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Page 108 Yang et al. Intell Robot 2024;4(1):107-24 I http://dx.doi.org/10.20517/ir.2024.07

1. INTRODUCTION
Overgroundwalkingisnecessaryandimportantforpatientswithgaitimpairments, whichcanbeimplemented
using the lower limb exoskeleton robots. Usually, it is hard for patients with insufficient upper body strength
to keep balance in the early rehabilitation stages; therefore, a mobile robotic walker is indispensable to assist
with the overground walking and gait training.

As shown in Figure 1, the human-exoskeleton-walker (HEW) system is presented. Note that in the following
sections, the human-exoskeleton system means a subject wearing the exoskeleton robot, and the HEW system
means the human-exoskeleton system with the robotic walker.As shown in Figure 1, the human-exoskeleton
system is connected to the mobile robotic walker with a solid cantilever; a support joint is attached to the
cantilever for the vertical movement assistance and weight supporting during walking. The robotic walker
ensures the stability of the human-exoskeleton system in the coronal plane, validly keeping balance and pre-
venting falls.

For patients with gait impairments, movement disorder can severely disrupt the performance of daily activi-
ties and increase the risk of falling. Although various existing walkers are owned by seniors, reported statistics
[1]
show that 33% of people over 60 years fell at least once . We argue that intelligence is essential for a robotic
walker to protect the safety of the patients, since primitive assistance devices, such as rollators and walkers, are
[2]
much more likely to fail . Present-day assistant devices require attentive control of the user while moving [3,4] ,
which could raise safety issues for many patients with insufficient upper body strength and cause bad coor-
dination between the human-exoskeleton system and the robotic walker. A few studies have investigated the
[5]
task enabling the walker to follow behind the user by detecting his/her trajectory beforehand . However, the
gait trajectory of patients with gait impairments is always varying during the rehabilitation stage. As a result,
the robotic walker may not be able to follow the patients well. The gait trajectory of the exoskeleton is prede-
fined, so we take the approach of adopting a Coordinated motion planning strategy to generate the appropriate
joint angle for the wheels of the robotic walker according to the predefined gait trajectory of the exoskeleton,
enabling our walker to follow the movement trend of the user; our walker can then automatically move be-
hind the user, providing mobility support. Furthermore, without an appropriate supporting force offered by
the support joint and coordinated movement of the wheels to follow the walking of the human-exoskeleton
system, the human-exoskeleton system has to pull or push the robotic walker forward during walking, which
leads to the bad walking posture and unnecessary energy-cost of the human-exoskeleton system. As a result,
battery life has always been a severe challenge to the exoskeleton robots. Among commercially available ex-
[6]
[7]
oskeletons, Indego and Ekso have only 4 h of battery life. Even with the largest battery capacity, ReWalk [8]
and SuitX [9] allow for continuous work for not more than 5 h. It is impossible to meet the hospital’s demand
for all-day rehabilitation training with these exoskeleton robots. Therefore, it is crucial to improve the energy
efficiency of the HEW system with an appropriate supporting force offered with the support joint and the
coordinated movement of the wheels to follow the natural walking of the human-exoskeleton system.

In the last decades, several approaches have been studied with the body weight supporting (BWS) system for
the energy-efficient walking assistance. Sun et al. proposed a BWS system for the three-dimensional walk-
ing in Cartesian space [10] , with the series elastic actuation structure to improve the human-robot interaction
performance and reduce the energy cost of the human. Wei et al. proposed a surplus force control strategy
named active loading compound control for the BWS system, which is used for estimating and improving the
loading accuracy [11] and reduces the surplus force and the energy cost. For the mobile robotic walker, Mun et
al. proposed a mobile robotic walker for the movement of the pelvis of humans, which can be used to facilitate
the over-ground walking without altering the normal gait dynamics [12] . Similar structure has been developed
in ref [13] . Chugo et al. developed a robotic walker to assist with the standing motion and simple walking
for the aged person in daily life, which estimates the load of the pelvis, knee and ankle joints of the human
body, and generate appropriate joint angle for the support joint of the robotic walker [14] . These mobile robotic