Feedback Control of Planar Linkages Using a Linearizing Filter: Theory and Experiments

The motion control of mechanical systems with flexible links is investigated. Issues addressed are the modeling and simulations, the design of a feedback control scheme and its implementation on an actual system. The modeling method used is a combination of a spline-based spatial discretization technique and the natural orthogonal complement of the kinematic constraints that eliminates the constraint forces and moments from the mathematical model. The control algorithm consists of two parts, namely, the uncoupling of nonlinear equations of motion and the filtering of consequent nonworking constraint forces. This control scheme is implemented on a prototype four-bar flexible mechanism. Results show that the proposed control scheme provides successful trajectory tracking while suppressing the vibration triggered by a doublet-type of disturbance.     

External force control of an industrial puma 560 robot

Deformations occurring in a robot working in contact with a rigid environment is a real problem in the industrial world. This problem can be solved by taking into account forces undergone by the robot at the end-effector level. The first part of this article is aimed at determining a force control scheme that satisfies this constraint and that can be implemented on a non-modified industrial robot controller. Various existing force control schemes are investigated and the reasons for discarding them are given. Then, the emphasis is put on a so-called external force control scheme, which seems to be a solution to our problem. The control law appearing in such a scheme is determined by means of a realistic robot simulator developed on a SUN workstation. A simple integral term on the force error gives acceptable results in various robot configurations. This is illustrated in the form of graphs. In a second part, the implementation of this external control scheme in a real PUMA 560 robot's UNIMATE controller is presented. Certain software issues and implementation solutions are pointed out. Several experiments are described. Once again, graphs are used to show the experimental results. © 1994 John Wiley & Sons, Inc.     

A COMPARATIVE STUDY ON COMPUTATIONAL SCHEMES FOR NONLINEAR MODEL PREDICTIVE CONTROL

This paper briefly reviews development of nonlinear model predictive control (NMPC) schemes for finite horizon prediction and basic computational algorithms that can solve the stable real‐time implementation of NMPC in space state form with state and input constraints. In order to ensure stability within a finite prediction horizon, most NMPC schemes use a terminal region constraint at the end of the prediction horizon — a particular NMPC scheme using a terminal region constraint, namely quasi‐infinite horizon, that guarantees asymptotic closed‐loop stability with input constraints is presented. However, when nonlinear processes have both input and state constraints, difficulty arises from failure to satisfy the state constraints due to constraints on input. Therefore, a new NMPC scheme without a terminal region constraint is developed using soften state constraints. A brief comparative simulation study of two NMPC schemes: quasi‐infinite horizon and soften state constraints is done via simple nonlinear examples to demonstrate the ability of the soften state constraints scheme. Finally, some features of future research from this study are discussed.     

Dynamic Modeling and Decentralized Control of a 3 PRS Parallel Mechanism Based on Constrained Robotic Analysis

In this paper, a novel dynamic model is developed for analysis and control of a 3-PRS (Prismatic, Revolute, Spherical) parallel mechanism. The dynamic model is formulated in the joint space and three holonomic constraint equations are derived to specify the coupling relationships between the actuated legs and the moving platform. The associated constraint forces are determined to be internal forces and it leads to a novel model where the dynamics of the moving platform can be separated from that of the actuated legs. By utilizing the developed model, the constraint forces can be computed more efficiently as compared to the conventional approach. After compensation of the coupling dynamics, each actuated legs could be considered as a decoupled system. The coupling behavior of the parallel mechanism can then be easily illustrated by the proposed model. To facilitate real-time control implementation, the derived model can be further simplified and a decentralized control scheme integrated with a disturbance observer is proposed. The computational efficiency and tracking performances for the proposed control method are then validated by computer simulations.     

未知环境下的机器人视觉/力觉混合控制

在实际的接触型作业任务中,需要在控制位置的同时控制末端执行器与环境之间的接触力.针对这一问题,通过力反馈信息对未知环境中的法线方向进行估计,并采用最小二乘方法估计图像雅可比矩阵.提出一种基于视觉/力觉的混合控制算法,给出了视觉/力觉混合控制的约束条件,对视觉采用变结构控制器,力控制采用PI控制器.实验结果表明,该策略具有较高的力控制精度和曲线跟踪能力.     

A unified approach for inverse and direct dynamics of constrained multibody systems based on linear projection operator: applications to control and simulation

This paper presents a unified approach for inverse and direct dynamics of constrained multibody systems that can serve as a basis for analysis, simulation, and control. The main advantage of the dynamics formulation is that it does not require the constraint equations to be linearly independent. Thus, a simulation may proceed even in the presence of redundant constraints or singular configurations, and a controller does not need to change its structure whenever the mechanical system changes its topology or number of degrees of freedom. A motion-control scheme is proposed based on a projected inverse-dynamics scheme which proves to be stable and minimizes the weighted Euclidean norm of the actuation force. The projection-based control scheme is further developed for constrained systems, e.g., parallel manipulators, which have some joints with no actuators (passive joints). This is complemented by the development of constraint force control. A condition on the inertia matrix resulting in a decoupled mechanical system is analytically derived that simplifies the implementation of the force control. Finally, numerical and experimental results obtained from dynamic simulation and control of constrained mechanical systems, based on the proposed inverse and direct dynamics formulations, are documented.     

Contractive model predictive control for constrained nonlinearsystems

This paper addresses the development of stabilizing state and output feedback model predictive control (MPC) algorithms for constrained continuous-time nonlinear systems with discrete observations. Moreover, we propose a nonlinear observer structure for this class of systems and derive sufficient conditions under which this observer provides asymptotically convergent estimates. The MPC scheme proposed consists of a basic finite horizon nonlinear MPC technique with the introduction of an additional state constraint, which has been called a contractive constraint. The resulting MPC scheme has been denoted contractive MPC. This is a Lyapunov-based approach in which a Lyapunov function chosen a priori is decreased, not continuously, but discretely; it is allowed to increase at other times. We show in this work that the implementation of this additional constraint into the online optimization makes it possible to prove strong nominal stability properties of the closed-loop system     

Move blocking strategies in receding horizon control

In order to deal with the computational burden of optimal control, it is common practice to reduce the degrees of freedom by fixing the input or its derivatives to be constant over several time-steps. This policy is referred to as ‘move blocking’. This paper will address two issues. First, a survey of various move blocking strategies is presented and the shortcomings of these blocking policies, such as the lack of stability and constraint satisfaction guarantees, will be illustrated. Second, a novel move blocking scheme, ‘Moving Window Blocking’ (MWB), will be presented. In MWB, the blocking strategy is time-dependent such that the scheme yields stability and feasibility guarantees for the closed-loop system. Finally, the results of a large case study that illustrate the advantages and drawbacks of the various control strategies discussed in this paper and the implementation of the MWB scheme on a mechanical system are presented.     

Energy optimal spacecraft attitude control subject to convergence rate constraints

Attitude control of operational satellites is still predominantly performed by standard controllers such as Proportional plus Derivative (PD) control laws, which are still preferred for implementation to the computationally intensive nonlinear optimal control techniques, representing higher implementation complexity. In this paper, an inverse optimal control approach based on phase space geometry is presented, which is easy to implement and free from numerical and computational issues. The optimal control objective is to minimize a norm of the control torque subject to a rapidity constraint on the convergence rate of a Lyapunov function, under the effect of a benchmark controller. The proposed optimization method is shown to significantly enhance the torque-rapidity trade-off compared to the benchmark controller, chosen to be a PD law then a sliding mode controller. The inverse optimal control scheme is implemented on an air bearing table experimental platform.     

Dual approach using a variant perimeter constraint and efficient sub-iteration scheme for topology optimization

To prevent numerical instabilities associated with the mesh-dependence, checkerboards and grey regions in topology optimization, a variant perimeter-constrained version of the SIMP algorithm is proposed using a smooth and quadratic function. In order to have an efficient implementation and to make sure the strict satisfaction of such an upper-bound perimeter constraint, a diagonal quadratic approximation of the perimeter constraint is used in the construction of each explicit optimization subproblem. The latter is then solved by a dual sub-iteration scheme. Numerical results show that the incorporation of such a sub-iteration scheme leads to a convergent solution without needs of move-limits or artificial control parameters. In addition to this, it is found that successive relaxations of the perimeter constraint by increasing the upper-bound tend to regularize the topology solution and result in a checkerboard free and satisfactory design solution without grey regions.     

Formalization and Analysis of Class Loading in Java

Since Java security relies on the type-safety of the JVM, many formal approaches have been taken in order to prove the soundness of the JVM. This paper presents a new formalization of the JVM and proves its soundness. It is the first model to employ dynamic linking and bytecode verification to analyze the loading constraint scheme of Java2. The key concept required for proving the soundness of the new model is augmented value typing, which is defined from ordinary value typing combined with the loading constraint scheme. In proving the soundness of the model, it is shown that there are some problems inside the current reference implementation of the JVM with respect to our model. We also analyze the findClass scheme, newly introduced in Java2. The same analysis also shows why applets cannot exploit the type-spoofing vulnerability reported by Saraswat, which led to the introduction of the loading constraint scheme.     

NICE h : a higher-order explicit numerical scheme for integration of constitutive models in plasticity

The article introduces, as a result of further development of the first-order scheme NICE, a simple and efficient higher-order explicit numerical scheme for the integration of a system of ordinary differential equations which is constrained by an algebraic condition (DAE). The scheme is based on the truncated Taylor expansion of the constraint equation with order h of the scheme being determined by the highest exponent in the truncated Taylor series. The integration scheme thus conceived will be named NICE ^h , considering both principal premises of its construction. In conjunction with a direct solution technique used to solve the boundary value problem, the NICE ^h scheme is very convenient for integrating constitutive models in plasticity. The plasticity models are defined mostly by a system of algebraic and differential equations in which the yield criterion represents the constraint condition. To study the properties of the new integration scheme, which, like the forward-Euler scheme, is characterised by its implementation simplicity due to the explicitness of its formulations, a damage constitutive model (Gurson–Tvergaard–Needleman model) is considered. The general opinion that the implicit backward-Euler scheme is much more accurate than the thus-far known explicit schemes is challenged by the introduction of the NICE ^h scheme. The accuracy of the higher-order explicit scheme in the studied cases is significantly higher than the accuracy of the classical backward-Euler scheme, if we compare them under the condition of a similar CPU time consumption.     

受限机器人神经形态的变结构位置/力控制

本文基于Ｊｅａｎ和Ｆｕ（１９９３）建立的受限机器人模型的降型阶形式，利用变结构系统理论，设计了具有未知动态的受限机器人轨道／力追踪控制，提出的学习方法仅仅利用了机器人动态模型的一般结构，不需要其精确信息，计算迅速，易于实现，仿真结果验证了提出的方法的有效性。     

Flow control in integrated local area networks

The modeling and optimal flow control in integrated local area networks is investigated. A generic integrated local area network with a double bus architecture, an adaptive TDMA access protocol, and a heterogenous community of users is considered. The class of control algorithms studied regulates the data flow between a pair of communicating stations. Since the data flow in the network exhibits transient behavior, dynamic optimal flow control strategies on a finite horizon are analyzed. It is shown that an adaptive window flow control mechanism maximizes the average throughput under an average system time delay constraint. The window size dynamically adapts to changes in the integrated traffic load that belongs to the other users accessing the network. Based on this analysis, a flow control mechanism is proposed for implementation that is asymptotically optimum. Simulation results showing the dynamics o of the window process and the effectiveness of the asymptotically optimum control scheme are also presented.     

针对时间距离约束的网络化控制系统带宽调度策略

网络化控制系统是一种具有时间距离约束的硬实时系统.根据该特点,在深入分析网络化控制系统时间距离约束关系的基础上,提出一种针对时间距离约束的网络化控制系统带宽调度策略.该策略能够更有效地与网络化控制系统的控制算法相配合,同时兼顾控制系统控制性能和网络服务质量,优化系统总体性能.最后通过仿真实例说明了该策略的有效性.     

网络化控制性能与带宽调度协调优化设计及其仿真

基于网络化控制系统是具有延迟约束的实时系统,首先建立了保证网络控制性能的延迟约束模型,以此为基础提出满足上述延迟约束的带宽调度策略,以兼顾控制系统控制性能和网络系统服务质量;利用时间需求分析,给出在该策略下的周期、偶发与非周期等３种网络数据的可调度条件及延迟估计的算法,给出调度策略与保证网络控制性能的协调优化设计方法;最后提出一种控制系统与网络调度的集成仿真平台,并举例说明了策略的有效性．     

工作流管理系统业务约束的研究

目前，工作流管理系统得到了广泛的应用，但现有的工作流管理系统只能模拟业务流转逻辑，缺少对业务约束的模拟。鉴此，本文提出了工作流管理系统业务约束的概念，建立了业务约束模型，并给出了整合业务约束和业务流转逻辑的方案。并提出了在已有工作流管理系统中实现业务约束的方案。     

Optimal topologies derived from a phase-field method

A topology optimization method allowing for perimeter control is presented. The approach is based on a functional that takes the material density and the strain field as arguments. The cost for surfaces is included in the functional that is minimized. Diffuse designs are avoided by introducing a penalty term in the functional that is minimized. Equilibrium and a volume constraint are enforced via a Lagrange multiplier technique. The extremum to the functional is found by use of the Cahn–Hilliard phase-field method. It is shown that the optimization problem is suitable for finite element implementation and the FE-formulation is discussed in detail. In the numerical examples provided, the influence of surface penalization is investigated. It is shown that the perimeter of the structure can be controlled using the proposed scheme.     

An And/Or-Parallel implementation of AKL

The Agents Kernel Language (AKL) is a general purpose concurrent constraint language. It combines the programming paradigms of search-oriented languages such as Prolog and process-oriented languages such as GHC. The paper is focused on three essential issues in the parallel implementation of AKL for shared-memory multiprocessors: how to maintain multiple binding environments, how to represent the execution state and how to distribute work among workers. A simple scheme is used for maintaining multiple binding environments. A worker will immediately see conditional bindings placed on variables, all workers will have a coherent view of the constraint stores. A locking scheme is used that entails little overhead for operations on local variables. The goals in a guard are represented in a way that allows them to be inserted and removed without any locking. Continuations are used to represent sequences of untried goals. The representation keeps the granularity of work more coarse. Available work is distributed among workers in such a way that hot-spots are avoided. And- and or-tasks are distributed and scheduled in a uniform way.     

Robust adaptive control for a class of semi‐strict feedback systems with state and input constraints

This paper proposes a dynamic surface control (DSC)–based robust adaptive control scheme for a class of semi‐strict feedback systems with full‐state and input constraints. In the control scheme, a constraint transformation method is employed to prevent the transgression of the full‐state constraints. Specifically, the state constraints are firstly represented as the surface error constraints, then, an error transformation is introduced to convert the constrained surface errors into new equivalent variables without constraints. By ensuring the boundedness of the transformed variables, the violation of the state constraints can be prevented. Moreover, in order to obtain magnitude limited virtual control signal for the recursive design, the saturations are incorporated into the control law. The auxiliary design systems are constructed to analyze the effects of the introduced saturations and the input constraints. Rigorous theoretical analysis demonstrates that the proposed control law can guarantee all the closed‐loop signals are uniformly ultimately bounded, the tracking error converges to a small neighborhood of origin, and the full‐state constraints are not violated. Compared with the existing results, the key advantages of the proposed control scheme include: (i) the utilization of the constraint transformation can handle both time‐varying symmetric and asymmetric state constraints and static ones in a unified framework; (ii) the incorporation of the saturations permits the removal of a feasibility analysis step and avoids solving the constrained optimization problem; and (iii) the “explosion of complexity” in traditional backstepping design is avoided by using the DSC technique. Simulations are finally given to confirm the effectiveness of the proposed approach.