Review Article

Electric-powered wheelchairwith stair-climbing ability

Weijun Tao1, Junyi Xu1 and Tao Liu2

AbstractAs 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 somevarious electric-powered wheelchairs with stair-climbing were developed. By using the developed electric-poweredwheelchairs with stair-climbing, many patients with walking difficulties are able to descend the stairs conveniently toparticipate in outdoor activities, which are beneficial to both their physical rehabilitation and mental health. In this article, areview of electric-powered wheelchair with stair-climbing current technology is given and its future tendency is discussedto inform electric-powered wheelchair with stair-climbing researchers in the development of more applicable and popularproducts. Firstly, the development history is reviewed and electric-powered wheelchairs with stair-climbing are classifiedbased on an analysis of their stair-climbing mechanisms. The respective advantages and disadvantages of different types ofelectric-powered wheelchairs with stair-climbing are outlined for an overall comparison of the control method, cost ofmechanical manufacture, energy consumption, and adaption to different stairs. Insights into the future direction of stabilityduring 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 aspecial review to the electric-powered wheelchairs with stair-climbing, it can provide a comprehensive understanding ofthe current technology about electric-powered wheelchairs with stair-climbing and serve as a reference for thedevelopment of new electric-powered wheelchairs with stair-climbing.

KeywordsElectric-powered wheelchair, stair-climbing mechanism, control method, stability evaluation

Date received: 18 January 2017; accepted: 22 May 2017

Topic: Service RoboticsTopic Editor: Manuel Angel Armada RodriguezAssociate 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, China2State 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 AdvancedRobotic Systems

July-August 2017: 1–13ª The Author(s) 2017

DOI: 10.1177/1729881417721436journals.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

further permission provided the original work is attributed as specified on the SAGE and Open Access pages (https://us.sagepub.com/en-us/nam/

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 classificationof 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 wheelcluster.20 (b) Three wheels on a wheel cluster.21 (c) Four wheelson a wheel cluster.22

2 International 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 GS323 Graz University of Technology,Austria

Hybrid Prototype proposed

1995–2002 Nagasaki Stair climber24–28 Nagasaki University, Japan Track based Commercialized1998–2000 Robotic hybrid wheelchair29,30 Nagasaki Institute of Applied Science,

JapanHybrid Prototype proposed

1999–2016 iBOT13 The inventor DEKA, USA Wheel cluster based Commercialized2000–2004 HELIOS V31,32 Tokyo Institute of Technology Japan Track based Commercialized2002 “Freedom”22 Tomo Co. Ltd, TamagawaUniversity,

JapanWheel cluster based Commercialized

2003–2016 TopChair17 TopChair Company, French Track based Commercialized2004–2010 WL-16 series33–37 Waseda University, Japan Leg based Prototype proposed2004–2010 Zero Carrier38,39 Tokyo Institute of Technology, Japan Leg based Prototype proposed2005 “I-foot”40 Toyota company, Japan Leg based Prototype proposed2006–2016 CALMOS wheelchair41 University of Castilla-La Mancha,

SpainHybrid Prototype proposed

2007–2013 TGR Explorer16 TGR Instrument of Freedomcompany, Italian

Track based Still in sales

2008 C-MAX U142 AAT The Stairclimber PeopleCompany, Germany

Leg based Still in sales

2009 The TGR Scoiattolo 200015 TGR Instrument of Freedomcompany, Italian

Wheel cluster based Still in sales

2009–2011 Wheelchair. Q43 Politecnico di Torino, Italy Wheel cluster based Prototype proposed2010 TBW-1 Matsushima44 Tohoku University, Japan Hybrid Prototype proposed2011–2014 The EPW-SC with VGSTM45 Chinese Academy of Science, China Track based Prototype proposed2011–2016 A compact EPW-SC12,46 Seoul National University, Republic

of KoreaHybrid Prototype proposed

2012 A planetary wheeled EPW-SC47 Chinese Academy of Science, China Wheel cluster based Prototype proposed2012 NOROROT48 University of Tokyo’s Kamata Lab,

JapanWheel cluster based Prototype proposed

2014 A leg-wheel EPW-SC49 Parallel Robot and MechatronicSystem Laboratory of HebeiProvince, China

Leg based Prototype proposed

2015 A novel EPW-SC robot50 Nanjing University of Science andTechnology, China

Wheel cluster based Prototype proposed

2015 LCSCW51 Universidade Federal do ABC, Brazil Wheel cluster based Prototype proposed2015 Novel design of a Wheelchair52 National Taiwan University, Taiwan Hybrid Prototype proposed2016 A novel wheel-tracked EPW-SC53 Nanjing University of Science and

Technology, ChinaTrack 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 typesof EPW-SCs

The control system and operation mode of EPW-SCs is

decided according to their stair-climbing structure. The

classification framework described in the section

“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-S18 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 namedCDM-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.

4 International 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, Hacker 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 cluster13,15

(Figure 8) and the dual cluster22,54 (Figure 9). For example,

the TGR Scoiattolo 2000 has a single wheel cluster

Figure 5. TopChair-S EPW-SC. (a) The TopChair-S providesmultiple options for users. (b) The TopChair-S collapses for easeof storage or travel. EPW-SC: electric-powered wheelchairs withstair-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 kmMax speed 9 km/hMax stair angel; Max step height 35�; 20 cmClimbing motor 2 � 400 WGround motor 2 � 350 WBattery capacity 2 � 60 A/h

Embedded

system

Control

module

Wheeled-tracked

switched drive

module

Seatadjustment

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 wheelchairswith 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-SC47 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-SC22 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 biped64

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 wheelchair49 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-SCnamed Freedom. (b) EPW-SC named LCSCWC. EPW-SC:electric-powered wheelchairs with stair-climbing.

6 International 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 climbingthe 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 mechanism28,45 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 hybrid EPW-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.

CategoryTypical prototype orProducts

Controlcomplexity

Cost of mechanicalmanufacture

Energyconsumption

Adaptability todifferent stairs

Track-based EPW-SC TopChair-S TGRExplorer

Low Moderate High Most high

Wheel cluster–basedEPW-SC

iBOT or TGRScoiattolo 2000

Moderate Low Low Moderate

Leg-based EPW-SC WL-16 series or I-foot Most high Most high Most high HighHybrid 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.

8 International 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 adaptabilityof 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 studies in which an evaluation

method of the EPW-SC’s stability in stair-climbing was

proposed53 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

proposed and verified by simulation 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 interfacetechnology

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 algorithms83 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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