8,241 research outputs found
Design requirements for laminar airflow clean rooms and devices
Laminar airflow and airborne contamination control concepts with clean room specifications and laminar flow facility design
Combining Boundary-Conforming Finite Element Meshes on Moving Domains Using a Sliding Mesh Approach
For most finite element simulations, boundary-conforming meshes have
significant advantages in terms of accuracy or efficiency. This is particularly
true for complex domains. However, with increased complexity of the domain,
generating a boundary-conforming mesh becomes more difficult and time
consuming. One might therefore decide to resort to an approach where individual
boundary-conforming meshes are pieced together in a modular fashion to form a
larger domain. This paper presents a stabilized finite element formulation for
fluid and temperature equations on sliding meshes. It couples the solution
fields of multiple subdomains whose boundaries slide along each other on common
interfaces. Thus, the method allows to use highly tuned boundary-conforming
meshes for each subdomain that are only coupled at the overlapping boundary
interfaces. In contrast to standard overlapping or fictitious domain methods
the coupling is broken down to few interfaces with reduced geometric dimension.
The formulation consists of the following key ingredients: the coupling of the
solution fields on the overlapping surfaces is imposed weakly using a
stabilized version of Nitsche's method. It ensures mass and energy conservation
at the common interfaces. Additionally, we allow to impose weak Dirichlet
boundary conditions at the non-overlapping parts of the interfaces. We present
a detailed numerical study for the resulting stabilized formulation. It shows
optimal convergence behavior for both Newtonian and generalized Newtonian
material models. Simulations of flow of plastic melt inside single-screw as
well as twin-screw extruders demonstrate the applicability of the method to
complex and relevant industrial applications
Structural dynamics branch research and accomplishments to FY 1992
This publication contains a collection of fiscal year 1992 research highlights from the Structural Dynamics Branch at NASA LeRC. Highlights from the branch's major work areas--Aeroelasticity, Vibration Control, Dynamic Systems, and Computational Structural Methods are included in the report as well as a listing of the fiscal year 1992 branch publications
A bibliography /with abstracts/ on gas-lubricated bearings Interim report
Gas lubricated bearings - annotated bibliograph
Accomplishing task-invariant assembly strategies by means of an inherently accommodating robot arm
Despite the fact that the main advantage of robot manipulators was always meant to
be their flexibility, they have not been applied widely to the assembly of industrial
components in situations other than those where hard automation might be used. We
identify the two main reasons for this as the 'fragility' of robot operation during tasks
that involve contact, and the lack of an appropriate user interface. This thesis describes
an attempt to address these problems.We survey the techniques that have been proposed to bring the performance of cur¬
rent industrial robot manipulators in line with expectations, and conclude that the
main obstacle in realising a flexible assembly robot that exhibits robust and reliable
behaviour is the problem of spatial uncertainty.Based on observations of the performance of position-controlled robot manipulators and
what is involved during rigid-body part mating, we propose a model of assembly tasks
that exploits the shape invariance of the part geometry across instances of a task. This
allows us to escape from the problem of spatial uncertainty because we are 110 longer
working in spatial terms. In addition, because the descriptions of assembly tasks that
we derive are task-invariant, i.e. they are not dependent on part size or location, they
lend themselves naturally to a task-level programming interface, thereby simplifying
the process of programming an assembly robot.the process of programming an assembly robot.
However, to test this approach empirically requires a manipulator that is able to control
the force that it applies, as well as being sensitive to environmental constraints. The
inertial properties of standard industrial manipulators preclude them from exhibiting
this kind of behaviour. In order to solve this problem we designed and constructed a
three degree of freedom, planar, direct-drive arm that is open-loop force-controllable
(with respect to its end-point), and inherently accommodating during contact.In order to demonstrate the forgiving nature of operation of our robot arm we imple¬
mented a generic crank turning program that is independent of the geometry of the
crank involved, i.e. no knowledge is required of the location or length of the crank.
I11 order to demonstrate the viability of our proposed approach to assembly we pro¬
grammed our robot system to perform some representative tasks; the insertion of a peg
into a hole, and the rotation of a block into a corner. These programs were tested on
parts of various size and material, and in various locations in order to illustrate their
invariant nature.We conclude that the problem of spatial uncertainty is in fact an artefact of the fact
that current industrial manipulators are designed to be position controlled. The work
described in this thesis shows that assembly robots, when appropriately designed,
controlled and programmed, can be the reliable and flexible devices they were always
meant to be
The kinematics and vibration of planar linkage mechanisms
PhD ThesisThis thesis reports an investigation into three problems
encountered in the design of linkage mechanisms, namely
kinematic synthesis, balancing of inertia forces and vibration
analysis.
A general method of synthesizing planar linkages with pin
and sliding joints using an Optimization approach has been
investigated. A concise but easily interpreted technique
for prescribing the topology of linkages formed by connecting
pairs of links together has been developed. The displacement
analysis of a linkage is achieved using a direct method which
is considerably faster than alternative techniques. A nonlinear
optimization algorithm has been modified to cater for
non-linear constraints such as transmission angle. These
techniques have been incorporated into a computer program.
Two case-studies of using the program are given. The
first is the synthesis of a six-bar linkage for a motorcycle
rear suspension such that a constant centre distance is
maintained between the chain-wheels as the suspension deflects.
The second concerns the modification of two linkages, containing
eight and ten links respectively, to give an improved knitting
action for a warp-knitting machine.
Operating linkages at high speeds can result in rapidly
varying forces acting on the frame due to the mass of the moving
links. A procedure to determine suitable counterweights to
balance these forces has been developed. Since adding the
counterweights may double the total mass of the linkage, the
links should have minimum mass.
If the mass of a link is reduced too far, the link may
vibrate and so detrimentally affect the performance of the
linkage. Accordingly the final part reports an investigation
into the forced vibration, assuming stability, of a 'Uniform,
pin-jointed, binary link. The equations of motion are derived
and stability boundaries determined. The theoretical predictions
are compared with experimental results from the coupler of a
four-bar linkage.Science Research Council:
Department of Industry
DEVELOPMENT OF A NOVEL Z-AXIS PRECISION POSITIONING STAGE WITH MILLIMETER TRAVEL RANGE BASED ON A LINEAR PIEZOELECTRIC MOTOR
Piezoelectric-based positioners are incorporated into stereotaxic devices for microsurgery, scanning tunneling microscopes for the manipulation of atomic and molecular-scale structures, nanomanipulator systems for cell microinjection and machine tools for semiconductor-based manufacturing. Although several precision positioning systems have been developed for planar motion, most are not suitable to provide long travel range with large load capacity in vertical axis because of their weights, size, design and embedded actuators. This thesis develops a novel positioner which is being developed specifically for vertical axis motion based on a piezoworm arrangement in flexure frames. An improved estimation of the stiffness for Normally Clamped (NC) clamp is presented. Analytical calculations and finite element analysis are used to optimize the design of the lifting platform as well as the piezoworm actuator to provide maximum thrust force while maintaining a compact size. To make a stage frame more compact, the actuator is integrated into the stage body. The complementary clamps and the amplified piezoelectric actuators based extenders are designed such that no power is needed to maintain a fixed vertical position, holding the payload against the force of gravity. The design is extended to a piezoworm stage prototype and validated through
several tests. Experiments on the prototype stage show that it is capable of a speed of 5.4 mm/s, a force capacity of 8 N and can travel over 16 mm
Swashplateless Helicopter Experimental Investigation: Primary Control with Trailing Edge Flaps Actuated with Piezobenders
Helicopter rotor primary control is conventionally carried out using a swashplate with pitch links. Eliminating the swashplate promises to reduce the helicopter's parasitic power in high speed forward flight, as well as may lead to a hydraulic-less vehicle. A Mach-scale swashplateless rotor is designed with integrated piezobender-actuated trailing edge flaps and systematically tested on the benchtop, in the vacuum chamber and on the hoverstand. The blade is nominally based on the UH-60 rotor with a hover tip Mach number of 0.64. The blade diameter is 66 inches requiring 2400 RPM for Mach scale simulation. The rotor hub is modified to reduce the blade fundamental torsional frequency to less than 2.0/rev by replacing the rigid pitch links with linear springs, which results in an increase of the blade pitching response to the trailing edge flaps. Piezoelectric multilayer benders provide the necessary bandwidth, stroke and stiffness to drive the flaps for primary control while fitting inside the blade profile and withstanding the high centrifugal forces.
This work focuses on several key issues. A piezobender designed from a soft piezoelectric material, PZT-5K4, is constructed. The new material is used to construct multi-layer benders with increased stroke for the same stiffness relative to hard materials such as PZT-5H2. Each layer has a thickness of 10 mils. The soft material with gold electrodes requires a different bonding method than hard material with nickel electrodes. With this new bonding method, the measured stiffness matches precisely the predicted stiffness for a 12 layer bender with 1.26 inch length and 1.0 inch width with a stiffness of 1.04 lb/mil. The final in-blade bender has a length of 1.38 inches and 1.0 inch width with a stiffness of 0.325 lb/mil and stroke of 20.2 mils for an energy output of 66.3 lb-mil. The behavior of piezobenders under very high electric fields is investigated. High field means +18.9 kV/cm (limited by arcing in air) and -3.54kV/cm (limited by depoling). An undocumented phenomenon is found called bender relaxation where the benders lose over half of their initial DC stroke over time. While the bender stiffness is shown not to change with electric field, the DC stroke is significantly less than AC stroke.
A two-bladed Mach-scale rotor is constructed with each blade containing 2 flaps each actuated by a single piezobender. Each flap is 26.5% chord and 14% span for a total of 28% span centered at 75% of the blade radius. Flap motion of greater than 10 degrees half peak-peak is obtained for all 4 flaps at 900 RPM on the hoverstand. So, the flaps show promise for the Mach-scale rotor speed of 2400 RPM. A PID loop is implemented for closed loop control of flap amplitude and mean position.
On the hoverstand at 900 RPM, the swashplateless concept is demonstrated. The linear springs used to lower the torsional frequency are shown to have minimum friction during rotation. 1/rev blade pitching of ±1 degree is achieved at a torsional frequency of 1.5/rev for each blade. At resonance, the blade pitching for each blade is greater than ±4 degrees. Primary control is demonstrated by measuring hub forces and moments. At resonance state, the flaps in conjunction with the blade pitching provide ±15 lbs of normal force at a mean lift of 15 lbs yielding ±100% lift authority. Significant hub forces and moments are produced as well.
For a production swashplateless helicopter, it may be prudent to eliminate the pitch links by reducing the blade structural stiffness. A novel wire sensor system network is proposed in order to measure blade elastic flap bending, lead-lag bending and torsion. The theory for measuring blade twist is rigorously derived. A blade is constructed with the wire sensor network and validated on the benchtop for blade elastic bending and twist.
This work is a step forward in achieving a swashplateless rotor system. Not only would this reduce drag in high speed forward flight, but it would lead to a hydraulic-less rotorcraft. This would be a major step in vertical flight aviation
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