Local joint information based active fault tolerant control for reconfigurable manipulator

This paper is concerned with the active fault tolerant control problem for reconfigurable manipulator actuator based on local joint information. It is considered that the entire reconfigurable manipulator system consists of a couple of independent joint modules as subsystems, which are controlled using unified radial basis function neural network adaptive algorithm using local joint information when actuators are fault free. For the subsystem in actuator fault situation, fault detection is achieved through comparing the user defined threshold to the residual between actual velocity value and nonlinear velocity observation value. The unknown input state observer is exploited for fault identification. Based on the information aforementioned, a compensation term is added to the proposed control algorithm for switching to realize active fault tolerant control when subsystem in fault. The advantages of the presented scheme are that unlike the complex control structure in centralized control, this scheme possesses simple control structure, as well as could isolate and tolerant the fault in subsystem. Furthermore, it can be easily applied to different configurations without any parameters modification. It means that the local fault could not affect the joint in normal situation. In order to demonstrate the effectiveness of the proposed method, two different 2-DOF reconfigurable manipulators are employed for simulation.     

Design of a single motor,tendon driven redundant manipulator with reduced driving joint torques

Lightweight manipulator design is one of the diverse and rich research fields in the area of robotics. It has become increasingly important to develop manipulators with reduced cost, high energy e?ciency and with low inertia. There are numerous design concepts proposed in the past decades such as designing lightweight joints or locating the actuators at the base. Reduction of number of actuators used has added advantages such as cost reduction, reduced power consumption, compact in design apart from reduction of weight. This paper presents a lightweight tendon drive redundant manipulator design with reduced joint torque using a single motor. The proposed design has reduced the number of actuators used. Thus the design is not only effective in reducing driving joint torques but also minimizes the number of actuators required and the power consumption. Driving joint torques computed for both conventional and for the proposed manipulator design highlight the significance of the proposed manipulator.     

刚弹耦合的机器人机构动力学方程及其解

本文给出了一般开链弹性机器人机构动力学方程。该方程是由关节广义坐标和杆件模态坐标联立的非线性微分方程组。对方程的刚性性质作了讨论,根据Rosenbrock的二级二阶半隐式Runge—Kutta公械,导出了一种求解弹性机器人机构动力学方程的算法。对一平面二杆操作手机构作了计算。对该例同时也用Gill方法作了计算,验证了上述算法的精度和效率。     

Controlling robot manipulators by dynamic programming

A certain number of considerations should be taken into account in the dynamic control of robot manipulators as highly complex non-linear systems. In this article, we provide a detailed presentation of the mechanical and electrical implications of robots equipped with DC motor actuators. This model takes into account all non-linear aspects of the system. Then, we develop computational algorithms for optimal control based on dynamic programming. The robot's trajectory must be predefined, but performance criteria and constraints applying to the system are not limited and we may adapt them freely to the robot and the task being studied. As an example, a manipulator arm with 3 degress of freedom is analyzed.     

带滑移铰空间机械臂运动规划的双向逼近方法

以双向逼近方法为基础,讨论了载体姿态与位置均不受控制的带滑移铰空间机器臂的运动规划问题．该方法利用系统的非完整动力学性质,仅通过对空间机器臂关节铰运动的控制,即可达到对载体姿态及机械臂关节位置的双重控制效果,从而减少了载体姿态控制燃料的消耗,有效延长了空间机械臂系统的使用寿命．系统数值仿真证明了该方法的有效性．     

On the Characterization of the Dynamic Performances of Planar Manipulators

The characterization of the dynamic performances of a manipulator is important both to compare different manipulators and to improve the dynamic performances of a manipulator during the design stage. In a previous paper the concepts of swiftness and of dynamic isotropy were used to characterize some dynamic performances of 3-dof manipulators. This paper analyzes the usefulness of these concepts for three-dof planar manipulators and shows that the concept of swiftness is still significant, whereas the concept of dynamic isotropy has no practical interest. Moreover, it introduces three new dynamic properties that are useful in the design of a 3-dof planar manipulator. Finally, it proposes some indices that measure both the swiftness and the three new dynamic properties and shows how to use them, both for evaluating the dynamic performances of a given 3-dof planar manipulator and for improving the dynamic performances during its design.     

Redundant actuators to achieve minimal vibration trajectory tracking of flexible multibodies: Theory and application

We address the problem of inverse dynamics for flexible multibodies, which arises, in trajectory tracking control of flexible multibodies such as space manipulators and articulated flexible structures. Previous research has resolved this trajectory tracking problem by computing the system inputs for feedforward control of actuators at the joints. Recently, the use of distributed actuators like electro-strictive actuators in flexible structures has introduced a new dimension to this trajectory tracking problem. In this paper we optimally utilize such actuators to aid joint actuators for tracking control, and introduce a new inverse dynamics scheme for simultaneously (1) tracking a prescribed trajectory and (2) minimizing ensuing elastic deflections. We apply this scheme for trajectory tracking of a two-link two-joint planar manipulator with joint motors and distributed electro-strictive actuators. Experimental results are presented to contrast our new scheme with other existing methods.     

Vibration analysis of a planar multilink manipulator using a reduced-order matrix formulation

The main scope of this paper is both to provide an efficient formulation for deriving natural frequencies and mode shapes of a multilink flexible manipulator in an arbitrary posture and discuss the importance of introducing axial deformations along with transversal ones in the same postures. The free vibration problem is solved through a global transfer matrix formulation, which is obtained by opportunely assembling the transfer matrices of links and joints associated with the desired configuration of the robot. The formulation has been chosen in order to keep the same matrix order of the problem, regardless of the number of links of the manipulator. Manipulator links are treated as Timoshenko beams by accounting for both axial loading capability and mass/inertial effects due to the joint actuators. Numerical comparisons between the solutions derived herein and those obtained through existing formulations highlight the importance of the proposed model within the frame of multilink flexible manipulators. A forced vibration analysis is finally presented for a two-link manipulator.     

Robust control of flexible-joint robots using voltage control strategy

So far, control of robot manipulators has frequently been developed based on the torque-control strategy. However, two drawbacks may occur. First, torque-control laws are inherently involved in complexity of the manipulator dynamics characterized by nonlinearity, largeness of model, coupling, uncertainty and joint flexibility. Second, actuator dynamics may be excluded from the controller design. The novelty of this paper is the use of voltage control strategy to develop robust tracking control of electrically driven flexible-joint robot manipulators. In addition, a novel method of uncertainty estimation is introduced to obtain the control law. The proposed control approach has important advantages over the torque-control approaches due to being free of manipulator dynamics. It is computationally simple, decoupled, well-behaved and has a fast response. The control design includes two interior loops; the inner loop controls the motor position and the outer loop controls the joint position. Stability analysis is presented and performance of the control system is evaluated. Effectiveness of the proposed control approach is demonstrated by simulations using a three-joint articulated flexible-joint robot driven by permanent magnet dc motors.     

A simple and quick control strategy for a class of first-order nonholonomic manipulator

This paper presents a simple and quick control strategy for a class of first-order nonholonomic manipulator: planar n-link manipulator with a passive first joint (no gravity). The control target is driving its endpoint from any initial position to any target position. First, we reduce the planar n-link manipulator to a planar three-link one by maintaining the states (angles and angular velocities) of n-3 active links in the initial value all the time. That is, we only adjust the states of the remaining two active links in the whole control process, and these two adjusted active links are chosen to guarantee that the target position is in the reachable region of the planar n-link manipulator by using the enumeration method. Then, we divide the whole control process into two stages for the reduced planar three-link manipulator. In each stage, the manipulator becomes a planar two-link one by maintaining the states of one adjusted active link to be the constant value. The state constraint existing between the passive first link and the adjusted active link is obtained by using the integral characteristics of a planar two-link manipulator. Meantime, the geometric constraint between the position of the endpoint and angles of all joints is obtained based on the homogeneous coordinate transformation method. According to the above two kinds of constraint, the target angles of the two adjusted active links are calculated by employing the particle swarm optimization algorithm. When the two adjusted active links are controlled to their target angles in turn, the control target of the planar n-link manipulator is completed. Finally, simulation results demonstrate that the proposed control strategy is valid and rapid.     

A New Parameter Design Method of a 6-DOF Parallel Motion Simulator for a Given Workspace

This paper introduces a new 9-DOF motion simulator that consists of a 3-DOF parallel manipulator and a 6-DOF parallel manipulator. For the 6-DOF manipulator, a new ‘3–3’-PSS parallel mechanism and a new parameter design method for a given workspace are presented. With the kinematic study of this parallel mechanism and the method of Lagrange multipliers, we have found several key points that represent the worst performance of the manipulator within the given workspace. When a position workspace is given, by checking the manipulator's performances at these key points, one can quickly find out whether the manipulator with certain parameters meets the requirements within this workspace. Furthermore, several figures that can find the appropriate parameters are plotted. This key point method can make the design of parameters much quicker and easier for manipulators.     

Experimental and Numerical Investigations of the Characteristics of Pulsed Thermal Actuators

The purpose of the study is to numerically, experimentally, and analytically investigate the characteristics of plasma pulsed thermal actuators (PT actuators) and to assess their possibilities in controlling flow around airfoils, wings, and configurations at large subsonic freestream velocities. For the PT actuators of the types considered the mathematical models adequately describing their effect on flow past bodies are developed. The characteristics of a prototype PT actuator are experimentally investigated on a specially developed rig. A new type of the PT actuator equippedwith a channel (PTC actuator) is proposed; it is designed to operate at a high pulse repetition frequency and at large flow velocities. Numerical investigations show that the PTC actuators are free of the essential and fundamental shortcoming of the PT actuators which consists in the working zone superheating at high pulse repetition frequencies.     

Global Bifurcation Analysis of a Controlled Underactuated Mechanical System

In this paper, bifurcation theory is employed to classify different dynamical behaviors arising in an underactuated mechanical system subject to bounded controls. The methodology is applied to an inertia wheel pendulum consisting of a simple pendulum with a rotating disk at the end. Restricting the magnitude of the control action places an important obstacle to the design of a continuous controller capable of swinging-up and stabilize the pendulum at the inverted position: the arm only can reach that position by means of oscillations of increasing amplitude. The controller is derived from a simple nonlinear state-feedback law, followed by a saturating device that limits the maximum amplitude of the control action applied to the system. This bound gives birth to a rich dynamical behavior, including pitchfork and Hopf bifurcations of equilibria, saddle-node bifurcations of periodic orbits, homoclinic and heteroclinic bifurcations. The global dynamics is analyzed in terms of certain control gains and a two-parameter bifurcation diagram is derived. It is shown that the dynamics on this bifurcation diagram is organized in a pair of codimension-two rotationally symmetric bifurcation points. Finally, it is found out that when the control gains lie on a certain region in the parameter space simultaneous stabilization of the upright position together with a large basin of attraction is obtained. Simulation results show that almost global stabilization of the system can be achieved.     

Study on electroelastic field concentration around the electrode tip in multilayer ferroelectric actuators of two designs and their optimizations

Distribution of electromechanical field near electrode tips is closely related to the reliability of ferroelectric multilayer actuators. In this paper, the deformation and stress concentrations around the electrode tip in two multilayer actuator designs, partially and fully cofired, are investigated by means of experimental measurement and numerical simulations. The digital speckle correlation method (DSCM) is used to measure the full displacement field near the electrode tip with the high spatial resolution. The paths of electric breakdown and cracks initiated from the edge of electrodes were observed. With the proposed Double Gibbs free energy criterion, a fully coupled nonlinear electromechanical finite element method based on domain-switching mechanisms is developed and the simulation results agree well with the experiments. It is found that the crack-like “defects” in the partially cofired layered actuators, i.e. the interlayer gaps filled with soft insulating wax, can significantly reduce the maximum tensile stress level compared with that in “perfect” fully cofired actuators, which implies that the partially cofired design is more reliable than the fully cofired one. Further optimization on geometrical dimension of actuators is also carried out.     

Modal analysis and dynamic responses of a rotating Cartesian manipulator with generic payload and asymmetric load

Abstract

Here, investigation to explore the effect of generic payload and externally applied asymmetric load on the calculation of modal parameters and dynamic performance of a rotating flexible manipulator under prismatic motion has been established. We thus have developed a dynamic model of a rotating Cartesian manipulator with a payload whose center of gravity doesn’t coincide with the point of attachment, to determine the modal parameters i.e., natural frequency and corresponding mode-shape. These modal parameters are then illustrated graphically upon varying parameters like offset parameters (i.e., offset mass, offset inertia, offset length), mass and stiffness of rotary actuator, and amplitude and frequency of asymmetric load. An investigation into the nonlinear dynamics of the system accounting of geometric nonlinearity has been executed while obtained results have been validated numerically within the permissible error at the assorted critical points in frequency characteristic curves. Current research further investigates the influences of offset parameters, mass and stiffness of the actuator, frequency and amplitude of axial force on the steady state responses for the primary and sub-harmonic resonance conditions to reveal the built-in saddle-node and pitchfork bifurcation due to which the system losses its structural stability. This work enables an insight into the modal characteristics and nonlinear behavior of a rotating-Cartesian manipulator with a generic payload under asymmetric axial force and prismatic motion.     

带空间机械臂航天器姿态运动规划的数值算法研究

自由漂浮空间机械臂系统在无外力矩作用时,系统的动量矩守恒而成为非完整系统。利用这一特性本文研究了自由漂浮空间机械臂系统的三维姿态运动规划问题。导出带空间机械臂的航天器三维姿态运动数学模型,将系统的非完整运动规划问题转化为非线性系统最优控制问题,在最优控制中利用小波逼近控制输入规律,提出基于遗传算法的最优控制数值算法。通过数值仿真,表明该方法对带空间机械臂航天器系统的非完整姿态运动规划是有效的。     

链状柔性多体机器人系统动力学研究

本文基于Ｊｏｕｒｄａｉｎ变分原理建立了具有链状拓扑结构柔性多体机器人系统动力学通用模型，用在一致质量有限单元法及正则模态分析基础上引入的模态坐标描述构件的弹性形，用独立坐标描述相邻板件间的大位移运动，每个铰容许１－６个自由度，组强非线性惯性耦合的封闭形式的系统动力学微分方程组，文末对单弹性臂和双弹性臂机器人操作手进行动力学仿真。     

Forward position analysis of 6-3 Linapod parallel manipulators

In this study closed-form solutions to the forward kinematic problems are obtained for a particular type of six degree-of-freedom parallel manipulator called 6-3 Linapod. The 6-3 Linapod parallel manipulators have a 6-3 PSS (or PUS) structure, and forward kinematic solutions are obtained by using the solution procedure for 6-3 SPS (or UPS) manipulators. In this procedure, a 6-3 Linapod is first transformed into its equivalent mechanism, namely an inclined 3RS manipulator, and then the condition that the three spherical joints on the moving platform form an equilateral triangle leads us to obtain three polynomial equations in three unknowns. These equations are solved by using Sylvester dialytic elimination method. Each set of real roots corresponds to a particular configuration of the manipulator. Solutions so obtained are verified by performing inverse position analysis. A method to identify configurations containing crossed links is presented in this study, which is based on the interpretation of link crossing as intersection of a link with a triangle, whose vertices are positions of joints on the corresponding links.     

Is Helmholtz Resonance a Problem for Micro-jet Actuators?

A theoretical analysis is described that determines the conditions for Helmholtz resonance for a popular class of self-contained microjet actuator used in both synthetic- and pressure-jump (pulse-jet) mode. It was previously shown that the conditions for Helmholtz resonance are identical to those for optimizing actuator performance for maximum mass flux. The methodology is described for numerical-simulation studies on how Helmholtz resonance affects the interaction of active and nominally inactive micro-jet actuators with a laminar boundary layer. Two sets of numerical simulations were carried out. The first set models the interaction of an active actuator with the boundary layer. These simulations confirm that our criterion for Helmholtz resonance is broadly correct. When it is satisfied we find that the actuator cannot be treated as a predetermined wall boundary condition because the interaction with the boundary layer changes the pressure difference across the exit orifice thereby affecting the outflow from the actuator. We further show that strong inflow cannot be avoided even when the actuator is used in pressure-jump mode. In the second set of simulations two-dimensional Tollmien–Schlichting waves, with frequency comparable with, but not particularly close to, the Helmholtz resonant frequency, are incident on a nominally inactive micro-jet actuator. The simulations show that under these circumstances the actuators act as strong sources of 3D Tollmien–Schlichting waves. It is surmised that in the real-life aeronautical applications with turbulent boundary layers broadband disturbances of the pressure field, including acoustic waves, would cause nominally inactive actuators, possibly including pulsed jets, to act as strong disturbance sources. Should this be true it would probably be disastrous for engineering applications of such massless microjet actuators for flow control.     

Development, modeling and deflection analysis of hybrid micro actuator with integrated thermal and piezoelectric actuation

Micro actuators are irreplaceable part of motion control in minimized systems. The current study presents an analytical model for a new Hybrid Thermo Piezoelectric micro actuator based on the combination of piezoelectric and thermal actuation mechanisms. The micro actuator structure is a double PZT cantilever beam consisting of two arms with different lengths. The presented micro actuator uses the structure of electrothermal micro actuator in which polysilicon material is replaced by PZT. Also the voltage and poling directions are considered in the lengthwise of PZT beams. As a result, the piezoelectric actuation mechanism is based on d ₃₃ strain coefficient. The tip deflection of micro actuator is obtained using Timoshenko beam theory. Analytical results are compared with FEM results along with other reported results in the literature. The effects of geometrical parameters and PZT material constants on actuator tip deflection are studied to provide an efficient optimization of HTP micro actuator.