Deadlock avoidance for a quadruped with free gait

Free gait is seen as a basis for the coordination of legs in locomotion over irregular terrain, in particular the terrain containing forbidden zones. However, deadlock, a situation in which further movement of a legged vehicle in the desired direction is impossible due to negative stability margin or zero kinematic margin, frequently occur to the free gait. It could be avoided by imposing restrictions on leg reachable volumes for foothold selection, but this may lead to another situation in which no foothold would be found. This paper introduces an alternative method to maintain the stability margin positive for free gait by moving manipulator arms equipped on the vehicle to shift the centre of gravity of the vehicle. The shift in the centre of gravity is measured by use of foot forces through foot force constraint equations. In this paper, an adaptive leg lift-forward time/velocity is also introduced to realize continuous locomotion using minimum stroke to achieve maximum stride. Simulations have demonstrated the feasibility of the proposed algorithms for a quadruped