Prismatic Actuators Research Articles

An Actuation Acceleration-Based Kinematic Modeling and Parameter Identification Approach for a Six-Degrees-of-Freedom 6-PSU Parallel Robot With Joint Clearances

Abstract The 6-prismatic-spherical-universal (6-PSU) parallel robot is useful for high-accuracy positioning, where the motors are installed on the robot base and the moving parts of the robot have low inertia. A highly accurate kinematic model of the robot is fundamental for the control. The joint clearances of all limbs often exist and have a significant influence on kinematic model accuracy. In this study, an actuation acceleration information-based kinematic modeling and identification method for a 6-PSU parallel robot with joint clearances is proposed. The direction of the joint clearance is related to the direction of the force acted on the joint, which is determined by the acceleration of the prismatic actuator. The existence of joint clearances is equivalent to the link length change. The joint clearances are identified from the experiments and compensated. Simulations and experiments show that the proposed method is effective and improves the accuracy of the kinematic model.

Kinematically redundant (6＋2)-dof HEXA robot for singularity avoidance and workspace augmentation

This paper introduces a novel (6＋2)-degree-of-freedom (dof) kinematically redundant parallel robot. The proposed architecture is based on the 6-dof HEXA parallel robot. The paper applies methodologies for avoiding type II singularities using kinematic redundancy that were originally proposed for robots with prismatic actuators to a new architecture with revolute actuators. The singularities of the HEXA robot are first examined based on previous work, and new instances of singularities are discovered. The singularity locus in the orientational workspace is identified using the Power Inspired Measure. Then, a novel kinematically redundant leg architecture is proposed to allow for the avoidance of these type II singularities. It is shown that the singularities present within the orientational workspace can successfully be avoided if two kinematically redundant legs are included in the robot architecture. The performance of the new architecture is then examined in various trajectories within the orientational workspace. Results show that the attainable orientational workspace is significantly increased with the proposed architecture.

Conjoined twins 3RPRS: a novel parallel spatial robot

In robotics, finding new structures of parallel robots to achieve higher kinematic and dynamic performance is a great matter. This article describes the introduction, design, and modeling of a novel parallel spatial robot. The name “conjoined twins 3RPRS” (CT-3RPRS) is chosen for the introduced robot as its structure is based on an innovative combination of two 3RPRS robots. Initially, the actuators of the 3RPRS robot were shifted from the base plane to the middle plane of the robot, and then three new links were added to balance the robot. Therefore, CT-3RPRS has six links, three on each side of the middle plane (actuator plane) that are connected to the prismatic actuators with revolute joints. The complete kinematic equations of the robot are extracted. Afterward, the singularity analysis is performed. The results of the simulations showed that CT-3RPRS provides a better reachable workspace feature than 3RPRS. The mentioned advantage is achieved while the size of the two robots is the same and the movement range of CT-3RPRS actuators is less.

On the Actuation Modes of a Multiloop Mechanism for Space Applications

A symmetric, double-tripod multiloop mechanism (DTMLM), intended for grabbing objects in outerspace, is the subject of this article. Actuation modes are analyzed while introducing a novel tool applicable to space mechanisms. The key issue here lies in establishing the criteria for selecting the optimum mode from multiple actuation possibilities. The evaluation procedure includes generalized-force values, power requirement, and actuation-strategy models, along with their optimization. Accordingly, the optimization procedure targets the mode(s) with 1) uniformity of generalized-force distribution, 2) uniformity of power-requirement distribution, and 3) the fewest working actuators in a given maneuver. In this way, a rather complex problem is formulated in a simple manner, whereby the optimum actuation mode is found by list-lookup, from a reduced number of candidates. The DTMLM, which carries three compound hinges plus three prismatic, and six revolute joints, has three degrees of freedom. To analyze the actuation modes of the mechanism, the dynamics model of the DTMLM is established; as well, five representative modes are selected from 84 possibilities. It turns out that the 3 <monospace xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">R</monospace> -type (three revolute actuators) shows a uniform power-requirement distribution among the three actuators in the bending motion mode; in the 3 <monospace xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">P</monospace> -type (three prismatic actuators), one single actuator is operational, and hence, takes all the load. Thus, the 3 <monospace xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">R</monospace> -type is the best from the power-distribution viewpoint, thereby providing strong power support, but complex from the actuation-strategy viewpoint, as illustrated in the article. Simulation results and prototype experiments of the DTMLM are reported, thereby verifying the analysis results.

An efficient combined local and global search strategy for optimization of parallel kinematic mechanisms with joint limits and collision constraints

The optimization of parallel kinematic manipulators (PKM) involve several constraints that are difficult to formalize, thus making optimal synthesis problem highly challenging. The presence of passive joint limits as well as the singularities and self-collisions lead to a complicated relation between the input and output parameters. In this article, a novel optimization methodology is proposed by combining a local search, Nelder–Mead algorithm, with global search methodologies such as low discrepancy distribution for faster and more efficient exploration of the optimization space. The effect of the dimension of the optimization problem and the different constraints are discussed to highlight the complexities of closed-loop kinematic chain optimization. The work also presents the approaches used to consider constraints for passive joint boundaries as well as singularities to avoid internal collisions in such mechanisms. The proposed algorithm can also optimize the length of the prismatic actuators and the constraints can be added in modular fashion, allowing to understand the impact of given criteria on the final result. The application of the presented approach is used to optimize two PKMs of different degrees of freedom.

Kinematic Analysis and Optimal Design of a Novel Schönflies-Motion Parallel Manipulator With Rotational Pitch Motion for Assembly Operations

Abstract This paper presents a novel Schönflies-motion Parallel Manipulator with RotationalPitch motion (SPM-RP) based on a single-platform fully parallel mechanism. An analysis of the position, workspace, velocity, and singularity of the SPM-RP is carried out in detail, and a dimensionless Jacobian is proposed to evaluate the manipulability of the SPM-RP. The analysis shows that the SPM-RP is with position-decoupled kinematics, a large singularity-free workspace, and excellent manipulability. The SPM-RP is actuated by four parallel prismatic actuators, enabling the manipulator to provide the identical kinematic performance at all generic cross sections perpendicular to the prismatic joint axes within its workspace. This paper thus proposes a reduced design optimization formulation, where the traditional optimization over the entire workspace is reduced to the optimization on a representative workspace cross section of the SPM-RP. According to this approach, the design optimization of the SPM-RP is carried out by maximizing its manipulability over the total orientation workspace, which is crucial for precision assembly. Based on the achieved optimal design, an SPM-RP prototype is developed. The mobility, orientation capability, total orientation workspace, and repeatability of the optimal design are tested and verified by the developed SPM-RP prototype. Experiments show that the SPM-RP can achieve a large total orientation workspace with excellent precision performance.

The Stewart Hand: A Highly Dexterous, Six-Degrees-of-Freedom Manipulator Based on the Stewart-Gough Platform

In this work, the design, modeling, dimensional synthesis, and experimental characterization of a dexterous robotic hand based on the Stewart-Gough parallel manipulator are presented. The mechanism consists of three parallel linkage fingers with six prismatic actuators that control manipulation. An additional actuator maintains stable grasping forces. A computational model to predict the hand's workspace is then utilized to determine the optimal design parameters that maximize the workspace size and manipulability. A physical prototype based on the optimized parameters is built and experimentally characterized to assess its performance (Figure 1).

A novel three-legged 6-DOF parallel robot with simple kinematics

This paper presents a novel three-legged six degrees of freedom (6-DOF) parallel robot with simple kinematics. The main idea behind this novel architecture is that each of the three identical legs is controlled by two prismatic actuators with parallel directions. As a result, it is possible to control simultaneously or separately the position and the orientation of a leg. The reduced number of legs leads to a simple mechanical design with reduced risk for mechanical interferences.

Modeling, identification and minimum length integral sliding mode control of a 3-DOF cartesian parallel robot by considering virtual flexible links

This article presents a method for modeling the dynamics and minimum vibration controller design of a 3-DOF parallel mechanism, the so-called Tripteron. The proposed comprehensive dynamic model consists of the dynamic model of the prismatic actuators, variable friction in them, and dynamics of flexibility in the limbs which caused vibration of the robot’s end-effector by considering a spring-damper element for each link based on the Tripteron structure. Then the identification procedure is carried out by collecting practical data from the robot. In order to design the oscillation damping controller, the Minimum Length Integral Sliding Mode Controller(MLISMC) is proposed in which the Phase Trajectory Length(PTL) approach is applied to ISMC for proper placement of closed-loop poles of the identified system. The foregoing method is based on numerical calculations to minimize the end-effector vibrations along the point-to-point control. Simulation and experimental results reveal that the PTL of the system have been reduced about 95% in simulation and 30% in the experimental test.

An Application of a 3-RRRS 6 DOF Parallel Manipulator

A new drive simulator is designed and fabricated through three legged, 6-DOF RRRS (Revolute- Revolute- Revolute- Spherical) parallel manipulator having three 1-DOF joints which are of revolute type and the middle joint being passive. The three legs are placed equilaterally on the base. Base joint rotates about a vertical axis whereas the other two joints rotate parallel to horizontal plane. The main objective of this drive simulator is to reduce the overall cost of a drive simulator which is done by replacing the conventional drive simulators having linear actuators with ones having rotatory actuators. Conventional drive simulators are 6 legged and each actuator is a linear actuator. The proposed design of drive simulator is having 3 legs which leads to increase in workspace and since prismatic actuators are replaced by rotary actuators the overall cost is also reduced. For the proposed drive simulator, inverse kinematics model is generated in a closed form way and workspace of the fabricated drive simulator is verified and workspace volume is calculated for all possible orientations and plotted. Kinematic control is done through Arduino based on inverse kinematics model.

Metamorphic tensegrity structure for pipe inspection

This paper proposes a new concept of metamorphic tensegrity structure for pipe inspection. A new structural deformation technique is developed that can be employed to design a non-complex and lightweight robot or vehicle. This concept is to deform the cross-shaped tensegrity structure by using the cooperation of prismatic actuators (PAs) and passive joints with locking mechanisms called restrictor mechanisms (RMs). Two control levels are designed to control the structural deformation: A high-level controller for motion patterns and low-level controllers to locally control the PA lengths and restrict the cable lengths of four RMs. Experiments show that the proposed deformation technique can deform the shape and size of the tensegrity structures with the aim of creating a new generation of pipe inspection robots.

A tri-state prismatic modular robotic system

Strut-type modular robots adopt an idealized truss mechanism composed of linear extensible struts (i.e., robotic modules) jointed by connector nodes. Conceptually, each strut is capable of passively rotating around its connected node, and all the struts linked by a same node share and intersect at a common center of rotation. Unfortunately, these two requirements pose difficulties in mechanically implementing such an ideal point-like compliant connector node. Meanwhile, motions of extensible struts composed of off-the-shelf prismatic actuators can be constrained in a parallel or networked robotic structure, leading to the stuck of the robotic structure and the failure of a given task. To tentatively tackle these issues, a tri-state (three-state) modular prismatic robotic system is designed with the ability to be rigid, back-drivable to external forces and to undertake self-powered motion. Two modular robotic concepts of (i) rigid node attachment and (ii) neighbor actuation are introduced and demonstrated. Specifically speaking, modular robotic structures are constituted of prismatic actuators linked by rigid connector nodes. Instead of placing revolute joints on the nodes, a passive revolute joint is in the middle of two prismatic actuators to deliver compliance. This arrangement removes many of the inherent limitations of complex connection joints and avoids the difficulty of designing and implementing point-like complaint connector nodes. In terms of neighbor actuation, modules are demonstrated that they have the ability to be self-powered or passively controlled from external forces applied by their neighboring modules. This allows robotic modules in a parallel or networked structure to change states to release physical constraints and prevent the whole structure from getting stuck. In this paper, the tri-state capability of the prismatic modules and the neighbor actuation of parallel modules are first experimentally demonstrated. Then, two physical modular robots are built and control algorithms are implemented. Experimental results validate the efficacy of the control strategy and substantiate communication and locomotion capabilities of the modular robots. Finally, a more complex robotic structure is established in a robot simulator to demonstrate that the tri-state prismatic actuators can be applied to release physical constraints of the modular robot during a locomotion task.

Unified Kinematics of Prismatically Actuated Parallel Delta Robots

SummaryThis paper presents a unified formulation for the kinematics, singularity and workspace analyses of parallel delta robots with prismatic actuation. Unlike the existing studies, the derivations presented in this paper are made by assuming variable angles and variable link lengths. Thus, the presented scheme can be used for all of the possible linear delta robot configurations including the ones with asymmetric kinematic chains. Referring to a geometry-based derivation, the paper first formulates the position and the velocity kinematics of linear delta robots with non-iterative exact solutions. Then, all of the singular configurations are identified assuming a parametric content for the Jacobian matrix derived in the velocity kinematics section. Furthermore, a benchmark study is carried out to determine the linear delta robot configuration with the maximum cubic workspace among symmetric and semi-symmetric kinematic chains. In order to show the validity of the proposed approach, two sets of experiments are made, respectively, on the horizontal and the Keops type of linear delta robots. The experiment results for the confirmation of the presented kinematic analysis and the simulation results for the determination of the maximum cubic workspace illustrate the efficacy and the flexible applicability of the proposed framework.

Redundancy resolution and control of a novel spatial parallel mechanism with kinematic redundancy

In this paper, a spatial kinematically redundant parallel mechanism consisting of three base prismatic actuators and six lateral prismatic actuators which control the six task space degrees of freedom is proposed. The main objective is to introduce a real-time redundancy resolution to actuate kinematically redundant parallel mechanisms. In the proposed method, the redundant coordinates are determined in such a way to improve the dynamic performance of the mechanism for tracking a given trajectory in order to avoid singular configurations. An inverse dynamic control strategy with a non-linear disturbance observer is designed. Kinematics and dynamics of the proposed mechanism are investigated and the control algorithm is implemented for motion control of the introduced mechanism. The effectiveness of the proposed controller is demonstrated through numerical simulations.

Dynamic Analysis of High-Speed Boat Motion Simulator by a Novel 3-DoF Parallel Mechanism with Prismatic Actuators Based on Seakeeping Trial

In this study, we focused on a novel parallel mechanism for utilizing the motion simulator of a high-speed boat (HSB). First, we expressed the real behavior of the HSB based on a seakeeping trial. For this purpose, we recorded the motion parameters of the HSB by gyroscope and accelerometer sensors, while using a special data acquisition technique. Additionally, a Chebychev high-pass filter was applied as a noise filter to the accelerometer sensor. Then, a novel 3 degrees of freedom (DoF) parallel mechanism (1T2R) with prismatic actuators is proposed and analyses were performed on its inverse kinematics, velocity, and acceleration. Finally, the inverse dynamic analysis is presented by the principle of virtual work, and the validation of the analytical equations was compared by the ADAMS simulation software package. Additionally, according to the recorded experimental data of the HSB, the feasibility of the proposed novel parallel mechanism motion simulator of the HSB, as well as the necessity of using of the washout filters, was explored.

Simplification of the dynamic model of a hydraulic rockbreaker for implementation in a model-based control scheme

A model-based approach to control the automation of hydraulic excavators and rockbreakers necessitates an adequate dynamic model to increase the robustness of the controller and improve its performance (e.g., reduce tracking error). Most previous efforts in developing dynamic models for excavators have assumed planar motion while neglecting the dynamic effects of hydraulically driven prismatic actuators. In this paper, a dynamic model of the mechanical subsystem of a hydraulic rockbreaker is developed using the Euler–Lagrange formulation. The model considers the contributions of the hydraulic actuators and does not assume planar motion. Potential simplifications to the dynamic model are then introduced to facilitate the model’s parameterization for developing an adaptive control algorithm. To evaluate their level of accuracy, these simplified dynamic models are then evaluated based on the required joint torques for specified trajectories. It is shown that the proposed simplifications reduce the complexity of the dynamic model while preserving its accuracy, which is attractive for real-time control applications.

An experimental dynamic identification & control of an overconstrained 3-DOF parallel mechanism in presence of variable friction and feedback delay

The closed-loop dynamic control of parallel mechanisms is a challenging field due to the high complexity of their dynamic behavior, especially those with prismatic actuators. Prismatic joints usually provide considerable amount of friction which could vary along its axis. This paper addresses the application of different control algorithms in order to tackle the challenges in closed-loop control of an overconstrained 3-DOF decoupled parallel mechanism which compromises three prismatic actuators with variable frictions along each axis. In addition, a Kinect vision sensor, as the position feedback, is installed. Since the Kinect RGB sensor performs at 30 frame per second, there is a 33 ms delay in the feedback of the control unit which restricts the control loop frequency to maximum value of 30 Hz. Then, based on the models obtained from the identifications of step response, kinetic friction and inverse dynamic model, various attempts have been made in order to obtain a controller with a reasonably performance. First, the conventional PID and sliding mode controllers are applied. Then, the position–velocity controller based on the obtained experience of the mechanism performance is proposed, in which a feedforward unit as the friction compensator is added to the latter feedback-based control units. Eventually, a feedback–feedforward controller based on PID controller and a compensator based on the identified inverse dynamic model is applied to the mechanism which was able to improve the performance of the control unit to a sufficient level.

Dynamic Simulation of a Mobile Manipulator with Joint Friction

Mission criticality in disaster search and rescue robotics highlights the requirement of specialized equipment. Specialized manipulators that can be mounted on existing mobile platforms can improve rescue process. However specialized manipulators capable of lifting heavy loads are not yet available. Moreover, effect of joint friction in these manipulators requires further analysis. To address these issues, concepts of model based design and concurrent engineering are applied to develop a virtual prototype of the manipulator mechanism. Closed loop manipulator mechanism actuated by prismatic actuators is proposed herein. The mechanics model of the manipulator is presented here as a set of equations and as multibody models. Mechanistic simulation of the virtual prototype has been conducted and the results are presented. Combined friction model that comprises Coulomb, viscous and Stribeck friction is used to compute frictional forces and torques generated at each one degree of freedom translational and rotational joints. Multidisciplinary approach employed in this work improves product design cycle time for complex mechanisms. Kinematic and dynamic parameters are presented in this paper. Friction forces and torques from simulation are also presented in addition to the visual representation of the virtual prototype.

Laminated micro-machine: Design and fabrication of a flexure-based Delta robot

Mesoscale electromechanical systems find applications in fields such as medical instrumentation, soft robotics, microscopy, flexible electronics and imagining. This paper implements the printed circuit MEMS (PC-MEMS) process [1] for the fabrication of a ‘pop-up’ flexure-based mesoscale system that exploits the simplicity of 2-D manufacturing techniques such as sheet-metal operations and laser cutting to realize a 3-D mechanism. The fabrication of a laminated Delta robot with prismatic actuation is presented to exemplify this process. A working device with actuation and functional components such as linear guides, stepper motors and limit switches is designed and fully realized. Because the mechanism is popped out of the plane to achieve its 3D shape, we present a stiffness analysis to arrive at the out-of-plane (or ‘pop-out’) angles that the planar system must accommodate so that constraints/limits on actuator torque/force can be can satisfied while producing an operational system. The simplicity of the processes makes it a candidate for the use in the emerging open-source hardware technologies for fabricating low-cost, complex, electromechanical systems.

A geometric approach for forward kinematics analysis of a 3-SPS/S redundant motion manipulator with an extra sensor using conformal geometric algebra

This paper proposes a geometric approach for forward kinematics analysis of a 3-SPS/S redundant motion mechanism using conformal geometric algebra (CGA). The forward kinematics of a parallel kinematic mechanism is generally very complicated and difficult to analyze. The proposed geometric method is useful because it provides simple, fast, and complete solutions to the problem and helps determine the relationship between the joints and end-effector without an iterative process. Thus, we introduce a geometric approach using CGA in an intuitive, stepwise fashion. We also use an extra sensor to provide more positional information, thereby allowing a unique solution to be selected geometrically from among the multiple solutions found using the geometric approach. In the mechanism considered herein, three identical legs linked by prismatic actuators are attached to a moving platform and to the base by two passive spherical joints. A passive leg is present at the center of the mechanism; it connects the center of the base to the platform, constraining the platform’s movement. The added components constrain the movement of the platform, making it possible to analyze the mechanism using a geometric approach. Herein we present performance comparisons that validate use of the proposed approach in real-time applications and demonstrate its low computational load.