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To increase safety and reduce the electric circuit cost, dielectric elastomer actuators (DEAs) must operate below the kV range. The simplest strategy to reduce the voltage operation is to decrease the dielectric elastomer membrane thickness. This research aims to demonstrate DEAs with a nanometric uniform thickness that can operate at a low voltage (below 70 V) and a high frequency. We use the roll-to-roll process to fabricate a 600 nm thick stretchable PDMS (polydimethylsiloxane) nanosheet and a 200 nm thick conductive nanosheet. These nanosheet-DEAs are tested in high-frequency operations of DC voltage below 70 V and in a frequency range of 1–30 kHz.

Frequency response functions (FRFs) are generally measured by investigating the input-output relationship of a target structure. An input is applied to the target structure and the output is measured. Typical vibration tests measure inputs as excitation forces using load cells or similar devices and outputs as velocities or accelerations using laser Doppler vibrometers or accelerometers. Each input or output has three translational degrees of freedom (DOF). Consequently, experimental models established based on such tests have three translational DOF. However, numerical models using finite element analysis (FEA) for existing structures have six DOF at each node (three translational and three rotational DOF). Therefore, numerical models using FEA and experimental models have gaps in their DOF, leading to diverse problems. This study realizes a non-contact measurement method for rotational DOF FRFs using an impulse excitation force generated by laser ablation (LA) where the input is a moment and the output is the velocity. A laser beam radiated from an Nd:YAG pulse laser emitter is divided using a half mirror, halving the laser pulse energy. Then, these laser beams are radiated on two points near a target structure's excitation point. The simultaneous occurrence of LA at these two points realizes a moment that acts on the target structure in a non-contact manner. This experiment uses a 5052 aluminum alloy plate where one end is fixed as a test piece. Both the auto FRFs and cross FRFs for the rotational DOF between two arbitrary points on the test piece are measured. The FRFs obtained by our method agree well with those obtained by FEA, demonstrating its efficacy. (C) 2019 The Authors. Published by Elsevier Ltd.

This study evaluates the basic characteristics of smart structures composed of dielectric elastomer actuators (DEAs) to suppress vibrations. A DEA, which is a lightweight, flexible polymer that can induce high deformations, should realize next-generation actuators. Additionally, DEA can achieve vibration control of structures with complex shapes or curved surfaces. Herein the performance and efficacy of DEAs are evaluated as an actuator for vibration control at high frequencies. First, the appropriate DEA structure is considered. Second, the control system for the smart structure using the DEA is modeled and designed as an actuator. Third, a method to determine DEA's optimum arrangement and shape is discussed by focusing on the structure's strain energy. Finally, a control simulation and a control experiment validate the vibration suppression effects and the efficacy of the DEA.

A balloon dielectric elastomer actuator (DEA) speaker is constructed by forming a disk-shaped DEA film into a sphere using air, increasing its surface area by 3033%. Sound is generated via expansion and contraction. Shaping the DEA film into a sphere decreases the film thickness and applies a significant amount of pre-strain (prestretch) to the film. In the experiment, sound pressure levels in an audible frequency response up to 16 kHz is the approximately same as both our previous hemispherical DEA speaker and this balloon DEA speaker, allowing an actuating voltage of the balloon speaker lower (800 V p-p signal with an offset of 400 V) than that of our previous hemispherical DEA speaker (2 kV). This is the first time to achieve the audible frequency response of the acrylic balloon DEA speaker. Furthermore, the acoustic radiation pattern of the speaker extends over a 270° range. Compared to polyhedron loudspeakers, which are currently used as omnidirectional sound sources in general acoustic tests, a balloon DEA speaker is compact and lightweight. Therefore, balloon DEA speakers have sufficient potential to contribute to the proper assessment of indoor acoustic characteristics using a scale model.

Many methods based on acoustic vibration characteristics have been studied to indirectly assess fruit ripeness via fruit firmness. Among these, the frequency of the 0S2 vibration mode measured on the equator has been examined, but soft-flesh fruit do not show the 0S2 vibration mode. In this study, a Rayleigh wave is generated on a soft mango fruit using the impulse excitation force generated by a laser-induced plasma shock wave technique. Then, the flesh firmness of mangoes is assessed in a non-contact and non-destructive manner by observing the Rayleigh wave propagation velocity because it is correlated with the firmness (shear elasticity), density, and Poisson’s ratio of an object. If the changes in the density and Poisson’s ratio are small enough to be ignored during storage, then the Rayleigh wave propagation velocity is strongly correlated to fruit firmness. Here, we measure the Rayleigh wave propagation velocity and investigate the effect of storage time. Specifically, we investigate the changes in firmness caused by ripening. The Rayleigh wave propagation velocity on the equator of Kent mangoes tended to decrease by over 4% in 96 h. The Rayleigh wave measured on two different lines propagated independent distance and showed a different change rate of propagation velocity during 96-h storage. Furthermore, we consider the reliability of our method by investigating the interaction of a mango seed on the Rayleigh wave propagation velocity.

In typical structures and machines, bolted joints are used for easy assembly, disassembly, and maintenance. However, axial force control is critical because a decrease in the axial force (viz., the fastening force) of bolted joints results in fatigue fractures due to the applied vibrations on the bolted joints and chronic degradation. This work assesses the feasibility of detecting weakly torqued bolted joints by studying the correlation between the variation in axial force of the bolt and the shift in the natural frequency of the protruding threaded part of the bolt. It has been shown that the rigidity of the jointed area at the base of the protruding thread part decreases in accordance with the decrease in the axial force of the bolt. This is attributed to a decrease in the natural frequency of the protruding thread part. Under these assumptions, the axial force on bolted joints is assessed. The accuracy with which axial force can be estimated depends on the extent of the shift in the natural frequency of the protruding thread part. To investigate the practical usability of our method on strength classification of bolts, we assessed the performance of loosening detection in three strength classifications (4.8, 8.8, and 10.9).

A health-monitoring system is proposed to detect holes drilled in a pipe based on laser plasma acoustic excitations and acoustic measurements. In this system, an acoustic excitation is applied to a pipe via a laser-induced plasma in air generated by a high-power Nd: YAG pulse laser. Laser-induced plasmas can realize non-contact acoustic impulse excitations. A microphone is used to measure the time response of the acoustic pressure. In this study, we focus on the detection of a hole in the pipe. The reflection of the acoustic wave due to a hole drilled in the pipe induces a change in the time response of the acoustic pressure. Applying a continuous wavelet transform to the measured time response data with/without the hole can locate the position of the hole. This study demonstrates the effectiveness of the present damage detection method based on an acoustic excitation using a laser-induced plasma.

A vibration experiment technique based on a dielectric elastomer actuator (DEA) excitation for apples is proposed. The characteristics of DEAs such as flexibility, lightweight, large deformation, and a fast response are conducive to vibration applications. Moreover, DEAs composed of soft rubber-like materials should be compatible with diverse objects. They can be applied to structures with curved surfaces and transfer excitation forces effectively. Herein, as an agricultural application, a DEA excitation technique is applied to assess the quality of apples, which have a spherical shape, using quality-related indices. Firmness is a function of the fruit's mass and its second-lowest resonance frequency. To evaluate the frequency responses and firmness of apples, we developed a vibration experiment system and the appropriate DEA composition. Then, the vibration responses of apples were measured by laser Doppler vibrometer (LDV) and evaluated by a spectrum analyzer to obtain the frequency responses. By assessing the firmness index of apples upon storing, we experimentally demonstrate the effectiveness of the proposed measurement technique based on the DEA excitation.

Various indexes have been used to assess the ripeness of fruit, including peel color and firmness because added value is given to fruit when grade selection is determined objectively. In this paper, we realize a non-destructive firmness assessment for apples by means of a non-contact vibration test method. We investigate their natural frequencies and vibration mode shapes because these factors influence the 0 S 2 mode, which is related to firmness. A laser-induced plasma shock wave generated with a high-output Nd:YAG pulsed laser is applied to apples as an excitation force. Firmness is assessed with this non-contact and non-destructive method using the apple’s vibration response spectra measured with a laser Doppler vibrometer. The effectiveness of this method is experimentally demonstrated through assessments of apples’ firmness, identification of the vibration mode shapes, and a follow-up survey on the flesh firmness of apples during storage.

Recently, the demand for higher performing underwater structures under diverse conditions has increased. Examples include improved precision and speed of the position control of robot manipulators. To prevent the control spillover problems when active controls are used, a control system is typically constituted with a low-pass filter to eliminate all modes except for the target modes. However, experimentally measuring the dynamic properties of an underwater structure in an environment where the structure and a fluid continuously influence each other is difficult. We have recently proposed a noncontact vibration testing method for dynamic characterizations of underwater structures in which the response to a laser ablation excitation force is measured by laser Doppler vibrometer. Integrating passive control using a vibration-damping material affixed onto the underwater structure and active control constituted with the low-pass filter may realize a more cost-effective system. To develop this combined control into a practical method, the reliability of the measured frequency response function must be validated. Additionally, the applicable frequency range must be expanded to encompass the high-frequency region (several tens of kHz) so that the vibration suppression quality of underwater structures can be evaluated. Herein we quantify the effect of random measurement errors on the measured frequency response function with a reliability factor based on the concept of coherence functions. Using the measured frequency response function with a reliability factor, we demonstrated that our method can evaluate passive vibration suppression effect of an underwater structure with a damping material in high-frequency ranges up to 20 kHz.

This paper proposes a non-contact damage detection method based on Lamb waves generated by laser ablation (LA). Previously, Lamb waves generated by contact-type sensors such as acoustic emission or piezoelectric zirconate titanate devices have been studied to detect damage. Lamb wave generation systems with embedded contact-type excitation devices to objective structures to be inspected may quickly realize large-area damage detection on a huge object such as an aircraft. However, replacing contact-type devices with non-contact devices in Lamb wave generation systems, the systems will have sufficient potential to excite under the specific conditions such as submerged target structures in liquid and high-temperature substances. The LA-generated Lamb waves that have amplitudes several hundred times larger than those generated by conventional laser-thermoelastically generated Lamb waves are of advantage from the viewpoint of the signal-to-noise ratio in the measurements. When the laser fluence reaches 1012–1014 W/m2, which is greater than that for laser-thermoelastic regime, a LA regime is induced. The amplitudes of the LA-generated Lamb waves might be higher than those of the laser-thermoelastically generated Lamb waves; this is within the scope of the assumption. Since the LA process entails a number of nonlinear processes such as melting, vaporization, and sublimation, it is important to confirm that LA could generate a Lamb wave and its mode. In this paper, Lamb waves that contain broadband frequency elements of more than several hundred kHz are generated by non-contact impulse excitation using LA, which is common in vibration tests in the high-frequency range, laser peening, propulsion of micro-aircraft, bolt loosening diagnosis, etc. The present method is evaluated by comparing the measured and calculated propagation phase and group velocities of the Lamb waves. Furthermore, the feasibility of our approach is demonstrated by non-contact damage detection against an aluminum alloy 2024 plate with a crack.

Soft robots with dynamic motion have a potential to apply for various purpose to treat fragile stuffs. On the other hand, rotational motors potentially are used as one of main actuators to provide functions for robots (such as vibration, locomotion, and suction). A rotational motor which does not prevent the soft robots from deforming is required for providing further applications for the soft robots. In this paper, we developed a deformable motor based on dielectric elastomer actuators (DEAs). The deformable motor has advantages in low-energy consumption, light-weight, and no-generation of magnetic field. We conducted on two experiments. In a visualize experiment, we confirmed that the internal stress from the DEA transmitted to the mechanism for rotating the motor. In a performance experiment, we demonstrated that performance of the deformable motor changed as deforming by external force. We observed the fact that the rotational performance is not likely to decrease when the external force is applied to deform as an elliptical shape. Our developed motor opens up a field for applying the rotational motion to the soft robots.

In recent years, polymer composites using natural fibers have attracted attention. To evaluate the aging process of the polymer composites caused by ultraviolet rays, one has to apply non-destructive tests. In this paper, we established an assessment system using laser-induced plasma shock wave as an impulse excitation to evaluate a Young’s modulus and a damping ratio of polymer composites composed of bamboo fiber and phenolic resin. A shock wave is generated when laser-induced plasma expands at a high velocity throughout the ambient. Measuring responses of the composites by a laser Doppler vibrometer, we obtain absolute value of Fourier spectra and their natural frequencies. We used those to determine dynamic Young’s moduli and damping ratios of the composites. We made the composites which were fabricated by a hot press using a mold. There were the same trend between the dynamic Young’s moduli obtained by our system and the ones measured by a three-point flexural bending test. We could additionally obtain the damping ratios of the composites.

A vibration suppression technique for lightweight and flexible membrane structures is proposed using smart structure technology with a dielectric elastomer actuator (DEA). DEA, which is a lightweight flexible polynomial material with a large deformation and high response time, can easily be applied to membrane structures for vibration suppression over a wide frequency range. Herein, active vibration control for membrane structures with DEA is examined. A non-contact excitation system with a high-power Nd:YAG pulsed laser precisely measures the vibration responses of membrane structures. A laminated DEA is fabricated and attached to the membrane surface, generating a highly reproducible ideal impulse excitation. Then a digital control system is constructed with the laser excitation system. To demonstrate the effectiveness of the proposed method, vibration control experiments are conducted using a controller designed by H∞ control theory with the identified membrane model in both atmospheric and vacuum environments.

A pulsed-laser ablation method for non-contact experimental vibration analysis of completely submerged underwater structures is proposed. Although impact testing with an impulse hammer is commonly used for vibration analysis due to its simplicity, impact testing has limited use in underwater conditions. An input-detection-free frequency response function measurement in water will greatly contribute to the development of high-precision and high-speed positioning autonomous underwater vehicles, underwater vehicle-manipulators, underwater robots, submarines, etc., which are used in dangerous conditions (e.g., deep oceans, under ice, and nuclear reactor plants). To achieve these high-performance underwater systems, vibrations due to hydrodynamic parameters (such as added mass, buoyant force, drag force, and damping coefficient) should be suppressed, and vibration tests should be conducted on the actual equipment submerged in water. The proposed method yields the frequency response function by applying a pulsed-laser-ablation excitation force to an underwater structure and measuring the output using a laser Doppler vibrometer. Because the direction, strength, and effective duration of the pulsed-laser-ablation force are essentially constant, this force can be estimated by measuring these properties in advance. Hence, the proposed method realizes input-detection-free frequency response function measurements in underwater conditions.

Herein a vibration test method is discussed based on a non-contact, non-destructive excitation input using a laser-induced plasma (LIP) shock wave and a non-contact input estimation using Newton’s second law of motion. We have previously introduced a non-contact vibration test method using an excitation force generated by laser ablation, but it cannot be used when the target structure is lightweight, fragile, and small-sized because a crater with a diameter of several micrometers is created. The LIP generates a highly reproducible shock wave, which become an excitation force to a target structure. This shock wave depends on the gas density, gas specific heat ratio, laser fluence, and ambient environment. If these parameters are constant, the LIP excitation force can be estimated beforehand, allowing only the output measurement to determine the frequency response measurement of an input–output relationship of the target structure. After calibrating the LIP excitation, the frequency response function of a target structure can be obtained.

A piece of paper has many useful characteristics; it is affordable, lightweight, thin, strong, and highly absorbent. These features allow inexpensive and flexible devices to be fabricated easily and rapidly. We have proposed a new field, “paper mechatronics,” which merges printed robotics and paper electronics, and to realize electronic and mechanical systems by printing. Herein, we develop a method to print an actuator and a structure on a sheet of paper. A trilayer electrothermal actuator is printed to activate a printed robot. The paper self-folds along the printed pattern to form the three-dimensional (3-D) structure of the robot body. We also investigate important factors necessary to develop a printed robot. Experiments, including finite element analysis (FEA), confirm our bimetal modeling assumption for the printed actuator and improve the locomotive ability. The key factors in self-folding are paper thickness and humidity. Our findings can improve the reliability of printed robot designs. A self-folding A7-sized paper robot demonstrates locomotion at 10 mm per step.

Although transparent materials with birefringent properties (e.g., solar panels and separator films for secondary cells) are common, damage detection during the manufacturing process is crucial to economically realize high-quality materials. Herein a method using a pulsed-laser and a high-speed polarization-imaging camera is proposed to rapidly detect damage, including scratches and dents, in transparent materials. Specifically, as stress waves, which are generated by a non-contact impulse excitation from laser ablation, propagate through a material, the stress concentrations induced around damage are measured as the two-dimensional birefringent phase differences using a high-speed polarization-imaging camera with a microsecond-order temporal resolution. When stress is dominant, the distribution of the measured birefringent phase difference can be considered the relative distribution of stress. Using acrylic plates as a representative transparent material with several hundred micrometers of damage (e.g., a dent or a scratch), we demonstrate that the proposed method detects damage in a very short timeframe of several microseconds.