IEEE Robotics & Automation Magazine - December 2017 - 38

is decided by points of completed tasks and the time taken to
achieve set goals. The device in this article was used for the
powered wheelchair category at the Cybathlon and won
fourth place in this competition.
Mechanisms can have high-mobility performance in
rough terrain, and the major mechanisms are the wheels,
crawler, leg, and combined mechanisms, and each mechanism type has pros and cons (Table 1).
Many studies have been conducted, and one of the
examples for combined mechanisms is the wheeleg robot
that has two front arms and two rear wheels [5]. Recent
study trends for the combined mechanism include a wheel
with a simple leg or transformable wheel [6]-[9], and they
describe a wheel mechanism with simple legs similar to a
claw [6]. In contrast, [7] and [8] propose a wheel mechanism that can transform into a leg, while [9] uses an amphibious locomotion using flipper legs.

Table 1. The strengths and limitations of the
mechanisms for rough terrain.
Wheel

Strength

Limitation

High speed capability,
high energy efficiency,
simple mechanism

Always in contact with
the ground

Crawler Higher capability for
Low speed capability,
rough terrain than wheel low energy efficiency
mechanism
Leg

The highest capability for Complicated mechanism,
rough terrain because
low speed capability,
the supporting point of low energy efficiency
a leg can be selected
separately

RT-Mover-Type Vehicle
Small Number
of DoF
Simple
Mechanism
- Reliability
- Cost

Premise:
- Four-Wheel Vehicle Like an Automobile
- Four-Wheel Drive for Mobility
Necessary Function:
- Steering
- Leg Motion
- Pitching and Rolling of the Seat

Main Target:
Paved Road
-> Wheel Drive

Subtarget:
Step, Gap, Stairs Stability
Comfort
(Barriers)
-> Seat Leveling
-> Leg Motion

Figure 1. The concept of the development of a personal mobility
vehicle for daily life.

IEEE ROBOTICS & AUTOMATION MAGAZINE

DECEMBER 2017

In the wheelchair field, many mobile mechanisms have
been proposed, such as the following [10]-[15]: the PerMMA Gen II robotics wheelchair, which can climb up a step
[10]; a wheelchair that consists of front and rear clusters
with four wheels [11]; a stair-climbing mobility system,
which combines four wheels and two sliding support
mechanisms for traction control to overcome stairs [12];
and a method to climb stairs by using laser distance sensors
[13]. In addition to the research field, there are commercial
wheelchairs with high-mobility performance, such as TopChair [14] and iBot. A user report for the iBot can be found
in [15], and the report states that the iBot has a standing
mode. By using this mode, a user's eye position is the same
as the eye level of the surrounding people so they can have
natural communication with the user. We hope that if people with and without disabilities use the same PMV vehicle,
they all can share the experience and understand each
other more, which is why we aim to develop PMVs that are
attractive to everyone.
For developing that kind of PMV, we should focus on
energy efficiency and speed in the drive mode. Figure 1
shows the concept of development for our device. The terrain corresponding to our daily activities is mainly paved
roads and partial steps, slopes, and stairs, i.e., barriers.
Therefore, the main mechanism should be a wheel because
of its high energy efficiency and high-speed capability. However, the PMV needs an additional mechanism for rough
terrain. Table 1 shows that the leg mechanism has the highest adaptability for rough terrain, because the leg can select
its contact point based on the terrain surface. Regarding
performance on rough terrain, while conventional vehicles
[10]-[15] have high-mobility performance, in the case of
steps or stairs, they must climb over them at a right angle
toward the front of the steps or stairs and not obliquely. In
the real world, it is very convenient for a PMV to be able to
climb a step not only at a right angle toward the front of the
step but also at various angles, i.e., obliquely. The proposed
device in this article has this feature to advance over a step at
various angles.
For a real-world device, stability, reliability, and cost are
important matters, and a simple mechanism is better to
address these issues. In consideration of this, the concept of
our device is as follows:
● The main mechanism is based on a wheel mechanism.
● The wheel mechanism has a leg function.
● The leg function is for one wheel to get over an obstacle to
cross a step obliquely.
● A seat is always kept horizontal even on rough terrain for
stability and comfort.
● The number of actuators should be small for simplicity of
the mechanism.
The main mechanism was discussed in [16], and the
smallest number of degrees of freedom (DoF) was introduced for achieving the previously described concept.
However, the number of DoF is still larger than the number
of DoF for commercial powered wheelchairs. In addition,

Table of Contents for the Digital Edition of IEEE Robotics & Automation Magazine - December 2017