Preview Control with Dynamic Constraints for Autonomous Vehicles
Abstract
:1. Introduction
2. Vehicle Lateral Dynamics
3. Controller Design
3.1. Preview Controller
3.2. Establishment of Vehicle Dynamic Constraints
- Mass center slip angle. Considering that the too large mass center slip angle β has a potential effect on the driving stability, the limited range of β is denoted as
- Tire slip angle. Due to the fact that the vehicle dynamics model is established under the small slip angle assumption, the tire slip angle beyond the linear region may incur lower model precision. Moreover, once the tire adhesion reaches saturation (namely beyond linear region), the vehicle may easily skid. For the sake of the aforementioned reasons, the following constraints are set to avoid vehicle instability:
- Input steering angle. This is a hard constraint based on vehicle physical limits and driving safety requirement that is imposed to ensure a reasonable range of steering angle:
3.3. Optimization on Preview Length Using SA Algorithm
- Step 1:
- Initialize temperature T, prediction length H, and energy function E. Initial T is set to 600. H is started at a random value within [1,30]; then, it is used to calculate corresponding E.
- Step 2:
- Select a neighbor of current H randomly and compute E.
- Step 3:
- If Metropolis criterion is satisfied, accept the new H and E; if not, discard them.
- Step 4:
- Come back to step 2 if the termination condition (a minimum temperature or a maximum iteration number) is not satisfied; otherwise, the procedure terminates.
4. Closed-Loop System Analysis
4.1. System Stability
4.2. Steady-State Response in the Time Domain
- The sub-optimal control coefficient λi affects the steady state ey by increasing the amplitude of γ. However, this situation only happens during system transient response. As the system reaches steady state, λi approaches 1. In other words, the sub-optimal control is able to achieve steady-state error similar to the optimal control.
- Feedback gain Kx and feedforward gain Kρ can only change the steady-state ey but independent of steady-state eψ. A higher k1 results in lowering ey. Furthermore, well-matched k3 and Kρ can lead ey to zero theoretically.
4.3. System Response in the Frequency Domain
- The sub-optimal control with constraints weakens the suppression of path tracking errors compared with the optimal preview control. A lower λi usually leads to higher errors, especially at low and medium frequency. It is worth to notice that the yaw angle error eψ is not influenced by λi at very low frequency, which is similar to the steady-state response.
- The proposed control has the capability of suppressing the constrained variables, resulting in the enhancement of vehicle stability. Unfortunately, this happens only in a limited frequency range, which is called the valid section. As the sub-optimal control coefficient λi decreases, the suppression of β, αf, and αr becomes more obvious, but at the sacrifice of the performance beyond the valid section.
5. Simulation and Results
5.1. Establishment of Simulation
5.2. Results and Analysis
5.2.1. Performance at Different Speeds under the High Road Adhesion Condition
5.2.2. Performance at Different Speeds under the Low Road Adhesion Condition
6. Conclusions
Author Contributions
Funding
Conflicts of Interest
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Symbol | Definition | Unit |
---|---|---|
Vehicle mass | kg | |
Yaw moment of vehicle inertia | kg·m2 | |
Lateral tire force of the front/rear wheel (in vehicle body-fixed coordinate system, oxy) | N | |
Lateral stiffness of the front/rear wheel | N/rad | |
Slip angle of the front/rear wheel | rad | |
Velocity angle of the front/rear wheel | rad | |
Distance from center of gravity to the front/rear wheel | m | |
Front wheel steering angle | rad | |
Vehicle speed (in inertial coordinate system, OXY) | m/s | |
Longitudinal speed (the projection of v in the x axis of oxy) | m/s | |
Lateral error from center of gravity to the reference trajectory | m | |
Yaw angle of vehicle | rad | |
Error of yaw angle with respect to reference path | rad |
Vehicle Speed (m/s) | Road Friction Coefficient μ | Preview Length H |
---|---|---|
10 | 0.3 | 17 |
0.9 | 4 | |
15 | 0.3 | 28 |
0.9 | 9 | |
20 | 0.3 | 33 |
0.9 | 17 | |
25 | 0.3 | 35 |
0.9 | 19 |
Parameter | Value/Description |
---|---|
Vehicle mass | 1620 kg |
Front wheelbase | 1.165 m |
Rear wheelbase | 1.535 m |
Yaw inertia | 3645 kg·m2 |
Powertrain | 125 kW front-wheel drive |
Transmission gear ratio | 4.1:1 |
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Li, R.; Ouyang, Q.; Cui, Y.; Jin, Y. Preview Control with Dynamic Constraints for Autonomous Vehicles. Sensors 2021, 21, 5155. https://doi.org/10.3390/s21155155
Li R, Ouyang Q, Cui Y, Jin Y. Preview Control with Dynamic Constraints for Autonomous Vehicles. Sensors. 2021; 21(15):5155. https://doi.org/10.3390/s21155155
Chicago/Turabian StyleLi, Rui, Qi Ouyang, Yue Cui, and Yang Jin. 2021. "Preview Control with Dynamic Constraints for Autonomous Vehicles" Sensors 21, no. 15: 5155. https://doi.org/10.3390/s21155155
APA StyleLi, R., Ouyang, Q., Cui, Y., & Jin, Y. (2021). Preview Control with Dynamic Constraints for Autonomous Vehicles. Sensors, 21(15), 5155. https://doi.org/10.3390/s21155155