Ralph L. Hollis

A six-degree-of-freedom magnetically levitated variable compliance fine-motion wrist: design, modeling, and control

A high-performance six-degree-of-freedom magnetically levitated fine-motion wrist with programmable compliance is described. Design considerations, a discussion of the major elements of the device, and issues of modeling, kinematics, dynamics, and control are presented. A prototype wrist which has been built and controlled successfully is discussed. Experimental results, including high bandwidth position control, compliant control, and the emulation of several mechanisms through software gain setting which establish the use of magnetically levitated robot wrists as an option for manipulation tasks requiring high precision and fine compliant motions, are included. >

A dynamically stable single-wheeled mobile robot with inverse mouse-ball drive

Multi-wheel statically-stable mobile robots tall enough to interact meaningfully with people must have low centers of gravity, wide bases of support, and low accelerations to avoid tipping over. These conditions present a number of performance limitations. Accordingly, we are developing an inverse of this type of mobile robot that is the height, width, and weight of a person, having a high center of gravity, that balances dynamically on a single spherical wheel. Unlike balancing 2-wheel platforms which must turn before driving in some direction, the single-wheel robot can move directly in any direction. We present the overall design, actuator mechanism based on an inverse mouse-ball drive, control system, and initial results including dynamic balancing, station keeping, and point-to-point motion

State transition, balancing, station keeping, and yaw control for a dynamically stable single spherical wheel mobile robot

Unlike statically stable wheeled mobile robots, dynamically stable mobile robots can have higher centers of gravity, smaller bases of support and can be tall and thin resembling the shape of an adult human. This paper concerns the ballbot mobile robot, which balances dynamically on a single spherical wheel. The ballbot is omni-directional and can also rotate about its vertical axis (yaw motion). It uses a triad of legs to remain statically stable when powered off. This paper presents the evolved design with a four-motor inverse mouse-ball drive, yaw drive, leg drive, control system, and results including dynamic balancing, station keeping, yaw motion while balancing, and automatic transition between statically stable and dynamically stable states.

Design and control of a force-reflecting teleoperation system with magnetically levitated master and wrist

A new approach to the design of teleoperation systems is presented. It is proposed that the teleoperation slave be a coarse-fine manipulator with a fine-motion wrist identical to the teleoperation master. By using a combination of position and rate control, such a system would require only small operator hand motions but would provide low mechanical impedance, high motion resolution and force feedback over a substantial volume. A new teleoperation system, consisting of a conventional manipulator and two identical magnetically levitated wrists, has been developed using this approach and is described in this paper. Aspects of mechanical, system and computational design are discussed. It is shown that the best way to position the slave is by decoupling position and rate control, with the conventional robot controlled in rate mode and its wrist in position mode. Kinesthetic feedback is achieved through wrist-level coordinated force control. Transparency is improved through feedforward of sensed hand forces to the master and environment forces to the slave. To maintain stability, the amount of damping in the system is controlled by the sensed environment forces. Experimental results demonstrating excellent performance are presented.

Trajectory planning and control of an underactuated dynamically stable single spherical wheeled mobile robot

The ballbot is a dynamically stable mobile robot that moves on a single spherical wheel and is capable of omnidirectional movement. The ballbot is an underactuated system with nonholonomic dynamic constraints. The authors propose an offline trajectory planning algorithm that provides a class of parametric trajectories to the unactuated joint in order to reach desired static configurations of the system with regard to the dynamic constraint. The parameters of the trajectories are obtained using optimization techniques. A feedback controller is proposed that ensures accurate trajectory tracking. The trajectory planning algorithm and tracking controller are validated experimentally. The authors also extend the offline trajectory planning algorithm to a generalized case of motion between non-static configurations.

One Is Enough

We postulate that multi-wheel statically-stable mobile robots for operation in human environments are an evolutionary dead end. Robots of this class tall enough to interact meaningfully with people must have low centers of gravity, overly wide bases of support, and very low accelerations to avoid tipping over. Accordingly, we are developing an inverse of this type of mobile robot that is the height, width, and weight of a person, having a high center of gravity, that balances dynamically on a single spherical wheel. Unlike balancing 2-wheel platforms which must turn before driving in some direction, the single-wheel robot can move directly in any direction. We present the overall design, actuator mechanism based on an inverse mouse-ball drive, control system, and initial results including dynamic balancing, station keeping, and point-to-point motion.

WYSIWYF Display: A Visual/Haptic Interface to Virtual Environment

To build a VR training system for visuomotor skills, an image displayed by a visual interface should be correctly registered to a haptic interface so that the visual sensation and the haptic sensation are both spatially and temporally consistent. In other words, it is desirable that what you see is what you feel (WYSIWYF). In this paper, we propose a method that can realize correct visual/haptic registration, namely WYSIWYF, by using a vision-based, object-tracking technique and a video-keying technique. Combining an encountered-type haptic device with a motion-command-type haptic rendering algorithm makes it possible to deal with two extreme cases (free motion and rigid constraint). This approach provides realistic haptic sensations, such as free-to-touch and move-and-collide. We describe a first prototype and illustrate its use with several demonstrations. The user encounters the haptic device exactly when his or her hand reaches a virtual object in the display. Although this prototype has some remaining technical problems to be solved, it serves well to show the validity of the proposed approach.

Lorentz magnetic levitation for haptic interaction : Device design, performance, and integration with physical simulations

A new Lorentz force magnetic levitation haptic interface device has been developed and integrated with real-time 3-D rigid-body simulations for detailed, responsive interaction with dynamic virtual environments. An ideal haptic interface device would enable remote or simulated objects to be sensed and manipulated in 6 degrees of freedom (DoF) in the same manner as real physical objects, with the same micrometer level of detail and kHz bandwidth response that the user can sense. To approach this level of performance requires stiff lightweight moving parts, responsive actuators, high-resolution sensors, a fast control system, and a real-time simulation closely integrated with the device. Lorentz levitation is well suited to high-performance, tool-based haptic interaction because noncontact actuation and sensing provide motion and force feedback in 6 DoF with the simple dynamics of a single moving part at high control bandwidths and sensitivity. New actuation and sensing subsystem designs greatly increase the user range of motion over previous maglev devices. The motion range of the new device is ± 12.5 mm in translation and ± 7.5 deg in rotation to accommodate haptic fingertip motion tasks. The device has a measured closed-loop position bandwidth of more than 100 Hz in each DoF, a maximum stiffness of 25.0 N/mm, and a position resolution of 5-10 μm. Virtual coupling and intermediate representation methods were implemented to combine the simulation and the device controller and maximize the realism of haptic user interaction when computational resources are limited. In our system, the device controller must cycle several times faster than the simulation to generate stiff constraints during stable levitation. With virtual coupling, the updated position and orientation data from the interface device and the simulated tool act as error and velocity feedback control setpoints for each other. With the contact point intermediate representation method, a list of current tool contact points and directions is generated at each update of the simulation so that the device controller can evalute contact constrains locally and change impedance to respond to user motions at the fast control rate rather than the slower simulation rate. The contact point intermediate representation provides a crisper, more responsive feeling than virtual coupling during interaction but is not as easily stabilized. Experimental data from the virtual coupling and a modified contact point intermediate representation are presented.

Contact sensor-based coverage of rectilinear environments

A variety of mobile robot tasks require complete coverage of an initially unknown environment, either as the entire task or as a way to generate a complete map for use during further missions. This is a problem known as sensor-based coverage, in which the robots sensing is used to plan a path that reaches every point in the environment. A new algorithm, CC/sub R/, is presented here which works for robots with only contact sensing that operate in environments with rectilinear boundaries and obstacles. This algorithm uses a high-level rule-based feedback structure to direct coverage rather than a script in order to facilitate future extensions to a team of independent robots. The outline of a completeness proof of CC/sub R/ is also presented, which shows that it produces coverage of any of a large class of rectilinear environments. Implementation of CC/sub R/ in simulation is discussed, as well as the results of testing in a variety of world geometries and potential extensions to the algorithm.

Interacting with virtual environments using a magnetic levitation haptic interface

A high-performance magnetic levitation haptic interface has been developed to enable the user to interact dynamically with simulated environments by holding a levitated structure and directly feeling its computed force and motion responses. The haptic device consists of a levitated body with six degrees of freedom and motion ranges of /spl plusmn/5 mm and /spl plusmn/3.5 degrees in all directions. The current device can support weights of up to 20 N and can generate a torque of 1.7 Nm. Control bandwidths of up to 50 Hz and stiffnesses from 0.01 to 23 N/mm have been achieved by the device using a digital velocity estimator and 1 KHz control on each axis. The response of the levitated device has been made successfully to emulate virtual devices such as gimbals and bearings as well as different dynamic interactions such as hard solid contacts, dry and viscous friction, and textured surfaces.