In many industries such as food, medicine, logistics and 3C electronics, a large number of high-speed robots for high-speed grasping, picking and packaging of products are required. These high-speed robots have become the core equipment for reducing labor costs and improving production efficiency in the logistics field, and are in great demand. The existing high-speed sorting robots are all rigid structures, with large motion inertia, high energy consumption, complex branch chain structures, high manufacturing costs, and bottlenecks in efficiency. The cable-driven parallel robot (CDPR) uses cables instead of rigid links to drive the terminal motion. It has the remarkable characteristics of simple structure, light weight and low cost, and has shown great potential in the field of high-speed motion.

However, the cable can only bear one-way tension, and the restraint ability is insufficient. The current high-speed CDPR adopts redundant structure to ensure the cable tension, and the energy consumption is high. There is a lack of a non-redundant high-speed CDPR with a simple structure and its design method. To solve this problem, we innovatively designed a new type of CDPR for pick-and-place applications. The key issues and performance assurance technologies in its configuration design, performance evaluation and parameter optimization, kinematic calibration, and dynamic performance assurance of the new CDPR are systematically studied. Developments of prototypes and the breakthrough in accuracy and dynamic characteristics of the high-speed CDPR were achieved.

Configuration innovation for CDPRs with parallel-cable chains

A new type of CDPR configuration scheme was proposed. We innovatively adopted the parallel-cable drive scheme and passive spring tension structure to improve the restraint ability of the cable-driven mechanism. A serialized non-redundant translational CDPR named TBot was obtained.

In the structural design of the TBot, the modular design concept is adopted. The actuation module, pulley guide module, passive tension kinematic chain module, and the end effector of this robot are all modular structures with different design schemes. According to the task requirements, the efficient reconfiguration and assembly of the robot can be realized. Finally, a series of high-speed CDPRs with different degrees of freedom and different kinematics/dynamic properties are obtained.

Motion/force transmission performance and optimization

Dimension design and configuration design constitute the two major aspects of robot design. reveal the essential characteristics of robot kinematics and dynamics, establish a performance index system and an integrated optimization design method, and ensure the excellent comprehensive performance of the mechanism is a key problem. To solve this problem, we started from the essence of the mechanism—motion/force transmission, and established an index system based on the terminal force transmission performance.

Aiming at the evaluation of the force transmission performance of the CDPR, based on the static analysis, a force parallelepiped representing the driving ability of the CDPR and a moment parallelepiped representing its restraining ability were established. The orthogonality between the force vectors defines the actuation transmission performance index OLAI and the constraint transmission performance index OLCI. The global transmission performance indices were defined as the average value over the entire workspace. In terms of dynamic performance evaluation, the overall inertia matrix of the robot was obtained through dynamic modeling, and a dynamic performance evaluation method based on the dynamic inertia index was established. On this basis, the robot dynamic optimization and inertia matching criteria were established. Based on the evaluation indexes of kinematics and dynamics, a comprehensive optimization method of parallel robots was established, which guides the design of robot parameters and the selection of actuation and transmission systems, and realizes the comprehensive performance optimization and actuation matching of kinematics and dynamics.

Kinematic calibration of CDPRs considering pulleys

Research work on kinematic calibration and accuracy assurance technology of CDPR was carried out. Due to the use of the pulleys, the position of a CDPR's outlet point changes with time the terminal poses. The traditional modeling method with point-to-point straight line model has poor accuracy. In order to improve the accuracy of the CDOPR, it is necessary to consider the kinematics of the pulley.

Aiming at this problem, the complete kinematics model of the CDPR considering the pulley kinematics was established first. The error model of the CDPR considering the kinematics of the pulley was established. The residual function was constructed by the difference between the calculated value and the measured value of the cable length, the error identification model was established and the identification matrix expression was deduced. Using the numerical characteristics of the identification matrix, corresponding indicators were constructed to analyze the error sensitivity of the CDPR, so as to guide the selection of measuring poses. Finally, the kinematics calibration method and operation process of the CDPR were established. The calibration experiment was carried out on the TBot robot. According to the comparison, the calibration method considering the pulley proposed in this project can improve the accuracy by 31.2% compared with the traditional method.

Pick-and-place trajectory planning

The trajectory planning method in joint space for practical high-speed pick-and-place applications is established based on the k-th order non-uniform rational B-spline. Then, the optimization models of trajectory planning with goals of time-optimal, energy-optimal, and smoothness-optimal are established considering the specific constraints of the CDPRs. The trajectory planning are carried out using genetic algorithms with these goals. The influence of each constraint point of the planar five-point constrained pick-and-place path is analyzed, and the trajectory planning methods based on spatial constraints and three-point constraints are proposed, which fully releases the potentials of the high-speed CDPRs. In order to solve the problems of real-time trajectory planning with variable paths, a quick trajectory planning method based on the distance and acceleration abilities is established, which laid the foundation for the engineering application of the CDPRs.

Prototypes and applications

Finally, a serialized CDPR equipment was developed. According to the needs of different working conditions, three specifications of the CDPR equipment, TBot-1200, TBot-800 and TBot-600, were designed and developed. The workspace diameters of the three CDPRs are 1200mm, 800mm and 600mm respectively. The developed robots have been successfully applied in the pick-and-place of different kinds of objects and various product production lines. The figure shows the application demonstration of TBot robot in the sorting of biscuits, chocolate and yogurt. Equipped with a self-developed soft gripper, the TBot robot can also grasp and sort various special-shaped objects, adapt to the requirements of flexible grasping conditions, and greatly expand the application scope of the CDPR.

Related Publications:

Zhaokun Zhang, Guangqiang Xie, Zhufeng Shao*, Clément Gosselin. Kinematic Calibration of Cable-Driven Parallel Robots Considering the Pulley Kinematics. Mechanism and Machine Theory, 2022, 169, 104648. DOI: 10.1016/j.mechmachtheory.2021.104648.

Guangqiang Xie, Zhaokun Zhang, Zhfueng Shao*, Liping Wang. Research on the Orientation Error of the Translational Cable-Driven Parallel Robots. Journal of Mechanisms Robotics, 2022, 14(3): 031003. DOI: 10.1115/1.4052848.

Jinhao Duan, Zhufeng Shao*, Zhaokun Zhang, Fazhong Peng. Performance simulation and energetic analysis of TBot high-speed cable-driven parallel robot. Journal of Mechanisms and Robotics, 2022, 14 (2), 024504. DOI: 10.1115/1.4052322.

Fazhong Peng, Jinhao Duan, Zhufeng Shao*, Zhaokun Zhang, Daoming Wang. Comparisons of a TBot high-speed cable-driven parallel robot with a classical Delta parallel robot. Journal of Tsinghua University (Science and Technology), 2021, 61(3): 183-192. DOI: 10.16511/j.cnki.qhdxxb.2020.26.027.

Zhaokun Zhang, Zhufeng Shao*, Fazhong Peng, Haisheng Li, Liping Wang. Workspace Analysis and Optimal Design of a Translational Cable-Driven Parallel Robot with Passive Springs. Journal of Mechanisms and Robotics, 2020, 12(5): 051005. DOI: 10.1115/1.4046030.

Zhaokun Zhang, Zhufeng Shao*, Liping Wang. Optimization and implementation of a high-speed 3-DOFs translational cable-driven parallel robot. Mechanism and Machine Theory, 2020, 145:103693. DOI: 10.1016/j.mechmachtheory.2019.103693.