Thesis defences

PhD Oral Exam - Meysam Effati, Electrical and Computer Engineering

Energy-Efficient Trajectory Planning for Skid-Steer Rovers

Date & time

Friday, July 3, 2020

Cost

This event is free

Where

Online

When studying for a doctoral degree (PhD), candidates submit a thesis that provides a critical review of the current state of knowledge of the thesis subject as well as the student’s own contributions to the subject. The distinguishing criterion of doctoral graduate research is a significant and original contribution to knowledge.

Once accepted, the candidate presents the thesis orally. This oral exam is open to the public.

Abstract

A skid-steer rover’s power consumption is highly dependent on the turning radius of its path. For example, a point turn consumes a lot of power compared to a straight-line motion. Thus, in path planning for this kind of rovers, turning radius is a factor that should be considered explicitly.

Based on the literature, there is a lack of globally energy-optimal path planning approaches for skid-steer rovers. This thesis addresses this problem for such rovers, specifically on obstacle-free hard ground. A key contribution of this thesis is an equivalency theorem that shows that, when using a popular power model for skid-steer rovers on hard ground, all minimum-energy solutions follow the same path irrespective of velocity constraints that may or may not be imposed. It is shown that one can choose velocity constraints to enforce constant power consumption, thus transforming the energy-optimal problem in an equivalent time-optimal problem. Existing theory, built upon the basis of Pontryagin’s minimum principle to find the extremals for time optimal trajectories for a rigid body, can then be used to solve the problem. Accordingly, another key contribution in this thesis is obtaining all the possible extremal paths for the energy-efficient path planning problem. As there is a finite number of extremals, they are enumerated to find the global minimum for a particular example.

Furthermore, the analysis identifies that the turns in optimal paths (aside from a small number of special cases, called whirls, considered separately) are to be circular arcs of a particular turning radius, R′, equal to half of a skid-steer rover’s slip track. R′ is the turning radius at which the inner wheels of a skid-steer rover are not commanded to turn, and its description and the identification of its paramount importance in energy-optimal path planning are another key contributions of this thesis. Experiments with a Husky UGV rover validate the energy-optimality of using R′ turns.

Finally, there are other contributions in this thesis: proposing a practical velocity constraint for skid-steer rovers; and, showing that almost always equal “friction requirement” can be used to obtain optimal traction forces for a common and practical type of 4-wheel rover.