Mechanical Cross-coupling Research Articles

Phased array probes for air-coupled ultrasonic testing based on cellular polymer

Air-coupled ultrasonic testing is an established alternative to conventional ultrasonic methods for testing of light-weight components in aerospace and automotive industry. Compared to direct coupled contact technique or immersion technique it is less invasive, so that it offers numerous financial incentives. While phased arrays are already an industrial standard in fluid-coupled mechanized ultrasonic testing, air-coupled testing is usually performed using two single-element probes and a transmission technique. Therefore, advantages of phased arrays such as active focussing and manipulation of the incidence angle are rarely used with air-coupled testing. This paper presents a newly developed air-coupled phased array device with the emphasis on the characterization of the probes. The device consists of a linear array probe, a transmitter unit and a receiver unit. The main novelty is that the array probe is based on high-voltage excitation of charged cellular polypropylene. Charged cellular polymers are referred to as ferroelectrets or as piezoelectrets. They exhibit an extremely low stiffness and acoustic impedance, so that a matching layer is not necessary. Extremely low mechanical cross-coupling enables independent excitation of the elements of the array. The development of the phased array probes is supported by computations of the sound field using point source synthesis. The main intended applications are transmission with focussing, transmission with an electronic sweep and testing with air-coupled guided waves by using beam steering, applied to carbon-fibre-reinforced plates and similar materials. Air-coupled guided waves enable testing of objects with one-sided access. This work was funded from the “Technologietransfer-Programm Leichtbau” by the German Federal Ministry for Economic Affairs and Climate Action, project 03LB1001A.

Design and modeling of decoupled miniature fast steering mirror with ultrahigh precision

To address the problem of low tracking precision caused by mechanical cross-coupling between the axes of a two-degree-of-freedom fast steering mirror (FSM), this study introduces an ultrahigh precision decoupled miniature fast steering mirror (MFSM) realized in a tiny volume. The proposed MFSM is driven by piezoelectric actuators and guided by a ring-type flexible hinge to achieve high-precision deflection and kinematic decoupling. A theoretical analysis of rotational stiffness was conducted to perform a detailed decoupling design. The output angle of the mirror was then evaluated. The relationship between the input voltage and the output angle was determined by modeling the dynamic characteristics of the MFSM. Owing to the mechanical decoupling property of the MFSM, two independent proportional–integral–derivative (PID) controllers were used separately for axis control, and experiments on trajectory tracking control were performed. The experimental results revealed that the angle traveled for more than ± 2 mrad for both axes, with an ultralow coupling ratio of less than 0.64 ‰. The underload natural frequency exceeded 3.24 kHz. The mirror exhibited a good closed-loop tracking accuracy, as indicated by the tracking error, which was less than 1 µrad for both the one-degree-of-freedom and two-degree-of-freedom trajectory tracking control.

Electrode Configurations for Piezoelectric Tube Actuators With Improved Scan Range and Reduced Cross-Coupling

The article describes two new methods for driving piezoelectric tube scanners. The first method aims to maximize horizontal and vertical scan range by driving the internal electrode rather than grounding it. This approach eliminates the need for a circumferential Z-electrode, which permits longer quadrant electrodes that develop greater deflection and vertical scan range. Experimental results demonstrate a 62% increase in lateral scan range and an 87% increase in vertical scan range. The second method aims to eliminate mechanical cross-coupling between the lateral deflection, tilt angle, and vertical extension. This method involves splitting the piezoelectric tube into eight external electrodes and a driven internal electrode, similar to the first method. This configuration results in less lateral deflection but significantly reduces the tilting and vertical motion induced by lateral deflection. Experimental results demonstrate an 44% increase in vertical displacement, 96% reduction in tilting, and 62% reduction in vertical cross-coupling.

Two-degrees-of-freedom piezo-driven fast steering mirror with cross-axis decoupling capability.

Because mechanical cross coupling between its axes would lead to degradation of the scanning precision of a piezo-driven fast steering mirror (PFSM), a two-degrees-of-freedom (2-DoF) PFSM with a cross-axis decoupling capability, in which 2-DoF flexure hinges are used, is proposed in this work. The overall structure of the proposed PFSM is first introduced and then both static and dynamic models are established analytically; in addition, the decoupling mechanism is described in detail and the low dynamic cross coupling ratios that occur between the two DoFs are shown. Because of the decoupling property of the PFSM, the 2-DoF motion is treated as a combination of two independent one-degree-of-freedom (1-DoF) motions and two independent proportional-integral-derivative controllers are thus used separately in the control of the two DoFs. Based on this control strategy, experiments involving both 1-DoF trajectory tracking and 2-DoF trajectory tracking are implemented. The test results show that the proposed PFSM can achieve the tilt range of ±7 mrad for both axes with the low coupling ratios that are less than 2% (-34 dB), and the bandwidths of both axes are higher than 810 Hz; in addition, the maximal tracking full scale range errors for 1-DoF trajectory tracking and 2-DoF trajectory tracking are less than 0.2% and 1%, respectively, where the larger error of 2-DoF trajectory tracking is mainly caused by the remaining cross coupling between axes.

Reducing mechanical cross-coupling in phased array transducers using stop band material as backing

Phased array transducers are widely used for acoustic imaging and surround sensing applications. A major design challenge is the achievement of low mechanical cross-coupling between the single transducer elements. Cross-coupling induces a loss of imaging resolution. In this work, the mechanical cross-coupling between acoustic transducers is investigated for a generic model. The model contains a common backing with two bending elements bonded on top. The dimensions of the backing are small; thus, wave reflections on the backing edges have to be considered. This is different to other researches. The operating frequency in the generic model is set to a low kHz range. Low operating frequencies are typical for surround sensing applications. The influence of the backing on cross-coupling is investigated numerically. In order to reduce mechanical cross-coupling a stop band material is designed. It is shown numerically that a reduction in mechanical cross-coupling can be achieved by using stop band material as backing. The effect is validated with experimental testing.

A dual-mass fully decoupled MEMS gyroscope with wide bandwidth and high linearity

A fully decoupled z-axis MEMS gyroscope with wide bandwidth and high linearity is presented in this paper. The mechanical structure consists of a dual proof mass, which are mechanically coupled together using a pair of double folded springs. The fully decoupled approach is employed to lower the mechanical cross coupling between the drive mode and sense mode. The gyroscope is fabricated using a simple two-mask process based on a silicon-on-insulator (SOI) substrate with a 30-μm-thick device layer. The electrostatic actuation and capacitive sensing gaps of around 1.1-μm provide an aspect ratio of close to 27. The narrow capacitive gap increases the electromechanical coupling and improves the signal to noise ratio (SNR) of the sensor. The fabricated MEMS gyroscope is vacuum packaged in a ceramic chip carrier in order to minimize the Brownian noise floor for achieving a better performance. A off-the-shelf integrated circuit with the excitation and sensing electronics is hybrid connected to the gyroscope. The MEMS gyroscope system demonstrates a bias instability of 9.6deg/h, an angular random walk (ARW) of 0.45deg/h, a bandwidth of greater than 120Hz, and a scale-factor nonlinearity of 770ppm in a full scale range of ±500deg/s at room temperature.

Micromachined tethered silicon oscillator for an endomicroscopic Lissajous fiber scanner.

This work reports micromachined tethered silicon oscillators (MTSOs) for endoscopic Lissajous fiber scanners. An MTSO comprises an offset silicon spring for stiffness modulation of a scanning fiber and additional mass for modulation of resonant scanning frequency in one body. MTSOs were assembled with a resonant fiber scanner and enhanced scanning reliability of the scanner by eliminating mechanical cross coupling. The fiber scanner with MTSOs was fully packaged as an endomicroscopic catheter and coupled with a conventional laparoscope and spectral domain OCT system. The endomicroscope was maneuvered with the integrated laparoscope and in vivo swine tissue OCT imaging was successfully demonstrated during open surgery. This new component serves as an important element inside an endoscopic Lissajous fiber scanner for early cancer detection or on-demand minimum lesional margin decision during noninvasive endoscopic biopsy.

Lissajous fiber scanning for forward viewing optical endomicroscopy using asymmetric stiffness modulation

We report a fully packaged and compact forward viewing endomicroscope by using a resonant fiber scanner with two dimensional Lissajous trajectories. The fiber scanner comprises a single mode fiber with additional microstructures mounted inside a piezoelectric tube with quartered electrodes. The mechanical cross-coupling between the transverse axes of a resonant fiber with a circular cross-section was completely eliminated by asymmetrically modulating the stiffness of the fiber cantilever with silicon microstructures and an off-set fiber fragment. The Lissajous fiber scanner was fully packaged as endomicroscopic catheter passing through the accessory channel of a clinical endoscope and combined with spectral domain optical coherence tomography (SD-OCT). Ex-vivo 3D OCT images were successfully reconstructed along Lissajous trajectory. The preview imaging capability of the Lissajous scanning enables rapid 3D imaging with high temporal resolution. This endoscopic catheter provides many opportunities for on-demand and non-invasive optical biopsy inside a gastrointestinal endoscope.

Eigenstructure assignment-based controllers applied to flexible spacecraft

The objective of this article is to evaluate the behaviour of a controller designed using a parametric eigenstructure assignment method and to evaluate its suitability for use in flexible spacecraft. The challenge of this objective lies in obtaining a suitable controller that is specifically designated to alleviate the deflections and vibrations suffered by external appendages in a flexible spacecraft while performing attitude manoeuvres. One of the main problems in these vehicles is the mechanical cross-coupling that exists between the rigid and flexible parts of the spacecraft. Spacecraft with fine attitude pointing requirements need precise control of the mechanical coupling to avoid undesired attitude misalignment. In designing an attitude controller, it is necessary to consider the possible vibration of the solar panels and how it may influence the performance of the rest of the vehicle. The nonlinear mathematical model of a flexible spacecraft is considered a close approximation to the real system. During the process of controller evaluation, the design process has also been taken into account as a factor in assessing the robustness of the system.

On Control System Design for the Conventional Mode of Operation of Vibrational Gyroscopes

This paper presents a novel control circuitry design for both vibrating axes (drive and sense) of vibrational gyroscopes, and a new sensing method for time-varying rotation rates. The control design is motivated to address the challenges posed by manufacturing imperfection and environment vibrations that are particularly pronounced in microelectromechanical systems (MEMS) gyroscopes. The method of choice is active disturbance rejection control that, unlike most existing control design methods, does not depend on an accurate model of the plant. The task of control design is simplified when the internal dynamics, such as mechanical cross coupling between the drive and sense axes, and external vibrating forces are estimated and cancelled in real time. In both simulation and hardware tests on a vibrational piezoelectric beam gyroscope, the proposed controller proves to be robust against structural uncertainties; it also facilitates accurate sensing of time-varying rotation rates. The results demonstrate a simple, economic, control solution for compensating the manufacturing imperfections and improving sensing performance of the MEMS gyroscopes.

Minimisation of mechanical cross-talk in periodic piezoelectric composite arrays

This paper describes an investigation into mechanical cross-talk within 1-3 and 2-2 connectivity piezoelectric composite array configurations, comprising a matrix of active piezoelectric elements embedded within a passive, polymeric, material. One way to take full advantage of the reported sensitivity and bandwidth improvements from single-crystal materials is to configure them as a piezoelectric composite. For this work, piezoelectric ceramic, lithium niobate and single-crystal pmn-pt materials are investigated as the active component in the piezocomposite array designs. Within these piezoelectric configurations, the generation of ultrasonic inter-pillar modes, which arise due to the periodicity of the active piezoelectric elements within the piezocomposite lattice, can be detrimental to the array performance. Consequently, finite element (FE) modelling, using PZFlex, is utilised to provide design techniques for the removal of these inter-pillar modes from the frequency band of interest and the realisation of unimodal piezocomposite transducer structures. Further FE modelling is used to generate dispersion data for 2-2, and doubly periodic 1-3, composite substrates. This dispersion data is used to design the linear arrays, with the objective of minimising mechanical inter-element cross-talk. A comparison between the FE predicted mechanical cross-coupling between array elements, for each composite material operating in air, is supported by experimentally measured data. Subsequently, the validated FE models are extended to include both operation into a solid load and the introduction of a backing material to simulate the operation of a practical NDE array transducer. The design techniques obtained from PZFlex are shown to produce arrays with low cross-talk and the extent of the cross-talk in manufactured and modelled ceramic and pmn-pt single-crystal arrays is compared.

Kinetic models and phenomenological analysis of passive lipid translocation in single-file.

Passive movement of lipids through a membrane-embedded pore is analysed with kinetic equations of transport in single-file. The number of lipids arranged along the translocation coordinate in the pore is not limited in the calculations. The assumption is made that the energetic state of a pore is independent of the sequence of lipids contained in it. The results are valid for an arbitrary number of species with identical kinetic constants. It is shown that infinitely fast diffusion of one vacant site is equivalent to the filled pore approximation, which has been used here. We introduce the concept of non-strict single-file, which allows also for exchanges of neighbouring lipids inside the pore at specified rates. The model successfully simulates the redistribution of lipids between the monolayers of red blood cell plasma membranes under operation of an active aminophospholipid translocase. Kinetic equations are related to linear flux force relations. Phenomenological coefficients are expressed and analysed in terms of kinetic constants. Plausible kinetic pore model parameters are derived from comparison with a reference simulation of a thermodynamic model of the erythrocyte transmembrane lipid distribution. Mechanical forces due to differences in compressions of the lipid molecules between the monolayers are incorporated in kinetic rate constants. It is seen how the active inward transport of aminophospholipids causes an unsymmetrical passive redistribution of the other components due to mechanical effects and cross-coupling of fluxes.

Ultrasonic transducer with reduced acoustic cross coupling

A continuous wave driven ultrasonic transducer for determining doppler frequency shift in reflected ultrasonic pressure waves in which the transmitter and receiver sections are constructed of a composite core having a plurality of segments of piezoelectric material separated by acoustic suppression material. Also disclosed is a method of reducing acoustic and mechanical cross coupling between piezoelectric transmitter and receiver sections of an ultrasonic transducer by arranging segments of piezoelectric material in a lateral array, and separating the piezoelectric segments with ultrasonic acoustic suppression material to produce a composite transducer core of reduced acoustic and mechanical cross coupling.

Biochemical organization of single motor units in two multi-tendoned muscles of the cat distal forelimb.

In anesthetized cats single motor units (MUs) of the extensor carpi ulnaris (ECU) and extensor digitorum communis (EDC) muscles were selectively activated by stimulation of cervical ventral root filaments. The distribution of force developed by single MUs at the four distal tendons of the EDC muscle and at three portions of the distal tendon of the ECU muscle was analysed. In general, single MUs of both muscles distributed force over all tendons in a unimodal pattern, with the maximal force levels generated at one specific tendon which was termed the best-tendon. Distributions of force were quantitatively described by a parameter representing the mean direction of force output (output-index) and a further one representing the dispersion of force over the distal tendons (divergence). Generally, these parameters and the best-tendon remained stable when a MU was stimulated at different frequencies, but varied from MU to MU. Despite the general stability of the force distribution, slight systematic changes were regularly found in EDC MUs, when they developed a higher amount of force due to a higher frequency of stimulation: the relative amount of force at the best-tendon increased; e.g. the MUs got more selective for the best-tendon. These changes were partly due to overcoming mechanical cross-coupling between neighbouring compartments of the EDC muscle. Such changes of force distribution were only found in a part of the ECU MUs; other ECU MUs did not change their force distribution at all or became less selective for the best-tendon. The phenomenon that MUs of multi-tendoned muscles distribute their force output to the distal tendons in specific patterns is probably due to mechanical partitioning of the parent muscles: the localization of spatial territories of MUs within different anatomical muscle compartments should correspond to the best-tendon. Complex mechanisms allowing passive transmission of force from limited territories along the transverse axis of both muscles must be assumed in order to explain why most MUs act on all tendons and why force distributions change with increasing stimulus frequency. In addition, specific relations between unit type and force distributions were found within both muscles. Fatigue-resistant EDC MUs have broader force distributions than fatigue-sensitive EDC MUs and slow ECU MUs were found to act predominantly on the most ulnar part of the distal tendon. These biomechanical properties of MUs are discussed as supporting the specific functions of the respective muscles.

An overview and general results of the Lopez island OBS experiment

The purpose of the experiment was to determine the effects of coupling and bottom currents on ocean bottom seismometers. Twelve operational OBSs, three specially designed three-component systems, and a hydrophone were compared with each other. Unlike seismometers placed on hard rock at land stations, ocean bottom seismometers can be affected by soft sediments (which act as lossy mechanical springs) and by buoyancy. Coupling through soft sediments can modify the response to ground motion much as a low pass filter does, and high buoyancy tends to counteract this effect. These effects are observed in the Lopez data, which consist of signals from mechanical transient tests, cap shots, airgun pulses, and general background noise. The modification of response is pronounced for some instruments and barely noticeable in others. Instruments that stand high in the water relative to their base width tend to be susceptible to rocking motion that shows up as a mechanical cross coupling between horizontal and vertical motion. Correlation of Lopez results with coupling theory suggests that it is possible to design ocean bottom seismometers that will couple well to any sediment. Current levels at the Lopez site (<5 cm s-1) were too small to produce noticeable effect on any of the instruments; however, the same design criteria that will minimize coupling problems will also lessen problems caused by ocean currents.

Autotrack Control Systems for Antenna Mounts with Non-Orthogonal Axes

The use of non-orthogonal, or conic, mounts for steerable antennas introduces some control system design problems not present in the more conventional orthogonal mounts. These problems result from both the geometrical and the mechanical cross-coupling which occurs between the two non-orthogonal axes of motion. This paper presents a general analysis and design of the control system for the open cassegrain antenna which can be readily applied to other non-orthogonal antenna structures. The form of the feedback controller for approximately non-interacting control of each axis is developed. Also described is a supplementary control strategy for providing tracking near the zenith region without excessively high slewing rates. A computer simulation o1 the system has verified the basic control strategy for non-orthogonal mounts and established the feasibility of operating compact antenna structures such as the open cassegrain design under severe wind conditions without a radome.