Jinwoo Jung

Open-loop position control of a polymer cable–driven parallel robot via a viscoelastic cable model for high payload workspaces

A polymer cable–driven parallel robot has a wide range of potential industrial applications by virtue of its light actuator dynamics, high payload capability, and large workspace. However, due to a viscoelastic behavior of polymer cable and difficulty in actual cable length measurement, there have been inevitable position and tracking control accuracy problems such as pick and place a high payload application. In this article, to overcome control problem, we propose a model-based open-loop control with the cable elongation compensation via experimentally driven cable model and switching control logic without additional Cartesian space feedback signal. The approach suggests a five-element cable model that is made with series combination of a linear spring and two Voigt models as a function of payload and cable length that are available to be measured in real-time. Experimental results show that using the suggested method, the cable length error due to viscoelastic effect can be compensated, and thus the position control accuracy of the polymer cable–driven parallel robot improved remarkably especially in gravity direction.

A Polymer Cable Creep Modeling for a Cable-Driven Parallel Robot in a Heavy Payload Application

A polymer cable driven parallel robot can be an effective system in many fields due to its fast dynamics, high payload capability and large workspace. However, creep behavior of polymer cables may yield a posture control problem, especially in high payload pick and place application. The aim of this paper is to predict creep behavior of polymer cables by using different mathematical models for loading and unloading motion. In this paper, we propose a five-element model of the polymer cable that is made with series combination of a linear spring and two Voigt models, to portray experimental creep in simulation. Ultimately, the cable creep can be represented by payloads and cable length estimated according to the changes of actual payloads and cable lengths in static condition.

Analysis of cable tension of high speed parallel cable robot: High speed position tracking of winch

Cable-driven parallel robots have a wide range of applications due to its unique characteristics of large workspace and high payload and dynamics. One of its promising industrial applications is high speed material handling allowed by high dynamics of the cable robots. The high speed material handling has been mostly performed by traditional parallel robots, such as delta robots whose workspace and speed are limited by the length and weight of the rigid links. Utilizing the characteristics of large workspace and high dynamics, a high speed cable robot is able to achieve high speeds over large workspace. However, high acceleration together with inertia for high speed manipulation can degenerate winch position tracking performance, which, in turn, causes tension distribution change. To achieve good performance at high speeds, it is desired to analyze and minimize the effects of the winch position tracking errors on tension distribution. In this paper, the effects of the position tracking error on tension have been analyzed. It is observed that approximately one half of tension error can be caused by the position tracking error during high speed manipulation.

Simulation of effect of cable robot configuration on natural frequency

Cable-driven parallel robots consist of non-rigid cables which are different from traditional serial robots. The lightweight of the cables allows them to have higher payload and larger workspace. However, the cables may cause significant vibration during operation because the cables are inevitably flexible. Increasing natural frequencies is one of the solutions to reduce the vibration of the cable robots. We propose that relationship between the shape of end-effector and the shape of frame can affect natural frequencies of planar cable driven parallel robots. To investigate the relationship, equations of motion for a planar cable robot is developed using Lagrange approach. Equations of motion are linearized for vibration analysis. Through mode analysis with different shapes of end-effector, it is verified that different combinations of end-effector shape and frame shape can cause different natural frequencies. Thus, it is possible to design a planar cable robot by utilizing the relationship between the shapes.

Geometric Parameter Calibration for a Cable-Driven Parallel Robot Based on a Single One-Dimensional Laser Distance Sensor Measurement and Experimental Modeling

A cable-driven parallel robot has benefits of wide workspace, high payload, and high dynamic response owing to its light cable actuator utilization. For wide workspace applications, in particular, the body frame becomes large to cover the wide workspace that causes robot kinematic errors resulting from geometric uncertainty. However, appropriate sensors as well as inexpensive and easy calibration methods to measure the actual robot kinematic parameters are not currently available. Hence, we present a calibration sensor device and an auto-calibration methodology for the over-constrained cable-driven parallel robots using one-dimension laser distance sensors attached to the robot end-effector, to overcome the robot geometric uncertainty and to implement precise robot control. A novel calibration workflow with five phases—preparation, modeling, measuring, identification, and adjustment—is proposed. The proposed calibration algorithms cover the cable-driven parallel robot kinematics, as well as uncertainty modeling such as cable elongation and pulley kinematics. We performed extensive simulations and experiments to verify the performance of the suggested method using the MINI cable robot. The experimental results show that the kinematic parameters can be identified correctly with 0.92 mm accuracy, and the robot position control accuracy is increased by 58%. Finally, we verified that the developed calibration sensor devices and the calibration methodology are applicable to the massive-size cable-driven parallel robot system.

Analysis of configuration of planar cable-driven parallel robot on natural frequency

This paper studies the configuration of a planar cable-driven parallel robot to reduce the vibration of an end-effector. Since cables are more flexible than rigid links, external disturbances may cause a significant vibration during operation. The characteristics of the vibration are closely related to the natural frequency of the end-effector. And, we consider that the shapes of the frame and the end-effector, and the connection methods of the cables can affect the vibration of the cable-driven parallel robot. Therefore, the first natural frequencies in a required workspace are analyzed by the configuration change of a planar cable-driven parallel robot. The simulation shows that planar cable robots can be designed to reduce vibration by utilizing robots configurations.

Geometric parameter calibration using a low cost laser distance sensor for a planar cable robot: MATLAB simulation

This paper proposes a calibration algorithm for a three degrees-of-freedom (DOF) planar cable-driven parallel robot. The developed calibration algorithm uses a low-cost laser distance sensor to measure the distance between an end-effector and a laser reflector, and also uses the cable lengths measured by using motor encoders. MATLAB simulation results show that using a single laser distance sensor we can calibrate the planar cable robot accurately, and also that selection of the desired end-effector poses influences the calibration accuracy.