Position and force tracking control of rigid-link electrically-driven robots

This paper introduces a framework for the design of tracking controllers for rigid-link electrically-driven (RLED) robot manipulators operating under constrained and unconstrained conditions. We present an intuitive nonlinear control strategy that can easily be reformulated for robots performing high precision tasks. The main emphasis is placed on the development of controllers that incorporate both motion in freespace and under constrained conditions. Another novelty is the combined treatment of force control and compensation for actuator dynamics. Based on models of the robot dynamics and environmental constraints, a reduced order dynamic model is obtained for the mechanical subsystem with respect to a set of constraint variables. A design procedure for tracking controllers is then formulated for the reduced order manipulator dynamics and the DC actuator dynamics. This paper concentrates on the theoretical aspects of the problem and, hence, is based on exact knowledge of the entire system. However, we have illustrated recently in [1] that this assumption can be generously relaxed in the design of a robust controller following a similar procedure as discussed in this paper.     

A simple and efficient computational approach for the forward dynamics of elastic robots

A simple and efficient Lagrangian formulation is presented for the forward dynamic analysis of elastic robots. The proposed method formulates the equations of motion with respect to a floating frame that follows the rigid motion of the links. By virtue of the proposed formulation the constraint conditions are inserted in the Hamilton's principle by means of a penalty formulation rather than by the classical Lagrange's multiplier technique. As a consequence, the number of equations that define the behavior of the robot does not increase. The numerical implementation of the new method is very simple and always leads to the solution of positive definite matrices. A series of elastic robots are analyzed and the results demonstrate the capabilities of the new formulation for the forward dynamics.     

Task-oriented programming of large redundant robot motion

Large robots are a new domain of advanced robotics. Examples of their application fields are tasks like operations on large free-form surfaces, especially aircraft cleaning and removing paint from hulls. They are equipped with a programmable robot control comparable to a control system used for industrial robots. However, conventional teach-in methods are not able to manage the complexity of programming large redundant robot operation on free-form geometries. The Fraunhofer IPA has developed an innovative off-line programming system that allows the creation of robot motion programs which satisfy time and energy optimization criteria. This system helps to avoid collisions within the workspace and to fulfill conditions that arise from the robot kinematics and dynamics. This advanced programming system has been successfully used to generate motion programs for the world's largest mobile robot, the aircraft cleaning manipulator SKYWASH. In this context offline programs for eleven different types of aircraft have been developed.     

Convergence and robustness of a discrete-time learning control scheme for constrained manipulators

The constrained motion control is one of the most common control tasks found in many industrial robot applications. The nonlinear and nonclassical nature of the dynamic model of constrained robots make designing a controller for accurate tracking of both motion and force a difficult problem. In this article, a discrete-time learning control problem for precise path tracking of motion and force for constrained robots is formulated and solved. The control system is able to reduce the tracking error iteratively in the presence of external disturbances and errors in initial condition as the robot repeats its action. Computer simulation result is presented to demonstrate the performance of the proposed learning controller. © 1994 John Wiley & Sons, Inc.     

Robust Hybrid Control of Constrained Robot Manipulators via Decomposed Equations

Basing on a constraint Jacobian induced orthogonal decomposition of the task space and by requiring the force controller to be orthogonal to the constraint manifold, the dynamics of the constrained robots under hybrid control is decomposed into a set of two equations. One describes the motion of robots moving on the constraint manifold, while the other relates the constraint force with the hybrid controller. This decomposition does not require the solution of the constraint equation in partition form. In this setting, the hybrid control of constrained robots can be essentially reduced to robust stabilization of uncertain nonlinear systems whose uncertainties do not satisfy the matching condition. A continuous version of the sliding-mode controller (from Khalil [12]) is employed to design a position controller. The force controller is designed as a proportional force error feedback of high gain type. The coordination of the position controller and the force controller is shown to achieve ultimately bounded position and force tracking with tunable accuracy. Moreover, an estimate of the domain of attraction is provided for the motion on the constraint manifold. Simulation for a planar two-link robot constraining on an ellipse is given to show the effectiveness of a hybrid controller. In addition, the friction effect, viewed as external disturbance to the system, is also examined through simulations.     

Feedback stabilization and tracking of constrained robots

Mathematical models for constrained robot dynamics, incorporating the effects of constraint force required to maintain satisfaction of the constraints, are used to develop explicit conditions for stabilization and tracking using feedback. The control structure allows feedback of generalized robot displacements, velocities, and the constraint forces. Global conditions for tracking, based on a modified computed-torque controller and local conditions for feedback stabilization, using a linear controller, are presented. The framework is also used to investigate the closed-loop properties if there are force disturbances, dynamics in the force feedback loops, or uncertainty in the constraint functions     

Model-based and neural-network-based adaptive control of two robotic arms manipulating an object with relative motion

In the study of constrained multiple robot control, the relative motion between the constraint object and the end effectors of manipulators are usually neglected in the literature. However, in many industrial applications, such as assembly and machining, the constraint object is required to move with respect to not only the world coordinates but also the end effectors of the robotic arms. In this paper, dynamic modelling of two robotic arms manipulating an object with relative motion is presented first, then a model-based adaptive controller and a model-free neural network controller are developed. Both controllers guarantee the asymptotic tracking of the constraint object and the boundedness of the constraint force. Asymptotic convergence of the constraint force can also be achieved under certain conditions. Simulation studies are conducted to verify the effectiveness of the approaches.     

Accurate dynamic modeling and control parameters design of an industrial hybrid spray-painting robot

To obtain higher performance for hybrid robots subject to nonlinear dynamics and friction, feedforward compensations have been ubiquitously utilized in the industrial robotic field to attenuate these disturbances. However, due to the complex friction model and the coupling and time-varying dynamic of hybrid robots, there is no effective approach to realize accurate feedforward compensations in industrial control systems. This paper investigated an accurate dynamic modeling and control parameters design method to address these issues all at once. Taking the friction of each joint into account, the accurate dynamic model of the hybrid robot is developed and verified by experiments. With the accurate dynamic model, an exact control parameter design method is proposed based on the mapping relationship between the dynamic model and the feedforward compensations. Additionally, the control system designed by the method proposed in this paper is compared with the one tuned by an experienced engineer. Particularly, the robot's position and motion accuracy are also tested by a third-party inspection agency. The experimental and test results show that the position and velocity accuracy of the robot is improved significantly when the control system is designed by using the method proposed in this paper, which proves the effectiveness of the proposed method.     

Inversion techniques for trajectory control of flexible robot arms

A general framework is given for computing the torques that are needed for moving a flexible arm exactly along a given trajectory. This torque computation requires a dynamic generator system, as opposed to the rigid case, and can be accomplished both in an open- or in a closed-loop fashion. In the open-loop case, the dynamic generator is the full or reduced order inverse system associated to the arm dynamics and outputs. In order to successfully invert the arm dynamics, the torque generator should be a stable system. The stability properties depend on the chosen system output, that is on the robot variables (e.g., joint or end-effector) to be controlled. The same inversion technique can be applied for closed-loop trajectory control of flexible robots. A simple but meaningful nonlinear dynamic model of a one-link flexible arm is used to illustrate different feasible control strategies. Simulation results are reported that display the effects of the system output choice on the closed-loop stability and on the overall tracking performance.     

Sliding mode force,motion control,and stabilization of elastic manipulator in the presence of uncertainties

This article considers the question of position and force control of three-link elastic robotic systems on a constraint surface in the presence of robot parameter and environmental constraint geometry uncertainties. The approach of this article is applicable to any multi-link elastic robot. A sliding mode control law is derived for the position and force trajectory control of manipulator. Unlike the rigid robots, sliding mode control of an end point gives rise to unstable zero dynamics. Instability of the zero dynamics is avoided by Controlling a point that lies in the neighborhood of the actual end point position. The sliding mode controller accomplishes tracking of the end-effector and force trajectories on the constrained surface; however, the maneuver of the arm causes elastic mode excitation. For point-to-point control on the constraint surface, a stabilizer is designed for the final capture of the terminal state and vibration suppression. Numerical results are presented to show that in the closed-loop system position and force control is accomplished in spite of payload and constraint surface geometry uncertainty. © 1995 John Wiley & Sons, Inc.     

Distributed Constraint Force Approach for Coordination of Multiple Mobile Robots

A new approach to coordination of multiple mobile robots is presented in this paper. The approach relies on the notion of constraint forces which are used in the development of the dynamics of a system of constrained particles with inertia. A familiar class of dynamic, nonholonomic robots are considered. The goal is to design a distributed coordination control algorithm for each robot in the group to achieve, and maintain, a particular formation while ensuring navigation of the group. The theory of constraint forces is used to generate a stable control algorithm for each mobile robot that will achieve, and maintain, a given formation. The advantage of the proposed method is that the formation keeping forces (constraint forces) cancel only those applied forces which act against the constraints. Another feature of the proposed distributed control algorithm is that it allows to add/remove other mobile robots into/from the formation gracefully with simple modifications of the control input. Further, the algorithm is scalable. To corroborate the theoretical approach, simulation results on a group of six robots are shown and discussed.     

视网膜显微手术机器人的约束运动规划及仿真

为了辅助医生完成视网膜显微手术中精细的手术操作,过滤颤抖、提高精度和稳定性,提出一种生成手术机器人空间运动约束的方法——虚拟固定器（VF）．首先,通过引入手术环境约束和任务约束,采用加权、线性化的多目标约束条件,根据用户的输入设置目标函数,构造了视网膜显微手术中所需的6个虚拟固定器基元．在此基础上,以远程运动中心虚拟约束（RCM VF）的生成为例,通过约束运动基元的组合,推导了复杂约束运动的实现方法．各约束运动基元算法及复杂约束运动算法的仿真结果表明,手术器械可以按照虚拟固定器的定义实现特定的约束运动．最后,在各手术步骤中引入约束运动基元的基础上,在乒乓球和离体猪眼球上进行了手术操作实验,证明了在该虚拟固定器的引导下,视网膜机器人可以完成高难度的手术操作,验证了所提出算法的合理性和有效性．     

能耗约束优化工业机器人作业轨迹

工业机器人作业过程中普遍需要较高的能耗。基于量子行为和差分进化的改进蜻蜓算法,实现能耗约束下优化工业机器人避障作业轨迹。基于工业机器人五次B样条曲线矩阵式和动力学模型,构建能耗约束模型;进行仿真实验,利用改进蜻蜓算法求解能耗约束模型为适应度评价函数的工业机器人轨迹,对比改进蜻蜓算法与原始蜻蜓算法和基于指数函数步长的精英反向蜻蜓算法的优化结果,表明改进蜻蜓算法具有更优的性能。     

基于加速度映射的跳跃机器人稳定性控制

针对腿式机器人,讨论了腿式机器人稳定性控制与稳定性判定准则,基于稳定约束条件零力矩点(ZMP),提出一种干扰下的动态稳定性控制方法.落地冲击是跳跃机器人必须考虑的外干扰.所提出的稳定性控制方法是基于驱动性能约束下腿式跳跃机器人的动态平衡,运用ZMP平面加速度正交映射方法,进行地面冲击力干扰下的稳定性控制,在满足轨迹跟踪的同时保证了系统的动态平衡性.跳跃运动过程中的仿真结果表明,该方法是一种有效可行的腿式机器人抗干扰控制策略.     

Transformative CAD based industrial robot program generation

Industrial robots are widely used in various processes of surface manufacturing, such as spray painting, spray forming, rapid tooling, spray coating, and polishing. Robot programming for these applications is still time consuming and costly. Typical teaching methods are not cost effective and efficient. There are many off-line programming methods developed to reduce the robot programming effort. However, these methods suffer many practical issues, such as cable/hose tangling, robot configuration, collision, and reachability. To solve these problems, this paper discusses a new method to generate robot programs. Since industrial robots have been used in production for decades, there are many robot programs for different parts generated by the robot programmers. These robot programs, which contain not only the robot paths, but also the programmers' knowledge and process parameters, can be transformed to generate new robot programs for similar parts. In this paper, a transformative robot program generation method is developed based on the existing ones in the database. Experiments were performed to validate the developed methodology. The results are very promising in reducing the programming efforts in surface manufacturing.     

Adaptive motion control of wheeled mobile robot with unknown slippage

As a major representative nonholonomic system, wheeled mobile robot (WMR) is often used to travel across off-road environments that could be unstructured environments. Slippage often occurs when WMR moves in slopes or uneven terrain, and the slippage generates large accumulated position errors in the vehicle, compared with conventional wheeled mobile robots. An estimation of the wheel slip ratio is essential to improve the accuracy of locomotion control. In this paper, we propose an improved adaptive controller to allow WMR to track the desired trajectory under unknown longitudinal slip, where the stabilisation of the closed-loop tracking system is guaranteed by the Lyapunov theory. All system states use neural network online weight tuning algorithms, which ensure small tracking errors and no loss of stability in robot motion with bounded input signals. We demonstrate superior tracking results using the proposed control method in various Matlab simulations.     

Application of a Perturbation Method for Realistic Dynamic Simulation of Industrial Robots

This paper presents the application of a perturbation method for the closed-loop dynamic simulation of a rigid-link manipulator with joint friction. In this method the perturbed motion of the manipulator is modelled as a first-order perturbation of the nominal manipulator motion. A non-linear finite element method is used to formulate the dynamic equations of the manipulator mechanism. In a closed-loop simulation the driving torques are generated by the control system. Friction torques at the actuator joints are introduced at the stage of perturbed dynamics. For a mathematical model of the friction torques we implemented the LuGre friction model that accounts both for the sliding and pre-sliding regime. To illustrate the method, the motion of a six-axes industrial Stäubli robot is simulated. The manipulation task implies transferring a laser spot along a straight line with a trapezoidal velocity profile. The computed trajectory tracking errors are compared with measured values, where in both cases the tip position is computed from the joint angles using a nominal kinematic robot model. It is found that a closed-loop simulation using a non-linear finite element model of this robot is very time-consuming due to the small time step of the discrete controller. Using the perturbation method with the linearised model a substantial reduction of the computer time is achieved without loss of accuracy.     

Momentum control with hierarchical inverse dynamics on a torque-controlled humanoid

Hierarchical inverse dynamics based on cascades of quadratic programs have been proposed for the control of legged robots. They have important benefits but to the best of our knowledge have never been implemented on a torque controlled humanoid where model inaccuracies, sensor noise and real-time computation requirements can be problematic. Using a reformulation of existing algorithms, we propose a simplification of the problem that allows to achieve real-time control. Momentum-based control is integrated in the task hierarchy and a LQR design approach is used to compute the desired associated closed-loop behavior and improve performance. Extensive experiments on various balancing and tracking tasks show very robust performance in the face of unknown disturbances, even when the humanoid is standing on one foot. Our results demonstrate that hierarchical inverse dynamics together with momentum control can be efficiently used for feedback control under real robot conditions.     

无机械辅助结构自行车机器人控制仿真及实现

为实现自行车机器人的平稳直线行驶,论证了无机械辅助结构、仅靠调整车把维持自平衡的后驱自行车机器人动力学建模、姿态控制、系统仿真及实物样机实验.针对具有典型对称性欠驱动非完整约束的自行车机器人系统难于实现平衡控制问题,首先基于拉格朗日方法分析系统力学机理,建立简化动力学模型.然后基于部分反馈线性化原理,对车体横滚角与转把力矩的欠驱动子系统进行线性化处理及模糊自适应控制.仿真及实验结果表明,有效地实现了自行车机器人直线运动自平衡控制,为进一步开展自行车机器人以及其他欠驱动系统平衡运动控制奠定理论基础.