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Research Papers

J. Dyn. Sys., Meas., Control. 2017;139(10):101001-101001-9. doi:10.1115/1.4036032.
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This paper presents a model to explain complex nonminimum phase (CNMP) zeros seen in the noncollocated frequency response of a large-displacement XY flexure mechanism, which employs multiple double parallelogram flexure modules (DPFMs) as building-blocks. Geometric nonlinearities associated with large displacement along with the kinematic under-constraint in the DPFM lead to a coupling between the X and Y direction displacements. Via a lumped-parameter model that captures the most relevant geometric nonlinearity, it is shown that specific combinations of the operating point (i.e., flexure displacement) and mass asymmetry (due to manufacturing tolerances) give rise to CNMP zeros. This model demonstrates the merit of an intentionally asymmetric design over an intuitively symmetric design in avoiding CNMP zeros. Furthermore, a study of how the eigenvalues and eigenvectors of the flexure mechanism vary with the operating point and mass asymmetry indicates the presence of curve veering when the system transitions from minimum phase to CNMP. Based on this, the hypothesis of an inherent correlation between CNMP zeros and curve veering is proposed.

Commentary by Dr. Valentin Fuster
J. Dyn. Sys., Meas., Control. 2017;139(10):101002-101002-12. doi:10.1115/1.4036234.

In this paper, we pay attention to studying the switched model of the hydroturbine governing system (HTGS) by introducing the concept of the switching of operational conditions. More specifically, utilizing the data of an existent hydropower station in China, we propose six nonlinear dynamic transfer coefficients of the hydroturbine, which can better describe the dynamic characteristics of the HTGS in the process of load rejection transient. Moreover, the elastic water hammer-impact of the penstock system and the nonlinearity of the generator for the process of load rejection transient are considered. Based on the combination of the different regulation modes of the governor and the corresponding running conditions of the hydroelectric generating unit, a novel nonlinear dynamic switched mathematical model of the HTGS is finally established. Meanwhile, the nonlinear dynamic behaviors of the governing system are exhaustively investigated using numerical simulations. These methods and analytical results will provide some theory bases for running a hydropower station.

Commentary by Dr. Valentin Fuster
J. Dyn. Sys., Meas., Control. 2017;139(10):101003-101003-10. doi:10.1115/1.4036364.

The design of nonlinear tracking controller for antagonistic tendon-driven joint has garnered considerable attention, whereas many existing control methodologies are impractical in the real-time applications due to complexity of computations. In this work, a robust adaptive control method for controlling antagonistic tendon-driven joint is mainly studied by combining adaptive control with mapping filtered forwarding technique. To enhance the robustness of the closed-loop systems, the true viscous friction coefficients are not needed to be known in our controller design. Typically, to tackle the problem of “explosion of complexity,” filters are introduced to bridge the virtual controls such that the controller is decomposed into several submodules. Mappings and their analytic derivatives are computed by these filters, and the mathematical operations of nonlinearities are greatly simplified. The block diagram of this controller of tendon-driven joint is provided, and controller performances are validated through simulations.

Commentary by Dr. Valentin Fuster
J. Dyn. Sys., Meas., Control. 2017;139(10):101004-101004-11. doi:10.1115/1.4036365.

This paper investigates a delay-dependent robust control problem of discrete-time uncertain stochastic systems with delays. The uncertainty considered in this paper is time-varying but norm-bounded, and the delays are considered as interval time-varying case for both state and input. According to the considerations of uncertainty, stochastic behavior, and time delays, the problem considered in this paper is more general than the existing works for uncertain stochastic systems. Via the proposed Lyapunov–Krasovskii function, some sufficient conditions are derived into the extended linear matrix inequality form. Moreover, Jensen inequality and free matrix equation are employed to reduce conservatism of those conditions. Through using the proposed design method, a gain-scheduled controller is designed to guarantee asymptotical stability of uncertain stochastic systems in the sense of mean square. Finally, two numerical examples are provided to demonstrate applicability and effectiveness of the proposed design method.

Commentary by Dr. Valentin Fuster
J. Dyn. Sys., Meas., Control. 2017;139(10):101005-101005-9. doi:10.1115/1.4036409.

This paper will study the exponential stable and state feedback stabilization of time delay singular systems with saturation actuators. Some sufficient conditions for existence of controller are obtained by using the linear matrix inequalities (LMIs) and integral inequality approach (IIA). When these LMIs are feasible, an explicit expression of controller is obtained. Based on Lyapunov–Krasovskii functional (LKF) techniques, a novel exponential stabilization criterion has been also derived in terms of LMIs which can be easily solved with efficient convex optimization algorithm. Our results are less conservative than some existing ones, and the decision variables involved in this paper are less than them. Examples illustrate our results as less conservative than those reported in the literature.

Commentary by Dr. Valentin Fuster
J. Dyn. Sys., Meas., Control. 2017;139(10):101006-101006-6. doi:10.1115/1.4036564.

This paper focuses on the problem of disturbance rejection for a class of interval type-2 (IT-2) fuzzy systems via equivalence-input-disturbance (EID)-based approach. The main objective of this work is to design a fuzzy state-feedback controller combined with a disturbance estimator such that the output of the fuzzy system perfectly tracks the given reference signal without steady-state error and produces an EID to eliminate the influence of the actual disturbances. By constructing a suitable Lyapunov function and using linear matrix inequality (LMI) technique, a new set of sufficient conditions is established in terms of linear matrix inequalities for the existence of fuzzy controller. Finally, a simple pendulum model is considered to illustrate the effectiveness and applicability of the proposed EID-based control design.

Commentary by Dr. Valentin Fuster
J. Dyn. Sys., Meas., Control. 2017;139(10):101007-101007-8. doi:10.1115/1.4036554.

A fundamental part of a digital fluid power (DFP) pump is the actively controlled valves, whereby successful application of these pumps entails a need for control methods. The focus of the current paper is on a flow control method for a DFP pump. The method separates the control task concerning timing of the valve activation and the task concerning the overall flow output control. This enables application of linear control theory in the design process of the DFP pump flow controller. The linearization method is presented in a general framework and an application with a DFP pump model exemplifies the use of the method. The implementation of a discrete time linear controller and comparisons between the nonlinear model and the discrete time linear approximation shows the applicability of the control method.

Commentary by Dr. Valentin Fuster
J. Dyn. Sys., Meas., Control. 2017;139(10):101008-101008-16. doi:10.1115/1.4036537.

This paper proposes, for the first time without using any linearization or order reduction, an adaptive and model-based discharge pressure control design for the variable displacement axial piston pumps (VDAPPs), whose dynamical behaviors are highly nonlinear and can be described by a fourth-order differential equation. The rigorous stability proof, with an asymptotic convergence, is given for the entire system. In the proposed novel controller design method, the specifically designed stabilizing terms constitute an essential core to cancel out all the stability-preventing terms. The experimental results reveal that rapid parameter adaptation significantly improves the feedback signal tracking precision compared to a known-parameter controller design. In the comparative experiments, the adaptive controller design demonstrates the state-of-the-art discharge pressure control performance, enabling a possibility for energy consumption reductions in hydraulic systems driven with VDAPP.

Commentary by Dr. Valentin Fuster
J. Dyn. Sys., Meas., Control. 2017;139(10):101009-101009-9. doi:10.1115/1.4036557.

This article presents an analysis of the damping and beating effects within the aggregate power demand of heterogeneous thermostatically controlled loads (TCLs). Demand response using TCLs is an appealing method to enable higher levels of penetration of intermittent renewable resources into the electric grid. Previous literature covers the benefits of TCL population heterogeneity for control purposes, but the focus is solely on the damping observed in these systems. This work, in contrast, characterizes the combined damping and beating effects in the power demand for different types of TCL parameter heterogeneity. The forced aggregate dynamics of TCLs have been shown to be bilinear when set point temperature adjustment is used as a control input. This motivates the article's use of free response dynamics, which are linear, to characterize both the damping and beating phenomena. A stochastic parameter distribution is applied to the homogeneous power demand solution, furnishing an analytic expression for the aggregate power demand. The time-varying damping ratios of this reduced-order model characterize the damping in the system. By analyzing a variety of case studies, it is determined that only a distribution of the TCL characteristic frequency creates damping in the aggregate power dynamics. The beating effect decays over time due to damping, and a relationship between the beat's amplitude and period is presented.

Commentary by Dr. Valentin Fuster
J. Dyn. Sys., Meas., Control. 2017;139(10):101010-101010-9. doi:10.1115/1.4036551.

A robust fuzzy sliding mode controller is presented for a multiple-input–multiple-output (MIMO) Dutch-Roll system with nonaffine inputs and external disturbances. An integrating factor with a nonlinear saturation function is introduced to construct a nonlinear integral sliding mode (NISM) surface to provide better transient response than traditional sliding mode control. Fuzzy logic systems are employed to approximate the unknown nonaffine part of the system directly. Based on Lyapunov method, the tracking errors are guaranteed to be asymptotically stable with the additional adaptive compensation terms. To verify the feasibility and effectiveness of the proposed controller, the Dutch-Roll system is presented for simulation.

Commentary by Dr. Valentin Fuster
J. Dyn. Sys., Meas., Control. 2017;139(10):101011-101011-9. doi:10.1115/1.4036539.

This paper presents experimental investigation results of an electric variable valve timing (EVVT) actuator using linear parameter varying (LPV) system identification and control. For the LPV system identification, a number of local system identification tests were carried out to obtain a family of linear time-invariant (LTI) models at fixed engine speed and battery voltage. Using engine speed and battery voltage as time-varying scheduling parameters, the family of local LTI models is translated into a single LPV model. Then, a robust gain-scheduling (RGS) dynamic output-feedback (DOF) controller with guaranteed H performance was synthesized and validated experimentally. In contrast to the vast majority of gain-scheduling literature, scheduling parameters are assumed to be polluted by measurement noises and the engine speed and battery voltage are modeled as noisy scheduling parameters. Experimental and simulation results show the effectiveness of the developed approach.

Commentary by Dr. Valentin Fuster
J. Dyn. Sys., Meas., Control. 2017;139(10):101012-101012-7. doi:10.1115/1.4036536.

In this paper, linear quadratic regulator (LQR) theory is applied to solve the inverse optimal consensus problem for a second-order linear multi-agent systems (MAS) under independent position and velocity topology. The optimal Laplacian matrices related to the topologies of position and velocity are derived by solving the algebraic Riccati equation (ARE). Theoretically, we obtain the optimal Laplacian matrices, which correspond to the directed strongly connected graphs, for the second-order multi-agent systems. Finally, two simulation examples are provided to verify the theoretical analysis of this paper.

Commentary by Dr. Valentin Fuster
J. Dyn. Sys., Meas., Control. 2017;139(10):101013-101013-7. doi:10.1115/1.4036538.

This paper presents a method for selecting the optimal transmission ratio for an electric motor for applications for which the desired torque and motion at the transmission output are known a priori. Representative applications for which the desired output torque and motion are periodic and known include robotic manipulation, robotic locomotion, powered prostheses, and exoskeletons. Optimal transmission ratios are presented in two senses: one that minimizes the root-mean-square (RMS) electrical current and one that minimizes the RMS electrical power. An example application is presented in order to demonstrate the method for optimal transmission ratio selection.

Commentary by Dr. Valentin Fuster
J. Dyn. Sys., Meas., Control. 2017;139(10):101014-101014-9. doi:10.1115/1.4036565.

In this paper, we study vehicle formations employing ring-structured communication strategies and propose a combinatorial approach for developing ring graphs for vehicle formations. In vehicle platoons, a ring graph is formed when each vehicle receives information from its predecessor, and the lead vehicle receives information from the last vehicle, thus forming a ring in its basic form. In such basic form, the communication distance between the first and the last vehicle increases with the platoon size, which creates implementation issues due to sensing range limitations. If one were to employ a communication protocol such as the token ring protocol, the delay in updating information and communication arises from the need for the token to travel across the entire graph. To overcome this limitation, alternative ring graphs which are formed by smaller communication distances between vehicles are proposed in this paper. For a given formation and a constraint on the maximum communication distance between any two vehicles, an algorithm to generate a ring graph is obtained by formulating the problem as an instance of the traveling salesman problem (TSP). In contrast to the vehicle platoons, generation of a ring communication graph is not straightforward for two- and three-dimensional formations; the TSP formulation allows this for both two- and three-dimensional formations with specific constraints. In addition, with ring communication structure, it is possible to devise simple ways to reconfigure the graph when vehicles are added/removed to/from the formation, which is discussed in the paper. Further, the experimental results using mobile robots for platooning and two-dimensional formations using ring graphs are shown and discussed.

Topics: Vehicles
Commentary by Dr. Valentin Fuster

Technical Brief

J. Dyn. Sys., Meas., Control. 2017;139(10):104501-104501-7. doi:10.1115/1.4036549.

We develop an observer-based boundary controller for the rotary table to suppress stick–slip oscillations and to maintain the angular velocity of the drill string at a desired value during a drilling process despite unknown friction torque and by using only surface measurements. The control design is based on a distributed model of the drill string. The obtained infinite dimensional model is converted to an ordinary differential equation–partial differential equation (ODE–PDE) coupled system. The observer-based controller is designed by reformulating the problem as the stabilization of an linear time-invariant (LTI) system which is affected by a constant unknown disturbance and has simultaneous actuator and sensor delays. The main contribution of the controller is that it requires only surface measurements. We prove that the equilibrium of the closed-loop system is exponentially stable, and that the angular velocity regulation is achieved with the estimations of unknown friction torque and drill bit velocity. The effectiveness of the controller is demonstrated using numerical simulations.

Commentary by Dr. Valentin Fuster
J. Dyn. Sys., Meas., Control. 2017;139(10):104502-104502-10. doi:10.1115/1.4036408.

This paper presents a new technique for design of full-state feedback controllers for linear dynamic systems in three stages. The new technique is based on appropriate partitioning of the linear dynamic system into linear dynamic subsystems. Every controller design stage is done at the subsystem level using only information about the subsystem (reduced-order) matrices. Due to independent design in each stage, different subsystem controllers can be designed to control different subsystems. Partial subsystem level optimality and partial eigenvalue subsystem assignment can be achieved. Using different feedback controllers to control different subsystems of a system has not been present in any other known linear full-state feedback controller design technique. The new technique requires only solutions of reduced-order subsystem level algebraic equations. No additional assumptions were imposed except what is common in linear feedback control theory (the system is controllable (stabilizable)) and theory of three time-scale linear systems (the fastest subsystem state matrix is invertible)). The local full-state feedback controllers are combined to form a global full-state controller for the system under consideration. The presented results are specialized to the three time-scale linear control systems that have natural decomposition into slow, fast, and very fast subsystems, for which numerical ill conditioning is removed and solutions of the design algebraic equations are easily obtained. The proposed three-stage three time-scale feedback controller technique is demonstrated on the eighth-order model of a fuel cell model.

Commentary by Dr. Valentin Fuster
J. Dyn. Sys., Meas., Control. 2017;139(10):104503-104503-6. doi:10.1115/1.4036563.

One important problem for unmanned aerial vehicles (UAVs) in mission applications is to track ground targets automatically. A major concern is how to keep the tracking process stable and efficient while the motion of the ground targets changes rapidly. In this brief, a new guidance strategy for the ground target “Search and Capture” based on a virtual target is proposed. First, a virtual trajectory, which is composed of straight lines and arcs, is generated based on the motion of the target. The straight lines are used to capture, while the arcs are used to search, and switch between straight line and arc when some condition is met; second, we design a new guidance law based on line-of-sight (LOS) which makes a UAV to track the virtual target automatically. This new method solves the following three problems simultaneously: (1) The UAV always keeps a constant speed to track the target with changing velocity, (2) the generated trajectory meets the flight constraints of the UAV, and (3) the speed range of the ground target can be from the stationary to almost the maximum cruising speed of the UAV. Simulation results show that the proposed guidance strategy can achieve stable tracking for various motions of the ground target.

Commentary by Dr. Valentin Fuster

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