Stewart Sherrit

Modeling and computer simulation of ultrasonic/sonic driller/corer (USDC)

Simulation and analytical models for the ultrasonic/sonic drill/corer (USDC) are described in this paper. The USDC was developed as a tool for in-situ rock sampling and analysis in support of the NASA planetary exploration program. The USDC uses a novel drive mechanism, which transfers ultrasonic vibrations of a piezoelectric actuator into larger oscillations of a free-flying mass (free-mass). The free-mass impact on the drill bit creates a stress pulse at the drill tip/rock interface causing fracture in the rock. The main parts of the device (transducer, free-mass, bit, and rock) and the interactions between them were analyzed and numerically modeled to explore the drive mechanism. Each of these interactions is normally described by a time-dependent 2- or 3-D model involving slowly converging solutions, which makes the conventional approach unsuitable for USDC optimization studies. A simplified integrated model using tabulated data was developed to simulate the operation of the USDC on desktop PC and successfully predicted the characteristics of the device under a variety of conditions. The simulated results of the model and the experimental data used to verify the model are presented.

Novel Horn Designs for Ultrasonic/Sonic Cleaning Welding, Soldering, Cutting and Drilling

A variety of Industrial applications exist where power ultrasonic elements such as the ultrasonic horn are used. These included the Automotive, Instruments, Foods, Medical, Textiles and Material Joining and Fabrication Industries. In many of these devices the ultrasonic horn is the key component. The standard transducer used in these devices consists of three main parts, the backing, the piezoelectric elements and the horn. Standard horn designs have changed very little since their inception. There are four common types of standard horns. They are; constant, linear, exponential and stepped, which refer to the degree to which the area changes from the base to the tip. A magnification in the strain occurs in the horn that in general is a function of the ratio of diameters. In addition the device is generally driven at resonance to further amplify the strain. The resonance amplification is in general determined by the mechanical Q (attenuation) of the horn material and radiation damping. The horn length primarily determines the resonance frequency. For a 22 kHz resonance frequency a stepped horn of titanium has a length of approximately 8 cm. Although these standard horns are found in many current industrial designs they suffer from some key limitations. In many applications it would be useful to reduce the resonance frequency however this would require device lengths of the order of fractions of meters which may be impractical. In addition, manufacturing a horn requires the turning down of the stock material (eg. Titanium) from the larger outer diameter to the horn tip diameter, which is both time consuming and wasteful. In this paper we will present a variety of novel horn designs, which overcome some of the limitations discussed above. One particular design that has been found to overcome these limitations is the folded horn. In this design the horn elements are folded which reduce the overall length of the resonator (physical length) but maintain or increase the acoustic length. In addition initial experiments indicate that the tip displacement can be further adjusted by phasing the bending displacements and the extensional displacements. The experimental results for a variety of these and other novel horn designs will be presented and compared to the results predicted by theory.

Comparison of the Mason and KLM equivalent circuits for piezoelectric resonators in the thickness mode

The parameters of the KLM and Masons equivalent circuits in the thickness mode are presented to include dielectric, elastic and piezoelectric loss. The models are compared under various boundary conditions with and without acoustic layers to the analytical solutions of the wave equation. We show that in all cases equivalence is found between the analytical solution and the KLM and Masons equivalent circuit models. It is noted that in order to maintain consistency with the linear equations of piezoelectricity and the wave equation care is required when applying complex coefficients to the models. The effect of the piezoelectric loss component on the power dissipated in the transducer is presented for loaded and unloaded transducers to determine the significance of the piezoelectric loss to transducer designers. The effect of the piezoelectric loss on the insertion loss was found to be small.

Modeling of horns for sonic/ultrasonic applications

JPL has a requirement for telerobotic tools for planetary sample acquisition, which require low power and have the ability to work in harsh environments. We are currently investigating the possibility of using ultrasonic horns to develop a family of ultrasonic tools for these environments. In an effort to determine control parameters a one-dimensional Masons model for a stepped ultrasonic horn assembly was developed which includes the effects of mechanical and electrical losses in the piezoelectric material and acoustic elements. The model is separated into three regions; the piezoelectric stack including stress bolt, the backing layer and the horn. The model is found to predict the impedance data of the horn assembly very accurately up to the first coupled (radial) resonance. The model also allows for the calculation of the velocity and force and power delivered to each acoustic element. FEM modeling and accelerometer data from the horn tip were used to corroborate the model. The difficulties associated with modeling the load impedance of various devices will be discussed and current directions noted.

Characterization of the Electromechanical Properties of EAP materials

Electroactive polymers (EAP) are an emerging class of actuation materials. Their large electrically induced strains (longitudinal or bending), low density, mechanical flexibility, and ease of processing offer advantages over traditional electroactive materials. However, before the benefits of these materials can be exploited, their electrical and mechanical behavior must be properly quantified. Two general types of EAP can be identified. The first class is ionic EAP, which requires relatively low voltages (<10V) to achieve large bending deflections. This class usually needs to be hydrated and electrochemical reactions may occur. The second class is Electronic-EAP and it involves piezoelectric, electrostrictive and/or Maxwell stresses. These materials can require large electric fields (>100MV/m) to achieve longitudinal deformations at the range from 4 - 360%. Some of the difficulties in characterizing EAP include: nonlinear properties, large compliance (large mismatch with metal electrodes), non-homogeneity (resulting from processing) and hysteresis. To support the need for reliable data, the authors are developing characterization techniques to quantify the electroactive responses and material properties of EAP materials. The emphasis of the current study is on addressing electromechanical issues related to the ion-exchange type EAP also known as IPMC. The analysis, experiments and test results are discussed in this paper.

Efficient Electromechanical Network Models for Wireless Acoustic- Electric Feed-throughs

There are numerous engineering design problems where the use of wires to transfer power and communicate data thru the walls of a structure is prohibitive or significantly difficult that it may require a complex design. Such systems may be concerned with the leakage of chemicals or gasses, loss of pressure or vacuum, as well as difficulties in providing adequate thermal or electrical insulation. Moreover, feeding wires thru a wall of a structure reduces the strength of the structure and makes the structure susceptibility to cracking due to fatigue that can result from cyclic loading. Two areas have already been identified to require a wireless alternative capability and they include (a) the container of the Mars Sample Return Mission will need the use of wireless sensors to sense pressure leak and to avoid potential contamination; and (b) the Navy is seeking the capability to communicate with the crew or the instrumentation inside marine structures without the use of wires that will weaken the structure. The idea of using elastic or acoustic waves to transfer power was suggested recently by Y. Hu, et al.1. However, the disclosed model was developed directly from the wave equation and the linear equations of piezoelectricity. This model restricted by an inability to incorporate head and tail mass and account for loss in all the mechanisms. In addition there is no mechanism for connecting the model to actual power processing circuitry (diode bridge, capacitors, rectifiers etc.). An alternative approach which is to be presented is a network equivalent circuit that can easily be modified to account for additional acoustic elements and connected directly to other networks or circuits. All the possible loss mechanisms of the disclosed solution can be accounted for and introduced into the model. The circuit model allows for both power and data transmission in the forward and reverse directions through acoustic signals at the harmonic and higher order resonances. This system allows or the avoidance of cabling or wiring. The technology is applicable to the transfer of power for actuation, sensing and other tasks inside sealed containers and vacuum/pressure vessels.

Wireless piezoelectric acoustic-electric power feedthru

There are numerous engineering applications where there is a need to transfer power and communication data thru the walls of a structure. A piezoelectric acoustic-electric power feedthru system was developed in this reported study allowing for wireless transfer of electric power through a metallic wall using elastic waves. The technology is applicable to the transfer of power for actuation, sensing and other tasks inside sealed containers and vacuum/pressure vessels. A network equivalent circuit including material damping loss was developed to analyze the performance of the devices. Experimental test devices were constructed and tested. The power transfer capability and the transfer efficiency were measured. A 100W feed though capability with 38 mm diameter device and 88% transmission efficiency were demonstrated. Both analytical and experimental results are presented and discussed in this paper.

The physical acoustics of energy harvesting

Energy harvesting systems based on the transformation of acoustic vibrations into electrical energy are increasingly being used for niche applications due to the reduction in power consumption of modern day electronic systems. Typically these applications involve extracting energy at remote or isolated locations where local long term power is unavailable or inside sealed or rotating systems where cabling and electrical commutation are problematic. The available acoustic power spectra can be in the form of longitudinal, transverse, bending, hydrostatic or shear waves of frequencies ranging from less than a Hz to 10s of kHz. The input stress/vibration power can be generated by machines, humans or nature. We will present a variety of acoustic energy harvesting modes/devices and look at the commonalities of these devices. The common elements of these systems are: an input mechanical power spectrum, an effective acoustic impedance matching, a conversion of the input mechanical energy into electrical energy using piezoelectric or biased electrostrictive transducers and a matched electrical load. This paper will focus of the physical acoustics of these energy harvesting systems and identify the elements of these devices and look at the current limits of the harvested electrical power from these devices. Recent results on an acoustic electric feed-through device demonstrated acoustic power conversions of the order of 70 W/cm2 and 25 W/cm3 using a pre-stressed stacked PZT ceramics operating at 16 kHz with an efficiency of 84%. These results suggest the piezoelectric is not the limiting element of these devices and we will show that the main impediment to increased power is the vibration source amplitude, frequency, inertia and the size limitations of the energy harvesting systems or in the case of human powered systems the requirement that the device remains unobtrusive. Although the power densities of these devices may be limited there are plenty of applications that are feasible within the available power densities due to the wonders of CMOS.

Studies of acoustic-electric feed-throughs for power transmission through structures

There are numerous engineering design problems where the use of wires to transfer power and communicate data thru the walls of a structure is prohibitive or significantly difficult that it may require a complex design. Using physical feedthroughs in such systems may make them susceptible to leakage of chemicals or gasses, loss of pressure or vacuum, as well as difficulties in providing adequate thermal or electrical insulation. Moreover, feeding wires thru a wall of a structure reduces the strength of the structure and makes the structure prone to cracking due to fatigue that can result from cyclic loading and stress concentrations. One area that has already been identified to require a wireless alternative to electrical feedthroughs would be the container of any Mars Sample Return Mission, which would need wireless sensors to sense a pressure leak and to avoid potential contamination. The idea of using elastic or acoustic waves to transfer power was suggested recently by [Y. Hu, et al., July 2003]. This system allows for the avoidance of cabling or wiring. The technology is applicable to the transfer of power for actuation, sensing and other tasks inside any sealed container or vacuum/pressure vessel. An alternative approach to the modeling presented previously [Sherrit et al., 2005] used network analysis to solve the same problem in a clear and expandable manner. Experimental tests on three different designs of these devices were performed. The three designs used different methods of coupling the piezoelectric element to the wall. In the first test the piezoelectric material was bolted using a backing structure. In the second test the piezoelectric was clamped after the application of grease. Finally the piezoelectric element was attached using a conductive epoxy. The mechanical clamp with grease produced the highest measured efficiency of 53% however this design was the least practical from a fabrication viewpoint. The power transfer efficiency of conductive epoxy joint was 40% and the stress bolts (12%). The experimental results on a variety of designs will be presented and the thermal and non-linear issues will be discussed.

Lemur IIb: a Robotic System for Steep Terrain Access

Purpose – Introduces the Lemur IIb robot which allows the investigation of the technical hurdles associated with free climbing in steep terrain. These include controlling the distribution of contact forces during motion to ensure holds remain intact and to enable mobility through over‐hangs. Efforts also can be applied to further in‐situ characterization of the terrain, such as testing the strength of the holds and developing models of the individual holds and a terrain map.Design/methodology/approach – A free climbing robot system was designed and integrated. Climbing end‐effector were investigated and operational algorithms were developed.Findings – A 4‐limbed robotic system used to investigate several aspects of climbing system design including the mechanical system (novel end‐effectors, kinematics, joint design), sensing (force, attitude, vision), low‐level control (force‐control for tactile sensing and stability management), and planning (joint trajectories for stability). A new class of Ultrasonic/S...