Force-closure workspace analysis of cable-driven parallel mechanisms

Reconfigurable fully constrained cable driven parallel mechanism for avoiding interference between cables

This paper presents a method for the dimensional synthesis of fully constrained spatial cable-driven parallel mechanisms (CDPMs), namely, the problem of finding a geometry whose wrench-closure workspace (WCW) contains a prescribed workspace. The proposed method is an extension to spatial CDPMs of a synthesis method previously published by the authors for planar CDPMs. The WCW of CDPMs is the set of poses for which any wrench can be produced at the end-effector by non-negative cable tensions. A sufficient condition is introduced in order to verify whether a given six-dimensional box, i.e., a box covering point-positions and orientations, is fully inside the WCW of a given spatial CDPM. Then, a nonlinear program is formulated, whose optima represent CDPMs that can reach any point in a set of boxes prescribed by the designer. The objective value of this nonlinear program indicates how well the WCW of the resulting CDPM covers the prescribed box, a null value indicating that none of the WCW is covered and a value greater or equal to one indicating that the full prescribed workspace is covered.

A new remote manipulator based on cable-driven parallel mechanism (CDPM) is designed for space long-distance operations (e.g. space capture/docking and other long-distance space activities) in this paper. By controlling the cables and thrusters which are equipped on the manipulator simultaneously, the new remote manipulator can achieve expected position, linear velocity, and angular velocity. The new manipulator has a larger controllable workspace compared with usual CDPMs. The structure and characteristics of this manipulator are discussed in this paper. The volume and characteristics of the workspace are also discussed. The influence of the distance on the static equilibrium is studied. The simulation results show that the workspace of this new manipulator is larger than usual CDPM's. The results also indicate that the cable forces and thruster vectors can completely constrain the manipulator and meet the requirements of space activities. The results of the simulation also show that the controllable workspace of the manipulator is not continuous at some regions. Hence, trajectory planning is necessary.

Workspace analysis and optimal design of dual cable-suspended robots for construction

Cable-driven parallel mechanism has been widely studied due to its advantages of fast response and large workspace. The structural uncertainties in the system often introduce internal disturbance and unavoidably influence its motion precision. Generally, to reduce the influence, robust control is adopted and the boundary of the disturbance is regarded as a fixed value given by rough estimation. This method often causes large vibration of the motion component, which certainly compromises the dynamic performance of the system. Therefore, to solve this problem, a variable boundary analytical scheme based on the decoupling of control law and internal disturbance is proposed in this article. Specifically, sufficient condition for the solvability of disturbance boundary is proved by the boundedness of the structural matrix error. Then, the disturbance boundary is modeled analytically, so that its particular value can be determined with respect to the system status. Following that, a robust adaptive control algorithm based on the variable boundary of disturbance is developed. It is verified that the tracking error is globally uniformly bounded. The experimental results show that the proposed control method can effectively reduce the tracking errors and attenuate the tension chattering compared with the conventional robust adaptive control scheme with a roughly estimated boundary.

The cable-driven parallel mechanism (CDPM) is known as an interesting application in industry to pick and place objects owing to its advantages such as large workspaces. In addition to the advantages of this mechanism, there are some challenges to improving performance by considering constraints in different components, such as the behavior of cables, shape, size of the end effector and base, and model of pulleys and actuators. Moreover, the impact of online geometry reconfiguration must be analyzed. This paper demonstrates the impact of these constraints on the performance of reconfigurable CDPM. The methodology is based on the systematic review and meta-analysis guidelines to report the results. The databases used to find the papers are extracted from Scopus and Google Scholar, using related keywords. As a result, the impact of physical constraints on system performance is discussed. A total of 90 and 37 articles are selected, respectively. After removing duplicates and unrelated papers, 88 studies that met the inclusion criteria are selected for review. Even when considering the physical constraints in modeling the mechanism, simplifications in designing a model for the reconfigurable CDPM generate errors. There is a gap in designing high-performance controllers to track desired trajectories while reconfiguring the geometry, and the satisfaction of physical constraints needs to be satisfied. In conclusion, this review presents several constraints in designing a controller to track desired trajectories and improve performance in future work. This paper presents an integrated controller architecture that includes physical constraints and predictive control.

We are developing impedance controlled twisted string actuators (TSA) for use in tensegrity robots, as an alternative to traditional spooled cable actuation. Tensegrity robots are composed of continuous tension and discontinuous compression elements, with no rigid joints between elements, which give them unique force distribution properties. The use of tensegrity robots is strongly motivated by biological examples, and they are capable of locomotion and manipulation by changing lengths of their continuous network of tensional elements, which is also the primary pathways for load transfer through the structure. TSA show the potential to address some of the unique engineering challenges faced by tensegrity structures, and provide unique qualities well suited to an actively controlled tension system, such as compact, light-weight mechanical structures, inherent compliance, variable “gearing”, and the ability to transmit high forces with a very low input torque. The inherent variable compliance of impedance control is essential for tensegrity robots to move through and manipulate the environment, and is a natural match to the unique qualities of TSA. This paper briefly introduces the tensegrity robots in the NASA Ames Intelligent Robotics Group and an overview of their future application to space planetary exploration. Then the effectiveness and robustness of TSA are verified through the performance of impedance control modes.

In recent researches relating to robotic fingers, a novel power transmission mechanism had been designed, and developed as Twist-drive Actuators or Twisted String Actuator systems (TSA). These actuation structures, state of the art, include compact designs, light-weight mechanical structures, inherent compliance, and variable gearing, resulting in the fabrication of anthropomorphic robotic hands. However, those studies mentioned position control of the joint, and focused on the control performance of the pulling forces of a twisted string. This paper, therefore, introduces a novel joint mechanism composed of an Antagonistically-twisted Round-belt Actuator (ARA), which is able to make rotating motion by means of contraction forces induced by twisting small-diameter elastic round-belts. A noteworthy point in this paper is that the ARA robot is extremely well-suited for contact force control, which is exerted on the tip of the single joint robot. First, we demonstrate the existence of hysteresis characteristics of static contraction forces induced by the round-belt twisting, in which three belts are simultaneously twisted at constant low-speed. In addition, there exists a stress relaxation phenomenon when strongly twisting the elastic round-belts. We reveal that such sorts of discontinuous and nonlinear properties do not influence control performance in either the contraction force on the belts or contact force on the tip of the robot. Finally, this paper clearly shows stable and accurate tracking performance of the contact force of the ARA robot. In these experiments, we newly develop a twin/triple-twisted round-belt structure for the agonist side actuator, thus enhancing the contact forces effectively.

Precision engineering industries demand devices with high-precision motion, the manipulation of micro-objects in particular, which is a highly challenging task. The microgripper is an essential device during the micromanipulation of objects. Precision manipulations of objects entirely depend on the design of the microgripper and are complicated to achieve using the conventional rigid link mechanism. The compliant mechanism is found to overcome the complexities of the rigid link mechanism in precision applications. The micro-object may slip or slide during gripping and releasing, which can be controlled through parallel movement of the gripping jaws. This research article focuses on the design of a compliant microgripper with parallel moving jaws by employing the Hoekens straight line mechanism. The Hoekens mechanism consists of binary and ternary revolute joints, which demand a special type of flexure hinges. Hence, the article introduces the cartwheel type of flexure hinges, as cartwheel-type flexure hinges are more suitable to achieve rotational deformation, values approximating nearly to that of the traditional revolute joint when it transfers motion between fixed and moving links. The outer rim of the cartwheel is modified to allow ternary and binary joints between moving links. Cartwheel is designed with curved flexure arms that are limited to eight numbers of flexures. The structural behavior of the cartwheel is analyzed by varying numbered flexures from 1 to 8. The minimum number of flexure arms required for having better rotational performance is determined through Finite Element Analysis (FEA). An appropriate type of cartwheel is positioned in the mechanism. The structural performance of the designed microgripper is carried out through FEA, and its parallel movement is verified. The microgripper is fabricated from structural steel through wire Electrical Discharge Machining (EDM) technique and actuated using Shape Memory Alloy wire. The displacement of the microgripper jaw is experimentally measured, and the results show a promising improvement.

In the last decade, cable-suspended parallel robots have attracted significant interest due to their large workspaces and high dynamic performances. However, a significant drawback is that cables must always be in tension to control the motion. Using launch motions to reach a target can enlarge the workspace of such robots. For a spatial translational cable robot suspended by six pairwise-parallel cables, an analytical method for planning point-to-point dynamic trajectories is proposed. Using a second-order Bézier curve trajectory, the mechanism starts from a static condition, passes through intermediate points, and finally launches an object towards a target. According to the kinematic constraint conditions on the position, the velocity and acceleration of the end-effector at a prescribed point, the parametric expressions for a dynamically-feasible trajectory can be determined. The feasibility of the trajectory is analyzed under the constraint that cable tensions must be positive at all times. By changing the position of the end point of the trajectory and the total motion time, the kinematic conditions on the position and velocity as well as the feasibility constraint can be satisfied. Finally, our point-to-point dynamic launch trajectories are verified by simulations and experiments.

[Abstract] This paper first presents a concept of using a 6-DOF cable robot for hardware-in-the-loop (HIL) dynamic simulation of microgravity contact-dynamics behavior of a space system such as a spacecraft or robot. It then addresses three basic problems of the cable robot in order to verify that the cable robot can meet the most critical requirements of such an HIL contact-dynamics simulator. The first problem is whether the dynamic motion of the end-effector being fully controllable under the constraint that all the cables must be in tension during a contact operation, i.e., under what condition the inverse dynamics problem has an all-positive force solution. A systematic method of checking force-closure condition is proposed. The second problem is to make sure that the cable robot can have sufficiently large workspace to simulate the motion of the space system. The force-closure workspace is determined using the systematic method of checking force-closure. The third problem is whether the potential vibration arising from the cable flexibility is acceptable for the intended simulation application. The study results of all the three problems support the feasibility of using such a cable robot for HIL simulation of contact-dynamics.

Due to the advantages of high stiffness, high precision, high load capacity and large workspace, hybrid robots are applicable to drilling and milling of complicated components with large sizes, for instance car panels. However, the difficulty in establishing an exact dynamic model and external disturbances affect the high accuracy control directly, which will decrease the machining accuracy and thereby affect the machining quality and efficiency of the system. Sliding mode control is an effective approach for high-order nonlinear dynamic systems since that it is very insensitive to disturbances and parameter variations. However, chattering may exist in traditional sliding mode control with fixed parameters, which results from a constant approaching speed. Besides, the approaching speed will affect the chattering strength directly. To solve these problems, a modified sliding mode controller with self-adaptive parameters is proposed to enhance the trajectory-tracking performance of a 5-degree-of-freedom hybrid robot. Firstly, the kinematic model of the robot is established. Then adopting the principle of virtual work, a rigid dynamic model of the robot is built. Based on the built dynamic model, a modified sliding mode control method is developed, of which the approaching speed is dependent on the system state. Finally, the sliding mode controller with self-adaptive parameters is created for a hybrid robot. The proposed sliding mode controller can achieve a rapid approaching speed and suppress chattering simultaneously. Simulation results demonstrate that the proposed modified sliding mode controller can achieve a comparatively accurate and smooth trajectory, which owns good robustness to external disturbances.

In this work, we focus on the design and analysis of a new modular hybrid parallel-serial manipulator for robotised deburring of large jet engine components such as the fan, compressor, and turbine discs. The manipulator consists of a 3-DOF (degree-of-freedom) planar parallel platform and a 3-DOF serial robotic arm. Benefiting from the hybrid kinematic structure, the manipulator exhibits good performance inherited from both serial and parallel robots, e.g. larger workspace and higher dexterity (comparing to a parallel robot), and higher rigidity and higher loading capacity (comparing to a serial robot). Such features are ideal for deburring applications of large jet-engine components. In order to rapidly deploy a demonstration system, the modularity design concept is employed in the system development, which is able to reduce the complexity of the overall design problem to a manageable level. Based on the specific hybrid manipulator design, closed-form symbolic solutions are derived for both forward and inverse displacement analysis. Computation examples are provided to verify the proposed displacement analysis algorithms. To obtain the relationship between the end-effector’s velocity and the active-joint rates, the instantaneous kinematics model of this hybrid manipulator is formulated. These analysis algorithms are essential for the design optimisation, trajectory planning, computer simulation, and real-time control of this hybrid manipulator. Utilising the 6-DOF hybrid parallel-serial manipulator, a robotised deburring system is investigated for large jet-engine components.

The force/moment capability of cable driven parallel mechanisms is characterized by force/moment polytopes resulting from a linear transformation between the space of cable tension and the space of the force/moment of the moving platform. Conventional method developed in computational geometry has been used to identify the vertices and faces of the polytopes. In this paper, we explored the relationship between the polytopes and the structure of a cable driven parallel mechanism and proposed a new method to identify the vertices and the faces of the polytopes. We analyzed the characteristics of the hyper cuboid defined by the minimum and maximum cable tensions and took into account inherent relationships between all vertices and faces of the polytopes. By utilizing the inherent relationships, we were able to show the relationships between the force/moment polytopes and the cable directions. These relationships are potentially useful for the design of cable driven parallel mechanisms. As an example, the polytopes of a 9-cable driven 6-DOF parallel mechanism are presented.

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