Machine Stiffness Research Articles

Control design for mechanical hardware-in-the-loop operation of dynamometers for testing full-scale drive trains

Advanced testing methods are becoming more and more prevalent to increase the reliability of wind turbines. In this field, dynamometers that allow for system level tests of full-scale nacelles will play an important role. Operating these test benches in a hardware-in-the-loop (HiL) set-up that emulates realistic drive train modes is challenging because of the relatively low stiffness of the load machines' drive trains. This paper proposes a control method for enabling the said operation mode. It is based on the idea that the HiL-controller overrides the present unrealistic dynamics and directly imposes desired realistic dynamics on the test bench. A solution for the control problem is given and applied in a design study with a generic wind turbine and a test bench model obtained from construction data of a real test bench. In the design study, the HiL-controller robustly imposes desired drive train dynamics on the test bench model. Despite measurement noise, unmodelled parametric uncertainty, and unmodelled delays, the first drive train mode is correctly reproduced. This is confirmed by a comparison with simulation results from a full servo-aero-elastic code. Furthermore, an implementation of the test bench model on a programmable logic controller showed the real-time feasibility of the proposed method. Copyright © 2016 John Wiley & Sons, Ltd.

Loop Stiffness of Grinding Machine Developed for 450 mm Silicon Wafers

Increasing the wafer diameter from φ300 mm to φ450 mm is required to enhance semiconductor devices productivity. A high-stiffness rotary grinding machine equipped with water hydrostatic bearings was developed for a φ450 mm silicon wafer. The grinding machine has an upper structure consisting of a wheel spindle system and a lower structure consisting of a rotary worktable system. The spindle shaft creates both rotary and axial feeding motion. The upper and lower structures are clamped together rigidly by three kinematic couplings. A higher loop stiffness is required for the grinding machine because grinding the larger wafer requires a higher grinding force. This paper investigates the loop stiffness of the developed wafer grinding machine.

Manufacturing at double the speed

The speed of manufacturing processes today depends on a trade-off between the physical processes of production, the wider system that allows these processes to operate and the co-ordination of a supply chain in the pursuit of meeting customer needs. Could the speed of this activity be doubled? This paper explores this hypothetical question, starting with examination of a diverse set of case studies spanning the activities of manufacturing. This reveals that the constraints on increasing manufacturing speed have some common themes, and several of these are examined in more detail, to identify absolute limits to performance. The physical processes of production are constrained by factors such as machine stiffness, actuator acceleration, heat transfer and the delivery of fluids, and for each of these, a simplified model is used to analyse the gap between current and limiting performance. The wider systems of production require the co-ordination of resources and push at the limits of human biophysical and cognitive limits. Evidence about these is explored and related to current practice. Out of this discussion, five promising innovations are explored to show examples of how manufacturing speed is increasing—with line arrays of point actuators, parallel tools, tailored application of precision, hybridisation and task taxonomies. The paper addresses a broad question which could be pursued by a wider community and in greater depth, but even this first examination suggests the possibility of unanticipated innovations in current manufacturing practices.

Axial splitting of rocks under uniaxial compression

Although uniaxial compression testing of rock is considered a simple technique for evaluation of rock strength, it induces a complicated fracture pattern in the specimen that is difficult to comprehend. In particular, axial splitting is observed in uniaxial compression tests. There have been some attempts to interpret the failure pattern in compression testing of rock. The rock non-homogeneity and existence of initial micro-cracks and pores have been considered the main causes for nucleation of axial cracks. Based on some physical uniaxial compression tests on Pennsylvania blue sandstone, we suggest another possible mechanism, which considers the rock–loading platen interaction for generation of axial cracks in rock. In particular, this failure mechanism can occur in rock pillars which are loaded by the roof or floor that is made of a rock with different elastic properties. The non-uniform penetration of specimen in the loading platens and the end friction produce shear strain that can be the cause of axial shear cracking at the specimen ends. Dislocation along these axial shear cracks and the induced material dilation generate radial stresses which result in confinement of the core of the specimen and radial cracks in the vicinity of the specimen end surfaces. As a consequence of axial and radial cracks, rock pieces in the form of cantilever beams are generated around the specimen free surface. These rock pieces can break in tension in mode I fracture due to the internal radial stresses. Based on the physical observations, a simple mathematical model is proposed to explain the fracture pattern observed in the physical tests. Furthermore, a bonded particle numerical model for rock, together with a finite element simulation of the loading machine, is used to show how the applied boundary conditions and loading machine stiffness can affect the failure pattern.

A new top-down design method for the stiffness of precision machine tools

This work describes a new top-down design method for the stiffness of precision machine tools that considers the entire machine stiffness to guarantee the stiffness requirements in the initial design stage. A stiffness modelling method and a stiffness matching design method are presented to achieve the top-down design of the stiffness. A new stiffness characterisation using the stiffness coefficients for characterising the stiffness of the structural parts and the functional units is proposed. The deformation model of the entire machine is established based on multi-body system theory, and the equations of the stiffness coefficients for the deformations of the components are established based on the simultaneous equations of the static equilibrium equations, the deformation compatibility equations and the physical equations. The three-direction (3D) stiffness model is obtained by substituting the equations into the deformation model that reflects the stiffness characteristics of the machine tool. Thus, the reliability of the stiffness model is verified by experiments. Next, the stiffness matching design is performed to confirm the reasonable stiffness values of the parts based on the stiffness model. The finite element method (FEM) is used to validate the proposed method. The contribution rate of the stiffness of the parts to the stiffness of the entire machine is analysed.

Intrinsic versus extrinsic effects on serrated flow of bulk metallic glasses

In this work, we systematically investigate the serrated flow behavior in the compression of bulk metallic glasses by varying the intrinsic composition and various extrinsic material and experimental factors including the sample size, the strain rate, and the testing machine stiffness. We find that the serrated flow, characterized by the amplitude of load serrations, can be suppressed for the higher Young's modulus, larger sample size, higher strain rate, and larger testing machine stiffness, respectively, and that it could completely disappear at certain critical strain rates. Meanwhile, the shape of serrated flow, which tends to become more “blunt”, manifests as the increasing ratio of the duration time to the awaiting time of the serrated events. The dependence of the serrated flow on these various factors is interpreted from the stick-slip dynamics of a single dominant shear band in compression process and can be condensed into a unified theoretical parameter k/kcr, where k is a parameter dependent on the Young's modulus, the sample size and the machine stiffness, and kcr is expressed as a function of temperature and testing strain rate. The implication of the stick-slip shear band dynamics together with the tuning of these material factors and test parameters will lead to the design of ductile BMGs.

Modeling and compensation of the interactions between geometrical errors and drive currents in directly driven gantry machine tools

Machine tools in gantry design are commonly used to produce very large workpieces e.g. in the aerospace industry. The increasing accuracy requirements of the workpieces demand a very stiff machine design. But high stiffness leads to increased force flux and tension within the machine tool due to existing geometric errors or thermal influences. Especially geometric errors in the rails impede the synchronous movement of the two gantry drive systems and lead to position errors. To achieve high positioning accuracy despite the named geometric errors, high drive forces are necessary. These drive forces tension the gantry bridge. However, high forces lead to increased drive currents and, thus, reduce the machine dynamics and increase the energy consumption. In this paper, the interactions between geometrical errors and drive currents are investigated. Based on that, a multi-variable control to compensate for these tensions is introduced. That way the tension currents have been reduced by 86 %.

Structural Changes of Surface Layers of Hard Turned Parts by Wiper and Conventional Geometry

Aims: The aim of this paper is to achieve information about surface and sub-surface layers after hard turning by mixed ceramic tool with different geometry – Wiper and conventional and to compare achieved results to find out advantages of its use. Second aim is to obtain graphs of the influence of the tool wear on the surface structural changes. Study Design: The experiment on hard turning was carried out with differently shaped mixed ceramic tools. Place and Duration of Study: Slovak University of Technology in Bratislava, Faculty of Materials Science and Technology, Institute of Production Technologies and Institute of Materials Science, between November 2013 and May 2014. Methodology: In the case of cutting tool wear and structural changes measurement this experiment was carried out on two different workpiece sizes – 1.125 mm long cylinder to achieve cutting tool wear, 2. 10 mm long rings for structural changes. Microscopy for cutting tool wear measurement and X – ray diffraction for structural changes measurement was used. Cutting tool wear was represented by VB parameter and structural changes were represented by structural Original Research Article Samardziova et al.;BJAST, 6(4): 335-341, 2015; Article no.BJAST.2015.092 336 phase content, crystallites and lattice. Results: Information about structural phase contents was achieved. There are some differences between surface and sub-surface layers after hard turning by mixed ceramic tool. Surface layer consists of more austenite phase than sub-surface layers. But there were not very big differences between used cutting tool geometry and its influence on the cutting tool wear. Both geometries were used more than 40 minutes till the flank wear parameter VB reached to 0.25 mm. The thickness of the surface heat influenced layer depends on the cutting tool wear and it is influenced by the cutting tool geometry – application of the Wiper geometry leads to thinner “white layer”. Conclusion: It is very important to take into consideration cutting tool geometry when dealing with surface integrity after hard turning. But all achieved results depend on the used cutting parameters, cutting tool material, workpiece material, cutting force components, machine stiffness in specific experiments.

Stiffness Analysis of a 6-DOF Hybrid Robot

To meet the demanding in modular design of robotized devices for welding, cutting, deburring and polishing, etc, a novel 6-DOF hybrid robot compose of a plane symmetrical 3-UPU parallel mechanism plus a 3-DOF rotating head is presented. With a brief introduction to the architecture of the robot, a semi-analytical stiffness model of the 3-UPU parallel mechanism is formulated by analyzing the wrenches systems imposed by the UPU limbs upon the platform. This leads to the complete formulation of the static and deformation equations, resulting in a comprehensive stiffness model that accounts for the compliances of all limb components and joints. With the aid of this approach, the stiffness of a prototype machine throughout the entire task workspace is evaluated. The computational results are validated by the ANSYS at two typical configurations.

Machine Stiffness Rating: Characterization and Evaluation in Design Stage

Stiffness of mechanical systems of machines with heavy demands on their accuracy, precision and productivity such as machine tools, coordinate measuring machines, and industrial robots presents one of the most important design criteria. However, stiffness evaluation in the general case when force-and-torque load coupling takes place leads to some problems. The problems are associated with physical distinction between translational and rotational stiffness values manifested, in particular, in their different units of measurement. To overcome a majority of the difficulties, a new performance index – the collinear stiffness value (CSV) presenting an equivalent stiffness (compliance) value during simultaneous linear and rotational displacements – is developed and represented in static and dynamic versions. In this presentation, the CSV is used to formulate a new design-related dimensionless criterion: the ratio of the minimal CSV to stiffness value of the drive system, which usually presents a weak point of the modern machines. The CSV-based approach is applied to quantitative formulation of the significant advantage from the stiffness viewpoint of the orthogonal serial-kinematics machines (SKM) compared with the parallel-kinematics machines (PKM): (a) the parameters-of-motion-depending variations of the minimal CSV of the SKM in their workspace are, as a rule, one-two orders of magnitude less than those of the PKMs; (b) the stiffness-limited workspace of the SKM is more than that of the PKM. Application examples are simulated.

The Coupling Dynamic Analysis and Field Test of TBM Main System under Multipoint Impact Excitation

Damage by excessive vibration is serious engineering problem in TBM boring process. Dynamic characteristic analysis is essential for TBM antivibration design. According to TBM horizontal support structure, a dynamic coupling nonlinear model is established, with consideration of time-varying impact load and multicomponent complex relationship from cutter to gripper shoe. A set of field vibration tests is set up to accurately collect data under extreme work conditions; then, field data is collected from Liaoning northwest engineering. Field data is applied to validate simulation model to make sure time-varying damping stiffness, support cylinder stiffness, and the TBM machine stiffness distribution are reasonable. Simulation indicates the weakest part of TBM in axial and torsional DOF is the cylinder hinge and the connection shaft between motor and pinion, and the horizontal and vertical weak parts are bull gear. It also shows that, in normal excavation conditions, the acceleration amplitude of the cutterhead in three directions ranges from 1.5 g to 2 g. These results provide theoretical basis for the antivibration design and structural optimization of TBM.

Research on Detection of Machine Stiffness

Machine tool stiffness is a principal factor affecting machine tool precision, traditional methods can only be used to detect limited categories of machine tools. The paper introduces a new scheme to detect machine tool stiffness on the basis of dynamic detection of machine tool stiffness considering its characteristics and stress state during processing. An experiment conducted in turn-milling machining center CH7516GS indicated by comparison that statics analysis of finite elements matched the experimental result well, which provided precise original data for design of improved machine tool precision and access to design of precision detection equipment for other types of machine tools.

Summary of the Machine Tool Stiffness Method

this paper discussed the significance of the machine tool stiffness and the definition of the machine tool stiffness classification. This paper studies the stiffness of the machine at home and abroad, the machine rigidity research methods, evaluation and performance optimization design research in recent years are discussed here in the machine tool stiffness, and the problems faced then solutions and suggestions are put forward. Finally, the problem of machine stiffness properties further study and future development trends are summed to improve tools, improve processing quality of products, and reduce production costs.

Chatter suppression methods of a robot machine for ITER vacuum vessel assembly and maintenance

In the process of assembly and maintenance of ITER vacuum vessel (ITER VV), various machining tasks including threading, milling, welding-defects cutting and flexible hose boring are required to be performed from inside of ITER VV by on-site machining tools. Robot machine is a promising option for these tasks, but great chatter (machine vibration) would happen in the machining process. The chatter vibration will deteriorate the robot accuracy and surface quality, and even cause some damages on the end-effector tools and the robot structure itself. This paper introduces two vibration control methods, one is passive and another is active vibration control. For the passive vibration control, a parallel mechanism is presented to increase the stiffness of robot machine; for the active vibration control, a hybrid control method combining feedforward controller and nonlinear feedback controller is introduced for chatter suppression. A dynamic model and its chatter vibration phenomena of a hybrid robot is demonstrated. Simulation results are given based on the proposed hybrid robot machine which is developed for the ITER VV assembly and maintenance.

A novel and versatile apparatus for brittle rock deformation

This paper describes a new biaxial rock deformation apparatus within a pressure vessel, consisting of a stainless steel vessel with an internal diameter of 40cm and six main access ports for electronics (~100 pins), pore fluids (three lines), and confining oil. The apparatus has to ability to work on large rock samples, up to 20×20cm2, with horizontal and vertical forces up to 1.5MN. The maximum confining pressure of the vessel is 70MPa, and fluid flow properties and permeability of large-samples can be tested using up to 8L of fluids that can be pressurized up to 30MPa. Sliding velocity during experiments is in the range 0.1m/s – 1.0cm/s. The machine stiffness is 0.91kN/μm and 1.3kN/μm for the vertical and horizontal axes, respectively. Measurements on friction, velocity dependence of friction and healing properties of reference material like granite and talc replicate the values presented in the literature. Triaxial stress state tests with controlled confining and pore fluid pressure, and fluid flow through the samples have been performed successfully. The machine is extremely versatile, as it works as a uniaxial, triaxial or true-triaxial apparatus, on rock samples with dimensions ranging from several to tens of centimetres. Due to the broad number of potential operating conditions, we decided to name the machine BRAVA: Brittle Rock deformAtion Versatile Apparatus.

Evaluation and optimization of dynamic stiffness values of the PKMs: Collinear stiffness value approach

Dynamic stiffness of the parallel-kinematics machines is considered and a new engineering index – dynamic collinear stiffness value (DynCSV) – is proposed. The DynCSV presents an extension of a concept derived by Portman for evaluation of the static stiffness values. The DynCSV depends on robot-configuration and inertia parameters and presents the function of the vibration frequency. Mathematically, the DynCSV is a quadratic form associated with the real symmetric matrix of the Newton–Euler equation. The minimal and maximal values of the DynCSV give natural criteria for evaluation, limitation, and optimization of dynamic systems of machines and robots. In the frequency range, where the dynamic stiffness matrix has a semipositive definition, the minimum DynCSV is applied to construct a PKM workspace satisfying preliminary given stiffness limits and compute a virtual protective barrier, keeping the PKM mechanism from approaching both singular configurations and resonance frequencies. As an application example, the DynCSV-based local and global stiffness features of the Gough–Stewart platform mechanism are simulated, investigated, and visualized.

Geometrical variations of tubes and their impact on freeform bending processes

In bending processes for tubes and profiles, the produced bending geometry depends on the characteristics of the semifinished product. Tubes are subjected to scatter and deviations from the standard’s dimensions caused by their fabrication process. This results in deviations of the forming behaviour. Depending on the bending process, the effect of this phenomenon is even larger, if the deviation of process forces leads to a significant deflection of the die elements. Particularly kinematic bending processes, e. g. the 3-roll-push-bending process have shown to be highly sensitive to low machine stiffness. The effect of this factor has not been identified among the influences of other disturbance factors so far. As an additional aspect, the tube dimensions affect the material characterization which leads to inaccurate material properties in numerical simulations. This results in wrong input data for the process design and has to be corrected in lengthy adjustment procedures. In this contribution the scatter of some geometrical tubes properties and its impact on the 3-roll-push-bending process is investigated. The tube geometry is measured by a tactile measurement system and is compared to the product standards. Observed deviations are categorized according to DIN 4760. The impact of the deviations on the bending process is examined by numerical variant calculations. Besides the tube dimensions, the stiffness of the machine is varied and the influence determined. By considering the errors resulting from wrong input data in the material characterization the overall error is quantified.

Effects of machine stiffness on the loading–displacement curve during spherical nano-indentation

Abstract

Inverse Identification of CDM Model Parameters for DP1000 Steel Sheets Using a Hybrid Experimental-Numerical Methodology Spanning Various Stress Triaxiality Ratios

A hybrid experimental-numerical methodology is presented for the identification of the model parameters regarding a mixed hardening anisotropic finite plasticity fully coupled with isotropic ductile damage in which the micro-crack closure effect is given account for, for steel sheets made of DP1000. The experimental tests involve tensile tests with smooth and pre-notched specimens and shear tests with specimen morphologies recently proposed by D.R. Shouler, J.M. Allwood (Design and use of a novel sample design for formability testing in pure shear, Journal of Materials Processing Technology, Volume 210, Issue 10, 1 July 2010, Pages 1304-1313). These tests cover stress triaxiality ratios lying between 0 (pure shear) and 1/√3 (plane strain). To neutralize machine stiffness effects displacements of the chosen material surface pixels are kept track of using the digital image correlation system ARAMIS, where recorded inputs are synchronized with force measurements. On the numerical part, developed constitutive model is implemented as user defined material subroutine, VUMAT, for ABAQUS/Explicit. FE models for the test cases are built using 3D brick elements (rather than thin shells) and devising developed VUMAT for the constitutive model, model parameters are identified using an inverse parameter identification procedure where the objective function relies on the difference of experimentally observed-numerically predicted forces for the selected pixel displacements. The validity of the material model and transferability of its parameters are tested using tests involving complex strain paths.

Discuss the Relevance of Heavy Truck Body and Modeling

For users and operators, the use function and mental function of the heavy truck have a great meaning, therefore, in a lot of designers’ eyes, heavy truck is no longer a stiff machine simply, on the contrary, it can be considered as a combination of science and art to some extent. Heavy truck’s body and formative correlation analysis shows the relationship between the technology and modeling so does the function and modeling in a large level, and at the same time, it is also reflects our understanding of the modern aesthetic in a certain extent. So, dealing with the problem of the relevance of heavy truck body and modeling correctly makes a great contribution to our heavy truck modeling design.