Current sensor for a vehicle

A vehicle includes a battery configured to operate at a current, during a drive cycle, in a nominal current range for a time greater than in a maximum current range. A controller is programmed to output a voltage based on the current according to a gain calibrated to be greater in the nominal current range than in the maximum current range to increase resolution in the nominal current range. The gain may be based on an exponential function or piecewise linear function of the current. The gain may increase as a magnitude of the current increases. A second controller receives the voltage and derives a battery current according to an inverse of the gain.

TECHNICAL FIELD

This application generally relates to current measurement systems for vehicles.

BACKGROUND

A hybrid-electric or all-electric vehicle includes a traction battery constructed of multiple battery cells in series and/or parallel. The traction battery provides power for vehicle propulsion and accessory features. Such vehicles may also include an electric machine driven by a power electronics module. To operate the battery and the electric machine, the amount of current flowing to or from the battery may be measured by a current sensor. The current sensor value may be used to compute a battery state of charge.

SUMMARY

A vehicle includes a battery configured to operate at a current, during a drive cycle, in a nominal current range for a time greater than in a maximum current range. The vehicle further includes a controller programmed to output a voltage based on the current according to a gain calibrated to be greater in the nominal current range than in the maximum current range to increase resolution in the nominal current range. The gain may be based on an exponential function of the current. The gain may be based on a piecewise linear function of the current. The gain may be such that a ratio between a change in the voltage and a change in the current decreases as a magnitude of the current increases. The vehicle may further include a second controller programmed to receive the voltage and operate the battery according to a battery current derived from the voltage. The battery current may be derived from the voltage based on an inverse of the gain. The second controller may be further programmed to, in response to conditions indicative of the current being zero, measure a zero current voltage and further derive the battery current based on the zero current voltage.

A battery management system includes a controller programmed to measure a current from a battery configured to operate, during a drive cycle, at the current that is in a nominal current range for a time greater than in a maximum current range, and output a voltage based on the current according to a gain calibrated to be greater in the nominal current range than in the maximum current range. The gain may be based on an exponential function of the current. The gain may be based on a piecewise linear function of the current. The gain may be such that a ratio between a change in the voltage and a change in the current decreases as a magnitude of the current increases. The battery management system may further include a second controller programmed to receive the voltage and operate the battery according to a battery current derived from the voltage. The battery current may be derived from the voltage based on an inverse of the gain. The second controller may be further programmed to, in response to conditions indicative of the current being zero, measure a zero current voltage and further derive the battery current based on the zero current voltage.

A method includes measuring, by a controller, a current from a battery configured to operate at the current that is in a nominal current range for a time greater than in a maximum current range. The method further includes outputting, by the controller, a voltage based on the current according to a gain calibrated to be greater in the nominal current range than in the maximum current range. The gain may be based on an exponential function of the current. The gain may be based on a piecewise linear function of the current. The gain may be such that a ratio between a change in the voltage and a change in the current decreases as a magnitude of the current increases. The gain may be calibrated such that a resolution of the current as represented by the voltage is greater in the nominal current range than in the maximum current range. The method may further include receiving, by a second controller, the voltage, and operating, by the second controller, the battery according to a battery current derived from the voltage.

The analog voltage as described herein improves the accuracy of the signal representing the current in a predefined current range in which the vehicle may operate more frequently. A signal to noise ratio of the analog voltage representing the current is increased for a range of currents. The improved accuracy in the current value improves the accuracy of those values derived from the current such as the battery state of charge.

DETAILED DESCRIPTION

FIG. 1depicts a typical plug-in hybrid-electric vehicle (PHEV). A typical plug-in hybrid-electric vehicle12may comprise one or more electric machines14mechanically connected to a hybrid transmission16. The electric machines14may be capable of operating as a motor or a generator. In addition, the hybrid transmission16is mechanically connected to an engine18. The hybrid transmission16is also mechanically connected to a drive shaft20that is mechanically connected to the wheels22. The electric machines14can provide propulsion and deceleration capability when the engine18is turned on or off. The electric machines14also act as generators and can provide fuel economy benefits by recovering energy that would normally be lost as heat in the friction braking system. The electric machines14may also reduce vehicle emissions by allowing the engine18to operate at more efficient speeds and allowing the hybrid-electric vehicle12to be operated in electric mode with the engine18off under certain conditions.

A traction battery or battery pack24stores energy that can be used by the electric machines14. A vehicle battery pack24typically provides a high voltage DC output. The traction battery24is electrically connected to one or more power electronics modules26. One or more contactors42may isolate the traction battery24from other components when opened and connect the traction battery24to other components when closed. The power electronics module26is also electrically connected to the electric machines14and provides the ability to bi-directionally transfer energy between the traction battery24and the electric machines14. For example, a typical traction battery24may provide a DC voltage while the electric machines14may operate using a three-phase AC current. The power electronics module26may convert the DC voltage to a three-phase AC current for use by the electric machines14. In a regenerative mode, the power electronics module26may convert the three-phase AC current from the electric machines14acting as generators to the DC voltage compatible with the traction battery24. The description herein is equally applicable to a pure electric vehicle. For a pure electric vehicle, the hybrid transmission16may be a gear box connected to an electric machine14and the engine18may not be present.

In addition to providing energy for propulsion, the traction battery24may provide energy for other vehicle electrical systems. A typical system may include a DC/DC converter module28that converts the high voltage DC output of the traction battery24to a low voltage DC supply that is compatible with other vehicle loads. Other high-voltage loads46, such as compressors and electric heaters, may be connected directly to the high-voltage without the use of a DC/DC converter module28. The low-voltage systems may be electrically connected to an auxiliary battery30(e.g., 12V battery).

The vehicle12may be an electric vehicle or a plug-in hybrid vehicle in which the traction battery24may be recharged by an external power source36. The external power source36may be a connection to an electrical outlet that receives utility power. The external power source36may be electrically connected to electric vehicle supply equipment (EVSE)38. The EVSE38may provide circuitry and controls to regulate and manage the transfer of energy between the power source36and the vehicle12. The external power source36may provide DC or AC electric power to the EVSE38. The EVSE38may have a charge connector40for plugging into a charge port34of the vehicle12. The charge port34may be any type of port configured to transfer power from the EVSE38to the vehicle12. The charge port34may be electrically connected to a charger or on-board power conversion module32. The power conversion module32may condition the power supplied from the EVSE38to provide the proper voltage and current levels to the traction battery24. The power conversion module32may interface with the EVSE38to coordinate the delivery of power to the vehicle12. The EVSE connector40may have pins that mate with corresponding recesses of the charge port34. Alternatively, various components described as being electrically connected may transfer power using a wireless inductive coupling.

One or more wheel brakes44may be provided for decelerating the vehicle12and preventing motion of the vehicle12. The wheel brakes44may be hydraulically actuated, electrically actuated, or some combination thereof. The wheel brakes44may be a part of a brake system50. The brake system50may include other components to operate the wheel brakes44. For simplicity, the figure depicts a single connection between the brake system50and one of the wheel brakes44. A connection between the brake system50and the other wheel brakes44is implied. The brake system50may include a controller to monitor and coordinate the brake system50. The brake system50may monitor the brake components and control the wheel brakes44for vehicle deceleration. The brake system50may respond to driver commands and may also operate autonomously to implement features such as stability control. The controller of the brake system50may implement a method of applying a requested brake force when requested by another controller or sub-function.

One or more electrical loads46may be connected to the high-voltage bus. The electrical loads46may have an associated controller that operates and controls the electrical loads46when appropriate. Examples of electrical loads46may be a heating module or an air-conditioning module.

The various components discussed may have one or more associated controllers to control and monitor the operation of the components. The controllers may communicate via a serial bus (e.g., Controller Area Network (CAN)) or via discrete conductors. A system controller48may be present to coordinate the operation of the various components.

A traction battery24may be constructed from a variety of chemical formulations. Typical battery pack chemistries may be lead acid, nickel-metal hydride (NIMH) or Lithium-Ion.FIG. 2shows a typical traction battery pack24in a series configuration of N battery cells72. Other battery packs24, however, may be composed of any number of individual battery cells connected in series or parallel or some combination thereof. A battery management system may have a one or more controllers, such as a Battery Energy Control Module (BECM)76that monitors and controls the performance of the traction battery24. The BECM76may monitor several battery pack level characteristics using various sensors and associate circuitry that may include a pack current sensor78, a pack voltage sensor80and pack temperature sensor82. The BECM76may have non-volatile memory such that data may be retained when the BECM76is in an off condition. Retained data may be available upon the next key cycle.

In addition to the pack level characteristics, there may be battery cell72level characteristics that are measured and monitored. For example, the terminal voltage, current, and temperature of each cell72may be measured. The battery management system may use a sensor module74to measure the battery cell72characteristics. Depending on the capabilities, the sensor module74may include sensors and circuitry to measure the characteristics of one or multiple of the battery cells72. The battery management system may utilize up to Ncsensor modules or Battery Monitor Integrated Circuits (BMIC)74to measure the characteristics of all the battery cells72. Each sensor module74may transfer the measurements to the BECM76for further processing and coordination. The sensor module74may transfer signals in analog or digital form to the BECM76. In some embodiments, the sensor module74functionality may be incorporated internally to the BECM76. That is, the sensor module74hardware may be integrated as part of the circuitry in the BECM76and the BECM76may handle the processing of raw signals.

The BECM76may include circuitry to interface with the one or more contactors42. The positive and negative terminals of the traction battery24may be protected by contactors42.

Battery pack state of charge (SOC) gives an indication of how much charge remains in the battery cells72or the battery pack24. The battery pack SOC may be output to inform the driver of how much charge remains in the battery pack24, similar to a fuel gauge. The battery pack SOC may also be used to control the operation of an electric or hybrid-electric vehicle12. Calculation of battery pack SOC can be accomplished by a variety of methods. One possible method of calculating battery SOC is to perform an integration of the battery pack current over time. This is well-known in the art as ampere-hour integration. The accuracy of the current measurement may affect the accuracy of the battery SOC calculation.

The current sensor78may be used to measure the current flowing to and from the battery pack24. The current sensor78may be based on a Hall effect device.

Current measurements may be used for a number of control features within the vehicle. The battery pack current may be used for calculating battery SOC and setting battery power limits. The battery pack current may be used in closed-loop control of the electric machines (e.g., Torque Control). Because of the extensive use of the battery pack current, it is desired to have an accurate current measurement. Improved current measurement accuracy may lead to improvements in vehicle performance, range, and fuel economy.

Prior art current sensors present an analog signal that is scaled in a linear fashion. The linear scaling has a uniform gain throughout the entire current range. That is, a ratio of the analog voltage output to the current input is typically constant throughout the measurement range. For example, a current sensor may have a range of −200 Amps to +200 Amps. The voltage output at +200 Amps may be 5 Volts. The uniform gain of the current sensor may be defined as a ratio of the maximum voltage range to the maximum current range. The gain provides a transfer function that may be applied to current values to determine the output voltage of the current sensor. The gain for a prior art current sensor may be constant.

An issue with the linear scaling is that all current values are treated the same. That is, the gain at low currents is the same as the gain at high currents. It may be observed that an electrified vehicle may spend more time operating at lower current ranges than at high current ranges. The traction battery may be configured to operate at a current in a nominal current range for a time greater than in a maximum current range greater than the nominal current range. The nominal current range may be a range of currents at which the vehicle most frequently operates. For example, the vehicle may operate more frequently with battery currents less than 50 Amps (e.g., nominal current range is 0 to 50 Amps). The maximum current magnitude may be up to 200 Amps (e.g., maximum current range is 100 Amps to 200 Amps). Based on this observation, it is apparent that improving current sensor accuracy at lower currents may improve the performance of functions that utilize the current sensor value. Any degradation in the accuracy at higher current values will have limited effect on the performance since less time is spent operating in the high current range.

The effects of a small decrease in accuracy at high currents (for instance, 100 Amps or 200 Amps) may be considered. One would not expect a serious degradation in the amp-hour integration accuracy due to the relatively small amount of time spent operating in the high current range. However, there may be accuracy impacts for other battery controls computations, such as instantaneous power, due to this small amount of additional error at higher currents. There are several factors that may allow a battery system designer to make this tradeoff and arrive at acceptable system performance. First, the acceptable error at high currents tends to be higher anyway (for instance, 2 or 3 Amps for a 200 Amp battery pack). Since the error may normally be stated as a percentage of the full-scale value, it is normal that the magnitude of the error is greater as the current approaches the full-scale value. Next, a limiting factor for current accuracy in the higher current range is likely to be instantaneous published power accuracy. So long as the required accuracy on the instantaneous published power is met, there is no net system effect. Since the accuracy of the measured pack voltage is not affected, it may still be possible to meet a typical published power accuracy target even with decreased pack current accuracy at high currents. Finally, the largest impact at high currents due to this change may be from electrical noise on the analog signal. Such electrical noise tends to be zero-average, so some attention to filtering (e.g., make sure to meet the Nyquist criterion) may alleviate this potential problem.

It should be noted that Plug-In Hybrid Vehicles (PHEV) and Battery Electric Vehicles (BEV) typically run higher average currents from the battery pack compared to a Full Hybrid-Electric Vehicle (FHEV). The numerical examples provided (for instance, the definition of the low-current range as 50 Amps and below, and the maximum current range of 100 Amps to 200 Amps) are with the FHEV use case in mind. However, the same discussion holds true for PHEV and BEV, but with a wider definition of the ranges. For instance, a BEV high current range might be 400 A and above. Any ranges discussed for high-current and low-current operation are exemplary and the ranges are expected to change depending the on the type of vehicle.

FIG. 3is a block diagram of a current sensor subsystem. The BECM76may receive an analog signal representing the current flowing in the battery. The current sensor200may interface with the BECM76via three conductors or wires. The conductors may be part of a wiring harness that couples the BECM76to the current sensor200. The three conductors may transfer signals between the BECM76and the current sensor200. The BECM76may provide a power supply and a precision voltage reference signal, CS_VREF212, to the current sensor200. The CS_VREF212signal may function as a combination power supply and reference voltage to the current sensor200. The BECM76may also provide a voltage or ground reference, CS_REF210, to the current sensor200. The voltage generated on the CS_VREF212signal may be referenced to the CS_REF210ground reference signal.

The current sensor200may provide an analog voltage signal, CS_AOUT216, to the BECM76. The CS_AOUT216signal may be an analog voltage that may have values from zero volts up to the reference voltage, CS_VREF212. The analog voltage on CS_AOUT216may be indicative of an amount of current flowing through the battery pack. The voltage output on CS_AOUT216and the current flowing through the battery may be related to one another by a mathematical relationship or function.

A current sensor module200may be present to measure the current flowing through the battery pack24. The current sensor module200may include a power supply module214. The power supply module214may receive the CS_VREF212and the CS_REF210signals and buffer the signals for distribution within the current sensor module200. The power supply module214may provide power for the current sensor processor208and a reference voltage for a digital-to-analog converter (DAC) module232.

The current sensor module200may further include an element206for concentrating flux. The flux concentrating element206may be a ferrite core or loop that is disposed around a conductor202that may carry current flowing through the battery pack. As current through the conductor202changes, the amount of flux passing through the flux concentrating element206may vary. A Hall effect sensor204may be included to provide an analog voltage that changes as the flux through the flux concentrating element206changes. A current sensing element may include the flux concentrating element206and the Hall effect sensor204. The output of the Hall effect sensor204may be coupled to an analog input of the current sensor processor208. The current sensor processor208may execute instructions to process the analog input from the Hall effect sensor204to provide an output to the DAC module232.

The DAC232may receive a digital value as input and convert the digital value to an analog voltage output. The DAC module232may further buffer the analog voltage output before providing the analog output voltage signal CS_AOUT216. The analog output voltage of the DAC232may be referenced to the reference voltage, CS_VREF212. For example, a full-scale digital value may provide an analog output voltage that is equal to CS_VREF212. Values that are less than the full-scale digital value may provide an analog output voltage that is equal to a percentage of the CS_VREF212defined by the ratio between the digital value provided to the DAC and the full-scale digital value. The full-scale digital value may vary depending on the resolution of the DAC.

The BECM76may include a microprocessor218and an analog to digital (A/D) converter222. The BECM76may further include a reference voltage module226to provide a precision reference voltage signal230. The precision reference voltage signal230may be provided to the A/D converter222. The BECM76may include an analog buffer/filter module224to process the analog signal, CS_AOUT216, received from the current sensor200.

FIG. 4depicts a block diagram of the reference voltage module226. The reference voltage module226may include a power supply module252that receives a battery voltage, B+254, from the vehicle. The battery voltage, B+254, may be a 12 Volt power signal from the auxiliary battery. The power supply module252may process and convert the battery voltage254to a desired voltage level and provide a voltage output to a precision reference module250.

The precision reference module250may convert a received voltage signal to a precision regulated voltage, PREF230. The precision regulated voltage230may have a lower magnitude than the received voltage signal. The precision regulated voltage230may be provided to a buffer module256. The buffer module256may provide an output voltage reference, CS_VREF212, that is isolated from the precision voltage reference230. As the reference voltage signal212may be connected to the current sensor200via a wiring harness, the buffer module256may isolate the precision voltage reference230signal from excessive current or impedance conditions that may occur within the wiring harness between the BECM76and the current sensor200. The buffer module256may function such that the output voltage, CS_VREF212, is substantially the same as the input voltage, PREF230.

The precision voltage reference230may be provided to the A/D converter222in the BECM76. The reference module226functions to ensure that the voltage reference, CS_VREF212, that is provided to the current sensor200is at substantially the same voltage level as the precision voltage reference230that is provided to the A/D converter222of the BECM76.

The reference module may also pass the ground reference, CS_REF210, to the current sensor200. A copper ground plane of the BECM76may be connected to a chassis reference ground220in the vehicle. The copper ground plane may be available within the reference module226. The CS_REF210connection may be connected to the copper ground plane and may be at an equal potential with a BECM ground220that is connected to a chassis of the vehicle.

The A/D converter222in the BECM76may be a separate component or may be part of the BECM microprocessor218. The A/D converter222operates to convert an analog voltage input into a digital value. The actual digital value may depend upon the voltage reference, PREF230and the resolution of the A/D converter222. For example, when a full-scale voltage that is equal to the reference voltage230is applied to the input of the A/D converter222, a full-scale digital value may be output. If the reference voltage230changes, the full-scale voltage input changes in response.

The BECM76may include an analog buffer/filter module224to process the analog signal, CS_AOUT216, that represents the battery pack current value. The analog buffer/filter module224may filter the signal to eliminate unwanted noise and may buffer the signal such that conditions within the BECM76do not affect the CS_AOUT216signal. In addition, the analog buffer/filter module224may protect the other components of the BECM76from damaging current or impedance conditions on the analog signal216conductor. The output of the analog buffer/filter module224may be coupled to an analog input of the A/D converter222.

The current sensor module200may periodically update the value of the analog output voltage216. The microprocessor218in the BECM76may periodically sample the value of the analog output voltage216. The sampling may be achieved by periodically transferring a value from the A/D converter222. The A/D converter222may provide a value that is the product of a full-scale digital value and a ratio between the analog output voltage216and the precision reference voltage230.

A typical current sensor may provide an analog output voltage that is linearly related to the current through the conductor being measured. To improve the signal being transferred to the BECM76, the current sensor processor208may execute instructions to rescale the analog voltage output as a function of the current. As discussed previously, a vehicle may operate more often in a range of currents below a predetermined current level. The analog voltage output216may only have a predetermined maximum range in which to express all values of the current and may be limited by the reference voltage212. A more thoughtful scaling of the analog voltage output216may allocate a larger voltage range to currents that are below the predetermined current level. Such a scaling may provide improved resolution of the analog voltage output216for currents below the predetermined current level. The improved resolution may improve control performance in functions in which the current feedback is utilized. For example, improved current control may be realized. In addition, better noise immunity may be realized as any noise induced signals will cause less deviation in the current measurement.

The current sensor processor208may convert the output of the Hall effect sensor204into a digital value. The current sensor processor208may scale the digital value into an actual current value in units of Amps. The output of the Hall effect sensor204may or may not be a linear function of the current. The current sensor processor208may include a lookup table to convert the digital value from the Hall effect sensor204to an actual current value in Amps. Alternatively, predetermined mathematical formulas may be implemented to perform the conversion. The current sensor processor208may then convert the actual current value into a digital output value for the DAC232. The digital output value may be scaled such that a full-scale value provides a maximum voltage output. Digital output values less than the full-scale value provide an analog voltage output216that is a percentage of the reference voltage212based on a ratio of the digital output value to the full-scale value.

The rescaling may be such that the analog output voltage216has a larger gain at lower values of current. That is, a given change in current yields a larger change in voltage output at lower current values than at high current values. The gain may also be considered as a rate of change of the analog output voltage with respect to the current. One example of such a gain function may be an exponential scaling function as depicted inFIG. 5. The graph300depicts an output voltage curve306and represents the voltage output for the range of current values of the current sensor. The y-axis302represents the analog output voltage, while the x-axis304represents the current sensor value. A zero current point308may be defined as a voltage on the curve306when the current is zero. The voltage at the zero current point308may be a non-zero voltage.

The rescaling may apply in the range of normal operating currents that occur when the current sensor output is not saturated. When the magnitude of the current is greater than a saturation current, the sensor output may no longer change in value. That is, an increase in current does not lead to an increase in the sensor output value.

The analog voltage output216may be scaled according to the following:

Vout⁢%=(2.5+[2*(1-ⅇ-Rf*HallCurrentFSC)])VCS_VREF⁢⁢for⁢⁢HallCurrent≥0(1)Vout⁢%=(2.5-[2*(1-ⅇRf*HallCurrentFSC)])VCS_VREF⁢⁢for⁢⁢HallCurrent<0(2)
where HallCurrent is the current flowing through the conductor202as measured by the Hall effect sensor in Amps, FSC is the full-scale current in Amps that is the largest current that the current sensor is designed to read accurately, VCS_VREFis the reference voltage of the analog output voltage, Rf is a rescaling factor that is a positive, dimensionless constant typically in the range of 1 to 5, and Vout% is the percentage of the reference voltage that should be output.

Equations (1) and (2) provide a percentage of the reference voltage that should be output as the analog output voltage216. The value of Vout% may be converted to a digital output value for the DAC232by multiplying Vout% by the full-scale digital value for the DAC232. The digital output value may be transferred to the DAC232. The DAC232may convert the digital output value into the analog output voltage216.

The current sensor processor208may implement the formulas expressed in Equations (1) and (2). Alternatively, one or more lookup tables may be implemented that approximate the formulas. The current sensor processor208may periodically sample the output of the Hall effect sensor204. The Hall effect sensor value may be converted to the actual current value. The voltage output value may then be calculated by executing the formulas described. The percentage voltage output value may be scaled for output to the DAC232. The result is an analog voltage output216with the characteristic as shown inFIG. 5.

To improve the resolution and noise immunity at lower measured currents, the voltage output may be such that a rate of change of the voltage output with respect to the current decreases as a magnitude of the current increases. The decrease in the rate of change may occur continuously as the current increases or may occur at discrete points. The current value may be reconstructed from the voltage output by applying the inverse function. For example, by dividing by the voltage received by the predetermined rate of change for the voltage, the original current may be reconstructed. The BECM76may store a table that relates the gain or rate of change to the voltage value. Alternatively, the table may relate the current value directly to the voltage value.

The BECM76may receive the analog output voltage216. The BECM76may execute a reverse transformation to the analog output voltage216to convert the value back to the actual current in Amps. The BECM76may apply an inverse of the gain to derive the battery current from the voltage signal.

The rescaling operation may substantially reduce electrical noise from the current value in the BECM76. The effectiveness of the rescaling may depend upon the value chosen for Rf. As Rf is increased (e.g., approaches 5), the current sensor200may become more immune to electrical noise at low current levels. For lower values of Rf (e.g., 1) there may be little difference in noise immunity when compared to a system without rescaling. The gain in the lower current range may be set by the choice of Rf. By increasing the gain in the nominal current range, the noise immunity of the amp-hour integration may be improved. Electrical noise and other factors, such as analog offsets and quantization errors, get mixed with the analog output voltage216at a specific magnitude of error. When the analog output voltage216is converted back to the original scale by the BECM76, the magnitude of any errors is reduced as the signal, along with any noise, is reduced back to the original level. In the nominal current range, the original current signal gain is increased and the inverse gain must be applied to reconstruct the original signal. However, any noise that enters into the measurement will also be multiplied by the inverse gain causing the amplitude of the noise to be reduced. The scheme described effectively reduces the magnitude of the noise signal in the nominal current range. The result is that noise immunity is improved in the nominal current range in which the vehicle may operate more frequently.

The A/D converter222may provide a value in counts that is a ratio between the signal voltage at the input and the reference voltage230. A percentage value may be expressed as the ratio between the measured counts and the full scale counts. The full-scale counts may be the number of counts that would be provided if the signal voltage is equal to the reference voltage.

The current measurement may be affected by offset errors and offset calibration errors in the A/D converter222. Offsets and biases in the A/D converter222may lead to errors in reading the current sensor value. A zero offset correction may be applied to correct for these issues. The zero offset correction may be computed by the BECM microprocessor218at a time when it is known that no current is flowing through the sensor. The A/D converter222may be sampled at ignition key-on while the contactors42are open. The analog output voltage216may be sampled under this zero-current condition to determine the zero offset correction.

The microprocessor218may execute instructions to convert the sampled signal into the current value in Amps. The current may be calculated using the following equations:

IAmps=-FSCRf*ln⁡(-1*[A⁢⁢2⁢D-ZC0.4*A⁢⁢2⁢Dmax-1]),when⁢⁢A⁢⁢2⁢DA⁢⁢2⁢Dmax≥0.5(3)IAmps=FSCRf*ln⁡(-1*[A⁢⁢2⁢D-ZC0.4*A⁢⁢2⁢Dmax-1]),when⁢⁢A⁢⁢2⁢DA⁢⁢2⁢Dmax<0.5(4)
where FSC and Rf are as described previously, A2Dmaxis the maximum A/D converter count, ZC is the zero offset correction value in A/D counts, A2D is the A/D value for the voltage measurement. The above scaling and rescaling is based on the reference voltage being approximately 5 Volts. Similar equations may be derived for different values of the reference voltage.

The rescaling may also be a piecewise linear scaling that has a different gain in different current ranges. One example of such a function may be a piecewise linear scaling function as depicted inFIG. 6. The graph400depicts a piece-wise linear output voltage curve406and represents the voltage output for the range of current values of the current sensor. The y-axis402represents the analog output voltage, while the x-axis404represents the current sensor value. A zero current point416may be defined as a voltage on the curve406when the current is zero. The voltage at the zero current point416may be a non-zero voltage. Breakpoints418,420may be defined as points at which the slope of the voltage curve changes. A positive current breakpoint418may be defined as the positive current408at which the slope changes. The voltage at the positive current breakpoint418may be defined as Vbph412. A negative current breakpoint420may be defined as the negative current410at which the slop changes. The voltage at the negative current breakpoint420may be defined as Vbpl414.

The voltage curve406exhibits a higher gain when a magnitude of the current is below a predetermined current408,410and a lower gain above the predetermined current. The percentage of the reference voltage that is output on the analog output voltage may be given by the following:

Vout⁢%={M⁢⁢1*HallCurrent+B⁢⁢1,HallCurrent≥IbpM⁢⁢2*HallCurrent+50,-Ibp<HallCurrent<+IbpM⁢⁢1*HallCurrent+B⁢⁢2,HallCurrent≤-Ibp(5)
where M1 and M2 are gain constants in units of percent per Amp, B1 and B2 are constants which set the offsets. The gain constants, M1 and M2, may be selected such that M2 is greater than M1. The offset constants, B1 and B2, may be selected such that each function associated with a current breakpoint yields the same percentage value at the piecewise linear current breakpoints418,420.

The current sensor processor208may convert the measured current value into an output voltage as defined by Equation (5). Using this function, the current is scaled such that currents less than a predetermined current (e.g., within the nominal current range) have a higher gain than those currents above the predetermined current (e.g., within the maximum current range).

The BECM76may convert the voltage to a current by implementing the inverse of the piecewise linear scaling as follows:

I={A⁢⁢2⁢D⁢%+ZOC⁢%-B⁢⁢1⁢%M⁢⁢1,A⁢⁢2⁢D≥Vbph⁢%*FSCA⁢⁢2⁢D⁢%+ZOC⁢%-50M⁢⁢2,Vbpl⁢%*FSC<A⁢⁢2⁢D<Vbph⁢%*FSCA⁢⁢2⁢D⁢%+ZOC⁢%-B⁢⁢2⁢%M⁢⁢1,A⁢⁢2⁢D≤Vbpl⁢%*FSC(7)
where A2D % is the A/D counts expressed as a percentage of the full-scale value (A2D*100/FSC), Z0C % is the zero offset correction expressed as a percentage of the full-scale value, Vbph% is the percentage of the full scale voltage at the positive current breakpoint418, Vbpl% is the percentage of the full scale voltage at the negative current breakpoint420, FSC is the number of counts at the full scale voltage, and A2D is the raw counts from the A/D converter.

The zero offset correction value416may be defined as a difference between one-half of the full scale A/D reading and the A/D counts measured when pack current is zero. This value corrects for any offsets between the current sensor200and the BECM76.

The BECM76may measure a zero offset correction as described above. The zero offset correction may impact each of the breakpoints by a fixed offset amount. Correcting for the offset voltage involves shifting the curve down by the zero offset amount.

The system described may be further generalized as a method that may be implemented in one or more controllers. The system as described measures a battery pack current, but the scheme is applicable to any type of measurement. The general method may include a first step of measuring a value. The value may be a current measurement. A next step may be to rescale the measured value according to a function that has a decreasing gain for the output value as the measured value increases. The rescaled value may then be output as an analog signal. A second controller may receive the analog signal and may convert the analog signal to the measured value by implementing an inverse of the function.