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AuthorsPo-Er Hsu (2013-01-28); recommended: Yeh-Liang Hsu (2013-02-10).
Note: This article is the Chapter 2 of Po-Er Hsu’s doctoral thesis “Development of an intelligent robotic wheelchair as the center of mobility, health care, and daily living of older adults.”

Chapter 2. Mobility assistance design of the intelligent robotic wheelchair

This chapter describes the mobility assistance design of the iRW. Maneuverability is of the highest concern when designing the omni-directional vehicle of the iRW. The omni-directional vehicle uses four Mecanum wheels to facilitate movement in all directions, in particular, the sideways movement which cannot be achieved by general electric wheelchairs. The efficiency of space utilization is discussed in this research for the iRW with the capability of moving sideways and zero radius of rotation.

2.1  Omni-directional vehicle of the iRW

The iRW is designed for home or nursing home use, mostly indoor or short-distance outdoor (such as taking a walk in the garden). Maneuverability is of the highest importance when designing the omni-directional vehicle, so that the iRW can be used in crowded indoor environments. Most electric wheelchairs are two-wheel drive and cannot move sideways, which was the main reason that Mecanum wheels were chosen for the omni-directional vehicle of the iRW.

Figure 2-1 shows the Mecanum wheels, which were invented by Swedish engineer Ilon in 1973 [Ilon, 1973]. Around the wheel’s tread are several barrel-shaped free rollers arranged at an angle of 45 degrees to the main axis, which enable omni-directional movement without using a classic steering mechanism. Mecanum wheels are commonly used where maneuverability is necessary in tight spaces, as with autonomous forklifts. Mecanum wheels also see applications in robots [De Villiers et al., 2012; Tsai et al., 2010] and electric wheelchairs [Hoyer et al., 1999]. One disadvantage of the Mecanum wheel is its energy consumption. Due to the rotation of the exterior rollers, only a component of the force at the perimeter of the wheel is applied to the ground [Adascalitei et al., 2011]. The other shortcoming is that Mecanum wheels are susceptible to slippage. As a result, with the same amount of wheel rotation, lateral travelling distance may differ from longitudinal travelling distance, according to the ground condition [Nagatani et al., 2000].

Figure 2-1. Free rollers in a Mecanum wheel at 45 degrees to the main axis

The omni-directional vehicle of the iRW uses four Mecanum wheels, one on each corner of the chassis, driven by four DC motors. Figure 2-2 shows how the components of the force vectors at every wheel sum up or eliminate each other by rotating either clockwise or anticlockwise to obtain the desired moving direction.

On the iRW, each Mecanum wheel is driven by one standard 24V DC Motor (3A, max) with a maximum speed of 4900 rpm. The motor control is realized by a central microcontroller (Arduino Mega 2560) and one full bridge motor driver (VNH3SP30) for each motor. The motor driver enables clockwise and anticlockwise rotation as well as rotational speed control by using high / low and pulse width modulation (PWM) signals sent from the microcontroller at a constant frequency of 490 Hz.

Table 2-1 shows the specifications for the omni-directional vehicle of the iRW. In contrast to the joystick control of commercial electric wheelchairs, which can move a vehicle forward / backward or turn it left / right, the joystick of the iRW can also maneuver sideways to the left / right or rotate clockwise / counterclockwise. When older adults use manual or power wheelchairs indoors, there is an inverse relationship between age and preferred movement speed [Tolerico et al., 2007; Karmarkar et al., 2011]. Tolerico et al. [2007] found that the speed of manual wheelchair users is 0.79 ± 0.19 m/s, and further Karmarkar et al. [2011] indicated that the speeds of manual and electric wheelchair users are 0.64 ± 0.13 m/s and 0.7 ± 0.3 m/s. Based on these results, the maximum forward speed of the joystick mode of the iRW is set at 50 m/min, which is close to walking speed of older adults, and the maximum backward speed and sideways speed are set at 40 m/min and 25 m/min, respectively. To foster operating safety and promote user comfort, teleoperation and indoor navigation modes are set at the constant speed 15 m/min, which is below the lowest manual operation speed suggested by Karmarkar et al. [2011].

Figure 2-2. Combined force vectors in a set of Mecanum wheels

Table 2-1. Specifications of the omni-directional vehicle of the iRW

Characteristic

Specification

L×W×H

800×610×870 mm

Forward speed

50 m/min (Max)

Backward speed

40 m/min (Max)

Sideways movement / Clockwise / Counterclockwise speed

25 m/min

Teleoperation / Indoor navigation speed

15 m/min

Mecanum wheel diameter

25.4 mm (10 inches)

Voltage / Current of the motor

24V / 3A (Max)

C-LiFePO4 Battery

24 V, 9.6 Ah, 3.4 kg,

400×133×72 (L×W×H, mm)

2.2      Mobility assistance design for different operators of the iRW

Figure 2-3 shows a prototype of the iRW. Three “operators”, the wheelchair user, caregivers, and the iRW itself, are considered in the mobility assistance design for the iRW. Five operation modes are developed:

Figure 2-3. Prototype of the iRW

(1)     Obstacle avoidance

When the iRW’s ultrasonic sensors detect an obstacle within a specific distance (10 cm), the iRW stops all motion and alerts the user with a beeping sound. The user can override the stopping command using joystick mode or handlebar mode.

(2)     Joystick mode

The joystick mounted on the right armrest of the iRW is the main operation mode used by the wheelchair user. The joystick includes three variable resistors that detect the movement of the joystick forward / backward to produce movement in that direction, left / right to produce sideways movement in that direction, or rotation clockwise / counterclockwise to produce rotation in that direction. Turning movement can be produced if the variable resistors detect forward and left / right movements of the joystick simultaneously. Releasing the joystick will cause the iRW to immediately stop.

(3)     Handlebar mode

The handlebar mode is designed for the caregiver who intends to push the iRW. The diameter of the Mecanum wheels makes the iRW more difficult to push than a manual wheelchair. To enable the caregiver to push the iRW in an intuitive way, three soft pressure-sensing units are implemented on the handlebar of the iRW, as shown in Figure 2-3. The caregiver can push the iRW to move forward, move backward, turn right, or turn left by applying pressure to the appropriate pressure units. If equal pressure is applied on the right and left units, the iRW will move forward; if higher pressure is applied on the right / left units, the iRW will turn right / left. When pressure is applied on the middle pressure unit, the iRW will move backward. Pressing a button on either side of the handlebar will cause the iRW to move sideways in the direction of that button.

(4)     Teleoperation

Teleportation mode is designed for the caregiver who intends to operate the iRW from a remote site. In teleoperation mode, the camera of the tablet mounted on the iRW’s armrest captures its environment, and a remote-control interface enables a caregiver at a remote site to view it. The caregiver can then operate the iRW by sending commands via the Internet to the tablet, which relays the commands to the central microcontroller via Bluetooth to guide the iRW to move to the desired location.

Figure 2-4 shows the user interface of teleoperation mode. The teleoperation user interface has seven operation commands to control the movement of the iRW: moving forward or backward, sideways to the right or left, rotating clockwise or anticlockwise, and stopping. Note that there is no command for turning right / left in teleoperation mode. Turning right / left is achieved by the series of commands “stop,” “rotate 90°,” then “move forward.”

Figure 2-4. The teleoperation user interface

(5)     Indoor navigation

A semi-autonomous indoor navigation mode uses a concept similar to automated guided vehicles (AGVs) to reduce the operation load of the wheelchair user or care givers. QR code (Quick Response code) labels are deployed on the ceiling as the “virtual AGV track.” When the iRW is steered under the track, the camera on the iRW will capture and recognize the QR code. Subsequently, the information conveyed by the QR code can be interpreted so that the iRW follows the virtual AGV track to move to the specific stop, such as bedroom, living room, or kitchen. The user then takes control to steer the iRW to the final location. The user interface and indoor navigation algorithm are implemented as an App in the tablet. Details will be described in Chapter 3.

Man-machine collaborative control is reflected in the assignment of three “operator priorities” (in descending sequence): the wheelchair user, caregivers, and finally the iRW itself. The functional test performed in this research compared the operational efficiency of the five operation modes.

Man-machine collaborative control of the iRW is reflected in the assignment of three “operator priorities” (in descending sequence): the wheelchair user, caregivers, and finally the iRW itself, to avoid different modes’ disrupting each other and fosters safety. Following this principle, the five operation modes are assigned the following precedence (in descending order): obstacle avoidance, joystick mode, handlebar mode, teleoperation, and indoor navigation. Emergency modes such as obstacle avoidance might be needed on the shortest notice and therefore were assigned the highest priority. The manual control modes give the highest control priority to the wheelchair user (joystick mode), next highest to the caregiver at the local site (handlebar mode), then to the caregiver at the remote site (teleoperation mode). Indoor navigation mode receives the lowest priority. The higher-priority modes can interrupt the lower-priority ones. Figure 2-5 illustrates the control scheme of the five operation modes.

Figure 2-5. Flow chart showing priority and processing of the operation modes

2.3      Space utilization assessment

According to the Wheelchair Skills Test (WST), “90° left / right turn,” “turns 180° in place,” and “maneuvers sideways” are the skills directly related to changing moving directions of the wheelchair for daily needs [Kirby et al., 2008; Fliess-Douer et al., 2010]. The turning space is used here to assess and compare space utilization of the iRW with that of general electric wheelchairs.

The specifications for 16 models of electric wheelchairs commercially available in Taiwan were examined. Their average size was 910×640 mm (L×W) and the turning radius ranged from 500 mm to 800 mm. The size of the iRW is 800×610mm (L×W) and its turning radius is 0 mm. Additionally, the iRW can move sideways, unlike the 16 electric wheelchairs.

Figure 2-6 and Table 2-2 show the details of the calculation of the turning space required by the electric wheelchairs for “90° left / right turn” and “turns 180° in place.” For “turns 180° in place,” an electric wheelchair may need a space up to 2m×2m. In contrast, the iRW can make a 180° turn within a 1m×1m space (actually, in a circle of diameter 1 m).

Figure 2-6. The turning space of commercial electric wheelchairs and the iRW

Table 2-2. The calculation of the turning space

 

Turning

Turning radius (mm)

Dimensions of turning space (L×W, mm)

Area of turning space (mm2)

Commercial electric wheelchairs

90°

500

1,730×1,730

1,667,450

800

2,030×2,030

1,969,280

iRW

0

Circle of diameter 1m

785,400

Commercial electric wheelchairs

180°

500

1,640×1,730

2,170,100

800

2,240×2,030

2,773,300

iRW

0

Circle of diameter 1m

785,400

“Transfer activities” involve moving to the transfer target and displacing the user’s body from one surface to another, such as from the wheelchair to a treatment table, a regular bed, a tub / shower bench, a toilet, or a car seat and vice versa [Fliess-Douer et al., 2010; Bromley, 1998]. The capability of the iRW to move sideways allows it to use much less space for transfer activities than do general electric wheelchairs.

Figure 2-7 is the enlarged individual toilet room for barrier-free lavatory recommended by the “Americans with Disabilities Act Accessibility Guidelines (ADAAG) for buildings and facilities.” [2003] The turning space is defined as a circle of minimum diameter 60 in. (1,524 mm) for wheelchairs to turn 180°, designated by the outer dashed circle, which is smaller than the turning space required by electric wheelchairs in Table 2-2. Figure 2-7, the standard positions for toilet transfer are position 1 (the side approach) and position 2 (the reverse diagonal approach). Position 3 is for using the sink. The minimum clear floor space is 1,524×1,422 mm2 for toilet, and 762×1,219 mm2 for sink. As shown in Figure 2-6, the turning space of the iRW is a circle of diameter 1 m. It can be easily turned and moved from one position to the other, for example, sideways directly from position 2 to position 3.

Figure 2-7. The recommended enlarged individual toilet room for barrier-free lavatory

References

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