Actuator Magnitude Research Articles

Handling actuator magnitude and rate saturation in uncertain over-actuated systems: a modified projection algorithm approach

This paper proposes a projection algorithm which can be employed to bound actuator signals, in terms of both magnitude and rate, for uncertain systems with redundant actuators. The investigated closed-loop control system is assumed to contain an adaptive control allocator to distribute the total control input among actuators. Although conventional control allocation methods can handle actuator rate and magnitude constraints, they cannot consider actuator uncertainty. On the other hand, adaptive allocators manage uncertainty and actuator magnitude limits. The proposed projection algorithm enables adaptive control allocators to handle both magnitude and rate saturation constraints. A mathematically rigorous analysis is provided to show that with the help of the proposed projection algorithm, the performance of the adaptive control allocator can be guaranteed, in terms of error bounds. Simulation results are presented, where the Aero-Data Model In Research Environment (ADMIRE) is used to demonstrate the effectiveness of the proposed method.

Dynamic folding of origami by exploiting asymmetric bi-stability

In this study, we examine a rapid and reversible origami folding method by exploiting a combination of resonance excitation, asymmetric bi-stability, and active control. The underlying idea is that, by harmonically exciting a bi-stable origami at its resonance frequencies, one can induce rapid folding between its different stable equilibria with a much smaller actuation magnitude than static folding. To this end, we use a bi-stable water-bomb base as an archetypal example to uncover the underlying principles of dynamic folding based on numerical simulation and experimental testing. If the water-bomb initially settles at its “weak” stable state, one can use a base excitation to induce the intra-well resonance. As a result, the origami would fold and remain at the other “strong” stable state even if the excitation does not stop. The origami dynamics starting from the strong state, on the other hand, is more complicated. The water-bomb origami is prone to show inter-well oscillation rather than a uni-directional switch due to a nonlinear relationship between the dynamic folding behavior, asymmetric potential energy barrier, the difference in resonance frequencies, and excitation amplitude. Therefore, we develop an active feedback control strategy, which cuts off the base excitation input at the critical moment to achieve robust and uni-directional folding from the strong stable state to the weak one. The results of this study can apply to many different kinds of origami and create a new approach for rapid and reversible (self-)folding, thus advancing the application of origami in shape morphing systems, adaptive structures, and reconfigurable robotics.

A transformed proportional-integral-derivative controller for a multi-vectored propeller aerostat with independent actuator magnitude and rate saturations.

The problem of designing a controller for a multi-vectored propeller airship with independent amplitude and rate saturations is addressed. First, a linear Proportional-Integral-Derivative (PID) controller is introduced for position control without considering the input saturations. Then, two design methods are applied to the traditional PID control output to satisfy the independent amplitude and rate constraints: the nested saturated PID controller (N-PID) and the transformed PID controller (T-PID). The bounded magnitudes and rate outputs of the modified controllers are given. Simulation results showed both controllers have good tracking performance while satisfying independent amplitude and rate saturations. However, the transformed PID controller has the advantage of expressing explicitly the relationship of the actuator magnitude and rate saturations with the parameters of the transformed function such that the actuator saturations are suppressed by calculation but not by trial and error.

Visible-Light-Driven, Nickel-Doped Cobalt Oxides/Hydroxides Actuators with High Stability.

Material-based, light-driven actuators have been a recent research focus for the development of untethered, miniaturized devices and microrobots. Recently introduced nickel hydroxide/oxyhydroxide (Ni(OH)2/NiOOH) and cobalt oxides/hydroxides (CoOx(OH)y) are promising light-driven actuators, as they exhibit large and rapid actuation response and are inexpensive to fabricate by fast electrodeposition. However, as their actuation is due to the volume change accompanying the light-induced desorption of intercalated water in their turbostratic structures, their actuation reduces over time as crystallization takes place slowly, which lowers the amount of water held. Here, we introduce nickel-doped cobalt oxides/hydroxides (NiCoOx(OH)y) actuator that exhibits similar turbostratic crystal structures and actuation magnitude as CoOx(OH)y, but with much slower crystallization and hence significantly more stable actuation than CoOx(OH)y or Ni(OH)2/NiOOH. The new actuator exhibits much better applicability than the Co and Ni counterparts, and the present work shows that a stabilized turbostratic structure is a key for achieving high light-driven actuation in transition-metal oxide/hydroxide actuators.

Finite-time spacecraft attitude control under input magnitude and rate saturation

This paper addresses attitude-stabilizing control for rigid spacecraft with actuator magnitude and rate saturation (MRS). Firstly, a continuous dynamical system is introduced to model the actuator MRS. Rigorous analysis is given to demonstrate that the model’s output can always meet the MRS constraints if the control scheme applied to the model derived here is continuous. Then, by using the proposed MRS model and the homogeneity property, two attitude-stabilizing control laws are presented. The measurement of angular velocity is required in the implementation of the first control law, but this requirement is unnecessary in the second one. The local finite-time stability of the resulting closed-loop system is ensured by employing the Lyapunov approach. Finally, several simulation examples are presented to validate the efficiency of the proposed method.

Dynamic surface control of trajectory tracking marine vehicles with actuator magnitude and rate limits

An n-degree of freedom nonlinear control law for the trajectory tracking of marine vehicles that operate in the presence of unknown time-varying disturbances, input saturation and actuator rate limits is developed using a disturbance observer and nonlinear dynamic surface control. In marine vehicle applications, n would typically be 3, 4 or 6. The disturbance observer provides estimates of the unknown time-varying disturbances and a continuously differentiable function is employed to model input saturation. The uniform ultimate boundedness of all signals in the closed-loop control system is proved. Trajectory-tracking simulations of an autonomous underwater vehicle demonstrate the performance of the proposed controller.

Control of non-cooperative spacecraft in final phase proximity operations under input constraints

This paper addresses the approaching trajectory control problem for the final phase of non-cooperative spacecraft proximity operations in the presence of disturbances and even input saturation. As a stepping stone, a novel line-of-sight tracking model is developed to describe attitude maneuver towards the target in the proximity. Then, a two-stage artificial potential function is employed to enable the pursuer spacecraft to comply the safety trajectory during approaching the target. In each stage, gradients of potential function are analyzed and the local minimum problem associated with potential function can be addressed. Furthermore, a saturated control law is proposed to ensure a reliable real-time tracking measurement of the target, and the controller rigorously enforces safe path and actuator magnitude constraints. The associated stability proof is accomplished through Lyapunov method. Simulations are carried out to evaluate the effectiveness of the proposed method.

Homogeneity for Shared Control in the Presence of Disturbances

This paper considers the shared control of a second order plant by a single machine and one person in the presence of exogenous disturbances. A mixed-initiative blending law that takes a passive measure of human intent as the input to a convex pair of exponential functions is proposed. The blending law is combined with a disturbance observer and homogeneous trajectory-tracking control law, which dynamically rescales to ensure that actuator magnitude constraints are respected. The globally uniform ultimate boundedness of the closed loop automatic controller is proved. The shared control system is tested in a simulation where a human user can impose a control input using a joystick. Even if large tracking errors are commanded by the user, the automatic controller is able to quickly return to tracking the desired path when the joystick is released.

Tuning of reversible actuation via ROMP-based copolymerization semicrystalline polymers

There exists a need for reversible actuation involving two-way shape memory polymers that can respond to changes in ambient temperature in the 20 °C–40 °C range, especially for soft actuators in applications requiring near-room-temperature environmental responsiveness. Herein, we report on the fabrication and analysis of a family of soft, semicrystalline copolymers based upon cis-cyclooctene and various melting point reducing comonomers via ring-opening metathesis polymerization (ROMP). Reversible actuation behavior of such copolymers was tuned by varying not only the species and the incorporation level of a second comonomer but also the concentration of an incorporated thermal initiator for crosslinking. Results revealed that actuation was tuned to occur best and near room temperature for a copolymer consisted of 97 mol% cis-cyclooctene and 3 mol% 1,5-cyclooctadiene and 0.50 wt% dicumyl peroxide initiator. Also, all crosslinked copolymers feature actuation magnitude greater than 50% and recovery magnitude greater than 94% at a single stress of 0.8 MPa, while thermal strain hysteresis was 17.5 °C in breadth, on average.

Two‐way multi‐shape memory properties of peroxide crosslinked ethylene vinyl‐acetate copolymer (EVA)/polycaprolactone (PCL) blends

Rare studies have investigated on the 2‐way shape memory crosslinked blends with multiple shape memory behavior up to date. To consider the merit of commercial cost‐competitive crystalline polymers, ethylene vinyl‐acetate copolymer (EVA)/polycaprolactone (PCL) blends (60/40 and 30/70) were peroxide‐cured to form the 2‐way multi‐shape memory crosslinked blends using a melt‐blending method. Both resins were selected to have a similar controlled crosslinking degree, which allowed us to distinctly evaluate their actuation contributions from the cooling‐induced elongation (crystallization) and from the entropy‐driven elongation during cooling process, respectively. In the 2‐way process for the 60/40 system, 2 respective peaks contributed from the cooling‐induced crystallization of EVA and PCL in the cooling curves based on the strain derivate rates at various temperatures were observed. After the cooling process under the loading stress of 150 kPa, the 2‐step heating‐induced contraction process with increasing temperature started at 54.1°C above the melting temperature of PCL at 52.3°C and EVA at 78.3°C, demonstrating 2‐way multi‐shape memory behavior. The multi‐step behavior was more prominent at higher PCL composition and higher load for the 30/70 system. It was found that the entropy‐driven contribution to the overall actuation magnitude increased with increasing nominal loads due to the increased orientation of molecular networks in the blends. The current approach offers numerous possibilities in preparing 2‐way multi‐shape memory crosslinked blends.

A Dynamic Window-Based Controller for Dynamic Positioning Satisfying Actuator Magnitude Constraints

This paper considers the use of a simplified dynamic window (DW) algorithm to handle actuator magnitude constraints for a 3 degrees-of-freedom dynamic positioning controller for ships. To accomplish this, we use the simplified DW algorithm to design a dynamic window-based controller (DWC) which guarantees that the velocities remain within a feasible set, while simultaneously respecting the actuator magnitude constraints. The DWC is compared with a benchmark motion controller which uses nonlinear position and velocity feedback terms. The comparison is made using performance metrics which consider both control accuracy and energy efficiency.

Robust attitude tracking control for a rigid spacecraft under input delays and actuator errors

ABSTRACTAttitude control of a rigid spacecraft under input delays, disturbances, parameter uncertainties, actuator errors, and constraints is a challenging problem. In this paper, these problems are considered simultaneously, and a robust control approach to attitude tracking of a rigid spacecraft is exploited. The design methodology is based on three steps: (1) compensating input delays by using the backstepping technique, (2) design of a disturbance observer for the delayed system by using the super-twisting algorithm to estimate unknown internal and external disturbances, then adding a feedforward compensation law based on the estimated signal to the backstepping controller to attenuate the effects of disturbances, (3) employing a robust least-square scheme to map the specified control command on the redundant actuators in the presence of actuator error, including actuator magnitude deviation and misalignment, with regard to actuator amplitude and rate constraints. The effectiveness of the proposed algorithm is shown by various numerical simulations.

Adaptive backstepping control for air-breathing hypersonic vehicle with actuator dynamics

This paper investigates the flight longitudinal control problem of a hypersonic vehicle subject to input constraints and aerodynamic uncertainties. A modified control-oriented model is first derived with explicit consideration of the actuator dynamics property. Then, based on the novel dynamics model developed, adaptive backstepping controller and dynamic inverse controller are designed for the altitude subsystem and the velocity subsystem, respectively. To avoid the complexity problem residing in traditional backstepping control caused by the repeated derivations of the virtual control variables, the dynamic surface control technique is involved in conjunction with the backstepping control approach, and a novel second-order sliding-mode-based integral filter instead of the conventional first-order filter is employed. Additionally, in order to deal with actuator magnitude constraints, two auxiliary systems are constructed to generate certain compensating signals to guarantee tracking errors suitable for adaptive parameter estimation. It is proved that the proposed control scheme can guarantee that the tracking errors converge to an arbitrarily small neighborhood around zero in the presence of actuator constraints and aerodynamic uncertainties as well. Finally, numerical simulations are presented to demonstrate the effectiveness of the proposed control scheme.

A reduced order model for dielectric elastomer actuators over a range of frequencies and prestrains

The actuation strain of an equibiaxially prestrained dielectric elastomer membrane is studied as a function of driving frequency and prestrain. Experimental data are gathered on the membrane's creep and recovery following DC actuation, as well as the steady state amplitude and phase for AC driving voltages ranging from 2 to 40 Hz. The effect of prestretch on steady state actuation was also investigated, using membranes of both 250% and 300% prestretch. A three-element generalized Kelvin-Voigt model is developed to capture the transient and steady-state actuation responses as a function of frequency and prestrain. We show that, despite its relative simplicity, this model captures the relevant timescales for the membrane behavior with good fidelity and can be used to accurately predict the actuation magnitude and phase as a function of time over a range of actuation configurations and driving conditions.

4D sequential actuation: combining ionoprinting and redox chemistry in hydrogels

The programmable sequential actuation of two-dimensional hydrogel membranes into three-dimensional folded architectures has been achieved by combining ionoprinting and redox chemistry; this methodology permits the programmed evolution of complex architectures triggered through localized out-of-plane deformations. In our study we describe a soft actuator which utilizes ionoprinting of iron and vanadium, with the selective reduction of iron through a mild reducing agent, to achieve chemically controlled sequential folding. Through the optimization of solvent polarity and ionoprinting variables (voltage, duration and anode composition), we have shown how the actuation pathways, rate-of-movement and magnitude of angular rotation can be controlled for the design of a 4D sequential actuator.

Smooth finite-time fault-tolerant attitude tracking control for rigid spacecraft

This paper investigates the problem of attitude tracking control for a rigid spacecraft subject to parametric uncertainties, external disturbances, actuator faults and actuator saturation constraints. By combining the finite-time passivity technique into adaptive sliding mode control approach, a novel smooth fault-tolerant control algorithm with finite time convergence is proposed. Then, the finite-time convergence of the relative attitude errors will be achieved by implementing the proposed smooth fault-tolerant controller, even under actuator faults and magnitude constraints. In particular, a solution to the unwinding phenomenon, arising from the usage of the redundant four-parameter attitude representation, is explored in the sense of finite-time passivity. Besides detailed controller design procedures and rigorous theoretical proofs of all related closed-loop finite-time stability, numerical simulation results are exhibited to demonstrate the effectiveness and superior control performance of the proposed control scheme.

Actuator stiction compensation via model predictive control for nonlinear processes

The problem of valve stiction is addressed, which is a nonlinear friction phenomenon that causes poor performance of control loops in the process industries. A model predictive control (MPC) stiction compensation formulation is developed including detailed dynamics for a sticky valve and additional constraints on the input rate of change and actuation magnitude to reduce control loop performance degradation and to prevent the MPC from requesting physically unrealistic control actions due to stiction. Although developed with a focus on stiction, the MPC‐based compensation method presented is general and has potential to compensate for other nonlinear valve dynamics which have some similarities to those caused by stiction. Feasibility and closed‐loop stability of the proposed MPC formulation are proven for a sufficiently small sampling period when Lyapunov‐based constraints are incorporated. Using a chemical process example with an economic model predictive controller (EMPC), the selection of appropriate constraints for the proposed method is demonstrated. The example verified the incorporation of the stiction dynamics and actuation magnitude constraints in the EMPC causes it to select set‐points that the valve output can reach and causes the operating constraints to be met. © 2016 American Institute of Chemical Engineers AIChE J, 62: 2004–2023, 2016

Output Feedback Attitude Tracking for Spacecraft Under Control Saturation and Disturbance

This paper proposes a class of velocity-free attitude stable controller using a novel finite-time observer for spacecraft attitude tracking, which explicitly takes into account control input saturation to assure fast and accurate response and to achieve effective compensation to the effect of external disturbance as well. First, a novel semiglobal finite-time convergent observer is proposed to estimate the angular velocity in a finite-time under external disturbance. Then, a simple global output feedback controller is proposed by adoption of the designed finite-time observer. Rigorous proofs show that the proposed observer can achieve the finite-time stability and the controller rigorously enforces actuator magnitude constraints. Numerical simulations illustrate the spacecraft performance obtained using the proposed controller.

Robust semi-global coordinated tracking of linear multi-agent systems with input saturation

SUMMARY This paper investigates the problem of coordinated tracking of a linear multi-agent system subject to actuator magnitude saturation and dead zone characteristic with input additive uncertainties and disturbances. Distributed consensus and swarm tracking protocols are developed from a low-and-high gain feedback approach. Under the assumption that each agent is asymptotically null controllable with bounded controls, it is shown that robust semi-global consensus tracking and swarm tracking of the multi-agent system can always be reached provided that the networks are connected. Numerical examples are provided to illustrate the theoretical results. Copyright © 2014 John Wiley & Sons, Ltd.

Generalized State Estimation and Model Predictive Guidance for Spiraling and Ballistic Targets

An extended Kalman filter based generalized state estimation approach is presented in this paper for accurately estimating the states of incoming high-speed targets such as ballistic missiles. A key advantage of this nine-state problem formulation is that it is very much generic and can capture spiraling as well as pure ballistic motion of targets without any change of the target model and the tuning parameters. A new nonlinear model predictive zero-effort-miss based guidance algorithm is also presented in this paper, in which both the zero-effort-miss as well as the time-to-go are predicted more accurately by first propagating the nonlinear target model (with estimated states) and zero-effort interceptor model simultaneously. This information is then used for computing the necessary lateral acceleration. Extensive six-degrees-of-freedom simulation experiments, which include noisy seeker measurements, a nonlinear dynamic inversion based autopilot for the interceptor along with appropriate actuator and sensor models and magnitude and rate saturation limits for the fin deflections, show that near-zero miss distance (i.e., hit-to-kill level performance) can be obtained when these two new techniques are applied together. Comparison studies with an augmented proportional navigation based guidance shows that the proposed model predictive guidance leads to a substantial amount of conservation in the control energy as well.