Part of the charm of the electric vehicle is the energy savings that derives from shedding much of the mass of the conventional car. The U.S. Department of Energy estimates that a 10% drop in vehicle weight translates into 6% to 8% better efficiency.
But lightweight vehicles, especially those under 800 kg (1764 lb), face safety concerns, said Junmin Wang, Associate Professor of Mechanical Engineering and Director of the Vehicle Systems and Control Laboratory at The Ohio State University. “Many EVs are so light that vehicle control becomes an issue,” he said. Large payloads—heavy cargoes or many passengers—can, for example, make them more prone to overturning on tight turns.
One way to improve control and stability in a lightweight EV is to place an electric motor in each of the wheels. The wheel-hub motors make for a four-wheel independently actuated (FWIA) car that can turn on a dime, spin around inside its own length and squeeze into tight parking spaces. There are also built-in redundancies that can be exploited to improve safety.
A good city car
The FWIA-enabled EV “is considered one of the promising future vehicle architectures,” he said. “It would make for a good city car—efficient and maneuverable, with no emissions.”
Several recently introduced EV designs feature wheel-hub motors. Each of the wheel in the Hiriko folding, urban two-seater from the Hiriko Driving Mobility consortium in the Basque region of Spain integrates a motor, steering actuators, suspension, and braking. SIM-Drive Corp. of Kawasaki City, Japan, is expected to bring its SIM-WIL car, a motor-in-wheel EV, to market in 2014.
And an FWIA research car was recently built and tested by a collaboration among Otago Polytechnic in New Zealand, the National Taiwan University of Science and Technology, and China’s Shenzhen Polytechnic. Wang expects that it will probably be another 10 years before fully enabled, commercial EVs of this type hit the road in any numbers.
For the same reasons of safety redundancy, the U.S. Dept. of Defense thinks that electric FWIA systems could also help make future military ground vehicles, even those using much heavier platforms, more survivable.
Wang and his OSU lab team have developed their vehicle control software using an 800-kg (1764-lb) experimental testbed that they built on the gutted chassis of a Big Muddy utility vehicle. Each wheel contains its own 7.5-kW (10 hp) permanent magnet, brushless dc motor that is powered by a 15-kW·h lithium-ion battery pack. A single cable connects all four motors to a central computer, which samples data from the steering wheel, accelerator pedal, and brake pedal 100 times a second and then actuates each wheel according to a set of sophisticated control algorithms.
“An EV with four in-wheel motors is an overactuated system,” Wang explained, one in which “the number of control actuators exceeds the number of degrees of freedom.” The four independently controlled and driven wheels thus open up multiple ways in which to give drivers greater control and more freedom of movement.
“Our task is to make a robust control system to keep it safe and reliable,” he said, noting that without the digital drive-by-wire, the test vehicle is difficult to drive. “You feel like you are driving something uncontrollable," Wang said. "But when the feedback system is active, the vehicle motion is controlled, and the handling is just smooth and reliable as the driver expects.”
By predicting the motion of the vehicle’s center of gravity and dynamics (the yaw motion, for instance) in real time with fault-tolerant adaptive controls, the computer calculates just how much torque the car needs for each of its four wheels.
Moreover, because each wheel is independent, “one wheel can be doing the braking, while another is doing the driving,” he said. “The computer gets signals from the driver from the steering wheel and pedal positions, then calculates the desired speed, or vehicle motion, based on a mathematical model” to provide the torque distribution that delivers optimal traction and motion control.
The result is better control than a commercial 4WD system, he claimed. Independent control of the left and right sides stops fish-tailing, for instance, and near-instantaneous interventions mean greater traction and better trajectory tracking and recovery.
“We’ve been making sure that we can sense with confidence and provide the appropriate feedback in all conditions as we open up the dynamic envelope of the vehicle,” Wang noted. In the recent past, for instance, the researchers estimated the tire-cornering stiffness coefficient, a key parameter for traction control and electronic stability control systems.
Of late the OSU team has concentrated on improving the car’s energy efficiency, which would augment its range. They applied three different control allocation methods to coordinate the redundant actuators. For the same speed tracking performance, the three schemes dictate different torque distribution strategies by considering the operating efficiencies of in-wheel motors in different ways, he said.
One scheme was a simple rule-based approach. The second was an adaptive system, and the third was based on nonlinear programming (Karush-Kuhn-Tucker, KKT). In simulations and experiments, the researchers evaluated the trio in terms of vehicle speed tracking performance, actuator dynamic response, and total energy consumption.
The adaptive and KKT-based methods used less energy than the simple rule-based system by incorporating the operating efficiencies in more elaborate formulations. The KKT-based scheme, which can theoretically achieve the global optimization within each sampling time, was the most energy-efficient of the three.
The Ohio State group is also trying to make the system more fault-tolerant so that if a motor, wheel, or brake malfunctions, it can compensate with the other drives to maintain safety. The more actuators, the greater chance of a fault—and fault detection and diagnosis is more challenging for overactuated systems. Equipment failures can cause conventional feedback control designs to respond inappropriately or even cause instability. Future work will focus on optimizing the weight distribution in the vehicle.
The Ohio State lab, which got its start when Wang received a 2009 Young Investigator Award from the U.S. Office of Naval Research for his work on vehicle control topics, today counts some 20 members, including three doctoral students, a master’s degree graduate student, two undergraduates, and three local-area high schoolers. The group also receives funding from the Honda-Ohio State University Partnership Program and the Ohio State University Transportation Research Endowment Program.
In 2012, he won a Faculty Early Career Development National Science Foundation CAREER award, a grant of $400,000 over five years that is intended to support active junior faculty who combine excellent research and teaching and integrate the two in their organization. As part of the grant, Wang’s lab hosted a summer program for high school students in which, among other things, they built radio-controlled electric model cars to better understand how EVs work.
In addition, students from the Columbus Metro School, a public STEM high school open to students from around the state, have participated in research internships on the experimental car.