1 Introduction
As a branch of the robot, the parallel robot has the advantages of high rigidity, strong load capacity, high precision, compact structure and easy control. It is widely used in some applications where the working space is limited or the working space is not high. The theoretical discussion and practical development of parallel robots have achieved certain results.
In order to improve the control performance of parallel robot trajectory tracking, various control algorithms are proposed. Nguyen et al. proposed an adaptive PID control algorithm to achieve high-precision trajectory tracking of 6-DOF parallel robots, but the algorithm is too complicated. Kim et al. used the Lya2punov method to design an adaptive controller to achieve high-precision trajectory tracking of a 6-DOF parallel robot, and achieved ideal results.
However, this method is not only complicated in algorithm, but also requires joint-based velocity measurement.
Based on the measurement of robot joint position, this paper uses nonlinear PID control algorithm to realize trajectory tracking control of 6-DOF parallel robot. The nonlinear PID controller is designed and experimentally studied to prove that the trajectory tracking control has high precision.
2. Motion control model
The mechanical structure of the parallel robot consists of a stationary platform, a movable lower platform and six ball screw actuators. The ball screw actuators are connected to the upper and lower platforms by Hook hinges and ball joints, respectively. The ball screw is driven by a servo motor to achieve the desired trajectory of the moving platform by changing its length. The system related structural parameters are as follows:
Among them: Ra and Rb are the radius of the upper and lower circular platforms, A is the angle between the hinge points of the same group, and B is the angle between the two sets of hinge points.
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