System for controlling electric motors as a virtual electronic limited-slip differential

Abstract

System that includes: a reference speed module (64) configured to: - to determine a reference speed range based on a speed of a left wheel (26, 50) of a pair of front or rear wheels (26, 28, 50, 52) of a vehicle (10) and a speed of a right wheel (28, 52) of the pair of front or rear wheels (26, 28, 50, 52), wherein the right wheel (28, 52) is separated from the left wheel (46, 50); - to select one of the speeds of the left and right wheels (26, 28, 50, 52) based on their sizes; - to set an average reference speed to the selected speed; and - to determine the reference speed range based on the mean reference speed and a permissible slip; and an engine control module (68) configured to control at least one of a first electric motor (14) and a second electric motor (16) based on the reference speed range, wherein: the first electric motor (16) is connected to the left wheel (26, 50); and the second electric motor (16) is connected to the right wheel (28, 52); characterized in that the reference speed module (64) is further configured: to set the average reference speed equal to a minimum value of the speeds of the left and right wheels (26, 28, 50, 52) when a driver of the vehicle (10) requests that the vehicle (10) accelerate; and to set the mean reference speed equal to a maximum value of the speeds of the left and right wheels (26, 28, 50, 52) when the driver requests that the vehicle (10) decelerate.

Description

INTRODUCTION

[0001] The present invention relates to a system according to the preamble of claim 1 for controlling electric motors to operate as a virtual electronic limited-slip differential.

[0002] Such a generic system is essentially described in DE 10 2018 214 763 A1. A comparable system is described in DE 40 11 291 A1.

[0003] Further prior art can also be found in the documents CN 2 08 544 105 U and US 7 906 919 B2.

[0004] A differential transfers torque from a power source, such as the engine or an electric motor, to a pair of wheels on an axle. An open differential always provides the same amount of torque to each of the wheels. Consequently, the wheels have different levels of traction, rotating at different speeds. In contrast, a limited-slip differential locks the wheels together as if the wheels were rotating on a common shaft, so that the wheels rotate at the same speed regardless of the levels of traction at each wheel. An electronic limited-slip differential contains an electronic solenoid that is operable to lock the wheels together or unlock the wheels from each other.

[0005] According to the invention, a system is presented which is characterized by the features of claim 1 or those of claim 6.

[0006] In one aspect, the reference speed range extends from a minimum reference speed to a maximum reference speed, the reference speed module is configured to determine the minimum reference speed by subtracting the allowable slip from the average reference speed, and the reference speed module is configured to determine the maximum reference speed by adding the allowable slip to the average reference speed.

[0007] In one aspect, the reference speed module is configured to determine whether the driver is requesting the vehicle to accelerate or decelerate based on at least one of an accelerator pedal position and a brake pedal position.

[0008] In one aspect, the reference speed module is configured to determine the allowable slip based on at least one driver input and whether the vehicle is turning.

[0009] In one aspect, the engine control module is configured to stop maintaining the axle torque request at the constant value, set the axle torque request to a driver torque request, and control both the first and second electric motors based on the axle torque request when (i) one of the left and right wheel speeds is within the reference speed range and (ii) the driver torque request is less than an estimated axle torque.

[0010] In one aspect, the motor control module is configured to control both the first and second electric motors based on the reference speed range.

[0011] Another system includes a reference speed module and a motor control module. The reference speed module is configured to determine a reference speed range based on at least one of a speed of a left wheel of a pair of front or rear wheels of a vehicle and a speed of a right wheel of the pair of front or rear wheels. The right wheel is disconnected from the left wheel. The motor control module is configured to control one of a first electric motor and a second electric motor based on the reference speed range when one of the speeds of the left and right wheels is outside the reference speed range. One of the first and second electric motors is connected to one of the speeds of the left and right wheels. The motor control module is configured to control the other of the first and second electric motors based on an axle torque request.The other of the first and second electric motors is connected to the other of the speeds of the left and right wheels.

[0012] In one aspect, the reference speed module is configured to set a mean reference speed to one of the left and right wheel speeds and to determine the reference speed range based on the mean reference speed and an allowable slip.

[0013] In one aspect, the reference speed range extends from a minimum reference speed to a maximum reference speed, the reference speed module is configured to determine the minimum reference speed by subtracting the allowable slip from the average reference speed, and the reference speed module is configured to determine the maximum reference speed by adding the allowable slip to the average reference speed.

[0014] In one aspect, the reference speed module is configured to set the average reference speed equal to a minimum value of the rotational speeds of the left and right wheels when a driver of the vehicle requests that the vehicle accelerate, and to set the average reference speed equal to a maximum value of the rotational speeds of the left and right wheels when the driver requests that the vehicle decelerate.

[0015] In one aspect, the motor control module is configured to control the one of the first and second electric motors connected to the one of the left and right wheels to adjust the speed of the one of the left and right wheels to a value within the reference speed range and maintain the axle torque request at a constant value.

[0016] In one aspect, the engine control module is configured to stop maintaining the axle torque request at the constant value, set the axle torque request to a driver torque request, and control both the first and second electric motors based on the axle torque request when (i) the speed of one of the left and right wheels is within the reference speed range and (ii) the driver torque request is less than an estimated axle torque.

[0017] Further, a vehicle is described. The vehicle includes a left wheel, a right wheel separate from the left wheel, a first electric motor connected to the left wheel, a second electric motor connected to the right wheel, a reference speed module, and a motor control module. The left and right wheels form a pair of front or rear wheels of the vehicle. The reference speed module is configured to determine a reference speed range based on at least one of a left wheel speed and a right wheel speed. The motor control module is configured to control one of the first and second electric motors based on the reference speed range when one of the left and right wheel speeds is outside the reference speed range. The one of the first and second electric motors is connected to the one of the left and right wheel speeds.The motor control module is configured to control the other of the first and second electric motors based on an axle torque request. The other of the first and second electric motors is connected to the other of the left and right wheel speeds.

[0018] In one aspect, the motor control module is configured to control the one of the first and second electric motors connected to the one of the left and right wheels to set the speed of the one of the left and right wheels to a value within the reference speed range and to maintain the axle torque request at a constant value when one of the speeds of the left and right wheels is outside the reference speed range.

[0019] In one aspect, the engine control module is configured to stop maintaining the axle torque request at the constant value, set the axle torque request to a driver torque request, and control both the first and second electric motors based on the axle torque request when (i) one of the left and right wheel speeds is within the reference speed range and the driver torque request is less than an estimated axle torque.

[0020] In one aspect, the first electric motor is configured to rotate the left wheel independently of the second electric motor rotating the right wheel, and the first electric motor is configured to rotate the right wheel independently of the first electric motor rotating the left wheel.

[0021] Further areas of applicability of the present invention will become apparent from the detailed description, claims, and drawings. The detailed description and specific examples are provided for illustrative purposes only. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] The present invention will be more fully understood from the detailed description and the accompanying drawings, in which: Fig. 1 is a functional block diagram of an exemplary vehicle according to the present invention; Fig. 2 is a functional block diagram of an exemplary powertrain control module according to the present invention; and Fig. 3 is a flowchart illustrating an exemplary powertrain control method according to the present invention.

[0023] In the drawings, reference symbols may be used multiple times to identify similar and / or identical elements. DETAILED DESCRIPTION

[0024] A vehicle according to the present invention includes a front or rear axle with a pair of electric motors that independently drive a pair of wheels on the axle. The electric motor and wheel on one side of the axle are mechanically separated from the electric motor and wheel on the other side of the axle, and vice versa. Consequently, the vehicle does not include a differential that transmits torque from the electric motors to the wheels. If the degree of traction at one of the wheels is different from the degree of traction at the other wheel, and the electric motors provide the same amount of torque to each wheel, the wheels may rotate at different speeds. Consequently, there is a potential for the speed of one of the wheels to fluctuate.

[0025] To avoid these problems, a control system according to the present invention controls the electric motors to drive the wheels in a manner that functions as a virtual electronic limited-slip differential. The control system achieves this by determining a reference speed range based on the speeds of the wheels and controlling at least one of the electric motors based on the reference speed range. In one example, the reference speed range is small and centered around the speed of one of the wheels, with the control system controlling the electric motor connected to the other wheel to maintain the speed of the other wheel within the reference speed range. In this manner, the control system controls the electric motors to function as a virtual electronic limited-slip differential by rotating the wheels at or nearly the same speed.

[0026] A virtual electronic limited-slip differential according to the present invention has several advantages over a physical or mechanical limited-slip differential. For example, a mechanical limited-slip differential allows for zero slip between the speeds of the wheels connected to the limited-slip differential. However, in certain situations, such as when the vehicle is turning, a small amount of slip may be desired. In contrast, the virtual electronic limited-slip differential can vary the allowable slip.

[0027] Additionally, a mechanical limited-slip differential may experience delays in engagement because the rotational speeds of the wheels connected to the differential must be brought below a threshold to physically engage. Furthermore, a mechanical limited-slip differential may experience delays in disengagement because the differential can lock at any wheel depending on the level of traction, which can lead to high axle loads and bouncing on dry roads. In contrast, the virtual electronic limited-slip differential can lock or unlock the wheels without any delay in engagement or disengagement by varying the allowable slip. Furthermore, a mechanical limited-slip differential may be noisy, whereas the virtual electronic limited-slip differential makes little to no noise.

[0028] In the following example, a vehicle includes a pair of rear electric motors that independently drive a pair of rear wheels, and a control system controls the rear electric motors to function as a virtual electronic limited-slip differential. Additionally, the vehicle includes a single front electric motor that drives a pair of front wheels through a physical electronic limited-slip differential. However, instead of just one front electric motor, the vehicle may include a pair of front electric motors that independently drive the front wheels, and the control system may control the front electric motors to function as a virtual electronic limited-slip differential. Additionally, the rear wheels may be driven by a single rear electric motor through a physical electronic differential, or the rear wheels may be driven independently by a pair of rear electric motors.In the latter case, the control system can control each pair of front and rear electric motors to function as a virtual electronic limited-slip differential.

[0029] In Fig. 1, a vehicle 10 includes a front electric motor 12, a left rear electric motor 14, a right rear electric motor 16, an electronic limited-slip differential 18, front half-shafts 20, a left rear half-shaft 22, a right rear half-shaft 24, a left front wheel 26, a right front wheel 28, a left rear wheel 50, and a right rear wheel 52. The front electric motor 12 outputs drive torque to rotate the left and right front wheels 26 and 28. The electronic limited-slip differential 18 transmits the drive torque from the front electric motor 12 to the left and right front wheels 26 and 28. In addition, the electronic limited-slip differential 18 is operable to lock the left and right front wheels 26 and 28 to each other so that the left and right front wheels 26 and 28 rotate at or nearly the same speed.The electronic limited-slip differential 18 includes a solenoid that is electronically controlled to lock or unlock the left and right front wheels 26 and 28.

[0030] The front half-shafts 20 connect the electronic limited-slip differential 18 to the left and right front wheels 26 and 28. In various implementations, the vehicle 10 includes front reduction gears (not shown) that transmit torque from the front electric motor 12 to the electronic limited-slip differential 18 with one or more gear ratios. The electronic limited-slip differential 18 and the front half-shafts 20 together form a front axle 34 of the vehicle 10. Additionally, the front electric motor 12, the reduction gears, and / or the left and right front wheels 26 and 28 may be considered part of the front axle 34.

[0031] The left rear electric motor 14 outputs drive torque to rotate the left rear wheel 50. The right rear electric motor 16 outputs drive torque to rotate the right rear wheel 52. The left rear half-shaft 22 connects the left rear electric motor 14 to the left rear wheel 50. The right rear half-shaft 24 connects the right rear electric motor 16 to the right rear wheel 52. In various implementations, the vehicle 10 includes rear reduction gears (not shown) that transmit torque from the left and right rear electric motors 14 and 16 to the left and right rear half-shafts 22 and 24, respectively, with one or more gear ratios. The left and right rear half-shafts 22 and 24 collectively form a rear axle 36 of the vehicle 10.The left and right rear electric motors 14 and 16 and / or the rear reduction gears can also be considered as part of the rear axle 36.

[0032] The left rear electric motor 14 rotates the left rear wheel 50 independently of the right rear electric motor 16, which rotates the right rear wheel 52. The right rear electric motor 16 rotates the right rear wheel 52 independently of the left rear electric motor 14, which rotates the left rear wheel 50. The left and right rear wheels 50 and 52 are located on the rear axle 36 and are aligned with each other in a longitudinal direction 38 of the vehicle 10. However, the left and right rear wheels 50 and 52 are separated from each other because the rear axle 36 does not connect the left and right rear wheels 50 and 52.

[0033] The vehicle 10 further includes an accelerator pedal 40, a brake pedal 42, a steering wheel 44, a left front wheel speed sensor 46, a right front wheel speed sensor 48, a left rear wheel speed sensor 40, a right rear wheel speed sensor 42, an accelerator pedal position sensor 54, a brake pedal position sensor 56, a steering wheel position sensor 58, a vehicle motion sensor 60, and a powertrain control module 62. A driver of the vehicle 10 depresses the accelerator pedal 40 to accelerate the vehicle 10. The driver depresses the brake pedal 42 to decelerate or stop the vehicle 10. The driver turns the steering wheel 44 to cause the vehicle 10 to turn.In various implementations, the vehicle 10 may be an autonomous vehicle, in which case the accelerator pedal 40, the steering wheel 44, and the brake pedal may be omitted and / or an autonomous driver module (not shown) may control the acceleration, steering, and braking of the vehicle 10.

[0034] The left front wheel speed sensor 46 measures the speed of the left front wheel 26 and generates a signal indicating the speed of the left front wheel. The right front wheel speed sensor 48 measures the speed of the right front wheel 28 and generates a signal indicating the speed of the right front wheel. The left rear wheel speed sensor 40 measures the speed of the left rear wheel 50 and generates a signal indicating the speed of the left rear wheel. The right rear wheel speed sensor 42 measures the speed of the right rear wheel 52 and generates a signal indicating the speed of the right rear wheel.

[0035] The accelerator pedal position sensor 54 measures the position of the accelerator pedal 40 and generates a signal indicating the accelerator pedal position. The brake pedal position sensor 56 measures the position of the brake pedal 42 and generates a signal indicating the brake pedal position. The steering wheel position sensor 58 measures the position of the steering wheel 44 and generates a signal indicating the steering wheel position.

[0036] The vehicle motion sensor 60 measures the longitudinal acceleration of the vehicle 10, the lateral acceleration of the vehicle 10, and the yaw rate of the vehicle 10. The vehicle motion sensor 60 may be an inertial measurement unit that may include accelerometers that measure the longitudinal and lateral acceleration of the vehicle and a gyroscope that measures the yaw rate of the vehicle. The vehicle motion sensor 60 generates a signal indicative of the longitudinal acceleration of the vehicle, the lateral acceleration of the vehicle, and the yaw rate of the vehicle.

[0037] The powertrain control module 62 controls the front electric motor 12, the left rear electric motor 14, the right rear electric motor 16, and the electronic limited-slip differential 18 based on inputs from the sensors of the vehicle 10. In one example, the powertrain control module 62 controls the left and right rear electric motors 14 and 16 based on the sensor inputs to independently drive the left and right rear wheels 50 and 52 in a manner that functions as a virtual electronic limited-slip differential. The powertrain control module 62 may accomplish this by determining a reference speed range based on the rear wheel speeds and controlling the left rear electric motor 14 and / or the right rear electric motor 16 based on the reference speed range.In one example, the reference speed range is relatively small and centered around the speed of one of the rear wheels, and the powertrain control module 62 controls the rear electric motor 14 or 16 connected to the other rear wheel to maintain the speed of the other rear wheel within the reference speed range. In this manner, the powertrain control module 62 controls the left and right rear electric motors 14 and 16 to function as a virtual electronic limited-slip differential by rotating the left and right rear wheels 50 and 52 at or near the same speed.

[0038] In Fig. 2, an exemplary implementation of the powertrain control module 62 includes a reference speed module 64, an estimated torque module 66, and an engine control module 68. The reference speed module 64 determines the reference speed range based on the left and right rear wheel speeds from the left and right rear wheel speed sensors 40 and 42. The reference speed range extends from a minimum reference speed to a maximum reference speed. The reference speed module 64 outputs the reference speed range.

[0039] In one example, the reference speed module 64 selects one of the rear wheel speeds, sets an average reference speed equal to the selected speed, and determines the minimum and maximum reference speeds based on the average reference speed and an allowable slip. The allowable slip is an allowable difference between the rear wheel speeds. The reference speed module 64 may determine the allowable slip based on a driver input and / or whether the vehicle 10 is turning.

[0040] In one example, the driver may touch a button or touchscreen (not shown) of the vehicle 10 to enable or disable a virtual electronic differential (eLocker) function. The reference speed module 64 may set the allowable slip to a small value, such as 1 revolution per minute (rpm). -1) when the virtual eLocker function is enabled. The reference speed module 64 can set the allowable slip to a large value when the virtual eLocker function is disabled.

[0041] In one example, the reference speed module 64 sets the allowable slip to a small value when the vehicle 10 is traveling in a straight line, and the reference speed module 64 increases the allowable slip when the vehicle 10 is turning to prevent locking. The amount by which the allowable slip is increased may be based on the size of the turn and / or the speed of the vehicle 10. The reference speed module 64 may determine whether the vehicle 10 is turning and the size of the turn based on the steering wheel position from the steering wheel position sensor 58.

[0042] The reference speed module 64 selects one of the rear wheel speeds based on the magnitudes of the rear wheel speeds before setting the average reference speed equal to the selected speed. In one example, the reference speed module 64 sets the average reference speed equal to a minimum value of the speeds of the left and right rear wheels when the driver requests that the vehicle 10 accelerate. Conversely, the reference speed module 64 sets the average reference speed equal to a maximum value of the speeds of the left and right rear wheels when the driver requests that the vehicle 10 decelerate. The reference speed module 64 may determine whether the driver is requesting vehicle acceleration or deceleration based on the accelerator pedal position and the brake pedal position from the accelerator pedal position sensor 54 and the brake pedal position sensor 56, respectively.

[0043] The estimated torque module 66 estimates a drive torque of the rear axle 36. The rear axle torque may be equal to the sum of the torques output by the left and right rear electric motors 14 and 16. The estimated torque module 66 may estimate the rear axle torque based on the speeds of the left and right rear electric motors 14 and 16 and the amount of power supplied to them using, for example, a function or map. Additionally or alternatively, the estimated torque module 66 may estimate the rear axle torque based on one or more of the speed of the vehicle 10, the longitudinal acceleration of the vehicle 10, the lateral acceleration of the vehicle 10, and the yaw rate of the vehicle 10.The estimated torque module 66 may determine the vehicle speed based on one or more of the wheel speeds from the left front wheel speed sensor 46, the right front wheel speed sensor 48, the left rear wheel speed sensor 40, and the right rear wheel speed sensor 42. The estimated torque module 66 outputs the estimated axle torque (the estimated torque of the rear axle 36).

[0044] The engine control module 68 controls the left and right rear electric motors 14 and 16. In one example, the engine control module 68 controls the amount of torque output by the left and right rear electric motors 14 and 16 to minimize a difference between the axle torque request and the estimated axle torque. The engine control module 68 may set the axle torque request equal to a driver torque request. The engine control module 68 may determine the driver torque request based on the accelerator pedal position from the accelerator pedal position sensor 54.

[0045] In one example, the engine control module 68 controls the left and right rear electric motors 14 and 16 based on the axle torque request as long as the rear wheel speeds are within the reference speed range. If the speed of one of the rear wheels 50 or 52 is outside the reference speed range, the engine control module 68 controls the associated rear electric motor 14 or 16 to set the speed of the one rear wheel 50 or 52 to a value within the reference speed range. Additionally, the engine control module 68 maintains the axle torque request at a constant value, controlling the other rear wheel 50 or 52 based on the axle torque request. The constant value may be the value of the rear axle torque request at the time the speed of the one rear wheel 50 or 52 is first outside the reference speed range.

[0046] The engine control module 68 continues to maintain the axle torque request at the constant value until the speed of one of the rear wheels 50 or 52 is within the reference speed range and the driver torque request is less than the estimated axle torque. At this time, the engine control module 68 may stop maintaining the axle torque request at the constant value, adjust the axle torque request to a driver torque request, and control both the left and right rear electric motors 14 and 16 based on the axle torque request. The engine control module 68 may adjust the axle torque request to the driver torque request at a predetermined rate.

[0047] In Fig.3, a method for controlling the left and right rear electric motors 14 and 16 to independently drive the left and right rear wheels 50 and 52 and to perform the virtual eLocker function begins at 70. At 72, the reference speed module 64 determines whether the driver of the vehicle 10 intends to accelerate the vehicle 10. The reference speed module 64 may make this determination based on the accelerator pedal position from the accelerator pedal position sensor 54. If the driver intends to accelerate the vehicle 10, the method continues at 74. Otherwise, the method continues at 76. At 74, the reference speed module 64 selects the minimum value of the rear wheel speeds as the average reference speed.

[0048] At 76, the reference speed module 64 determines whether the driver intends to decelerate the vehicle 10. The reference speed module 64 may make this determination based on the brake pedal position from the brake pedal position sensor 56. If the driver intends to decelerate the vehicle 10, the method continues at 78. Otherwise, the method continues at 80. At 78, the reference speed module 64 selects the maximum value of the rear wheel speeds as the average reference speed.

[0049] At 80, the reference speed module 64 determines the minimum and maximum reference speeds based on the average reference speed and the allowable slip. The reference speed module 64 determines the minimum reference speed by subtracting the allowable slip from the average reference speed. The reference speed module 64 determines the maximum reference speed by adding the allowable slip to the average reference speed.

[0050] At 82, the engine control module 68 determines whether any of the rear wheel speeds are outside the reference speed range. If any of the rear wheel speeds are outside the reference speed range, the method continues at 84. Otherwise, the method continues at 86.

[0051] At 84, the engine control module 68 enables the virtual eLocker feature if the virtual eLocker feature is not already enabled. At 88, the engine control module 68 freezes the axle torque request at its value corresponding to the time the virtual eLocker feature is enabled and / or the time the speed of the one rear wheel is initially outside the reference speed range. At 90, the engine control module 68 brings the speed of the one rear wheel into or maintains it within the reference speed range using proportional-integral-derivative (PID) control. The reference speed module 64 sets the reference speed range based on the speed of the other rear wheel while the virtual eLocker feature is enabled.The reference speed module 64 accomplishes this by setting the average reference speed equal to the speed of the other rear wheel and determining the minimum and maximum reference speeds based on the average reference speed and the allowable slip.

[0052] At 92, the engine control module 68 determines whether the speed of the one rear wheel is within the reference speed range. If the speed of the one rear wheel is within the reference speed range, the method continues at 94. Otherwise, the method returns to 90.

[0053] At 94, the engine control module 68 determines whether the driver torque request is less than the estimated axle torque. If the driver torque request is less than the estimated axle torque, the method continues at 96. Otherwise, the method returns to 90. At 96, the engine control module 68 disables the virtual eLocker feature. In turn, the engine control module 68 stops freezing the axle torque request, sets the axle torque request to the driver torque request, and controls both the left and right electric motors 14 and 16 based on the axle torque request.

[0054] At 86, the engine control module 68 determines whether the virtual eLocker feature is already enabled. If the virtual eLocker feature is already enabled, the method continues at 90. Otherwise, the method continues at 98. The method ends at 98.

Claims

[1] System that includes: a reference speed module (64) configured to: - to determine a reference speed range based on a speed of a left wheel (26, 50) of a pair of front or rear wheels (26, 28, 50, 52) of a vehicle (10) and a speed of a right wheel (28, 52) of the pair of front or rear wheels (26, 28, 50, 52), wherein the right wheel (28, 52) is separated from the left wheel (46, 50); - to select one of the speeds of the left and right wheels (26, 28, 50, 52) based on their sizes; - to set an average reference speed to the selected speed; and - to determine the reference speed range based on the mean reference speed and a permissible slip; and a motor control module (68) configured to control at least one of a first electric motor (14) and a second electric motor (16) based on the reference speed range, wherein: the first electric motor (16) is connected to the left wheel (26, 50); and the second electric motor (16) is connected to the right wheel (28, 52); characterized by , that the reference speed module (64) is further configured: to set the average reference speed equal to a minimum value of the speeds of the left and right wheels (26, 28, 50, 52) when a driver of the vehicle (10) requests that the vehicle (10) accelerate; and to set the mean reference speed equal to a maximum value of the speeds of the left and right wheels (26, 28, 50, 52) when the driver requests that the vehicle (10) decelerate. [2] The system of claim 1, wherein: the reference speed range extends from a minimum reference speed to a maximum reference speed; the reference speed module (64) is configured to determine the minimum reference speed by subtracting the allowable slip from the average reference speed; and the reference speed module (64) is configured to determine the maximum reference speed by adding the allowable slip to the average reference speed. [3] The system of claim 1, wherein the reference speed module (64) is configured to determine whether the driver requests that the vehicle (10) accelerate or decelerate based on at least one of an accelerator pedal position and a brake pedal position. [4] The system of claim 1, wherein the reference speed module (64) is configured to determine the allowable slip based on at least one of a driver input and whether the vehicle (10) is turning. [5] The system of claim 1, wherein the engine control module (68) is further configured to control both the first and second electric motors (14, 16) based on the reference speed range. [6] System that includes: a reference speed module (64) configured to determine a reference speed range based on a speed of a left wheel (26, 50) of a pair of front or rear wheels (26, 28, 50, 52) of a vehicle (10) and a speed of a right wheel (28, 52) of the pair of front or rear wheels (26, 28, 50, 52), wherein the right wheel (28, 52) is separated from the left wheel (26, 50), and a motor control module (68) configured to control at least one of a first electric motor (14) and a second electric motor (16) based on the reference speed range, wherein: the first electric motor (14) is connected to the left wheel (26, 50); and the second electric motor (16) is connected to the right wheel (28, 52); wherein, when one of the speeds of the left and right wheels (26, 28, 50, 52) is outside the reference speed range, the engine control module (68) is configured: controlling one of the first and second electric motors (14) connected to one of the left and right wheels (26, 28, 50, 52) to set the rotational speed of the one of the left and right wheels (26, 28, 50, 52) to a value within the reference rotational speed range; and control the other of the first and second electric motors (14) based on an axle torque request; characterized by , that the engine control module (68) is further configured to maintain the axle torque request at a constant value when one of the left and right wheel speeds is outside the reference speed range. [7] The system of claim 6, wherein the engine control module (68) is further configured to stop maintaining the axle torque request at the constant value, set the axle torque request to a driver torque request, and control both the first and second electric motors based on the axle torque request when: one of the speeds of the left and right wheels (26, 28, 50, 52) is within the reference speed range; and the driver torque request is less than an estimated axle torque.

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