Position control of a flexible gantry robot arm using smart material actuators

This article presents new feedback actuators that achieve accurate position control of a flexible gantry robot arm. Translational motion in the plane is generated by two dc motors and controlled by applying electric fields to electro‐rheological (ER) clutch actuators. On the other hand, during control action of translational motion, a flexible arm attached to the moving part produces undesirable oscillations due to its inherent flexibility. Oscillations are actively suppressed by employing feedback voltage to the piezoceramic actuator attached to the surface of the flexible arm. Consequently, an accurate position control at the end‐point of the flexible arm can be achieved. To accomplish this control goal, governing equations of the proposed system are derived and written as transfer functions. Transfer functions are used in design of a set of robust H_∞ controllers. Electric fields to be applied to ER clutch and control voltage for the piezoceramic actuator are determined via H_∞ methodology which is incorporated with classical loop shaping design technique. To evaluate effectiveness of the proposed control system, experiments for both regulating and tracking controls are undertaken. ©1999 John Wiley & Sons, Inc.     

A New Control Scheme of Single-link Flexible Manipulators Robust to Payload Changes

A new scheme is presented in this paper to control single-link flexiblemanipulators. The objective is to control the tip position of the flexiblearm in the presence of joint friction and payload changes. The controlscheme is based on two nested loops: an inner feedback loop to control themotor position which is robust to joint friction, and an outer loop tocontrol the tip position which is robust to payload changes. This outer loopis composed of a feedforward term and a feedback term. This results in asimple control law that needs minimal computing effort and, thus, can beused for real time control of flexible arms. The proposed method is generalin the sense that it can be applied to very different arm structures anddiverse sensor systems configurations. Results corresponding to twodifferent arm setups are presented.     

Adaptive control of flexible link manipulators using a pseudolink dynamic model

This paper presents a novel adaptive control scheme for a lightweight manipulator arm governed by electric motors. The controller design is based on the dynamic model of the arm in a quasi-static approximation which consists of the transports subsystem and the motor equations corrected for the elastic compliance of the plant. A passivity property of the flexible electromechanical system is established and an adaptive motor controller is developed which contains the rigid manipulator controller as a part. The motor controller updates all unknown rigid manipulator parameters as well as elastic parameters and ensures global asymptotic stability of the tracking errors with all signals in the system remaining bounded. Projecting of parameter estimates is used in the update law to avoid possible singularities when generating control input. Simulation results for a single-link elastic arm confirm the validity and demonstrate advantages of the proposed method.     

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.     

Robustness Comparative Study of Two Control Schemes for 3-DOF Flexible Manipulators

This article describes a comparative study of two control schemes designed for a new three-degree-of-freedom flexible arm. This arm has been built with light links, has most of its mass concentrated on the tip, and its special mechanical configuration uncouples tip motions in spherical coordinates. This special configuration simplifies the dynamic modeling and control of the arm. A compliance matrix is used to model the oscillations of the structure. A consequence of this simple dynamics is that minimum sensing effort is required (only direct motor and tip measurements), and the use of complex observers is avoided because the state of the system can be very easily obtained from these measurements; then its control becomes very simple. Two two-nested control loop schemes are used to control the tip position, by using a joint position and tip acceleration feedback, measured with accelerometers placed at the tip (first control scheme), or tip deflexions feedback, measured with strain gauges placed at the bars of the mechanism near the joints (second control scheme). Both control systems can be considered as equivalents when nominal payload is used for designing them. It can be proved that the use of strain gauges is more robust than the use of accelerometers as tip sensors if the tip mass differs from the nominal one. Simulated results are presented for both control schemes and different payload conditions. Comparative results between the controlled and non-controlled tip responses are also shown.     

The design and control of a robot finger for tactile sensing

This article describes the design and control of a lightweight robot finger intended for tactile sensing research. The finger is a three-link planar chain with the joints actuated through cables by two motors. Kinematic coupling of the three joints provides two degrees of freedom for finger tip manipulation, and a curling action of the finger for enclosing an object. Hall effect sensors in each joint provide position feedback, and strain gage sensors on each cable provide tension information. To minimize weight and power consumption, a high speed low torque motor together with a 172:1 speed reducer is used as the actuator. A force control loop around the motor speed reducer system reduces the effect of the friction inherent in the speed reducer. Flat mounting plates are provided on each link for special purpose grasping surfaces and sensors.     

Modified PID Control of a Single-Link Flexible Robot

The use of flexible links in robots has become very common in different engineering fields. The issue of position control for flexible link manipulators has gained a lot of attention. Using the vibration signal originating from the motion of the flexible-link robot is one of the important methods used in controlling the tip position of the single-link arms. Compared with the common methods for controlling the base of the flexible arm, vibration feedback can improve the use of the flexible-link robot systems. In this paper a modified PID control (MPID) is proposed which depends only on vibration feedback to improve the response of the flexible arm without the massive need for measurements. The arm moves horizontally by a DC motor on its base while a tip payload is attached to the other end. A simulation for the system with both PD controller and the proposed MPID controller is performed. An experimental validation for the control of the single-link flexible arm is shown. The robustness of the proposed controller is examined by changing the loading condition at the tip of the flexible arm. The response results for the single-link flexible arm are presented with both the PI and MPID controller used. A study of the stability of the proposed MPID is carried out.     

Asynchronous control of rotation and translation for a robot vehicle

Robot arms require an ‘arm controller’ to command joint motors to achieve a coordinated motion in an external Cartesian coordinate space. In the same sense, robot vehicles require a ‘vehicle controller’ to command the motors to achieve a coordinated motion specified in terms of an external Cartesian coordinate space. This paper presents the design of a general purpose vehicle controller.

The vehicle controller is designed as a three-layer structure. The top layer is an interpreter which assures a control protocol based on asynhcronous commands and independent control of orientation and forward displacement. The middle layer is a control loop which maintins an estimate of the vehicle's position and orientation, as well as their uncertainties. The control loop generates estimates and commands translation and rotation in terms of a ‘virtual vehicle’. The bottom layer is a translator between the ‘virtual vehicle’ and whatever physical vehicle on which the controller is implemented.     

Flexible-link robot arm control by a feedback linearization/singular perturbation approach

A nonlinear tracking controller for the link-tip positions and velocities of a multi-link flexible robot arm is designed that gives guaranteed performance. The controller has three parts: a model-based trajectory generator, an inner loop based on input-output feedback linearization, and an outer loop that stabilizes the internal dynamics (e.g., the flexible modes) using a singular perturbation design. We show how to stabilize the internal dynamics by selecting a physically meaningful modified performance output for tracking; this output is the slow portion of the link-tip motions. That is, the tracking requirement is relaxed so that the internal dynamics are stabilizable through a boundary layer correction that attenuates the flexible mode vibrations. © 1994 John Wiley & Sons, Inc.     

A new controller design for a flexible one-link manipulator

A novel controller design for noncollocated flexible one-link manipulator arm tip position control based on variable structure sliding mode control is presented. Using the assumed-mode method, the plant model is derived. The discontinuous control law based on the variable structure system theory for the noncollocated manipulator tip position control is then designed. The position state variables are obtained directly from the inversion of the output submatrix multiplied by the sensor measurements. The velocity state variables are estimated through decoupled estimators-a separate first-order observer for each of the system's modes under consideration. Different sampling periods are used for the estimator and the controller. The performance of the controller is evaluated through a series of simulations, followed by an analysis of the designed control system     

Intelligent desktop NC machine tool with compliant motion capability

In this paper, a new desktop NC machine tool with compliance control capability is presented for finishing metallic molds with small curved surface. The NC machine tool consists of three single-axis robots. Tools attached to the tip of the z-axis are ball-end abrasive tools. The control system of the NC machine tool is composed of a force feedback loop, position feedback loop and position feed-forward loop. The force feedback loop controls the polishing force consisting of tool contact force and kinetic friction force. The position feedback loop controls the position in pick feed direction. Further, the position feed-forward loop leads the tool tip along cutter location data. In order to first confirm the application limit of a conventional industrial robot to a finishing task, we evaluate the backlash that causes the position inaccuracy at the tip of an abrasive tool, through a simple position/force measurement. Through a similar measurement and a surface following control experiment along a lens mold, the basic position/force controllability with high resolutions is demonstrated. This work was presented in part at the 13th International Symposium on Artificial Life and Robotics, Oita, Japan, January 31–February 2, 2008     

Position control of very lightweight single-link flexible arms with large payload variations by using disturbance observers

In this article, a robust control scheme for trajectory tracking of very lightweight single-link flexible arms is discussed. Since the payload is one of the most variable parameters in a manipulator, the control is designed to achieve an accurate tracking of the desired tip trajectory for any value of the robot tip mass, or even for a tip mass changing during the maneuver. The proposed controller also guarantees stability for small uncertainties in parameters such as stiffness or motor friction. In addition, the effect of spillover on the performance of the controlled system is analyzed, and it is proven that stability and a good performance are preserved independently from the non-modeled high-order dynamics. The control scheme is based on a two nested loops structure. Each of these loops implements a Generalized Proportional Integral (GPI) controller. Moreover, the outer loop includes a disturbance compensation term based on a disturbance observer, which achieves the required insensitivity to payload changes. The theoretical analysis is supported by an extensive set of numerical simulations which shows controlled system response when variations in the robot payload, or dynamics neglected in the controller design, are considered. Finally, some experiments have been carried out in order to test the performance of the tip trajectory tracking of the proposed control system.     

Experimental Testing of a Gauge Based Collision Detection Mechanism for a New Three‐Degree‐of‐Freedom Flexible Robot

The use of flexible robots can be easily justified in two main cases: (1) when the weight of the robot has to be minimized and (2) when collisions between the robot and the environment are foreseen, since a flexible, lightweight robot implies less impact energy. The position control of these robots has already been analyzed in previous communications. However, the second of these cases justifying the use of flexible robots requires further consideration, leading to the development of a force controller. Inmost up to date analysis the force control is studied beginning from a known contact point at a given collision time. In a more realistic approach, however, an accurate detection of the collision would be needed prior to dealing with the force control. After developing a reliable position controller for a three‐degree‐of‐freedom flexible robot using strain gauges placed over the robot structure, in this paper we deal with the possibility of carrying out the collision detection for the same prototype. This has been easily achieved by analyzing some estimated signals such as tip position and tip velocity. However, a complete analysis of the information obtained with the sensors has been required to obtain those estimates and a signal processing scheme had to be devised for a previous filtering of the original signals from the sensors (encoders and strain gauges). This work has been carried out as a first step towards the position/force control. Experimental results on a three‐degree‐of‐freedom flexible arm prototype are presented to verify how well this method performs. © 2003 Wiley Periodicals, Inc.     

Design,dynamic modelling and experimental validation of a 2DOF flexible antenna sensor

A two-degree-of-freedom flexible antenna sensor platform was designed to physically simulate the ability of a robotic arm, which rapidly reorients and targets itself towards specific surfaces from different approachable angles. An accurate antenna model involves non-linear expressions that represent the system dynamics. Therefore, a comprehensive study along with experimental work has been carried out in order to achieve accurate system identification and validate the dynamic model. The model developed has proven useful in controlling the antenna tip, minimising the effects of the non-linear flexural dynamics and the Coulomb friction. The system was driven by servo motors. Algebraic controllers were developed for the antenna tip to track the reference trajectory. The platform system used encoders to measure the joint angles and a loadcell sensor to obtain the flexible link tip position. To validate the sensory information, the results obtained by the integrated sensors were compared to that of an external camera system.     

On the decoupling of position and force controllers in constrained robotic tasks

In hybrid control of robot manipulators separate controllers are designed for force and position errors control. Controllers are designed either in task or joint space and their outputs combine to provide input torque to the manipulator. Position and force controllers performance in a constrained robotic task is affected by their interaction to a degree dependent on the controller's ability to reject disturbances. Ideally, decoupling of the two control loops is desired to achieve the best performance in position and force directions. In this article, analysis of control loop interactions is performed for contact and noncontact phases, and controller design requirements are developed to achieve maximum decoupling. Design requirements involve output subspace of each controller leading to control discontinuities for contact and noncontact phases. In the noncontact phase, satisfaction of design requirements leads to a fully linearized and decoupled system. When in contact with the constraining surface, design requirements eliminate disturbances in the force loop, but minimize disturbances in the position loop to an extent dependent on force loop performance. Known hybrid control schemes analysis is performed to reveal existence of control loop interactions in these schemes. Confirmation of theoretical analysis is done through simulation of a three revolute planar manipulator. © 1998 John Wiley & Sons, Inc.     

Adaptive Controller for Single-Link Flexible Manipulators Based on Algebraic Identification and Generalized Proportional Integral Control

In this paper, we propose a fast online closed-loop identification method combined with an output-feedback controller of the generalized proportional integral (GPI) type for the control of an uncertain flexible robotic arm with unknown mass at the tip, including a Coulomb friction term in the motor dynamics. A fast nonasymptotic algebraic identification method developed in continuous time is used to identify the unknown system parameter and update the designed certainty equivalence GPI controller. In order to verify this method, several informative simulations and experiments are shown.     

An inverse kinematics algorithm for interaction control of a flexible arm with a compliant surface

This paper deals with the problem of controlling the interaction of a multilink flexible arm in contact with a compliant surface. For a given tip position and surface stiffness, the joint and deflection variables are computed using a closed-loop inverse kinematics algorithm. This is based on a suitable Jacobian matrix which includes terms accounting for the static deflections due to gravity and contact force. The computed variables are used as the set-points for a simple joint PD control, thus achieving regulation of the tip position and contact force via a joint-space controller. The scheme is tested in a simulation case study for a planar two-link manipulator.     

Trajectory control of a two DOF rigid–flexible space robot by a virtual space vehicle

Model based control schemes use inverse dynamics of the robot arm to produce the main torque component necessary for trajectory tracking. For a model-based controller one is required to know the model parameters accurately. This is a very difficult job especially if the manipulator is flexible. This paper presents a control scheme for trajectory control of the tip of a two arm rigid–flexible space robot, with the help of a virtual space vehicle. The flexible link is modeled as an Euler–Bernoulli beam. The developed controller uses the inertial parameters of the base of the space robot only. Bond graph modeling is used to model the dynamics of the system and to devise the control strategy. The efficacy of the controller is shown through simulated and animation results.     

机器人串联弹性关节驱动器的刚度控制方法

机器人关节的柔顺性在人机协作过程中具有重要作用，然而固定的关节柔性无法满足动态变化的人机协作需求，因此对机器人的关节驱动器提出了具有刚度调节能力的要求.本文采用阿基米德螺旋线平面涡卷弹簧作为机器人关节的柔性元件，并提出一种可用于具有固定刚度的串联弹性驱动器的刚度控制方法.根据关节刚度的定义，将测量得到的弹簧输出端角度用于计算弹簧的输入端转角，使得机器人关节驱动器的等效刚度可以被调整到所期望的大小.该方法以电机位置控制为内环，关节刚度控制为外环，简化了控制器设计，并实现了解耦控制.对所设计的刚度控制器进行了分析.最后在自主设计的单自由度薄型串联弹性驱动器实验平台上进行了刚度调节实验，包括刚度的双向阶跃、零刚度和正弦变化的刚度，实验结果表明关节等效刚度能准确跟踪期望值，验证了该方法的有效性.