2010 Vol.23(4)

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Dynamic Manipulability and Optimization of a Two DOF Parallel Mechanis
2010, 24(4).
[Abstract](2411) [PDF 312KB](104)
Abstract:
The dynamic dexterity is an important issue for manipulator design, some indices were proposed for analyzing dynamic dexterity, but they can evaluate the dynamic performance just at one pose in the workspace of the manipulator, and can’t be applied to dynamic design expediently. Much work has been done in the kinematic optimization, but the work in the dynamic optimization is much less. A global dynamic condition number index is proposed and applied to the dynamic optimization design the parallel manipulator. This paper deals with the dynamic manipulability and dynamic optimization of a two degree-of-freedom (DOF) parallel manipulator. The particular velocity and particular angular velocity matrices of each moving part about the part’s pivot point are derived from the kinematic formulation of the manipulator, and the inertial force and inertial movement are obtained utilizing Newton-Euler formulation, then the inverse dynamic model of the parallel manipulator is proposed based on the virtual work principle. The general inertial ellipsoid and dynamic manipulability ellipsoid are applied to evaluate the dynamic performance of the manipulator, a global dynamic condition number index based on the condition number of general inertial matrix in the workspace is proposed, and then the link lengths of the manipulator is redesigned to optimize the dynamic manipulability by this index. The dynamic manipulability of the origin mechanism and the optimized mechanism are compared, the result shows that the optimized one is much better. The global dynamic condition number index has good effect in evaluating the dynamic dexterity of the whole workspace, and is efficient in the dynamic optimal design of the parallel manipulator.
Comparative Analysis of Characteristics of the Coupled and Decoupled Parallel Mechanisms
2010, 24(4).
[Abstract](2449) [PDF 508KB](239)
Abstract:
The existence of coupling makes the parallel mechanism possess some special advantages over the serial mechanism, while it is just the coupling that brings about the parallel mechanism some limitations, such as complex workspace, high nonlinear relationship between input and output, difficulties in static and dynamic analysis, and the development of control system, which restricts its application fields. The decoupled parallel mechanism is currently one of the research focuses of the mechanism fields, while the study on the different characteristics between the decoupled and coupled parallel mechanisms has not been reported. Therefore, this paper performs the systematic comparative analysis of the 3-RPUR and the 3-CPR parallel mechanisms. The features of the two mechanisms are described and their movement forms are analyzed with screw theory. The inverse and forward displacement solutions are solved and the Jacobian matrices are obtained. According to the Jacobian matrices and by using the theory of physical model of the solution space, the workspace, dexterity, velocity, payload capability, and stiffness of the mechanisms are analyzed with plotting the indices atlases. The research results prove that the effects of the coupling on the parallel mechanism are double-side, and then the adoption of the decoupled parallel mechanism should be determined by the requirements of the concrete application situation. The contents of this paper should be useful for the type synthesis and practical application of the parallel mechanism.
Analytical Solution for Three-dimensional Forging Taking into Account Bulging of Sides by Mean Yield Criterion
2010, 24(4).
[Abstract](2370) [PDF 327KB](68)
Abstract:
Currently, for some complex plastic deformations, the analytical solution can not be obtained by using Mises yield criterion, because Mises yield criterion is nine dimensions, the velocity field is complex, and the solving methods are not innovative. Corresponding solutions of these problems are that yield criterion is linearized to reduce the variable numbers, and the velocity field and the solving methods are reasonably simplified, respectively. In this paper, a new linear yield criterion—mean yield(MY) criterion and inner-product of strain rate vector are used to analytically solve 3D forging taking into account bugling of sides. The velocity field is expressed as a vector in three dimensions, and rotation and divergence are applied to confirm that the velocity field is kinematically admissible. Then, the corresponding strain rate tensor of the velocity field is transformed into principal one by making the determinant of coefficients of the tensor cubic equation be zero. By using MY criterion, the plastic power is term by term integrated and summed according to inner-product of strain rate vector. An upper bound analytical solution is obtained for the forging, and verified by a pure lead press test. The test result turns out that the total pressure calculated by MY criterion is higher by 2.5%15% than measuring value. In addition, a measuring formula of bulging parameter (a) is proposed, but the values of a measured by the formula are lower than those optimized by the golden section search. The total pressure calculated by MY criterion is compared with the ones by twin shear, Trasca yield, and Mises yield criterion. The comparing result shows that the total pressure calculated by MY criterion is slightly higher than the mean value of that by twin shear and Trasca yield criterion, and lower than that by Mises yield criterion, but more close to that by Mises yield criterion compared with that by other two. The proposed analytical solving methods can be effectively used to other complex plastic deformation, simplifying the solving process and obtaining the reasonable results
Reliability Analysis of Electromechanical Systems with Degraded Components Containing Multiple Performance Parameters
2010, 24(4).
[Abstract](2347) [PDF 133KB](71)
Abstract:
Components of electromechanical systems usually contain multiple performance parameters and degrade over time. In previous studies, the reliability of these electromechanical systems was analyzed by the traditional method, and the system reliability was estimated based on the reliability of components and the structures of the systems. The system reliability estimated by the traditional method could not reflect the performance of the systems. A new method is proposed in this paper to analyze the system reliability according to the data of multiple performance degraded processes of components. The performance distribution of a degraded component is obtained by the performance degradation analysis, and then states of the component are defined and corresponding state probabilities are estimated. The universal generating function method is proposed and extended to compute the performance distribution and reliability of the system based on the performances of components. A numerical example illustrates the proposed method. The results of the example show that the proposed method can relate the performance of the system to the performances of components and absolutely reflect the relationship between reliability and performance. Compared with the exact values of the system reliability, the results obtained by the proposed method is almost the same with the exact values, and the results obtained by the traditional method are conservative. The proposed method overcomes the shortcomings of the traditional method and provides a new approach to analyze the reliability of electromechanical systems with degraded components containing multiple performance parameters.
New Method to Measure the Fill Level of the Ball Mill I—Theoretical Analysis and DEM Simulation
2010, 24(4).
[Abstract](2175) [PDF 396KB](92)
Abstract:
The accurate measurement of the fill level in the ball mill has not been resolved because of the interplay of many variable factors, which led the mill to be operated under the uneconomical condition and lost a lot of energy. At present, some methods, such as vibration method and acoustic method, have been applied for measuring the fill level by the researchers. Aiming at the problem of the traditional methods for measuring the fill level, that is, the feature variables of the fill level suffer the influences of the ball load and the water content of the coal, a novel method to measure the fill level is proposed and a possible relation between the fill level and the angular position of the maximum vibration point on the mill shell is investigated. The angular positions of the maximum vibration point on the mill shell for different fill level cases are calculated theoretically under two assumptions, respectively. Meanwhile the charge motions of the mill for different fill level cases are simulated with the discrete element method (DEM). And the simulation results are verified by comparing the motion trajectories of steel balls and power draft of the mill. The simulated movement trajectories of the outmost layer steel balls in the mill are monitored and analyzed to obtain the angular positions of the maximum vibration point on the mill shell. Both the results of the theoretical calculation and the 3D DEM simulation show that the position of the maximum vibration point on the mill shell moves to a lower angular positions as the fill level decreasing, which provides a new idea for measuring the fill level accurately.
Characterization of the Elastic-plastic Region Based on Magnetic Memory Effect
2010, 24(4).
[Abstract](2203) [PDF 263KB](145)
Abstract:
Detecting stress concentration, especially critical stress state leading to structure damage or failure, is one of the most important tasks of equipment diagnosis. Metal magnetic memory technique needs further research to evaluate stress concentration quantitatively due to ambiguous physical mechanism, though it has potential to detect early defects in ferromagnetic materials. Mild Q235 steel defective specimens in demagnetization state were loaded in tension up to visible necking, with magnetic memory signals measurement made at increasing stress levels. Magnetic signals varied greatly under first several loadings and subsequently tended to stability in the elastic region, which showed that the magnetization always approaches the anhysteretic magnetization curve and was explained by the theory of magnetomechanical effect. In the plastic stage, an abnormal wave occurred in the stress concentration zone and its height value was sensitive to plastic deformation levels and dependent on the distance between the probe and defect, in accordance with the simulation results based on the magnetic dipole model. Different magnetic signal characteristics in the elastic-plastic region indicate that the magnetic memory technique can identify macroyielding and early damage, which is of profound significance for ensuring safe operation of equipment in service.
Modeling and Verification of an Astronaut Handling Large-mass Payload
2010, 24(4).
[Abstract](2230) [PDF 385KB](104)
Abstract:
The experiments on astronaut motions are difficult to conduct due to the limitation and high cost of constructing or simulating the microgravity environment of space. Therefore, the method of computer simulation on astronaut extravehicular activity is broadly promoted. However, validations and verifications for these simulations stated in related literatures are incomplete such as comparing with the limits of human body movements or reconstructing a three-dimensional movement for some parts of EVA video. Novel modeling and verification methods for the task of an astronaut handling large-mass payload during EVA were revealed. A simplified model of an astronaut was constructed, and the astronaut motion was conceived as a planar movement of a multi-body system which includes seven segments with six revolute joints in the human body sagittal plane. The inverse kinematics method was used to calculate joint angles, joint velocities, and joint accelerations in time domain. The solution of joint torques using the inverse recursive dynamics was achieved. Furthermore, a virtual model with the ADAMSTM software was developed and implemented to verify the results by adding the kinematical data calculated to joints in order to achieve the trace of the center of mass of the hand. Additionally, the joints kinematics and kinetics data with time in the virtual model were obtained and compared with the corresponding calculated data. This result indicates that the modeling methods proposed can be employed as a solid algorithm to conduct the simulation of astronaut’s tasks in EVA, and verification using the virtual model can be easily operated and has a good accuracy. This study provides an effective and economical way of modeling and simulation for extravehicular missions.
Wheel-rail Profiles Matching Design Considering Railway Track Parameters
2010, 24(4).
[Abstract](2327) [PDF 328KB](230)
Abstract:
The profile of wheel/rail has great concern with the vehicle running safety, the wheel/rail wear and the rolling contact fatigue between wheel and rail, due to its severer impact on the dynamic behavior of both the railway vehicle/track, and the wheel/rail rolling contact status. However, recent studies in this respect are mainly explored in reverse methods, where track parameters are predetermined and invariable during the optimizing process. This paper attempts to propose a wheel-rail profiles matching design method considering multi-parameter, through optimizing wheel/rail profile under different rail cants and track gauges, based on the existed optimization technology for the normal gap of wheel/rail. The method presented in this paper can also, compared with the prior reverse methods, be called “forward solution method” in which the riding comfort, wheel unloading rate and wheel/rail contact stress of the speed-up railway passenger car are calculated by means of a vehicle-track coupling dynamic model, with the range of the rail cant varying from 1/20 to 1/40 and the rail gauge from 1 433 mm to 1 441 mm. These results show that the distribution status of the pairs of contact points can be obviously improved and the contact stress can be reduced significantly; a great influence is exposed by the rail cant and track gauge on the dynamic behavior of the high speed passenger car, and an optimal vehicle dynamics behavior are obtained with the optimized wheel/rail profile when the rail cant is 1/30 and the track gauge is 1 435 mm. This research can provide important references for the investigation of the wheel-rail profiles matching design method considering multi-parameter.
Design of Robot Welding Seam Tracking System with Structured Light Vision
2010, 24(4).
[Abstract](2287) [PDF 330KB](691)
Abstract:
Robot welding is an important developing direction of welding automation and intelligentization, and automatic seam tracking technology is one of principal research domains. Nowadays, seam tracking system with structured light vision becomes a hot research. Structured light vision seam tracking products abroad are generally very expensive and can only be applied on special occasions. In China, the research of structured light vision seam tracking system is still just on the stage of experiments. A robot real-time seam tracking system with line structured light vision is designed. The hardware system is set up, a filtering method for line structure seam image is improved, and compared with common filtering, it has better effect and characteristic of real time. Two methods, fast template matching and fast Hough transform, to recognize the image coordinates of seam center are improved. Two new image recognition methods, structure element matching and corner detecting, are proposed. The comparison of seam image recognition shows that fast template matching and corner detecting are more precise and stable than the other two methods, and corner detecting is the best in real time. A simultaneous calibration for camera parameters and robot hand-eye is also proposed, and calculation shows that the calibration is effective and feasible. The robot seam tracking tests for linear and folded lap-joint are performed, which are based on the above four image recognition methods, and the results indicate that four image recognition methods are all applicable to real-time seam tracking, and the whole system sufficed for the requirements of real-time seam tracking. Automatic seam tracking with line structured light vision is feasible and has good versatility.
New Hybrid Parallel Algorithm for Variable-sized Batch Splitting Scheduling with Alternative Machines in Job Shops
2010, 24(4).
[Abstract](2382) [PDF 396KB](118)
Abstract:
The batch splitting scheduling problem has recently become a major target in manufacturing systems, and the researchers have obtained great achievements, whereas most of existing related researches focus on equal-sized and consistent-sized batch splitting scheduling problem, and solve the problem by fixing the number of sub-batches, or the sub-batch sizes, or both. Under such circumstance and to provide a practical method for production scheduling in batch production mode, a study was made on the batch splitting scheduling problem on alternative machines, based on the objective to minimize the makespan. A scheduling approach was presented to address the variable-sized batch splitting scheduling problem in job shops trying to optimize both the number of sub-bathes and the sub-batch sizes, based on differential evolution(DE), making full use of the finding that the sum of values of genes in one chromosome remains the same before and after mutation in DE. Considering before-arrival set-up time and processing time separately, a variable-sized batch splitting scheduling model was established and a new hybrid algorithm was brought forward to solve both the batch splitting problem and the batch scheduling problem. A new parallel chromosome representation was adopted, and the batch scheduling chromosome and the batch splitting chromosome were treated separately during the global search procedure, based on self-adaptive DE and genetic crossover operator, respectively. A new local search method was further designed to gain a better performance. A solution consists of the optimum number of sub-bathes for each operation per job, the optimum batch size for each sub-batch and the optimum sequence of sub-batches. Computational experiments of four test instances and a realistic problem in a speaker workshop were performed to testify the effectiveness of the proposed scheduling method. The study takes advantage of DEs distinctive feature, and employs the algorithm as a solution approach, and thereby deepens and enriches the content of batch splitting scheduling.
General Model of Fuzzy Plasticity
2010, 24(4).
[Abstract](2225) [PDF 541KB](64)
Abstract:
The transition between the elastic and plastic states is sharp in the classical plasticity theory. To overcome this problem, many constitutive models, such as multi-yield-surface model and two-surface model, have been developed. However, these models can not represent the true deformation process in a material. In order to capture nonlinear hardening behavior and smooth transition from elastic to plastic state, a general model of fuzzy plasticity is developed. On the basis of the theory of fuzzy sets and TAKAGI-SUGENO fuzzy model, a fuzzy plastic model for monotonic and cyclic loadings in one dimension is established and it is generalized to six dimensions and unsymmetric cycles. The proposed model uses a set of surfaces to partition the stress space with individual plastic modulus. The plastic modulus between two adjacent surfaces is determined by a membership function. By means of a finite number of partitioning surfaces, the fuzzy plastic model can provide with a more realistic and practical description of the materials behavior than the classical plasticity model. The validity of the fuzzy plastic model is investigated by comparing the predicted and experimental stress-strain responses of steels. It was found that the fuzzy plasticity has the ability to handle many practical problems that cannot be adequately analyzed by the conventional theory of plasticity.
Water-Assisted Injection Molding System Based on Water Hydraulic Proportional Control Technique
2010, 24(4).
[Abstract](2292) [PDF 324KB](223)
Abstract:
Water-assisted injection molding(WAIM), an innovative process to mold plastic parts with hollow sections, is characterized with intermittent, periodic process and large pressure and flow rate variation. Energy savings and injection pressure control can not be attained based on conventional valve control system. Moreover, the injection water can not be supplied directly by water hydraulic proportional control system. Poor efficiency and control performance are presented by current trial systems, which pressurize injection water by compressed air. In this paper, a novel water hydraulic system is developed applying an accumulator for energy saving. And a new differential pressure control method is proposed by using pressure cylinder and water hydraulic proportional pressure relief valve for back pressure control. Aiming at design of linear controller for injection water pressure regulation, a linear load model is approximately built through computational fluid dynamics(CFD) simulation on two-phase flow cavity filling process with variable temperature and viscosity, and a linear model of pressure control system is built with the load model and linearization of water hydraulic components. According to the simulation, model based feedback is brought forward to compensate the pressure decrease during accumulator discharge and eliminate the derivative element of the system. Meanwhile, the steady-state error can be reduced and the capacity of resisting disturbance can be enhanced, by closed-loop control of load pressure with integral compensation. Through the developed experimental system in the State Key Lab of Fluid Power Transmission and Control, Zhejiang University, China, the static characteristic of the water hydraulic proportional relief valve was tested and output pressure control of the system in Acrylonitrile Butadiene Styrene(ABS) parts molding experiments was also studied. The experiment results show that the dead band and hysteresis of the water hydraulic proportional pressure relief valve are large, but the control precision and linearity can be improved with feed-forward compensation. With the experimental results of injection water pressure control, the applicability of this WAIM system and the effect of its linear controller are verified. The novel proposed process of WAIM pressure control and study on characteristics of control system contribute to the application of water hydraulic proportional control and WAIM technology.
Multidisciplinary Design Optimization with a New Effective Method
2010, 24(4).
[Abstract](2295) [PDF 154KB](140)
Abstract:
Collaborative optimization (CO) is one of the most widely used methods in multidisciplinary design optimization (MDO), which is an effective methodology to solve modern complex engineering problems. CO consists of two-level optimization problems which are system optimization problem and subspace optimization problem. The architecture of CO can reserve the autonomy of individual disciplines in maximum, while providing a mechanism for coordinating design problem. However, CO has low computation efficiency and is easy to diverge. For the purpose of solving these problems, the former improved methods were studied. The relaxation factors were used to change the system consistency constraints to inequality constraints, or the response surface estimation was used to surrogate the system consistency constraints. However, these methods didn’t avoid the computational difficulties very well, furthermore, some new problems arose. The concept of optimum constraint sensitivity was proposed, and the quadratic constraints in system level were reformed. Hence, a new collaborative optimization was developed, which is called system level dynamic constraint collaborative optimization (DCCO). The novel method is able to increase the exchange of information between system level and disciplinary level. The optimization results of each disciplinary optimization can be feedback to system level with the optimum constraint sensitivity. On the basis of the information, the new system level linear dynamic constraints can be constructed; it is better to reflect the effect of disciplinary level optimizations. The system level optimizer can clearly capture the boundary where disciplinary objective functions become zero, and considerably enhance the convergence. Two standard MDO examples were conducted to verify the feasibility and effectiveness of DCCO. The results show that DCCO can save the solving time, and is much better in terms of convergence and robustness, hence, the new method is more efficient.
Methods for Integrating Energy Consumption and Environmental Impact Considerations into the Production Operation of Machining Processes
2010, 24(4).
[Abstract](2257) [PDF 266KB](442)
Abstract:
Energy consumption and environmental impact considerations of machining processes are viewed as important issues for the global trends towards sustainable manufacturing. The existing research of reducing energy consumption and environmental impacts of machining processes greatly focuses on design and planning activities, but is reasonably sparse for production operation activities. This paper explores a systematic methodology that incorporates energy consumption and environmental impact considerations into the production operation of machining processes. Firstly, the framework of the methodology is proposed to establish the generic procedures for integrating the above considerations in production operation activities. As the two key issues of the framework, the profile index value matrix is determined by valuing the individual quantity of energy consumption and environmental impacts of machining each job on each machine, and the multi-criteria models are constructed by the operational methods. Furthermore, with the guideline of the framework, the specific formulations are modeled by two sub-models for the parallel machine scheduling problem, in which makespan and energy consumption are the optimizing objectives as well as the constraints of environmental impact considerations. The specific formulations provide a practical method to integrate energy consumption and environmental impact considerations into the scheduling activity, and also can serve as a reference to other activities in the production operation. The case study for a batch of jobs, including seven kinds of gears in the machining shop floor, is presented to demonstrate the application of the specific formulations of the methodology. The proposed methodology provides potential opportunities for reducing energy consumption and environmental impacts in machining processes, and helps production managers in decision-making on the issues of energy consumption and environmental impacts in the production operation.
Improved Hybrid Robust Control Method for the Electromechanical Actuator in Aircrafts
2010, 24(4).
[Abstract](2284) [PDF 583KB](103)
Abstract:
In the flight process of aircrafts, their electromechanical actuators(EMA) must have the ability of enduring uncertainties caused by factors such as load disturbance, the variation of work temperature and the EMA’s nonlinearity. At present, in order to increase the EMA’s robustness on the uncertainties, the H∞ control method has been applied in aircrafts. The major problems with standard H∞ control lie in the large overshoot of step response and the high orders of the controller. For the purpose of addressing the two problems, this paper investigates several kinds of robust control strategies of the EMA. A mathematical model of the EMA is first built, and then with MATLAB software a H∞ controller and an improved hybrid robust controller composed of a reduced order H∞ controller and a lead compensator are designed. In order to make a scientific comparison of the control effects of H∞ controller, hybrid controller and classic proportion-integral-differential(PID) controller, a simulation research is made in respect of the open loop frequency response and the closed loop step response of the three controllers. For comparing the robustness of the three controllers, the load torque is entered as a disturbance and the disturbance response of error and control input are thus obtained. The experiments with the three controllers are also conducted. Through giving the EMA a command and a disturbance torque successively, the transient response and disturbing process of EMA are recorded. The simulation and experiment results show that with the help of the hybrid controller, the EMA not only guarantees good dynamic characteristics, but also has strong robustness of disturbance rejection. Therefore, the excogitated H∞ hybrid control method effectively solves the problem of large overshoot in dynamic response, and moderately meets the requirement of overcoming the uncertainties in the EMA of aircrafts.
Stiffness of Postural Alignment System Based on 3-Axis Actuators for Large Aircraft Components
2010, 24(4).
[Abstract](2281) [PDF 395KB](108)
Abstract:
Aircraft digital flexible assembly fixture and technologies are widely used in developed countries, while the traditional jig-based assembly mode is still used in China. The application study of aircraft digital flexible assembly system is just beginning in our country recently. To meet the requirements of automated posture alignment and join in digital assembly system for large aircraft components, a novel fitting fixture called 3-axis actuator is developed. On the basis of the actuators, three kinds of posture alignment system for large aircraft components are proposed, including the non-redundant system, the redundant actuating system, and the redundant leg system, and their constitutions and properties are introduced. Through deriving the feeding transmission stiffness model of single actuator and analyzing the inverse kinematics of these systems, the relationship between the external force and the changes of position and orientation of large aircraft component is obtained, and then the postural alignment stiffness models are established. With the method mentioned above, the postural alignment stiffness of three systems is computed by using the algebraic formulate, and the results show that redundant properties can increase system’s postural alignment stiffness. As an example, a optimized layout of the assembly system for a given model of aircraft is developed, the results of application show that the layout has many advantages, such as high accuracy, stiffness, stability, reliability, efficiency and flexible, which can satisfy the requirement of aircraft digital assembly system well. The proposed study of postural alignment stiffness for different systems can supply the theoretic support for the optimization layout design of aircraft digital assembly system, and contribute to evaluate the system working performance of systems.
Entrance Effect on Load Capacity of Orifice Compensated Aerostatic Bearing with Feeding Pocket
2010, 24(4).
[Abstract](2393) [PDF 423KB](282)
Abstract:
The current research of the aerostatic thrust bearing mainly focuses on the porous material bearing and inherent compensated air bearing, which aims at obtaining small physical dimension and large load capacity. Although porous material bearing appears larger load capacity, materials anisotropy itself and void content distortion caused in heat-treating, and machining processes add greater complexity to internal flow transfer process. Inherent compensated air bearing has the advantages of simple structure and good stability, but its load capacity and static stiffness is not worth somewhat. In this paper, based on hydrostatic lubrication theory, finite volume method is presented for taking entrance effects into account in computing pressure distribution, load capacity and mass flow rates of circular aerostatic thrust bearings. Technical analysis, numerical simulations and laboratory demonstration tests of influence of pocket diameter and pocket depth on loading capacity of aerostatic thrust bearing are carried out on simple orifice compensated air bearings with feeding pockets. The static parameters, such as air consumption and pressure distributions, are measured as a function of supply pressure and air gap height for several different orifices and pockets size. Entrance effects are described in term of typical throttling types, and the effect of pocket diameter and pocket depth on load capacity is systematically described respectively. The proposed research results uncover the causation of throttling action of the orifice compensated air bearing with feed pocket and further develop and improve the design theory of air bearing.