After Gutenberg

TM4

The Advanced Technology Trasportation Institute explains that an in-wheel motor assembly consists of:

1. Highly efficient electric motor
2. Motor-Wheel Slave Controller (MWSC) including power and control electronics
3. Brake
4. Wheel bearings
5. Steerable front suspension interface
6. Heat sink embedded in the stator.

The configuration of the 3-phase synchronous motor consists of a central stator which supports the windings and the inverter, surrounded by an external rotor which supports the permanent magnets. The motor assembly is liquid-cooled to sustain high continuous power demands.

Currently, one of the more interesting designs for an electric drivetrain, the motor-wheel assembly is an elegant integration of an electric motor and other components into a package that fits inside a regular-size tire. Mounting the wheel directly on the rotor provides for direct transmission of torque, enhanced freewheeling, regenerative braking, and more economical inclusion of vehicle control, e.g., braking, traction, and stability systems.

Note: Nissan has an even more economical approach: two motors doing the job of four, nevertheless, this design relies upon a more traditional four wheel drive arrangement.

These in-wheel motors are becoming the norm in personal mobility and robots. Mitsubishi has set a precedent by offering cars with in-wheel motors; it remains to be seen whether the majority of carmakers will adopt this design.

There are advantages and disadvantages to providing propulsion in the wheel and removing a tremendous amount of mechanical devices from a main engine compartment. Some of the advantages include:

• Provides control over each wheel individually, which can result in enhanced handling and performance
• Eliminates the need for a differential and drive shaft (or chains and sprockets)
• Allows for different design of vehicles. “Designers can allocate more space to the driver and passengers without increasing the overall size and weight of the vehicle.”

A V is for Voltage story about the Bridgestone Dynamic-Damping, In-Wheel Motor states the major disadvantage of in-wheel motor assemblies:

A stubborn drawback of in-wheel drive motors has been the weight that they add to each wheel, which adversely affects comfort and road-holding performance.

The article goes on to explain how the Bridgestone in-wheel motor overcomes the disadvantage of additional unsprung weight.

Bridgestone’s new technology overcomes this drawback by using the motors to absorb vibration. The motors themselves function as vibration dampers. Their own vibration offsets the vibration from the road and tires, allowing for better traction and a more-comfortable ride.

Bridgestone provides evidence that this approach is successful. And, while also less than unbiased reporting, Mitsubishi is, according to Green Car Congress, road-testing its electric cars with in-wheel motors to identify and resolve “any deterioration in road holding and ride comfort due to increases in un-sprung weight, as well as reliability and durability issues in the in-wheel motor system and its peripheral components (suspension, wheels, tires).” Nevertheless, it would be better to see some reports from “independent” road tests. I look forward to reading reviews of the MIEV after the 39th Tokyo Motor Show opens.

Such development seems consistent with increase sophistication of automotive control electronics, e.g., “drive-by-wire” and “brake-by-wire”. Unquestionably, control electronics are making their way from stationary applications, such as the servomechanisms that are becoming prevalent in industrial robotics and have made their way from flight control and radio control applications to the automotive industry. It would seem that such designs can withstand the rigors of use in drivetrains, as there is increased experience in the field of very heavy transportation. I believe that these new electric cars from Japan do get better mileage than a Crawler-Transporter, which gets 0.007 miles to the gallon.

Crawler-Transporter