I. Yalda-Mooshabad

Nonholonomic camera-space manipulation

The method of camera-space manipulation is extended to wheeled systems. A minimum of two cameras is required to place points on end-effectors (or objects in their grasp) of n-degree-of-freedom manipulators relative to other bodies where the nonholonomic degrees of freedom on a mobile manipulator base may be included. The target bodies do not have a precisely known location relative to the environment. The method is illustrated experimentally, though not in real time, using a point placement task. It is then generalized to rigid-body positioning tasks. Although the experimental point placement illustrations make use of a very simple trajectory planning scheme for the wheels of the base, a smoother optimal trajectory planning scheme that makes use of the Pontryagin maximum principle is also developed and illustrated. In a departure from the practice of using time as the independent variable for estimation and optimal trajectory planning algorithms, the present development is time independent and instead introduces the forward rotation of the drive wheel of the base as the independent variable. >

Backscattered microstructural noise in ultrasonic toneburst inspections

A model is presented which relates the absolute backscattered noise level observed in an ultrasonic immersion inspection to details of the measurement system and properties of the metal specimen under study. The model assumes that the backscattered noise signal observed for a given transducer position is an incoherent superposition of echoes from many grains. The model applies to normal-incidence, pulse-echo inspections of weakly-scattering materials using toneburst pulses from either a planar or focused transducer. The model can be used in two distinct ways. Measured noise echoes can be analyzed to deduce a “Figure-of-Merit” (FOM) which is a property of the specimen alone, and which parameterizes the contribution of the microstructure to the observed noise. If the FOM is known, the model can be used to predict the absolute noise levels that would be observed under various inspection scenarios. Tests of the model are reported, using both synthetic noise echoes, and measured noise echoes from metal specimens having simple and complicated microstructures.

Modeling Ultrasonic Microstructural Noise in Titanium Alloys

Ultrasonic echoes from small or subtle defects in metals may be masked by competing “noise” echoes which arise from the scattering of sound by grains or other microstructural elements. Algorithms for estimating the detectability of such defects consequently require quantitative models for microstructural noise. In previous work [1,2] we introduced an approximate noise model for normal-incidence immersion inspections using tone-burst pulses, and we used the model to estimate signal/noise ratios for brittle (hard-alpha) inclusions in titanium alloys. In the present work we consider an extension of that noise model to inspections using broadband incident pulses. Like its predecessor, the broadband noise model neglects multiple scattering events, and applies to low-noise, low-attenuation materials. The broadband model provides an expression for the root-mean-square (rms) average amplitude of a given spectral component of the noise, computed on a finite time interval greater than the duration of the pulse. The model can be used to analyze backscattered noise to extract a Figure-of-Merit (FOM) for noise severity which is a property of the specimen and is independent of the measurement system. Conversely, if the FOM of the specimen is known, the model can be used to predict average noise spectral characteristics and average noise levels for various inspection scenarios.

The Practical Application of Grain Noise Models in Titanium Billets and Forgings

For pulse/echo ultrasonic inspections of metal components, mathematical models have been developed which can be be used to assess the likelihood of flaw detection. For example, for various classes of simple defects (e.g., flat cracks, spheroidal inclusions), measurement models [1] based on Auld’s reciprocity relationship can predict the RF echo seen on an oscilloscope when the defect is present at some given location in the component. Other models can predict the average level of backscattered microstructural noise that is seen when no defect is present [2]. Together, the models can be used to estimate signal-to-noise ratios for defects of various types, sizes, and locations [3]. Even when all model assumptions are satisfied, accurate predictions require accurate knowledge of transducer characteristics and pertinent properties of the metal specimen. For the prediction of grain noise levels using the Independent-Scatterer Noise Model [2], for example, the required material properties are the density, wave speed, attenuation coefficient, and Figure-of-Merit (FOM) describing the contribution of the metal microstructure to the backscattered noise. In addition to the above metal properties, the geometrical focal length and effective diameter of the transducer must also be be known. All of these model inputs must be deduced by auxilary measurements.

Monte-Carlo Simulation of Ultrasonic Grain Noise

In ultrasonic inspections for small or subtle defects in metals, defect signals may be obscured by grain noise echoes which arise from the scattering of sound by the microstructure of the metal. Models for predicting microstructural noise levels are consequently essential for accurately assessing the reliability of the ultrasonic inspections. Existing noise models, like the independent scatterer model (ISM) [1], are capable of predicting only average noise characteristics, such as the root-mean-square (rms) noise level. Average noise levels, although useful, are not sufficient for assessing detection reliability. One needs to know the manner in which noise signals are distributed about their average level. The expected peak noise level, for example, effects the rate of “false calls”, in which noise signals are mistaken for echoes from critical defects. In this work, we present a Monte-Carlo method for simulating time-domain noise signals observed in pulse/echo immersion inspections of metal components. The method predicts simulated time-domain noise signals, and hence can be used to determine both average and peak noise levels. We assume that the backscattered noise is dominated by the single- scattering of the incident beam by individual metal grains. The metal volume is represented as an ensemble of spherical, single-crystal grains whose centers and orientations are randomly chosen. Grain radii are determined by the nearest-neighbor distances and volume conservation. The backscattered voltage signal from each grain is calculated by treating the grain as an anisotropic scatterer in the homogeneous average medium formed by the other grains. Backscattered signals from all grains are summed to determine the total noise signal.

Reflection of ultrasonic waves from imperfect interfaces: a combined boundary element method and independent scattering model approach

A numerical technique for obtaining interface reflection coefficients for imperfect bonds between similar materials for a wide range of distributed defects is developed. A numerical boundary element method is utilized to find the far-field scattering amplitudes of a single defect for a normally incident plane wave. Then, the normal incidence reflection coefficient for a planar distribution of such defects is obtained from the independent scattering model. As a validation, the reflection coefficients are compared to the quasi-static model results where the latter are available. This establishes the basis for one application of the new model, the determination of spring constants which are not available. Other applications of the model, including studies of the response at frequencies beyond the quasi-static limit, the ratio of longitudinal to transverse wave reflectivities, and the effects of selected multiple scattering are discussed.

Model Applications to Predict Ultrasonic Signals from Honeycomb Panels

The engine fan ducts used in the F100 fighter are titanium alloy structures consisting of inner and outer plates (“skins”) and intervening interior “side” walls that connect the two plates and are arranged in a “honeycomb” fashion. The panels are ultrasonically inspected in immersion in pulse-echo or through-transmission modes to look for common defect conditions: interior water in the honeycomb cells, disbonds between the skins and side walls, and tears in the honeycomb side walls. In such C-scan inspections, focussed transducers are used with the beam(s) normal to the inner and outer skins.