Control of redundant manipulators considering order of disturbanceobserver

A manipulator control method using a disturbance observer with no inverse dynamics has been proposed. The order of the disturbance observer affects the performance of control system, because nominalization of plant is dependent on the control object and the order of the disturbance observer. This paper proposes a unique choice of disturbance observers of different order in joint and task space to improve system performance of hybrid position/force control of redundant manipulators. Also, it has been shown that a proper selection of a coefficient of the disturbance observer is capable to improve robust stability, while not influencing basic performance. The proposed strategy can realize acceleration control and second derivative of force control in the task space, which realizes robust and precise control of manipulators. Experimental results using a redundant manipulator show the effectiveness of the proposed strategy     

A dexterity comparison for 3-DOF planar parallel manipulators with two kinematic chains using genetic algorithms

In this research work, a dexterity comparison is performed for seven three-degree-of-freedom (3-DOF) Planar Parallel Manipulators with two kinematic chains (PPM2KCs) using genetic algorithms. Introducing a new design approach, one of the three kinematic chains of seven possible fully planar parallel manipulators (FPPMs) is disconnected in order to design the new seven PPM2KCs, and one active actuator is added to the first leg to maintain the 3-DOF. These manipulators are designed on the basis of two purposes: (i) increasing the workspace area, and (ii) reducing the chance of interference since the three independent kinematic chains not only cause interference among the mechanical components but also reduce the workspace, which are the major challenges for parallel manipulators. Afterwards base distances are optimized using genetic algorithms (GAs) to compute the performance indices κ and GDI for the PPM2KCs. The results have been examined regarding especially two objectives: (i) selecting the best possible architecture among the seven new designed configurations for a specific task, and (ii) analyzing the correlation between joint types (prismatic or revalue) and dexterous maneuverability for PPM2KCs. It has been concluded that the PPR (prismatic–prismatic–revolute) planar parallel manipulator is the best configuration, providing the best dexterous maneuverability among the others.     

An optimization-based approach for elasticity-aware trajectory planning of link-elastic manipulators

Elastic manipulators often result from lightweight construction or safety requirements in human–robot-collaboration scenarios. In many cases, vibration-damping becomes necessary to enable stable and precise operation, which imposes high demands on controllers. A fundamental challenge for link-elastic manipulators with general kinematics and no dedicated damping actuators is the potential inability of a joint to control the vibration of a link depending on the manipulator’s configuration. Numerous control concepts assume a sufficiently high ability to control vibrations by, for example, dedicated actuators or special kinematic structures. This work presents an online, optimization-based motion planning approach with three methods for maximizing this ability for link-elastic manipulators. The planning algorithm utilizes modified cost functions to incorporate the controllability of vibrations by prioritization and adaptive activation of competing objectives. The effectiveness of the approach is demonstrated on a real manipulator with 3 actuated DoFs and two elastic links in different disturbance scenarios. The results show that the amplitudes of vibrations caused by disturbances decay noticeably faster than with conventional motion planning.     

A fully neural-network-based planning scheme for torqueminimization of redundant manipulators

The aim of this paper is to develop a new method for minimizing joint torques of redundant manipulators in the Chebyshev sense and to present a fully neural-network-based computational scheme for its implementation. Minimax techniques are used to determine the null space acceleration vector which can guarantee to minimize the maximum joint torque. For real-time implementation, we transform the proposed method into a computation of a recurrent neural network. At each time step, the neural network is adopted for both the solution of the least-norm joint acceleration and the determination of the optimum null space acceleration vector. Compared with previous torque minimization schemes, the proposed method enables more direct monitoring and control of the magnitudes of the individual joint torques than does the minimization of the sum of squares of the components. Simulation results demonstrate that the proposed method is effective for the torque minimization control of redundant manipulators     

Development of a new 4-DOF endoscopic parallel manipulator based on screw theory for laparoscopic surgery

Development of rigid manipulators for Minimally Invasive Surgery (MIS) becomes essential to replace wire driven manipulators which are problematic due to possible cutting of the wire during the surgery. In this paper, a 4-DOF dexterous endoscopic parallel manipulator for MIS is designed and implemented. The manipulator is developed based on the concept of virtual chain and screw theory. The manipulator consists of four links; two links are 2-PUU (each leg consists of one active prismatic joint and two passive hook joints); the other two links are 2-PUS (each leg consists of one active prismatic join, one passive hook joint and one passive spherical joint). The inverse and forward kinematics solutions are derived analytically and numerically, respectively. The singularity analysis is investigated using screws algebra. The manipulator workspace is obtained using MATLAB software. The known problem of limited bending angles found in previous existing surgical manipulators was solved as the proposed manipulator can reach ±90° in any direction. The proposed surgical manipulator is designed, manufactured and tested successfully. The system model utilizing PID and PI controllers has been built using MATLAB software. Co-simulation using ADAMS/MATLAB software is implemented to validate the achieved bending angles and the proposed tracking control. The results show that the performance of the tracking control is satisfactory since the tracking error is about 2.5%.     

A decentralized kinematic control architecture for collaborative and cooperative multi-arm systems

In this paper a two-layer decentralized framework for kinematic control of cooperative and collaborative multi-robot systems is developed. The motion of the system is specified at the workpiece level, by adopting a task-oriented formulation for cooperative tasks. The first layer computes the motion of the single arms in the system. In detail, the control unit of each robot computes the end-effector motion references in a decentralized fashion on the basis of the knowledge of the assigned cooperative task and the motion references computed by its neighbors. Then, in the second layer, each control unit computes the reference joint motion of the corresponding manipulator from the end-effector reference motion. The approach is, then, tested in simulation on a work-cell composed by several manipulators, and experimentally on a dual-arm kinematically redundant work-cell composed by industrial manipulators.     

Disturbance-observer-based motion control of redundant manipulatorsusing inertially decoupled dynamics

In this paper, a robust motion control method for kinematically redundant manipulators is addressed to control the motion of the end-effector, as well as the null-space motion. To reduce the disturbance effects on the system performance, a new disturbance observer is proposed to improve the performance of the conventional structure. A minimal parametrization of the null space is performed to visualize the null-space motion explicitly using the weighted decomposition of joint space. Augmenting this null motion with conventional velocity relations, an extended task space formulation is obtained. The proposed disturbance observer is adopted in the extended task space formulation and a motion control method is devised based on the passivity concept. The performance of the proposed controller is verified through experiments with a three-link planar direct-drive manipulator     

A frequency-domain approach to learning control: implementation fora robot manipulator

A frequency-domain approach to the analysis and design of learning control laws for achieving a desired repetitive behavior in a dynamical system is presented. The scheme uses two separate filters in order to obtain rapid improvement in a specified bandwidth, while cutting off possibly destabilizing dynamic effects that would bar learning convergence. In this way the trade-off between global convergence conditions and approximate learning of trajectories is made explicit. The synthesis is presented for single-input, single-output (SISO) linear systems, but the method is of general application. The proposed learning controller has been used for exact tracking of repetitive trajectories in robot manipulators. In particular, actuator inputs that enable accurate reproduction of robot joint-space trajectories are learned in a few iterations without the knowledge of the robot dynamic model. Implementation aspects are discussed, and experimental results are reported     

Optimum design of 2-DOF parallel manipulators with actuation redundancy

The optimum design of 2-DOF redundant parallel manipulators is investigated in this paper. The planar 2-DOF parallel manipulator with actuation redundancy is treated as non-dimensional structure. The physical model of the solution space is studied. Based on the kinematic model and Jacobian matrix, the global conditioning index, global velocity index and global stiffness index of the 2-DOF parallel manipulators are investigated, and the geometrical parameters without dimension are determined. Based on the optimum non-dimensional result, the optimum dimensional parameters are achieved. The result of this paper is useful for the development of 2-DOF 3-RRR redundant parallel manipulators.     

Path planning for robotic manipulators with redundant degrees offreedom

The problem of path planning for robotic manipulators with redundant and nonredundant degrees of freedom is addressed. It is assumed that the motors for each joint are capable of achieving the commanded velocity within limits. Thus, the dynamic model is simplified and the main complexity is that of the kinematic relationships. Of primary interest is the problem of moving the end effector from point A to point B in an efficient manner, possibly in the presence of obstacles. A suboptimal solution is proposed and discussed. Examples are presented in order to compare the performance of the redundant and the nonredundant manipulators     

Tool-Center-Point control of the KAI manipulator using constrained QP optimization

The KAI manipulator is a four joint mobile manipulator, which will be used within the German road clearance package to investigate improvised explosive devices and ordnance from within an armored vehicle. To improve handling of the manipulator, a Tool-Center-Point (TCP) control is implemented. By using constrained quadratic optimization (cQP) it is possible to allow for the control of the manipulator within three different operating spaces. The QP is formulated to account for constraints in the joint angular rates and TCP velocities, as well as additional velocity constraints, e.g. on the movement of the center of mass of the manipulator. The proposed algorithm is able to handle redundant as well as non redundant manipulator kinematics. By using an efficient QP solver the algorithm can be used within a real-time trajectory generation scheme. The performance of the algorithm is demonstrated using simulation results and validated by measurements of the TCP control.     

Force feedback control of robot manipulator by the accelerationtracing orientation method

The authors propose a novel approach to force and compliance control of multi-degree-of-freedom (DOF) robot manipulators. The acceleration tracing orientation method (ATOM) is applied to both controllers. The control law is described in the Cartesian space; however, the final command is the acceleration in the joint space. The interactive terms in each joint disturb and deteriorate the joint motion. The disturbance observer cancels out the total sum of these terms and enables each joint to trace the acceleration command. As a result, a robust control is possible in the force task. The testing of the proposed system in a three-DOF robot manipulator is discussed     

System Design and Control of Anthropomorphic Walking Robot LOLA

This paper presents the 25-DOF full-size humanoid robot LOLA . Our goal is to realize fast and human-like walking. Furthermore, we want to increase the robot’s autonomous, vision-guided walking capabilities. LOLA is characterized by a redundant kinematic configuration, an extremely lightweight design, joint actuators with brushless motors and an electronics architecture using decentralized joint control. Special emphasis was put on an improved mass distribution to achieve good dynamic performance. Center of mass trajectories are calculated in real-time from footstep locations using a spline collocation method. Reference trajectories are modified by a stabilizing control system based on hybrid force/position control with an inner joint position control loop.      

Position control of a Stewart platform using inverse dynamics control with approximate dynamics

Configuration-dependent nonlinear coefficient matrices in the dynamic equation of a robot manipulator impose computational burden in real-time implementation of tracking control based on the inverse dynamics controller (IDC). However, parallel manipulators such as a Stewart platform have relatively small workspace compared to serial manipulators. Based on the characteristics of small motion range, nonlinear coefficient matrices can be approximated to constant ones. The modeling errors caused by such approximation are compensated for by H_∞ controller that treats the error as disturbance. The proposed IDC with approximate dynamics combined with H_∞ control shows good tracking performance even for fast tracking control in which computation of full dynamics is not easy to implement.     

A new homogeneous manipulability measure of robot manipulators, based on power concept

Kinetostatic performance indices have been commonly used for many potential applications in robotics, i.e. optimal design purpose, trajectory planning, manipulation programming, redundancy treatment and dexterity analysis. This has been successful when the mechanism has either fully rotational or translational joints. However, in case of a mechanism having both rotational and translational degrees of freedom; performance indices, such as Jacobian matrix, manipulability or condition number, may not be used due to the dimensional inconsistency with its elements. In this paper, by means of the power concept, a new kinetostatic performance index of robot manipulators is proposed. The power has the same physical units in either translations or rotations. Therefore, we can make use of it as a homogeneous or natural performance index of manipulators. Although it has never been considered as a subject matter of kinetostatic performance criteria, exploiting the behaviour of its basic components namely, force and speed, along the mechanism was likely interesting. On the other hand, the new concept of Oriented Power was introduced, in order to formulate the quadrivector of apparent power, leading to the final homogeneous performance index, which is no longer susceptible to the physical units’ choice. The main benefits of this approach are as follows: firstly there is no need for any restriction on the kinematics’ limits; secondly, the new formalism is insensitive to joint types of mechanism, as well as it has the possibility of grouping translations and rotations of the operational space together. The new index is then both applied in a simple serial mechanism dexterity analysis, and extended to a planar parallel mechanism.     

Inverse-free computation for infinity-norm torque minimization of robot manipulators

The minimum infinity-norm redundancy resolution, also known as the minimum-effort solution, explicitly minimizes the largest component of joint vectors. It is useful in situations where focuses are on low individual magnitude, even distribution of workload, and analysis of motion diversity. The minimum-effort solution is well investigated at the velocity level. However, because of the weaknesses of torque instability and matrix computation of O(n³) operations, the infinity-norm torque minimization of redundant manipulators seemed less favorable. In this paper, an elegant treatment is presented by resolving redundancy at the acceleration level and reformulating the task as the online solution to a quadratic programming problem. The new computation scheme incorporates three levels of joint physical limits, thus naturally remedying the torque-instability problem. In addition, this new scheme does not entail any matrix inversion or matrix–matrix multiplication, which was entailed in others’ researches with expensive O(n³) operations. The validity and advantages of the new computation scheme are substantiated by simulation results performed based on the PUMA560 robot arm.     

A new formulation technique for local torque optimization ofredundant manipulators

A new formulation technique called “torque-based formulation” for the local torque optimization of redundant manipulators is introduced in this paper. The formulation based on joint torques makes the pseudoinverse and recursive techniques possible to local torque optimization. The local torque optimization approaches that are formulated on the basis of the proposed formulation technique are simple in formulation and are efficient in computational cost. Equivalence between the proposed solutions for local torque optimization and the conventional solutions is also proven analytically     

Optimal design and development of a five-bar finger with redundant actuation

In order to develop a human hand mechanism, a five-bar finger with redundant actuation is designed and implemented. Each joint of the finger is driven by a compact actuator mechanism having an ultrasonic motor and a gear set with a potentiometer, and controlled by a VME bus-based control system. Optimal sets of actuator locations and link lengths for cases of a minimum actuator, one-, two-, and three-redundant actuators are obtained by employing a composite design index which simultaneously considers several performance indices, such as workspace, isotropic index, and force transmission ratio. According to the optimization result, several finger-configurations optimized for a special performance index are illustrated, and it is concluded that the case of one redundant actuator is the most effective in comparison to the cases of more redundant actuators, and that the case of two redundant actuators is the most effective in multi-fingered operation in which the force characteristic is relatively important, as compared to the kinematic isotropy and the workspace of the system.     

A cooperative optimization strategy for distributed multi-robot manipulation with obstacle avoidance and internal performance maximization

Reactive multi-robot manipulation can be of great assistance in many applications. In this article a distributed cooperation strategy for object manipulation with multiple robots in un-modelled environments is proposed to tackle workpiece transportation and dynamic obstacle avoidance simultaneously. A decentralised optimisation process will run first to generate robot pose references and then a joint-level control is responsible for tracking. Since in multi-arm manipulation systems the available workspace for each robot is more restricted, necessary internal performance optimisation is also included in the planning phase. A velocity-level optimum will be solved and then transformed to a position-level control reference so that it can be combined with many compliant interactive controllers such as impedance control. Although no torque or force sensor is required and each robot only has access to local information, when avoiding obstacles the coordination strategy can generate synchronised group motion so that internal forces are avoided. Only task-space variables will be communicated, therefore it is easy to team up different types of robots, making the system more flexible. Simulation and experiments with two redundant manipulators have been done to validate the proposed design, and data show that the online computation speed is fast.     

A Design Example of a Fractional-Order Kerwin–Huelsman–Newcomb Biquad Filter with Two Fractional Capacitors of Different Order

Design, realization and performance studies of continuous-time fractional order Kerwin–Huelsman–Newcomb (KHN) biquad filters have been presented. The filters are constructed using two fractional order capacitors (FC) of orders α and β (0<α, β≤1). The frequency responses of the filters, obtained experimentally have been compared with simulated results using MATLAB/SIMULINK and also with PSpice (Cadence PSD 14.2), where the fractional order capacitor is approximated by a domino ladder circuit. It has been observed that fractional order filters can give better performance in certain aspects compared to integer order filters. The effects of the exponents (α and β) on bandwidth and stability of the realized filter have been examined. Sensitivity analysis of the realized fractional order filter has also been carried out to investigate the deviation of the performance due to the parameter variation.