Guoliang Zhong

Dynamic Hybrid Control of a Hexapod Walking Robot: Experimental Verification

In this paper, a walking robot that consists of its body and six legs is considered for trajectory control. The leg mechanism is a 1-UP&2-UPS (U-universal joint, P-prismatic joint, and S-spherical joint) parallel mechanism with the arm-leg conversion function. Due to the nonlinearity and uncertainties of the robot, it is difficult to obtain its exact dynamic model, therefore this paper utilizes the decentralized control strategy to design a dynamic hybrid controller, in which the outer loop is an impedance controller, the middle and inner loops are designed by using the sliding-mode control and the nonlinear proportional-integral-derivative (PID) control, respectively. The inverse kinematics of one leg is derived for mathematic model, based on which three control methods including the classical PID control, position-velocity loops cascade control, and dynamic hybrid control are examined. Contrast tracking experiments were performed with one robotic leg. The experimental results are shown to identify their differences and prove the effectiveness and applicability of the proposed dynamic hybrid control.

Approaches based on particle swarm optimization for problems of vibration reduction of suspended mobile robot with a manipulator

Mobile robots with suspension systems can absorb vibration induced by rough roads, but due to center-of-gravity (CG) shift, the suspended platform is subject to vibration when the platform moves with acceleration. This paper presents approaches based on the particle swarm optimization (PSO) algorithm to overcome the following vibration problems: (1) when the suspended platform moves with a static manipulator, vibration of the suspended platform occurs due to CG shift, (2) when the suspended platform and the manipulator move simultaneously, the vibration is caused by the dynamic manipulator. For the first problem, a method for the optimization of multi-input shapers using PSO is adopted with chaos to reduce the residual vibration. For the second problem, an approach based on PSO with chaos is developed to suppress the vibration by searching for the time-jerk synthetic optimal trajectories of the manipulator. Finally, the authors perform the resulting shapers and optimal trajectories on the presented models and demonstrate the vibration can be controlled to a desired level effectively in both problems.

An Improved Force-Angle Stability Margin for Radial Symmetrical Hexapod Robot Subject to Dynamic Effects

This paper presents a study on stability monitoring for a radial symmetrical hexapod robot under dynamic conditions. The force-angle stability margin (FASM) measure method has been chosen as the stability criterion. This is because it is suitable for the stability analysis, in terms of external forces or manipulator loads acting on the body. Considering that a radial symmetrical hexapod robot can tumble along the contact point besides tip-over axis, this paper proposes an improved FASM measure method. Furthermore, it provides the method for calculating the stability angle of contact point and simplifies the algorithm of FASM. To verify the improved FASM measure method, three potential dynamic situations have been simulated. The simulation results confirm that, under dynamic conditions, the improved FASM is efficient, simple in terms of calculation cost and sensitive to manipulator loads and external disturbances. This means it has practical value in on-line controllers.

Hierarchical Kinematic Modelling and Optimal Design of a Novel Hexapod Robot with Integrated Limb Mechanism

This paper presents a novel hexapod robot, hereafter named PH-Robot, with three degrees of freedom (3-DOF) parallel leg mechanisms based on the concept of an integrated limb mechanism (ILM) for the integration of legged locomotion and arm manipulation. The kinematic model plays an important role in the parametric optimal design and motion planning of robots. However, models of parallel mechanisms are often difficult to obtain because of the implicit relationship between the motions of actuated joints and the motion of a moving platform. In order to derive the kinematic equations of the proposed hexapod robot, an extended hierarchical kinematic modelling method is proposed. According to the kinematic model, the geometrical parameters of the leg are optimized utilizing a comprehensive objective function that considers both dexterity and payload. PH-Robot has distinct advantages in accuracy and load ability over a robot with serial leg mechanisms through the formers comparison of performance indices. The reachable workspace of the leg verifies its ability to walk and manipulate. The results of the trajectory tracking experiment demonstrate the correctness of the kinematic model of the hexapod robot.

Trajectory tracking of wheeled mobile robot with a manipulator considering dynamic interaction and modeling uncertainty

This paper proposes an adaptive control strategy for trajectory tracking of a Wheeled Mobile Robot (WMR) which consists of a suspended platform and a manipulator. When the WMR moves in the presence of friction and external disturbance, the trajectory can hardly be tracked accurately by applying the backstepping approach. For addressing this problem, considering the dynamic interaction, a dynamic model of the system is constructed by using Direct Path Method (DPM). An adaptive fuzzy control combined with backstepping approach based on the dynamic model is proposed. To track the trajectory accurately, a fuzzy compensator is proposed to compensate modeling uncertainty such as friction and external disturbance. Moreover, to reduce the approximation error and ensure the system stability, a robust term is added to the adaptive control law. Simulation results show the effectiveness and merits of the proposed control strategy in the counteraction of modeling uncertainty and the trajectory tracking.

Dynamic analysis of a hexapod robot with parallel leg mechanisms for high payloads

A novel hexapod robot with 3-DOF parallel leg mechanisms is presented in this paper. The payload capability of the robot has a high promotion due to the utilizing of parallel mechanisms. Meanwhile, a novel force distribution approach of joint space distribution for redundant actuation system is proposed to optimize joint forces in order to promote the payload capability of the robot. Usually, the dynamic load of the robot is distributed over the contact space, then map the forces of contact space onto joint space. Different to the usual way of contact space distribution, in this paper, the optimal force distribution for the supporting legs is directly solved in the joint space using the Moore-Penrose pseudo-inverse matrix. The simulation results demonstrate that the joint space distribution scheme results in a much more efficient distribution compared with the conventional scheme of contact space distribution due to the good use of friction to reduce the required joint forces.

Gait and trajectory rolling planning and control of hexapod robots for disaster rescue applications

Hexapod robots have stronger adaptability to dynamic unknown environments than wheeled or trucked ones due to their flexibility. In this paper, a novel control strategy based on rolling gait and trajectory planning, which enables hexapod robots to walk through dynamic environments, is proposed. The core point of this control strategy is to constantly change gait and trajectory according to different environments and tasks as well as stability state of robot. We established a gait library where different kinds of gaits are included. Zero moment point, which indicates the stability of robot, is estimated by a Kalman filter. According to this control strategy, a hierarchical control architecture consisting of a manmachine interface, a vision system, a gait and trajectory planner, a joint motion calculator, a joint servo controller, a compliance controller and a stability observer is presented. The control architecture is applied on a hexapod robot engaging in disaster rescue. Simulation and experimental results show the effectiveness of our control strategy. A complete control architecture based on gait and trajectory rolling planning method is proposed for hexapod robots.A method based on COG Jacobian is proposed to calculate joint motion depending on desired the robots COG trajectory and end-point trajectories of each leg.Typical gaits are obtained according to environmental adaptability and ZMP stability margin.

Measure and control of stability for mobile robots

The stability of mobile robots would be degraded due to the complex environment in rough terrain and the requirements of operation. In this paper, we investigate the improvement of stability from two aspects: measure and control. To measure the stability, we put forward a new method called force-angle stability margin (FASM), then use it and its modified form to evaluate the stability for a three-wheeled robot and a six-legged robot, respectively. To control the stability, for the three-wheeled robot particle swarm optimization (PSO) method is used to search the optimum semi-active damping characteristics for reducing the vibration from roads, the cost function is defined by considering the FASM method. For the six-legged robot, when its stability is beyond the acceptable range obtained by the FASM, a leg makes one step forward to support the robot and prevent tipping over. To verify and examine the effectiveness of the FASM method and control approaches, we perform them in simulative pavement and external environment. The obtained results show the proposed FASM method is feasible and the control approaches yield substantially improved stability when robots negotiate rough terrain.

Gait and Trajectory Rolling Planning for Hexapod Robot in Complex Environment

Hexapod robots have stronger adaptability to dynamic unknown environment than wheeled or trucked ones due to their flexibility. In this paper, a control strategy based on rolling gait and trajectory planning that enables a hexapod robot to walk in dynamic environment is proposed. The core content of the control strategy is to constantly change the gait and trajectory according to the dynamic environment and predicted stability margin of robot. Kalman filter is employed to compute predicted zero moment point (ZMP) monitoring the stability of robot in order to keep balance with adjusting gait and trajectory. A hierarchical control architecture consisting of high-level gait planner, low-level trajectory planner, joint servo controller and compliance controller is presented. The control strategy is applied to a hexapod robot engaging to disaster rescue. Experiment results show the efficiency of our control strategy over challenging terrain.

A comparison of tracking control methods in multi-legged robots

In this paper, a hexapod robot that consists of its body and six legs is considered. The leg mechanism is a 1-UP&2-UPS (U-universal joint, P-prismatic joint and S-spherical joint) parallel mechanism with three degree-of-freedoms (DOFs). A comparative investigation of tracking control methods in the hexapod robot with parallel legs is presented. The methods include standard PID control, position-velocity loops cascade control, and hybrid sliding mode control. It is difficult to get the accurate model of the parallel legs due to the nonlinear uncertainties, therefore this paper applies decentralized control strategy to design the controllers. The inverse kinematics of one leg is derived for mathematic model, based on which three control methods are examined. The results are shown to identify the difference of them and prove the effectiveness and applicability of hybrid sliding mode control.