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Platform design for the intelligent robotic wheelchair

Authors: Yeh-Liang Hsu, Po-Er Hsu, Chau-Heng Tu, Chia-Hung Lu, Yan-Wei Chen (2010-08-22); recommended: Yeh-Liang Hsu (2010-08-22).
Note: This paper is presented at the 25th World Electric Vehicle Symposium and Exposition (EVS 25), Nov 5-9, 2010, Shenzhen, China.

Abstract

In recent years, research in electrical wheelchairs has been emphasizing on how to implement the sensing and judgment capabilities commonly seen on robots, so that the wheelchairs can perform autonomous behaviors to a certain extent in order to improve its usability. This paper is the 2nd paper of a sequence of paper describing the design concepts and current progress of an on-going research project conducted in the Gerontechnology Research Center in Yuan Ze University, Taiwan, in constructing an intelligent robotic wheelchair (iRW), which intends to redefine the wheelchair as the center of mobility, everyday living and healthcare of the senior users, based on the concept of robotic wheelchairs. This paper describes the design of the moving platform of iRW. The iRW is to be used in the home or nursing home. Nimble maneuver ability in the crowded indoor environment is the basic requirement for iRW. The seat mechanism is required to have high rigidity and versatile adjustment capability for multiple transfer assists. To answer these design requirements, the design concept of the platform of the iRW is basically “a nimble moving parallel robot in the form of a chair”. The platform is mainly consisting of a Mecanum-wheeled moving vehicle and a seat adjusting mechanism resembling a Stewart platform. The design concept and technical details of platform are described.

Keywords: robotic wheelchair, Mecanum wheel, Stewart platform.

1.    Introduction

The rapidly aging society has brought an increasing demand in living support and healthcare. Lower limb disability is one of the most common disabilities among seniors, and wheelchair is the most common and important mobility aids. Electrical wheelchair is an option for senior users who cannot move themselves on a wheelchair. However, operating an electrical wheelchair is often difficult for senior users. In recent years, research in electrical wheelchairs has been emphasizing on how to implement the sensing and judgment capabilities commonly seen on robots, so that the wheelchairs can perform autonomous behaviors to a certain extent in order to improve its usability. Miller and Slack [1] first used the term “robotic wheelchair” in their research. They applied various sensing and navigation technologies that are often used in robotics and built two prototypes of robotic wheelchairs, which were able to assist users to pass through narrow paths and avoid obstacles. Since then, many research projects have been carried out to develop robotic wheelchairs or “smart wheelchairs”. Simpson presented a survey of related research projects through 2005 [2].

This paper is the 2nd paper of a sequence of paper describing the design concepts and current progress of an on-going research project conducted in the Gerontechnology Research Center in Yuan Ze University, Taiwan, in constructing an intelligent robotic wheelchair (iRW), which intends to redefine the wheelchair as the center of mobility, everyday living and healthcare of the senior users, based on the concept of robotic wheelchairs. Figure 1 describes the overall design concept of the iRW developed in the study. Centered on the needs of the senior users, the core of the iRW is a user interface designed specifically for the senior users’ declining ability in perception, motor control and cognition, a moving vehicle. The iRW is composed of a moving vehicle, a sensing and control module, and an information/communication module, to provide the senior users three types of functions: mobility, everyday living, and healthcare. The iRW is expected to assist the senior users to interact with the environment more effectively, including physical interaction, environmental control, information exchange, and most importantly, interpersonal communication. The final aim is to enhance independence and social participation, and to improve the quality of life of the senior users.

Figure 1. Design concept of the iRW

This paper describes the design of the moving platform of iRW. The iRW is to be used in the home or nursing home. Nimble maneuver ability in the crowded indoor environment is the basic requirement for iRW. The seat mechanism is required to have high rigidity and versatile adjustment capability for multiple transfer assists. To satisfy these requirements, Mecanum wheel and a parallel mechanism resembling a Stewart platform were used in the iRW.

The rest of the paper is organized as follows. In Section 2, the design concept of platform, the integration of a Mecanum-wheeled vehicle and a seat adjustment mechanism based on Stewart platform, is described first. Section 3 and 4 describe the technical details of the platform. Finally, Section 5 describes the performance and specification of the platform to conclude the paper.

2.    Design concept of the moving platform of the iRW

There are several important design requirements for the moving vehicle of the iRW:

(1)  Nimble maneuverability is of the highest priority for iRW to be used in the crowded indoor home environment.

(2)  The iRW should be capable of minor movement adjustments in all directions for the senior users to adjust their positions when approaching table, bed, toilet, etc.

(3)  The seat mechanism of the iRW should have multiple degrees of freedom in seat adjustments needed in various situations encountered in everyday living, including comfortable seating positions and transfer assists from wheelchair to bed, toilet, etc. In particular, transfer assist often requires the seat to be able to move vertically and sideways simultaneously.

(4)  Light weight and high stiffness are also desired for the structure of the moving vehicle of the iRW.

To answer these design requirements, the design concept of the platform of the iRW is basically “a nimble moving parallel robot in the form of a chair”. The platform is mainly consisting of a moving vehicle and a seat adjusting mechanism. Most electrical wheel chairs use 2 driving wheels, and control the rotational speed of the 2 driving wheels for moving forward/backward and turning. In order to achieve nimble maneuverability, the moving vehicle of the iRW employs 4 Mecanum wheels, which is a type of omni-directional wheels. The Mecanum-wheeled vehicle has a zero radius of gyration, and can move in all directions (including moving sideways) by controlling the direction of rotation of the 4 Mecanum wheels. This technique is generally used in stacker in narrow warehouse, and has also seen applications in electrical wheelchairs, as shown in Figure 2.

Figure 2. Applications of Mecanum-wheeled platform [http://en.wikipedia.org/wiki/Mecanum_wheel]

Seat adjustment mechanisms in electrical wheelchairs (or any type of chairs) often put actuators in the required degree of freedom based on a fixed structure. Sideway adjustment of the seat is not commonly seen in electrical wheelchairs.

Stewart platform is a parallel structure robot which has the advantages of high stiffness and high positioning accuracy compared to the serial structure robots. The geometry of this parallel robot, illustrated in Figure 3, is composed of a fixed base, a movable platform, and 6 variable length actuators connecting the fixed base and the movable platform. This is a 6 degrees-of-freedom universal-prismatic-spherical mechanism, including heave, surge, sway, yaw, pitch, and roll. No additional structural members are needed in Stewart platform because the actuators also perform as structural members.

Figure 3. Schematic diagram and degrees-of-freedom of parallel robot

Based on the concept of the Stewart platform, Figure 4 shows the design concept of the seat adjustment mechanism integrating with the Macanum-wheeled moving vehicle of the iRW. As shown in Figure 4, the seat plate is the movable plate of the Stewart platform, and the L-shape structure of the moving vehicle is the fixed plate of the Stewart platform. The number of linear actuators is reduced from 6 to 4 because only 4 degrees of freedom of seat adjustments are required. Note that two linear actuators are in tension and the other two are in compression, which results in an unsymmetrical work space required for seat adjustment. The 4 Mecanum wheels and motors are installed at the corners of the chassis. A joystick-type controller is used to control the iRW to move forward/backward, sway right/left, and spin clockwise/counter clockwise.

Figure 4. Design concept of the moving platform of the iRW

3.    Mecanum wheel platform design of the iRW

A Swiss inventor Bengt Ilon invented Mecanum wheel in a company which was named Mecanum in 1973. His idea was to invent a car which can move in all directions with 4 Mecanum wheels. The outer ring of every Mecanum wheel has free rollers, which make a 45 degree angle with the wheel’s axis, as shown in Figure 5. In this design, the free rollers touch the ground, and the ground gives 45 degree friction to the Mecanum wheel when it rotates. The 4 Mecanum wheels in a vehicle can be divided into 2 groups, the front and the back. The angles of the free rollers of the Mecanum wheels should be arranged as shown in Figure 6.

Figure 5. Free rollers in a Mecanum wheel make a 45 degree angle with the axis[3]

Figure 6. Arrangement of Mecanum wheels in a vehicle

Figure 7 shows the direction of the friction forces acting on the 4 Mecanum wheels (T1, T2, T3, T4). By controlling the individual wheels to rotate forward or backward, the Mecanum-wheeled vehicle can move forward/backward (Figure 7(a) and (b)), sway left/right (Figure 7(c) and (d)), and spin clockwise/counter clockwise (Figure 7(e) and (f)).

       

(a) Move forward                                  (b) Move backward

    

(c) Sway left                                   (d) Sway right

 

(e) Spin clockwise                                          (e) Spin counter clockwise

Figure 7. Direction of the friction forces

Table 1 shows the specifications of the 4 Mecanum wheels and motors used in the moving vehicle of the iRW. Control of the moving vehicle is achieved by an analog joystick WJ-200 by DeviceMart. As shown in Figure 8, the stick can be rotated, and swung to left, right, forward and backward. There are 3 variable resistors in the joystick to reflect the movement of the stick in these 3 directions, which in turn control the motors of the 4 Mecanum wheels thru a motor controller.

Table 1. Specifications of the 4 Mecanum wheel and motor

Mecanum wheel

Load

200 kg

motor

Voltage/Current

24V/3A, Max.

Radius

8 inch

Speed

4900 rpm

Number of free roller

16

Reduction ratio

25:1

Weight of motor

3 kg

 

Figure 8. WJ-200 Joystick[http://www.devicemart.co.kr/mart7/]

For safety reasons, the maximum forward speed of the moving vehicle is set at 40cm/sec. The maximum speed moving backward and sideways is set at 10cm/sec. In our evaluation, the Mecanum-wheeled moving vehicle can be easily maneuvered in a crowded environment to pass through narrow paths. It can also easily climb up the wheelchair ramp and over small obstacles such as a door sill. A 24V lithium iron phosphate battery is used on the iRW. It can last for about 2 hours in continuous operation of the Mecanum wheel based moving vehicle, and can be fully charged in about 1 hour.

4.    Stewart platform seat mechanism design of the iRW

Stewart platform is a parallel structure robot which has the advantages of high stiffness and high positioning accuracy compared to the serial structure robots. According to the definition of Stewart platform by Dasgupta and Mruthyunjaya[4]

“The generalized Stewart platform consists of two rigid bodies (referred to as the base and the platform) connected through six extensible legs, each spherical joints at both ends or with spherical joint at one end and with universal joint at the other.”

Referring to Figure 4, the iRW uses 4 variable length actuators to design the seat adjustment mechanism as a four-axis Stewart platform. Table 2 shows the specification of the linear actuators. The parallel mechanism with carefully designed joints creates 4 degrees of freedom in the seat adjustment of iRW, including heave, surge, sway and pitch. In order to enhance the stability of the seat, a locking-mechanism is designed to constrained uncontrolled degree of freedoms (surge and sway) of the seat.

Table 2. The specification of the linear actuator

Tensile force

Thrust force

Self-locking force

Extension

Speed

Current

Resolution

1200

(Max, N)

1200

(Max, N)

800

(Max, N)

150

(mm)

12

(mm/s)

2.5

(A, Max)

0.3175

(mm/pulse)

Figure 9 shows the detail coordinates of the joints of the seat adjustment mechanism. In our current prototype, the range of the seat adjustment in the heave-and-surge degree of freedom is 13 cm (± 6.5cm from the initial position). The range of the seat adjustment in the sway degree of freedom is 26 cm, and the range of seat adjustment in the pitch degree of freedom is 30 degrees. Figure 10 and Table 3 shows the workspace of the seat adjustment mechanism. With this parallel mechanism, iRW provides adjustment capability for multiple transfer assists, as well as postural adjustment for seating comfort.

Table 3. Feasible workspace of the seat adjustment algorithm at the center of the seat

Coordinates of the center of the seat (x, y, z)

Range of sway distance

Range of pitch angle

(0, 0, 0)

130mm

-15o~15o

(-5, 0, 10), (5, 0, -10)

125mm

-15o~15o

(-10, 0, 20), (10, 0, -20)

115mm

-15o~15o

(-15, 0, 30), (15, 0, -30)

110mm

-10o~15o

(-20, 0, 40), (20, 0, -40)

90mm

-5o~10o

(-25, 0, 50), (25, 0, -50)

60mm

-5o~5o

(-30, 0, 60), (30, 0, -60)

20mm

0

(-30, 0, 65), (30, 0, -65)

0mm

0

Figure 9. The detail coordinates of the joints of the seat adjustment mechanism

Figure 10. Feasible workspace of the seat adjustment algorithm at the center of the seat

Control strategy of the seat adjustment mechanism also has to be carefully considered. There are two position control schemes for Stewart platform. One is forward kinematics which defines the length of each variable length actuators to manipulate the position of movable platform. The other is inverse kinematics which defines the position and orientation of movable platform to manipulate length of each variable length actuators. However, both control schemes are not suitable to be an intuitive user interface in this application. Moreover, in addition to reaching the desired end positions, special attention is paid to maintaining the orientation of the seat plate, so that the senior user can maintain a safe and comfortable position during the transferring process.

The workspace in Figure 10 is digitized into 2,028 discrete positions. Feasible paths on these positions are planned first, following the 3 rules below:

(1)     Beginning the vertical adjustment, sway distance and pitch angle of the seat has to be zero.

(2)     Beginning the horizontal adjustment, pitch angle of the seat has to be zero and highest position of the seat has to be maintained.

(3)     Beginning the pitch adjustment, the sway distance has to be zero.

For example, if the seat is moving from the initial position to point A in Figure 10, it has to pass through all intermediate positions along the path to assure that the seat maintains in a safe and comfortable position during the transferring process. It the seat is moving from the initial position to point B in Figure 10, it has to lift to the same height as point B, then move horizontally to point B. The discrete positions (and the corresponding lengths of the actuators) are stored in the controller. These rules assure that there is a unique path for moving to a certain position.

5.    Conclusions

Table 4 shows the functions and specifications of the first prototype of the iRW developed in this project. This prototype is currently under intensive testing for functions. Human factor evaluations and industrial design are also being carried out. After the functional tests are completed, a field test of the iRW will be conducted in a nursing home to get feed back from the senior users and further improve its usability.

Table 4. Functions and specifications of the first prototype of the iRW

Function

Specification

Moving vehicle

forward speed

40cm/s

Backward/sway/rotate speed

10cm/s

Adjustment of comfort

Height of the seat

(Distance from the bottom of the seat to the ground)

Highest position: 498mm

Initial position: 433mm

Lowest position: 368mm

Adjustment of transfer assist

Stand assist

+15°(Pitch)

Back/hip pressure variability

+15°~ -15°(Pitch)

Transfer assist

130mm (left/right)

C-LiFePO4 Battery

24V, 9.6Ah, 402×133.5×71.5 (mm, L×W×H), 3.4 kg

References

[1]     Miller, D., Slack, M., 1995. “Design and testing of a low-cost robotic wheelchair prototype,” Autonomous Robots, v. 2, pp. 77-88.

[2]     Simpson, R. C., 2005. “Smart Wheelchairs: A Literature Review,” J. Rehabilitation Res. & Dev., v. 42, n.4, pp. 423-438.

[3]     Jefri, E. M. S., Mohamed, R., Sazali, Y., Abdul, H. A., Mohd, R. M., 2005, “Designing Omni-Directional Mobile Robot with Mecanum Wheel,” American Journal of Applied Sciences , v. 5, pp. 1831-1835.

[4]     Dasguptaa, B. and Mruthyunjayab, T. S. 2000. “The Stewart platform manipulator: a review”, Mechanism and Machine Theory, v. 35, pp. 15-40.