Arturo Baltazar

Active sensing and damage detection using piezoelectric zinc oxide-based nanocomposites

This study investigated the design and performance of piezoelectric nanocomposite-based interdigitated transducers (IDTs) for active sensing and damage detection. First, thin films that are highly piezoelectric and mechanically flexible were designed by embedding zinc oxide (ZnO) nanoparticles in a poly(vinylidene fluoride-trifluoroethylene) (PVDF-TrFE) piezo-polymer matrix. Second, the suspended nanoparticle solutions were then spin coated onto patterned comb electrodes to fabricate the IDTs. The films were then poled to align their electric domains and to increase their permanent piezoelectricity. Upon IDT fabrication, its sensing and actuation of Lamb waves on an aluminum pipe was validated. These results were also compared to data obtained from commercial Macro Fiber Composite IDT transducers. In the last phase of this work, damage detection was demonstrated by mounting these nanocomposite sensors and actuators (using a pitch-catch setup) onto an aluminum pipe and plate. Damage was simulated by tightening a band clamp around the pipe and by drilling holes in the plate. A damage index calculation was used to compare results corresponding to different levels of damage applied to the plate (i.e., different drilled hole depths), and good correlation was observed. Thus, ZnO/PVDF-TrFE transducers were shown to have the potential for use as piezoelectric transducers for structural health monitoring and damage detection.

Determination of surface mechanical properties using a hertzian contact and ultrasound sensor

Nondestructive monitoring of contact properties for biological and engineering materials is important in several engineering areas. Also, in robotics, manipulation of fragile objects requires precise determination of instantaneous contact area and forces with minimum intrusion. In this paper, a contact probe based on hertzian contact and ultrasonic energy transmission for characterization of surface properties is proposed. The probe is built from a solid semi-spherical head which is sonified from the top by an ultrasonic transducer with a center frequency of 1 MHz. Force range was 0–23N and displacements as small as 0.05 millimeters were reached with a proposed automatic control system. A quasi-static spring model to describe the interfacial properties at contact is studied. The results indicate that due to its high sensitivity, the probe could be used for measurements of surface mechanical response.

Detection of damage in multiwire cables based on wavelet entropy evolution

Multiwire cables are widely used in important engineering structures. Since they are exposed to several dynamic and static loads as well as detrimental environmental conditions, their structural health can be compromised. Due to the critical role played by multiwire cables, it is necessary to develop a non-destructive health monitoring method to maintain their structure and proper performance. Ultrasonic inspection using guided waves is a promising non-destructive damage monitoring technique for rods, single and multiwire cables. However, the propagated guided waves are composed of an infinite number of dispersive vibrational modes making their analysis difficult. In this work, an entropy-based method to identify small changes in non-stationary signals is proposed. An experimental system to capture and post-process acoustic signals is implemented. The discrete wavelet transform is computed in order to obtain the reconstructed wavelet coefficients of the signals and to analyze the energy at different scales. The use of the concept of entropy evolution of non-stationary signals to detect damage in multiwire cables is evaluated. The results show that there is a correlation between the entropy value and level of damage of the cable including breaking of single wires and change in the mechanical contact conditions among the wires. It is found that the studied method has low sensitivity to signal noise and can reduce the computational complexity encountered in a typical time–frequency analysis.

Determination of the instantaneous initial contact point on a parallel gripper using a multi input fuzzy rules emulated network controller with feedback from ultrasonic and force sensors

Many applications of robotic manipulators require a precise applied force control to determine the instantaneous initial contact (force approaching zero) as accurate as possible. Thus, the transient state from free to restricted motion needs to be taken into account. In this work, a multi input fuzzy rules emulated network (MiFREN) control scheme with adaptation is developed to find the first contact position between the fingers of a parallel gripper and a soft object. We propose the use of an ultrasonic sensor (with Hertzian contact) working simultaneously with a high sensitivity load cell. The IF-Then rules for MiFREN controller and a new cost function using both the ultrasonic signal and the contact force are proposed. The results show that the proposed controller is capable of finding the instantaneous initial contact without any knowledge about the object, material properties and/or its location on the working space.

Damage detection using the signal entropy of an ultrasonic sensor network

Piezoelectric ultrasonic sensors used to propagate guided waves can potentially be implemented to inspect large areas in engineering structures. However, the inherent dispersion and noise of guided acoustic signals, multiple echoes in the structure, as well as a lack of an approximate or exact model, limit their use as a continuous structural health monitoring system. In this work, the implementation of a network of piezoelectric sensors randomly placed on a plate-like structure to detect and locate artificial damage is studied. A network of macro fiber composite (MFC) sensors working in a pitch–catch configuration was set on an aluminum thin plate 1.9 mm in thickness. Signals were analyzed in the time-scale domain using the discrete wavelet transform. The objectives of this work were threefold, namely to first develop a damage index based on the entropy distribution using short time wavelet entropy of the ultrasonic waves generated by a sensor network, second to determine the performance of an array of spare MFC sensors to detect artificial damage, and third to implement a time-of-arrival (TOA) algorithm on the gathered signals for damage location of an artificial circular discontinuity. Our preliminary test results show that the proposed methodology provides sufficient information for damage detection, which, once combined with the TOA algorithm, allows localization of the damage.

Ultrasonic elastic modes in solid bars: an application of the plane wave expansion method.

Ultrasonic elastic modes in solid bars are investigated theoretically and experimentally using the plane wave expansion method to calculate the dispersion curves k=k(omega) for longitudinal, torsional, and flexural waves. The plane wave extension method allows to consider rods of circular and square cross sections. The technique, which has received attention in the study of photonic and phononic crystals, is adapted in order to identify the various types of modes. Results are compared with predictions from semi-analytical models. The numerical approximation is validated with the experimental determination of the time-frequency dispersion curves. The technique based on the plane wave expansion method presented here could be a numerical alternative used to determine the wave propagation and modal vibration with high precision in structures like bars and cylinders. Practical applications of this study could include the inspection of long-span engineering systems with bar or cylinder like characteristics.

Development of an estimated force feedback controller based on hertzian contact and ultrasound

In this paper, the design of a novel force controller which combines a neuro-fuzzy algorithm and a new contact ultrasonic probe is proposed. The fuzzy rules emulated network (FREN) has a control algorithm structure based on fuzzy IF-THEN rules. The contact probe is integrated in a Cartesian robotic system with the FREN force control. The contact probe is equipped with a 1 MHz frequency ultrasonic transducer. The probe is constructed by a semi-spherical head and an ultrasonic transducer located at the top of the head. The contact between the probe and the medium is modeled by Hertz theory. The experimental results show that the proposed system exhibits high sensitivity for touching contact, instantaneous contact and reaction force estimation. To test the system, experiments were conducted in elastic and elastic-plastic materials. The results indicate that the proposed system is able to effectively control the contact force.

THICKNESS DETERMINATION OF A PLATE WITH VARYING THICKNESS USING AN ARTIFICIAL NEURAL NETWORK FOR TIME-FREQUENCY REPRESENTATION OF LAMB WAVES

Thickness estimation of a varying‐thickness media is carried out using an algorithm acting as an artificial neural network for time‐frequency representation (TFR) of Lamb waves. Dispersion curves are reconstructed using a self adjustable network multi‐input fuzzy rules emulated network (MIFREN). The uncertainty in the time‐frequency determination is compared with a typical spectrogram technique. The proposed algorithm is computationally less complex than others used in the past. Experimental results were obtained by exciting Lamb waves on an aluminum plate with varying thickness; these were compared with numerical estimations.

Determination of Contact Evolution on a Soft Hemispherical Probe Using Ultrasound

In the area of robotic sensing, determination of parameters of a solid object, such as contact area, pressure distribution, or deformation, is an important step for the sensing and manipulation of an object with an unknown mechanical behavior. In this paper, a soft contact probe based on ultrasound is developed using a commercial elastomer. The emphasis is placed on the study of the contact mechanics of the probe. The probe is equipped with an ultrasonic sensor and then implemented in a 3 degrees of freedom manipulator robotic system. The correlation between the normal contact force and the ultrasonic signal reflected from the interface between the soft probe and the object is precisely studied. Experimental results are presented for loading and unloading cycles of the soft contact probe against a flat surface. Tests are performed on aluminum, elastomer, and plasticine test samples. It is shown that the probe can potentially be used as an end effector of a robotic system which will use the recorded ultrasonic signal as the feedback to a force control algorithm.

ULTRASONIC LAMB WAVE TOMOGRAPHY OF NON‐UNIFORM INTERFACIAL STIFFNESS BETWEEN CONTACTING SOLID BODIES

In this work, the interfacial condition between two contacting solid bodies is determined using an ultrasonic Lamb wave tomography. An ultrasonic measurement setup based on parallel projection tomography that uses broadband transducers is presented. Time‐frequency representation (TFR) is employed to improve identification of the arrival time and frequency contents of the transmitted wave modes. Experiments are carried out on an aluminum plate where the flat end of a solid aluminum bar is pressed perpendicular to the surface; then, the level and distribution of the interfacial stiffness is controlled by applied force on the bar. The tomographic reconstructions are performed during repeated loading cycles. Finally, reconstructed tomographic images of the contact area versus applied pressure are obtained. The method presented has potential applications in determining the loss of contact in mechanically joined structural components.