Active Control Framework Research Articles

Trajectory Deformation With Constrained Optimization for Bilateral Rehabilitation Robots

Robot-aided bilateral treatment has been verified to be an effective training program for hemiplegic rehabilitation. In this article, a reference-free active control framework based on optimal trajectory deformation is proposed to ensure the safety requirements in the leader–follower paradigm of bilateral treatment. A constrained optimization method is developed to handle the motion constraints, which are constructed by quantitative assessments of typical impairment in stroke patients, such as reduced range of motion, resistance to passive movement, and disturbed quality of movement. Then, by optimally deforming the robotic trajectory, abnormal motion patterns that lead to safety issues can be rectified. Furthermore, the physically interactive trajectory deformation is employed to achieve active control without a predefined trajectory. At last, all approaches are verified on a robotic platform with a 2-DoF lower-limb exoskeleton. Experimental results demonstrate the effectiveness of proposed control scheme in rectifying abnormal motion patterns and enhancing human–robot interaction.

PID-like active impedance control for electroacoustic resonators to design tunable single-degree-of-freedom sound absorbers

Sound absorption at low frequencies still remains a challenge in both scientific research and engineering practice. Natural porous materials are ineffective in this frequency range, as well as acoustic resonators which present too narrow bandwidth of absorption, thus requiring alternative solutions based on active absorption techniques. In the present work, we propose an active control framework applied on a closed-box loudspeaker to enable the adjustment of the acoustic impedance at the loudspeaker diaphragm. More specifically, based on the proportionality between the pressure inside the enclosure and the axial displacement of the loudspeaker diaphragm at low frequencies, we demonstrate both analytically and experimentally that a PID-like feedback control approach allows tuning independently the compliance, the resistance and the moving mass of the closed-box loudspeaker to implement a prescribed impedance of a single-degree-of-freedom resonator. By considering different control combinations to tailor the resonator characteristics, a perfect absorption (with absorption coefficient equal to 1) is achievable at the target resonance frequency, while enlarging the effective absorption bandwidth. Moreover, the proposed feedback control strategy shows an excellent control accuracy, especially compared to the feedforward-based control formerly reported in the literature. The mismatches between the performance of experimental prototype and the model, likely to result from the control time delay and the inaccuracy in estimating the loudspeaker parameters, can be compensated directly by tuning the control parameters in the control platform. The active resonators implemented through the reported control scheme can be used to build more complex acoustic devices/structures to enable high-efficiency broadband sound absorption or other types of acoustic phenomena such as wavefront shaping.

Improving Sound Absorption Through Nonlinear Active Electroacoustic Resonators

Absorbing airborne noise at frequencies below 300 Hz is a particularly vexing problem due to the absence of natural sound absorbing materials at these frequencies. The prevailing solution for low-frequency sound absorption is the use of passive narrow-band resonators, whose absorption level and bandwidth can be further enhanced using nonlinear effects. However, these effects are typically triggered at high intensity levels, without much control over the form of the nonlinear absorption mechanism. In this study, we propose, implement, and experimentally demonstrate a nonlinear active control framework on an electroacoustic resonator prototype, allowing for unprecedented control over the form of non-linearity, and arbitrarily low sound intensity thresholds. More specifically, the proposed architecture combines a linear feedforward control on the front pressure through a first microphone located at the front face of the loudspeaker, and a nonlinear feedback on the membrane displacement estimated through the measurement of the pressure inside the back cavity with a second microphone located in the enclosure. It is experimentally shown that even at a weak excitation level, it is possible to observe and control the nonlinear behaviour of the system. Taking the cubic nonlinearity as an example, we demonstrate numerically and experimentally that in the low frequency range ([50 Hz, 500 Hz]), the nonlinear control law allows improving the sound absorption performance, i.e. enlarging the bandwidth of optimal sound absorption while increasing the maximal absorption coefficient value, and producing only a negligible amount of nonlinear distortion. The reported experimental methodology can be extended to implement various types of hybrid linear and/or nonlinear controls, thus opening new avenues for managing wave nonlinearity and achieving non-trivial wave phenomena.

Rear Vehicle Tracking on a Bicycle Using Active Sensor Orientation Control

This paper focuses on the development of an active sensing system for a bicycle to accurately detect and track rear vehicles. A collision detection sensor on a bicycle is required to be inexpensive, small, and lightweight. A single beam laser sensor that meets these constraints is mounted on a rotationally controlled platform for this sensing mission. The rotational orientation of the laser sensor needs to be actively controlled in real time in order to continue to focus on a rear vehicle, as the vehicle’s lateral and longitudinal distances change. This tracking problem requires controlling the real-time angular position of the laser sensor without knowing the future trajectory of the vehicle. The challenge is addressed using a novel receding horizon framework for active control and an interacting multiple model framework for estimation. The features and benefits of this active sensing system are shown first using simulation results. Then, extensive experimental results are presented using an instrumented bicycle to show the performance of the system in detecting and tracking rear vehicles during both straight and turning maneuvers.

Toward wideband steerable acoustic metasurfaces with arrays of active electroacoustic resonators

We introduce an active concept for achieving acoustic metasurfaces with steerable reflection properties, effective over a wide frequency band. The proposed active acoustic metasurface consists of a surface array of subwavelength loudspeaker diaphragms, each with programmable individual active acoustic impedances allowing for local control over the different reflection phases over the metasurface. The active control framework used for controlling the reflection phase over the metasurface is derived from the Active Electroacoustic Resonator concept. Each unit-cell simply consists of a current-driven electrodynamic loudspeaker in a closed box, whose acoustic impedance at the diaphragm is judiciously adjusted by connecting an active electrical control circuit. The control is known to achieve a wide variety of acoustic impedances on a single loudspeaker diaphragm used as an acoustic resonator, with the possibility to shift its resonance frequency by more than one octave. This paper presents a methodology for designing such active metasurface elements. An experimental validation of the achieved individual reflection coefficients is presented, and full wave simulations present a few examples of achievable reflection properties, with a focus on the bandwidth of operation of the proposed control concept.

Theoretical framework of feedback aerodynamic control of flutter oscillation for long-span suspension bridges by the twin-winglet system

This paper presents an active aerodynamic control method of flutter oscillation for long-span suspension bridges with a newly designed twin-winglet system. The key point of this paper is to establish the theoretical framework for active control of bridge flutter by a pair of rotatable winglets beyond the deck, of which motions are determined by the feedback control algorithm. Through utilizing aerodynamic forces generated by these winglets, this active control system can improve aerodynamic stability of long-span suspension bridges to some extent. The modeling and control of this system mainly focus on practical feasibility, in which the relative rotations rather than driving forces of both winglets are selected as the control variables in the paper. State space expression of the control system is proposed to handle problems of high order terms elimination and decoupling for control variables. Since the unobservability of aerodynamic states, model reduction technique is adopted in the design of observer and controller. The active control algorithm presented in this paper is verified by numerical simulations, and the stabilizing mechanism and energy consumption are discussed as well. It shows good effectiveness and robustness of this active aerodynamic control device in a wide range of wind speeds for flutter suppression.

Alternation of Flex Cable Vibration Modes in High Precision Hard Disk Drives

For fast and accurate data accessing operations, flex cable's vibratory response has been identified to be the key development focus for magnetic hard disk drives to meet near nanometer precision accuracy. Dampening the cable's dynamics with damper layers and revising the seek servo designs have been the common approaches found in the literature to reach the requirement. In this paper, vibrations of nonuniform flex cable, which is composed of baseline flex cable layers and addition of piezoelectric layer such as polyvinylidene fluoride film, are investigated through experiments and non-classical, reduced-order finite element analysis. It is found that the addition of the piezofilm changes the cable's natural frequencies only within 3% error range but alters the cable's bending vibration mode shapes significantly. The change in the cable's mode shapes leads to change of the known cable/actuator dynamic coupling reported in the literature. Results presented in this paper establish mechanical framework for active control of the flex cable onto the rotary actuator.

Active Compressor Stability Management Via a Stall Margin Control Mode

An active engine control scheme for protection against compressor instabilities such as rotating stall and surge is presented. Compressor stability detection is accomplished via a parameter known as the correlation measure, which quantifies the repeatability of the pressure fluctuations in the tip region of a compressor rotor. This work investigates the integration of the correlation measure with an aircraft engine control system through the use of a stall margin control mode. The development and implementation of the stall margin mode are described. The effectiveness of the overall active control framework—an active compressor stability management system—is assessed using a computer simulation of a high-bypass, dual-spool, commercial-type turbofan engine.

Sound pressure estimation at an electroacoustic transducer's voicing face by way of all‐electrical sensing

When it comes to designing active control disposals, sound pressure sensing is usually required. In the framework of active control of acoustic impedance, this work aims at conceiving a digital estimator that processes voltage and current measurement to compute an estimation of the pressure in the vicinity of the diaphragm of an electro‐dynamic speaker. After dealing with modeling concerns of the actuator, the bilinear transform is used to obtain equations of two IIR filters. An adaptive identification using the LMS algorithm and a SVD‐based identification are tested to compute filters coefficient, both leading to poor experimental results. Accordingly, the actual transfer functions of the speaker are measured and optimal IIR filters are designed using the simulated annealing algorithm. It is then shown that good results can be achieved experimentally when the speaker is loaded electrically by a short circuit or an open circuit. Finally, performances of the process are discussed regarding other operating conditions.

Active Shielding and Control of Noise

We present a mathematical framework for the active control of time-harmonic acoustic disturbances. Unlike many existing methodologies, our approach provides for the exact volumetric cancellation of unwanted noise in a given predetermined region of space while leaving unaltered those components of the total acoustic field that are deemed friendly. Our key finding is that for eliminating the unwanted component of the acoustic field in a given area, one needs to know relatively little; in particular, neither the locations nor structure nor strength of the exterior noise sources need to be known. Likewise, there is no need to know the volumetric properties of the supporting medium across which the acoustic signals propagate, except, perhaps, in the narrow area of space near the boundary (perimeter) of the domain to be shielded. The controls are built based solely on the measurements performed on the perimeter of the region to be shielded; moreover, the controls themselves (i.e., additional sources) are also concentrated only near this perimeter. Perhaps as important, the measured quantities can refer to the total acoustic field rather than only to its unwanted component, and the methodology can automatically distinguish between the two. In the paper, we construct a general solution to the aforementioned noise control problem. The apparatus used for deriving the general solution is closely connected to the concepts of generalized potentials and boundary projections of Calderon's type. For a given total wave field, the application of Calderon's projections allows us to definitively split its incoming and outgoing components with respect to a particular domain of interest, which may have arbitrary shape. Then the controls are designed so that they suppress the incoming component for the domain to be shielded or alternatively, the outgoing component for the domain, which is complementary to the one to be shielded. To demonstrate that the new noise control technique is appropriate, we thoroughly work out a two-dimensional model example that allows full analytical consideration. To conclude, we very briefly discuss the numerical (finite-difference) framework for active noise control that has, in fact, already been worked out, as well as some forthcoming extensions of the current work: optimization of the solution according to different criteria that would fit different practical requirements, applicability of the new technique to quasi-stationary problems, and active shielding in the case of the broad-band spectra of disturbances. In the future, the aforementioned finite-difference framework for active noise control is going to be used for analyzing complex configurations that originate from practical designs.