Collision Avoidance in Tightly-Constrained Environments without Coordination: a Hierarchical Control Approach (ICRA 2021)
We present a hierarchical control approach for maneuvering an autonomous vehicle (AV) in a tightly-constrained environment where other moving AVs and/or human-driven vehicles are present.
A two-level hierarchy is proposed: a high-level data-driven strategy predictor and a lower-level model-based feedback controller. The strategy predictor maps a high-dimensional environment encoding into a set of high-level strategies. Our approach uses data collected on an offline simulator to train a neural network model as the strategy predictor. Depending on the online selected strategy, a set of time-varying hyperplanes in the AV's motion space is generated and included in the lower-level control. The latter is a Strategy-Guided Optimization-Based Collision Avoidance (SG-OBCA) algorithm where the strategy-dependent hyperplane constraints are used to drive a model-based receding horizon controller towards a predicted feasible area. The strategy also informs switching from the SG-OBCA control policy to a safety or emergency control policy.
We demonstrate the effectiveness of the proposed data-driven hierarchical control framework in simulations and experiments on a 1/10 scale autonomous car platform where the strategy-guided approach outperforms a model predictive control baseline in both cases.
Xu Shen*, Edward L. Zhu*, Yvonne R. Stürz, Francesco Borrelli
Model Predictive Control Lab
University of California, Berkeley
ICRA 2021
* equal contribution
In the actual implementation, we define 7 operation states:
Free-Driving: Solve a nominal path tracking problem since there is no target vehicle in the local maneuver region.
Safety-Confidence: Select the Safety Control policy when the strategy predictor outputs low confidence for all strategies.
Safety-Yield: Select the Safety Control policy when the strategy predictor predicts "Yield" with high confidence.
Safety-Infeasible: Select the Safety Control policy when the SG-OBCA is infeasible.
HOBCA-Unlocked: Select the SG-OBCA policy without locking the strategy (i.e. the strategy predictor can change the strategy between "Pass Left" and "Pass Right" at any time).
HOBCA-Locked: Select the SG-OBCA policy with a locked strategy (i.e. does not allow the strategy predictor to change the strategy between "Pass Left" and "Pass Right" anymore since the maneuver is already in progress. The sudden change of strategy will lead to infeasibility).
Emergency-Brake: Select the Emergency Brake policy since a collision is anticipated and cannot be resolved.
In paper, we abstract them into 3 policies (SG-OBCA, Safety, Emergency-Brake) for simplicity;
SG-OBCA: include "Free-Driving", "HOBCA-Unlocked", "HOBCA-Locked"
Safety: include "Safety-Confidence", "Safety-Yield", "Safety-Infeasible"
Emergency-Brake: "Emergency-Brake"
We have logged the time that the Forces Pro-compiled NLP solver takes to return an optimal solution of SG-OBCA. The total time has a mean of 30ms and 99.8% of feasible solutions are returned in less than 100ms, which is real-time feasible.
ParkPredict: Motion and Intent Prediction of Vehicles in Parking Lots (Dataset for training)
More Research about "Learning in MPC" at MPC Lab, UC Berkeley