Mixed kinematic and dynamic sideslip angle observer for accurate control of fast off-road mobile robots

Automation in outdoor applications (farming, surveillance, military activities, etc.) requires highly accurate control of mobile robots, at high speed, although they are moving on low-grip terrain. To meet such expectations, advanced control laws accounting for natural ground specificities (mainly sliding effects) must be derived. In previous work, adaptive and predictive control algorithms, based on an extended kinematic representation, have been proposed. Satisfactory experimental results have been reported (accurate to within ±10 cm, whatever the grip conditions), but at limited velocity (below 3 m·s-1). Nevertheless, simulations reveal that control accuracy is decreased when vehicle speed is increased (up to 10 m·s-1). In particular, oscillations are observed at curvature transition. This drawback is due to delays in sideslip angle estimation, unavoidable at high speed because only an extended kinematic representation was used. In this paper, a mixed backstepping kinematic and dynamic observer is designed to improve observation of these variables: the slow-varying data are still estimated from a kinematic representation, which is then injected into a dynamic observer to supply reactive and reliable sliding variable (namely sideslip angle) estimation, without increasing the noise level. The algorithm is evaluated via advanced simulations (coupling Adams and MatLab software) investigating high-speed capabilities. Actual experiments at lower speed (experimental platform maximum velocity) demonstrate the benefits of the proposed approach

Mobile robot control on uneven and slippery ground: An adaptive approach based on a multi-model observer

International audienceThis paper proposes an algorithm dedicated to off-road mobile robot path tracking at high speed. In order to ensure a high accuracy, a predictive and adaptive approach is developed to face the various perturbations due to this context (mainly the bad grip conditions and the terrain geometry). The control law is based on previous work, and requires the knowledge of sideslip angles, which cannot be directly measured. As a result, an observer based on two levels of modeling (kinematic and dynamic) is proposed to ensure a relevant and fast estimation. If the kinematic part is independent from the terrain geometry, the dynamic model used in this paper requires to take explicitly into account the influence of the terrain geometry on mobile robot dynamic. It is achieved by the introduction of the lateral robot inclination, which is on-line estimated via a kalman filter and integrated in the dynamical model. The advantages of the proposed contribution to path tracking control are investigated through full-scale experiments achieved at high speed (up to 6m/s) on an uneven and grass field

Ride and directional dynamic analysis of articulated frame steer vehicles

ABSTRACT Pazooki Alireza, Ph.D. Concordia University, 2012 Articulated frame steer vehicles (ASVs), widely employed in different off-road sectors, are generally unsuspended vehicles. Owning to their complex operating environment, high mass center, relatively soft and large diameter tires, wide load variations and load distribution, and kineto-dynamics of the frame steering mechanism, these vehicles transmit higher magnitudes of low frequency whole-body vibration (WBV) to the operators and also exhibit lower roll and directional stability limits. While the superior performance potentials of axle suspension in limiting the WBV exposure have been clearly demonstrated, the implementations in ASVs have been limited due to the complex design challenges associated with conflicting requirements posed by the ride and roll/directional stability requirements. Growing concerns on human driver comfort and safety, and increasing demands for higher speed ASVs such as articulated dump trucks, however, call for alternate suspension designs for realizing an improved compromise between the ride and stability performance. This dissertation research is aimed at analysis of a torsio-elastic axle suspension concept for achieving improve ride, while preserving the directional stability limits of the ASV. For this purpose a comprehensive three-dimensional model of the articulated frame steer vehicles is developed for design and analysis of the proposed axle suspension concept. The model is formulated considering a three-dimensional tire model, tire lag, coherent right- and left-terrain track roughness, and kinematics and dynamics of the steering struts. Field measurements were performed to characterize the ride properties of a conventional forestry skidder and that of a skidder retrofitted with the rear-axle torsio-elastic suspension under different load conditions. The measured data were analyzed to assess the ride performance potential of the suspension and to examine validity of the simulation model. Both the field measured and simulation results revealed that the proposed suspension could yield significant reductions in the magnitudes of vibration transmitted to the operator location, irrespective of the load and speed conditions. A simple yaw-plane model of the vehicle is also formulated to study the role of steering system design including the steering valve flows, kineto-dynamics of the steering struts and leakage flows on the snaking stability limits of the ASV. The results showed that the critical speeds are strongly dependent upon the kineto-dynamics of the articulated steering system. The comprehensive three-dimensional model subsequently used for analysis of integrated ride and roll/directional stability limits of the vehicle and the axle suspension designs. The stability performance measures are defined to assess the vehicle stability limits under steady as well as transient directional maneuvers. The results show that the proposed rear-axle suspension deteriorates the stability performance only slightly, irrespective of the load condition. It is concluded that the proposed suspension concept could yield a very good compromise in ride and stability performance. The proposed model could serve as an effective and efficient tool for integrated ride and handling analysis and to seek primary suspension designs for an improved compromise between the ride and stability performance of ASVs

Dynamic Modelling and Stability Controller Development for Articulated Steer Vehicles

In this study, various stability control systems are developed to remove the lateral instability of a conventional articulated steer vehicle (ASV) during the oscillatory yaw motion or “snaking mode”. First, to identify the nature of the instability, some analyses are performed using several simplified models. These investigations are mainly focused on analyzing the effects of forward speed and of two main subsystems of the vehicle, the steering system and tires, on the stability. The basic insights into the stability behavior of the vehicle obtained from the stability analyses of the simplified models are verified by conducting some simulations with a virtual prototype of the vehicle in ADAMS. To determine the most critical operating condition with regard to the lateral stability and to identify the effects of vehicle parameters on the stability, various studies are performed by introducing some modifications to the simplified models. Based on these studies, the disturbed straight-line on-highway motion with constant forward speed is recognized as the most critical driving condition. Also, the examinations show that when the vehicle is traveling with differentials locked, the vehicle is less prone to the instability. The examinations show that when the vehicle is carrying a rear-mounted load having interaction with ground, the instability may happen if the vehicle moves on a relatively good off-road surface. Again, the results gained from the analyses related to the effects of the vehicle parameters and operating conditions on the stability are verified using simulations in ADAMS by making some changes in the virtual prototype for any case. To stabilize the vehicle during its most critical driving condition, some studies are directed to indicate the shortcomings of passive methods. Alternative solutions, including design of different types of stability control systems, are proposed to generate a stabilizing yaw moment. The proposed solutions include an active steering system with a classical controller, an active torque vectoring device with a robust full state feedback controller, and a differential braking system with a robust variable structure controller. The robust controllers are designed by using simplified models, which are also used to evaluate the ability to deal with the uncertainties of the vehicle parameters and its variable operating conditions. These controllers are also incorporated into the virtual prototype, and their capabilities to stabilize the vehicle in different operating conditions and while traveling on different surfaces during the snaking mode are shown