IEEE Robotics & Automation Magazine - December 2017 - 79

where i k (t) is the knee-joint angle and i k and i k
are the lower and upper bounds of the knee-joint angle,
respectively.
In addition to the knee-joint contracture, the RoM of the
pilot's ankle joints was also limited. Unlike the knee joint, the
characteristics of the RoM of the ankle joints were complicated because the contracture resulted from the stiffened gastrocnemius muscle, which biarticularly connects the thigh
and the foot. Therefore, the RoM of the ankle joint is
expressed as
ia

1 i a (t) 1 min ^i k (t), i ah,

(2)

where ia is the ankle-joint angle measured between the shank
and the foot and i a and i a are the upper and lower bounds
of the ankle-joint angle, respectively. The RoM of the anklejoint angle is a function of the knee-joint angle. These
mathematical models of the joint contracture, (1) and (2),
were considered in the generation of joint-angle trajectories to
prevent any damage to the pilot's body.
Knee-Ankle-Foot Orthosis
A knee-ankle-foot orthosis (KAFO) is a device that connects the pilot's body parts and the robotic legs, as shown
in Figure 2(a); thus, it is one of the most important components in determining the safety and usability of a powered exoskeleton. Because the actuation force of the
WalkON Suit is transferred to the human body through
the KAFO, the KAFO must be perfectly fitted to the
human body shape. In particular, as the WalkON Suit is
for use by people with complete paraplegia, the pilot does
not generate any voluntary muscular force at all. Therefore, unless the human body part is tightly fastened to the
robotic legs through the KAFO, the actuation force may
not be transferred to the human body part properly, which
causes an abnormal gait or even gait failure. More seriously, the use of an inappropriate KAFO may cause a serious
chronic pressure sore as well as scratches or bruises on the
skin. Because pressure sores can be fatal to people with
complete paraplegia, the KAFO must be customized for
each user by an orthotist.
For the design of the WalkON Suit KAFO, the pilot's
body shape was accurately measured using a three-dimensional (3-D) scanning method. Through the 3-D-scanned
object, the pilot's musculoskeletal structure and anthropometric data were accurately identified. Based on the anthropometric data, a KAFO consisting of metal uprights, bands,
and cuffs was designed and manufactured. As the KAFO
was fabricated based on accurate measurements, it allowed a
perfect fit between the human body and the robotic legs and
was also easy for the pilot to put on and take off. The inner
and outer surfaces of the bands were all covered with shockabsorbent materials such that any impact applied to the
pilot's body was minimal.
As in the case of the WalkON Suit pilot, many complete paraplegics experience joint contracture, which

θk

θa
(a)

(b)

Figure 3. The contracture of legs: (a) the knee joint with full
extension and (b) a schematic model.

limits the RoM, as shown in Figure 3; thus, the KAFO
should be designed taking into account the limited RoM.
Even with a design that compensates for the contracture,
however, the misalignment of joint positions is unavoidable
during the operation of powered exoskeletons. Once the misalignment occurs, the gait stability may be lost, and the operation must be stopped. To
avoid misalignment of
the knee joints during
A KAFO is one of the most
walking, additional ergonomically designed kneeimportant components
caps were installed in the
WalkON Suit system. In
in determining the safety
particular, patients with
complete paraplegia due
and usability of a powered
to cauda equina injury,
who have frail legs withexoskeleton.
out spasticity, are liable to
wear-and-tear injuries of
weight-bearing joints while using a powered exoskeleton for
assisted gait.
Control System
Processes of the Overall Control System
The processes of the WalkON Suit's overall control system consist of start-up mode, safe mode, locomotion
mode, and shutdown mode. These processes are shown
in Figure 4; detailed explanations of each process are
as follows.
1) Start-up mode is the process used to initiate the robot's
operation after the mechanical power switches are all
turned on. (This should be done by the pilot.) Once
on, the battery power is supplied to the CompactRIO
DECEMBER 2017

IEEE ROBOTICS & AUTOMATION MAGAZINE

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