Applied Mechanics and Materials Vols. 284-287

Abstract: In this paper, a robust adaptive control strategy is proposed for trajectory control of an omnidirectional vehicle with three omni-wheels, which have the ability to move simultaneously with independently rotational motion. Actuators of the omnidirectional vehicle contain uncertainties and their parameters are unknown. By the Lyapunov stability, the asymptotic tracking performance can be assured. The proposed control scheme is demonstrated by actual tracking experiments using the omnidirectional vehicle system. Experimental results showed promising tracking performance for the proposed method as compared to traditional sliding mode controller.

Abstract: Generally, the NC format is the description for the positioning and/or the movement of its linear and rotary axes. As the multi-axis machine tools have a variety of configurations, their NC codes are not exchangeable. This issue leads to some inconvenience and confounding in the manufacturing processing schedule. Furthermore, when the specifications of tool such as length, diameter or shape are reset, the NC program must be regenerated accordingly. That is to say, the exchangeability of NC program among different five-axis machine configurations is an important issue in making better usage of industrial five-axis machine tools for efficient applications. An APT program records the tool path, tool vector and cutting information, etc. In particular, the recent development of APT format can provide the capability recording the motion posture of the tool such as the tool orientation, the position and its normal vector of the tool contact point. Therefore, it can solve the problems of the exchangeability for the different machine tools as well as the online resetting of tool specifications, even the tool posture. In this paper, a new method was proposed to interpret the APT code into tool movement data including toolpath, location, tool orientation, the contact point and its contact vector, etc., which can be applied to the conversion of different NC codes, or be connected to the controller of the machine tool so as to proceed the interpolation calculation for directly machining control. Moreover, the application scope can be extended to the verification of machining and to drive a virtual machine tool for previewing. Since the APT format varies according to different CAD/CAM systems, a common intermediate interchange standard (CMIS) was proposed, designed and verified in this paper as a feasible solution for the exchangeability of different APT formats. The process of the proposed method includes interpreting a variety of APT program into a common standard format, and then transforming this intermediate standard code into various NC programs for the corresponding machine configurations. An example was used to demonstrate how to convert an APT generated by CATIA software into intermediate code for a Table-Table five-axis machine tool with two rotary axes attached on table (XYZAC configuration). As the APT contains the definition of inclined plane, so the homogeneous coordinate transformation was adopted to transform the coordinate system of the inclined plane into the work coordinate system; it was further transformed into the corresponding NC program via an inverse kinematics transformation. This example has shown the feasibility of the method proposed. Moreover, the research can be applied not only to the exchangeability of different APT format but also to the other related applications such as the verification of machining error and the drive of virtual machine tool.

Abstract: Strong coupling is one of the prominent features of the general parallel mechanisms(Par. Mec.), which has led to difficulty in the trajectory planning and precision control. To solve this problem, the designing of motion decoupled parallel mechanisms(Dec. Par. Mec.) has become a hot topic. This paper, based on the work achieved in our pre-papers, is to make an improvement on the criterion for a branch type synthesis of the rotational decoupled parallel mechanisms(Rot. Dec. Par. Mec.), which ensures the decoupling of the rotations in each limb. This paper focuses on a type synthesis of the decoupled parallel mechanisms with two degree of freedoms (DOFs). Decoupled parallel manipulators with two parallel kinematic chains, one of which is of type 2R(R represents rotation), are taken into consideration in this paper. A large number of novel decoupled architectures are already obtained, some of which have got an application for a China Patent. What has been done in this paper is carried out by means of the screw theory, which has effectively avoided complex equations by synthesis.

Abstract: This paper describes the application of system identification techniques and robust control strategies to a pneumatic muscle actuator system. Due to the inherent nonlinear and time-varying characteristics of this system, it is difficult to achieve excellent performance using conventional control methods. Therefore, we apply identification techniques to model the system as linear transfer functions and regard the un-modeled dynamics as system uncertainties. Because robust control is well-known for its capability in dealing with system uncertainties, we then apply robust control strategies to guarantee system stability and performance for the system. This work is carried out in three parts. First, the pneumatic muscle actuator system was modeled as linear transfer functions. Second, robust control theorem were utilized to design a Hinf robust controller to deal with system uncertainties and performance requirements. Finally, the designed controller was implemented for experimental verifications and compared with a conventional PID controller. From the experimental results, the proposed Hinf robust controller is deemed effective.

Abstract: A novel shuttling polymerase chain reaction (PCR) system is assembled to make temperature uniform in the reaction chamber. The chamber is oscillated by a servo motor and contacted with three different isothermal zones to complete several thermal cycles. The home-made computer code is utilized to investigate the influences of operational parameters on the temperature inside the chamber. Numerical results show that the contact resistances between the heating blocks and the reaction chamber dominate the temperatures inside the PCR chamber. In this work a PCR system that is composed of the PID controller, the moving stage, three aluminum blocks for three different isothermal zones and a reaction chamber is also developed. Experimental results demonstrated that the stability of this shuttling PCR system is confirmed. And results show that DNA templates provided with the yT&A® cloning vector are amplified successfully in this PCR system.

Abstract: A brake pedal simulator in a Brake-by-Wire system is studied for fault tolerant control of the brake pedal signals. This study is conducted for the pedal simulator installed with a sensor that generates two analogue signals. Several realistic fault modes recognized by automotive experts have been analyzed. To solve the fault modes, we propose a fault tolerant output selector that can handle transient, intermittent, or permanent faults. The fault tolerant output selector, based on a fault detection algorithm, uses the BLS(brake light switch) signal and the Acc(acceleration pedal) signal to find faults and isolate them. To confirm the system performance, the fault modes were simulated. The result showed the reliability and safety of the pedal simulator for dealing with unexpected faults.

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 difficulties in kinematics and dynamic analysis, the development of control system, and the trajectory planning. Therefore the research on the decoupled parallel mechanism becomes one of the hot of the mechanism fields. While whether the parallel mechanism can realize decouple is the premise for synthesis and analysis of the parallel mechanism. Based on screw theory, the existence of the three degrees of freedom (3-DoF) rotational fully-decoupled parallel mechanism is distinguished. Then taking the 6-PUS/UPU parallel mechanism as example, the rotation angles of the moving platform are measured, which is verified the impossibility of the 3-DoF rotation decoupling. The contents of this paper should possess theoretical significance for the innovative configuration synthesis and structure design of rotational decoupled parallel mechanism.

Abstract: INS/GNSS integration scheme can overcome the shortcoming of GNSS or INS alone to provide superior performance. The position and velocity from GNSS is an excellent external aid to update the INS with improving its long-term accuracy. Adaptive Kalman Filter (AKF) is based on the maximum likelihood criterion for choosing the most appropriate weight and thus to adjust Kalman gain factors online. The conventional Extended Kalman Filter (EKF) implementation suffers uncertain results while the update measurement covariance matrix R does not meet the case. The primary advantage of AKF is that the filter has less relationship with the priori statistical information because the R varies with time. The innovation sequence is used to derive the measurement weights through the measurement covariance matrices, innovation-based adaptive estimation (IAE) in this study. There are two non-holonomic constraints (NHC) available for land vehicle navigation. Land vehicles will not jump off or slid on the ground under normal condition. Using these constraints, the velocity of the vehicle in the plane perpendicular to the forward direction is almost zero. EKF and AKF based tightly-coupled scheme with NHC are implemented in the study. To validate the performance of EKF and AKF based tightly-coupled INS/GNSS integration scheme with NHC, field scenarios were conducted in the downtown area of Tainan city. The test platform was mounted on the top of a land vehicle and the data fusion of INS/GNSS/NHC can be used as stand-alone positioning tool during GNSS outages of over 1 minute. The preliminary results presented in this study illustrated that AKF based tightly-coupled INS/GNSS integration scheme can provide more stable solutions combined with NHC during GNSS outages of over 1 minute. Generally speaking, the improvement ratio in term of 3D positioning of proposed algorithm reach 40% compared to EKF based tightly-coupled INS/GNSS integration scheme.

Abstract: The air squeeze film damping effect on the dynamic responses of clamped micro- electromechanical resonators is investigated in this study. A dynamic model for a clamped micro- electromechanical resonator with the damping consideration is derived using Lagrange’s equation. The corresponding resonator eigen solutions are formulated and solved by employing the assumed-mode method. The effect of different parameters; i.e. the resonator size, ambient temperature and pressure on the squeeze film damping characteristics were simulated and investigated. The results indicate that the squeeze film damping effect may significantly affect the dynamic responses of micro-scale electromechanical resonator.

Abstract: The aim of this study is to derive the governing equation of an electrostatically actuated micro system by use of the Hamilton principle, and then the natural frequencies of a micro fixed-fixed beam are derived as the solutions to a boundary value problem with prescribed boundary conditions through the differential transformation method (D.T.M.). The differential transformation employed is a transformed function based on the Taylor series that is effective in solving nonlinear problems with fast convergence. The numerical results of the calculated natural frequencies are compared with the analytical data and were found to be in good agreement. Hence, the differential transformation method is one of the most efficient methods of simulating the electrostatic behavior of a micro-structure system, and it has a great potential for use in the analysis of the micro fixed-fixed beam.