Wheel Torque and Speed In A Vehicle with In-Wheel Motors

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Abstract: Wheel Torque And Speed In A Vehicle With In-Wheel Motors

Conventional passenger vehicles transfer power from an engine or electric motor to the wheels via a differential. The simplest form is the open differential which ensures that each wheel receives equal torque while allowing the two wheels on an axle to rotate at different speeds. In modern vehicles this behaviour is modified with application of the brakes or more complex differentials to avoid loss of traction. Vehicles driven by in-wheel electric motors do not have a differential, rather they deliver torque directly and independently to the two wheels. If equal torque is demanded of each motor the behaviour of the wheels and vehicle is identical to the behaviour in a vehicle with an open differential. As with conventional vehicles the brakes can be used to prevent a wheel losing traction. Unlike traditional vehicles the in-wheel motors can provide much more responsive and comprehensive traction control than is possible with differentials and brakes alone, offering the opportunity to improve handling and safety and reduce cost in the vehicle

 Wheel Torque And Speed In A Vehicle With In-Wheel Motors – Introduction

Most road vehicles are powered by a single engine or motor with a drive-line that transfers that power to the wheels, generating torque at the wheel hubs. The wheels must be free to move at different speeds to one another to allow for cornering and variations in road surface. This is achieved with the differential, a mechanical device which, in its simplest form, delivers equal torque to both wheels on an axle while allowing them to rotate at different speeds. Wheels can then rotate at their natural speeds as determined by the kinematics of the vehicle. The so-called ‘open differential’ allows any amount of wheel speed difference across an axle. Where the friction force available between tyre and road is very different on one wheel to the other the wheel with lower friction can lose traction and spin up rapidly. This can occur during cornering, when the vehicle’s weight shifts to the outside wheels so that the inside wheels have little traction on the road surface, or when one wheel is on a surface with poor grip, for example n ice or loose stones. This situation is clearly undesirable and so several systems have been developed to counteract the loss of traction while still allowing torque to be applied to the wheel with good road friction. Most commonly a ‘limited slip differential’ is employed, or a Traction Control Systems (TCS) is used to prevent wheel spin-up. The former is a more elaborate mechanical equivalent of the open differential that limits the wheel speed differential while the latter applies the brake to a wheel that is losing traction to prevent it spinning up. More complicated ‘torque vectoring’ systems provide further enhancements to vehicle handling but are rare due to their complexity and cost. Vehicles powered by in-wheel motors do not have a differential so the question arises as to how the vehicle will behave in terms of wheel speeds and how problems associated with lack of traction can be addressed. The answers, as explained below, are rather straight-forward. If the wheel-motors are controlled by making the same torque demand of each of the motors the vehicle will behave exactly as if there were an open differential. As with a conventional vehicle the traction control system can be used to control loss of traction. On the other hand, enhanced vehicle ride and handling can be achieved by dynamically varying the torque distribution amongst the wheel motors