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IN THIS ISSUE

### Review Article

J. Dyn. Sys., Meas., Control. 2018;141(2):020801-020801-16. doi:10.1115/1.4041063.

This paper reviews the state of the art of directly driven proportional directional hydraulic spool valves, which are widely used hydraulic components in the industrial and transportation sectors. First, the construction and performance of commercially available units are discussed, together with simple models of the main characteristics. The review of published research focuses on two key areas: investigations that analyze and optimize valves from a fluid dynamic point of view, and then studies on spool position control systems. Mathematical modeling is a very active area of research, including computational fluid dynamics (CFD) for spool geometry optimization, and dynamic spool actuation and motion modeling to inform controller design. Drawbacks and advantages of new designs and concepts are described in the paper.

Commentary by Dr. Valentin Fuster

### Research Papers

J. Dyn. Sys., Meas., Control. 2018;141(2):021001-021001-7. doi:10.1115/1.4041299.

The working conditions of the propulsion system of ships are affected by many factors and partially by hull deformations and lubricating oil film. In order to solve the problem of engineering application of reliability assessment and control of ship propulsion system on heavy sea, a mechanical model of ship shafting-oil film-stern structure coupled system is established. The hull and shafting are studied as a whole, and a test rig with the wave loads system is assembled. By carrying out the integrative analysis and physical experiment, the motion characteristics of the system are analyzed. According to the various types of wave loads which ship faces on ocean, the influence of the stern structure on the vibration characteristics of the shafting is obtained. It is concluded that the coupling degree of shafting and stern structure is correlated with the natural frequency of the coupled system, and the wave-induced loads response is correlated with wave encountering frequency. The characteristics of the shafting-oil film-stern structure system, such as the maximum amplitude of tail shaft and the minimum oil film thickness of bearing, are significantly modified under the influence of stern structure.

Commentary by Dr. Valentin Fuster
J. Dyn. Sys., Meas., Control. 2018;141(2):021002-021002-11. doi:10.1115/1.4041448.

A new approach to modeling and linearization of nonlinear lumped-parameter systems based on physical modeling theory and a data-driven statistical method is presented. A nonlinear dynamical system is represented with two sets of differential equations in an augmented space consisting of independent state variables and auxiliary variables that are nonlinearly related to the state variables. It is shown that the state equation of a nonlinear dynamical system having a bond graph model of integral causality is linear, if the space is augmented by using the output variables of all the nonlinear elements as auxiliary variables. The dynamic transition of the auxiliary variables is investigated as the second set of differential equations, which is linearized by using statistical linearization. It is shown that the linear differential equations of the auxiliary variables inform behaviors of the original nonlinear system that the first set of state equations alone cannot represent. The linearization based on the two sets of linear state equations, termed dual faceted linearization (DFL), can capture diverse facets of the nonlinear dynamics and, thereby, provide a richer representation of the nonlinear system. The two state equations are also integrated into a single latent model consisting of all significant modes with no collinearity. Finally, numerical examples verify and demonstrate the effectiveness of the new methodology.

Commentary by Dr. Valentin Fuster
J. Dyn. Sys., Meas., Control. 2018;141(2):021003-021003-10. doi:10.1115/1.4041445.

Pneumatic artificial muscles (PAMs) are an interesting type of actuators as they provide high power-to-weight and power-to-volume ratio. However, their efficient use requires very accurate control methods taking into account their complex and nonlinear dynamics. This paper considers a two degrees-of-freedom platform whose attitude is determined by three pneumatic muscles controlled by servovalves. An overactuation is present as three muscles are controlled for only two degrees-of-freedom. The contribution of this work is twofold. First, whereas most of the literature approaches the control of systems of similar nature with sliding mode control, we show that the platform can be controlled with the flatness-based approach. This method is a nonlinear open-loop controller. In addition, this approach is model-based, and it can be applied thanks to the accurate models of the muscles, the platform and the servovalves, experimentally developed. In addition to the flatness-based controller, which is mainly a feedforward control, a proportional-integral (PI) controller is added in order to overcome the modeling errors and to improve the control robustness. Second, we solve the overactuation of the platform by an adequate choice for the range of the efforts applied by the muscles. In this paper, we recall the basics of this control technique and then show how it is applied to the proposed experimental platform. At the end of the paper, the proposed approach is compared to the most commonly used control method, and its effectiveness is shown by means of experimental results.

Commentary by Dr. Valentin Fuster
J. Dyn. Sys., Meas., Control. 2018;141(2):021004-021004-9. doi:10.1115/1.4041300.

In this paper, a mathematical model to simulate the pressure and flow rate characteristics of a spool valve is derived. To improve the simulation accuracy, the discharge coefficient through the spool valve ports is assumed to be a function of both the Reynolds number and the orifice geometry rather than treating it as a constant. Parameters of the model are determined using the data obtained by computational fluid dynamics (CFD) analyses conducted on two-dimensional axisymmetric domains using ANSYS Fluent 15® commercial software. For turbulence modeling, shear stress transport (SST) k–ω model is preferred after a comparison of performance with the other available turbulence model options. The resulting model provides consistent pressure and flow rate estimations with CFD analyses and a smooth transition between different geometrical conditions. The ultimate aim of this study is to fulfill the need for a model to precisely determine the geometrical tolerances of spool valve components for optimum performance. Estimations of the developed model is compared with the experimental data of a spool valve, and the model is proved to be able to accurately estimate the maximum leakage flow rate, the pressure sensitivity, and the shapes of leakage flow/load pressure curves.

Commentary by Dr. Valentin Fuster
J. Dyn. Sys., Meas., Control. 2018;141(2):021005-021005-10. doi:10.1115/1.4041447.

This paper presents tracking strategies for the attitude dynamics of a rigid body that are global on the configuration space SO(3) and semiglobal over the phase space $SO(3)×ℝ3$. It is well known that global attractivity is prohibited for continuous attitude control systems on the special orthogonal group. Such topological restriction has been dealt with either by constructing smooth attitude control systems that exclude a set of zero measure in the region of attraction or by introducing discontinuities in the control input. This paper proposes nonmemoryless attitude control systems that are continuous in time, where the region of attraction guaranteeing exponential convergence completely covers the special orthogonal group. This provides a new framework to address the topological restriction in attitude controls. The efficacy of the proposed methods is illustrated by numerical simulations and an experiment.

Commentary by Dr. Valentin Fuster
J. Dyn. Sys., Meas., Control. 2018;141(2):021006-021006-9. doi:10.1115/1.4041504.

To make motion perception more realistic, a current-implemented classical washout motion cueing algorithm (CWMCA) is extended to a model predictive motion cueing algorithm (MPMCA) for a seven-cylinder pneumatically actuated Stewart platform. Through this enhancement, not only are potential predictive signals taken into account, but also comprehensive information and data sets relating to the mechanical limitations of the simulator platform. The significantly increased information content enables the calculation of far more specific targeted requirements for the platform. First, the platform kinematics are derived and its physical platform constraints are examined. Furthermore, the CWMCA is extended and transformed into a state space-based motion cueing algorithm for the purpose of setting up a linear quadratic MPMCA. Finally, the MPMCA is simulated and evaluated with respect to its degree of realism.

Commentary by Dr. Valentin Fuster
J. Dyn. Sys., Meas., Control. 2018;141(2):021007-021007-12. doi:10.1115/1.4041358.

An approach for building a real-time simulation and testing platform for a novel seamless two-speed automated manual transmission (AMT) for electric vehicles (EVs) is proposed and experimentally evaluated. First, the structure of the AMT and the dynamic model of an EV powertrain system equipped with the AMT are presented. Then, according to the testing requirements, a prototype of the AMT, hardware components and software system of the platform are designed. Unlike a real-time transmission test bench, of which the real-time simulation and control system (RSCS) is built based on a dedicated simulator, the RSCS of the platform is built based on a standard desktop personal computer (PC) by using a useful and low-cost solution from matlab/simulink®. Additionally, a simulation model of EV, which is equipped with the AMT and is more suitable for hardware-in-the-loop (HIL) simulation, has been developed. In particular, for conducting various dynamic mechanical tests, the platform is combined with induction motors (IMs), which are adopted with direct torque control (DTC) technique to emulate the dynamic driving conditions of the transmission. The designed platform can be used for different test techniques, including rapid simulation, rapid control prototyping, HIL simulation as well as dynamic mechanical tests. The work expands the capability of the platform and makes the test conditions become closer to reality. Simulation and experimental results indicate that the platform responds well to the real-time dynamic requirements, and it is very useful for developing the proposed transmission.

Commentary by Dr. Valentin Fuster
J. Dyn. Sys., Meas., Control. 2018;141(2):021008-021008-14. doi:10.1115/1.4040977.

The problem of robust and optimal output feedback design for interval state-space systems is addressed in this paper. Indeed, an algorithm based on set inversion via interval analysis (SIVIA) combined with interval eigenvalues computation and eigenvalues clustering techniques is proposed to seek for a set of robust gains. This recursive SIVIA-based algorithm allows to approximate with subpaving the set solutions $[K]$ that satisfy the inclusion of the eigenvalues of the closed-loop system in a desired region in the complex plane. Moreover, the LQ tracker design is employed to find from the set solutions $[K]$ the optimal solution that minimizes the inputs/outputs energy and ensures the best behaviors of the closed-loop system. Finally, the effectiveness of the algorithm is illustrated by a real experimentation on a piezoelectric tube actuator.

Commentary by Dr. Valentin Fuster
J. Dyn. Sys., Meas., Control. 2018;141(2):021009-021009-8. doi:10.1115/1.4041449.

Series elastic actuators (SEA) are widely used for impact protection and compliant behavior, but they typically fall short in tasks calling for accurate position control. In this paper, we propose a simple and effective heuristic for tuning series elastic actuator controllers to a high impedance position control behavior, which compares favorably with previous publications. Our approach considers two models, an ideal model and a nonideal model with time delays and filtering lag. The ideal model is used to design cascaded proportional-derivative (PD)-type outer impedance and inner force loops as a function of critically damped closed-loop poles for the force and impedance loops. The nonideal model provides an estimate of the phase margin of the position controller for each candidate controller design. A simple optimization algorithm finds the best high-impedance behavior for which the nonideal model meets a desired phase margin requirement. In this way, the approach automates the trade-off between force and impedance bandwidth. The effect of important system parameters on the impedance bandwidth is also analyzed and the proposed method verified with a physical actuator.

Commentary by Dr. Valentin Fuster
J. Dyn. Sys., Meas., Control. 2018;141(2):021010-021010-12. doi:10.1115/1.4041383.

The full- and reduced-order fault detection filter design is examined for fault diagnosis in linear time-invariant (LTI) systems in the presence of noise and disturbances. The fault detection filter design problem is formulated as an $H∞$ problem using a linear fractional transformation (LFT) framework and the solution is based on the bounded real lemma (BRL). Necessary and sufficient conditions for the existence of the fault detection filter are presented in the form of linear matrix inequalities (LMIs) resulting in a convex problem for the full-order filter design and a rank-constrained nonconvex problem for the reduced-order filter design. By minimizing the sensitivity of the filter residuals to noise and disturbances, the fault detection objective is fulfilled. A reference model can be incorporated in the design in order to shape the desired performance of the fault detection filter. The proposed fault detection and isolation (FDI) framework is applied to detect instrumentation and sensor faults in fluid transmission and pipeline systems. To this end, a lumped parameter framework for modeling infinite-dimensional fluid transient systems is utilized and a low-order model is obtained to pursue the instrumentation fault diagnosis objective. Full- and reduced-order filters are designed for sensor FDI. Simulations are conducted to assess the effectiveness of the proposed fault detection approach.

Commentary by Dr. Valentin Fuster
J. Dyn. Sys., Meas., Control. 2018;141(2):021012-021012-10. doi:10.1115/1.4041444.

Additive manufacturing (AM) processes fabricate parts by adding material in a layer-by-layer fashion. In order to enable closed-loop process control—a major hurdle in the adoption of most AM processes—compact models suitable for control design and for describing the layer-by-layer material addition process are needed. This paper proposes a two-dimensional modeling framework whereby the deposition of the current layer is affected by both in-layer and layer-to-layer dynamics, both of which are driven by the state of the previous layer. The proposed framework can be used to describe phenomena observed in AM processes such as layer rippling and large defects in laser metal deposition (LMD) processes. Further, the proposed framework can be used to create two-dimensional dynamic models for the analysis of layer-to-layer stability and as a foundation for the design of layer-to-layer controllers for AM processes. In the application to LMD, a two-dimensional linear–nonlinear–linear (LNL) repetitive process model is proposed that contains a linear dynamic component, which describes the dynamic evolution of the process from layer to layer, cascaded with a static nonlinear component cascaded with another linear dynamic component, which describes the dynamic evolution of the process within a given layer. A methodology, which leverages the two-dimensional LNL structure, for identifying the model process parameters is presented and validated with quantitative and qualitative experimental results.

Commentary by Dr. Valentin Fuster
J. Dyn. Sys., Meas., Control. 2018;141(2):021013-021013-7. doi:10.1115/1.4041355.

This paper proposes a detection method of driver fatigue by use of electrocardial signals. First, lifting wavelet transform (LWT) was used to reduce signal noise and its effect was confirmed by applying it to the denoising of a white-noise-mixed Lorenz signal. Second, phase space reconstruction was conducted for extracting chaotic features of the measured electrocardial signals. The phase diagrams show fractal geometry features even under a strong noise background. Finally, Kolmogorov entropy, which is a factor reflecting the uncertainty in and the chaotic level of a nonlinear dynamic system, was used as an indicator of driver fatigue. The effectiveness of Kolmogorov entropy in the judgment of driver fatigue was confirmed by comparison with a semantic differential (SD) subjective evaluation experiment. It was demonstrated that Kolmogorov entropy has a strong relationship with driver fatigue. It decreases when fatigue occurs. Furthermore, the influences of delay time and sampling points on Kolmogorov entropy were investigated, since the two factors are important to the actual use of the proposed detection method. Delay time may have significant influence on fatigue determination, but sampling points are relatively inconsequential. This result indicates that real-time detection can be realized by selecting a reasonably small number of sampling points.

Commentary by Dr. Valentin Fuster
J. Dyn. Sys., Meas., Control. 2018;141(2):021014-021014-12. doi:10.1115/1.4041356.

Inspired by fast model predictive control (MPC), a new nonlinear optimal command tracking technique is presented in this paper, which is named as “Tracking-oriented Model Predictive Static Programming (T-MPSP).” Like MPC, a model-based prediction-correction approach is adopted. However, the entire problem is converted to a very low-dimensional “static programming” problem from which the control history update is computed in closed-form. Moreover, the necessary sensitivity matrices (which are the backbone of the algorithm) are computed recursively. These two salient features make the computational process highly efficient, thereby making it suitable for implementation in real time. A trajectory tracking problem of a two-wheel differential drive mobile robot is presented to validate and demonstrate the proposed philosophy. The simulation studies are very close to realistic scenario by incorporating disturbance input, parameter uncertainty, feedback sensor noise, time delays, state constraints, and control constraints. The algorithm has been implemented on a real hardware and the experimental validation corroborates the simulation results.

Commentary by Dr. Valentin Fuster
J. Dyn. Sys., Meas., Control. 2018;141(2):021015-021015-9. doi:10.1115/1.4041530.

An integrated and general methodology is required to define an ideal relation between input controls and structural parameters of a system in trajectory tracking problems. For underactuated manipulators, a synergistic optimal design should be able to reduce elastic deformations, mass of the structure, and actuation forces. The key advantage of such integrated approach is the capability to search in a feasible design space, to account for many dynamic couplings in an early design stage, and to avoid simplifying assumptions which would induce to suboptimal design. Particularly, some advances considering underactuated manipulators are the possibility to treat nonminimum phase systems, then lighter structures could be selected, since bounded and smoother solution can be generated. A synergistic consideration, in order to find the desired requirements and realize the specified task through an optimal control problem, is in evidence, where a generalization of an inverse dynamics problem is defined. A planar underactuated manipulator is considered for the methodology application.

Commentary by Dr. Valentin Fuster