Bahareh Ghotbi

Effect of normal force dispersion on the mobility of wheeled robots operating on soft soil

A number of applications of wheeled robots, including planetary exploration rovers and rescue missions, require that the vehicle operates in a non-structured environment. Optimizing the vehicle mobility is of key importance in such applications. Reduced mobility can limit the ability of the robot to achieve the mission goals, or even render it immobile in extreme cases. In this paper, the effect of normal contact forces on mobility is reported. A performance indicator based on the force distribution is defined and used to compare different vehicle configurations. The validity of this indicator was assessed using both simulation and experimental results obtained for a six-wheel rover prototype. Results suggest that modifying the robot configuration to alter the normal force distribution can lead to increased traction force available at the wheel-terrain interfaces, thus improving the mobility.

Vehicle-terrain interaction models for analysis and performance evaluation of wheeled rovers

In this work, a multibody dynamics model of a wheeled mobile robot is developed to characterize the terrain reaction forces in terms of the physical and control parameters of the system. A common strategy for simulating the motion of mobile robots on soft soil is to compute the soil reaction forces using terramechanics models and to solve a forward dynamics problem by considering the soil reactions as a set of forces applied to the system. This intends to provide an accurate computation of the forces involved in the wheel-soil interaction; however, a series of factors such as the sensitivity of reaction forces to soil parameters limits the applicability of the existing terramechanics models in unstructured environments. We propose an alternative approach which does not rely on the soil properties, but at the same time does not intend to provide an exact computation of wheel-soil interaction forces. The main objective of this approach is to estimate the effect of changes in control and design parameters on the performance of the system, using the information provided by the dynamics model of the vehicle. To this end, the reaction forces for the wheel-terrain interaction in the ideal limit case of pure rolling and no penetration are obtained upon the specification of the motion at the contact points, via kinematic constraints. The validity of the analysis results obtained using the proposed paradigm is verified by simulation runs and experiments. The experimental results suggest that this approach is successful in predicting the variation of a set of important performance indicators in terms of the changes in the parameters of the system.

Mobility Assessment of Wheeled Robots Operating on Soft Terrain

Optimizing the vehicle mobility is an important goal in the design and operation of wheeled robots intended to perform on soft, unstructured terrain. In the case of vehicles operating on soft soil, mobility is not only a kinematic concept, but it is related to the traction developed at the wheel-ground interface and cannot be separated from terramechanics. Poor mobility may result in the entrapment of the vehicle or limited manoeuvring capabilities. This paper discusses the effect of normal load distribution among the wheels of an exploration rover and proposes strategies to modify this distribution in a convenient way to enhance the vehicle ability to generate traction. The reconfiguration of the suspension and the introduction of actuation on previously passive joints were the strategies explored in this research. The effect of these actions on vehicle mobility was assessed with numerical simulation and sets of experiments, conducted on a six-wheeled rover prototype. Results confirmed that modifying the normal load distribution is a suitable technique to improve the vehicle behaviour in certain manoeuvres such as slope climbing.

Sensitivity Analysis of Mobile Robots for Unstructured Environments

Simulating mobile robots in unstructured environments requires knowledge of the wheel/terrain interaction phenomena. Even assuming that the terramechanics models available accurately represent the physics of the interaction, estimation of soil parameters can be a source of error. In applications where high robot reliability is mandatory, it is important to realize the influence of possible sources of error on the system behavior. The effect of small variations of parameters on system performance can be studied under sensitivity analysis. In this work, sensitivity analysis is conducted to investigate the effect of perturbations in the soil parameters on the behavior of a single rigid wheel and a vehicle on soft terrain. For the first system the two widely used terramechanics models, Bekker’s and Wong and Reace’s are studied, sensitivity analysis being conducted using direct differentiation. The second system is modeled using Bekker model, sensitivity being obtained using finite differences.Copyright