Slip Modelling, Estimation and Control of Omnidirectional Wheeled Mobile Robots with Powered Caster Wheels

Abstract

Wheel slip problem has been mainly studied in the fields of vehicle dynamics and outdoor mobile robot navigation. Different from these areas that usually consider non-holonomic Wheeled Mobile Robots (WMRs), this research focuses on the wheel slip problem in the case of omnidirectional WMRs with Powered Caster Wheels (PCWs). PCW-based WMRs are chosen because they are omnidirectional, singularity free and redundantly actuated.

Most existing modelling methodologies of WMRs are based on the "pure rolling without slipping" assumption, thus most existing motion control schemes of WMRs assume that there is no slip and traction between the wheel and the ground is always maintained. However, it is observed that slip often occurs in WMRs with PCWs. Moreover, in mission critical tasks such as planetary exploration, traction between the wheel and the ground must always be maintained and the wheel slip critically determines the traction performance of the robot. These are the main motivations for this research.

This research distributes the efforts on three main aspects of the wheel slip problem for WMRs with PCWs: slip modelling, slip detection and slip control.

By removing the assumption of "pure rolling without slipping", we model WMRs with slip for both the kinematic and dynamic models. Borrowing ideas from vehicle dynamics, a new wheel-ground interaction model is developed that describes the explicit relation between slip ratio and traction force. For the convenience of describing wheel slip and internal force analysis for WMRs with PCWs, longitudinal and lateral velocities of wheel center are chosen as the generalized velocities of the robot, rather than the rolling and steering velocities of the wheel.

Several slip detection and estimation schemes are proposed in this research. For the purpose of explicit slip estimation, sliding mode observer based on the vehicle dynamic model is proposed to estimate the actual vehicle velocity using only joint angle measurements. All the proposed slip detection and estimation schemes are easily realized and demonstrated to be suitable for real time implementation. The performance of the proposed slip detection schemes is validated by both simulations and real time experiments.

The main contribution of this research is the proposition of several slip control schemes for effectively controlling the wheel slip effects. Sliding mode slip compensation scheme is proposed to achieve much better wheel motion synchronization. Slip constraint force control scheme is proposed based on the internal force analysis for WMRs with PCWs. Actuation redundancy of the mobile robot is used in the slip constraint force control scheme to minimize wheel slip. In the slip constraint force control scheme, the operational space space is decoupled with the internal force space so that multi-objective control is achieved. Extensive simulation and experimental results are presented to validate the performance of the proposed slip constraint force control.

To extend the applications of the proposed slip detection and control schemes, those schemes have been incorporated into the unified force/motion control framework for a mobile manipulator. Testing for a force controlled wheeled mobile robot is presented with the slip constraint force control implemented. Slip control techniques that are suitable for rough terrain navigation are also studied. Sliding mode slip ratio control and adaptive terrain identification are proposed to achieve reliable rough terrain navigation.