Eddy currents induced by rotating magnetic dipole fields can produce forces and torques that enable dexterous manipulation of conductive nonmagnetic objects. This paradigm shows promise for application in the remediation of space debris. The induced force from each rotating-magnetic-dipole field source always includes a repulsive component, suggesting that the object should be surrounded by field sources to some degree to ensure the object does not leave the dexterous workspace during manipulation. In this article, we show that it is possible to fully control the position of an object in a workspace near the midpoint between just two stationary field sources. A given position controller requires a low-level force controller. We propose two new force controllers, and compare them with the state-of-the-art method from the literature. One of the new force controllers is particularly good at not inducing parasitic torques, which is hypothesized to be beneficial for future tasks manipulating and detumbling rotating resident space objects. We perform experimental verification using numerical and physical simulators of microgravity.

Many tasks in image-guided surgery require a clinician to manually position an instrument in space, with respect to a patient, with five or six degrees of freedom (DOF). Displaying the current and desired pose of the object on a 2D display such as a computer monitor is straightforward. However, providing guidance to accurately and rapidly navigate the object in 5-DOF or 6-DOF is challenging. Guidance is typically accomplished by showing distinct orthogonal viewpoints of the workspace, requiring simultaneous alignment in all views. Although such methods are commonly used, they can be quite unintuitive, and it can take a long time to perform an accurate 5-DOF or 6-DOF alignment task. In this article, we describe a method of visually communicating navigation instructions using translational and rotational arrow cues (TRAC) defined in an object-centric frame, while displaying a single principal view that approximates the human's egocentric view of the physical object. The target pose of the object is provided but typically is used only for the initial gross alignment. During the accurate-alignment stage, the user follows the unambiguous arrow commands. In a series of human-subject studies, we show that the TRAC method outperforms two common orthogonal-view methods-the triplanar display, and a sight-alignment method that closely approximates the Acrobot Navigation System-in terms of time to complete 5-DOF and 6-DOF navigation tasks. We also find that subjects can achieve 1 mm and 1° accuracy using the TRAC method with a median completion time of less than 20 seconds.

In this paper, we describe a soft-robotic actuation concept to enable a mesoscale medical robot to navigate the natural lumens of the body, such as blood vessels and intestines. The concept comprises a simple soft robot with two embedded permanent magnets with alternating magnetic polarity, and a rotating (nonuniform) dipole magnetic field that is swept over the robot, resulting in a traveling-wave undulatory motion that propels the robot forward and backward. This soft-actuation technology can be fabricated in a wide range of sizes due to its simplicity, and has the potential to be applied in a variety of diagnostic and therapeutic contexts. We conduct experiments and numerical simulations to verify the movement of the soft robot. Then, we confirm the benefits of using nonuniform dipole fields over using uniform fields, as well as the benefits of alternating the polarity of the magnets embedded in the device.

This paper presents a method for closed-loop propulsion of a screw-type magnetic capsule with embedded Hall-effect sensors using a single rotating actuator magnet. The method estimates the six-degree-of-freedom (6-DOF) pose of the capsule while it is synchronously rotating with the applied field. It is intended for application in active capsule endoscopy of the intestines. An extended Kalman filter, which uses a simplified 2-DOF process model restricting the capsule to forward or backward movement and rotation about its principle axis, is used to provide a full 6-DOF estimate of the capsule's pose as the capsule travels through a lumen. The capsule's movement in the applied field is constantly monitored to determine if the capsule is synchronously rotating with the applied field. Based on this information, the rotation speed of the external source is adjusted to prevent a loss in the desired magnetic coupling. We experimentally demonstrate, for the first time, simultaneous localization and closed-loop propulsion of a capsule through a lumen using a single rotating magnet. Prior work assumed the capsule had no net motion during the localization phase, requiring decoupled localization and propulsion. This closed-loop performance results in a three times speed up in completion time, compared to the previous decoupled approach.

Magnet-tipped, elastic rods can be steered by an external magnetic field to perform surgical tasks. Such rods could be useful for a range of new medical applications because they do not require either pull wires or other bulky mechanisms that are problematic in small anatomical regions. However, current magnetic rod steering systems are large and expensive. Here, we describe a method to guide a rod using a robot-manipulated magnet located near a patient. We solve for rod deflections by combining permanent-magnet models with a Kirchhoff elastic rod model and use a resolved-rate approach to compute trajectories. Experiments show that three-dimensional trajectories can be executed accurately without feedback and that the system's redundancy can be exploited to avoid obstacles.

A variety of magnetic devices can be manipulated remotely using a single permanent “actuator” magnet positioned in space by a robotic manipulator. This paper describes the spherical-actuator-magnet manipulator (SAMM), which is designed to replace or augment the singularity-prone spherical wrist used by prior permanent-magnet manipulation systems. The SAMM uses three omniwheels to enable holonomic control of the heading of its magnet's dipole and to enable its magnet to be rotated continuously about any axis of rotation. The SAMM performs closed-loop control of its dipole's heading using field measurements obtained from Hall-effect sensors as feedback, combined with modeled dynamics, using an extended Kalman filter. We describe the operation and construction of the SAMM, develop and characterize controllers for the SAMM's spherical magnet, and demonstrate remote actuation of an untethered magnetic device in a lumen using the SAMM.

An admittance-type robotic manipulator is a nonbackdrivable device whose motion is controlled to move in response to a user-applied force, typically with velocity proportional to force. This study characterizes the ability of ten human subjects to accurately and precisely control the velocity of such a device, using force applied by the index finger, as the user is provided visual feedback of device motion and a target velocity on a display. The admittance, the velocity, and the visualization scale factor are varied in a full factorial design, with parameter levels representative of microsurgery/micromanipulation tasks. The results indicate that: visual scaling has no effect, for the levels tested; low velocity at high admittance results in reduced precision and accuracy; high velocity at low admittance results in reduced accuracy; and an admittance-dependent velocity exists at which accuracy is maximized. The results suggest that gain scheduling will result in improved performance.

This paper presents the design concept and fabricated prototype of a device that swings the arms for use in gait rehabilitation. The device is designed to be used in conjunction with a body-weight-support treadmill. The device is backdrivable, wearable, capable of assisting the user's arm swing in the sagittal plane, and has unhindered kinematics in the remaining unactuated degrees of freedom. Tests are performed to validate the shoulder-angle prediction equations based on the non-collocated motor-angle sensor measurements, to validate the device's ability to provide adequate torque to induce arm-swing in a passive user, and to investigate whether or not the user's active involvement can be determined by examining sensor data. The results show that the device does provide sufficient torque to move the arms with a factor of safety, but that the model-based shoulder-angle estimates obtained from the motor measurements have non-negligible error with the current prototype. It is shown that the controlled device generates low RMS tracking error and is able to diagnose user-assistance level (i.e., if the user is passive or actively assisting arm swing) online by observing shoulder-angle amplitudes and peak motor torques.

Untethered magnetic devices (UMDs) driven by rotating magnetic fields have been proposed for applications in minimally invasive medicine. Previous work has explored the propulsion and guidance of a single UMD using a rotating magnet dipole source, and actuation of multiple UMDs has focused on the use of uniform magnetic fields. This work seeks to extend the methods for remote actuation using a single magnet dipole source to generate two rotating magnetic fields with arbitrary independent rotation axes at any two points with a common rotation frequency. The solutions found show that there are multiple positions and rotation axes of the magnet dipole that can generate these rotating fields. The method is demonstrated with two screw-type UMDs propelled independently within an agar gel block using a single permanent-magnet actuator.

This is the first demonstration of a modular and reconfigurable magnetic-manipulation system with integral ferromagnetic material. This system-which includes multiple Omnimagnets, each comprising three orthogonal solenoids and a spherical ferromagnetic core-is capable of dexterous manipulation of a magnetic tool. The magnetization coupling of an arbitrary arrangement of spherical ferromagnetic cores is solved, enabling an analytical solution for the magnetic forces and torques. Thus, the system does not require extensive field maps or in situ field characterization. If the positions and orientations of the Omnimagnets are sensed, the Omnimagnets can be actively rearranged during manipulation while maintaining control of the tool. This could enable new capabilities in medical procedures because the manipulation system can be modified to accommodate the imaging systems and enable physician access without loss of control. The capabilities of this system are demonstrated through five-degree-of-freedom manipulation (position and heading) of a permanent-magnetic tool with several arbitrary Omnimagnet configurations.

A variety of magnetic devices can be manipulated remotely using a single permanent “actuator” magnet positioned in space by a robotic manipulator. This paper presents the spherical-actuator-magnet manipulator (SAMM), which is designed to replace or augment the singularity-prone spherical wrist used by prior permanent-magnet manipulation systems. The SAMM uses three omniwheels to enable holonomic control of a spherical magnet's heading and enable the magnet's instantaneous axis-of-rotation to be set arbitrarily. The SAMM performs closed-loop control of its dipole using field measurements obtained from Hall-effect sensors. We describe the operation and construction of the SAMM, develop and characterize a controller for the SAMM's spherical magnet, and demonstrate remote actuation of an untethered magnetic device in a lumen.

To date, untethered magnetic devices actuated with a single rotating permanent magnet, such as active capsule endoscopes and magnetic microrobots, have been constrained to operate in positions where the rotating dipole field behavior is simple and easy to visualize. In this paper, we show how to generate a rotating magnetic field with any desired rotation axis, for magnetic device actuation, at any device position in space using a single rotating-magnet actuator. The methods presented can control untethered rotating magnetic devices, while the rotating actuator magnet follows trajectories independent of the untethered devices themselves. We demonstrate our methods by actuating rotating magnetic devices in a lumen. Applications include minimally invasive medical tasks requiring an untethered magnetic device to operate in natural lumen pathways of the body (e.g., the gastrointestinal system, the subarachnoid space of the nervous system, or vasculature).

In this paper, we examine several magnetic control methods that utilize the fully controllable dipole field generated by the single stationary dipole source. Since the magnetic field generated by a dipole source is nonuniform, it applies both forces and torques to magnetic objects and can be used to manipulate magnetic tools. Recently, the Omnimagnet, a computer-controlled magnetic dipole source capable of varying both its dipole-moment direction and magnitude, was developed to perform magnetic manipulation. The equations and methods are developed generally; therefore, they can be applied to any omnidirectional dipole source, but their effectiveness is demonstrated using the Omnimagnet.

Untethered magnetic devices, such as magnetic microrobots and magnetically actuated capsule endoscopes, stand to significantly impact the field of minimally invasive medicine. These devices, which we refer to as magnetically actuated tools (MATs), are often controlled using electromagnets, which can be expensive to scale clinically. Due to their potential for low-cost and high strength, permanent magnets are being considered for MAT actuation. Great care must be taken for in vivo medical applications, however, as systems using single permanent magnets typically generate an attractive force, which may cause trauma if uncontrolled. In this paper, we present techniques for managing the attractive magnetic force. We find that in the case of rotating MATs actuated using a single rotating permanent magnet (RPM), for which magnetic torque is the primary form of actuation, the attractive magnetic force can be substantially reduced in magnitude and the time-averaged attractive component eliminated when operating the MAT such that magnetic torque is maximized. We provide experimental demonstration that validates and illustrates the force management strategies presented.

Previous work in the open-loop behavior of magnetically driven screws in soft tissue has focused on the impact of magnetic field rotation speed and pitch angle, measured between the field's rotation axis and the screw's principle axis, on the average magnetic torque, both in-plane and out-of-plane. However, prior work did not rigorously consider the role of screw geometry and tissue material properties on the resulting trajectory. This study seeks to develop a plausible empirical model from experimental measurements in an agar gel tissue phantom for use in further research and feasibility studies of magnetic screws for in vivo applications. Non-dimensional parameters representing rotation speed, pitch angle, screw size, magnetic strength, and tissue stiffness are varied orthogonally. Circular trajectories are fitted to the resulting recorded trajectories. Using physical insight and observation of the trends of the non-dimensional turning radius/curvature, simple models for each parameter group are proposed. A final empirical model is then proposed to unify the individual models.

An Omnimagnet is an omnidirectional electromagnet comprised of a ferromagnetic core inside of three orthogonal nested solenoids. It generates a magnetic dipole-field with both a variable dipole-moment magnitude and orientation with no moving parts. The design of an Omnimagnet, in which each solenoid has the same dipole moment and minimizes the differences between each of the solenoid's fields, is provided and optimized for strength by tailoring the size of the spherical core used to amplify the solenoids' field. This design is then analyzed using FEA tools and shown to be dipole-like in nature. Various magnetic control methods are then motivated by providing the necessary equations relating the three applied currents to applied field, torque, or force on an adjacent magnetic device. Finally, the optimal design is constructed and its utility is demonstrated by driving a helical capsule endoscope mockup through a transparent lumen.

Previous research on the localization of wireless capsule endoscopes with magnetic fields and sensors has typically utilized incremental methods. This paper provides a non-iterative solution to determine the six degree-of-freedom (6-DOF) position and orientation of a wireless capsule endoscope being actuated by a rotating magnetic dipole. Non-iterative solutions in the past have only been used to locate immobile objects. We experimentally demonstrate that our algorithm calculates the 6-DOF position and orientation of capsules that are truly stationary as well as those that are operated in the “step-out” regime, where the magnetic field is rotated too quickly for the capsule to rotate synchronously, but the capsule does undergo chaotic movement.

It has been shown that when a magnetic dipole, such as a permanent magnet, is rotated around a fixed axis such that the dipole is perpendicular to the axis of rotation, the magnetic field vector at every point in space also rotates around a fixed axis. In this paper, we reformulate this phenomenon using linear algebraic techniques, which enables us to find the necessary dipole rotation axis to make the magnetic field at any desired point in space rotate about any desired axis. To date, untethered magnetically actuated tools (e.g., capsule endoscopes, rolling spheres, and helical-propeller microswimmers) controlled with a single rotating permanent magnet have been constrained to operate in positions where the rotating field behavior is simple and easy to visualize. We experimentally demonstrate that the results of this paper can be used to control a variety of untethered, rotating magnetic devices in any position even while the rotating permanent magnet follows trajectories independent of the devices themselves. This method constitutes a substantial step toward making a great deal of prior laboratory research regarding rotating magnetic microrobots and capsule endoscopes clinically feasible.

This paper considers the requirements for stable control of simple devices that consist of a permanent magnet rigidly attached to a screw operating in soft tissue. Placing these devices in a rotating magnetic field causes the device's permanent magnet to rotate synchronously with the applied field, making the attached screw generate forward propulsion. Although there has been significant research in the area of magnetic screws, this paper describes magnetic phenomena that have not been addressed in prior work, yet will be critical in the design of future steering and control strategies for medical applications. Using a time-averaged model of magnetic torque, we analyze the magnetic torque present when intentionally steering the screw or due to naturally occurring disturbances in the screw's heading. We predict and experimentally demonstrate the existence of magnetic torques that simultaneously act to stabilize and destabilize magnetic screws operating in tissue. We find that there exists an applied field rotation speed, above which screws may become unstable, and we find that the interaction between the stabilizing and destabilizing magnetic torques may cause potentially undesired but predictable behavior while steering.

The small intestine is the longest part of the gastrointestinal (GI) tract, but it is difficult to examine with traditional endoscopic procedures. To resolve this, swallowable capsules have been created that slowly pass through the entire GI tract taking images. Research is currently being done in magnetic actuation and localization to increase the speed and control of these smart pills. This paper focuses on a technique to more efficiently control a capsule endoscope based on proprioceptive sensing of the magnetic field. This sensing provides knowledge of the applied field in the frame of the capsule, and enables determination of its current state.

Cochlear implants have become a standard treatment for many with severe to profound sensorineural hearing loss. However, delicate cochlear structures can be damaged during surgical insertion, which can lead to loss of residual hearing and decreased implant effectiveness. We propose a magnetic guidance concept in which a magnetically tipped cochlear implant is guided as it is inserted into the cochlea. In a scaled in vitro experimental study, we record insertion forces for nonguided and magnetically guided insertion experiments and compare the results. Results indicate that magnetic guidance reduced insertion forces by approximately 50%.

Flying robots capable of perch-and-stare are desirable for reconnaissance missions. Current solutions for perch-and-stare applications utilize various methods to create an aircraft that can land on a limited set of surfaces that are typically horizontal or vertical planes. This paper presents a bio-inspired concept that allows for passive perching on cylindrical-type surfaces. The prototype provides compliant gripping through the use of an underactuated foot. A mechanism inspired by songbird anatomy is integrated that utilizes rotorcraft weight as a way to passively actuate the foot. Successful perching trials on two rods of differing diameters were performed and are discussed. The purpose of this initial design is to act as a proof of concept for the mechanical action of the mechanism; our results demonstrate that passive perching can be achieved through the integration of underactuated gripping with mechanism-generated mechanical advantage.

Commercial micro/nano-manipulators, which utilize piezoelectric stick-slip actuators to achieve high precision over a large workspace, are currently controlled by a human operator at the joint level, leading to unintuitive and time-consuming teleoperation. Prior work has considered the use of computer-vision-feedback to close a control loop for improved performance, but computer-vision-feedback is not a viable option for many end users. In this paper, we discuss how open-loop models of the micro/nano-manipulator can be used to achieve desired end-effector movements, and we explain the process of obtaining open-loop models. We propose a rate-control teleoperation method that utilizes the obtained model, and we experimentally quantify the effectiveness of the method using a common commercial manipulator (the Kleindiek MM3A).

Magnetic helical medical microrobots are promising for use in open fluid, lumen, and soft-tissue environments and will enable minimally invasive access to hard-to-reach locations in the body. Prior work focused on control via uniform magnetic fields from orthogonal arrangements of electromagnetic coils, which are difficult to scale up to the size required for clinical applications. We propose the use of nonuniform magnetic fields emanating from a single rotating-permanent-magnet (RPM) manipulator for the control of magnetic helical microrobots. We theoretically and experimentally demonstrate that the RPM manipulator is a viable option for wireless control of magnetic helical microrobots that warrants further investigation.

We demonstrate five-degree-of-freedom (5-DOF) wireless magnetic control of a fully untethered microrobot with a magnetic steering system we call OctoMag. Although only occupying a single hemisphere, this system is capable of isotropically applying forces on the order of 1-40 μN with unrestricted control of the 2 orienting DOF. These capabilities are enabled through the use of soft-magnetic-cores which provide an increase of approximately 20× that of air cores in magnetic-field strength, but comes at the cost of more complicated interactions between coils. We propose a modeling mechanism that assumes the field contributions of the individual currents superimpose linearly when using cores with large linear regions and negligible hysteresis. When designing the system, the locations and quantity of electromagnets were optimized with regards to the force generation in the worst-case direction predicted by the model. The resultant system is capable of both open and closed-loop operation over a workspace of 4 cm3. OctoMag was primarily designed for the control of intraocular microrobots for delicate retinal procedures, but also has potential uses in other medical applications or micromanipulation under an optical microscope.

We demonstrate five-degree-of-freedom (5-DOF) wireless magnetic control of a fully untethered microrobot (3-DOF position, 2-DOF pointing orientation). The microrobot can move through a large workspace, and is completely unrestrained in the rotation degrees of freedom. We accomplish this level of wireless control with an electromagnetic system that we call OctoMag. OctoMag's unique abilities are due to its utilization of complex nonuniform magnetic fields, which capitalizes on a linear representation of the coupled field contributions of multiple soft-magnetic-core electromagnets acting in concert. OctoMag was primarily designed for the control of intraocular microrobots for delicate retinal procedures, but it also has potential uses in other medical applications or micromanipulation under an optical microscope.

We demonstrate five-degree-of-freedom (5-DOF) wireless magnetic control of a fully untethered microrobot (3-DOF position and 2-DOF pointing orientation). The microrobot can move through a large workspace and is completely unrestrained in the rotation DOF. We accomplish this level of wireless control with an electromagnetic system that we call OctoMag. OctoMag's unique abilities are due to its utilization of complex nonuniform magnetic fields, which capitalizes on a linear representation of the coupled field contributions of multiple soft-magnetic-core electromagnets acting in concert. OctoMag was primarily designed to control intraocular microrobots for delicate retinal procedures, but it also has potential uses in other medical applications or micromanipulation under an optical microscope.

Successful ophthalmic surgeries using intraocular untethered microrobots or tethered robotic microtools require methods to robustly track the microdevices in the posterior of the human eye. The dimensions and specularities of the microdevices are major obstacles for accurate tracking. In addition, the optical structure of the human eye makes it challenging to keep the objects of interest constantly in focus, resulting in blurred images. In this paper, the advantages of using different colorspaces for intraocular tracking are examined. After selection of the appropriate colorspace, thresholds that ensure maximum separation of the device from the background are calculated. Based on trained color histograms, level sets are used to track in real time, and the use of statistical shape information is incorporated in the existing tracking framework. The efficacy of the algorithm is demonstrated by tracking a microrobot in a model eye, using a custom made ophthalmoscope and off-the-shelf ophthalmoscopy lenses. With the appropriate colorspace and threshold selection, tracking errors are minimized and are further diminished using shape information.

Future retinal therapies will be partially automated in order to increase the surgeons' ability to operate near the sensitive structure of the human eye retina. Untethered robotic devices that achieve the desired precision have been proposed, but require localization information for their control. Since the interior of the human eye is externally observable, vision can be used for localization. Previously, a focus-based paraxial localization algorithm using a mechatronic vitreoretinal ophthalmoscope(MVO) was proposed and evaluated by the authors. In this paper, the first algorithm for wide-angle intraocular localization is presented. The effectiveness of this new localization approach is demonstrated by experiments using a model eye and a customized MVO, and there is clear improvement over previously reported results.

Self-assembly is a process in which individual components form an organized structure as a consequence of local interactions. When using magnetics to create interaction forces, the magnetic flux distribution of a self-assembling system changes as its assembly state varies. Since Hall effect sensors are a convenient and effective means to detect changes in the magnetic field, we explore their applicability to monitoring the morphology of such magnetically self-assembling systems. We find that optimal positions for the sensor can be found where the flux changes maximally. Our analysis is applied to two different systems by deriving the flux changes for all possible states, and theoretical flux changes are verified with experiments. In addition, we show that a small number of sensors is sufficient for robust state determination. In addition to state detection, experiments show the potential for angle measurement for compliant cylindrical magnet joints using a single Hall sensor.

Artificial bacterial flagella (ABF) are swimming microrobots that mimic the swimming motion of bacteria. The helical swimmer consists of an InGaAs/GaAs/Cr helical nanobelt tail fabricated by a self-scrolling technique with dimensions similar to a natural flagellum, and a thin soft-magnetic metal ¿head¿ consisting of a Cr/Ni/Au multi-layer. The swimming locomotion of ABF is precisely controlled in 3-D by external rotating magnetic fields. Microsphere manipulation is performed by ABF, and experimental results show that both the position and the orientation of microspheres can be precisely controlled. The propelling force of ABF is in the pico-Newton range. We also describe a swarm-like behavior in which three ABF swim in a pack, indicating the potential to handle several micro objects in parallel. Self-propelled devices such as these are candidates for wireless 6-DOF micro and nanomanipulation tools for handling cellular and sub-cellular objects.

Capitalizing on advances in CMOS and MEMS technologies, microrobots have the potential to dramatically change many aspects of medicine by navigating bodily fluids to perform targeted diagnosis and therapy. Onboard energy storage and actuation is very difficult at the microscale, but externally applied magnetic fields provide an unparalleled means of wireless power and control. Recent results have provided a model for accurate real-time control of soft-magnetic bodies with axially symmetric geometries. In this paper, we extend the model to consider the real-time control of assembled-MEMS devices that may have significantly more complex geometries. We validate the model through FEM and experiments. The model captures the characteristics of complex 3-D structures and allows us, for the first time, to consider full 6-DOF control of untethered devices, which can act asinvivomicrorobots or as end-effectors of micromanipulation systems.

Retinal vein occlusion is an obstruction of blood flow due to clot formation in the retinal vasculature, and is among the most common causes of vision loss. Currently, the most promising therapy involves injection of t-PA directly into small and delicate retinal vessels. This procedure requires surgical skills at the limits of human performance. In this paper, targeted retinal drug delivery with wireless magnetic microrobots is proposed. We focus on four fundamental issues involved in the development of such a system: biocompatible coating of magnetic microrobots, diffusion-based drug delivery, characterization of forces needed to puncture retinal veins, and wireless magnetic force generation. We conclude that targeted drug delivery with magnetic microrobots is feasible from an engineering perspective, and the idea should now be explored for clinical efficacy.

Few rotational actuators currently exist with the ability to transmit motion at different speeds, torques, and directions at the nanometer scale. We present work regarding the application of helical nanobelts as linear-to-rotary and rotary-to-linear motion converters. We discuss their ability to rectify device rotation to linear motion for untethered microrobotic applications as well as their application as rotary sample stages for nanoscale imaging.

There is a clear trend toward the miniaturization of medical devices for minimally invasive medical procedures, ranging from diagnosis and targeted drug delivery to complex surgical interventions. Current research focuses on increasing the functionality of commercially successful capsule endoscope technology by developing active locomotion and telemetry. The size of such a capsule must not be larger than what a person can swallow without difficulty. One approach to increase functionality while still working within this size constraint is to build a modular robotic system in which the modules are swallowed one at a time, and the final assembly is performed inside the gastrointestinal (GI) tract. This paper addresses a fundamental challenge that must be be met for the success of such swallowable modular robots-their self-assembly. We propose to use magnets in a specific configuration on the mating faces of the modules. Our results show that high success rates can be achieved and snake-type robots can be self-assembled with compliant magnetic joints allowing them to adapt to highly irregular paths, such as the small intestine.

Microrobots experience physical phenomena that are difficult to model analytically and that are not completely captured with macro-scale prototypes. In this paper we present a reconfigurable robotic measurement system to characterize the magnetic and hydrodynamic properties of assembled-MEMS microrobots. The system consists of a powerful permanent magnet that is position controlled with a linear stage. The magnetic field is accurately characterized. Precision sensors are used to measure magnetic force as a function of applied field. The system is first used to validate an existing model for the magnetic force on a soft-magnetic ellipsoid. Next, the magnetic force on a soft-magnetic assembled-MEMS microrobot as a function of the applied field is measured experimentally. Finally, a vision tracking system is integrated with the setup to measure the hydrodynamic properties of the microrobot. The coefficient of viscous friction for the microrobot is obtained experimentally.

We calculate the torque and force generated by an arbitrary magnetic field on an axially symmetric soft-magnetic body. We consider the magnetization of the body as a function of the applied field, using a continuous model that unifies two disparate magnetic models. The continuous torque and force follow. The model is verified experimentally, and captures the often neglected region between weak and saturating fields, where interesting behavior is observed. We provide the field direction to maximize torque for a given field magnitude. We also find an absolute maximum torque, for a given body geometry and material, which can be generated with relatively weak applied fields. This paper is aimed at those interested in systems-level analysis, simulation, and real-time control of soft-magnetic bodies.

The "virtual wall" is the most common building block used in constructing haptic virtual environments. A virtual wall is typically based on a simple spring model, with unilateral constraints that allow the user to make and break contact with a surface. There are a number of factors (sample-and-hold, device dynamics, sensor quantization, etc.) that cause virtual walls to demonstrate active (nonpassive) behavior, destroying the illusion of reality. In this paper, we find an explicit upper bound on virtual wall stiffness that is a necessary and sufficient condition for virtual wall passivity. We consider a haptic display that can be modeled as a mass with Coulomb-plus-viscous friction, being acted upon by two external forces: an actuator and a human user. The system is equipped with only one sensor, an optical encoder measuring the position of the mass. We explicitly model the effects of position resolution, which has not been done in previous work. We make no assumptions about the human user, and we consider arbitrary constant sampling rates. The main result of our analysis is a necessary and sufficient condition for passivity that relies on the Coulomb friction in the haptic device, as well as the encoder resolution. We experimentally verify our results with a one-degree-of-freedom haptic display, and find that the system can display nonpassive behavior in two decoupled modes that are predicted by the necessary and sufficient condition. One mode represents instability, while the other mode results in active tactile sensations.

A forbidden-region virtual fixture (FRVF) is a computer-generated constraint that displays position or force limitations to a robot manipulator or operator, in order to prevent motion into forbidden regions of the workspace. We compare nine FRVFs on each of four common telemanipulator control architectures: position forward, position exchange, position forward/force feedback, and position exchange/force feedback. A one-degree-of-freedom telemanipulation system was used in an experiment designed to simulate users working near a known forbidden region. The metrics of tracking, safety, and submittance were used to analyze the performance of the system with six different users. The results indicate that different FRVF architectures perform best for each of the three metrics. No single FRVF scheme is the best over all metrics, so selection of an FRVF architecture should be an application-dependent weighting of the three metrics. Across all control architectures, the results indicate that a very strong FRVF at the slave device in combination with no FRVF at the master device leads to poor telepresence.

A forbidden-region virtual fixture (FRVF) is a constraint, implemented in software, that keeps the slave manipulator of a master/slave telemanipulation system from entering into a forbidden region of the workspace. In this paper, we consider the problem of unstable vibrations of the slave against the FRVF for a general class of telemanipulator control architectures. The master and slave equilibrium positions resulting from a constant human input force are found, and the system is evaluated around this equilibrium. We consider two methods of analyzing the stability of the system around this equilibrium point. The first method uses tools developed for analysis of two-port networks. The second method converts the system to its discrete state-space form, and then uses the position of the eigenvalues to analyze system stability. We find that the discrete state-space method agrees with simulations, and can be easily used to design and analyze the stability and transient behavior of the telemanipulator.