Ji Chul Ryu

Power mobility training for a 7-month-old infant with spina bifida

Purpose: Power mobility is a critical assistive technology for many children with special needs. Our previous work suggests that certain infants younger than the age 1 year of age can participate in formal power mobility training. Key Points: This case report describes the feasibility of providing a power mobility training program with a young infant with spina bifida. Specifically, we longitudinally quantified the infant’s driving ability with a joystick-controlled device (UD1), using UD1’s onboard computer and video camera from an infant’s age of 7 to 12 months. During the training period, the infant improved in all driving variables. The infant’s Bayley III cognition and language scores also increased at a rate greater than his chronological age. Conclusions/Implications for Clinical Practice: These results suggest that power mobility training within the first year of life may be appropriate for certain populations at risk of immobility.

Differential-Flatness-Based Planning and Control of a Wheeled Mobile Manipulator—Theory and Experiment

This paper presents a differential-flatness-based integrated point-to-point trajectory planning and control method for a class of nonholonomic wheeled mobile manipulator (WMM). We demonstrate that its kinematic model possesses a feedback-linearizable description due to the flatness property, which allows for full-state controllability. Trajectory planning can then be simplified and achieved by polynomial fitting method in the flat output space to satisfy the terminal conditions, while control design reduces to a pole-placement problem for a linear system. The method is then deployed on our custom-constructed WMM hardware to evaluate its effectiveness and to highlight various aspects of the hardware implementation.

Dynamics and Control of a Helicopter Carrying a Payload Using a Cable-Suspended Robot

In this paper we present a study of the dynamics and control of a helicopter carrying a payload through a cable-suspended robot. The helicopter can perform gross motion, while the cable suspended robot underneath the helicopter can modulate a platform in position and orientation. Due to the underactuated nature of the helicopter, the operation of this dual system consisting of the helicopter and the cable robot is challenging. We propose here a two time scale control method, which makes it possible to control the helicopter and the cable robot independently. In addition, this method provides an effective estimation on the bound of the motion of the helicopter. Therefore, even in the case where the helicopter motion is unknown, the cable robot can be stabilized by implementing a robust controller. Simulation results of the dual system show that the proposed control approach is effective for such a helicopter-robot system.

Differential flatness-based robust control of mobile robots in the presence of slip

Slip between the ground and wheel often cannot be avoided in most applications of mobile robots. However, a majority of controllers developed so far make a no-slip assumption with non-holonomic constraints. To achieve desired performance in the presence of slip, controllers that are robust to slip are required. In this paper, we discuss robust trajectory-tracking control for a differentially driven two-wheeled mobile robot. The structure of a differential flatness controller, which has shown distinctive advantages providing an integrated framework for planning and control, is extended to account for slip disturbances. It is shown that the differential flatness framework can be extended to develop a robust controller based on a dynamic as well as a kinematic model with slip. Simulation results for both kinematic and dynamic controllers are presented to demonstrate the effectiveness of the robust controllers. Experiments with the kinematic controller which is suited to typical laboratory and field mobile robots were conducted to validate the proposed robust controller. The simulation and experimental results show that the proposed robust controllers are effective in the presence of slip.

Control of Nonprehensile Rolling Manipulation: Balancing a Disk on a Disk

This paper presents feedback stabilization control of a rolling manipulation system called the disk-on-disk. The system consists of two disks in which the upper disk (object) is free to roll on the lower disk (hand) under the influence of gravity. The goal is to stabilize the object at the unstable upright position directly above the hand. We show that it is possible to stabilize the object at the upright position, while the hand or object rotates to a specific orientation or spins at a constant velocity. We use full-state feedback linearization to derive control laws. We present simulation as well as experimental results demonstrating the controllers.

Control of a Passive Mobility Assist Robot

In this paper, a control methodology for a mobility assist robot is presented. There are various types of robots that can help persons with disabilities. Among these, mobile robots can help to guide a subject from one place to the other. Broadly, the mobile guidance robots can be classified into active and passive types. From a users safety point of view, passive mobility assist robots are more desirable than the active robots. In this paper, a two-wheeled differentially driven mobile robot with a castor wheel is considered the assistive robot. The robot is made to have passive mobility characteristics by a specific choice of control law, which creates damperlike resistive forces on the wheels. The paper describes the dynamic model, the suggested control laws to achieve the passive behavior, and proof of concept experiments on a mobile robot at the University of Delaware. From a starting position, the assistive device guides the user to the goal in two phases. In the first phase, the user is guided to reach a goal position while pushing the robot through a handle attached to it. At the end of this first phase, the robot may not have the desired orientation. In the second phase, it is assumed that the user does not apply any further pushing force while the robot corrects the heading angle. A control algorithm is suggested for each phase. In the second phase, the desired heading angle is achieved at the cost of deviations from the final position. This excursion from the goal position is minimized by the controller. This control scheme is first verified in computer simulation. Then, it is implemented on a laboratory system that simulates a person pushing the robot, and the experimental results are presented.

Control of a passive mobility assistive robot

In this paper, a control methodology for a mobility assistive robot is presented. There are various types of robots that can help the disabled. Among these, mobile robots can help to guide a subject from one place to the other. Broadly, the mobile guidance robots can be classified into active and passive type. From a users safety point of view, passive mobility assistive robots are more desirable than the active robots. In this paper, a two-wheeled differentially driven mobile robot with a castor wheel is considered as the assistive robot. The robot is made to have passive mobility characteristics by a specific choice of control law which creates damper-like resistive forces on the wheels. The paper describes the dynamic model, the suggested control laws to achieve a passive behavior, and experiments on a mobile robot facility at the University of Delaware. From a starting position, the assistive device guides the user to the goal in two phases. In the first phase, the user is guided to reach a goal position while pushing the robot through a handle attached to it. At the end of this first phase, the robot may not have the desired orientation. In the second phase, it is assumed that the user does not apply any further pushing force while the robot corrects the heading angle. A control algorithm is suggested for each phase. In the second phase, the desired heading angle is achieved at the cost of deviation from the final position. This excursion from the goal position is minimized by the controller. This control scheme is first verified in computer simulation. Then, it is implemented on a laboratory system and the experimental results are presented.© 2006 ASME

Control of nonprehensile rolling manipulation: Balancing a disk on a disk

This paper presents stabilization control of a rolling manipulation system called the disk-on-disk. The system consists of two disks in which the upper disk (object) is free to roll on the lower disk (hand) under the influence of gravity. The goal is to stabilize the object at the unstable upright position directly above the hand. We use backstepping to derive a control law yielding global asymptotic stability. We present simulation as well as experimental results demonstrating the controller.

Differentially Flat Designs of Underactuated Mobile Manipulators

This paper presents a methodology for design of mobile vehicles, mounted with underactuated manipulators operating in a horizontal plane, such that the combined system is differentially flat. A challenging question of how to perform point-to-point motions in the state space of such a highly nonlinear system, in spite of the absence of some actuators in the arm, is answered in this paper. We show that, by appropriate inertia distribution of the links and addition of torsion springs at the joints, a range of underactuated designs is possible, where the underactuated mobile manipulator system is differentially flat. The differential flatness property allows one to efficiently solve the problem of trajectory planning and feedback controller design for point-to-point motions in the state space. The proposed method is illustrated by the example of a mobile vehicle with an underactuated three-link manipulator.

HLPR Chair: A Novel Indoor Mobility-Assist and Lift System

This paper describes a novel Home Lift, Position, and Rehabilitation (HLPR) Chair, designed at National Institute of Standards and Technology (NIST), to provide independent patient mobility for indoor tasks, such as moving to and placing a person on a toilet or bed, and lift assistance for tasks, such as accessing kitchen or other tall shelves. These functionalities are currently out of reach of most wheelchair users. One of the design motivations of the HLPR Chair is to reduce back injury, typically, an important issue in the care of this group. The HLPR Chair is currently being extended to be an autonomous mobility device to assist cognition by route and trajectory planning. This paper describes the design of HLPR Chair, its control architecture, and algorithms for autonomous planning and control using its unique kinematics.