Collision-Detecting Device for Omnidirectional ElectricWheelchair

An electric wheelchair is the device to support the self-movement of the elderly and people with physical disabilities. In this paper, a prototype design of an electric wheelchair with a high level of mobility and safety is presented. e electric wheelchair has a high level of mobility by employing an omnidirectional mechanism. Large numbers of mechanisms have been developed to realize omnidirectional motion. However, they have various drawbacks such as a complicated mechanism and difficulty of employment for practical use. Although the ball wheel drivemechanism is simple, it realizes stable motion when negotiating a step, gap, or slope. e high level of mobility enhances the freedom of users while increasing the risk of collision with obstacles or walls. To prevent collisions with obstacles, some electric wheelchairs are equipped with infrared sensors, ultrasonic sensors, laser range �nders, or machine vision. However, since these devices are expensive, it will be difficult for them to be widely used with electric wheelchairs. We have developed a prototype design of collision-detecting device with inexpensive sensors.is device detects the occurrence of collisions and can calculate the direction of the colliding object. A prototype has been developed to perform motion experiments and verify the accuracy of the device. e results of experiments are also presented in this paper.


Introduction
e reduced physical functions associated with aging or disability make independent living more difficult.Lower extremity function ability limits the scope to take part in vocational and educational opportunities and many also negatively affect self-esteem.If people with reduced physical functions cannot receive support, they may become bedridden.A wheelchair can compensate for a lower extremity function, by allowing users to move freely by themselves.An electric wheelchair is the device to support the self-movement of the elderly and people with physical disabilities.Previous research topics based on electric wheelchairs can be classi�ed into projects to develop increasing a high level of mobility and projects to add intelligent functions to wheelchairs.e conventional wheel-type mechanism needs to switch the drive when negotiating narrow spaces.An omnidirectional vehicle has no limits to its direction of motion and is expected to have a wide range of applications.Omnidirectional vehicles are an active area of research in robotics and a numerous mechanisms have been developed.To realize omnidirectional motion, vehicles so far have been equipped with an omniwheel consisting of a large number of free rollers [1] or a spherical ball wheel [2][3][4][5].Several vehicles have been developed for use as electric wheelchairs [5].However, they must be tested for practical use.
Various input methods are used with electric wheelchairs.e traditional input method is the joystick.Voice recognition [6] and eye-and head-tracking [7][8][9] have oen been used.Intelligent functions must be based on safety.However, the unintended motion of electric wheelchairs may be caused by user errors.To ensure the safety of users, many studies have focused on the avoidance of obstacles by detecting potential hazards in the local environment.e sensors that have been used by electric wheelchairs are ultrasonic sensors, infrared sensors, laser range �nders, and force feedback joysticks   [10][11][12][13][14][15][16].In recent years, machine vision systems have been developed that use an omnidirectional camera system [17,18], a �sheye camera system [19], or a stereo omnidirectional system [20] based on computer vision technology.ese systems cannot only help avoid collisions, but also enable the realization of additional intelligent functions such as autonomous and semiautonomous wheelchairs.However, there are still many problems to be solved.Because inexpensive and reliable systems are re�uired in the �elds of public health and welfare, as reported in this paper, we have developed an omnidirectional wheelchair with a ball wheel drive mechanism.As a measure to ensure its safety, we have also developed a collision-detecting device without expensive sensors.

Outline of Prototype
Photographs of the prototype are shown in Figure 1(a).e drive unit of the prototype employs a ball wheel drive mechanism with the following features: (1) the ball wheel is a suitable shape for omnidirectional motion and do not generate vibration or noise, and (2) a high level of ability for negotiating a step, gap, or slope.e input device of the prototype is a joystick using Arduino chipset.A battery (Ni-MH, 24 [V], 6.7 [Ahr]) and control unit are mounted on the body.e user can move in any direction by operating the joystick.
Figure 1(b) shows the prototype collision-detecting device on the body of the vehicle.e role of this device is not to avoid collisions but to detect them.Although various types of sensors and cameras have been used to improve collision avoidance performance, it is not realistic to install these expensive devices in welfare equipment such as electric wheelchairs.We proposed a new device that can physically detect a collision and contribute to improve operation safety.Collision detection is realized by installing a bumper around the wheelchair.When the bumper collides with an obstacle, the obstacles does not come in contact with the main body of the wheelchair.e bumper can move omnidirectionally in the horizontal plane with the vehicle body.e device can calculate the collision direction from the measured displacement (position and orientation) of the bumper from the initial position.To measure the displacement, the device is equipped with three potentiometers.Moreover, so that the bumper can rerun to its initial position, the device is equipped with three tension springs and dampers.e collision-detecting device is composed of low-cost sensors.

Ball Wheel Drive Mechanism
We have developed a holonomic omnidirectional vehicle with a simple mechanism consisting of three ball wheels and three actuators [2].e layout of the mechanical parts is shown in Figure 2.Each actuator can drive two rotors simultaneously by using pulleys and belts.e rotation of each ball wheel is supported by two rollers.e mechanism does not cause overconstraint, because the number of actuators is equal to the number of degrees of freedom of motion on a �at surface.
Photographs of the prototype omnidirectional vehicle with the ball wheel drive mechanism are shown in Figure 3 and the speci�cations of the vehicle are shown in Table 1.e ability to negotiate more difficult terrain steps, gaps, and slopes is con�rmed by motion experiments with an adult of approximately 56 kg in the wheelchair.e omnidirectional vehicle with the ball wheel drive mechanism was able to (1) overcome a step of 14 mm, (2) traverse a gap of 50 mm, and (3) climb a slope of 15 deg.ese abilities are necessary for practical use and allow the electric wheelchair to be used an indoor environment.A sensor unit consisting of a potentiometer, a tension spring, and a damper is installed on the frame, as shown in Figure 4(a).A rotary knob is attached to a link with a slit, as shown in Figure 5.When the bumper is moved by a collision, it causes the link to rotate around the pin.e arrangement of the link and pin is shown in Figure 4(b).Using each measurement value, the system can calculate the position and orientation of the bumper.Aer the collision, the bumper returns to the initial position, and the force for which is provided by a tension spring connected to the bumper and the vehicle body.To support the smooth movement of the bumper in the horizontal plane, a free caster is installed on each short side of the bumper.e free caster rotates passively on the vehicle chassis to realize the smooth motion of the bumper.

Kinematics.
We de�ned the �xed coordinate Σ  of the vehicle as shown in Figure 6, in which  1 and  2 are the distances from the vehicle center o  to the pin and potentiometer, respectively.(  ,   ) and (  ,   ) in Figure 7 When it is assumed that  is sufficiently small, sin  ≒ , cos  ≒ 1. Aer rearranging these relationships, we obtain where, We can calculate the bumper movement       , which is in the direction of the collision, using 4.3.Accuracy of the Device.In this section, we describe a set of experiments conducted to con�rm the accuracy of the collision-detecting device.We intentionally applied an external force to the bumper from the translational and rotational directions.We compared the true and calculated displacement of the bumper using (7) for three directions.Note that the maximum of movement range are ±18.5 mm in the front-back direction, ±20 mm in the horizontal direction, and ±7.5 deg in the rotational direction.Figure 8 shows the movement of the bumper when an external force is applied to the bumper in the following cases: e experimental results are shown in Figures 9-11.In these experiments, we applied an external force in particular direction.Ideally, the displacement in the other directions is zero.However, as the movement range increases, the displacement in other directions also increases.e desired directions, as shown in Figures 9(a), 10(b), and 11(c), exhibit satisfactory accuracy when the movement is small.e maximum errors are ±2.5 mm in the front-back direction, ±6.0 mm in the horizontal direction, and ±2.5 deg in the rotational direction.e problem is that the link is passivelydeformed when the movement is big.To reduce the error, we should change the link material from MC nylon to a metal.

Conclusion
In this study, we developed a prototype with the aim of realizing high mobility and safety.e developed wheelchair is an omnidirectional vehicle with a high level of ability when negotiating a step, gap, or slope.Collision detection is realized by installing a bumper around the vehicle.When the bumper collides with an obstacle, the obstacle does not come in contact with the main body of the vehicle.e collisiondetecting device can calculate the collision direction from the measured displacement (position and orientation) of the bumper from the initial position.e collision-detecting device is composed of low-cost sensors.In future work, we will implement the collision detecting device in an electric wheelchair and add an intelligent function to the wheelchair such as an operation assistance system.

F 1 :
Photographs of the prototype.

F 3 :F 4 :F 5 :
Whole view of ball wheel drive mechanism.Sensor unit of collision detecting device.Detecting of the displacement.

3 F 6 :F 7 :
Layout of the sensor unit.Bumper movement by collision.

4. 1 .
Concept of Collision-Detecting Device.e basic part of the collision-detecting device is the frame surrounding the vehicle body.Each potentiometer in the device is arranged on the long side of the bumper and can measure different directions.
which is the origin of the vehicle coordinate.erotation angles   are formed by pin  and the vehicle coordinate.Here, counterclockwise rotation is assumed to be positive.epositions of pin  and potentiometer  are as fellows:  ,      1 cos   ,  1 sin      ,      2 cos   ,  2 sin    .
(a) designate the center positions of each pin and potentiometer, respectively (  1, 2, ).First, to evaluate the motion of the bumper, we de�ne the initial position.�e�neΣasthe�xed coordinate of the bumper.In the initial condition, o  coincides with o  -axis, :   -axis, orientation : o  ).e center of each potentiometers (  ,   ) is given by   ,       cos     sin   ,   sin     cos    .(2)Let     be the rotation angle from the   -axis to the line segment joining the pin and potentiometer center.Here,   is the sum of the bumper rotation angle  and potentiometer rotation angle   .e relationships between   and   are tan                  .