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### Editorial

J. Dyn. Sys., Meas., Control. 2017;140(2):020201-020201-4. doi:10.1115/1.4038637.
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Commentary by Dr. Valentin Fuster

### Review Article

J. Dyn. Sys., Meas., Control. 2017;140(2):020801-020801-17. doi:10.1115/1.4037295.

This paper delivers an in-depth review of the state-of-the-art technologies relevant to rail flaw detection giving emphasis to their use in detection of rail flaw defects at practical inspection vehicle speeds. The review not only looks at the research being carried out but also investigates the commercial products available for rail flaw detection. It continues further to identify the methods suitable to be adopted in a moving vehicle rail flaw detection system. Even though rail flaw detection has been a well-researched area for decades, an in-depth review summarizing all available technologies together with an assessment of their capabilities has not been published in the recent past according to the knowledge of the authors. As such, it is believed that this review paper will be a good source of information for future researchers in this area.

Topics: Flaw detection , Rails , Waves
Commentary by Dr. Valentin Fuster

### Research Papers

J. Dyn. Sys., Meas., Control. 2017;140(2):021001-021001-14. doi:10.1115/1.4037329.

The design of a robust fixed low-order controller for uncertain decoupled multi-input multi-output (MIMO) systems is proposed in this paper. The simplified decoupling is used as a decoupling system technique. In this work, the real system behavior is described by a linear model with parametric uncertainties. The main objective of the control law is to satisfy, in presence of model uncertainties, some step response performances such as the settling time and the overshoot. The controller parameters are obtained by resolving a min–max nonconvex optimization problem. The resolution of this kind of problems using standard methods can generate a local solution. Thus, we propose, in this paper, the use of the generalized geometric programming (GGP) which is a global optimization method. Simulation results and a comparison study between the presented approach, a proportional integral (PI) controller, and a local optimization method are given in order to shed light the efficiency of the proposed controller.

Commentary by Dr. Valentin Fuster
J. Dyn. Sys., Meas., Control. 2017;140(2):021002-021002-12. doi:10.1115/1.4037270.
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This paper investigates the supervisory-level, fault tolerant control of a 2004 Prius powertrain. The fault considered is an interturn short circuit (ITSC) fault in the traction drive (a surface mount permanent magnet synchronous machine (SPMSM) for which its rotor is part of the vehicle's driveline). ITSC faults arise from electrical insulation failures in the stator windings where part of a phase winding remains functional while the remaining decoupled windings form a self-contained loop. Because the permanent magnets on the rotor (driveline) shaft are able to induce very large eddy currents in this self-contained loop if its rotational velocity is left unchecked, the maximum allowable driveline speed, and consequently, vehicle speed, must be reduced to avoid exceeding the drive's operational thermal limits. A method for detecting these ITSC faults and the induced eddy current in an SPMSM using a moving horizon observer (MHO) is reviewed. These parameters then determine which previously computed, fault-level-dependent SPMSM input–output power efficiency map and maximum safe operating speed is utilized by the supervisory-level controller. The fault tolerant control is demonstrated by simulating a Prius over a 40 s drive velocity profile with fault levels of 0.5%, 1%, 2%, and 5% detected at the midpoint of the profile. For comparison, the Prius is also simulated without a traction motor fault. Results show that the control reduces vehicle velocity upon detection of a fault to an appropriate safe value.

Commentary by Dr. Valentin Fuster
J. Dyn. Sys., Meas., Control. 2017;140(2):021003-021003-9. doi:10.1115/1.4037333.

This paper considers the problem of reliable finite-time robust control for uncertain mechanical systems with stochastic actuator failures and aperiodic sampling. A novel model of actuator failure capable of depicting various faulty modes is developed on the basis of homogenous Markov variable. To guarantee the finite-time stability (FTS) and boundedness, a novel fault-tolerant switching controller is developed by virtue of Lyapunov–Krasovskii functional and stochastic analysis technique, simultaneously, the finite-time H performance is also ensured to attenuate the mechanical vibration caused by external disturbances. With convex optimization algorithm, the anticipated controller can be procured by solving a set of linear matrix inequalities (LMIs). Finally, two practical examples of mechanical systems, one of which is governed by lumped parameters and the other is described by distributed parameters, are proposed to prove the effectiveness of the theoretical developments of this study.

Commentary by Dr. Valentin Fuster
J. Dyn. Sys., Meas., Control. 2017;140(2):021004-021004-10. doi:10.1115/1.4037271.

Commentary by Dr. Valentin Fuster
J. Dyn. Sys., Meas., Control. 2017;140(2):021005-021005-12. doi:10.1115/1.4037287.

A method is presented for slip analysis of a wheeled mobile manipulator. The said system consists of an industrial manipulator mounted on a mobile platform performing aircraft manufacturing tasks. Unlike tracked/legged mobile robots that may slip when negotiating slopes or climbing stairs, a wheeled mobile manipulator may slip resulting from the manipulator movement or the forces from the end-effector during fastening. Slip analysis is crucial to ensure pose accuracy for operation. In this study, first a universal friction constraint is used to derive the slip condition of the system. Three cases are considered, with the first case considering the reaction force in relation to the stand-off distance between the mobile manipulator and the workpiece. The second case deals with the joint speeds to investigate the effect of coupling terms including centrifugal forces and gyroscopic moments on slip. The third case deals with the joint accelerations to investigate the effect of inertia forces and moments on slip. Simulations and experiments are carried out to verify the proposed method.

Commentary by Dr. Valentin Fuster
J. Dyn. Sys., Meas., Control. 2017;140(2):021006-021006-8. doi:10.1115/1.4037297.

Developing a flywheel energy storage system (FESS) with permanent magnetic bearing (PMB) and spiral groove bearing (SGB) brings a great challenge to dynamic control for the rotor system. In this paper, a pendulum-tuned mass damper is developed for 100 kg-class FESS to suppress low-frequency vibration of the system; the dynamic model with four degrees-of-freedom is built for the FESS using Lagrange's theorem; mode characteristics, critical speeds, and unbalance responses of the system are analyzed via theory and experiment. A comparison between the theoretical results and the experiment ones shows that the pendulum-tuned mass damper is effective, the dynamic model is appropriate, and the FESS can run smoothly within the working speed range.

Commentary by Dr. Valentin Fuster
J. Dyn. Sys., Meas., Control. 2017;140(2):021007-021007-11. doi:10.1115/1.4037388.

This paper deals with passive stabilization of thermoacoustic dynamics in a Rijke tube using a Helmholtz resonator. Thermoacoustic instabilities result from the dynamic coupling between the heat release and pressure in a chamber. Helmholtz resonators are used akin to vibration absorbers to suppress unwanted pressure oscillations in such structures and prevent instabilities. The first contribution of the paper is a state-space representation of the thermoacoustic dynamics for the resonator-mounted Rijke tube. This relationship happens to be in the class of linear time invariant, neutral multiple time delay systems (LTI-NMTDS). Then, benefiting from the cluster treatment of characteristic roots (CTCR) paradigm, we investigate the effect of resonator location on suppression of thermoacoustic instability. CTCR is a mathematical tool that determines the stability of LTI-NMTDS exhaustively and nonconservatively in the parameter space of the system. This capability provides a novel tool for the futuristic design concepts of combustors. These analytically obtained findings are also supported with experimental results from a laboratory-scale Rijke tube. In addition, a conceptual case study is presented where the stabilizing contributions of the resonator to the dynamics are investigated under strong thermoacoustic coupling.

Commentary by Dr. Valentin Fuster
J. Dyn. Sys., Meas., Control. 2017;140(2):021008-021008-9. doi:10.1115/1.4037528.

This paper focuses on the robust control problem for a class of linear uncertain systems by using frequency techniques. The controller/observer dynamics are analyzed using Lyapunov techniques, in terms of the state and state estimation error, for an uncertainty constrained over a specified range. A Popov-type criterion, a “circle criterion,” defined as the Popov frequency condition and the uncertainty circle, is formulated. It is proved that the closed-loop system is robustly stable if the Popov condition holds at all frequencies. The proposed method is validated against a robust controller for a balancing robot (BR).

Commentary by Dr. Valentin Fuster
J. Dyn. Sys., Meas., Control. 2017;140(2):021009-021009-8. doi:10.1115/1.4037386.

This paper presents a linear robust output reference trajectory tracking controller, addressed here as a flat filtering controller (FFC), for nonlinear differentially flat systems. Here, we illustrate the controller's performance, via digital computer simulations and, also, via laboratory experiments, carried out on a single link-direct current (DC) motor driven robot manipulator undergoing a reference trajectory tracking task. The proposed linear FFC only requires the output to be regulated of the composite system and none of the internal states of the resulting third-order nonlinear system. The controller is designed on the basis of a drastic simplification of the combined single link-DC motor dynamics to a, perturbed, third-order pure integration system. This demonstrates the robustness of the proposed scheme with respect to ignored nonlinear state-dependent, endogenous, disturbances and, also, to independent unstructured exogenous disturbances inevitable in an experimental setup. Simulation and experimental results, as well as comparisons with other controllers, are presented.

Commentary by Dr. Valentin Fuster
J. Dyn. Sys., Meas., Control. 2017;140(2):021010-021010-8. doi:10.1115/1.4037735.

Researchers often use mechanisms that consist of massless rods and concentrated masses in order to capture the dynamics of robotic locomotors. A kinematic prototyping tool that captures all possible locomotion modes of a given kinematic mechanism can be very useful in conceiving and designing such systems. Previously, we proposed a family of mechanisms that consist of two types of primitive building units: a single mass with a built-in revolute joint and a massless connection rod. This family starts from a single bouncing mass and progressively evolves into more complex generations. In this paper, we present a prototyping tool that generates all possible locomotion cycles of particle-based linear chain mechanisms. A new skip impact concept is introduced to describe the relative motion of the moving masses and the masses on the ground. Also, the paper represents a graphical user interface (GUI) that facilitates data input and the visualization of the locomotion modes.

Commentary by Dr. Valentin Fuster
J. Dyn. Sys., Meas., Control. 2017;140(2):021011-021011-10. doi:10.1115/1.4037652.
OPEN ACCESS

An efficient and power dense high pressure air compressor/expander (C/E) is critical for the success of a compressed air energy storage (CAES) system. There is a tradeoff between efficiency and power density that is mediated by heat transfer within the compression/expansion chamber. This paper considers the optimal control for the compression and expansion processes that provides the optimal tradeoff between efficiency and power. Analytical Pareto optimal solutions are developed for the cases in which hA, the product of the heat transfer coefficient and heat transfer surface area, is either a constant or is a function of the air volume. It is found that the optimal trajectories take the form “fast-slow-fast” where the fast stages are adiabatic and the slow stage is either isothermal for the constant-hA assumption, or a pseudo-isothermal (where the temperature depends on the instantaneous hA) for the volume-varying-hA assumption. A case study shows that at 90% compression efficiency, power gains are in the range of $500−1500%$ over ad hoc linear and sinusoidal profiles.

Commentary by Dr. Valentin Fuster
J. Dyn. Sys., Meas., Control. 2017;140(2):021012-021012-8. doi:10.1115/1.4037529.

In this paper, we propose a new coupled dynamical model of a horizontal axis wind turbine (HAWT). The proposed model takes into consideration the dynamic coupling of the flexible tower with both bending and torsion of the flexible blades. This model also accounts for the dynamics of an additional point mass located in one of the blades to simulate a crack. In addition, a finite element model (FEM) analysis along with an experimental study is conducted in this research to validate the modal analysis of a HAWT prototype. Data from the analytical, numerical, and experimental investigations were collected and showed comparable findings. Using the analytical model, the modal analysis and the steady-state response of the HAWT prototype are performed for two configurations: with and without a crack. In this paper, we also propose a new model-based technique for the detection of cracks in the HAWT.

Commentary by Dr. Valentin Fuster

### Technical Brief

J. Dyn. Sys., Meas., Control. 2017;140(2):024501-024501-7. doi:10.1115/1.4037288.

Real-time detection and decision and control of thermoacoustic instabilities in confined combustors are challenging tasks due to the fast dynamics of the underlying physical process. The objective here is to develop a dynamic data-driven algorithm for detecting the onset of instabilities with short-length time-series data, acquired by available sensors (e.g., pressure and chemiluminescence), which will provide sufficient lead time for active decision and control. To this end, this paper proposes a Bayesian nonparametric method of Markov modeling for real-time detection of thermoacoustic instabilities in gas turbine engines; the underlying algorithms are formulated in the symbolic domain and the resulting patterns are constructed from symbolized pressure measurements as probabilistic finite state automata (PFSA). These PFSA models are built upon the framework of a (low-order) finite-memory Markov model, called the D-Markov machine, where a Bayesian nonparametric structure is adopted for: (i) automated selection of parameters in D-Markov machines and (ii) online sequential testing to provide dynamic data-driven and coherent statistical analyses of combustion instability phenomena without solely relying on computationally intensive (physics-based) models of combustion dynamics. The proposed method has been validated on an ensemble of pressure time series from a laboratory-scale combustion apparatus. The results of instability prediction have been compared with those of other existing techniques.

Commentary by Dr. Valentin Fuster
J. Dyn. Sys., Meas., Control. 2017;140(2):024502-024502-7. doi:10.1115/1.4037530.

In this paper, we propose a new method for an optimal systematic determination of models' base for multimodel identification. This method is based on the neural classification of data set picked out on a considered nonlinear system. The obtained cluster centers are exploited to provide the weighting functions and to deduce the corresponding dispersions and their models' base. A simulation example and an experimental validation on a chemical reactor are presented to evaluate the effectiveness of the proposed method.

Commentary by Dr. Valentin Fuster