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Review Article
Electric-powered wheelchair
with stair-climbing ability
Weijun Tao
1
, Junyi Xu
1
and Tao Liu
2
Abstract
As an autonomic and convenient assistance device for people with disabilities and the elderly climbing up and down stairs,
electric-powered wheelchairs with stair-climbing ability have attracted great attention in the past two decades and some
various electric-powered wheelchairs with stair-climbing were developed. By using the developed electric-powered
wheelchairs with stair-climbing, many patients with walking difficulties are able to descend the stairs conveniently to
participate in outdoor activities, which are beneficial to both their physical rehabilitation and mental health. In this article, a
review of electric-powered wheelchair with stair-climbing current technology is given and its future tendency is discussed
to inform electric-powered wheelchair with stair-climbing researchers in the development of more applicable and popular
products. Firstly, the development history is reviewed and electric-powered wheelchairs with stair-climbing are classified
based on an analysis of their stair-climbing mechanisms. The respective advantages and disadvantages of different types of
electric-powered wheelchairs with stair-climbing are outlined for an overall comparison of the control method, cost of
mechanical manufacture, energy consumption, and adaption to different stairs. Insights into the future direction of stability
during stair-climbing are discussed as it is an important aspect common to all electric-powered wheelchairs with stair-
climbing. Finally, a summary of electric-powered wheelchairs with stair-climbing discussed in this article is provided. As a
special review to the electric-powered wheelchairs with stair-climbing, it can provide a comprehensive understanding of
the current technology about electric-powered wheelchairs with stair-climbing and serve as a reference for the
development of new electric-powered wheelchairs with stair-climbing.
Keywords
Electric-powered wheelchair, stair-climbing mechanism, control method, stability evaluation
Date received: 18 January 2017; accepted: 22 May 2017
Topic: Service Robotics
Topic Editor: Manuel Angel Armada Rodriguez
Associate Editor: Ismael Garcia Varea
Introduction
As an important mobility assistance device for aged and
physically disabled people, electric-powered wheelchairs
(EPWs) have been widely used for many years. Nowadays,
many patients and users utilize EPWs as their primary
means of mobility during indoor and outdoor daily activi-
ties.
1,2
However, it is still a challenging task for a standard
EPW to overcome the existing environmental barriers such
as the building or civil infrastructure stairs.
3
Especially for
some patients living in a building without an elevator, it is
1
School of Mechanical Engineering, Nanjing University of Science and
Technology, Nanjing, China
2
State Key Laboratory of Fluid Power and Mechatronic Systems, College
of Mechanical Engineering, Zhejiang University, Hangzhou, China
Corresponding author:
Tao Liu, State Key Laboratory of Fluid Power and Mechatronic Systems,
College of Mechanical Engineering, Zhejiang University, 310027
Hangzhou, China.
Email: liutao@zju.edu.cn
International Journal of Adva nced
Robotic Systems
July-August 2017: 1–13
ªThe Author(s) 2017
DOI: 10.1177/1729881417721436
journals.sagepub.com/home/arx
Creative Commons CC BY: This article is distributed under the terms of the Creative Commons Attribution 4.0 License
(http://www.creativecommons.org/licenses/by/4.0/) which permits any use, reproduction and distribution of the work without
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open-access-at-sage).
difficult for them to travel up and down the stairs using a
standard EPW. This results in patients staying indoors for
prolonged period of time, which is not conducive to their
mental health and physical rehabilitation. In this case, the
development of an electric wheelchair that allows them to
travel up and down stairs is very helpful to expand their
scope of activities and improve their quality of life. As a
result, the development of an electric-powered wheelchair
with stair-climbing (EPW-SC) ability is highly necessary
for many patients and elderly people with walking dysfunc-
tion and has already attracted great attention in the past
several decades.
In general, staircases have many continuous steps on a
slope with a large angle, which presents a challenge for an
EPW with a rider to ascend or descend the stairs. Compared
with the common EPWs, EPW-SCs will require larger driv-
ing forces, better maneuverability, and stronger obstacle
crossing ability.
3,4
In addition, prevention of potential tip
over and ensuring the rider’s safety during stair-climbing
are also necessary for EPW-SCs.
4,5
The affordability and
convenience of use is important for the EPW-SCs to be
widely adopted by users. In summary, the four character-
istics of an ideal EPW-SC are a suitable stair-climbing
mechanism maintaining low cost and lightweight,
6
a smart
motion control system with high stability, a reliable tip over
preventing strategy, and a convenient operation interface.
Since the 1990s, many research results on EPW-SC have
been achieved and a variety of EPW-SC prototypes have
been developed.
7–12
In the past two decades, several types
of commercial EPW-SCs were deployed to the market,
including the iBOT series,
13,14
TGR series,
15,16
and
TopChair series.
17,18
Although all of these prototypes and
commercial EPW-SCs have stair-climbing abilities, few
are able to meet the four characteristics of an ideal EPW-
SC and satisfy user requirements. For example, the most
popular EPW-SC product iBOT sold by Independence Tech-
nology, L.L.C., has a high price and requires additional care
in ascending and descending stairs.
19
Likewise, the latest
EPW-SC prototype with a wheel-leg combined mechanism
for stair-climbing developed by Chang-Hyuk Lee has not
achieved the low cost and lightweight requirements.
12
So
far, new EPW-SC technologies and prototypes are still in
the research stage for many investigators and companies.
The primary purpose of the current article is to review
current EPW-SC technologies and provide an outlook for
their future development. The section “Development
history and classification of EPW-SCs” reviews the devel-
opment history of EPW-SCs and provides a classification
of devices based on their various stair-climbing mechan-
isms. The section “Analysis and comparison of different
types of EPW-SCs” provides an overall comparison of
these different types of EPW-SCs and analyzes the current
technologies of EPW-SCs. In the section “Stability analysis
of EPW-SC in climbing the stairs,” stair-climbing stability
is discussed as it is an important common requirement. In
the section “Development trend of EPW-SC,” insights into
the future direction of EPW-SCs are provided. The conclu-
sions to this article are presented in the section “Summary.”
Development history and classification
of EPW-SCs
In the development of EPW-SCs, it is crucial to design a
suitable stair-climbing mechanism. The stair-climbing
mechanism design will directly affect the cost, stability,
control complexity, and energy consumption of EPW-
SCs. Therefore, an in-depth analysis of stair-climbing
mechanisms is provided for EPW-SC prototypes and com-
mercial products. The mechanisms are classified into four
categories: track-based, wheel cluster–based, leg-based,
and hybrid stair-climbing mechanisms.
Track-based stair-climbing mechanism: The outer teeth
of the track press heavily on the sharp corner of a step
while the motors driving the track wheel cause it to
rotate along the inner surface of the track. The track-
based stair-climbing mechanism enables the EPW-SC
to climb up or down the stairs.
Wheel cluster–based stair-climbing mechanism: A
wheel cluster is a component with multiple wheels
uniformly distributed in the same plane around a com-
mon center. Wheel clusters typically have two to four
wheels, as shown in Figure 1. While using a stair-
climbing mechanism, the wheels rotate around the
central axis of the wheel cluster and propel the
EPW-SC up or down the stairs.
Leg-based stair-climbing mechanism: This type of stair-
climbing mechanism originates from the imitation of
humans’ and animals’ stair-climbing techniques,
Figure 1. Wheel cluster mechanism. (a) Two wheels on a wheel
cluster.
20
(b) Three wheels on a wheel cluster.
21
(c) Four wheels
on a wheel cluster.
22
2International Journal of Advanced Robotic Systems
using legs and feet to walk on various steps facilitat-
ing efficient and stable stair-climbing.
Hybrid stair-climbing mechanism: This type of stair-
climbing mechanism uses a combination of a special
deformation mechanism and a wheeled mobile
mechanism and is defined in this article as a hybrid
stair-climbing mechanism. It has an innovative and
more complex mechanical structure and control
method.
Based on the understanding and analysis of working
principles and characteristics, EPW-SCs developed in the
past two decades and their stair-climbing mechanisms
are listed in Table 1. This enables further study of the
occurrence of different stair-climbing mechanisms and
observation of their developmental trends.
According to Table 1, we can see that track-based
EPW-SCs have attracted the most attention from research-
ers since the 1990s, followed by the wheel cluster–based
EPW-SCs with a variety of prototypes and several com-
mercialized products. Leg-based EPW-SCs had less proto-
types and commercialized products, and hybrid EPW-SCs
are the least studied. It can also be seen that the track-based
EPW-SCs were developed the earliest and in the last two
decades have been the main concern of researchers. Con-
cern from researchers and product application of the leg-
based EPW-SCs and hybrid EPW-SCs are substantially
lower than the other two types of EPW-SCs. Considering
that the development of these four categories of wheel-
chairs should have a direct relationship with their respec-
tive performance and characteristics, in the next section,
the current technology of each category will be analyzed
Table 1. The developed EPW-SCs and their classification.
Period Name of EPW-SC Research unit Type of EPW-SC Commercialized or not
1995 GS2 and GS3
23
Graz University of Technology,
Austria
Hybrid Prototype proposed
1995–2002 Nagasaki Stair climber
24–28
Nagasaki University, Japan Track based Commercialized
1998–2000 Robotic hybrid wheelchair
29,30
Nagasaki Institute of Applied Science,
Japan
Hybrid Prototype proposed
1999–2016 iBOT
13
The inventor DEKA, USA Wheel cluster based Commercialized
2000–2004 HELIOS V
31,32
Tokyo Institute of Technology Japan Track based Commercialized
2002 “Freedom”
22
Tomo Co. Ltd, TamagawaUniversity,
Japan
Wheel cluster based Commercialized
2003–2016 TopChair
17
TopChair Company, French Track based Commercialized
2004–2010 WL-16 series
33–37
Waseda University, Japan Leg based Prototype proposed
2004–2010 Zero Carrier
38,39
Tokyo Institute of Technology, Japan Leg based Prototype proposed
2005 “I-foot”
40
Toyota company, Japan Leg based Prototype proposed
2006–2016 CALMOS wheelchair
41
University of Castilla-La Mancha,
Spain
Hybrid Prototype proposed
2007–2013 TGR Explorer
16
TGR Instrument of Freedom
company, Italian
Track based Still in sales
2008 C-MAX U1
42
AAT The Stairclimber People
Company, Germany
Leg based Still in sales
2009 The TGR Scoiattolo 2000
15
TGR Instrument of Freedom
company, Italian
Wheel cluster based Still in sales
2009–2011 Wheelchair. Q
43
Politecnico di Torino, Italy Wheel cluster based Prototype proposed
2010 TBW-1 Matsushima
44
Tohoku University, Japan Hybrid Prototype proposed
2011–2014 The EPW-SC with VGSTM
45
Chinese Academy of Science, China Track based Prototype proposed
2011–2016 A compact EPW-SC
12,46
Seoul National University, Republic
of Korea
Hybrid Prototype proposed
2012 A planetary wheeled EPW-SC
47
Chinese Academy of Science, China Wheel cluster based Prototype proposed
2012 NOROROT
48
University of Tokyo’s Kamata Lab,
Japan
Wheel cluster based Prototype proposed
2014 A leg-wheel EPW-SC
49
Parallel Robot and Mechatronic
System Laboratory of Hebei
Province, China
Leg based Prototype proposed
2015 A novel EPW-SC robot
50
Nanjing University of Science and
Technology, China
Wheel cluster based Prototype proposed
2015 LCSCW
51
Universidade Federal do ABC, Brazil Wheel cluster based Prototype proposed
2015 Novel design of a Wheelchair
52
National Taiwan University, Taiwan Hybrid Prototype proposed
2016 A novel wheel-tracked EPW-SC
53
Nanjing University of Science and
Technology, China
Track based Prototype proposed
EPW-SC: electric-powered wheelchairs with stair-climbing.
Tao et al. 3
and compared to provide reference for researchers regard-
ing selection of EPW-SC design.
Analysis and comparison of different types
of EPW-SCs
The control system and operation mode of EPW-SCs is
decided according to their stair-climbing structure. The
classificationframeworkdescribedinthesection
“Development history and classification of EPW-SCs” will
be used to summarize the main technologies of stair-
climbing wheelchairs. Aspects of mechanical structure,
control system and method, energy consumption, and oper-
ation convenience will be compared to provide a reference
for the future development of EPW-SCs.
Track-based EPW-SC
Due to the interlocking effect between the track’s outer
teeth and the steps’ sharp corner, the track-based stair-
climbing mechanism enables the EPW-SC to climb up or
down the stairs at a constant speed in a stable manner. As a
result, the track-based stair-climbing wheelchair is the most
widely applied of the four types of EPW-SCs.
The first successful and popular application of a track-
based stair-climbing mechanism in an EPW-SC was the
Nagasaki stair climber series developed in Japan.
24–28
They
performed experiments and optimization work on a proto-
type and achieved commercialization of the EPW-SC prod-
ucts. Their products include different models with the
option of a single section track or dual section track, as
shown in Figures 2 and 3. Hirose et al. of the Tokyo Insti-
tute of Technology in Japan also developed a track-based
EPW-SC named Helios < in 2000.
31,32
Their successful
work laid a lasting foundation for the development of addi-
tional EPS-SC technologies. Yu et al. designed a new
wheelchair robot with a variable geometry single-tracked
mechanism, which can easily adapt to the transition
between the stairs and the top flat platform as shown in
Figure 2.
45
In addition to its complicated mechanism
design, the crawler easily slips due to the change in track
tension during stair ascent. Tao et al. developed a track-
based EPW-SC, which facilitates the transition between the
stair-climbing mode and the wheel mobile mode on flat
ground, as shown in Figure 4.
53
Several companies have released a number of EPW-
SC products with a track-based stair-climbing mechan-
ism and an integrated wheeled mobile mechanism. This
type of wheelchair incorporates both functions of
stair-climbing and moving on level ground. Among
them, the most successfully commercialized products
are TopChair-S
18
and TGR Explorer,
54
as shown in Fig-
ures 5 and 6, respectively. These products have good
performance for riding comfort, maneuverability, and
stair-climbing. However, their slightly more complicated
structure and higher price partially hinder a more exten-
sive application among the elderly population and per-
sons with disabilities.
The control system of a track-based EPW-SC is mainly
used for motion control, automatic seat leveling, and com-
mand receiving from the operation interface, which aims
at ensuring the EPW-SC ascends and descends stairs
safely. As the TopChair-S is a widely used track-based
EPW-SC, its specifications and control system structure
will be described as a typical example,
18
as shown in
Table 2 and Figure 7.
Figure 2. Track-based EPW-SC. (a) EPW-SC Sunwa named
CDM-2. (b) The Nagasaki EPW-SC named KSC-A-11. EPW-SC:
electric-powered wheelchairs with stair-climbing.
Figure 3. The variable geometry single-tracked EPW-SC. EPW-
SC: electric-powered wheelchairs with stair-climbing.
Figure 4. Track-based EPW-SC prototype designed by Tao et al.
EPW-SC: electric-powered wheelchairs with stair-climbing.
4International Journal of Advanced Robotic Systems
Based on the control structure, an adaptive control
method is also necessary to ensure the EPW-SC can climb
up and down the stairs in a stable and smooth manner. For
the track-based EPW-SC, the Proportion Integration Deriva-
tion (PID) control algorithm is sufficient for controlling the
speed of its driven wheels and rotation angle of the seat, as
applied in the TopChair-S.
3
In addition, an active optimal
control for track-based EPW-SCs with non-holonomic sys-
tem was designed by Wang et al. to obtain the expected
reference of constraint forces and verification through the
simulation.
55
In addition, a few researchers have considered
stair recognition technology and gravity center adjustment
methods, which are beneficial to the EPW-SC’s stability in
stair-climbing. For example, Ha¨cker et al. developed a
stair-sensing system for EPW-SCs based on optical three-
dimensional (3-D) data.
56
Through the appropriate algo-
rithm, it can automatically identify and locate stairs made
from different materials. This result shows an immense step
forward for autonomous and assisted mobility of EPW-SCs.
In terms of its motion characteristics, the track-based
EPW-SC can climb up or down the stairs at a constant
speed in a stable manner due to the interlocking effect
between the track’s outer teeth and the steps’ sharp corner.
On the other hand, the deformation and lower transmission
efficiency from the track drive results in high-energy con-
sumption during stair-climbing.
Wheel cluster–based EPW-SC
The wheel cluster–based stair-climbing mechanism is
relatively compact and can easily switch to wheeled
mobile mode when running on level ground. This results
in the wheel cluster–based EPW-SC having a relatively
simple structure and low cost.
The wheel cluster–based mechanisms for stair-climbing
can be subdivided into two categories: single cluster
13,15
(Figure 8) and the dual cluster
22,54
(Figure 9). For example,
the TGR Scoiattolo 2000 has a single wheel cluster
Figure 5. TopChair-S EPW-SC. (a) The TopChair-S provides
multiple options for users. (b) The TopChair-S collapses for ease
of storage or travel. EPW-SC: electric-powered wheelchairs with
stair-climbing.
Figure 6. EPW-SC named TGR Explorer. EPW-SC: electric-
powered wheelchairs with stair-climbing.
Table 2. The specifications of TopChair-S.
Feature Specification
Electronic VSI controller (PG company)
Max mileage 45 km
Max speed 9 km/h
Max stair angel; Max step height 35;20cm
Climbing motor 2 400 W
Ground motor 2 350 W
Battery capacity 2 60 A/h
Embedded
system
Control
module
Wheeled-tracked
switched drive
module
Seat
adjustment
drive module
Wheel drive
module
Track drive
module
Joystick and
switch buttons
Stairs detection
State detection
Obstacle detection
Figure 7. The control system structure of TopChair-S.
Figure 8. Single wheel cluster–based EPW-SCs. (a) iBOT. (b)
TGR Scoiattolo 2000. EPW-SC: electric-powered wheelchairs
with stair-climbing.
Tao et al. 5
mechanism with three wheels and two additional omnidir-
ectional wheels in the front.
15
Also, there are similar
mechanisms such as an economic EPW-SC
47
and iBOT
4000.
57
A dual wheel cluster–based stair-climbing mechan-
ism with two wheels on each wheel cluster is used in the
“Freedom” series EPW-SC
22
and a low-cost EPW-SC pro-
totype named “LCSCWC”.
51
Generally, compared with the
single wheel cluster–based EPW-SC, the dual wheel clus-
ter–based mechanism can reduce the speed fluctuation and
improve both stability and safety. However, it also has a
larger size and more complex control method.
The control system structure of a wheel cluster–based
EPW-SC is similar to that of a track-based EPW-SC. To
obtain this control system structure, the track drive seen in
Figure 6 is changed to a wheel cluster drive. Despite dif-
ferent types of EPW-SCs having certain similarities in func-
tion, their control methods or strategy in stair-climbing may
be very different. Shino et al. developed the drive motors’
control method by designing a wheel cluster mechanism
equivalent to an inverted pendulum and proposing an inte-
grated control system based on a linear–quadratic regulator.
This consisted of a gravity-center and a wheel-linkage
control stage, which was applied in stair-climbing success-
fully.
10
Several fuzzy-based control structures and methods
were developed by Ghani et al. to control a wheel cluster–
based EPW-SC similar to the iBOT for stair climbing, with
controls verified through simulations.
58–63
In addition, since the wheel cluster–based EPW-SC
climbs one step at a time, the movement speed is constantly
fluctuating, which results in the rider feeling uncomforta-
ble. The complexity of its control is significantly higher
than that of the track-based EPW-SC. However, its energy
consumption and mechanical structure cost are also con-
siderably less than that of the track-based EPW-SC.
Leg-based EPW-SC
Leg-based EPW-SCs have adopted several kinds of
leg-based stair-climbing mechanisms, including biped
64
and multiped stair-climbing mechanisms,
65
as shown
in Figure 10. For example, a biped stair-climbing mechan-
ism based on the parallel mechanism was adopted in the
WL-16 series locomotors.
33–37,64,66
Likewise, a biped
stair-climbing mechanism based on the human-like legs
and feet mechanism consisting of three to four rotary
joints is applied in a stair-climbing device named
“I-foot.”
40
In addition, a stair-climbing vehicle named
“Zero Carrier” with eight legs was proposed by Yuan
et al. and a corresponding experimental prototype was
developed.
67
Wang et al. also proposed a concept of an
eight-legged wheelchair
49
aiming at improving the
limitations of the Zero Carrier design.
Due to the complex structure of the leg-based EPW-SCs
and the number of required control motors, its stair-
climbing trajectory needs to be combined with the planned
walking trajectory. For the WL-16 series, an improved
control method for model-based dynamic walking of the
bipedal humanoid was studied from 1996. The virtual com-
pliance control method was used to moderate its landing
impact, which resulted in successful stair-climbing with
human carrying.
This type of stair-climbing device is essentially a biped
robot with human carrying function. Its control method is
similar to the general biped robot, which is mainly based on
zero moment point (ZMP) analysis and feedback control to
achieve stable walking and stair-climbing.
33–37
Yuan et al.
designed an EPW-SC called Zero Carrier that controls its
eight legs and feet to locate at the right step in sequence,
ensuring the stability of stair ascent and descent.
68,69
Due to
the large number of legs, the design can easily have
decreased efficiency during stair-climbing and difficulty
in adapting to different stair types.
Although these leg-based stair-climbing vehicles are
complex, have high costs, and have unconventional
appearances, they are able to achieve the core function
of stair ascent and descent and provide some innovations
for EPW-SC design.
Hybrid EPW-SC
The hybrid EPW-SC can adapt to the stairs and perform
stair-climbing through the combination of a unique trans-
mission mechanism and a wheeled-type mobile mechan-
ism. The number of hybrid stair-climbing mechanisms is
relatively less than the three previously described mechan-
ism types due to its complexity. The hybrid EPW-SC
named “TBW-1” is composed of two four-bar linkage
mechanisms with a wheel at every vertex,
44
as shown
in Figure 11. Through the four-bar linkage mechanisms’
locomotion, the wheels’ positions can be adjusted to
maintain a suitable contact state with the steps, therefore
allowing it to climb up and down stairs. Another hybrid
EPW-SC designed by Morales et al. in 2007 utilizes two
four-bar linkages consisting of two linear actuators, a series
of connecting rods, and two driving wheels to accomplish
Figure 9. Dual wheel cluster–based EPW-SC. (a) EPW-SC
named Freedom. (b) EPW-SC named LCSCWC. EPW-SC:
electric-powered wheelchairs with stair-climbing.
6International Journal of Advanced Robotic Systems
positioning and climbing on the stairs separately,
41
as
shown in Figure 12.
Morales et al. studied hybrid EPW-SC stair adaptive con-
trol, obstacle surpassing, and posture control and proposed
the stability control method based on the kinematic model
and climbing strategies.
70
These were further incorporated
with the dynamic model into the feedback control. A control
system based on the Stair-Climbing Mobility Systems
(SCMS) transition diagram and a linear Proportion Deriva-
tion (PD) action with a nonlinear compensation of gravita-
tional terms was then proposed to accomplish the dynamic
control for stair-climbing.
71–73
These control methods were
mainly designed to let the stair-climbing mechanism
accurately reach the appropriate stair, and, therefore, they
can only be applied in this hybrid EPW-SC. Although these
hybrid EPW-SCs have been greatly innovative, they have
larger sizes and greater difficulty in stair-climbing stability
compared with other EPW-SCs.
Comparison of various types of EPW-SCs
Each stair-climbing mechanism has its own characteristics
and advantages. From a practical point of view, the track-
based and wheel cluster–based mechanisms are widely
accepted by the users in terms of cost, size, and stability.
However, a novel stair-climbing mechanism still needs to
be developed, and currently some researchers and compa-
nies have started to focus on new methods of control,
Figure 10. Leg-based EPW-SCs. (a) WL-16 II. (b) I-foot. (c) Zero Carrier. EPW-SC: electric-powered wheelchairs with stair-climbing.
Figure 11. TBW-1 with two four-bar linkage mechanisms. Figure 12. The EPW-SC while climbing on the stairs. EPW-SC:
electric-powered wheelchairs with stair-climbing.
Tao et al. 7
stability, and human–computer interface technology. The
detailed performance comparison is shown in Table 3.
As can be seen from Table 3, the track-based and wheel
cluster–based EPW-SCs in general have better overall
performance. Comparing these two types of EPW-SCs,
the track-based EPW-SCs have simpler control systems
and better adaptability to different stairs, while the wheel
cluster–based EPW-SCs have advantages in cost and
energy consumption.
Stability analysis of EPW-SC in climbing
the stairs
Preventing the wheelchair from tipping over is a crucial
step in maintaining stability during stair climbing. There
are three main aspects to solve this stability problem
including design of the tip over prevention mechanism,
control strategies for preventing tip over, and center of
gravity (COG) adjustment and stability evaluation.
Design of the tip over prevention mechanism
The stair-climbing mechanism allows the EPW-SC to
climb up and down stairs; however, it must also prevent
it from tipping over. Especially during the initiation of stair
ascent and descent, it is easy for the EPW-SC to tip over
due to the sharp change in attitude. Thus, in addition to
lowering the COG of the EPW-SC and its rider, implemen-
tation of auxiliary support mechanisms for tip over preven-
tion is necessary. An EPW-SC named C-MAX U1was
designed with a stable support at the front portion of the
stair-climbing mechanism to prevent tip over.
42
Yokota
et al. presented a caster-fixed design with a new type of
assistive plate to prevent tipping from the stairs.
74
The TGR
Explorer also has two feet-like members at the front and
back of the stair-climbing mechanism that act as an assis-
tive support to improve the stability between the platform
and stairs’ transition,
54
as shown in Figure 13.
For the dual- or multitrack-based stair-climbing mechan-
ism, one of the tracks can perform an auxiliary role similar to
the TGR’s four feet-like mechanism
28,45
by controlling the
attitude angle between different steps. Some corresponding
auxiliary mechanisms are also designed in the wheel cluster–
based stair-climbing mechanism to prevent its tip over.
21,75
In fact, the method for designing a set of reliable and effec-
tive auxiliary support institutions is an important problem
that needs to be solved in the development of track-based or
wheel cluster–based EPW-SCs.
Control strategies for preventing tip over
For some leg-based and hybrid EPW-SCs, preventing tip over
is mainly achieved by control strategies aiming the feet’s
locomotion. For example, the EPW-SC called Zero Carrier
controls its eight legs and feet to move the feet and ensure
stability.
65,67,68
Some biped EPW-SCs are mainly controlled
by ZMP to achieve stability on stairs and prevent tipping in
any direction, such as the WL-16 series.
33–37,64,66
Sugahara
et al. also developed a hybridEPW-SC named “TBW-1” with
a controlling mechanism that deforms step by step to achieve
stability during the stair-climbing action.
76
The TopChair
with integrated crawler and wheel mobile modes also imple-
mented a smooth transition during stair climbing between the
top platform and the stairs. This was done through control of
the wheels’ height to adjust and change the attitude of the
track-based stair-climbing mechanism.
77
COG adjustment and stability evaluation
In the process of climbing up and down stairs, the COG
locomotion of the EPW-SC with a rider and the related
Table 3. The detailed performance comparison of four types of EPW-SC.
Category
Typical prototype or
Products
Control
complexity
Cost of mechanical
manufacture
Energy
consumption
Adaptability to
different stairs
Track-based EPW-SC TopChair-S TGR
Explorer
Low Moderate High Most high
Wheel cluster–based
EPW-SC
iBOT or TGR
Scoiattolo 2000
Moderate Low Low Moderate
Leg-based EPW-SC WL-16 series or I-foot Most high Most high Most high High
Hybrid EPW-SC TBW-1 High High Moderate Low
EPW-SC: electric-powered wheelchairs with stair-climbing.
Figure13. Two feet-like assistive mechanism of TGR Explorer.
8International Journal of Advanced Robotic Systems
overturning moment are the main factors in determining
whether or not the EPW-SC is going to tip over. The auto-
matic adjustment of seat posture is also a dynamic position
adjustment to the rider’s COG. Therefore, it is necessary to
give sufficient consideration to the possible COG positions
of the EPW-SC and rider in the stair-climbing mechanism
design and seat position determination. The iBOT series
EPW-SCs performed the feedback control to its COG posi-
tion similar to the inverted pendulum control and laid a
foundation of stable stair-climbing.
20,78
In the aspect of stability analysis and evaluation, Fang
et al. analyzed the stair-climbing stability of their wheel
cluster–based EPW-SC and gave a simulation verification
based on the usage of stability margin and the position
analysis of the COG’s projection point in the support
polygon.
4
Yu et al. proposed a tip over and slippage sta-
bility criterion for a variable geometry single-tracked
EPW-SC based on the geometric model, the static model,
and the track–stair interaction analysis, for partial
tracked-based EPW-SCs.
5,79
Once tip over has occurred during the process of stair
climbing, it can be very dangerous and cause personal
injury to the rider. Therefore, analysis on stair-climbing
stability is very important. From reviewing existing stair-
climbing stability and related results, it was deemed nec-
essary to further study prediction and assessment methods
on stability and to provide the auxiliary safety protection
before stability problems occur.
Development trend of EPW-SC
Due to the low cost and high spatial efficiency of staircases
when connecting areas of differing vertical elevations, the
presence of stairs will most likely be a reality in our society
for the next several decades. EPW-SCs designed for the
elderly and persons with disabilities still need improve-
ments to meet the wider range of demands for future use.
After the review of the existing EPW-SC technology and
common EPWs, the current situation and trend of EPW-
SCs will be discussed.
Low cost, lightweight, and high adaptability
of EPW-SC
The existing EPW-SC products can meet the basic func-
tional demands of the users, but there is a need to lower the
cost and the weight for affordability. The complexity and
cost of an EPW-SC is often determined by its mechanical
structure. Therefore, it is necessary that related innovative
design and optimization is performed on stair-climbing
mechanism. Moreover, better adaption to various sizes of
stairs should be considered by the EPW developer. Once
these improvements are obtained, the EPW-SC will be
more widely favored and attract broader interest.
Stair-climbing stability evaluation and prediction
In order to prevent tip over of EPW-SCs in stair-climbing,
it is necessary for the researchers to provide some safety
precautions through mechanism design and control strat-
egy. On the other hand, the EPW-SCs also need to com-
bine the latest sensor technology and stability evaluation
methods to perform real-time analysis and detection on
stair-climbing stability. This can be used to predict possi-
ble tip over and immediately trigger the protection
mechanism. There are few studiesinwhichanevaluation
method of the EPW-SC’s stability in stair-climbing was
proposed
53
or the real-time monitoring of the EPW-SC’s
stability and feedback to the motion control was
applied.
3,12
A fuzzy model-based gravity center adjust-
ment and inclination control method for EPW-SCs was
proposedandverifiedbysimulation and experiments by
Wang et al. This method can efficiently compensate for
the changing of the gravity center to prevent the EPW-
SC’s tip over.
80
From the intelligence and security of
EPW-SCs, a main development trend of stair-climbing
stability is the automatic evaluation, prediction, and
adjustment methods of stair-climbing stability.
Control system and intelligent control algorithms
For the EPW-SC, the adopted control system is determined
according to its stair-climbing mechanism. The control sys-
tem of various types of EPW-SCs may be largely different,
but their main purpose in ensuring the stability and safety
of stair-climbing is common. As a result, the control sys-
tems embedded stability evaluation, prediction, and adjust-
ment should be the future development trend.
In addition, the development of the autonomous naviga-
tion technology, self-positioning algorithm, and intelligent
control algorithm in the field of robotics are likely applied
to EPW-SCs. To many elderly persons and persons with
disabilities, future EPW-SCs will not only be a tool to aid in
stair-climbing, but could also be developed as an intelligent
system providing entertainment, communication, and other
service functions.
Operation mode and human–machine interface
technology
The rocker operation mode was adopted in most of the
existing EPW-SCs, by which the user can realize the con-
trol to the EPW-SC based on a correct rocker operation. A
new type of EPW-SC has been equipped with a wheelchair
status display, which can facilitate use and improve the
correction and safety in their rocker operation. Because
some users of EPW-SCs are not able to use the operating
handle, user-friendly human–machine technologies, such
as brain machine interfaces,
81
language interfaces, and
other intelligent human–machine interfaces, are needed to
Tao et al. 9
control the EPW-SCs in the future. Some of these features
are already available options in common EPWs.
82
In addition, taking into account that many users of
EPW-SCs are elderly and may have cognitive or behavioral
dysfunction, more user-friendly interfaces and applied
machine learning algorithms
83
should be designed to make
the EPW-SCs easier to use. This is also an important trend
of future EPW-SC design.
Summary
As a useful tool both in clinical practice and robot research,
EPW-SC has attracted an increasing amount of attention
from the clinicians and researchers since the 1990s.
Although a variety of prototypes have been developed suc-
cessfully, and several commercial EPW-SCs are available
on the market, it is necessary to develop more preferable
devices and promote their usage among elderly populations
and persons with disabilities.
The purpose of this article was to provide an overview of
the EPW-SC development for the last two decades and to
give insight into future trends. A classification of EPW-SCs
was provided based on an analysis of various stair-climbing
mechanisms. A summary of advantages and disadvantages
was provided based on the following four aspects: control
method, cost of mechanical manufacture, energy consump-
tion, and adaption to different stairs. Lastly, common tech-
nologies used to maintain stability during stair-climbing
were discussed to give insights into their future direction.
Overall, this article is a summary of the current EPW-SCs
and prospective technology based on research and may be
able to provide insight in the development of more appli-
cable and popular products.
Acknowledgment
The financial support from National Natural Science Foundation
of China (NSFC) and Zhejiang Provincial Natural Science
Foundation of China are gratefully acknowledged.
Declaration of conflicting interests
The author(s) declared no potential conflicts of interest with
respect to the research, authorship, and/or publication of this
article.
Funding
The author(s) disclosed receipt of the following financial support
for the research, authorship, and/or publication of this article: This
work was supported in part by the National Natural Science Foun-
dation of China (NSFC) under grant nos 51275244, 61428304 and
Zhejiang Provincial Natural Science Foundation of China under
grant no. LR15E050002.
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