Motion Control And Stability Analysis Of Cable-Driven Camera Robots

Cable-driven parallel robots(CDPRs)are a special class of parallel robots.Their end-effectors are connected to the bases through cables,the movements and forces being provided from actuators to end-effectors via cables.Compared to rigid parallel manipulators,cable-driven parallel robots have some advantages,such as simple structure,light weight,low inertia of movement components,large workspace,large loading capacity,fast moving speed,small cumulative error,small chance of interference,removable and so on.They have been successfully applied in rehabilitation,material handling,sport simulation,astronomical observation,wind tunnel experiment and other fields,greatly expanding the application scope of parallel robots.Hence,they have a wide application prospects and significant research value.Cable-driven parallel robots lead to unstable with single way force of cables.Hence,it is important to study the stability of the cable-driven parallel robots.The stability of the cable-driven parallel robots mainly includes structural stability and motion stability.The factors affecting the structural stability are mainly cable tension and stiffness.Cable tension factor plays a main role,which refers to the minimum tension threshold keeping positive tensions to ensure CDPRs balance external forces.The other one is the stiffness factor,which ensures that the mechanism has sufficient capacity to resist elastic deformation.It is an important factor that cannot be ignored.And because the high speed and high maneuvering of the cable-driven parallel robots have a great influence on their motion performance,the motion stability of the cable-driven parallel robots has become a research focus.Motion stability requires cable-driven parallel robots to achieve a smooth start-stop and fast commutation and smooth high-speed operation;it is necessary to design a reasonable control strategy to achieve high-speed stable operation.In order to accurately reflect the actual problems of the cable-driven parallel robots,and verify the correctness of the dynamic model and the feasibility of the control method,a 5m prototype model is studied in this paper.The main work includes analyzing the dynamic of the cable-driven parallel robots,establishing the evaluation index of structural stability,proving the stability of the control strategy under high speed,and solving the cable pseudo-drag problem.The aim is to improve the performance of the cable-driven parallel robots and provide technical support for practical engineering applications.The main contents of this paper are as follows:Studying the structural stability based on the cable tension factor and the stiffness factor.Defining the cable tension factor and stiffness factor based on minimum cable tension and minimum stiffness singular value.Then putting forward the evaluation index to evaluate structural stability of cable-driven parallel robots,which lay the foundation for a more comprehensive evaluation and measure for the stability of the cable-driven parallel robots.A modified PD feedforward control strategy based on higher-order polynomial interpolation motion planning with disturbance observer is designed.According to the motion characteristics of the cable-driven parallel camera robots,a high-order polynomial interpolation programming is used to solve the problem of unsmoothness or discontinuity at start-stop;and a modified PD feedforward control strategy with disturbance observer is used to ensure the robot's high-speed and stable operation.The stability and consistency of the control strategy are proved by Lyapunov stability theory.An example is used to analyze the tracking characteristics after the start-stop transition planning.The motion stability control strategy considering the cable inertia is studied.The dynamic model of the cable is established by the finite element method.The dynamic model of the end-effector is established by the Newton Euler method.Combined with the dynamic model of the driving system,the dynamic model of the cable-driven parallel robots is obtained.And the modified PD feedforward control law based on the end space position is proposed.The stability and uniform continuity of the control law are proved by the Lyapunov stability theory.An example is given to illustrate the stable tracking performance of cable-driven parallel camera robots under high speed.A force/position hybrid control strategy based on cable force optimization is proposed to avoid the cable pseudo-drag problem.When the actual horizontal cable force is less than the theoretical horizontal cable force,there will be a cable pseudo-drag phenomenon.In order to solve the cable pseudo-drag problem,a synchronous active tension control based on the modified PD forward control strategy with cable tension optimization is presented.That is,the force/position hybrid controls,so as to achieve active control of cable tension,and to avoid the cable pseudo-drag problem under high speed.A 5m principle prototype model of cable-driven parallel camera robots is studied.The movement technical indicators of the model are obtained through experiments.And the maneuverability and stability of camera robot are analyzed.Through the trajectory tracking experiment,the validity of the dynamic model is verified.The comparison between numerical calculations and experimental data shows that the numerical calculation results are reliable.