WeP01-32
Proceedings of the 2005 IEEE 9th International Conference on Rehabilitation Robotics June 28 - July 1, 2005, Chicago, IL, USA
User-Environment Based Navigation Algorithm for an Omnidirectional Passive Walking Aid System Naemeh Nejatbakhsh Student Member, IEEE and Kazuhiro Kosuge Senior Member, IEEE
[email protected]
Abstract— This work introduces a new indoor navigation method for an omnidirectional passive walking aid system, called Omni RT Walker-II. Omni RT Walker-II is the second version of a newly manufactured walking aid system with an omnidirectional platform which is controlled through merely control of MR rotary brakes and excludes actuators, aiming at higher safety and better functionality and maneuverability of the system. The proposed navigation method is unique in that it uses both environment information and human characteristics in order to provide its user with further freedom in handling the walker, and avoids abrupt movements of the system. The new method can assist the users with gait disorder and avoid turning over or falling while navigating the user to a destination. Experimental results are presented to evaluate the accuracy and quality of navigation.
M
I. INTRODUCTION
ANY people, due to severe damage, lose the ability of full use of their lower extremities such as loss of strength, control, dexterity or any combination. The same problem is reported as a major impairment in the elderly population. Conventional ambulatory devices including canes, crutches and walkers are mostly used in their primitive form with although useful but insufficient functions. Robotic devices can be used to assist such people in daily life tasks or in a vocational setting. This area of robotic application is often referred to as Rehabilitation Robotics. Several efforts have been made on the research and development of assistive systems with abilities more than that of the conventionally used versions, such as physical support [1], obstacle avoidance, navigation [2] and guidance [1], [3]. Most of these systems perform assistive tasks using actuators as their initial locomotive assemblies, especially in devices with power assisting objectives. Systems operating with actuators usually become quite heavy which is a shortcoming especially in practical usage. Most of these devices, however, are far from getting popular because of the problem of possible actuator malfunction. In this research we propose a passive system; a system excluding motion generating parts which operates under external energy sources. Among the passive walking aid systems developed so far, [4] presents a passive system using actuators only to steer walker front wheels and the system presented in [5] uses servo brakes to control wheels of a nonholonomic walker system. Most of these prototypes however, have a
The authors are with the department of Bioengineering and Robotics, Tohoku University, Aobayama 01, Sendai 980-8579, Japan.
0-7803-9003-2/05/$20.00 ©2005 IEEE
178
nonholonomic structure requiring extra motions of their users in order to compensate walkers’ lack of maneuverability in sideway directions and rotation on the spot, especially in highly dense environments with narrow corridors. This becomes a serious problem for the cases in which the users severely suffer from lack of lower extremities muscular energy or gait disorders. Omni RT Walker-II is a passive omnidirectional walker with better maneuverability, which has integrated functionality, safety and user-friendly characteristics. A major problem arises about elderly is the need of personal nursing care. Elderly or disabled living or staying in indoor environments such as hospitals or welfare facilities, mostly require nursing care while transferring from a place to another, whereas using walking aid devices with a carefully designed navigation system can fulfills the need of assistance. In the following, we briefly introduce the structure of Omni RT Walker-II and propose an environment and user-based navigation system for indoor translation. II. OMNI RT WALKER-II PROTOTYPE Omni RT Walker-II (ORTW-II) consists of an omnidirectional platform with four wheel assemblies shown in Fig.1. Each wheel assembly is composed of an omniwheel, a rotary encoder to detect wheel rotation, and a Magneto-Rheological brake [6] coupled with the omniwheel. The first prototype [7] used commercially available omniwheels which tended to be bumpy due to their small roller diameter and roller substance. Omniwheels used in ORTW-II use material with higher elasticity and have larger diameter and the contact point with floor is reduced to one. A simple suspension mechanism is also devised in each assembly to help absorb vibration and surmount small ledges.
Fig.1 Wheel Assembly
Fig.2 ORTW-II Prototype
MR brakes show excellent rise-time and linear current-torque characteristic. The MR brake in each assembly can apply torque to omniwheel main axis, four of which can control all direction motion of the platform. The system
motion is caused by human force in a walker’s case. The COR (Center of Rotation) of the platform is designed inside the area of the platform, giving the capability of rotation on the spot. Problems such as path planning, navigation or tracking can be simplified in the absence of nonholonomic constraints. The omnidirectional platform performs a better maneuverability, (sideway motion and on the spot rotation becomes possible) which is a major advantage of ORTW-II. This feature decreases extra movements of the user which mostly happen due to the nonholonomic constraints in conventionally used walkers. ORTW-II is equipped with a control system mounted on the platform and a laser range finder (SICK LMS-200) mounted on the secondary plate as shown in Fig.2. The total weight is about 41 Kg. and the minimum force needed to move the walker varies from 11.8 N to 34.3 N depending of omniwheel contact configuration and floor substance. III. NAVIGATION Navigation can be interpreted as generation of a path from an initial to a destination position of an agent, and guiding the agent on the generated path. Conventional path planners can be used for path generation but the problem arises is how to conduct a walker on a certain path which excludes energy sources. We use MR brakes in order to constrain the walker movements which mislead the user. However, application of the path tracking algorithms developed for most robotic agents, requiring the robot to move strictly on the generated trajectory is not necessarily sufficient for a passive walking aid system. These algorithms highly constrain robot movements, carrying off user’s freedom and making walker movements coarse and stressful to the user. For systems with high human interaction similar to walkers, the systems’ behavior is desired to be compatible with human movement pattern to some extent. Potential canal is a perception of paths which increases user dependability of walker’s motion. We introduce the path planner used in the following experiments and the potential canal concept successively. A. Path Planner A conventional path planner is used to generate the shortest path from an initial to a destination configuration based on grid map information of the environment. The A* algorithm is used for efficient graph search and smoothing algorithms are used to remove slacks in the generated path. An example of a path generated by the path planner is shown in Fig.3. B. Potential Canal In the human walking pattern, the position of the center of gravity of human switches at each step from one gait to another, since unlike a wheeled robot, human supports his weight by one of lower extremities while lifting and swinging the other one. Therefore a conventional human walking pattern can be paraphrased to walking in a pathway rather than walking on a single line. Some previously performed experiments on ORTW-II showed that when conventional
179
path tracking methods were applied to ORTW-II its motion pattern was jagged and unsmooth. The user with no knowledge about the path to be tracked tended to push the walker away from its desired trajectory, and the brake force applied by the walker to correct the direction caused discontinuous motion, reported to be quite stressful to the user. Motivated by Arkin’s approach [8], we propose potential canal as an artificial pathway allowable for walker motion, which is very similar to a street sideway or a corridor rather than to a trajectory (or a path). The main advantage of this approach is that it provides the user with further freedom of handling and control of the walker. A potential canal is initialized by a collision-free path pinitial, designed by a conventional path planner. The geometric locus of all points in the motion configuration space with a distance less than or equal to dmin to pinitial, construct a pathway, where 2dmin is called potential canal width. The walker is assumed to move in a field of force; the integration of human force and an artificial potential force applied by brakes. The potential force is the gradient of an artificial potential field, set around the pathway. The instantaneous desired position of the walker on the pathway is an attraction pole and any point in the configuration space outside the pathway is a repulsion pole. The artificial potential field concept is originally used also for realtime modification of paths in order to avoid obstacle collision in [9] and [10]. With the assumptions above, the control of the walker can be achieved by subjecting it to an artificial potential field as follows: U (x) U path (x) (1) where ( x) ( x, y , T ) is the walker configuration. A polynomial of 3rd degree is chosen as the path potential function due to its continuity at initial and final values and smooth rise pattern. The potential function is described as follows, ° , if ( d d min ) ° 03 ° i U path (x) ®¦ ai (d d min ) , if (d min d d max ) (2) °i 0 ° 3 i , if ( d max d ) °¦ ai (d max d min ) ¯i 0 where ai
max f i (d min , d max , U path ) , d is the distance from walker
configuration to walker’s desired configuration on the initial path and dmax is the maximum distance for which the potential function value increases. fi are calculated by subjecting the potential function in eq.2 to the following conditions: max °U path (d min ) 0 U path (d max ) U path ® °¯U path (d min ) 0 U path (d min ) 0
(3)
U max path should be an appropriately high value in order to
restrict the walker inside the potential canal when a large force is applied by the user. A potential canal for the path generated by the path planner is illustrated in Fig.4. The
constrain control force to be applied by brakes is calculated by eq.4: F (x) grad [U path (x)] (4)
If an obstacle is detected closer than U max to the walker, the total potential value increases and the resultant force increases as a gradient function of total potential function.
800 700 600 500 y_g [cm]
increases. The total potential value at a point x of configuration space can be achieved as follows: (6) U total (x) U path (x) U obst (x)
Final Position
400
V. HUMAN ADAPTIVE MODIFICATION OF POTENTIAL CANAL One of the major deficiencies of conventional walkers is in lack of giving support and avoiding slippage and tumbling of its users. This problem becomes serious when the users do not have enough control on one or both of their lower extremities due to severe damage. In these cases, a complete control of the user over the walker seems ineffective. We present another modification method of potential canal in order to give support while walking and constrain abrupt movements of the walker. In its initial definition, potential canal width is a constant value. We modify this value as a realtime function of walker velocity. An excessive amount of walker velocity denotes a possible slippage or tumbling of the user, therefore potential canal width is decreased constraining the movements and giving support to the user. This can be presented as follows: d min f min (x ) (7) where fmin is a decreasing function with respect to walker velocity.
300
Initial Position
200 100
Generated Path
0 0
100
200
300 400 x_g [cm]
500
600
700
800
Fig.3 The map information and generated path (pinitial)
Fig.4 Potential Canal for pinitial
VI. EXPERIMENTS
IV. ENVIRONMENT ADAPTIVE MODIFICATION OF POTENTIAL CANAL Reactive modification of navigation route is one of the important issues on the reliability of a navigation method. The path generated according to the static map information of an indoor environment can still conduct collisions when dynamic features such as moving people exist in the environment. We propose a potential canal modification method which can guarantee collision-free motions of the walker. An artificial potential field is assigned to each obstacle on the pathway. The configurations on the pathway with distance shorter than U max to the obstacle are the repulsive poles, where U max represents the limit distance of obstacle potential field influence. A polynomial of 3rd degree is used as the obstacle potential function as follows: ° , if ( U max d U ) ° 30 1 1 ° U obst (x) ®¦ ai ( ) i , if ( U min U U max ) (5) U U i 0 max ° ° 3 1 1 i , if ( U d U min ) ° ¦ ai ( U U ) min max ¯i 0 where U is the shortest distance to the obstacle and U min is the minimum distance for which obstacle potential function value
180
Some experiments are performed in order to evaluate navigation quality and examine the effect of possible obstacles, existing inside potential canal. Potential canal width is varied in Exp.1 and 2 (Table I) and the walkers position is plotted in Fig.5. The position deviation from the initial path is clearly very small in the first experiment at the expense of discontinuous and rough motion pattern. The deviation is more when dmin is increased, while a good tracking pattern is achieved. The same experiment is executed in the presence of obstacle inside potential canal. The walker position in Exp.2 and 3 are plotted together in Fig.6 to clarify the difference in motion pattern. Collision is avoided and navigation is well achieved at the same time. Objects such as conventional chair or table are positioned on a sideway of potential canal as an obstacle, while potential canal width was a fixed value. It is clear that when an obstacle size is large enough to block the potential canal, the user either needs a large amount of force in order to circumnavigate the obstacle or the potential canal width is to be increased so that the user can drive the walker away from the obstacle. This problem is to be solved for the practical use in the future. Fig.7 shows the walker position when dmin is changed to a time-variant function of velocity. dmin and velocity are both plotted in figure 8. In Exp.4, dmin is a discontinuous variable
45
dmin, Exp.4
40
Velocity [cm/s], Dmin [cm]
of walker velocity, therefore when walker velocity exceeds a certain amount dmin value declines and break force is applied, which result in fast decline of walker velocity successively. A point of concern in the experiments above is that sensor can only detect the objects inside its view while a 3D scanning mechanism can be devised for safer control scheme. Since the experiments are performed initially to evaluate the control mechanism, scanning precision is not strictly taken into consideration at this stage.
Walker Velocity, Exp.4
35 30 25 20 15 10 5
Table1. Experimental Data Exp1 10 15 No No
dmin [cm] dmax [cm] Obstacle Existence dmin Variation
Exp2 30 60 No No
0
Exp3 30 60 Yes No
0
Exp4 30 60 No Yes
y_g [cm]
150
Walker Position, Exp.1 Initial Path
0 0
100
200
300 400 x_g [cm]
500
600
700
Fig.5 ORTW-II position in Exp.1 and Exp.2 300
Walker Position, Exp.2
250
Obstacl
y_g [cm]
200 150 100
Walker Position, Exp.3 50
200
300 400 x_g [cm]
500
600
700
Fig.6 ORTW-II position in Exp.2 and Exp.3 [1]
Walker Position, Exp.2
[2] [3]
200 y_g [cm]
[4] 150
[5]
Walker Position, Exp.4
100
[6] [7]
Initial Path
50
[8] 0
[9] 0
100
200
300 400 x_g [cm]
500
70
REFERENCES
300 250
60
The Authors would like to thank Mr. Amin Saeedfar and Mr. Oscar Chuy for their careful and supportive comments.
0 100
50
ACKNOWLEDGMENT
Initial P h 0
40 Time [s]
In this paper, an omnidirectional passive type walking aid system (ORTW-II) was introduced, composed of an omnidirectional platform, an onboard control unit and a frame with MR brakes as the only controllable units of it. Techniques mostly used in the field of robotics are adapted in order to increase the functionality and improve the performance of ORTW-II. A human-environment adaptive navigation method was proposed for indoor use. The walker was provided with a static map of environment and a conventional path planner was used to generate path from the initial to the final configuration of the walker. The path was used to initialize a pathway called potential canal. Walker motion outside potential canal was restricted by generating potential forces as a function of walker distance from the pathway. A modification method of potential canal was discussed in order to avoid collision by incorporating static and dynamic map information of the environment. Another modification of potential canal was discussed in order to avoid abrupt movements to the users with tendency to fall, as well as giving support while walking.
200
50
30
VII. CONCLUSION AND OUTLOOK
Walker Position, Exp.2
100
20
Fig.8 ORTW-II Velocity and dmin in Exp.4
300 250
10
600
700
[10]
Fig.7 ORTW-II position in Exp.2 and Exp.4
181
H. Yu, M. Spenko, S. Dubowsky, “An Adaptive Shared Control System for an Intelligent Mobility Aid for the Elderly”, Autonomous Robots 5, Vol. 15, pp. 53-66, 2003. B. Graf, “reactive Navigation of an Intelligent Robot Walking Aid”, Proceeding of ROMAN2001, pp. 353-358, 2001. S. McNamara, G. Lacey, “A Smart Walker for the Frail Visually Impaired”, Proceeding of the 2000 IEEE International Conference on Robotics & Automation, pp. 1354-1359, 2000. G. Wasson, J. Gunderson, S. Graves, R. Felder, “An Assistive Robotic Agent for Pedestrian Mobility”, International Conference on Autonomous Agent 2001, pp. 169-173, 2001. Yasuhisa Hirata, Asami Hara, Kazuhiro Kosuge, “Passive-type Intelligent Walking Support System “RT Walker””, Proceeding of 2004 IEEE/RSJ International Conference on Intelligent Robots and Systems, 3871-3876, 2004. Lord Corporation, http://www.lord.com/Default.aspx. Naemeh Nejatbakhsh and Kazuhiro Kosuge, “Passive Omnidirectional Walker -Design and Control-”, to appear in the proceedings of 12th Int. Conf. on Advanced Robotics, 2005. Arkin R.C., “Motor Schema-Based Mobile Robot Navigation”, The Int. Journal of Robotics Research, pp.92-112., August 1989. Oussama Khatib, “Real-Time Obstacle Avoidance for Manipulators and Mobile Robots”, The International Journal of Robotics Research Vol.5, No.1, pp.90-98, 1986. Sean Quinlan, “Real-Time Modification of Collision-Free Paths”, Ph.D. Thesis Dissertation, Department of Computer Science, Stanford University, 1994.
