Study Of Redundantly Actuated Bionic Mastication Robot With Point Contact Higher Kinematic Pair

Mastication robot can be widely used in dentistry, food science, biomechanics and prosthetics, etc. Furthermore, it can provide a useful tool for studying biomechanical behavior of human masticatory system. So far, the movement characteristic of the temporomandibular joint (TMJ) and actuation redundancy feature in masticatory system is not given sufficient consideration when a mastication robot is designed. While, the TMJ not only can affect the mandibular movement, but also can affect the force of the mandible. Consequently, the designed mastication robot can not reproduce the movement trajectory and the occlusal force at the same time. Ignoring the TMJ will result in shortcomings in bio-imitability of mastication robot. For this situation, a novel spatial redundantly actuated parallel bionic mechanism with point contact higher kinematic pair is proposed to reproduce the actuation redundancy and complex movement behavior of mandible. The kinematics, trajectory planning, dynamics, driving force optimization, etc. of the novel mechanism is studied. The experimental verification is carried out. The main contents are as follows.Based on the biomechanical findings about physical structures of the masticatory system and movement feature of the TMJ, a novel spatial redundantly actuated parallel masticatory robot is proposed. This novel mechanism contains point contact higher kinematic pair (HKP), and has four degrees of freedom (DOFs), but is driven by six actuators. In the mechanism, six PUS parallel chains are used to model the six major groups of masticatory muscles and two HKPs to model the two TMJs. Combining the HKPs and the parallel mechanism, the 6PUS-2HKP mechanism is designed. The HKP allows the mastication robot to be able to simulate influence of the TMJ on mandibular force and movement trajectory. It can reveal the advantages of the actuation redundancy of the masticatory system and improve the bio-imitability of the mastication robot. So far, the redundantly actuated parallel mechanism is mainly composed of lower kinematic pairs. The kinematics, dynamics and control issues of the redundantly actuated parallel mechanism with HKP haven't formed a systematical theory.The kinematics modelling method of the novel 6PUS-2HKP mechanism with HKPs is proposed. The number of DOFs of the novel mechanism is larger than the number of the driving motors. Four independent generalized parameters correspond to six joint variables. First, the general form of kinematics of the redundantly actuated parallel mechanism with HKP are given. Second, based on the constraint equations of the two HKPs and the four independent parameters in the generalized coordinate system, the six position and orientation of the end effector (mandible) in Cartesian space are solved. Then the model of kinematics is derived with attention on the closed-loop constraint of the branched chain. Third, the differential kinematics is analyzed. The mapping between the independent velocity parameters and the joint velocity is established and the Jacobian matrix is obtained. Different with the general parallel mechanism, the Jacobian matrix is a non-square matrix. On this base, the workspace, singularity, and force transmission kinematic performances of the 6PUS-2HKP mechanism are analyzed. Last, compared with the kinematic performance of non-redundant 6-PUS parallel mechanism, the advantage of the 6PUS-2HKP in force transmission is revealed.Considering the bilateral correlated movement of the TMJ, The trajectory planning method of the mandible, which is also constrained by the occlusal surface shape of the teeth, is proposed. After the trajectory, velocity and border movement of subjects'lower incisor point are measured by using the mandibular kinesiograph, the movement relation of the mandible structure, occlusal contact and artificial TMJ (HKP) is analyzed. According to the bilateral movement relations of the HKPs, the kinematics of the HKPs, such as contact position, velocity and acceleration, are analyzed. Based on the kinematics of the HKPs, the functional movement (protrusive movement, lateral movement and opening movement) and chewing movement of the masticatory robot are planned, in order to fulfill the purpose of reproducing the complex movement cycle of the mandible.A method of driving force optimization for redundantly actuated parallel mechanism is proposed based on the dynamics of the 6PUS-2HKP. The quasi-static force analysis of the masticatory robot when the teeth are in occlusion is analyzed firstly, finding that the TMJ palys an important role in the force system of the mandible. And then, the inverse dynamics using the Lagrangian formulation is established. For a given occlusal trajectory and teeth force, the driving force of every linkage will have infinite solution due to actuation redundancy. Therefore, there are optimization methods and corresponding optimization goals that can optimize the dirving fore of the six linkages. As a result, the mechanism can achieve the best performance. The pseudo-inverse matrix method is proposed to optimize the driving force of the redundantly actuated parallel mechanism. According to the modeling and simulation result, the advantage of the redundantly actuated parallel mechanism is revealed.The bionic masticatory robot experiment platform is built to verify the trajectory and force of the mandible. The related trajectories, such as functional movements, workspace and mouth-opening movement, are verified by experiment. Simulated foods made of different materials are put in the mouth of the masticatory robot when the robot perform chewing movement. The chewing force and HKP force during the chewing process are analyzed. It verifies the output force can meet the requirements and offers a good reference to the study of the biomechanics of the human masticatory system. A force/position hybrid control method is proposed for the trajectory tracking control experiment. The lateral pterygoid motors are controlled by force mode and the masseter and temporalis motors are controlled by position mode. Through experiments and comparison with position control, it is shown that the fluctuation of torque curves of the lateral pterygoid motors get a significant improvement by using hybrid force/position control.