Control of Power-Assist Exoskeleton Robots with Biological Signals
as artificial muscles are required to be
practically wearable for daily living.
Upper-limb motion is involved in
many important activities in daily living,
so assistance here is important for
physically weak persons. Figure 2
shows an example of 4 DOF (degrees
of freedom) upper-limb motion (i.e.,
shoulder vertical flexion/extension,
shoulder horizontal flexion/extension,
elbow flexion/extension, and forearm
In this case, the exoskeleton robot is
attached to the mobile wheelchair.
Therefore, the user does not carry any of
the weight at all. The exoskeleton mainly consists of a shoulder motion support
part, an elbow motion support part, and
a forearm motion support part.
The shoulder motion support part
is composed of an upper arm link, driver and driven pulleys (one for shoulder
horizontal flexion/extension motion,
another one for shoulder vertical flexion/extension motion), two DC motors,
two potentiometers, an arm holder, and
the mechanism for the center of
rotation (CR) of the shoulder joint. The
1 DOF elbow motion part consists of a
forearm link, pulleys, a DC motor, and a
potentiometer. The forearm motion support consists of a wrist frame, an inner
and outer wrist holder, a wrist cover, a
wrist force sensor, and potentiometers.
Usually, the movable range of the
human shoulder is 180° in flexion, 60° in
extension, 180° in abduction, 75° in
adduction, 100-110° in internal rotation,
and 80-90° in external rotation. The limitation of the movable range of the forearm motion is 50-80° in pronation and
80-90° in supination, and elbow motion
is 145° in flexion and - 5° in extension.
Considering the minimal amount of
motion required in everyday life and the
safety of the user, shoulder motion of the
4 DOF exoskeleton is limited to 0° in
extension and adduction, 90° in flexion,
and 90° in abduction. Limitation of the
forearm motion is 50° in pronation and
80° in supination, and 120° in flexion and
0° in extension for the elbow motion.
In order to activate the exoskeleton in accordance with the user’s
intended motion, the EMG-based control can be applied as explained next.
FIGURE 3. Location of electrodes.
FIGURE 2. 4DOF power-assist exoskeleton.
In order to control the 4 DOF upper-limb motion, 12 kinds of EMG signals
should be used, as shown in Figure 3.
Control with Biological
In order to assist the motion of the
user, the exoskeleton robot must determine the generating motion in realtime.
The user’s motion can be estimated in
real time by monitoring the EMG signals
of the certain muscles. Since the
amount of EMG signal indicates the
activity level of the muscles, the amount
of generating force by the user can be
estimated by monitoring these signals.
However, this EMG-based controller is not very easy to be realized,
because: 1) obtaining the same EMG
signal for the same motion is difficult
even with the same person since the
signal is biologically generated; 2) the
activity level of each muscle and the
way they’re used for certain motion is
different among individuals; 3) activity
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