This paper presents the design of a bioinspired artificial fingertip that resembles the mechanical behavior of a human fingertip under conditions of both static deformation and high frequency excitation. The artificial fingertip is constructed around a deformable spherical membrane filled with a cellulose sponge, itself connected to a rigid structure that acts as a bone. Force-deformation characteristics and response to a transient mechanical perturbation are both shown to be in good qualitative agreement with those of a real finger. More importantly, the fingertip exhibits friction reduction when interacting with TPads (variable friction tactile displays based on transverse ultrasonic vibrations). Comparison with artificial fingertips that do not exhibit friction reduction suggests that mechanical damping characteristics play a key role in the amount of friction reduction achieved.

Texture modeling strives to encapsulate the important properties of texture in a concise representation for interpretation, storage, and rendering. Models for tactile texture have yet to describe a representation that is both perceptually complete and sufficiently compact. In this work, we take inspiration from models of visual and auditory texture and propose a spatial spectrogram representation of tactile texture that separates localized features from textural aspects using a windowed Fourier decomposition. We investigate the length scales at which humans can perceive localized features, and represent textures as spectrograms that capture those local features. Additionally, we demonstrate a reconstruction algorithm capable of recreating texture from a spectrogram representation with no perceptual consequence.

This work aims to demonstrate and explain a nearly century old electrostatic haptic effect in human fingertips, which has since gone unreported. This effect, based on the original work of Johnsen and Rahbek [1], as well as research on electrostatic chucking devices [2], is capable of producing electrostatic forces on the finger an order of magnitude greater than those previously reported in literature. It is also capable of working with DC excitation, an aspect which stands out against previous reports which utilize purely AC excitation. This work also proposes a unified force model for this effect, drawn from electrostatic chuck research, and resolves this model with those in previous reports. We briefly discuss the background and specifics of the Johnsen-Rahbek effect, and include measurements made with our own electroadhesive surface and experimental apparatus. Finally, we discuss how this model fits in with previous observations, and its implications going forward.

This work investigates how haptic percepts are combined across two fingertips. Two single-degree-of-freedom haptic interfaces were used to present virtual bumps to the thumb and index finger of subjects' right hand. As subjects slid the two interfaces from left to right while maintaining a fixed finger separation, they would encounter one bump with the index finger and one with the thumb. The objective bump locations were varied randomly, from spatially coincident to separated by slightly more than the fingertips. Subjects were asked to report the number of bumps in the objective world and the number of times they encountered each bump, and also to point to the locations of the bumps. We found that subjects exhibited a strong bias toward reporting a single objective bump in the virtual world. However, the percept varied from one bump encountered twice, when the two virtual bumps were nearly spatially coincident, to one bump encountered once, which occurred when the two virtual bumps were close to finger separation and therefore encountered nearly simultaneously. The latter result is evidence for a kinesthetic grouping phenomenon: temporally synchronized sensations at multiple fingers can be perceptually collapsed into a single percept in both time and space.

The human fingertip is extremely sensitive to lateral (shear) forces that arise in surface exploration. We and others have developed haptic displays that work by modulating surface friction via electrostatic attraction. Despite the demonstrated ability of these displays to render haptic environments, an understanding of the fingertip-surface interface is lacking. We have developed a tribometer for measuring the lateral frictional forces on a fingertip under well-controlled conditions. We show an expected square law dependence of frictional force (and inferred electrostatic normal force) on actuation voltage, although we observe a large person to person variability. We model an expected dependence of the frictional force on the frequency of the actuation voltage, predicting a first order cut off below about 500Hz. However, our measurements are unambiguously at odds with the model's predictions.

Certain electric fish use a self-generated AC electric field to navigate and hunt. Thousands of sensors on the surface of the fish's body detect the pattern of amplitude and phase distortions of the field caused by nearby objects. Prior research has suggested that phase distortions may be especially useful for recognition of live objects. Here we present the first study of the utility of phase information in a robotic implementation of active electrosense. Using our robotic implementation, we investigated how the phase information depends on the frequency of the emitted field, the conductivity of the surrounding water, and object properties. An analytical model was developed serving as qualitative explanation of the dependency. We show that in certain situations phase information enables discrimination between two objects that are otherwise very similar in the amplitude of their electric images. We also show the utility of probing objects with multiple frequencies.

Touch interactions have refreshed some of the 'glowing enthusiasm' of thirty years ago for direct manipulation interfaces. However, today's touch technologies, whose interactions are supported by graphics, sounds or crude clicks, have a tactile sameness and gaps in usability. We use a Large Area Tactile Pattern Display (LATPaD) to examine design possibilities and outcomes when touch interactions are enhanced with variable surface friction. In a series of four studies, we first confirm that variable friction gives significant performance advantages in low-level targeting activities. We then explore the design space of variable friction interface controls and assess user reactions. Most importantly, we demonstrate that variable friction can have a positive impact on the enjoyment, engagement and sense of realism experienced by users of touch interfaces.

Many of those who survive a stroke develop a gait disability known as stiff-knee gait (SKG). Characterized by reduced knee flexion angle during swing, people with SKG walk with poor energy efficiency and asymmetry due to the compensatory mechanisms required to clear the foot. Previous modeling studies have shown that knee flexion activity directly before the foot leaves the ground, and this should result in improved knee flexion angle during swing. The goal of this research is to physically test this hypothesis using robotic intervention. We developed a device that is capable of assisting knee flexion torque before swing but feels imperceptible (transparent) for the rest of the gait cycle. This device uses sheathed Bowden cable to control the deflection of a compliant torsional spring in a configuration known as a Series Elastic Remote Knee Actuator (SERKA). In this investigation, we describe the design and evaluation of SERKA, which includes a pilot experiment on stroke subjects. SERKA could supply a substantial torque (12 N· m) in less than 20 ms, with a maximum torque of 41 N·m. The device resisted knee flexion imperceptibly when desired, at less than 1 N·m rms torque during normal gait. With the remote location of the actuator, the user experiences a mass of only 1.2 kg on the knee. We found that the device was capable of increasing both peak knee flexion angle and velocity during gait in stroke subjects. Thus, the SERKA is a valid experimental device that selectively alters knee kinetics and kinematics in gait after stroke.

Machines might physically interact with humans more smoothly if we better understood the subtlety of human-human physical interaction. We recently reported that two people working cooperatively on a physical task will quickly negotiate an emergent strategy: typically subjects formed a temporal specialization such that one member commands the early parts of motion and the other the late parts. In our study, we replaced one of the humans with a robot programmed to perform one of the typical human specialized roles. We expected the remaining human to adopt the complementary specialized role. Subjects did believe that they were interacting with another human but did not adopt a specialized behavior as subjects would when physically working with another human; our negative result suggests a very subtle negotiation takes place in human-human physical interaction.

We propose a novel control method for lowerlimb assist that produces a virtual modification of the mechanical impedance of the human limbs. This effect is accomplished through the use of an exoskeleton that displays active impedance. The proposed method is aimed at improving the dynamic response of the human limbs, while preserving the user’s control authority. Our goal is to use active-impedance exoskeleton control to improve the user’s agility of motion, for example by reducing the average time needed to complete a movement. Our control method has been implemented in a 1-DOF exoskeleton designed to assist human subjects performing knee flexions and extensions. In this paper we discuss an initial study on the effect of negative exoskeleton damping (a particular case of active-impedance control) on the subject’s time to complete a target-reaching motion. Experimental results show this effect to be statistically significant. On average, subjects were able to reduce the time to complete the motion by 16%.

The haptic sense provides continuous information during physical human-machine interaction. Humans can respond very quickly and effectively to such feedback, such as a driver making rapid steering adjustments when his vehicle hits a pothole. It may be possible for designers to take advantage of this interaction by providing users with well- designed haptic communications to assist in manual control tasks. In this paper, we describe an experiment conducted to test this idea. Subjects were instructed to step on a modified stair climber at a consistent workout pace while simultaneously completing a mental distraction task. They were provided with one of five different haptic communications to assist in maintaining their workout intensity level. Results show that haptic communications helped subjects step as much as 1.7% faster with as much as 3.95% more power output than without the haptic communications, although none of the communications beneficially affected velocity or power standard deviations. Four of the five communications had no significant impact on mental task performance.

In the Cobotic Hand Controller, we have introduced an admittance display that can render very high impedances (up to its own structural stiffness). This is due to its use of infinitely variable transmissions. While admittance displays typically excel at rendering high impedances, the incorporation of infinitely variable transmissions in the Cobotic Hand Controller allows the stable display of a wide dynamic range, including low impedances. The existence of a display that excels at rendering high-impedance constraints, but has high-fidelity control of low impedances tangent to those constraints, has led us to describe an admittance control architecture not often examined in the haptics community. In this paper, we develop a comprehensive approach that enables rendering of rigid motion constraints while simultaneously preserving the physical integrity of the intended inertial dynamics tangent to those constraints. This is in contrast to conventional impedance-control algorithms that focus primarily on rendering reaction forces along contact normals with constraints. We present this algorithm here, which is general to all admittance displays, and report on its implementation with the Cobotic Hand Controller. We offer examples of rigid bodies and linkages subject to holonomic and/or nonholonomic constraints

Haptic interaction between people and machines might benefit from an understanding of haptic communication between one person and another. We recently reported results showing that two people performing a physically shared dyadic task can outperform either person alone, even when the perception of each participant is that the other is a hindrance. Evidently a dyad quickly negotiates a more efficient motion strategy than is available to individuals. This negotiation must take place through a haptic channel of communication, and it is apparently at a level below the awareness of the participants. Here we report results on the motion strategy that emerged. By recording forces and motions we show that the dyads "specialized" temporally such that one member took on early parts of the motion and the other late parts. Tests in which one participant's contribution was surreptitiously replaced by a motor did not elicit a similar cooperative response from the remaining human participant, showing that the language of haptic communication between people must be rather subtle

In this paper we are concerned with allowing the operator of a haptic display to interact with virtual systems having significant inertial dynamics and realistic constraints. We review the mathematical structure arising from the kinetic energy metric, required to create a virtual dynamics simulation consisting of rigid-body dynamics along with holonomic and/or nonholonomic motion constraints. We develop an admittance controller composed of feedforward and feedback terms, while preserving the integrity of the intended virtual dynamics simulation. This controller is implemented on the Cobotic Hand Controller, an admittance-type haptic display, and two examples are discussed.

Several species of animals use whiskers to accurately navigate and explore objects in the absence of vision. We have developed inexpensive arrays of artificial whiskers based on strain-gage and Flex Sensor technologies that can be used either in passive (“dragging”) mode, or in active (“whisking”) mode. In the present work we explore the range of functions that whisker arrays can serve on a rover. We demonstrate that when mounted on a rover, whisker arrays can (1) Detect obstacles and determine obstacle distance (2) Map terrain features (3) Determine ground and surface texture (4) Provide an estimate of rover speed (5) Identify “slip” of the rover wheels, and (6) Perform 3-dimensional extraction of object shape. We discuss the potential use of whisker arrays on planetary rovers and as an investigative tool for exploring the encoding of sensory information in the nervous system of animals.

A novel method based on deformable length of elastic element control (DLEEC) to realize the stiffness display for perception of virtual soft object is proposed, and the stiffness display interface device has been developed and is presented. The stiffness display interface device is composed of a thin elastic beam and an actuator to adjust the length of the beam. The deformation of the beam under a force is proportional to the third power of the beam length. By controlling the beam length, the stiffness display device can reproduce the stiffness of the virtual object from very soft to hard, so that the human fingertip can feel it as if he directly touches with the virtual object by interacting with the device. A real time position control algorithm is employed to guarantee the real time stiffness display.

We consider teleoperation in which a slave manipulator, seen in one or more video images, is controlled by moving a master manipulandum. The operator must mentally transform (i.e., rotate, translate, scale, and/or deform) the desired motion of the slave image to determine the required motion at the master. Our goal is to make these mental transformations less taxing in order to decrease operator training time, improve task time/performance, and expand the pool of candidate operators. In this paper, we introduce a framework for describing the transformations required to use a particular teleoperation setup. We analyze in detail the mental transformations required in an interface consisting of one camera and display. We then expand our discussion to setups with multiple cameras/displays and discuss the results from an initial experiment.

A novel, six-degree-of-freedom active haptic device is introduced. A parallel kinematic design and the use of non-holonomic continuously variable transmissions provide extremely high stiffness in directions that would violate a virtual constraint. At the same time, smooth motion is permitted tangential to virtual constraints and in open space. High quality constraint surfaces having one to five dimensions can be displayed. A notable feature of this device is the mutual coupling of all six linear actuators to a common rotating cylinder, which can, optionally, be powered. The resulting mechanism is simple to control, and allows several new control strategies in Cobotic haptics.

This study investigates natural single-arm interaction with kinematic constraints. Smooth, frictionless, kinematic constraints reduce the degrees-of-freedom of motion at the hand, but add force freedoms. These force freedoms allow the hand to push and pull against the constraints with no effect on the task. Understanding how subjects take advantage of kinematic constraints will be useful in designing constraint surfaces for assisted manipulation. This paper reports the results of an experiment studying how subjects make use of the presence of a kinematic constraint in a static planar single-arm task. Subjects are asked to hold a handle that is free to slide on a linear rail, and to apply a force tangent to the rail to resist a pulling force. Thus the goal of the task is to hold the handle stationary. Subjects are also free to apply any force normal to the rail, as these forces have no effect on the task. This freedom does not exist without a kinematic constraint. We find that subjects make use of the force freedom by applying significant forces against the constraint in a consistent and constraint-configuration-dependent fashion. We show that the constraint forces can be predicted by a convex, scale-invariant objective function on the hand force space. The level curves of this objective function can be found directly from the experimental data without any biomechanical modeling.

Remote systems are essential for reducing risk to human workers from hazardous radiation and difficult work environments, while improving productivity and reducing costs. The major drawback of currently available remote manipulator systems is that teleoperation is slow and imprecise. The presented work focuses on enhancing remote operation of tools for D&D tasks by introducing teleautonomy and telecollaboration. In teleautonomy, the robot performs a given task autonomously, while the human operator intervenes in the process as a supervisor. In telecollaboration, the human operator is passively constrained by a virtual fixture, but is responsible for the motion. This work, sponsored by the US Department of Energy (DOE) Environmental Management Science Program (EMSP), builds on a reactive, agent-based control architecture and robot control technology.

Often two people must work together physically on a common task, such as lifting and positioning; a long board, or, in our model experimental system, turning a two-handled crank. Such tasks involve communication between the people, mediated by the task kinematics and dynamics: each person feels forces and motions produced by the other and derive some meaning from them. Tasks may include a degree of competition: the two people may not have exactly the same goal in mind, and must negotiate a compromise. Understanding human-human communication is important in designing robots for interaction with humans, and for robots that provide powered assistance for human-human tasks (such as physical therapy). In this paper we describe early experiments in human-human physical interaction, with a 1 dof robot included in order to give experimental access to the exchange of forces and motions between the people. We report on Fitts' law-like tasks, in which the two people cooperate to move a cursor to a common target, or to targets that do not completely overlap. Our results suggest that human-human physical communication may be a rich area of study.

We have been exploring the use of passive robotic mechanisms for the display of virtual surfaces. Cobots are one way of producing virtual surfaces using a passive mechanism. Unlike powered robots, the nonlinear dynamics of the passive mechanism (e.g., an arm) can be felt by the user as a spatially varying apparent inertia. This effect occurs in many passive designs, including but not limited to cobots. We explain the variable apparent inertia as the projection of the spatially-varying inertia matrix onto the direction of motion, and discuss several ways to control the apparent inertia. We explore apparent inertia in detail for the unicycle two link arm, a cobot we have developed for experiments in single-arm motor control studies and rehabilitation. Special paths ("iso-mass contours") are found for this mechanism along which the apparent inertia is constant.

The state-of-the-art in intelligent assist devices (IADs) is reviewed. IADs are computer-controlled tools that enable production workers to lift, move and position payloads quickly, accurately, and safely. Several examples of industrial applications are given, illustrating typical configurations and functionality, including strength amplification and virtual surfaces. The concept of human intent sensing is introduced and discussed, as are IAD safety and control considerations.

Cobots are devices for human/robot interaction, in which axes of motion are coupled to one another by computer-controlled continuously variable transmissions rather than individually driven by servomotors. We have recently built a cobot with a three-dimensional workspace and a 3-revolute parallelogram-type mechanism. Here we present the control methods for the display of virtual surfaces and for free mode in which the cobot endpoint moves as if it were unconstrained. We provide experimental results on the performance of the free, virtual path, and virtual surface controllers.

Cobots are devices for human/robot interaction, in which axes of motion are coupled to one another by computer-controlled continuously variable transmissions rather than individually driven by servomotors. We have recently built a 3R parallelogram cobot. Here we present a control method for the display of virtual surfaces and for free-mode in which the cobot endpoint moves as if it were unconstrained. We show experimental results on the performance of the cobot in these modes.

Of several possible forms of human-robot collaborative manipulation, we focus on the case where the human and the robot jointly manipulate a common load. In our formulation, the robot's role is to provide a constraint surface to guide the motion of the load. The value of this form of interaction, in terms of ergonomics, accuracy, or speed, depends on how humans make use of such constraints. We are studying natural single-arm manipulation of a load constrained to move along a guide rail. In this paper we present results of experiments showing that subjects apply significant forces against the rail, depending on the configuration of the arm and the orientation of the rail. These forces are unnecessary for the manipulation task, and we hypothesize that humans apply forces against the constraint to simplify the manipulation task.

A general framework is presented for the design and analysis of cobot controllers. Cobots are inherently passive robots intended for direct collaborative work with a human operator. While a human applies forces and moments, the controller guides motion by tuning the cobot's set of continuously variable transmissions. In this paper, a path-following controller is developed that steers the cobot so as to asymptotically approach and follow a preplanned path. The controller is based on feedback linearization. Generality across cobot architectures is assured by designing the controller in task space and developing transformations between each of four spaces: task space, joint space, a set of coupling spaces, and steering space.

We describe a new robot architecture: the collaborative robot, or cobot. Cobots are intended for direct physical interaction with a human operator. The cobot can create smooth, strong virtual surfaces and other haptic effects within a shared human/cobot workspace. The kinematic properties of cobots differ markedly from those of robots. Most significantly, cobots have only one mechanical degree of freedom, regardless of their taskspace dimensionality. The instantaneous direction of motion associated with this single degree of freedom is actively servo-controlled, or steered, within the higher dimensional taskspace. This paper explains the kinematics of cobots and the continuously variable transmissions (CVT) that are essential to them. Powered cobots are introduced, made possible by a parallel interconnection of the CVT. We discuss the relation of cobots to conventionally actuated robots and to nonholonomic robots. Several cobots in design, prototype, or industrial testbed settings illustrate the concepts discussed.

Repetitive manual materials handling of heavy loads is common in assembly and is a common cause of low back disorders. The manual manipulation of a heavy load may be made more comfortable by constraining the load to move along a guide. The frictionless guide directs the motion of the load to the goal configuration as the human operator provides forces in directions that are comfortable. We present our first experimental results in guided manipulation with the purpose of understanding motions and forces that are comfortable for human operators.

Intelligent assist devices (IADs) are a new class of hybrid devices for direct, physical, interaction with a human operator in shared workspaces. These devices-designed for the assembly line worker-can reduce ergonomics concerns that arise due to on-the-job physical and cognitive loading, while improving safety, quality and productivity. Cobots, a sub-set of IADs, implement software defined virtual guiding surfaces while providing some amplification of human power. They exemplify the central theme of this paper-that humans are critical in many assembly operations and ergonomics tools that enable them to perform their duties are necessary. The paper describes broad design principles for human-machine interaction in these industrial settings. Prototype industrial cobots designed for testing and validation are described.

A general framework is presented for the design and analysis of cobot controllers. Cobots are inherently passive robots intended for direct collaborative work with a human operator. While a human applies forces and moments, the controller guides motion by tuning the cobot's set of continuously variable transmissions. In this paper, a path following controller is developed that steers the cobot so as to asymptotically approach and follow a pre-planned path. The controller is based on feedback linearization. Generality across cobot architectures is assured by designing the controller in task space and developing transformations between each of four spaces: task space, joint space, a set of coupling spaces, and steering space.

Conventional force sensors are overdesigned for use in measuring human force inputs, such as is needed in research and application of human/robot interaction. A new type of force sensor is introduced that is suited to human-robot interaction. This sensor is based on optoelectronic measurements rather than strain gauges. Criteria for material selection and dimensioning are given, and results for linearity, noise, and drift are reported.

Cobots are capable of producing virtual surfaces of high quality, using mechanical transmission elements as their basic element in place of conventional motors. Most cobots built to date have used steerable wheels as their transmission elements. We describe how continuously variable transmissions (CVTs) can be used in this capacity for a cobot with revolute joints. The design of an "arm-like" cobot with a three-dimensional workspace is described . This cobot can implement virtual surfaces and other effects in a spherical workspace approximately 1.5 meters in diameter. Novel elements of this cobot include the use of a power disk that couples three CVTs directly.

Cobots are a class of mechanically passive robotic devices, intended for direct physical collaboration with a human operator. The operator supplies all motive power while the cobot enforces software-defined guiding surfaces, or constraints. Cobots are intrinsically passive, safe devices. This is because, rather than employ powered actuators to produce constraint forces, cobots use "steerable" nonholonomic joints. Constraint forces are mechanical in origin, yet software defined. The simplest possible cobot is a unicycle which is steered by a servo system acting under computer control, but which is moved by a human operator. The unicycle cobot requires essentially no consideration of kinematics. Two fundamental control modes of the unicycle cobot, "virtual caster" and "constraint tracking", are reviewed. More complicated cobots, such as the three-wheeled "Scooter", require a set of kinematic tranformations relating configuration space to joint space. These transformations play a role in cobot control like that of the Jacobian in robot control.

The impedance matrices of independent point fingers of a multifingered gripper map to the impedance matrix of a grasped workpart. We find that in a planar geometry, three fingers are enough to allow an unrestricted range of workpart impedances, if finger impedances are selectable. In a spatial geometry however, five fingers are necessary for the broadest range of workpart impedances, and even so there is one impedance matrix that a workpart cannot attain regardless of the number of fingers that grasp it. We find this "unattainable" impedance matrix. We also characterize the impedance restrictions on workparts grasped with fewer than five spatial or three planar fingers.

Conventional approaches to haptic interface rely on high gain servos to implement virtual constraints. The role of the servo is to reduce the apparent degrees of freedom in such a way as to effectively constrain a human operator's motion. A significant drawback of this approach, however, is that the operator must interact directly with a high power system that is not inherently passive, and which may become unstable. In this paper, we present a novel approach to haptic display which allows virtual constraints to be implemented in a manner that is completely passive and therefore intrinsically safe. The key idea is to begin with a device having zero or one degree of freedom, and to use feedback control to increase the apparent degrees of freedom as necessary. This becomes possible with the use of nonholonomic joints, which have fewer degrees of freedom than generalized coordinates. The design and feedback control of several "programmable constraint machines" (PCMs) of this type are discussed.

Describes a new architecture for passive robots and haptic displays, which the authors call a programmable constraint machine (PCM). An n-DOF PCM can, under computer control, exhibit constraints (smooth, impenetrable virtual surfaces of dimensionality

Peshkin and Sanderson (1988) showed that parts can be aligned as they move on a conveyor belt against a passive sequence of fences. In this paper the authors describe the first complete algorithm to design such sequences for a given convex polygonal part. The algorithm is complete in the sense that it is guaranteed to find a design if one exists and to terminate with a negative report otherwise. Based on an exact breadth-first search of the design space, the algorithm is also guaranteed to find the design requiring the fewest fences. The authors describe the algorithm and compare results with those previously reported. The authors conjecture that a fence design exists to orient any convex polygonal part defined by a sequence of rational vertices.

Previously, force-assembly has been defined as an assembly process for which a single admittance control law (i.e., a single nominal velocity in conjunction with a single mapping of forces to motions) can guarantee the proper assembly of a given pair of mating parts. As a testbed application of force-assembly, the insertion of a workpiece into a fixture consisting of multiple rigid fixture elements (fixels) is addressed. Previous work in this area has shown that, when workpiece/fixture contact is frictionless and positional error is infinitesimal, there always exists an admittance control law that will ensure the proper insertion of a workpiece into a deterministic fixture. When workpiece/fixture contact is frictionless, the workpiece/fixture contact force contains the relative positional information required to identify error-reducing motions. Friction between the workpiece and fixture, however, provides a disturbance to the geometrical information contained in the contact force. This paper addresses: 1) the identification of the conditions that must be satisfied for force-assembly with friction, and 2) the formulation and results of an optimization of the admittance control law to obtain the maximum value of friction that will satisfy the force-assembly conditions for a given workpiece/fixture combination. Results indicate that force-assembly fails when the contact forces are no longer characteristic. Forces are characteristic if the possible contact forces that may occur for one type of misalignment are unique to that type of misalignment.>

The design of an admittance control law for reliable force-guided assembly is addressed. The overall objective is to design the manipulator's mechanical performance so that, at each possible bounded part misalignment, the contact force always leads to a motion that alleviates the misalignment. The implementation of such an approach using a single nominal velocity and a single admittance function is referred to as force-assembly. Previous work addressing force-assembly of a workpiece into a fixture has shown that when workpiece/fixture contact is frictionless there always exists an admittance control law that ensures the proper insertion of a workpiece into a deterministic future. The authors discuss the identification of the condition that must be satisfied for force-assembly with friction and the optimization of the admittance control law to obtain the maximum value of friction that satisfies the force-assembly conditions for a given workpiece-fixture combination.>

The authors address the design of manipulator admittance for reliable force-guided assembly. Their goal is to design the admittance of the manipulator so that, at all possible bounded part misalignments, the contact forces always lead to error-reducing motions. If this objective can be accomplished for a given pair of mating parts, the parts are called force-assemblable. As a testbed application of manipulator admittance design for force-guided assembly, the authors investigate the insertion of a workpiece into a fixture consisting of multiple rigid fixture to be one for which there exists an admittance matrix that ensures the unique positioning of a workpiece despite initial positional error. It is shown that, in the absence of friction, all deterministic fixtures are linearly force-assemblable. How to design an admittance matrix that guarantees that the workpiece will be guided into the deterministic fixture by the fixel contact forces alone is shown.>

The authors describe the concept of force-assemblability, which may be applied to fixture insertion as well as other assembly tasks. They define a linear force assemblable fixture to be one for which there exists a linear motion control law (examples of which include compliance and accommodation) which necessarily results in workpiece contact with all fixture elements (fixels) despite initial positional error. For reliable insertion, the fixture should have the property that contact with all fixels insures a unique workpiece position (i.e. the fixture should be deterministic) and the property that, after the inserting motion terminates, contact with all fixels is insured. It is shown that all deterministic fixtures are force-assemblable when friction is less than some value dictated by the workpiece/fixture geometry. It is also shown how to generate a motion control law that satisfies force-assemblability at this value of friction.>

A passive wrist, of fixed design, can be programmed to execute a wide range of useful control laws. Considered in particular are wrists whose actuators are unpowered hydraulic cylinders, the ports of which are coupled to one another via variable-conductance constrictions. The wrist is programmed by selection of these conductances, much as an analog computer is programmed. The range of control laws such a device can compute is characterized mathematically.>

A methodology is described by which erroneous contact configurations for a given task are used to synthesize an appropriate accommodation matrix (either diagonal or nondiagonal) such that the motions which result from contact with the environment are error corrective. The methodology is verified by the synthesis and experimental validation of a nondiagonal accommodation matrix sufficient for an error-corrective place-in-detent robotic assembly operation, an operation that fails if a diagonal accommodation matrix is used.>

The following problem is addressed. Suppose that before an assembly task commences, one can specify at will the manipulator's response to assembly forces by providing a single compliance (or damping) matrix to be used for the duration of the operation. Can one choose the matrix elements so that the force that characterizes every possible error condition maps into a motion that reduces it? If so, it is assured that the operation will evolve toward decreased errors and eventual success. A framework and a method of synthesis for an error-corrective matrix are described.>

Energetic formulations of Newton's laws are valuable for mechanics problems involving multiple constraints. The following energic principle for quasistatic systems is discussed: a quasistatic system chooses that motion, from among all motions satisfying the constraints, which minimizes the instantaneous power. This minimum power principle states that a system chooses at every instant the lowest energy, or 'easiest', motion in conformity with the constraints. It is shown that the principle is in general false. For example, if viscous forces act, the motion predicted by the minimum power principle will be incorrect. The authors proved that the principle is correct if there are no forces with velocity-dependent magnitude. This allows its application to many systems with Coulomb friction.>

Quasistatic mechanical systems, in which mass or acceleration is sufficiently small for the inertial term ma in F=ma to be negligible compared to dissipative forces, are discussed. It is pointed out that many instances of robotic manipulation can be well approximated as quasistatic systems, with the dissipative force being dry friction. Energetic formulations of Newton's laws have often been found useful in the solution of mechanics problems involving multiple constraints. An intuitive minimum power principle is outlined which states that a system chooses at every instant the lowest-energy, or 'easiest', motion in conformity with the constraints. Surprisingly, the principle is in general false; but it is proved that the principle is correct in the useful special case that Coulomb friction is the only dissipative or velocity-dependent force acting in the system.>

The authors consider the automated planning of manipulation strategies for workpieces able to slide on their work surface. The aim is to generate open-loop (i.e. sensorless) strategies which succeed in aligning or grasping a workpiece, in the face of two kinds of uncertainty: (1) the initial configuration of the workpiece may have some bounded error, and (2) the details of the contact between workpiece and work surface may be unknown, precluding deterministic solution for the motion of the workpiece even were its initial configuration exactly known. Configuration maps are defined which map all configurations of a workpiece before elementary manipulative operation to all possible outcomes. Using elementary manipulative operations (represented by configuration maps) as primitives, appropriate search techniques are applied to find operations sequences which are guaranteed to succeed despite uncertainty. As a concrete example, the authors demonstrate the automated design of a class of passive parts-feeder consisting of multiple sequential fences across a conveyor belt.>

It occurs frequently in robotic applications that a robot manipulates a workpiece which is free to slide on a work surface. Because the pressure distribution supporting the workpiece on the work surface cannot in general be known, the motion of the workpiece cannot be calculated uniquely. The authors find the locus of centers of rotation of a workpiece for all possible pressure distributions. The results allow a quantitative understanding of open-loop robot motions which guarantee the alignment of a workpiece. Several sample problems are solved using the results, including the distance that a flat fence, or robot finger, must push a polygonal workpiece to assure that a facet of the workpiece comes into alignment with the fence.>

A configuration map is defined and computed, mapping all configurations of a part before an elementary manipulative operation to all possible outcomes. Configuration maps provide a basis for planning the operation sequences which occur in parts-feeder designs or in more general sensorless manipulation strategies for robots. Sequences of elementary operations are represented as matrix-products of configuration maps for the individual operations. Efficient methods for searching the space of all operations sequences are described. As an example we consider a class of parts feeders based on a conveyor belt. Parts arrive on the belt in random initial orientations. By interacting with a series of stationary fences angled across the belt, the parts are aligned into a unique final orientation independent of their initial orientation. The planning problem is to create (given the shape of a part) a sequence of fences which will align that part. We demonstrate the automated design of such parts feeders.

Planning manipulation of an object free to slide on a surface is an important problem in many robotic applications. Physical analysis of the object's motion is made difficult by the absence of information about the distribution of support of the object on the surface, and of the resulting frictional forces. Here we describe a new approach to the analysis of sliding motion. We present results for the locus of centers of rotation for all possibie distributions of support. In one application to robotic manipulation, bounds on the distance an object must be pushed to come into alignment with a robot finger or a fence are determined.

An algorithm that finds the externally visible vertices of a polygon is described. This algorithm generates a new geometric construction, termed the convex ropes of each visible vertex. The convex ropes give the range of angles from which each vertex is visible, and they give all the pairs of vertices which are reachable by a straight robot finger. All of the convex ropes can be found in expected time order n, where n is the number of vertices of the polygon. We discuss the application of this geometric construction to automated grasp planning. The algorithm may also be useful in image interpretation and graphics where efficient computation of visible points is important. The direct application of the algorithm is restricted to two dimension since sequential ordering of vertices is required. Extension to three dimension would rely on well chosen intersecting or projective planes.