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IN THIS ISSUE

### Research Papers

J. Dyn. Sys., Meas., Control. 2017;139(11):111001-111001-16. doi:10.1115/1.4036535.

A Lyapunov stable control scheme is designed and implemented for bilateral haptic teleoperation of a single-rod hydraulic actuator. The proposed controller is capable of reducing position errors at master and slave sides, as well as perceiving the interaction force between the actuator and the task environment without a need for direct measurement of force. The controller only requires the actuator's line pressures and displacements of the master and slave. Stability of the proposed controller incorporating hydraulic nonlinearities and operator dynamics is analytically proven. Simulation studies demonstrate that the proposed system can reach an equilibrium point while interacting with an environment exhibiting stiffness. Experimental results confirm that the controller is able to effectively maintain stability, while having good position tracking by the hydraulic actuator as well as perceiving the contact force between the actuator and the task environment without direct measurement. This kind of haptic feedback force is a suitable choice for applications where mounting a force sensor at the end-effector is not feasible, such as excavators and backhoes. This work contributes to enhancing the operator's ability to perform stable haptic-enabled teleoperation of hydraulic manipulators.

Commentary by Dr. Valentin Fuster
J. Dyn. Sys., Meas., Control. 2017;139(11):111002-111002-15. doi:10.1115/1.4036808.

Actively controlled gun-launched projectiles require a means of modifying the projectile flight trajectory. While numerous potential mechanisms exist, microspoiler devices have been shown to be a promising control actuator for fin-stabilized projectiles in supersonic flight. These devices induce a trim force and moment generated by the boundary layer–shock interaction between the projectile body, rear stabilizing fins, and microspoilers. Previous investigations of microspoiler mechanisms have established estimates of baseline control authority, but experimental results have been restricted to cases in which the mechanism was statically deployed. This paper details the design and flight testing of a projectile equipped with a set of active microspoilers. A mechanical actuator is proposed that exhibits unique advantages in terms of robustness, simplicity, gun-launch survivability, and bandwidth compared to other projectile actuator mechanisms considered to date. A set of integrated test projectiles is constructed using this actuator design, and flight experiments are performed in which the microspoilers are oscillated near the projectile roll frequency. Data obtained from these flight tests are used in parameter estimation studies to experimentally characterize the aerodynamic effects of actively oscillating microspoilers. These predictions compare favorably with estimates obtained from computational fluid dynamics (CFD). Overall, the results presented here demonstrate that actively controlled microspoilers can generate reasonably high levels of lateral acceleration suitable for trajectory modification in many smart-weapons applications.

Commentary by Dr. Valentin Fuster
J. Dyn. Sys., Meas., Control. 2017;139(11):111003-111003-7. doi:10.1115/1.4036807.

A design of linear matrix inequality (LMI)-based fractional-order surface for sliding-mode controller of a class of uncertain fractional-order nonlinear systems (FO-NSs) is proposed in this paper. A new switching law is achieved guaranteeing the reachability condition. This control law is established to obtain a sliding-mode controller (SMC) capable of deriving the state trajectories onto the fractional-order integral switching surface and maintain the sliding motion. Using LMIs, a sufficient condition for existence of the sliding surface is derived which ensures the $t−α$ asymptotical stability on the sliding surface. Through a numerical example, the superior performance of the new fractional-order sliding mode controller is illustrated in comparison with a previously proposed method.

Commentary by Dr. Valentin Fuster
J. Dyn. Sys., Meas., Control. 2017;139(11):111004-111004-11. doi:10.1115/1.4036546.

A method to estimate ground reaction forces (GRFs) in a robot/prosthesis system is presented. The system includes a robot that emulates human hip and thigh motion, along with a powered (active) transfemoral prosthetic leg. We design a continuous-time extended Kalman filter (EKF) and a continuous-time unscented Kalman filter (UKF) to estimate not only the states of the robot/prosthesis system but also the GRFs that act on the foot. It is proven using stochastic Lyapunov functions that the estimation error of the EKF is exponentially bounded if the initial estimation errors and the disturbances are sufficiently small. The performance of the estimators in normal walk, fast walk, and slow walk is studied, when we use four sensors (hip displacement, thigh, knee, and ankle angles), three sensors (thigh, knee, and ankle angles), and two sensors (knee and ankle angles). Simulation results show that when using four sensors, the average root-mean-square (RMS) estimation error of the EKF is 0.0020 rad for the joint angles and 11.85 N for the GRFs. The respective numbers for the UKF are 0.0016 rad and 7.98 N, which are 20% and 33% lower than those of the EKF.

Commentary by Dr. Valentin Fuster
J. Dyn. Sys., Meas., Control. 2017;139(11):111005-111005-12. doi:10.1115/1.4036553.

Utilizing flow rate and pressure data in and out of the fluid circulation loop provides a driller with real-time trends for early detection of well-control problems that impact the drilling efficiency. Due to limited number of sensors and time delay in processing and measurements, the flow rate and pressure along the annulus and drill string need to be estimated. This paper presents state and parameter estimations for infinite-dimensional models used in automated managed pressure drilling (MPD). The objective is to monitor the key process variables associated with process safety by designing a nonlinear adaptive observer that use the available information coming from the continuous-time online process measurements at the outlet of the well. The adaptive observer consists of a copy of the infinite-dimensional model plus output injection terms where the gain is computed analytically in terms of the Bessel function of the first kind. The design is tested using field data from a drilling commissioning test by Statoil ASA, Stavanger, Norway. The results show that the nonlinear adaptive observer estimates the flow rate and pressure of the drilling fluid accurately.

Commentary by Dr. Valentin Fuster
J. Dyn. Sys., Meas., Control. 2017;139(11):111006-111006-7. doi:10.1115/1.4036560.

Robots are being increasingly used by physical therapists to carry out rehabilitation treatments owing to their ability of providing repetitive, controlled, and autonomous training sessions. Enhanced treatment outcomes can be achieved by encouraging patients' active participation besides robotic assistance. Advanced control strategies are required to be designed and implemented for the rehabilitation robots in order to persuade patients to contribute actively during the treatments. In this paper, an adaptive impedance control approach is developed and implemented on a parallel ankle rehabilitation robot. The ankle robot was designed based on a parallel mechanism and actuated using four pneumatic muscle actuators (PMAs) to provide three rotational degrees-of-freedom (DOFs) to the ankle joint. The proposed controller adapts the parallel robot's impedance according to the patients' active participation to provide customized robotic assistance. In order to evaluate performance of the proposed controller, experiments were conducted with stroke patients. It is demonstrated from the experimental results that the robotic assistance decreases as a result of patients' active participation. Similarly, increased robotics assistance is recorded in response to decrease in patient's participation in the rehabilitation process. This work will aid in the further development of customized robot-assisted physical therapy of ankle joint impairment.

Commentary by Dr. Valentin Fuster
J. Dyn. Sys., Meas., Control. 2017;139(11):111007-111007-10. doi:10.1115/1.4036885.

Regulating indoor air environment is one of the core functions of building energy management system. Heating, ventilation, and air-conditioning (HVAC) control systems play an important role in adjusting the room temperature to provide occupants a desired level of comfort. Occupant comfort has a direct effect on the energy consumption and providing an optimal balance between comfort and energy consumption is a challenging problem. This paper presents a framework for control of building HVAC systems using a methodology based on power-shaping paradigm that exploits the passivity property of a system. The system dynamics are expressed in the Brayton–Moser (BM) form which exhibits a gradient structure with the mixed-potential function, which has the units of power. The power-shaping technique is used to synthesize the controller by assigning a desired power function to the closed-loop dynamics so as to make the equilibrium point asymptotically stable. The proposed methodology is demonstrated on HVAC subsystems: RC network building zone model and a heat exchanger system.

Commentary by Dr. Valentin Fuster
J. Dyn. Sys., Meas., Control. 2017;139(11):111008-111008-12. doi:10.1115/1.4036884.

Tractor–trailer vehicles will suffer from nonholonomic constraint, uncertain disturbance, and various physical limits, when they perform path tracking maneuver autonomously. This paper presents a composite path tracking control strategy to tackle the various problems arising from not only vehicle kinematic but also dynamic levels via two powerful control techniques. The proposed composite control structure consists of a model predictive control (MPC)-based posture controller and a direct adaptive fuzzy-based dynamic controller, respectively. The former posture controller can make the underactuated trailer midpoint follow an arbitrary reference trajectory given by the earth-fixed frame, as well as satisfying various physical limits. Meanwhile, the latter dynamic controller enables the vehicle velocities to track the desired velocities produced by the former one, and the global asymptotical convergence of dynamic controller is strictly guaranteed in the sense of Lyapunov stability theorem. The simulation results illustrate that the presented control strategy can achieve a coordinated control effect for the sophisticated tractor–trailer vehicles, thereby enhancing their movement performance in complex environments.

Commentary by Dr. Valentin Fuster
J. Dyn. Sys., Meas., Control. 2017;139(11):111009-111009-7. doi:10.1115/1.4036880.

This paper explores the fault-tolerant control problem for uncertain impulsive singular linear time-variant (LTV) systems in the presence of bounded-power or L disturbances. Here, a saturation avoidance mechanism is employed to prevent faulty actuators from overloading. Also, the conflict between attenuating the effect of L disturbances and enlarging the domain of attraction is tackled by proposing a nonconstant state feedback controller. In addition, the proposed method is capable of tolerating time-varying faults. The suggested method is implemented on a mechanical viscoelastic model of cortical bone with impulsive and time-varying nature to evaluate its competency.

Commentary by Dr. Valentin Fuster
J. Dyn. Sys., Meas., Control. 2017;139(11):111010-111010-10. doi:10.1115/1.4036886.

A general design approach is presented for model-based control of piston position in a free-piston engine (FPE). The proposed approach controls either “bottom-dead-center” (BDC) or “top-dead-center” (TDC) position. The key advantage of the approach is that it facilitates controller parameter selection, by the way of deriving parameter combinations that yield both stable BDC and stable TDC. Driving the piston motion toward a target compression ratio is, therefore, achieved with sound engineering insight, consequently allowing repeatable engine cycles for steady power output. The adopted control design approach is based on linear control-oriented models derived from exploitation of energy conservation principles in a two-stroke engine cycle. Two controllers are developed: A proportional integral (PI) controller with an associated stability condition expressed in terms of controller parameters, and a linear quadratic regulator (LQR) to demonstrate a framework for advanced control design where needed. A detailed analysis is undertaken on two FPE case studies differing only by rebound device type, reporting simulation results for both PI and LQR control. The applicability of the proposed methodology to other common FPE configurations is examined to demonstrate its generality.

Commentary by Dr. Valentin Fuster
J. Dyn. Sys., Meas., Control. 2017;139(11):111011-111011-8. doi:10.1115/1.4037168.

Semi-active systems provide an attractive solution for the structural vibration problem. A useful approach, aimed to simplify the control design, is to divide the control system into two parts: an actuator and a controller. The actuator generates a force that tracks a command which is generated by the controller. Such approach reduces the complexity of the control law design as it allows for complex properties of the actuator to be considered separately. In this study, the semi-active control design problem is treated in the framework of optimal control theory by using bilinear representation, a quadratic performance index, and a constraint on the sign of the control signal. The optimal control signal is derived in a feedback form by using Krotov's method. To this end, a novel sequence of Krotov functions which suits the multi-input constrained bilinear-quadratic regulator problem is formulated by means of quadratic form and differential Lyapunov equations. An algorithm is proposed for the optimal control computation. A proof outline for the algorithm convergence is provided. The effectiveness of the suggested method is demonstrated by numerical example. The proposed method is recommended for optimal semi-active feedback design of vibrating plants with multiple semi-active actuators.

Commentary by Dr. Valentin Fuster

### Technical Brief

J. Dyn. Sys., Meas., Control. 2017;139(11):114501-114501-3. doi:10.1115/1.4036550.

This paper proposes a simple reduced-order model for a general flexure-guided piezoelectrically actuated nanopositioner and employs it to derive the upper limit of achievable bandwidth for a specified travel range. It is shown that flexure-based motion amplification enables achieving higher bandwidth than that obtained when they are used for guiding motion alone. The optimal amplification and the corresponding maximum bandwidth are studied as functions of the mass carried by the positioner and the stiffness of the flexure. Simple analytical expressions are derived for the two in case of stiff flexures carrying small mass. The proposed reduced-order model is validated by means of finite element analysis.

Commentary by Dr. Valentin Fuster
J. Dyn. Sys., Meas., Control. 2017;139(11):114502-114502-5. doi:10.1115/1.4036555.

Finding natural frequencies and mode shapes for flexible structures can be a challenging problem. Although well-known approaches exist for single flexible links, the problem becomes increasingly more complex when dealing with multiple links. Spatial configurations add an additional layer of difficulty. This work presents a systematic method for finding the natural frequencies and mode-shapes for n-link serial structures using a transfer matrix approach. The method is validated by finite element analysis and experiments.

Commentary by Dr. Valentin Fuster
J. Dyn. Sys., Meas., Control. 2017;139(11):114503-114503-6. doi:10.1115/1.4036665.

This paper proposes sliding mode control of vibration in three types of single-degree-of-freedom (SDOF) fractional oscillators: the Kelvin–Voigt type, the modified Kelvin–Voigt type, and the Duffing type. The dynamical behaviors are all described by second-order differential equations involving fractional derivatives. By introducing state variables of physical significance, the differential equations of motion are transformed into noncommensurate fractional-order state equations. Fractional sliding mode surfaces are constructed and the stability of the sliding mode dynamics is proved by means of the diffusive representation and Lyapunov stability theory. Then, sliding mode control laws are designed for fractional oscillators, respectively, in cases where the bound of the external exciting force is known or unknown. Furthermore, sliding mode control laws for nonzero initialization case are designed. Finally, numerical simulations are carried out to validate the above control designs.

Commentary by Dr. Valentin Fuster
J. Dyn. Sys., Meas., Control. 2017;139(11):114504-114504-7. doi:10.1115/1.4036664.

Accounting for the autopilot as second-order dynamics, an observer-based guidance law with terminal impact angle constraint is designed using the dynamic surface control method. Some first-order low-pass filters are introduced into the designing process to avoid the occurrence of high-order derivatives of the line of sight (LOS) angle in the expression of the guidance law such that the guidance law can be implemented in practical applications. The proposed guidance law is effective in compensating for the second-order autopilot lag. In simulation of intercepting targets with sinusoidal acceleration, the guidance law is compared with the biased proportional navigation guidance (BPNG) law in the presence of missile autopilot lag. Simulation results show that the proposed observer-based guidance law with terminal impact angle constraint is able to guide a missile with large autopilot lag to impact a target with a desired angle and achieve a small miss distance, even if the target escapes in a great and fast maneuver.

Commentary by Dr. Valentin Fuster
J. Dyn. Sys., Meas., Control. 2017;139(11):114505-114505-6. doi:10.1115/1.4036873.

Micro- and millimeter-scale resonant mass sensors have received widespread attention due to their robust and sensitive performance in a wide range of detection applications. A key performance metric for such systems is the sensitivity of the resonant frequency of a device to changes in mass, which needs to be calibrated. This calibration is complicated by the fact that the position of the added mass on a sensor can have an effect on the measured sensitivity—therefore, a spatial sensitivity mapping is needed. To date, most approaches for experimental sensitivity characterization are based upon the controlled addition of small masses, e.g., the direct attachment of microbeads via atomic force microscopy or the selective microelectrodeposition of material, both of which are time consuming and require specialized equipment. This work proposes a method of experimental spatial sensitivity measurement that uses an inkjet system and standard sensor readout methodology to map the spatially dependent sensitivity of a resonant mass sensor—a significantly easier experimental approach. The methodology is described and demonstrated on a quartz resonator. In the specific case of a Kyocera CX3225 thickness-shear mode resonator, the location of the region of maximum mass sensitivity is experimentally identified.

Topics: Resonance , Sensors , Inks
Commentary by Dr. Valentin Fuster
J. Dyn. Sys., Meas., Control. 2017;139(11):114506-114506-6. doi:10.1115/1.4036874.

In this note, the analysis of time delay systems (TDSs) using Lambert W function approach is reassessed. A common canonical (CC) form of time delay systems is defined. We extended the recent results of Cepeda–Gomez and Michiels (2015, “Some Special Cases in the Stability Analysis of Multi-Dimensional Time-Delay Systems Using the Matrix Lambert W Function,” Automatica, 53, pp. 339–345) for second-order into nth order system. The eigenvalues of a time delay system are either real or complex conjugate pairs and therefore, the whole eigenspectrum can be associated with only two real branches of the Lambert W function. A new class of time delay systems is characterized to extend the applicability of the above-said method. Moreover, this approach has been exploited to design a controller which places a subset of eigenvalues at desired locations. Stability is guaranteed by using a new algorithm developed in this paper, which is based on the Nyquist plot. The approach is validated through numerical examples.

Commentary by Dr. Valentin Fuster