Hybrid Parallel Kinematic Machine Research Articles

Sequential Measurement of Position-independent Geometric Errors in the Rotary and Spindle Axes of a Hybrid Parallel Kinematic Machine

We propose a technique to measure position-independent geometric errors (PIGEs) in the rotary and spindle axes of a hybrid parallel kinematic machine (PKM). The PKM investigated here includes one more rotary axis than an Exechon PKM, which is used to improve the productivity of hybrid processes, such as machining and direct-energy-deposition three-dimensional metal printing. Errors in the measured position and orientation of the rotary axis, and the orientation of the spindle axis produce volumetric errors in the processed workpiece. If accuracy is to be improved, the deviation of each axis must be measured and compensated. In our approach, errors are measured using three methodologies that require only control of the rotary axis: in the first, no offset is applied to account for positional deviation of the rotary axis; in the second, an offset is used to correct the orientation of the rotary axis; and in the third, a tool offset is used to correct the orientation of the spindle axis. We developed an algorithm that uses the three measured datasets to identify PIGEs. The proposed method was applied to a hybrid PKM and the PIGEs were measured and compensated. This technique uses simple measurement paths and sequential measurements to correct rotary and spindle axis errors, and could therefore be widely used in industry.

Inverse kinematic analysis of 5-axis hybrid parallel kinematic machine using CAD and regression analysis approach

Since three decades for their potentially desirable fast dynamic performance, rigidity, and acceptable accuracy parallel kinematic machines (PKM) attracted interest from industry and academia in the machine tool/robot sectors. PKMs are highly used for their higher accuracy as it relies on system stiffness and rigidity. In PKM, the inverse kinematic analysis for finding the velocity and acceleration of a limb having more than two degree of freedom (DOF) manually is tedious. Also, generation of transformation matrix is too complex. In present work, six degrees of freedom 5-axis hybrid parallel kinematic machine (HPKM) with hemisphere workspace has been modelled and assembled in CATIA. Secondly, inverse kinematic analysis of PKM was carried out in digital mockup unit (DMU), CATIA. The velocities and accelerations of all the three limbs at three different feed rates and variations in joint angles were found. On the other hand, the regression equations were generated for velocity and acceleration of three limbs, joint angles with respect to position and time, while the tool travels along the semi circular contour trajectory.

Inverse kinematic analysis of 5-axis hybrid parallel kinematic machine using CAD and regression analysis approach

Inverse kinematic analysis of 5-axis hybrid parallel kinematic machine using CAD and regression analysis approach

Optimization design for a compact redundant hybrid parallel kinematic machine

Abstract Designing a machine tool that simultaneously satisfies the requirements of high rigidity, large dexterous workspace and small footprint is challenging. This paper proposed a redundant hybrid manipulator as a machine tool with these features. The design attempted to integrate the advantages of a PKM (parallel kinematic mechanism), SKM (serial kinematic mechanism) and redundancy mechanism. The geometric parameters of the manipulator were elaborately tuned to overcome intrinsic and extrinsic singularities with an optimal machining plane. High-accuracy and complex-shape machine results have been experimentally verified. The advantage is the ability to shorten the required strokes of the linear actuators effectively. This change not only improves the performance in terms of rigidity, accuracy and dynamics of the entire mechanism but also reduces the cost of the materials and the performance ratio. The second is to decrease the overall machinery-footprint-to-workspace ratio and lower the manufacturing cost via occupied floor space reduction and layout flexibility of the production line.

Effect of machining parameters over surface roughness in contour milling of aluminium 2024-T351 using 5-axis parallel kinematic machine

Surface roughness is reduced by optimising the machining parameters. This research analyses the effect of machining parameters on surface quality of aluminium 2024 T351 in milling of semi-circular pocket with carbide tool on 5-axis hybrid parallel kinematic machine (HPKM). Three different segmented surfaces were selected on semi-circular pocket and investigated the influence of machining parameters. The selected parameters are spindle speed, feed rate and depth of cut. Investigation of this study results in the optimum parameters that could produce good surface finish. The effect of these cutting parameters is evaluated using L9 Taguchi orthogonal array, s/n ratio, analysis of variance (ANOVA). Based on the result analysis, good surface finish can be obtained at high cutting speed, low feed rate, and low depth of cut. Also, the maximum influence of feed rate among the selected surface segments was ascertained.

An experimental investigation of control parameters in five-axis hybrid parallel kinematic machine in milling of aluminium 6061-T3

Surface roughness is an important factor in predicting performance of any machining operation. The experiment has been performed in milling a V-shaped pocket on aluminium 6061-T3 by Exechon parallel kinematic machine using carbide tool. Three different segmented surfaces are selected on V-block and the effect of control parameters were investigated over it. In this research, Taguchi method is used to identify the optimal combination of spindle speed, feed, and depth of cut. To analyse the effect of the control parameters for fine surface finish, Taguchi L9 orthogonal array, signal-to-noise (S/N) ratio and analysis of variance (ANOVA) are employed. Control parameters such as high spindle speed, low feed rate and low depth of cut plays a decisive role in the result of surface roughness. Also, the obtained results from PKM were compared to that of CNC machining, and the surface topologies were examined under optical and scanning electron microscope.

An experimental investigation of control parameters in five-axis hybrid parallel kinematic machine in milling of aluminium 6061-T3

An experimental investigation of control parameters in five-axis hybrid parallel kinematic machine in milling of aluminium 6061-T3

Volumetric compensation of accuracy errors in a multi-robot surgical platform.

A multi-robot platform, made of a hybrid parallel kinematic machine and 2 KUKA LWR arms, is dedicated to open skull neuro-surgical tasks. Sub-millimeter accuracy is clearly required for both the absolute tool tracking and for good performances in motion compensation when the head is set free to move. An analysis of the sources of inaccuracies, mostly derived from the calibration phase, illustrates that errors are insufficiently reduced by stand-alone calibrations of the single robots. A method for volumetric compensation of errors is reported. A compensation transform is, in fact, computed during an offline training phase for a set of discretized subregions of the constrained head workspace. At runtime, a compensation motion is applied to robots so as to reach the desired real targets on anatomical parts. The resulting end-to-end static accuracy is distributed with median 0.75 mm and below 1 mm for the 95% of tests, with a 1:36 reduction factor from the starting conditions. The accuracy is evaluated also in dynamic tests with mild oscillatory patterns.

Experimental identification of the static model of the HPKM Tricept industrial robot

The present paper addresses the modelling and the experimental identification of the static behaviour of the Tricept robot, a hybrid parallel kinematic machine. Mass properties of robot links are initially hypothesized from solid modelling and then incorporated in the identification procedure. Coulomb friction and gravity contributions to motor torques are taken into account: their identification is carried out by means of ordinary least-squares algorithms based on motor currents measurements during several slow motion tests. Moreover, the effect of external forces applied at the end-effector is introduced in the model and analysed by driving the robot end-effector against a calibrated compliant cell. Eventually, the static model is profitably used in an industrial operation of Friction Stir Welding to estimate the external forces applied at the tool mechanical interface providing some benefits: a deeper understanding of the technological process parameters and the possibility to realize model-based controls.

Design of a 4-DOF hybrid PKM module for large structural component assembly

This paper presents a novel 4-DOF hybrid parallel kinematic machine (PKM), named Bicept, comprising a 2-DOF parallel mechanism plus a 2-DOF rotating head. The PKM is designed as a rigid yet compact module that can act as a robot cell moving along a long track for aircraft structural component assembly, a wing box for example. Dimensional synthesis of the 2-DOF parallel mechanism is carried out to achieve a relatively good kinematic performance within a prescribed task workspace that has a large width/height ratio. Then, using commercial CAE software, rigid body dynamics and stiffness analyses are carried out for motor sizing and performance evaluation of a full-size virtual prototype.

Kinematics analysis of a novel parallel platform with passive constraint chain

A novel 4-DOF (degree of freedom) parallel platform with the passive constraint chain is proposed in this paper, according to the topology of the mechanism 4-UPS/1-RPU (4-Universal-Prismatic-Spherical joints and 1-Revolute-Prismatic-Universal joints). The inverse kinematics is studied in detail. In the formulation of Jacobian matrix of the presented parallel mechanism, the full Jacobian matrix with 6×6 dimensional matrix that consists of actuated Jacobian matrix and the constraint Jacobian matrix is formulated by the screw theory. The positional and orientational workspace is analysed using the method of boundary mapping. The dexterity analysis is then carried out based on the full Jacobian matrix under different orientations. As an application example of the proposed parallel mechanism a 5-DOF hybrid parallel kinematic machine is developed, verifying that the proposed novel parallel mechanism has the desired 4-DOFs and the dexterity of the mechanism is good enough to use in the machining process.

Compensation of thermal deformation of a hybrid parallel kinematic machine

Many different Parallel kinematic machine architectures have been proposed, but few have actually been used in production environments. The class of machine structures referred to as hybrid parallel kinematic machines have been more successful than most and have been employed in a wide variety of applications. These machines are however still affected by many of the limitations associated with purely parallel structures. One of these is temperature-related error due to thermal expansion of the parallel machine links. This paper proposes and demonstrates a method of compensating for this error. The paper presents details of the method, developed, the development of the required kinematic model and its test and validation using a real production machine, the NEOS Tricept.

Applications of equivalent components concept on the singularity analysis of TRR–XY hybrid parallel kinematic machine tools

The design of using a parallel kinematic mechanism as a structure of machine tools has several good points compared with the traditional machine tools. However, the problem of singularity is usually encountered during real applications. Therefore, singularity analysis is very essential for using a parallel kinematic mechanism in real machining applications. Screw theory based on the Plucker coordinate system is one of the effective methods for analyzing the singularity of a parallel kinematic mechanism. A TRR–XY five DOF parallel kinematic machine tool that was developed in this research is adopted as an example for investigating the feasibility of this method. A 6×6 matrix calculation is usually required in screw theory for analyzing a six DOF mechanism. For a three DOF mechanism, the analysis using the screw theory becomes more complicated. An equivalent component method is addressed in this research to simplify the singularity analysis for a three DOF mechanism. The effects of differently designed dimensions of machine tools on the singularity are also investigated in this research.