Minimum-lap-time Control Strategies for All-wheel Drive Electric Race Cars via Convex Optimization

TL;DR

This paper develops a convex optimization framework to determine minimum-lap-time control strategies for AWD electric race cars, optimizing motor and battery control while considering tyre grip limitations, validated through simulations on a race track.

Contribution

It introduces a convex optimization approach for real-time control of AWD electric race cars, incorporating grip constraints and validated through simulation and design studies.

Findings

Torque vectoring improves tyre grip utilization and reduces lap time by over 4%.

Rear motor sizing has a greater impact on lap time due to load transfer effects.

The convex optimization framework efficiently computes globally optimal control strategies.

Abstract

This paper presents a convex optimization framework to compute the minimum-lap-time control strategies of all-wheel drive (AWD) battery electric race cars, accounting for the grip limitations of the individual tyres. Specifically, we first derive the equations of motion (EOM) of the race car and simplify them to a convex form. Second, we leverage convex models of the electric motors (EMs) and battery, and frame the time-optimal final-drives design and EMs control problem in space domain. The resulting optimization problem is fully convex and can be efficiently solved with global optimality guarantees using second-order conic programming algorithms. Finally, we validate our modeling assumptions via the original non-convex EOM, and simulate our framework on the Formula Student Netherlands endurance race track. Thereby, we compare a torque vectoring with a fixed power split configuration, showing that via torque vectoring we can make a better use of the individual tyre grip, and significantly improve the achievable lap time by more than 4%. Finally, we present a design study investigating the respective impact of the front and rear EM size on lap time, revealing that the rear motor sizing is predominant due to the higher vertical rear tyre load caused by the center of pressure position and rearwards load transfer under acceleration.