Page 284 - Control Theory in Biomedical Engineering

256 Control theory in biomedical engineering

based on the classical regressor matrix. These types of controllers assume that
the robot is linear in a set of physical parameters and find a control law able to
ensure the stability of this linear system only around its operating points
(Yao, 1996). In fact, the manipulator is highly nonlinear. So, the integration
of this adaptation law may affect the stability of the system in the presence of
even small disturbances (Yao, 1996). Adaptive visual or image-based track-
ing control (Hutchinson et al., 1996; Deng et al., 2002; Espiau et al., 1992;
Gans et al., 2003; Malis and Chaumette, 2002; Liu et al., 2006) is one of the
powerful approaches that has been developed to transact with the kine-
matic/dynamic uncertainties. This is due to their robustness practically to
modeling and calibration errors (Deng et al., 2002). However, these con-
trollers are concentrated on uncertainties in nonlinear transformation func-
tions or image Jacobian matrix, but they ignored the uncertain kinematic/
dynamic effects. Additionally, few stability analyses are provided in the lit-
erature for visual tracking control with the uncertainties of kinematics/
dynamics and in the presence of uncertainties in visual system (camera)
parameters (Cheah et al., 2006).

5 Discussion

Over the last two decades, numerous exoskeletons have been developed to
rehabilitate people with upper limb disability and researchers have been
extensively and constantly working to advance the hardware design and
control approaches for such robot-aided therapeutic devices.
This chapter reviewed developments in hardware design to deliver better
HEI in upper limb exoskeletons and advancements in control approaches to
ensure safe and desired running of the exoskeleton. The developments of
exoskeletons were compared based on safety, comfort of wearing, align-
ment, actuation, power transmission mechanism, singularity, and backdriva-
bility. The main challenges while building an exoskeleton are that its joints
should be aligned with the corresponding human anatomical joints properly,
accommodating different size wearers/patients, providing safe and natural-
istic movements. Most existing exoskeletons did not include mobility of
shoulder, elbows and wrist joints during upper limb movements. To allow
movements of shoulder, elbows and wrist joints, a more novel mechanism
should be developed without compromising ROM.
Furthermore, exoskeletons need more sophisticated actuators that are
light, compact, easily mountable, have high operating bandwidth, and are
durable, reliable, and require low maintenance. The power transmission