Page 278 - Control Theory in Biomedical Engineering

250 Control theory in biomedical engineering

Most of the existing prototypes of rehabilitation exoskeletons have used
electric actuators as they are advantageous in terms of high power, commer-
cial availability, reliability, easy mounting, and little to no maintenance. In
pneumatic actuation, compressed air is used to produce mechanical motion.
Pneumatic actuators have inherently low impedance and considerably
reduce weight of the exoskeleton. However, they require a compressor
to be installed on, and an air tube and valve, which limits their portability.
On top of that, they require regular maintenance to keep the right air pres-
sure. The major limitation of pneumatic actuators is the bandwidth they are
operating on, which is relatively low (5 Hz). This low bandwidth limits the
rate at which they can respond to command signals (Lo and Xie, 2012).
In hydraulic actuation, pressure of liquid is used to produce required energy
for actuation. The commercially available hydraulic actuators are large,
bulky, and noisy, and are not suitable to use in clinical settings. Moreover,
they have high impedance and liquid leakage. Pneumatic muscle actuators
are composed of stretchable bladders and flexible braided mesh. These
muscle-like actuators vary their diameter during actuation and this variation
produces tension at their ends, ultimately leading to slow and nonlinear
responses (Chou and Hannaford, 1996). In series elastic actuators (e.g., Har-
mony robot; Kim and Deshpande, 2017), a passive mechanical spring is
placed between the motor and load to reduce interface stiffness to provide
greater shock tolerance with the tradeoff of operating bandwidth (Pratt and
Williamson, 1995). The desired characteristics for choosing actuators for
upper limb exoskeleton are: (a) light weight, (b) high power-to-weight
ratio, (c) high operating bandwidth, and (d) considerable impedance to deal
with unwanted movements such as shock, (e) safe and easy operation, (f )
reliability, (g) durability, and (h) low maintenance. For further study, the
interested reader may want to see the review by Manna and Dubey
(2018), which compares the different actuation systems used in upper limb
exoskeletons.

Power transmission mechanism

Transmitting power from motors to exoskeleton joints is one of the key
challenges in upper limb exoskeleton research. Continuous variable power
transmission is required to provide therapy uninterruptedly to patients. The
power transmission can be done by cable drive, wire rope drive, gear train
transmission, harmonic drive, belt drive, and so on. Rotation produced at
motors is considerably high and needs to be reduced before it transmits to