Method and aparatus for monitoring a junction between electrical devices

A method and a test fixture for evaluating a junction between an electrical lead trace and a busbar are described, and include an electric power supply disposed to supply electric power to the electrical lead trace and an electric monitoring device disposed to monitor electrical potential across the junction. A mechanical stress-inducing device is disposed to apply mechanical stress proximal to the junction. The electric monitoring device monitors the electrical potential across the junction of the electrical lead trace coincident with the mechanical stress-inducing device applying mechanical stress proximal to the junction when the electric power supply is supplying electric power to the electrical lead trace. Electrical integrity of the junction is evaluated based upon the monitored electrical potential across the junction.

TECHNICAL FIELD

This disclosure relates to junctions between electrical devices, and monitoring thereof.

BACKGROUND

A battery pack typically includes multiple rechargeable battery cells that are connected in series or parallel to store and supply electric power to a distribution system. Terminals of adjacent battery cells are joined at busbars, and electrical lead traces may electrically connect to the busbars for monitoring purposes. Joining of electrical lead traces to busbars may be accomplished employing a mechanical fastener, e.g., a rivet. There are opportunities for improvement of methods and test fixtures to evaluate electrical integrity of such a mechanical fastener at assembly. Known methods for evaluating electrical integrity include static impedance tests and visual inspections.

SUMMARY

A method and a test fixture for evaluating a junction between an electrical lead trace and a busbar are described, and include an electric power supply disposed to supply electric power to the electrical lead trace and an electric monitoring device disposed to monitor electrical potential across the junction. A mechanical stress-inducing device is disposed to apply mechanical stress proximal to the junction. The electric monitoring device monitors the electrical potential across the junction of the electrical lead trace coincident with the mechanical stress-inducing device applying mechanical stress proximal to the junction when the electric power supply is supplying electric power to the electrical lead trace. Electrical integrity of the junction is evaluated based upon the monitored electrical potential across the junction.

DETAILED DESCRIPTION

Referring now to the drawings, which are provided for the purpose of illustrating certain exemplary embodiments only and not for the purpose of limiting the same,FIG. 1schematically illustrates a battery cell interconnect board10including a plurality of electrically conductive busbars30that are disposed in a non-conductive frame12, and an accompanying monitoring circuit40that includes an electrical connector50. The battery cell interconnect board10is advantageously disposed on a second frame portion (not shown) that houses a plurality of rechargeable battery cells (not shown). The battery cell interconnect board10and the second frame portion together form a battery pack. The battery cells each have a positive terminal and a negative terminal, and subsets of the positive or negative terminals are welded to one of the busbars30. The battery cells may be connected in series or parallel through the battery cell interconnect board10to store and supply electric power to an electric power distribution system. The battery pack may be disposed on a vehicle in one embodiment to supply electric power to propulsion systems and other systems, depending upon the specific application. Those having ordinary skill in the art will recognize that terms such as “horizontal”, “vertical”, “above,” “below,” “upward,” “downward,” “top,” “bottom,” etc., are used descriptively for the figures, and do not represent limitations on the scope of the disclosure, as defined by the appended claims. Furthermore, the disclosure, as illustrated and described herein, may be practiced in the absence of any element which is not specifically disclosed herein.

Each busbar30is fabricated from conductive material, e.g., copper, and preferably includes a web portion31and side portions32, with a first aperture33and a second aperture34formed in the web portion31. The first aperture33is preferably centrally disposed on the web portion31along its longitudinal axis, and the second aperture34is preferably disposed near one of the ends of the web portion31. Subsets of the positive or negative terminals of the battery cell are welded to side portions32of the busbars30.

The battery cell interconnect board10includes the electrically conductive busbars30disposed in the non-conductive frame12. The frame12may be a rectangularly-shaped rigid device that is fabricated from non-conductive thermoplastic material, e.g., polycarbonate, using injection molding or another suitable process. The frame12includes an outer peripheral portion14, a central bridge16, a plurality of reinforcement ribs19and a plurality of side bridges20. A plurality of apertures18are formed between adjacent reinforcement ribs19and side bridges20. The apertures18accommodate either the positive terminals or the negative terminals from a subset of the battery cells, which extend therethrough to permit welding to the side portions32of one of the busbars30during a subsequent assembly process. In one embodiment, the battery cell interconnect board10is formed by overmolding the frame12onto the plurality of busbars30, wherein each of the busbars30is oriented with the second aperture34proximal to the central bridge16. Each of the busbars30preferably includes protrusion portions that secure the busbars30into the frame12as part of the overmolding process. Alternatively, the frame12molded such that each of the side bridges20includes a protrusion portion22that extends upwardly from the surface of the side bridge20. The busbars30are assembled onto the side bridges20such that the web portion31of each busbar30is contiguous with the side bridge20. Each busbar30is oriented to have its second aperture34proximal to the central bridge16, and the protrusion portion22of the side bridge20is inserted into the first aperture33. Heat is then applied to plastically deform the protrusion portion22and fixedly secure the busbar30to the side bridge20.

The monitoring circuit40includes a plurality of electrical lead traces44that are fabricated onto a non-conductive flexible web material45, wherein the electrical lead traces44electrically connect between the busbars30and terminal pins52of the connector50for monitoring and signal communication. One of the electrical lead traces44electrically connects between one of the busbars30and one of the terminal pins52of the connector50, and preferably includes an in-series fuse46. A single electrical lead trace44, fuse46and terminal pin52are shown for ease of illustration. In use, the connector50communicates electrical information gathered from the subsets of the battery cells via the busbars30for purposes of monitoring, load balancing, fault detection, etc. The electrical connector50includes a plurality of terminal pins52that are arranged in a structured body to effect connection to another device, such as a monitoring controller.

A portion of each electrical lead trace44and a portion of the web material45is formed into a tab42that preferably overlaps with one of the busbars30such that the tab42is adjacent with the second aperture34of the web portion31of the busbar30. The tab42is fixedly secured to the busbar30employing a permanent mechanical fastener36, such as a rivet, which forms a junction38between the tab42and the busbar30. Each mechanical fastener36forms the junction38by applying a normal force that compresses adjoining surfaces of the busbar30and the portion of the electrical lead trace44formed into the tab42. The normal force may be applied by deforming a portion of the mechanical fastener36in one embodiment. Each junction38has two components, including a mechanical joining of the tab42and the busbar30, and an electrically-conductive joining of the portion of the electrical lead trace44and the busbar30. Mechanical fasteners36such as rivets are known.

Assembly processes associated with mechanically coupling a plurality of the tabs42to corresponding busbars30employing a plurality of mechanical fasteners36may be subject to variation. Such variation may be associated with the magnitude of the applied normal force due to the deformation of the mechanical fastener36during fabrication, wherein the variation may be non-obvious. Furthermore, one of the junctions38formed between one of the tabs42and one of the busbars30may appear to be mechanically sound but have a non-obvious difference that introduces variation in the electrical conductivity across the junction38. This variation in the electrical conductivity may be immediately discernible, may be discernible after time and use, or may be discernible in response to an induced stress.

Each of the electrical lead traces44has a characteristic resistance that is determined based upon the trace length and pattern on the flexible web material45, the fuse46, solder and/or other interfaces, the terminal pin52and the junction38formed between the tab42and the busbar30by the mechanical fastener36. The characteristic resistance may vary due to variation resistance at the junction38, which may depend upon the magnitude of normal force that is applied by the mechanical fastener36to form the junction38. By way of a non-limiting example, if a rivet is not formed properly during assembly, the rivet may have low clamping force, which may increase resistance at the junction38.

FIG. 2schematically shows a test fixture75associated with evaluating one of the electrical lead traces44, and more specifically evaluating the electrical conductivity of one of the junctions38formed between the busbar30and the electrical lead trace44by the mechanical fastener36. The junction38and an accompanying in-line fuse46are evaluated as resistive devices. The test fixture75is preferably configured to non-destructively evaluate the junction38formed between the busbar30and the electrical lead trace44by the mechanical fastener36on a workpiece. The test fixture75includes an electric power supply80, an electric monitoring device85, a mechanical stress-inducing device90, and an associated controller95. Overall, the controller95employs the test fixture75to measure a dynamic change in resistance at the junction38while the mechanical fastener36is subjected to an external non-destructive mechanical stress.

The electric power supply80is preferably a low-power device that electrically connects to a workpiece in the form of one of the electrical lead traces44to supply electric power at a preset voltage level. A load resistor82is placed in series with the electrical lead trace44to limit the current. The load resistor82is preferably selected to facilitate detecting a change in the monitored voltage that may occur due to a change in the overall resistance of the electrical lead trace44, wherein the change in the monitored voltage may be caused by a change in the resistance across the junction38. In one non-limiting embodiment, the preset voltage level is 0.5 volts DC.

The electric monitoring device85may be a voltmeter that is incorporated into a digital data acquisition device that monitors voltage, e.g., at a 1 kHz rate. The electric monitoring device85is preferably disposed to monitor a voltage between leads86and87, which thus provides a voltage drop across the junction38and the in-line fuse46. Connection to the lead86may be made via a pogo pin, and connection to the lead87may be made via the corresponding terminal pin52in the electrical connector50.

The mechanical stress-inducing device90may be a device that is configured to apply mechanical stress proximal to the junction38. The magnitude of the mechanical stress is sufficient to induce a discernible change in electrical resistance across the junction38when the junction38was not formed in accordance with specification, but limited so as to not induce new stress in the junction38. The mechanical stress may be in the form of a burst of pressurized airflow that is applied to the junction38, e.g., from a high-pressure source via a nozzle that is aimed towards the junction38. Alternatively, the mechanical stress may be in the form of a direct mechanical tapping onto the junction38, e.g., from a hammer device that is disposed to tap on the junction38. Alternatively, the mechanical stress may be in the form of an induced vibration at the junction38, e.g., from a horn of an ultrasonic welding device that is placed in contact with the junction38. Alternatively, the mechanical stress may be in the form of another suitable mechanical stress.

The test fixture75preferably includes a controller95that executes one or more control routines to evaluate the integrity of the corresponding junction based upon the monitored electrical conductivity when applying the mechanical stress, which may be described as follows. In operation, one of the electrical lead traces44is secured and electrically connected to the electric power supply80and the electric monitoring device85of the test fixture75. The mechanical stress-inducing device90is activated and the electric monitoring device85monitors voltage across the junction38. The monitored voltage is evaluated. When there is a change in the monitored voltage associated with the induced stress, it may be indicative of an increased resistance in the junction38caused when the junction38was not formed in accordance with specification.

By way of a non-limiting example, a resistance level associated with an embodiment of the junction38that was formed in accordance with the specification may be in the order of magnitude of 2 milli-ohms, and a resistance level associated with an embodiment of the junction38that was not formed in accordance with the specification may be in the order of magnitude of 50 milli-ohms when mechanical stress is induced. As such, the electric monitoring device85must be configured to detect and discern between such levels. Preferably, there is a threshold voltage level, with minor changes in the increased resistance in the junction38not causing rejection of a workpiece. When there is no change in the monitored voltage associated with the induced stress, it may indicate that the junction38was formed in accordance with the specification.

FIGS. 3-1 and 3-2graphically show results associated with employing the test fixture75described with reference toFIG. 2to evaluate one of the electrical lead traces44described with reference toFIG. 1. Voltage levels indicated by the electric monitoring device85are shown on the vertical scale302, in relation to time, which is shown on the horizontal scale304. The embodiment of the mechanical stress-inducing device90is in the form of a burst of pressurized airflow that is applied to the junction38. The voltage spikes310shown inFIG. 3-1include a plurality of voltage spikes312, which are associated with repeated operation of an embodiment of the mechanical stress-inducing device90described with reference toFIG. 2, wherein the spikes312may indicate an increased resistance in the junction38caused when the junction38was not formed in accordance with specification. The voltage levels320shown inFIG. 3-2do not include any voltage spikes that are associated with repeated operation of an embodiment of the mechanical stress-inducing device90described with reference toFIG. 2. This may indicate no increased resistance in the junction38, i.e., the junction38was formed in accordance with specification and the workpiece may be deemed acceptable.

The terms controller, control module, module, control, control unit, processor and similar terms refer to any one or various combinations of Application Specific Integrated Circuit(s) (ASIC), electronic circuit(s), central processing unit(s), e.g., microprocessor(s) and associated non-transitory memory component in the form of memory and storage devices (read only, programmable read only, random access, hard drive, etc.). The non-transitory memory component is capable of storing machine readable instructions in the form of one or more software or firmware programs or routines, combinational logic circuit(s), input/output circuit(s) and devices, signal conditioning and buffer circuitry and other components that can be accessed by one or more processors to provide a described functionality. Input/output circuit(s) and devices include analog/digital converters and related devices that monitor inputs from sensors, with such inputs monitored at a preset sampling frequency or in response to a triggering event. Software, firmware, programs, instructions, control routines, code, algorithms and similar terms mean any controller-executable instruction sets including calibrations and look-up tables. Each controller executes control routine(s) to provide desired functions, including monitoring inputs from sensing devices and other networked controllers and executing control and diagnostic instructions to control operation of actuators. Routines may be executed at regular intervals, for example each 100 microseconds during ongoing operation. Alternatively, routines may be executed in response to occurrence of a triggering event. Communication between controllers, and communication between controllers, actuators and/or sensors may be accomplished using a direct wired point-to-point link, a networked communication bus link, a wireless link or any other suitable communication link. Communication includes exchanging data signals in any suitable form, including, for example, electrical signals via a conductive medium, electromagnetic signals via air, optical signals via optical waveguides, and the like. The data signals may include discrete, analog or digitized analog signals representing inputs from sensors, actuator commands, and communication between controllers. The term “signal” refers to any physically discernible indicator that conveys information, and may be any suitable waveform (e.g., electrical, optical, magnetic, mechanical or electromagnetic), such as DC, AC, sinusoidal-wave, triangular-wave, square-wave, vibration, and the like, that is capable of traveling through a medium.

FIG. 4schematically shows a plan view of an electrical circuit for an integrated test fixture100and selected elements of the battery cell interconnect board10that are described with reference toFIG. 1. The integrated test fixture100includes elements of the test fixture75described with reference toFIG. 2, and is configured to monitor and evaluate a plurality of junctions38formed between corresponding tabs42and corresponding busbars30of the battery cell interconnect board10. The integrated test fixture100includes an electric power supply180, a plurality of electric monitoring devices185, a mechanical stress-inducing device190, an associated controller195, and a plurality of load resistors182that may be placed in series with individual electrical lead traces44of the battery cell interconnect board10. A single load resistor182is indicated in series with a single junction38, electrical lead trace44, and fuse46of the battery cell interconnect board10, and is monitored via leads186and187.

Each of the electrical lead traces44has a distinct characteristic resistance that is determined based upon the length, width and thickness of the electrical lead trace44, its pattern on the flexible web material45, the fuse46, solder and/or other interfaces, the terminal pin52and the junction38formed between the tab42and the busbar30by the mechanical fastener36. As such, the characteristic resistance may differ between individual ones of the electrical lead traces44in the battery cell interconnect board10. As such each of the load resistors182is preferably selected to limit current draw such that all the electrical lead traces44of the battery cell interconnect board10have approximately the same current draw when all of the junctions38have been fabricated in accordance with the specification. As such, the voltage changes measured across the plurality of junctions38of the plurality of the electrical lead traces44will be equivalent. This configuration facilitates employing a single voltage threshold for monitoring all of the electrical lead traces44to detect whether the junctions38function in accordance with specification. This facilitates detecting a change in the monitored voltage that may occur due to a change in the overall resistance of the electrical lead trace44, wherein the change in the monitored voltage may be caused by a change in the resistance across the junction38. The monitored leads186and187are monitored via a voltmeter185, which measures the voltage drop across the junction38. The voltmeter185may be a stand-alone device that communicates with the controller195, or may be electrically integrated into the controller195. Connections to the leads186may be made via a pogo pin, and connections to the leads187may be made via the corresponding terminal pins152in the electrical connector150. Although not shown in detail, the integrated test fixture100includes leads186and187and associated voltmeter185for each of the junctions38of the battery cell interconnect board10. Overall, the integrated test fixture100is configured to apply mechanical stress to each of the junctions38of the battery cell interconnect board10employing the mechanical stress-inducing device190and monitor the electrical connections across the monitored leads186and187via the controller195. The mechanical stress may be applied serially or simultaneously to the junctions38. The controller195includes circuits and/or control routines that monitor signals from the voltmeter185to evaluate the battery cell interconnect board10. The controller195determines that the battery cell interconnect board10has been fabricated in accordance with specifications only when the all of the junctions38function in accordance with specification, as indicated by the voltage drops thereacross.