Frequency analysis and experimental validation for stiction phenomenon in multi-loop processes

The existence of static friction, also known as stiction nonlinearity, in a control valve may lead the entire system to limit cycles. Despite numerous studies on modeling, detection and compensation of this phenomenon, lack of a reliable methodology to predict occurrence and properties of such oscillations, particularly in multi-loop settings, is quite sensible. This work focuses on frequency analysis of multi-loop processes oscillating due to stiction. Before using any of the existing stiction models for the analysis, a comparison between existing stiction models and actual lab data is carried out. Derivation of a mathematical representation of the condition, under which stiction-induced oscillations occur in a multi-loop system, is the main achievement of the proposed analysis. This condition enables users to predict and compare the severity of the oscillations, i.e. values of frequencies and magnitudes, in different situations. Results of the theoretical discussion are validated by both simulation and experimental studies.     

A friction compensator for pneumatic control valves

A procedure that compensates for static friction (stiction) in pneumatic control valves is presented. The compensation is obtained by adding pulses to the control signal. The characteristics of the pulses are determined from the control action. The compensator is implemented in industrial controllers and control systems, and the industrial experiences show that the procedure reduces the control error during stick-slip motion significantly compared to standard control without stiction compensation.     

控制阀粘滞特性的频域检测方法研究

控制阀粘滞特性是导致控制回路振荡的主要原因之一,对控制阀粘滞特性进行了详细分析,在此基础上使用离散傅里叶变换分析振荡的偏差信号,提出了一种频域分析、适用范围较广的控制阀粘滞特性检测方法。通过仿真研究,表明了该方法的有效性。     

Frequency analysis and compensation of valve stiction in cascade control loops

Valve stiction is often a common problem in control loops and stiction induced oscillation is the main cause of poor performance in control systems. Cascade control is extensively applied in process industry as an effective strategy to restrain disturbances and compensate process nonlinearities. In recent years many studies have been performed on the detection and quantification of valve stiction in single feedback control loops. However, there is a lack in developing a mechanism which can analyze stiction induced oscillation in cascade control loops. This work focuses on the frequency analysis of stiction induced oscillations in cascade control loops and proposes a mechanism of oscillation compensation through outer and inner controller tuning. The effect of oscillation compensation by changing control strategies is also discussed. The theoretical analysis is evaluated through simulation examples and a pilot-scale flow-level cascade control experiment.     

克服阀粘滞特性的控制方法研究

控制阀粘滞特性在工业过程中非常普遍,本文针对控制阀粘滞特性而引起的回路振荡现象,提出了1种改进的PI控制方法,在一定的误差范围内只用比例控制从而消除振荡,而在误差比较大时采用了自适应PI控制来提高系统性能。同时将改进的PI控制方法转换成模糊控制经验,采用模糊控制克服回路中控制阀粘滞特性,同样达到消除误差的目的。最后在Matlab Simulink平台上,通过设计S函数,实现了控制阀粘滞特性模块和改进的PI控制器,使用模糊控制工具箱实现了模糊控制器的设计,在此基础上分析了在扰动存在的情况下2种控制方法的有效性。     

An autonomous valve stiction detection system based on data characterization

This paper proposes a valve stiction detection system which selects valve stiction detection algorithms based on characterizations of the data. For this purpose, novel data feature indexes are proposed, which quantify the presence of oscillations, mean-nonstationarity, noise and nonlinearities in a given data sequence. The selection is then performed according to the conditions on the index values in which each method can be applied successfully. Finally, the stiction detection decision is given by combining the detection decisions made by the selected methods. The paper ends demonstrating the effectiveness of the proposed valve stiction detection system with benchmark industrial data.     

A shape-analysis approach for diagnosis of stiction in control valves

The paper describes a method for automatic diagnosis of stiction in control valves. The diagnosis is performed using a shape analysis of the wave form of the oscillations that appear in the process output during stick-slip motion. The procedure is automatic in the sense that no process information is assumed except the one that is already available in the controllers. The procedure can be used both online and off-line. Results from industrial tests are provided in the paper.     

Detection and quantification of valve stiction by the method of unknown input estimation

Valve stiction is one of the most common causes of oscillations in industrial process control loops. Such oscillations can degrade the overall performance of the loop and eventually the final product quality. The detection and quantification of valve stiction in industrial process control loops is thus important. From previous studies in the literature, a sticky valve has been shown to have a distinct signature of the stiction phenomena in its valve positioner data. However, the position of the modulating control valves is seldom available. We consider the problem of estimating the valve position as an unknown input estimation problem. In this work, we propose a novel application of the unknown input estimator in order to estimate the valve position given the process model and the data of the process variable and controller output. Using the estimated valve position, we can detect and also quantify the amount of stiction. We demonstrate the efficacy of the method through simulation examples where a sticky valve is deliberately introduced in the closed loop using a two-parameter stiction model available in the literature. Application of the proposed methodology to a laboratory scale flow control loop is presented. An industrial case study is also presented in which the algorithm accurately detects and quantifies stiction in the level control loop of a power plant.     

显式抑制测量误差的改进滑模控制

反馈控制系统中模型不确定性和测量误差的同时出现, 给高精度控制器的设计带来挑战. 经典滑模控制能抵抗一定程度的模型不确定性和输入干扰, 但引入的高增益使得其性能对测量噪声极为敏感, 也容易引起系统强烈抖振. 为此, 本文针对一种典型的角度和角速率测量分别包含高频和低频测量误差条件, 提出了一种改进的基于趋近律的角度跟踪控制方案. 本方案采用低通滤波器来应对高频测量噪声, 同时采用一种新颖的基于模型的测量误差估计器, 来补偿低频测量漂移. 采用Quanser Aero平台进行两自由度角轨迹跟踪控制仿真和实验验证, 并与自抗扰控制等几种典型鲁棒控制方案进行了全面对比, 证实了本文方案性能上的优越性和调参的便捷性.     

气动控制阀控制系统爬行和滞后问题的解决方法

本文提出了一种气动控制阀控制系统非线性因素（静摩擦O的补偿方法,就是根据实际情况在控制信号中加入某种额外脉冲信号。这有利于减小或消除爬行和需后现象,减小系统误差,从而改善系统的精度和稳定性。     

A unified framework for the study of anti-windup designs

We present a unified framework for the study of linear time-invariant (LTI) systems subject to control input nonlinearities. The framework is based on the following two-step design paradigm: ‘design the linear controller ignoring control input nonlinearities and then add anti-windup bumpless transfer (AWBT) compensation to minimize the adverse effects of any control input nonlinearities on closed loop performance’. The resulting AWBT compensation is applicable to multivariable controllers of arbitrary structure and order. All known LTI anti-windup and/or bumpless transfer compensation schemes are shown to be special cases of this framework. This unification of existing schemes for AWBT compensation under a general framework is the main result of the paper.     

A simple model-free butterfly shape-based detection (BSD) method integrated with deep learning CNN for valve stiction detection and quantification

Control valve stiction is a long-standing problem within process industries. In most methods for shape-based stiction detection, they rely heavily on the traditional controller output (OP) and process variable (PV) plot (i.e. PV-OP plot) that tends to produce an “elliptical” shape which is the widely acknowledged pattern indication for the presence of stiction. However, many of the methods suffered from unsatisfactory generalization capability when subjected to different loop dynamics. In this paper, a “butterfly” shape derived from the manipulation of the standard PV and OP data, which is more robust towards different loop dynamics, is developed for stiction detection. This simple model-free butterfly shape-based detection (BSD) method uses Stenman's one parameter stiction model, which results in a distinctive ‘butterfly’ pattern in the presence of stiction. The proposed BSD is tested on simulated data, as well as 26 benchmark industrial case studies and has shown a relatively higher generalization capability with relatively higher successful detection rate on stiction loops and on non-stiction loops. A simple quantification algorithm based on BSD-convolutional neural network (BSD-CNN) framework is then developed to quantify the stiction severity. Based on the 15 benchmark industrial loops with stiction, the proposed BSD-CNN quantification algorithm has shown reasonable accuracy when compared to other published quantification methods in literature.     

Coordination and control of batch-based multistage processes

Run-to-run (R2R) process control has attracted much attention in research and has been widely used in practice. It has been proved effective at compensating for process disturbances by using R2R controllers at a single stage. However, most manufacturing processes span across multiple stages; variation in earlier stages can be magnified stage by stage if they are not properly eliminated. In addition, products are processed batch by batch in certain manufacturing processes. In such cases, the traditional EWMA controller might not effectively reduce the variation. This paper focuses on developing a process control strategy for batch production in a multistage process. In the newly proposed framework, a batch-allocation operation is introduced to group products into similar clusters before each stage; an R2R controller is then implemented to generate customized recipes for each batch. This framework emphasizes better coordination among the stages in a multistage process. Simulation results show that the proposed strategy is effective for the reduction of variation.     

T-S模糊控制系统的稳定性分析及系统化设计

研究了输入采用双交叠模糊分划的模糊控制系统的性质,提出了一个新的判定T—S模 糊控制系统稳定的充分条件.该条件只需在各最大交叠规则组内分别寻找公共的正定矩阵，减 小了以往稳定性判定方法的局限性和难度.运用并行分布补偿法(PDC)进一步探讨了闭环T-S 模糊控制系统的稳定性分析和模糊控制器系统化设计方法.通过两个例子的仿真研究验证了本 文方法的有效性.     

Multiple neural-network-based adaptive controller using orthonormalactivation function neural networks

A direct adaptive control scheme is developed using orthonormal activation function-based neural networks (OAFNNs) for trajectory tracking control of a class of nonlinear systems. Multiple OAFNNs are employed in these controllers for feedforward compensation of unknown system dynamics. Choice of multiple OAFNNs allows a reduction in overall network size reducing the computational requirements. The network weights are tuned online, in real time. The overall stability of the system and the neural networks is guaranteed using Lyapunov analysis. The developed neural controllers are evaluated experimentally and the experimental results are shown to support theoretical analysis. The effects of network parameters on system performance are experimentally evaluated and are presented. The superior learning capability of OAFNNs is demonstrated through experimental results. The OAFNNs were able to model the true nature of the nonlinear system dynamics characteristics for a rolling-sliding contact as well as for stiction.     

Estimation of valve stiction in control loops using separable least-squares and global search algorithms

This contribution presents a new procedure for quantifying valve stiction in control loops based on global optimisation. Measurements of the controlled variable (PV) and controller output (OP) are used to estimate the parameters of a Hammerstein system, consisting of a connection of a two-parameter stiction model and a linear low-order process model. As the objective function is non-smooth, gradient-free optimisation algorithms, i.e., pattern search (PS) methods or genetic algorithms (GA), are used for fixing the global minimum of the parameters of the stiction model, subordinated with a least-squares estimator for identifying the linear model parameters. Some approaches for selecting the model structure of the linear model part are discussed. Results show that this novel optimisation-based technique recovers accurate and reliable estimates of the stiction model parameters, dead-band plus stick band (S) and slip jump (J), from normal (closed-loop) operating data for self-regulating and integrating processes. The robustness of the proposed approach was proven considering a range of test conditions including different process types, controller settings and measurement noise. Numerous simulation and industrial case studies are described to demonstrate the applicability of the presented techniques for different loops and for different amounts of stiction.     

Control of mechanical motion systems with non-collocation of actuation and friction: A Popov criterion approach for input-to-state stability and set-valued nonlinearities

The presence of friction in mechanical motion systems is a performance limiting factor as it induces stick-slip vibrations. To appropriately describe the stiction effect of friction, we adopt set-valued force laws. Then, the complete motion control system can be described by a Lur’e system with set-valued nonlinearities. In order to eliminate stick-slip vibrations for mechanical motion systems, a state-feedback control design is presented to stabilize the equilibrium. The proposed control design is based on an extension of a Popov-like criterion to systems with set-valued nonlinearities that guarantees input-to-state stability (ISS). The advantages of the presented controller is that it is robust to uncertainties in the friction and it is applicable to systems with non-collocation of actuation and friction where common control strategies such as direct friction compensation fail. Moreover, an observer-based output-feedback design is proposed for the case that not all the state variables are measured. The effectiveness of the proposed output-feedback control design is shown both in simulations and experiments for a typical motion control system.     

Nonlinear output‐feedback control of torsional vibrations in drilling systems

This paper considers the design of a nonlinear observer‐based output‐feedback controller for oil‐field drill‐string systems aiming to eliminate (torsional) stick–slip oscillations. Such vibrations decrease the performance and reliability of drilling systems and can ultimately lead to system failure. Current industrial controllers regularly fail to eliminate stick–slip vibrations under increasingly challenging operating conditions caused by the tendency towards drilling deeper and inclined wells, where multiple vibrational modes play a role in the occurrence of stick–slip vibrations. As a basis for controller synthesis, a multi‐modal model of the torsional drill‐string dynamics for a real rig is employed, and a bit–rock interaction model with severe velocity‐weakening effect is used. The proposed model‐based controller design methodology consists of a state‐feedback controller and a (nonlinear) observer. Conditions, guaranteeing asymptotic stability of the desired equilibrium, corresponding to nominal drilling operation, are presented. The proposed control strategy has a significant advantage over existing vibration control systems as it can effectively cope with multiple modes of torsional vibration. Case study results using the proposed control strategy show that stick–slip oscillations can indeed be eliminated in realistic drilling scenarios in which industrial controllers fail to do so. Moreover, key robustness aspects of the control system involving the robustness against uncertainties in the bit–rock interaction and changing operational conditions are evidenced. Copyright © 2017 John Wiley & Sons, Ltd.     

时滞L PV 系统的稳定新判据及控制器设计

针对一类具有随参数变化状态时滞的线性参数变化系统，提出一种新的依赖于参数的Lyapunov稳定条件，该准则通过引入两个附加矩阵，解除了系统矩阵与依赖于参数的Lyapunov函数之闻的耦合，更易于系统的分析与综合，在此基础上设计了此类系统的状态反馈控制器，采用线性矩阵不等式技术，将控制器存在的充分条件转化为参数线性矩阵不等式的解存在条件，数值仿真验证了所提出算法的可行性。