1,486 research outputs found
Sufficient Conditions for Admittance to Ensure Planar Force-assembly in Multi-point Frictionless Contact
An important issue in the development of force guidance assembly strategies is the specification of an appropriate admittance control law. This paper identifies procedures for selecting the appropriate admittance to achieve reliable planar force-guided assembly for multi-point contact cases. Conditions that restrict the admittance behavior for each of the various types of two-point contact are presented. These conditions ensure that the motion that results from contact reduces part misalignment for each case. We show that, for bounded misalignments, if the conditions are satisfied for a finite number of contact configurations, the conditions ensure that force guidance is achieved for all configurations within the specified bounds
Admittance Selection for Force-guided Assembly of Polygonal Parts Despite Friction
An important issue in the development of force guidance assembly strategies is the specification of an appropriate admittance control law. This paper identifies conditions to be satisfied when selecting the appropriate admittance to achieve force-guided assembly of polygonal parts for multipoint contact with friction. These conditions restrict the admittance behavior for each of the various one-point and two-point contact cases and ensure that the motion that results from contact reduces part misalignment for each case. We show that, for bounded friction and part misalignments, if the identified conditions are satisfied for a finite number of contact configurations and friction coefficients, the conditions ensure that force guidance is achieved for all configurations and values of friction within the specified bounds
Admittance Selection Conditions for Frictionless Force-Guided Assembly of Polyhedral Parts in Two Single-Point Principal Contacts
The admittance of a manipulator can be used to improve robotic assembly. If properly selected, the admittance will regulate a contact force and use it to guide the parts to proper positioning. In previous work, procedures for selecting the appropriate admittance for single principal contact (PC) cases were identified. This paper extends this research for some of the two PC cases-those for which each contact occurs at a single point. The conditions obtained ensure that the motion that results from frictionless contact always instantaneously reduces part misalignment. We show that, for bounded misalignments, if an admittance satisfies the misalignment-reducing conditions at a finite number of contact configurations, then the admittance will also satisfy the conditions at all intermediate configurations
Combustion instabilities: mating dance of chemical, combustion, and combustor dynamics
Combustion instabilities exist as consequences of
interactions among three classes of phenomena: chemistry and chemical dynamics; combustion dynamics; and combustor dynamics. These dynamical processes take place simultaneously in widely different spatial scales characterized by lengths roughly in the ratios (10^(-3)
- 10^(-6)):1:(10^3-10^6). However, due to the wide differences in the associated characteristic velocities, the corresponding time scales are all close. The instabilities in question are observed as oscillations having a time scale in the range of natural acoustic oscillations. The apparent dominance of that single macroscopic time scale must not be permitted to obscure the fact that the relevant physical processes occur on three disparate length scales. Hence, understanding combustion instabilities at the practical level of design and successful operation is ultimately based on understanding three distinct sorts of dynamics
Q-based design equations for resonant metamaterials and experimental validation
Practical design parameters of resonant metamaterials, such as loss tangent,
are derived in terms of the quality factor of the resonant effective medium
permeability or permittivity. Through electromagnetic simulations of loop-based
resonant particles, it is also shown that the of the effective medium
response is essentially equal to the of an individual resonant particle.
Thus, by measuring the of a single fabricated metamaterial particle, the
effective permeability or permittivity of a metamaterial can be calculated
simply and accurately without requiring complex simulations, fabrication, or
measurements. Experimental validation shows that the complex permeability
analytically estimated from the measured of a single fabricated
self-resonant loop agrees with the complex permeability extracted from
parameter measurements of a metamaterial slab to better than 20%. This
equivalence reduces the design of a metamaterial to meet a given loss
constraint to the simpler problem of the design of a resonant particle to meet
a specific constraint. This analysis also yields simple analytical
expressions for estimating the loss tangent of a planar loop magnetic
metamaterial due to ohmic losses. It is shown that
is a strong lower bound for magnetic loss tangents for frequencies not too far
from 1 GHz. The ohmic loss of the metamaterial varies inversely with the
electrical size of the metamaterial particle, indicating that there is a loss
penalty for reducing the particle size at a fixed frequency
Force control of heavy lift manipulators for high precision insertion tasks
Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, June 2005."May 2005." Leaf 81 blank.Includes bibliographical references (leaves 67-70).The inherent strength of robotic manipulators can be used to assist humans in performing heavy lifting tasks. These robots reduce manpower, reduce fatigue, and increase productivity. This thesis deals with the development of a control system for a robot being built for this purpose. The task for this robot is to lift heavy payloads while performing complex insertion tasks. This task must be completed on the deck of a naval vessel where possible disturbances include wind, rain, poor visibility, and dynamic loads induced by a swaying deck. The primary objective of the controller being designed here is to allow for insertion of the payload despite tight positioning tolerances and disturbances like surface friction, joint friction, and dynamic loads from ship motions. A control structure designed for intuitive interaction between the robot and operator is analyzed and shown to be stable using an established environment interaction model. The controller is shown to perform within established specifications via numerical simulation based on simple user inputs. An additional objective of this controller design is to prevent part jamming during the insertion task. With a large, powerful manipulator, the chances of a jam occurring is high. Without the use of bilateral force feedback, it will be difficult for the operator feel when these jams will occur and there will be no information about how to prevent them. This thesis analyzes the geometry and mechanics of the jamming problem and derives a control system to assist the user in preventing these jams. These methods can be extended to other insertion tasks simply by specifying the appropriate geometry.by Matthew A. DiCicco.S.M
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 geographies of recruiting a partner from abroad. An exploration of Swedish data
International marriages are both a result and a driver of higher levels of global mobility and interconnectivity. Increasing ease of air travel for work and leisure, rising numbers of individuals studying, working and travelling abroad, and the emergence of international partnering websites have expanded traditionally local marriage fields – the geographical areas where people meet the partner – to global proportions. This expansion has increased the chance of meeting a potential partner from abroad resulting in an increase in international marriage migration. Recruiting a partner from abroad is surrounded by prejudice and stigma. ‘Knowledge’ about the characteristics of the individual ‘importing’ a partner from abroad is often based on anecdotic evidence and myths. In this paper we explore the factors that determine the probability that a native Swede recruits a partner from abroad. Along with various demographic and socioeconomic characteristics of the Swede we will pay specific attention to the geographies of marriage migration: the opportunity structure. This study uses longitudinal population data for the whole of Sweden, containing information on all individuals who lived in Sweden between 1994 and 2004. The results from multinomial logistic regression models shed a unique light on gendered and geographic patterns of partner recruitment.Migration; International marriage; Marriage migartion; Demographic characteristics; Socioeconomic characteristics; Globalisation; Sweden
High-speed electrical connector assembly by structured compliance in a finray-effect gripper
Fine assembly tasks such as electrical connector insertion have tight
tolerances and sensitive components, requiring compensation of alignment errors
while applying sufficient force in the insertion direction, ideally at high
speeds and while grasping a range of components. Vision, tactile, or force
sensors can compensate alignment errors, but have limited bandwidth, limiting
the safe assembly speed. Passive compliance such as silicone-based fingers can
reduce collision forces and grasp a range of components, but often cannot
provide the accuracy or assembly forces required. To support high-speed
mechanical search and self-aligning insertion, this paper proposes monolithic
additively manufactured fingers which realize a moderate, structured compliance
directly proximal to the gripped object. The geometry of finray-effect fingers
are adapted to add form-closure features and realize a directionally-dependent
stiffness at the fingertip, with a high stiffness to apply insertion forces and
lower transverse stiffness to support alignment. Design parameters and
mechanical properties of the fingers are investigated with FEM and empirical
studies, analyzing the stiffness, maximum load, and viscoelastic effects. The
fingers realize a remote center of compliance, which is shown to depend on the
rib angle, and a directional stiffness ratio of . The fingers are
applied to a plug insertion task, realizing a tolerance window of mm and
approach speeds of m/s.Comment: Under review. arXiv admin note: substantial text overlap with
arXiv:2301.0843
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