Research On The Design And Gait Planning Of Multi-locomotion Modes Of A Bionic Leg-wheel Hybrid Mechanisms

With the ever-growing requirements of ground mobile systems with high mobility performance, attention should be paid to expanding new movement platforms to enhance the locomotion performance of robots in environments, such as grasslands, marshes and mountains. Legs and wheels are two widely selected methodologies utilized by ground locomotion systems. Legged robots are excellent for rough and unstructured terrain, but are slow and energy inefficient. While wheeled vehicles are usually used for smooth terrain due to their high speeds, high energy efficiency and simple controls, but most cannot overcome uneven terrain. Therefore, a leg-wheel hybrid locomotion mechanism, which would provide the merits of speedy motion of wheels on flat ground and efficient movement of legs on uneven terrain, could be thought as an appropriate motion platform with great mobility in the natural environment,In this paper, by summarizing and analyzing the characteristics of mechanism of leg-wheel hybrid robots at home and abroad, a leg-wheel hybrid transformable robot, called HyTRo-I, has been developed to integrate mammalian legs and wheels in a mechanically decoupled manner. The legs that imitate the legged locomotion of mammals would traverse irregular terrain with its main body at a high height that assures adequate ground clearance and largely improves the traversability. Inspired from the structural characteristics and legged locomotion of tetrapod, a biologically designed quadruped robot, which has four modular legs actuated by ball-screws with better mechanical property, has been proposed. The wheel mobile vehicle was suspended from the abdomen of a quadrupedal robot’s body while the four modular leg mechanisms were attached to the torso. The four legs were assembled with the front leg knee joints pointing forward and the hind leg knee joints pointing backward with respect to the direction of motion. This configuration avoids range of motion interference between the leg and wheel mechanisms, but results in a more compact structure. HyTRo-I possesses various movement modes to satisfy different requirements of different terrain conditions and tasks:high speed of wheeled rolling (WR) on flat ground, the adaptability of quadrupedal walking (QW) over rough terrain, the legged-wheeled traversing (LWT) mode to overcome complicated terrain by coordinating locomotion of the legs and wheels simultaneously.The kinematics and dynamics of HyTRo-I in three locomotion modes were analysed. Firstly, based on the D-H kinematics model of one leg, the inverse kinematics equation of the joints variations and the concrete relationships between axial-direction working strokes of ball-screw actuators and joint angles of one leg are given. Secondly, under the condition of satisfying the constraints of the non-holonomic mobile robot, the relationships among the movement speed of body, driving speed and steering speed of wheels were described. Thirdly, to realize the LWT locomotion mode, the kinematic relationship between the foot-end of leg and driven wheels about the position, velocity and acceleration is built by the derivation of Jacobian Matrix.To realize the QW mode of HyTro-I, a modified gait model, which released several constraints of the wave gait model and proposed new parameters for the lateral offset and displacements of the center of gravity (COG), was proposed to formulate a new reinforced wave gait, called reinforced wave gait. It was more applicable and stable for QW mode over irregular terrain. Additionally, the longitudinal and lateral stability margins of the new gait were formulated. To illustrate the superiority of these new gait parameters, alternative and more efficient reinforced wave gait patterns are described and strategies were proposed to address the asymmetric disposal carrying loads on the body and the loss of stability that results from the massive leg motion of the quadruped robot. Finally, the modified calculation of stability margin of the reinforced wave gait was formulated for the real motion control of HyTRo-I in QW mode.With respect to the low energy efficiency of QW mode, a method of optimizing the gait parameters was proposed to reduce the total energy expenditure in a period of HyTRo-I in the QW mode. The model of total energy consumption was related to the dynamics and inertia of legs, quasi-static static equilibrium equation and constraint, friction cone constraints of feet, the constraints of joint torque and system balance. The energy efficiency analyses were conducted by taking the minimum specific resistance, which refers to the energy consumption per length of travel for a quadruped robot, as the criterion for evaluation. According to a lot of simulated results, the new reinforced wave gait was more efficient than the standard wave gait. Under the same duty factor and step length, the lateral offset could reduce the energy consumption, and the larger the lateral offset, the less the energy consumption.The transitions between the QW and WR modes which respect joint range limitations could be achieved by using simple statically stable motions. Additionally, the longitudinal stability margin of the transformation gait was derived to guide the gait generation and motion control.The adaptive gait to traverse over the obstacles such as the protrusions and holes for HyTRo-I using the LWT mode was generated. Based on the known dimensions of obstacles and the model of HyTRo-I, a general gait strategy that relates the motion regulation and stability analysis was formulated to guide HyTRo-I or quadruped robot to ascend protrusions. Additionally, the all-leg-supporting gait was proposed for HyTRo-I to overcome the depressions.Finally, the first prototype of mechanically decoupled leg and wheel hybrid transformable system was set up. The experiments of wheels rolling on flat ground, quadruped walking with onboard loads, the reciprocal transformation of WR and QW locomotion modes, obstacles crossing including climbing up and down a high protrusion, traverse over ditches and slopes were carried out and had testified the effectiveness of the proposed hybrid mechanisms design, gait generation and control, and mobility and adaptive on uneven terrain.