查看更多>>摘要：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.
查看更多>>摘要：The dynamic friction coefficient of a wet clutch affects the vehicle shift quality and changes with factors such as the friction plate abrasion and oil and friction plate temperatures. Herein, an adaptive control strategy for dual clutch transmission (DCT) shifting is proposed. Based on the experimentally obtained dynamic friction coefficient of a wet clutch with a definite service mileage, the dynamic friction coefficient of a wet clutch is estimated online, considering the current service mileage and the oil and friction plate temperatures, using the recursive least squares method. By establishing a linear time-varying model of the DCT powertrain and using linear time-varying model predictive control to design the upper controller, the optimal shift reference curves of the clutch pressure and engine torque are obtained. Thereafter, the clutches and engine track the optimal reference curves through lower-layer proportional-integral-derivative controllers. The co-simulation results obtained using Simulink and AMESim show that the proposed control strategy can achieve good shift quality. It can also adapt to changes in the dynamic friction coefficient caused by clutch abrasion and oil and friction plate temperatures and ensure the shift quality of DCT vehicle.
查看更多>>摘要：The potential applications of constant-force mechanisms (CFMs) have increased in recent years with the development of new designs. This paper focuses on the experimental evaluation and the mathematical modelling of a CFM. The mechanism is based on the use of cams, rollers and springs, which allows for a large stroke and user-adjustable force value. The evaluation and validation of the mechanism is carried out by performing a series of tests examining the constancy of the input force, and where the influence of friction forces and the manufacturing process on the performance of the mechanism is revealed. A multibody model of the constant force mechanism is developed and calibrated with the help of the experimental results. This model is based on the assumption of rolling without sliding between the cam and the roller, what makes the multibody model to include velocity constraints. The calibration of the model pursues the development of a mathematical tool to study the performance of the mechanism in conditions that are different from those of the experiments presented. The CFM model is used to perform a sensitivity analysis of some important design parameters of the mechanism.
查看更多>>摘要：Cutting forces cause machine tool vibration, deformation and tool wear during machining pro-cesses, due to the complexity of the generating kinematic relationship between the head cutter and hypoid gear, great challenges are brought to the prediction of cutting forces. This study focused on a vectorized closed-form solution to cutting force prediction for face-hobbing of hypoid gears. Based on a thorough investigation of the relationships between the series of tooth surface families machined by head cutter during the face-hobbing, a vectorization model for undeformed cutting chip geometry is established. A closed-form solution to cutting forces pre-diction of face-hobbing of hypoid gears is firstly presented by combining the oblique cutting theory and the established vectorized undeformed chip geometry model. Results shows that the formularized cutting forces prediction model are more accuracy and efficiency than the semi-analytical method. Thus, the proposed model will be suitable for analyzing mechanisms of chatter, and optimization of gear cutting systems.
查看更多>>摘要：The kinematic accuracy analysis of harmonic drive (HD) is very complex since there is a compliant cup. To explicitly figure out the effects of errors on the kinematic accuracy of HD, an equivalent mechanism in kinematics of HD is proposed, in which a spatial gear-linkage with 3-R, 2-S pin, 1-P and 1-Gear pair is equivalent to the HD with single pair of teeth of flexspline and circular spline. The kinematic model/equations of the mechanism under error condition are established by means of the mechanism topological structure analysis. For multiple tooth pairs' engagement in HD with load, a similar parallel mechanism with multi-branch of the single tooth pair is then established and a compatibility equation is derived from the identical output motion caused by all branches. With the mechanism model, the kinematic accuracy/transmission error (TE) of HD can be investigated with consideration of errors and deformation caused by load. The TE effects of assembly errors and geometry errors are simulated. The FE analysis and test of a HD are conducted to validate the proposed mechanism model. The FE and test results show good agreement with the simulations', which indicates that the proposed mechanism model is reasonable and effective.
查看更多>>摘要：Grinding is an important finishing process for gear teeth to eliminate the surface deformation after heat treatment and improve accuracy. Grinding face-hobbed hypoid gears is especially challenging because the extended epicycloid curve of the generating gear does not match with the circular curve of the conventional disc-type bevel grinding wheel. In this paper, we propose a novel tooth grinding method based on continuous indexing, which uses a conical grinding tool to replace the blade on the face-hobbing cutter head. It includes two types of generating motions between the conical grinding tool and the generating gear, and between the generating gear with the workpiece. The mathematical model is developed to determine the geometric parameters of the conical grinding tool and to avoid interference between the conical grinding tool and nongrinding flanks. Finally, a numerical example is presented and a computer numerical control (CNC) virtual experiment is conducted to validate the proposed method. The results show that the tooth surface accuracy can be effectively maintained.
查看更多>>摘要：Simulation of complex systems requires the division to subsystems modelling each subsystem separately, and then interfacing and coupling them. The individual subsystems are often modelled in separate environments using different formulations and integrations. One way to couple subsystems is through co-simulation. For interactive, real-time simulation, non-iterative coupling is the main possibility. Such non-iterative co-simulation is very prone to instability problems that relate to the communication and information exchange between subsystems. This work focuses on non-smooth mechanical subsystem subjected to unilateral contacts and introduces an efficient way to interface it with the rest of the system using a reduced interface model. The proposed interface model accounts for contact attachments/detachments during the macro time step which can lead to better functionality and more accurate results. The proposed method and model is illustrated using an example of interfacing mechanical and hydraulic subsystems. The obtained results confirm that the proposed interface model enhances simulation accuracy, functionality and stability.
查看更多>>摘要：A new method to increase the tooth width and a specific face gear called non-orthogonal asymmetric helical (NAH) face gear is presented, which can avoid pointing by increasing the pressure angle of the driven side. Based on the enveloping process of the asymmetric helical shaper, the tooth surface equation of the NAH face gear is deduced, the effect of shaper parameters on tooth surface morphology is obtained by calculating the vertical deviation point by point. The influence of the pressure angle on the tooth width of NAH face gears is studied, and a practical method to avoid overcutting of tooth root is put forward. A simplified model of the tooth contact analysis (TCA) equations is established, of which the equation variables are reduced from 7 to 4. According to Hertz contact theory, the loaded tooth contact analysis (LTCA) of gears is carried out, and the finite element method is used to verify it. In addition, in order to obtain good contact performance and avoid tooth edge contact, the optimization results of NAH face gear drives are presented.
查看更多>>摘要：This paper presents an analytical description and experimental results for a reconfigurable field robot that can climb inside circular and rectangular pipes. The robot is fitted with two mechanisms that allow it to change its width and height and shift its center of mass (COM) to adapt itself to the size of the pipe. We start by describing the kinematic model of the robot as a function of its sprawl and four bar extension mechanism (FBEM). Next, we develop a force analysis based on the robot's geometry, its configuration, the position of its center of mass (COM), the diameter of the pipe, and the coefficient of friction (COF). We then develop strategies for driving, climbing and transitioning between the two modes. Although a high COF increases the robot's grip, it reduces its ability to reconfigure its shape, which it needs to transition between its climbing/driving modes. Based on this analysis, we designed a control algorithm comprised of actuation sequences to automatically drive the robot inside pipes, including the transition phases. The results show that the robot successfully executed its climbing tasks (see video).
查看更多>>摘要：Reciprocating mechanisms are widely used in industry because a complex movement is achieved by a simple rotation of the driven axis. Given the tendency to evolve towards more energy efficient machines and flexible production, motion profile optimization offers a cost-effective solution as it results in large energy savings without any hardware adaptions. However, the existing optimizers are used off-line because the position-dependent parameters such as load torque and inertia of the system model must be known in advance. When the actual machine differs from the model, or when parameters change during operation due to process flexibility, the off-line determined motion profile is no longer optimal and results in unnecessary energy consumption. This paper therefore presents an on-line approach in which the varying inertia is estimated on the actual machine and used for updating the motion profile. The sliding discrete Fourier transform is proposed for real-time estimation and a gradient-based algorithm combined with Chebyshev polynomials is proposed for on-line optimization. Experimental validation on an industrial pick-and-place unit proves that the presented method leads to similar energy savings as off-line optimizers, but without prior knowledge of the parameters, and is moreover capable of handling mass changes during operation.
NSTL
Elsevier