Vivake M. Asnani

Geotechnical Properties of JSC-1A Lunar Soil Simulant

For the success of planned missions to the moon in the near future, it is essential to have a thorough understanding of the geotechnical behavior of lunar soil. However, only a limited amount of information is available about geotechnical properties of lunar soils. In addition, the amount of lunar soils brought back to Earth is small. To help the development of new regolith moving machines and vehicles that will be used in future missions, a new lunar soil similant JSC-1A has been developed. A group of conventional geotechnical laboratory tests was conducted to characterize the geotechnical properties of the simulant, such as particle size distribution, maximum and minimum bulk densities, compaction characteristics, shear strength parameters, and compressibility.

Robotic Lunar Geotechnical Tool

Rover-mounted geotechnical systems are of paramount importance to lunar trafficability assessment, construction, and excavation/mining toward establishing permanent human presence on the Moon. These tools can also be used to determine density, when the regolith is used as radiation shield, for example. Two popular insitu devices for establishing geotechnical properties of soil are the Static Cone Penetrometer (SCP) and Dynamic Cone Penetrometer (DCP). However, both systems have shortcomings that may prevent them from being robotically-deployed in a low gravity environment. In this paper we describe an alternative system, called the Percussive Dynamic Cone Penetrometer (PDCP) that can be used to roboticallymeasure geotechnical soil properties in a low gravity environment. It is shown that PDCP data correlates well with the data obtained from both SCP and DCP testing, and by extension with California Bearing Ratio (CBR) and soil bearing strength. ACRONYMS

Inching locomotion for planetary rover mobility

New articulated planetary rovers offer alternative locomotion modalities beyond conventional rolling wheel mobility.12 These new modalities should be explored to overcome the limitations of traditional rolling mobility, and expand the areas of planetary surfaces amenable to exploration. The topic of this study is a hybrid push-roll locomotion mode called inching. Static (non-rolling) wheels are used in conjunction with the rolling wheels of a vehicle in order to increase net traction potential. Preliminary experiments have shown an approximate doubling in drawbar pull for the inching locomotion mode relative to pure rolling. This improvement is not accounted for by reductions to wheel motion resistance alone, and furthermore evidence is provided that static wheels are capable of reacting more ground thrust than rolling wheels. Further investigations using a transparent soil tank, and novel image processing techniques, reveal key differences in the soil shear failure interface under rolling and static wheels. For the cases studied, static wheels generated much deeper and more unified soil failure masses than rolling wheels. Further investigation is recommended to clarify the physics of these thrust development processes, and ultimately to populate the vehicle design space for inching locomotion.

Dynamically tuned magnetostrictive spring with electrically controlled stiffness

This paper presents the design and testing of an electrically controllable magnetostrictive spring that has a dynamically tunable stiffness (i.e., a magnetostrictive Varispring). The device enables in situ stiffness tuning or stiffness switching for vibration control applications. Using a nonlinear electromechanical transducer model and an analytical solution of linear, mechanically induced magnetic diffusion, Terfenol-D is shown to have a faster rise time to stepped voltage inputs and a significantly higher magnetic diffusion cut-off frequency relative to Galfenol. A Varispring is manufactured using a laminated Terfenol-D rod. Further rise time reductions are achieved by minimizing the rods diameter and winding the electromagnet with larger wire. Dynamic tuning of the Varisprings stiffness is investigated by measuring the Terfenol-D rods strain response to dynamic, compressive, axial forces in the presence of sinusoidal or square wave control currents. The Varisprings rise time is ms for 1 A current switches. Continuous modulus changes up to 21.9 GPa and 500 Hz and square wave modulus changes (dynamic effect) up to 12.3 GPa and 100 Hz are observed. Stiffness tunability and tuning bandwidth can be considerably increased by operating about a more optimal bias stress and improving the control of the electrical input.

Magnetostrictive Vibration Damper and Energy Harvester for Rotating Machinery

Vibrations generated by machine driveline components can cause excessive noise and structural dam- age. Magnetostrictive materials, including Galfenol (iron-gallium alloys) and Terfenol-D (terbium-iron- dysprosium alloys), are able to convert mechanical energy to magnetic energy. A magnetostrictive vibration ring is proposed, which generates electrical energy and dampens vibration, when installed in a machine driveline. A 2D axisymmetric finite element (FE) model incorporating magnetic, mechanical, and electrical dynamics is constructed in COMSOL Multiphysics. Based on the model, a parametric study considering magnetostrictive material geometry, pickup coil size, bias magnet strength, flux path design, and electrical load is conducted to maximize loss factor and average electrical output power. By connecting various resistive loads to the pickup coil, the maximum loss factors for Galfenol and Terfenol-D due to electrical energy loss are identified as 0.14 and 0.34, respectively. The maximum av- erage electrical output power for Galfenol and Terfenol-D is 0.21 W and 0.58 W, respectively. The loss factors for Galfenol and Terfenol-D are increased to 0.59 and 1.83, respectively, by using an L-C resonant circuit.

Experimental comparison of piezoelectric and magnetostrictive shunt dampers

A novel mechanism called the vibration ring is being developed to enable energy conversion elements to be incorporated into the driveline of a helicopter or other rotating machines. Unwanted vibration is transduced into electrical energy, which provides a damping effect on the driveline. The generated electrical energy may also be used to power other devices (e.g., health monitoring sensors). PZT (‘piezoceramic’) and PMN-30%PT (‘single crystal’) stacks, as well as a Tb0.3Dy0.7Fe1.92 (‘Terfenol-D’) rod with a bias magnet array and a pickup coil, were tested as alternative energy conversion elements to use within the vibration ring. They were tuned for broadband damping using shunt resistors, and dynamic compression testing was conducted in a high-speed load frame. Energy conversion was experimentally optimized at 750Hz by tuning the applied bias stress and resistance values. Dynamic testing was conducted up to 1000Hz to determine the effective compressive modulus, shunt loss factor, internal loss factor, and total loss factor. Some of the trends of modulus and internal loss factor versus frequency were unexplained. The single crystal device exhibited the greatest shunt loss factor whereas the Terfenol-D device had the highest internal and total loss factors. Simulations revealed that internal losses in the Terfenol-D device were elevated by eddy current effects, and an improved magnetic circuit could enhance its shunt damping capabilities. Alternatively, the Terfenol-D device may be simplified to utilize only the eddy current dissipation mechanism (no pickup coil or shunt) to create broadband damping.

Dynamic Characterization of Galfenol

A novel and precise characterization of the constitutive behavior of solid and laminated research-grade, polycrystalline Galfenol (Fe81:6Ga18:4) under under quasi-static (1 Hz) and dynamic (4 to 1000 Hz) stress loadings was recently conducted by the authors. This paper summarizes the characterization by focusing on the experimental design and the dynamic sensing response of the solid Galfenol specimen. Mechanical loads are applied using a high frequency load frame. The dynamic stress amplitude for minor and major loops is 2.88 and 31.4 MPa, respectively. Dynamic minor and major loops are measured for the bias condition resulting in maximum, quasi-static sensitivity. Three key sources of error in the dynamic measurements are accounted for: (1) electromagnetic noise in strain signals due to Galfenols magnetic response, (2) error in load signals due to the inertial force of fixturing, and (3) time delays imposed by conditioning electronics. For dynamic characterization, strain error is kept below 1.2 % of full scale by wiring two collocated gauges in series (noise cancellation) and through lead wire weaving. Inertial force error is kept below 0.41 % by measuring the dynamic force in the specimen using a nearly collocated piezoelectric load washer. The phase response of all conditioning electronics is explicitly measured and corrected for. In general, as frequency increases, the sensing response becomes more linear due to an increase in eddy currents. The location of positive and negative saturation is the same at all frequencies. As frequency increases above about 100 Hz, the elbow in the strain versus stress response disappears as the active (soft) regime stiffens toward the passive (hard) regime.

Frequency-dependent, dynamic sensing properties of polycrystalline Galfenol (Fe81.6Ga18.4)

This paper presents the first measurement of Galfenols frequency-dependent strain and magnetic flux density responses to controlled dynamic stress, from which frequency-dependent, effective material properties relating these quantities are calculated. Solid and laminated Galfenol (Fe81.6Ga18.4) rods were excited by 2.88 MPa compressive stresses up to 1 kHz under constant field and constant current conditions. Due to magnetic diffusion cut-off frequencies of only 59.3 to 145.7 Hz, the dynamic properties of the solid rod are found to vary significantly; this illustrates the inaccuracy of frequency-independent dynamic properties calculated via linear piezomagnetic models from experimental responses to electrical excitation. Conversely, the sensing properties of the laminated rod exhibit a weak dependence on frequency over the measurement range (i.e., a cut-off >1 kHz). The data are used to validate an existing model for mechanically induced magnetic diffusion. Loss factors and magnetomechanical energy densities are also presented and discussed in terms of loss separation, magnetic diffusion, and energy conservation.

Quasi-static major and minor strain-stress loops in textured polycrystalline Fe81.6Ga18.4 Galfenol

The ΔE effect (Youngs modulus variation of magnetostrictive materials) is useful for tunable vibration absorption and stiffness control. The ΔE effect of iron-gallium (Galfenol) has not been fully characterized. In this study, major and minor strain-stress loops were measured under different bias magnetic fields in solid, research grade, ⟨100⟩-oriented, highly-textured polycrystalline Fe81.6Ga18.4 Galfenol. A 1 Hz, constant amplitude compressive stress was applied from −0.5 MPa to −63.3 MPa for major loop responses. Minor loops were generated by simultaneously applying a 4 Hz, 2.88 MPa amplitude sinusoidal stress and different bias stresses ranging from −5.7 MPa to −41.6 MPa in increments of about 7.2 MPa. Bias magnetic fields were applied in two ways, a constant field in the sample obtained using a proportional-integral (PI) controller and a constant current in the excitation coils. The ΔE effect was quantified from major and minor loop measurements. The maximum ΔE effect is 54.84% and 39.01% for consta...

Design and testing of a dynamically tuned magnetostrictive spring with electrically controlled stiffness

This paper details the development of an electrically-controlled, variable-stiffness spring based on magnetostrictive materials. The device, termed a magnetostrictive Varispring, can be applied as a semi- active vibration isolator or switched stiffness vibration controller for reducing transmitted vibrations. The Varispring is designed using 1D linear models that consider the coupled electrical response, mechanically-induced magnetic diffusion, and the effect of internal mass on dynamic stiffness. Modeling results illustrate that a Terfenol-D-based Varispring has a rise time almost an order of magnitude smaller and a magnetic diffusion cut-off frequency over two orders of magnitude greater than a Galfenol-based Varispring. The results motivate the use of laminated Terfenol-D rods for a greater stiffness tuning range and increased bandwidth. The behavior of a prototype Varispring is examined under vibratory excitation up to 6 MPa and 25 Hz using a dynamic load frame. For this prototype, stiffness is indirectly varied by controlling the excitation current. Preliminary measurements of continuous stiffness tuning via sinusoidal currents up to 1 kHz are presented. The measurements demonstrate that the Youngs modulus of the Terfenol-D rod inside the Varispring can be continuously varied by up to 21.9 GPa. The observed stiffness tuning range is relatively constant up to 500 Hz, but significantly decreases thereafter. The stiffness tuning range can be greatly increased by improving the current and force control such that a more consistent current can be applied and the Varispring can be accurately tested at a more optimal bias stress.