current output conductors414,416, and the bipolar calibration current input conductor418are all electrically connected to the bipolar calibration conductor408. The first and second bipolar calibration current output conductors414,416are spaced apart from each other, and the bipolar calibration current input conductor418is disposed between, and is spaced equidistant from, the first and second bipolar calibration current input conductors414,416. The bipolar calibration current input conductor418is also preferably aligned with the axis of symmetry413.

As may be readily appreciated, when the bipolar calibration current input conductor418is supplied with a drive current (IDRIVE), half of the drive current (½ IDRIVE) will flow through the bipolar calibration conductor408in a first direction422, and flow out the first bipolar calibration current output conductor414, thereby generating a first magnetic field. The other half of the drive current (½ IDRIVE) will flow through the bipolar calibration conductor408in a second direction424, and flow out the second bipolar calibration current output conductor416, thereby generating a second magnetic field. As a result, the first half of the sensors114(114-1,114-3) will be exposed to the first magnetic field, and the second half of the sensors114(114-2,114-4) will be exposed to the second magnetic field. Because the currents are equal but flowing in opposite directions, the first and second magnetic fields will have equal magnitudes, but opposite directions. That is, the first magnetic field will have a first magnitude and a first direction, while the second magnetic field will also have the first magnitude but will have a second direction that is opposite the first direction.

It will be appreciated that in some embodiments, the first and second bipolar calibration current output conductors414,416could be first and second bipolar calibration current input conductors, and the bipolar calibration current input conductor418could instead be a bipolar calibration current output conductor. In these embodiments, when the first and second bipolar calibration current output conductors414,416are supplied with a drive current (IDRIVE), half of the drive current (½ IDRIVE) will flow through the bipolar calibration conductor408in the first and second directions422,424toward and out bipolar calibration current input conductor418, thereby generating first and second magnetic fields.

The unipolar calibration current input and output conductors422,424are spaced apart from each other and are electrically connected to the unipolar calibration conductor412. These conductors422,424are disposed such that when the unipolar calibration current input conductor422is supplied with a drive current (IDRIVE), all of the current flows through the unipolar calibration conductor412in the same direction, and flow out the unipolar calibration current output conductor424, thereby generating a magnetic field. Because of the configuration and disposition of the unipolar calibration current input and output conductors422,424, the relatively wide size of the unipolar calibration conductor412, and the relative locations of the sensors114, all of the sensors114will be exposed to the same magnetic field that is generated.

The unipolar calibration conductor412and the bipolar calibration conductor408are used to calibrate the current sensor104, and may be used, after calibration and during operation, to monitor the health of the current sensor104. The manner in which the calibration is carried out will now be described. In doing so, reference should be made toFIG. 8, which depicts a block diagram of a circuit that may be used to implement a particular calibration process.

The calibration circuit800includes a controller802that is electrically coupled to the current sensor104and is configured to selectively energize the unipolar calibration conductor412and the bipolar calibration conductor408. The controller802is also coupled to selectively receive the responses supplied from the sensors114. The controller802selectively receives the responses by controlling current amplifiers804that are connected to a different one of the sensors114. Specifically, the controller802is configured to selectively switch the amplifiers804on and off, as needed or desired. The controller802is also configured to control the currents applied to the calibration conductors408,412via control lines806and808, respectively, and to adjust the gain and offsets of the individual sensors114via control lines812(e.g., I2C or SPI) that are connected to individual sensor gain and offset control inputs814(e.g.,814-1,814-2,814-3,814-4).

The controller802selectively energizes the calibration conductors408,412, and selectively obtains the responses from various ones of the sensors114. The sensors114are then adjusted, based on the obtained responses, to appropriately calibrate the current sensor104. More specifically, and as will now be described, the unipolar calibration conductor412is used to equalize the sensor gains and remove any offset, and the bipolar calibration conductor408is used to conduct sensor health checks and drift correction.

Initially, the controller802will energize the unipolar calibration conductor412to zero the offsets for the sensors114. To do so, the controller802will energize the unipolar calibration conductor412with a first current magnitude. The controller802will then switch on consecutively the first, second, third, and fourth current amplifiers804-1,804-2,804-3, and804-4, and switch off the other amplifiers and obtain the response. The controller802will then repeat this using another current magnitude. The offset may be calculated from a linear extrapolation of the sensor output at the two current magnitudes. The offsets for the sensors114may then be adjusted via, for example, a digital potentiometer until the calibrations described above yield substantially zero offsets. It will be appreciated that in some embodiments, the current amplifiers804could be implemented using switches. In still other embodiments, this functionality could be implemented in the controller802.

After the offsets are zeroed out, the gains of the sensors114are equalized. Specifically, the controller802energizes the unipolar calibration conductor412with a current magnitude, while switching on the first and second amplifier804-1,804-2, and switching off the third and fourth current amplifiers804-3,804-4. The gains of the first and second sensors114-1,114-2are then equalized until the difference between their outputs is substantially equal to zero. This difference may be measured using differential amplifier816. Thereafter, with the unipolar calibration conductor412still being energized with the same current magnitude, the third and fourth current amplifiers804-3,804-4are switched on, and the first and second current amplifiers804-1,804-2are switched off. The gains of the third and fourth sensors114-3,114-4are then equalized.

Before proceeding further, it is noted that the sensor gains are equalized so that the sensors114will better reject common mode magnetic interference. Such magnetic interference could come from other nearby current conductors or from the earth magnetic field that may get amplified by nearby ferromagnetic materials, such as iron.

Returning now to the calibration process, after the sensor offsets are zeroed and the gains are equalized, the current sensor104gain is trimmed by placing it in the opening110in the conductor102(seeFIG. 1), and passing a relatively high current, such as 50 amps, through the conductor102. While the current is passing through the conductor102, the gains of the first and second sensors114-1,114-2are iteratively adjusted, preferably the same amount to maintain equalization, until the current sensor104outputs the correct value (e.g., 50 amps). Thereafter, the gains of the third and fourth sensors114-3,114-4are iteratively adjusted, preferably the same amount, until the current sensor104outputs the correct value (e.g., 50 amps).

Finally, with the relatively high current (e.g., 50 amps) again passing through the conductor102, the controller802supplies enough current to the bipolar calibration conductor408to bring the sensor114output to zero. In the depicted embodiment, the sensor output is the voltage drop across the output load818, such as a resistor. This amount of current can then be used to perform sensor health checks and drift corrections by applying the calibration current determined above to the bipolar calibration conductor408preferably during a startup of the sensor114when there is no current flowing in the conductor102. If the sensor output has not changed from the calibrated value, the sensor114is deemed to be functioning properly and there is no drift. It will be appreciated that sensor drift could also be corrected by performing the full calibration process described above to check if the sensor gains are equalized and the offsets are equal to zero. The gains and offsets could then be returned to the optimal values by the digital potentiometers to preserve the high performance of the sensor114.

The current sensor system disclosed herein is robust, accurate, can sense relatively large current magnitudes, provides health monitoring capability, and does not rely on a cumbersome calibration procedure. Moreover, the configuration of the bipolar calibration conductor408allows for periodic in-situ testing of the current sensor104to ensure proper sensor operation and may also detect, for example, sensor drift.