Master Thesis Adaptive Slip Based Control Allocation

Master Thesis Proposal - Adaptive Slip Based Control Allocation of BEV Combinations with the Safe State Limits Obtained from the Phase Portraits 

Trucks with different trailer combinations can have several motion actuators that can be used for controlling the desired vehicle motions. The number of motion actuators such as steerable axles, mechanical brakes, and auxiliary brakes in combination with available powertrain actuators exceeds the number of controlled motions, so-called “over-actuation”. This allows several feasible solutions to achieve the desired longitudinal, lateral, yaw, and roll motion of each unit within the vehicle combination. One way to solve the over-actuation is to use Control Allocation (CA) within the control system. Control allocation is an option for coordination when the system has more input signals than virtual signals controlled.

Allocation of global longitudinal force requests into different units can be done by considering the safety and energy efficiency:

Safety: Allocation of very large braking or propulsion forces on driven axles, while keeping most of the other axles unbraked or unpropelled may cause yaw instabilities. As an example, if the tractor drive axle is (regeneratively) braked too much in a slippery curve, keeping all other axles unbraked may cause “jackknifing”. Allocation of too large forces on a slippery surface to the e-trailer drive axle may cause “trailer swing”. The control allocation algorithm should allocate forces in a safe way, which leads to no yaw instability.

Energy Efficiency: Even if only one unit is electrified, energy efficiency is an important concern. For example, if only tractor is electrified, but trailer is conventional (non-electrified), most of the braking should be done in a regenerative way, which means braking only the driven axle of the tractor. If more than one unit is propelled, then optimal force distribution may be different, for different speeds and different force requests. This depends on the efficiencies of the electric motors, powertrains, batteries, inverters, and other losses.

As explained above, allocation with safety concern and allocation with energy efficiency concern may result in different distributions. Hence, “safe and energy-efficient” control allocation becomes a more complicated optimization problem.  In other words, it can be considered as a “trade-off”. For the simplest example with e-tractor and conventional trailer, the most energy-efficient braking strategy is to (regeneratively) brake the driven axle of the tractor only. But in a slippery curve, this is not safe and may cause jackknifing. From the safety point of view, the most optimal solution is usually braking all axles proportional to their normal loads.

Control allocation is mostly performed with physical model-based optimization methods [1,2,3]. In this thesis, it is aimed to obtain the safe state limits for the electric tractor and electric semitrailer combination shown in Fig. 2, where both units can both propel and regeneratively brake. The advantage of electric powertrains is that the controllers can request a certain amount of torque from the electric machines, while also requiring the longitudinal slips to be below a certain limit. Hence, force/torque control with slip/speed limits can be achieved. In [4], a slip controller using a slip polytope is studied. This MSc thesis will be an extension of [4]. In [4], a novel approach to ensure the yaw stability of combination vehicles is proposed. Longitudinal slip limits of the electric motor are reduced for the deviations from the reference side-slip angle. This resulted in a superior acceleration and stability performance compared to fixed slip limits.

In this thesis work:

• Propulsion 
  • with only e-tractor electric motor
  • with only e-trailer electric motor

with both units

• Regenerative braking
  • with only e-tractor electric motor
  • with only e-trailer electric motor

with both units

will be investigated by using 
•    single-track or
•    two-track models
with a combined slip tire model.

Instead of having a reference side-slip angle generator, and a fixed side-slip angle margin as implemented in [4], phase portraits of tractor-semitrailer combination will be derived.

Phase portrait for a passenger vehicle, from Mats Jonasson, Chalmers TME102 Vehicle Motion and Control Lecture Notes

The aim is to obtain the corresponding phase portraits as shown in Fig. 3, for the tractor semitrailer combination. The phase portraits can be in different combinations of states, such as

•    Lateral speed vs yaw rate
•    Side-slip angle vs side-slip angle rate
•    Side-slip angle vs yaw rate

It is envisioned that multiple phase portraits are needed to address the complexity of all possible state combinations of a two-unit vehicle combination.

The safe limits obtained from the phase portraits will be used in the control allocation of combination vehicles. The vehicle combination will be simulated with Volvo Transport Model (VTM, high-fidelity model) developed with Simscape Multibody™. The accuracies of the safe limits will be tested in different scenarios. The safe limits should result in safe and energy efficient driving.

The thesis work will require control theory, vehicle dynamics and signal processing skills. Interest in mathematical modeling can be seen as a benefit. The thesis is recommended for two students with control analysis profile with good mathematical skills. Thesis start date: Jan 2025.

Supervisor:
Umur Erdinc – Volvo GTT

References

[1]. Uhlén, K., Nyman, P., Eklöv, J., Laine, L., Kati, M. and Fredriksson, J., “Coordination of Actuators for an A-double Heavy Vehicle Combination”, 17th International IEEE Conference on Intelligent Transportation Systems, 2015. 

[2]. Hjelte Ulmehag R., “Energy management for a tractor and semitrailer combination using control allocation”, Master thesis, Chalmers, 2016. 

[3] Axel Hansson, Erik Andersson, Leo Laine, Maliheh Sadeghi Kati, Umur Erdinc and Mats Jonasson, Control envelope for limiting actuation of electric trailer  in tractor-semitrailer  combination, to be presented in ITSC 2022 Conference, October 8-12, in Macau, China,2022. 

[4] U. Erdinc, M. Jonasson, M. S. Kati, L. Laine, B. Jacobson, and J. Fredriksson, ”Yaw stability control of vehicles using a slip polytope validated with real tests,” 16th International Symposium on Advanced Vehicle Control, Milan, Italy, Sep. 2024. Available in 
https://drive.google.com/file/d/1Nd0o0gX6dq0Q4ZmjhChqhibV2nOsCPyT/view?usp=sharing