Dynamic pushing strategies for dynamically stable mobile manipulators

This paper presents three effective manipulation strategies for wheeled, dynamically balancing robots with ar- ticulated links. By comparing these strategies through analy- sis, simulation and robot experiments, we show that contact placement and body posture have a significant impact on the robot's ability to accelerate and displace environment objects. Given object geometry and friction parameters we determine the most effective methods for utilizing wheel torque to perform non-prehensile manipulation. Index Terms— dynamic stability, mobile manipulation, fric- tion, contact forces I. INTRODUCTION In this paper we present, analyze and evaluate three contact strategies for dynamically balancing mobile manipulators that push large and heavy objects. Such a control strategy is useful for transporting a hospital stretcher, a warehouse crate or a fallen beam that needs to be pushed in order to reach a target goal or to create space for the robot to move. Robots designed for mobile manipulation are similar to humans since they can navigate and manipulate their environments in order to satisfy such a task. However, under these circumstances, humans try different postures and contacts. They lean and use their entire bodies to perform the task effectively. In contrast, typical robots do not consider how their posture or contact will affect their ability to perform the task. (1-4) In this paper we use dynamic analysis and experimental validation to prove that robots can also choose the best configuration for a desired push. Our work focuses on Sparky, a two-link mobile manipu- lator that dynamically balances on two wheels. Articulation between the links allows the robot to bend forward and back in order to make contact with environment objects. Prior to contact, an LQR derived PD controller keeps the robot balanced and allows it to navigate. In this paper we choose the contact and analyze the result of applying wheel torque to perform manipulation. We compare moving an object by pushing against a vertical edge, lifting and pushing against a horizontal edge, and colliding with the object. The performance of these techniques is shown via dynamic analysis, dynamic simulation, and experimental results.