Curvature-Based Ground Vehicle Control of Trailer Path Following Considering Sideslip and Limited Steering Actuation
In this paper, a curvature-based control method capable of both forward and backward path following is developed for off-axle hitching trailers. Compared with earlier methods, this controller handles varying path curvature better while also being easy to implement and tune. The controller also addresses two major issues that deteriorate tracking accuracy: limited steering actuation and sideslip. An active speed limiter brakes the vehicle when the steering angle error is too high and therefore allows the same set of control gains to work across a relatively large range of reference speeds without significantly increasing lateral error or losing stability, which further simplifies controller tuning. An extended Kalman filter (EKF) sideslip estimator is designed to allow sideslip compensation, which considers the varying noise magnitude in GPS measurements caused by varying vehicle speeds so that the braking maneuver will not significantly deteriorate the estimations. To our knowledge, this paper presents the first publication of these two algorithms with trailer control. This method is compared with results from two earlier hitch-angle-based control methods in simulation and experiment. The active speed limiter and the EKF are also implemented on these control methods for fair comparison, during which it is found that the two algorithms can directly work with other control methods and benefit their performance. The results show the curvature-based controller provides better maneuverability and reduced tracking error.
There are numerous works on trailer control in the literature. While a wealth of traditional techniques based upon nonlinear control theory exists, as discussed by Pradalier et al. , these often provide highly complicated solutions that are difficult to implement on real systems with delay and disturbances –. Some early works – use vehicle path tracking control with paths planned specifically for the controlled vehicle-trailer system as a solution, which is an open loop control strategy for the trailer. Their performance can easily deteriorate in an outdoor environment, where positioning accuracy is usually lower and disturbances are more common. More recent simple trailer control methods use two nested control loops to steer the trailer by manipulating the hitch angle , . These works offer trailer control methods that are robust and easy to implement. For these controllers, hitch angle must reach a specific steady state to steer the trailer into a desired motion. Since trailer curvature is governed by both hitch angle and vehicle curvature, these controllers have difficulty tracking a path with varying curvature as the desired hitch angle changes constantly and the assumed steady state mapping is invalid. Hence, hitch angle based control leads to tracking error on paths with varying curvature.
This paper presents a curvature based tracking controller. A two tiered algorithm is proposed since subtleties in achieving trailer curvature must be considered. In the first tier (named “Path Tracking Controller”), desired trailer curvature is calculated according to trailer tracking error. The second tier (named “Trailer Curvature Controller”) then attempts to control the vehicle to achieve the desired trailer curvature. Our analysis shows, however, that the relationship between steering angle, hitch angle, and trailer curvature becomes inverted in certain configurations, and thus counter steering maneuvers are required. Hence, the second tier analyzes these conditions and decides whether to achieve desired trailer curvature directly or indirectly using different sub-controllers. Two other issues that most earlier works overlooked are limited steering actuation and sideslip. Steering rate limitations are commonly neglected, however, limited steering actuation always appears in real systems and can cause significant tracking error or instability. Ignoring this limitation requires lower reference speeds, which is less efficient, or lower control gains, which lead to worse tracking performance. Thus, an active speed limiter is necessary for better trailer control. Sideslip, which is normally caused by sideslope or cornering behavior, adds steady state error to path tracking. Estimating and compensating sideslip is necessary for maintaining high tracking accuracy, especially on sloped terrain.
A bidirectional curvature based trailer controller is designed for simple implementation and tuning. It considers and compensates sideslip of the vehicle and trailer. It has been proven in experiments on a full size vehicle that the controller can follow a path with good accuracy in the presence of sideslip. Comparisons with existing methods show that the method has advantages in handling path curvature change. An active speed limiter is designed to ensure that the steering wheel angle convergence process is fast enough to have an insignificant impact on lateral error convergence. It is proven to be effective and applicable to all the simple trailer controllers implemented in this research.
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