Fast forward kinematics algorithm for real-time and high-precision control of the 3-RPS parallel mechanism

A new forward kinematics algorithm for the mechanism of 3-RPS (R: Revolute; P: Prismatic; S: Spherical) parallel manipulators is proposed in this study. This algorithm is primarily based on the special geometric conditions of the 3-RPS parallel mechanism, and it eliminates the errors produced by parasitic motions to improve and ensure accuracy. Specifically, the errors can be less than 10^–6. In this method, only the group of solutions that is consistent with the actual situation of the platform is obtained rapidly. This algorithm substantially improves calculation efficiency because the selected initial values are reasonable, and all the formulas in the calculation are analytical. This novel forward kinematics algorithm is well suited for real-time and high-precision control of the 3-RPS parallel mechanism.     

Infrared Thermographic Testing of Hybrid Materials Using High-Power Ultrasonic Stimulation

Ultrasonic infrared thermography is a rapid and informative method of nondestructive testing, used to inspect materials and products in aviation and rocket and space industries. High-power piezoelectric emitters stimulate various materials by ultrasound and help revealing hidden flaws by local temperature changes, which can amount to tens of degrees. The possibility of optimizing the procedure of ultrasonic infrared thermography by choosing the frequency of acoustic waves so as to match the frequencies of local resonances of defects of different sizes has been investigated.     

Recent advances in finite element modeling of the human cervical spine

The human cervical spine is a complex structure that is the most frequently injured site among all spinal injuries. Therefore, understanding of the cervical spine injury and dysfunction, and also biomechanical response to external stimuli is important. Finite element (FE) modeling can help researchers to access the internal stresses and strains in the bones, ligaments and soft tissues more realistically, and it has been widely adopted for spine biomechanics research. Although in recent years numerous techniques have been developed, there are no recent literature reviews on FE models of the cervical spine. Our objective was to present recent advances in FE modeling of the human cervical spine in terms of component modeling, material properties, and validation procedures. Model applications and further development are also discussed. The integration of new technologies will allow us to generate more accurate and comprehensive model of the cervical spine, which can increase efficiency and model applicability. Finally, the FE modeling can help to facilitate diagnosis, treatment, and prevention technologies for cervical spine injuries.     

Optimal design of a six-axis vibration isolator via Stewart platform by using homogeneous Jacobian matrix formulation based on dual quaternions

A six-axis vibration isolation system is essential to high-precision space systems for attenuating vibrations on precise instruments. The kinematic optimal design is researched for the space six-axis vibration isolator via Stewart mechanism. Jacobian matrix is the basis of the kinematic performance index. However, the conventional Jacobian matrix is not usually dimensionally homogeneous due to the inhomogeneous physical units, caused by the different mathematical representations of the rotation and translation. In this paper, we propose a dual quaternion approach to derive the dimensionally homogeneous Jacobian matrix of a general six-axis parallel mechanism. Two quaternions are used to parameterize the rotation and translation of the platform. The dimensionally scaling factor for the generalized Jacobian matrix is defined as the ratio of the norms of the two quaternions. The dimensionally homogenous Jacobian matrix is then obtained and applied to the optimal design of the six-axis vibration isolator. The performance index of isotropy is considered to make the isolator minimum kinematic coupling in its working configuration.     

Chute design of mackerel automatic grader using multibody dynamics

An automatic mackerel grader is a machine that automatically sorts mackerel according to size. The chute spreads the mackerel to allow the latter to enter each roller evenly. Sorting is simulated to confirm that the multibody dynamics model of the automatic grader correctly sorts the mackerel. This process is completed according to the shape of the chute to confirm the effect on performance. A new shape of the chute and the application are proposed and evaluated by a comparison with conventional chute shapes. RecurDyn, a multi-body dynamics program, is used for the sorting simulation. The contact parameters are defined by Hertzian contact theory using the material information of mackerel and C/S steel. A rigid-body mackerel model is used to express the behavior of mackerel. Sorting is simulated for each shape. The sorting accuracy and error of the new shape are compared with those of two existing shapes. The new shape shows a 0.29 mm sorting error and better sorting accuracy than the existing shapes.     

Rolling element bearings absolute life prediction using modal analysis

In this paper, the authors introduce an experimental procedure for predicting the fatigue life of each individual rolling element bearing separately using vibration modal analysis. The experimental procedure was developed based on a statistical analysis. A statistical analysis was performed to find an empirical model that correlates the dynamic load capacity of rolling bearings to their dynamic characteristics (Natural frequencies and damping). These dynamic characteristics are obtained from the frequency response function of each individual bearing that results from vibration modal analysis. A modified formula to the already known Lundberg-Palmgren life formula is proposed for rolling element bearings. Given the modified formula, one can predict the fatigue life of each individual rolling element bearing based on its dynamic characteristics. The paper compares the results from the modified formula with those from Lundberg-Palmgren formula. The modified formula provides an accurate prediction for the fatigue life of each individual bearing based on its dynamic characteristics. The experimental validation of the modified formula is considered for future work. Therefore, it can be used in various applications of rolling element bearings in machinery systems.     

Accurate measurement of residual stresses of 2219-T87 aluminum alloy friction stir welding joints based on properties of joints

In order to avoid the deviation caused by calculating the residual stresses of welding joints with the release coefficients of the same parent metal, a new method has been proposed based on the properties of weld joints. Since the mechanical property of 2195-F Al alloy is close to that of 2219-T87 Al alloy Friction stir welding (FSW) joint, 2195-F Al alloy is selected as the substitute material of 2219-T87 Al alloy FSW joint in the calibration test. Release coefficients of 2195-F Al alloy and proper correction coefficient are used to calculate the residual stresses of 2219-T87 Al alloy FSW joints. Compared to the results calculated with release coefficients of its parent metal, it has been proved that the proposed correction method is more precise in residual stresses measurement of weld joints. The mathematical analysis and the results of verification tests have shown that this new method is reliable, and it will provide guidance in academic research and engineering application of the spherical indentation strain-gauge method.     

Numerical investigation on radial impeller induced vortex rope in draft tube under partial load conditions

The vortex rope phenomena appearing in a draft tube under partial load operations were investigated. The efforts were carried out by using a homogeneous equilibrium vapor-liquid two-phase model and a blending SST turbulence model, and the former takes account of the weak compressibility of mixed media. The vortex rope phenomena were induced by the radial impeller and three kinds of flow regimes were illustrated. Vortex rope was visible at some conditions and it displayed various patterns, while no obvious vortex rope was found under some conditions. It was found that the plant cavitation number determines that whether the vortex rope was visible or invisible. An interesting profile was that a secondary vortex filament ring entwining the vortex rope appeared clearly near the design condition. Both flow structures and pressure characteristics were studied to depict the vortex mechanism and the characteristics of the η-σ curves. Further analysis found that the shedding frequency of the vortex ring is similar with that of the precession movement.     

Generalized model of vibro-isolation systems with seated human body for single-axis transmission of vibration

A generalized model of vibration isolation systems with a human body in a seated position is developed. The system dynamics is modeled for a single-axis transmission of vibration, that is, longitudinal x, lateral y and vertical z, to limit whole-body vibration exposure. The developed model can be successfully employed to describe the propagation of vibration waves from the source to a vibrating object. The proposed mathematical model can be widely used to protect a human body against vibration.