Lyndon S. Stephens

Mechanical property evaluation and failure analysis of cantilevered LIGA nickel microposts

An experimental apparatus has been built to measure the elastic modulus and bending strength (modulus of rupture) of LIGA nickel posts. The apparatus uses the static cantilever beam bending approach to measure mechanical properties in a direction parallel to the growth direction. Experimental results are presented for two sets of largely identical posts constructed using an overplating method. One set was electroplated using a Watts bath, and the other set was electroplated using a sulfamate bath. For the Watts bath, the measured modulus of elasticity was slightly lower than that of bulk nickel (182 GPa), while, for the sulfamate bath, it was approximately half (93 GPa). The strength properties of the two sets of posts also differ dramatically. Microhardness measurements, Focused Ion Beam (FIB) images of grain structure, and scanning electron microscopy (SEM) micrographs of failure regions are used to further characterize and explain the differences in the results. This integrated testing approach yields a consistent set of data regarding material properties, grain size/structure and failure mechanisms. Potential sources of experimental error are also identified and improvements in experiment design are suggested to reduce these errors.

Deterministic Micro Asperities on Bearings and Seals Using a Modified LIGA Process

Deterministic micro asperities show potential for enhancement of lubrication in conformal contacts as found in many bearing and seal designs. Several manufacturing methods have been proposed for deterministic micro asperities. Of these, laser texturing has emerged as the most viable option. This paper proposes the LIGA MEMs manufacturing method as an alternative. Using LIGA, surfaces with patterned micron sized surface features of arbitrary cross section (cylindrical, hex, triangular, etc.) can be fabricated from electroplated nickel, gel-cast silicon nitride, or plastic. The resulting asperities can be positive (protuberances) or negative (recesses) and can have heights (depths) from 1-1000 microns and be patterned over surface areas up to about 150 mm X150 mm. In this paper, the LIGA method is used to fabricate a sample thrust bearing surface with a hexagonal array of positive asperities. The resulting asperities are 550 μm in average diameter, 165 μm in edge-to-edge spacing and have heights of 3-100 μm. Surface metrology indicates submicron accuracy of form and 13 nm Ra roughness on the asperity tops (land). Tribology testing in a nonpressurized oil bath indicates full film conditions and shows a 14-22% reduction in friction coefficient for a thrust surface covered with the micro asperities. A model confirms the experimental trends and indicates the potential to further reduce the friction coefficient by about 60% through optimization of the asperity geometry and layout.

Soft elastohydrodynamic analysis of radial lip seals with deterministic microasperities on the shaft

A numerical analysis is conducted to investigate the elastohydrodynamic effect of deterministic microasperities on the shaft of a lip seal. Various geometries of microasperities (triangular, square, hexagonal, and circular) are put into a 100-by-100-μm unit cell and are investigated using Reynold’s Equation. For each shape, the area fraction of the microasperity is varied between 0.2 and 0.8, and the asperity height is varied between 0.3 and 5 micron. The calculation for load capacity and friction coefficient indicates that there are values for asperity height where load capacity and friction coefficient are optimized. These optimum heights were reached at 1–3 μm. Although the lip seal surface is considered to be smooth, reverse pumping can still be obtained using an oriented triangular design. The Couette flow rate for this asperity showed lubricant is reverted back toward the seal side 2.6 times more than using a conventional lip seal. The presence of all designs in the lip seal shows significant improvement on lubrication characteristics, i.e. increasing load support and reducing friction coefficient.Copyright

Force and torque characteristics for a slotless Lorentz self-bearing servomotor

A new type of self-bearing motor is presented that uses Lorentz-type forces to produce both bearing force and motoring torque. The design eliminates the tradeoff between motoring torque and bearing force with respect to permanent-magnet thickness found in many previous designs. The self-bearing motor layout and servo control are described including alternative winding schemes. Permeance and flux models are presented and used to derive expressions for torque and force production. As with conventional magnetic bearings, these expressions are quadratic with both rotor position and control current. A linearized force-current- displacement relationship is then derived for a general operating point. Experimental results are presented for a prototype servomotor. These results show good agreement with theory and, hence, validate the permeance and flux models, and the linearized force-current relationship. The prototype servomotor, 152 mm in outer diameter and 25.4 mm, in length, was a peak hearing force capacity of 213.6 N and a peak torque capacity of 24 N/spl middot/m, Which is limited by power amplifier saturation.

Closed-loop performance of a six degree-of-freedom precision magnetic actuator

The slotless permanent-magnet self-bearing motor (SBM) technology is used to fully provide radial bearing and motoring functionality simultaneously in a six degree-of-freedom magnetic actuator for potential precision pointing and tracking applications in space such as intersatellite laser communications. This novel precision magnetic actuator incorporates two SBMs to produce both radial bearing forces and motoring torque, and one active magnetic bearing to provide axial support, and thus allows for a complete electromagnetic suspension and precision noncontact pointing. An analytical representation of the overall dynamic system is presented for controller design. Six decoupled proportional-integral-derivative controllers are designed, and a feedback control system is implemented. The closed-loop performance confirms experimentally that the precision magnetic actuator is capable of smooth angular slewing while maintaining good stabilization. The most encouraging result is that the actuator achieves a high angular resolution of 754 nrad over a large azimuth range of /spl plusmn/45/spl deg/.

A pin fin microheat sink for cooling macroscale conformal surfaces under the influence of thrust and frictional forces

Conventional microheat sink design primarily focuses on the use of continuous fin arrays to optimally dissipate thermal energy from electronic components. By contrast, this paper experimentally measures the thermal and structural performance of two micro pin fin heat sinks designed for use in load bearing applications such as mechanical seals and thrust bearings. One pin fin array is of low porosity, which is more optimal for load bearing capacity, and the other is of high porosity, which is more optimal for heat dissipation. By using these two extreme cases, the thermal-structural tradeoff found in load bearing microheat sinks is demonstrated. The heat sinks are constructed of nickel, electrodeposited onto a stainless steel thrust ring using a modified LIGA technique. Under forced air cooling, the thermal performance of each is compared to a simple model based on a combination of macroscale pin fin heat sink results and classical correlations for fins in cross flow. The low porosity design is also tested under the application of a 44.5 N thrust load at 2500 rpm and found to be structurally sound. Experimental temperature profiles demonstrate a substantial benefit of the microheat sink in cooling the load bearing surface.

Experimental Results for a Heat-Sink Mechanical Seal

This work presents experimental results for a heat-sink mechanical seal installed in a 1 × 1.5 × 6 in. ANSI centrifugal water pump. The heat-sink seal is constructed of a stainless steel substrate with an electrodeposited pin fin micro-heat sink located within 3 mm of the end face. Each pin has a ten-sided polygon cross section with a flat-to-flat diameter of 675 μm, a height of 856 μm, and a 300-μm edge-to-edge spacing. The end face is coated with a WC thin film that forms the wear surface for the rotating ring. The heat-sink seals effectiveness is demonstrated in a significant reduction of both the seal interface and seal chamber temperature when compared to a similar industry standard seal. The heat-sink seal removes 750 W of heat and reduces the stationary ring temperature by 80°C under dead-head conditions using only 1.6 W of coolant pumping power.

Actuator gains for a toothless permanent-magnet self-bearing motor

Permanent-magnet self-bearing motors provide independent bearing and motoring functionality in a single magnetic actuator. Typically, self-bearing motor designs use toothed stators to provide minimum reluctance flux paths that create the magnetic bearing forces necessary to support the rotor. These toothed designs can have significant cogging torque, rendering them ineffective for smooth torque applications such as those found in aerospace. A toothless permanent-magnet self-bearing motor can provide smooth torque production and adequate bearing force for low-gravity environments. Characterization of the open-loop gains for this actuator is necessary for linear controller development. In this paper simple algebraic equations are derived for the motoring and bearing current gains, and an analytical method is presented for computing the negative stiffness. The analytical method solves the Dirichlet boundary value problem (BVP) in the eccentric annulus for the magnetomotive force (MMF) in the air gap subject to harmonic boundary conditions. A conformal transformation to bipolar coordinates is used, yielding a BVP that is solvable by separation of variables. Expressions for the flux density, Maxwell force on the rotor, and the negative stiffness in terms of the MMF are presented. A sample problem is presented that illustrates the flux distribution in the air gap and the operating principals of this actuator type.

Force characteristics and gain determination for a slotless self-bearing motor

In this paper, we use the Maxwell stress tensor method to derive the analytical force and torque expressions for a large-scale slotless permanent-magnet (PM) self-bearing motor actuator that uses common coils to produce both the torque and radial forces, in order to include all the possible interactions among the PMs, currents, and stator/rotor back iron. We solve the radial and angular components of the two-dimensional instantaneous magnetic field distribution in the low-permeability region induced from the rotor PMs and stator windings separately, using the unwrapped geometry, and then superimpose them. Instead of being incorporated into the boundary conditions, the general winding current and PM magnetization distributions are expanded into the Fourier series in the separate source layers, with respect to one motor revolution and one PM pole pair, respectively. For each source, we first solve one homogenous Laplaces equation in the planar layer without source and one nonhomogenous Laplaces equation in the planar layer with source simultaneously in Cartesian coordinates for a centered rotor and then add the rotor eccentricity. We compare the analytical solutions for the individual magnetic field distributions, as well as the total force and torque production, to those from the finite-element analysis (FEA), and find excellent agreement between the two. We characterize the open loop current and negative stiffness gains of the SBM from the linearized force-current-displacement relationship, which forms the basis for the linear system modeling and controller design, and validate our results by comparison with the FEA results.

Influence of stator slot geometry and rotor eccentricity on field distribution in cylindrical magnetic actuators

The paper presents an analytical method for computing the instantaneous air-gap flux distribution in cylindrical magnetic actuators with slotted stators and rotor eccentricity. The method does not rely on permeance functions; therefore, the resulting flux distribution can be used to compute actuator properties such as negative stiffness, cogging torque, and back-emf. The method uses Fourier series expansions to model current distributions and permanent magnets as sources of magnetomotive force (MMF) on the air-gap boundary and then solves the Dirichlet boundary value problem in the eccentric annulus for the MMF within the permanent magnet and air gap regions. It accounts for the slotting effect by adding a boundary perturbation step to the solution that includes the decreased permeance of the slots. The method is applied to a sample problem that examines both the effects of slot length and slot width on negative stiffness coefficients. The method is benchmarked against finite-element solutions; the two models agree well up to a certain critical slot length. This critical slot length corresponds to the traditional infinite slot length (that slot length beyond which no appreciable change in flux distribution occurs). While the method is applied to a permanent magnet motor of internal rotor construction, it is generally applicable to many actuator topologies.