Experimental control of a single-link flexible robot arm using energy shaping

Flexible-link robotic manipulators are mechanical devices whose control can be rather challenging, among other reasons because of their intrinsic under-actuated nature. This paper presents the application of an energy-based control design methodology (the so-called IDA-PBC, interconnection and damping assignment passivity-based control) to a single-link flexible robotic arm. It is shown that the method is well suited to handle this kind of under-actuated device not only from a theoretical viewpoint but also in practice. A Lyapunov analysis of the closed-loop system stability is given and the design performance is illustrated by means of a set of simulations and laboratory control experiments, comparing the results with those obtained using conventional control schemes for mechanical manipulators.     

Vibration control of a two-link flexible robot arm

Analysis and experimentation is described for a two-link apparatus in which both members are very flexible. Attention is focused on endpoint position control for point-to-point movements, assuming a fixed reference frame for the base, with two rotary joints. Each link is instrumented with acceleration sensing and is driven by a separate motor equipped with velocity and position sensing. The control perspective adopted is to implement a two-stage control strategy in which the vibration control problem for fine-motion endpoint positioning is considered separate from the gross-motion, large-angle slew problem. In the first stage the control law shapes the actuator inputsfor the large-angle movement in such a way that minimal energy is injected into the flexible modes, while in the second phase an endpoint acceleration feedback scheme is employed in independent joint controlfor vibration suppression at the link endpoints.     

Regulation of a one-link flexible robot arm using sliding-mode technique

This paper is concerned with the application of sliding control’ to the regulation of a one-link flexible robot arm, with an arbitrary number of flexible modes. Slidingmode technique is applied to achieve a robust feedback linearization of the highly non-linear dynamic equation of the arm. Pole placement is then used to attain good dynamic response. An example is given to demonstrate the potential of the sliding method. This work serves as a first step towards a practical solution in the feedback control of flexible arms using the sliding technique.     

Experimental two-axis vibration suppression and control of a flexible robot arm

This article focuses on the implementation of a dual-mode controller for the maneuver of a two-axis flexible robotic arm. The joint angle trajectory tracking is accomplished by proportional and derivative and feedforward controllers. Based on the pole placement technique, a linear stabilizer is designed for elastic mode stabilization in the plane perpendicular to each joint axis. The stabilizer is switched on when the trajectory reaches the vicinity of the terminal state. The effect of switching time of the stabilizer and varying payload on arm vibration are investigated. With the proposed control system, accurate joint angle tracking and elastic mode stabilization can be accomplished. © 1993 John Wiley & Sons, Inc.     

Adaptive control of a robot arm using driver programs

Most successful state-of-the-art robotic manipulators have the characteristic of producing precise, fast, smooth and reproducible movements. Their drawback is that they tend to have a limited repertoire which can only be extended by costly inverse kinematics calculations or direct teach-in sessions. The goal of our project is to develop a flexible open world robot, which adapts to its task and environment progressively and in an iterative manner. It begins carefully and slowly, with small jagged movements, but, after repetition, reaches an acceptable level of smoothness and execution speed. It can be placed in new surroundings with new task definitions, requiring only a further practice time before becoming expert. Similarities from other sub-tasks are recognized and may be transferred to the new domains. The basic constituents of the adaptive robot are simple iterative inverse kinematics and driver programs. In this paper the idea of driver programs for robot arm control is introduced and their properties investigated.     

Distributed machining control and monitoring using smart sensors/actuators

The study of smart sensors and actuators led, during the past few years, to the development of facilities which improve traditional sensors and actuators in a necessary way to automate production systems. In another context, many studies have been carried out aimed at defining a decisional structure for production activity control and the increasing need of reactivity leads to the autonomization of decisional levels close to the operational system. We study in this paper the natural convergence between these two approaches and we propose an integration architecture, dealing with machine tool and machining control, that enables the exploitation of distributed smart sensors and actuators in the decisional system.     

On the use of accelerometers in iterative learning control of a flexible robot arm

Iterative learning control (ILC) is applied to a robot arm with joint flexibility. The ILC algorithm uses an estimate of the arm angle, where the estimate is computed using measurements of the motor angle and the arm angular acceleration. The design of the ILC algorithm is evaluated experimentally on a laboratory scale robot arm with good results.     

Learning local linear Jacobians for flexible and adaptive robot arm control

Successful planning and control of robots strongly depends on the quality of kinematic models, which define mappings between configuration space (e.g. joint angles) and task space (e.g. Cartesian coordinates of the end effector). Often these models are predefined, in which case, for example, unforeseen bodily changes may result in unpredictable behavior. We are interested in a learning approach that can adapt to such changes—be they due to motor or sensory failures, or also due to the flexible extension of the robot body by, for example, the usage of tools. We focus on learning locally linear forward velocity kinematics models by means of the neuro-evolution approach XCSF. The algorithm learns self-supervised, executing movements autonomously by means of goal-babbling. It preserves actuator redundancies, which can be exploited during movement execution to fulfill current task constraints. For detailed evaluation purposes, we study the performance of XCSF when learning to control an anthropomorphic seven degrees of freedom arm in simulation. We show that XCSF can learn large forward velocity kinematic mappings autonomously and rather independently of the task space representation provided. The resulting mapping is highly suitable to resolve redundancies on the fly during inverse, goal-directed control.     

The development of a fault-tolerant control approach and its implementation on a flexible arm robot

This article presents a robust sensor fault-tolerant control (FTC) scheme and its implementation on a flexible arm robot. Sensor faults affect the system's performance in the closed loop when the faulty sensor readings are used to generate the control input. In this article, the non-faulty sensors are used to reconstruct the faults on the potentially faulty sensors. The reconstruction is subtracted from the faulty sensors to generate a 'virtual sensor' which (instead of the normally used faulty sensor output) is then used to generate the control input. A design method is also presented in which the virtual sensor is made insensitive to any system uncertainties (which could corrupt the fault reconstruction) that cannot fit into the framework of the model used. Two fault conditions are tested: total failure and incipient faults. Then the scheme robustness is tested and evaluated through its implementation on two flexible arm systems, one with a flexible joint and the other with a flexible link. Excellent results have been obtained for both cases (joint and link); the FTC scheme produced system performance almost identical to the fault-free scenario, whilst providing an indication that a fault is present, even for simultaneous faults.     

Kinematics of an infinitely flexible robot arm

This article describes a research effort to develop a command and control algorithm for a proposed flexible robot arm. This robot arm, unlike previous arms, is assumed to be extremely flexible, possessing a large number of degrees of freedom and functioning as a “tentacle.” Algorithms for commanding smooth motion of this arm in the presence of obstacles are developed, including both forward and inverse kinematics. The approach is based on the use of Catmull-Rom splines and local radius of curvature commands to discrete actuators along the arm's length. Several example trajectories are presented and explained. According to the proposed method, a spline curve algorithm is successfully applied for the configuration of the flexible robot and the accuracy and the collision avoidability of the flexible arm are verified through simulation.     

Minimum-time control of a two-link robot arm

Minimum time paths for a two-link robot arm are determined so that the tip moves a specified distance, starting from rest and ending at rest. Two torquers are used, one at the shoulder and one at the elbow, each having bounded torque. The optimal locations of the two end points in the robot coordinate system are determined as part of the solution. The paths are found to be “bang-bang” in both controls. For distances that are small compared to the reach of the arm, the minimum time paths are asymmetric, changing to symmetric paths when the distance is just slightly less than the maximum reach of the arm. A new gradient algorithm for systems with multiple bounded controls was developed to solve the problem.     

Model-free controller design for a single-link flexible smart materials robot

In this paper, controller design is investigated for a single-link flexible smart materials robot which combines the advantages of both flexible link robots and piezoelectric materials. To avoid the drawbacks resulting from model uncertainties and/or model truncations, model-free controllers (both decentralized and centralized) are proposed for tip regulation and residual vibration suppression. In contrast to traditional model-based methods, the controllers presented in the paper are derived from the basic energy-work relationship and can guarantee the asymptotic stability of the damped truncated system with arbitrarily any finite number of flexible modes. Furthermore, the controllers are easily implementable because all the signals can be chosen to be readily measurable. Simulations are carried out to show the effectiveness of the approach presented.     

Optimal robot arm control using the minimum variance model

Models of human movement from computational neuroscience provide a starting point for building a system that can produce flexible adaptive movement on a robot. There have been many computational models of human upper limb movement put forward, each attempting to explain one or more of the stereotypical features that characterize such movements. While these models successfully capture some of the features of human movement, they often lack a compelling biological basis for the criteria they choose to optimize. One that does provide such a basis is the minimum variance model (and its extension—task optimization in the presence of signal‐dependent noise). Here, the variance of the hand position at the end of a movement is minimized, given that the control signals on the arm's actuators are subject to random noise with zero mean and variance proportional to the amplitude of the signal. Since large control signals, required to move fast, would have higher amplitude noise, the speed‐accuracy trade‐off emerges as a direct result of the optimization process. We chose to implement a version of this model that would be suitable for the control of a robot arm, using an optimal control scheme based on the discrete‐time linear quadratic regulator. This implementation allowed us to examine the applicability of the minimum variance model to producing humanlike movement. In this paper, we describe our implementation of the minimum variance model, both for point‐to‐point reaching movements and for more complex trajectories involving via points. We also evaluate its performance in producing humanlike movement and show its advantages over other optimization based models (the well‐known minimum jerk and minimum torque‐change models) for the control of a robot arm. © 2005 Wiley Periodicals, Inc.     

Using FORTH to control a robot arm

The characteristics of the Cyber 3 robot arm are outlined, with a brief history of programming languages used to control it. The advantages of FORTH over its rivals are given. A program sequence for a robot to reorient an object is given.     

Tracking control of a flexible robot link

The formulation of an appropriate hybrid lumped/distributed model of a flexible robot link makes it possible to formulate and study tracking problems without the necessity for a priori approximations in the flexibility model for the link. The use of secondary support beam and active tip control provides perfect tracking with hypothetical exact measurements, and tracking to arbitrary accuracy with use of a variant of the acceleration feedback technique     

Simultaneous optimization of a two‐link flexible robot arm

Integrated structure–control design of a two‐link flexible robot arm is investigated in this article. The whole arm consists of two flexible links, a fixed joint, a moving joint, and a tip load. The arm is driven by the torque motors at the two joints to reach predefined tip positions and to suppress residual flexural vibrations. The links of the arm are modeled using the finite element method and the cross‐sectional dimensions of the beam elements are used as structural design variables. A sliding mode controller and a linear stabilizer are used to regulate the arm position. The structural and the control parameters of the whole arm system are optimized simultaneously using a genetic algorithm and the performance is compared with that of an arm with uniform links and an optimized control system. The simulation result shows that faster regulation and less weight of the arm system can be achieved by simultaneous optimization. © 2001 John Wiley & Sons, Inc.     

The path precompensation method for flexible arm robot

This paper constructed a closed loop path precompensation method for a flexible arm robot. A torque computation method taking care of the elastic arm deformation was first proposed and discussed. A concept of partial deformation compensation was subsequently proposed to improve the torque profiles and the trajectory fidelity. The advantage of this concept was first shown by examples of planar trajectory. After the construction of the closed-loop path precompensation method for a flexible arm, the torque method and partial deformation compensation were incorporated to track the spatial trajectory. Numerical simulations were given to show the usefulness of the proposed concept and method.     

融合运动想象脑电与眼电信号的机械臂控制系统开发

脑机接口(brain computer interface, BCI)旨在通过脑电信号与外部设备通信,以实现对外部设备的控制。针对目前脑机接口系统中混合多种复杂生理电信号,并且输出控制指令较少的问题,本文提出融合运动想象(motor imagery, MI)脑电与眼电信号方法扩充控制指令的轻量级机械臂控制系统。该系统分阶段融合脑电和眼电信号两种生物信号,使用双次眼电作为任务开关,运动想象脑电信号控制机械臂运动,单次眼电控制阶段切换,实现了二分类运动想象生成多种控制指令,完成了对机械臂的连续控制。其中运动想象脑电信号使用提升小波变换(lifting wavelet transform, LWT)和共空间模式(common spatial pattern, CSP)结合的方法提取特征,并采用支持向量机(support vector machines, SVM)进行分类;眼电信号通过分析无意识眼电和有意识眼电的峰值来设置阈值进行区分。为了验证系统的可行性,设计了一项脑控机械臂自主服药实验,通过在线实验测试,被试通过使用脑电信号和眼电信号实现了机械臂控制,并完成了服药流程,有利于进一步推广脑机接口技术的实际应用。     

Modeling and feedback control of a flexible arm

When a flexible arm is rotated by a motor about an axis through the arm's fixed end, transverse vibration may occur. The motor torque should be controlled in such a way that the motor rotates by a specified angle, while simultaneously stabilizing vibration of the flexible arm so that it is arrested as soon as possible at the end of rotation. In this paper, we first derive a partial differential equation and a set of boundary conditions governing the vibration. Then, a feedback control system which incorporates a dynamic compensator is designed using sensor outputs. A set of experiments has been constructed to demonstrate control strategies for a flexible arm, where a strain gage was used as a vibration sensor and a microcomputer was equipped as a controller. Several satisfactory experimental results are shown.     

Infinite dimensional observer for a flexible robot arm with a tip load

This note addresses observer design for a flexible robot arm with a tip load. The robot arm is modeled as an Euler–Bernoulli beam. The beam is clamped to a motor at one end and attached to a force actuator at the other. Based on boundary measurements, an exponentially stable observer is proposed. The existence, uniqueness, and stability of solutions of the observer are proven using semigroup theory. The results are illustrated by simulation. Copyright © 2008 John Wiley and Sons Asia Pte Ltd and Chinese Automatic Control Society