Zhaoheng Liu

Reliability optimization using multiobjective ant colony system approaches

The multiobjective ant colony system (ACS) meta-heuristic has been developed to provide solutions for the reliability optimization problem of series-parallel systems. This type of problems involves selection of components with multiple choices and redundancy levels that produce maximum benefits, and is subject to the cost and weight constraints at the system level. These are very common and realistic problems encountered in conceptual design of many engineering systems. It is becoming increasingly important to develop efficient solutions to these problems because many mechanical and electrical systems are becoming more complex, even as development schedules get shorter and reliability requirements become very stringent. The multiobjective ACS algorithm offers distinct advantages to these problems compared with alternative optimization methods, and can be applied to a more diverse problem domain with respect to the type or size of the problems. Through the combination of probabilistic search, multiobjective formulation of local moves and the dynamic penalty method, the multiobjective ACSRAP, allows us to obtain an optimal design solution very frequently and more quickly than with some other heuristic approaches. The proposed algorithm was successfully applied to an engineering design problem of gearbox with multiple stages.

Understanding and modelling the torsional stiffness of harmonic drives through finite-element method

Abstract Torsional stiffness or rigidity is a crucial characteristic in the design of transmission devices, including harmonic drives (HDs). Among the various design aspects constituting a reduction mechanism in robotic systems, torsional stiffness is an important factor for positioning accuracy and control issues. One of the major advantages of HDs is their capacity to present a high reduction ratio while maintaining a small hardware size. However, manufacturing these drives remains a complex and costly process due to the high precision of its machined components; as a result, the use of such drives is still limited only to high-end mechanical products and technologies. Given these costs, numerical analysis becomes an effective alternative for obtaining valuable data through simulations, without the need for prototypes. This article presents a finite-element model to reproduce the behaviour of the torsional stiffness of an HD. The numerical model allows an evaluation of the effects of various geometrical parameters on the torsional stiffness of the HD. The numerical model of the HD can be used for optimization purposes, i.e. to develop an HD with a high torque capacity combined with a high-rated lifespan.

Global bifurcation analysis of a nonlinear road vehicle system

This paper investigates the global stability behavior present near a bifurcation point of a nonlinear road vehicle system. The nonlinear behavior of the system is determined by reducing its dimensions according to the center manifold theory applied to a nongeneric case. A generalized Hopf bifurcation is analyzed by unfolding the limit cycle mean amplitude equation into a two-parameter space. The numerical application of the analytical framework demonstrates the coexistence of two limit cycles for certain ranges of physical and driver parameter values.

A unified framework for dynamics and Lyapunov stability of holonomically constrained rigid bodies

A new dynamic model for interconnected rigid bodies is proposed here. The model formulation makes it possible to treat any physical system with finite number of degrees of freedom in a unified framework. This new model is a non minimal realization of the system dynamics since it contains more state variables than is needed. A useful discussion shows how the dimension of the state of this model can be reduced by eliminating the redundancy in the equations of motion, thus obtaining the minimal realization of the system dynamics. With this formulation, we can for the first time explicitly determine the equations of the constraints between different elements of the mechanical system corresponding to the interconnected rigid bodies in question. One of the advantageous coming with this model is that we can use it to demonstrate that Lyapunov stability and control structure for the constrained system can be deducted by projection in the submanifold of movement from appropriate Lyapunov stability and stabilizing control of the corresponding unconstrained system. This procedure is tested by some simulations using the model of two-link planar robot

A comparison of two control methods for vehicle stability control by direct yaw moment

This paper proposes a comparison study of two vehicle stability control methods by direct yaw-moment control (DYC): a PID and a sliding controller. For the purpose of this study, control systems are based solely on vehicle side-slip angle state feedback and the lateral dynamics of the 2 DOF vehicle model are used to establish the desired response. Close-loop dynamics of the PID controller are determined with the pole placement method, and an anti-windup strategy is adopted to respond to the tire’s nonlinear characteristics. The comparison study was performed by computer simulations with a 14 DOF nonlinear vehicle model validated with experimental data. The controllers are evaluated for typical severe manoeuvres on low friction road surfaces. It is found that despite their fundamental differences, the control methods provide comparable performances for the cases studied.

Characterization, modeling and vibration control of a flexible joint for a robotic system

This paper presents the experimental characterization and vibration control of a flexible robotic system. For this work, a test bench was built to characterize the harmonic drive (HD) and joint components, while control algorithms were designed and compared to minimize vibration. Encoder accuracy was critical since the difference in the measurements between two encoders was used to evaluate the vibrational behavior of the test set-up. Therefore, a laser tracker was used to characterize the error of the output encoder. Real-time compensation using this technique achieved an angular position accuracy of 50 µrad. Four rosette strain gauges were fixed to the HD’s flexible spline to determine its torsion. To reduce torque ripple, a real-time correcting function was applied. It was thus possible to reduce the error to 0.3% of the full-scale error. Two vibration control strategies were developed, namely, singular perturbation and feed-forward control. Simulation results showed that both control strategies greatly reduced vibration response compared to a common rigid control. However, test results showed that good vibration control could only be achieved with the feed-forward approach: the singular perturbation technique generated too much torque ripple to the motor. A feed-forward controller can quickly stabilize the link, achieving the same settling time as with the rigid control algorithm.

IMPACT-CUTTING AND REGENERATIVE CHATTER IN ROBOTIC GRINDING

This paper presents a study on the dynamic behavior of a flexible robot performing a grinding process. The ultimate goal is to understand whether regenerative chatter is the source of divergent vibrations observed when machining with a compliant robot. An important nonlinear characteristic of the dynamic response of the system is found and is included in the conventional approach to chatter analysis. Robotic machining is represented by a SDOF model. The steady-state response of this model to external forces is found to be repetitive impacts. The existence of this process is justified theoretically without invoking any self-exciting regenerative effect. High-speed camera observations during operation confirm the existence of such a vibro-impact process. To investigate stability, the robotic holder’s dynamic equation is excited by a forcing function representing impulse forces during cutting impacts. Response to regenerative impact cutting forces is simulated. Zones of stable/unstable cutting were identified. This suggests that the regenerative mechanism may explain the onset of divergent vibrations in the application under study. Established regenerative chatter theory predicts an extensive stable cutting zone for a flexible robotic holder. A regenerative mechanism then would not be a probable source of instability. Considering that conventional analysis is based on linear responses, the existence of vibro-impact nonlinearity is illustrated and its effect is analyzed. This results in a more realistic stable cutting zone, better matching our experience.Copyright

Fault feature extraction and classification based on WPT and SVD: Application to element bearings with artificially created faults under variable conditions:

Achieving a precise fault diagnosis for rolling bearings under variable conditions is a problematic challenge. In order to enhance the classification and achieves a higher precision for diagnosing rolling bearing degradation, a hybrid method is proposed. The method combines wavelet packet transform, singular value decomposition and support vector machine. The first step of the method is the decomposition of the signal using wavelet packet transform and then instantaneous amplitudes and energy are computed for each component. The Second step is to apply the singular value decomposition to the matrix constructed by the instantaneous amplitudes and energy in order to reduce the matrix dimension and obtaining the fault feature unaffected by the operating condition. The features extracted by singular value decomposition are then used as an input to the support vector machine in order to recognize the fault mode of rolling bearings. The method is applied to a bearing with faults created using electro-discharge machining under laboratory conditions. Test results show that the proposed methodology is effective to classify rolling bearing faults with high accuracy.

Modeling and parameter identification of a tractor semitrailer/driver closed-loop system using the Simulated Annealing (SA) optimization approach

Identifying the dynamics and parameters for an articulated heavy vehicle/driver (tractor semitrailer/driver) system is presented in this paper. The dynamic behaviour of the vehicle system is studied by setting up a closed-loop control model of a vehicle/driver system that included a three degree of freedom (3-DOF) tractor semitrailer lateral dynamic model and a driver steering model. It is assumed that the driver steering control model responds to the vehicle state vector following a time delay. The inherent driver steering control parameters to be identified are the state vectors coefficients, here defined as an optimal control vector. The Simulated Annealing (SA) optimization method is used to search for this control vector. The results show that this method (SA) gives a good convergence rate and robustness for the closed-loop vehicle/driver system under study. Various simulations and sensitivity analysis are also performed and presented.

Road vehicle suspension and performance evaluation using a two-dimensional vehicle model

A two-dimensional model with eight Degrees Of Freedom (DOF) is developed to simulate and animate the response of a vehicle to different road, traction, braking and wind conditions. An algorithm is developed to compute the normal force acting on each tyre depending on its position with the road, allowing multiple contact regions along the tyre circumference. The vehicle is animated in real-time in a 3D VRML environment to visualise its behaviour. A model validation is conducted by comparing a 0-100 km/h acceleration run against a Honda Accord car equipped with an accelerometer and engine rpm recorder.