Degrees Of Freedom Control Scheme Research Articles

Resonance compensation research of tip-tilt mirror’s 2-DOF tracking-disturbance rejection problem

This paper discusses the target tracking problem with high-frequency disturbance rejection demand for the tip-tilt mirrors component in electro-optical systems, where the mechanical vibration disturbances always exist near the system’s resonance. After analyzing the influence of resonance to the tracking performance, a modified two degrees of freedom control scheme is proposed. Specifically, utilizing the decoupling capacity, the feedback control without resonance compensation is designed to attenuate system resonance’s neighboring disturbances, and a feedforward controller is added to make up for the sacrifice in tracking performance. As the tracking performances are difficult to be fused and judged, the parameters’ tuning problems are skillfully translated into a proportion optimization problem with a genetic algorithm. By this method, the controller can be finely designed and tuned to match particular systems and meet the performance demands. A series of simulations and experiments illustrate the effectiveness of this new 2-DOF control scheme.

A New Approach to Model Predictive Control Based on Two Degrees of Freedom Control and B-Splines Input Shaping

The purpose of this article is to reduce some technical difficulties related to the complexity of stability and feasibility analysis of Model Predictive Control (MPC) as well as to reduce the complexity of the relative optimization procedure. The new approach is based on a two degrees of freedom control scheme where the output $r(k)$ of a feedforward input estimator is used as input forcing the closed-loop system $\Sigma _f$ . This latter is given by the feedback connection of a Linear Time Invariant (LTI) plant with a dynamic output controller. The task of the controller is to guarantee the stability of $\Sigma _f$ , as well as the fulfillment of hard constraints for any $r(k)$ satisfying an “ a priori ” determined admissibility condition. The input $r(k)$ is computed through the online minimization of a quadratic cost functional and is applied to $\Sigma _f$ according to the usual MPC strategy. To simplify the constrained optimization problem, the input $r(k)$ forcing $\Sigma _f$ is assumed to be given by a B-spline function. This greatly decreases the number of decision variables of the online optimization procedure because B-splines are universal approximators which admit a parsimonious parametric representation. Moreover such parameterization allows us to reformulate the minimization of the cost functional as a box Constrained Least Squares (CLS) problem. It is shown that stability and recursive feasibility of the adopted MPC strategy are guaranteed in advance, regardless the chosen prediction horizon.

Trajectory Tracking Based on Adaptive Sliding Mode Control for Agricultural Tractor

Trajectory tracking is one of the key technologies for high-precision agriculture and has been the research focus by a large number of academics. In order to design a simple trajectory tracking control scheme, an adaptive sliding mode control based on lateral deviation is proposed. To reduce the chattering phenomenon, an adjustable parameter associated with the sliding surface and system error is introduced, by which the forward gain value can be regulated with the state change. According to the established tractor model, the desired front wheel angle can be obtained as the output of the sliding mode controller. A modified Smith predictor structure based two degree of freedom control scheme is designed to follow the desired front wheel angle. The effectiveness of the designed trajectory tracking control system is demonstrated by simulations, especially the performance of preventing chattering phenomenon.

Towards resolving the co-existing impacts of multiple dynamic factors on the performance of EMG-pattern recognition based prostheses

Background and ObjectiveMobility of subject (MoS) and muscle contraction force variation (MCFV) have been shown to individually degrade the performance of multiple degrees of freedom electromyogram (EMG) pattern recognition (PR) based prostheses control systems. Though these factors (MoS-MCFV) co-exist simultaneously in the practical use of the prosthesis, their combined impact on PR-based system has rarely been studied especially in the context of amputees who are the target users of the device. MethodsTo address this problem, this study systematically investigated the co-existing impact of MoS-MCFV on the performance of PR-based movement intent classifier, using EMG recordings acquired from eight participants who performed multiple classes of targeted limb movements across static and non-static scenarios with three distinct muscle contraction force levels. Then, a robust feature extraction method that is invariant to the combined effect of MoS-MCFV, namely, invariant time-domain descriptor (invTDD), was proposed to optimally characterize the multi-class EMG signal patterns in the presence of both factors. ResultsExperimental results consistently showed that the proposed invTDD method could significantly mitigate the co-existing impact of MoS-MCFV on PR-based movement-intent classifier with error reduction in the range of 7.50%~17.97% (p<0.05), compared to the commonly applied methods. Further evaluation using 2-dimentional principal component analysis (PCA) technique, revealed that the proposed invTDD method has obvious class-separability in the PCA feature space, with a significantly lower standard error (0.91%) compared to the existing methods. ConclusionThis study offers compelling insight on how to develop accurately robust multiple degrees of freedom control scheme for multifunctional prostheses that would be clinically viable. Also, the study may spur positive advancement in other application areas of medical robotics that adopts myoelectric control schemes such as the electric wheelchair and human-computer-interaction systems.

Steering Tracking Control Based on Assisted Motor for Agricultural Tractors

One of the most important tasks in tractor navigation is the accurate steering tracking during field operations. Steering tracking is to achieve desired steering angle under the governance of the control action, it is essential for the operation quality of the tractor. The objective of this paper is to develop a steering tracking system based on motor control and propose steering tracking control method for automatically guided agricultural tractors. The steering column installed power motor is set up and the electronic control unit (ECU) is designed. As for the established steering system model, a modified Smith predictor structure based two degree of freedom control scheme is proposed. Steering tracking controller and disturbance rejection controller are derived by the analytical formulae on the basis of the internal model control (IMC) design principle and obtained with a simple and practical PID form. Simulation results demonstrated the efficiency of the proposed method for the steering tracking control system.

IMC-PID Fractional Order Filter Multi-loop Controller Design for Multivariable Systems Based on Two Degrees of Freedom Control Scheme

An IMC-PID fractional order filter multi-loop controller design method based on two degrees of freedom paradigm is proposed for Multiple Input-Multiple Output systems with time delays. The interactions among the control loops are considered as disturbances. Thus, a two degrees of freedom control scheme, used for monovariable system to ensure the disturbance rejection, is extended to multivariable systems in order to reduce the effect of the coupling among the control loops. The proposed controller design method requires the control pairing selection with the least interactions and a set-point controller is calculated. An interactions reduction effect controller is calculated for each loop by defining a suitable complementary sensitivity function. The proposed controller design method is simple and systematic in relation with the desired closed loop specifications of each loop. The controllers obtained ensure robustness to process variations. Two illustrative examples are presented to show the merits of the proposed method.

Two degree of freedom control scheme for unstable processes with small time delay

In this work, a recently reported two degree of freedom control structure namely the Parallel Control Structure (PCS) is modified to achieve improved control performance for unstable processes. New tuning rules are proposed using a direct synthesis approach for unstable first order plus time delay (UFOPTD), unstable second order plus time delay (USOPTD) and integrating and unstable plus time delay (IUPTD) process models. The proposed method has two tuning parameters whose suitable values are recommended after studying their effect on the system performance and robustness. This is an advantage of the present work over the other reported control schemes where the authors provide suitable ranges of the tuning parameters values. Simulation examples show that the present scheme results in improved nominal and perturbed responses as compared to the recently reported methods. Also it is observed that satisfactory performance is achieved when the proposed tuning rules are applied on an isothermal chemical reactor which exhibits multiple steady state solutions.

Improving feedforward disturbance compensation capabilities in Generalized Predictive Control

This paper deals with the measurable disturbance rejection problem in Generalized Predictive Control (GPC). First, it is analyzed how the unconstrained GPC algorithm with implicit disturbance compensation can be interpreted as a typical feedback plus feedforward control scheme, where the main feature is that the feedforward action includes future estimations of the measurable disturbances. Then, it is shown that classical GPC cannot always eliminate the effect of measurable disturbances even using perfect disturbance models and having exact disturbance estimations along the prediction horizon. To overcome this problem a particular GPC tuning condition is proposed, which allows the improved GPC controller to eliminate the disturbance effect even in those cases where causality and instability problems can appear in the relation between the dynamics of the load disturbance and the process output with the dynamics of the control signal and the process output. Since the new tuning condition for disturbance compensation in GPC leads to a high bandwidth in the feedback loop, a two degrees of freedom control scheme within the Filtered Smith Predictor (FSP)-based GPC framework has been implemented to improve the robustness capabilities of the control law. Simulation examples are presented to show the main advantages of the proposed control scheme, including a realistic simulation based on a greenhouse climate control problem where estimators for the main process disturbances are also designed.

The input amplitude saturation problem in QFT: A survey

In this work the input amplitude saturation problem is analysed in the Quantitative Feedback Theory (QFT) framework. This paper reviews previous works in the literature dealing with the input amplitude saturation problem in the presence of an uncertain plant in the frequency domain using QFT. The objective of this paper is to compare the different available approaches and summarize the design process for each case so that this paper can be used as a tutorial; there are six main approaches to this problem. Two of these approaches use the classical two degrees of freedom control scheme for QFT; in both of these, the design constraints of a linear QFT compensator are added in the loop shaping stage: they are added in the first approach to avoid excitation of the actuator saturation and in the second one to guarantee global stability. The other three techniques are considered as anti-windup (AW) approaches. Starting from a base design in QFT with two degrees of freedom, the first AW approach introduces a third degree of freedom that guarantees the stability of the system, allowing for base designs for high performance. The other two AW approaches also introduce a third degree of freedom, but they take simple stability considerations into account and focus on the performance of the system. The last solution consists of using a reference governor technique, which guarantees the computation of a reference signal for an inner control loop that is shaped using QFT in such a way that robust stability will be guaranteed. The reference governor technique is a time domain approach that implies the resolution of an optimization problem. The rest of the approaches are frequency domain techniques based on a loop shaping method in the traditional QFT sense.

Pressure control of hydraulic servo system using proportional control valve

The purpose of this study is to build up control scheme that promptly control pressure in a hydraulic cylinder having comparatively small control volume, using a PCV (proportional control valve) and a digital computer. Object pressure control system has the character to be unstable easily, because the displacement-flow gain of the PCV is too large considering the small volume of the hydraulic cylinder and the time delay of response of the PCV is comparatively long. Considering the above-mentioned characteristics of the object pressure control system, in this study, control system is designed with two degree of freedom control scheme that is composed by adding a feed-forward control path to I-PDD control system, and a reference model is used on the decision of control parameters. And through some experiments on the control system with FF-I-PDD controller, the validity of this control method has been confirmed.