Figure 2.3: Classification of exoskeleton robots. a Methods of classification of upper-limb exoskeleton robots. [4]
Figure 2.4: Classification of exoskeleton robots. b A classification of upper-limb exoskeleton robots based on the actuators used in mechanical designs. [4]
2.1.4 Actuation System
Several actuation technologies are available for actuating a system. Electric motors, pneumatic actuators, hydraulic actuators, ultrasonic motor, IC engines, static electric
actuators, and shape memory alloy are some of them. DC motors are the commonly used actuator for upper-limb exoskeleton robots. It has high speed and precision. Therefore, DC
motors can be used to actuate upper-limb exoskeleton robots from advanced motion control methods. However, gears are required to obtain required torques for upper-limb
motions using less weight and small sized DC motors. Otherwise heavy motors causing a burden for the function of upper-limb exoskeleton robot have to be used for generating
required upper-limb joint torques. [4]
2.1.5 Power Transmission System
Power transmission method of upper-limb exoskeleton robot depends on the actuator. With an electric motor, gear drives, cable wire drives andor linkage
mechanisms can be used to transmit power. Although electric motors can be used as direct drives, they are rarely used as direct drive in upper limb exoskeleton robots, since the size
of the existing motors which can generates required upper-limb joint torques are rather large. Gear drives andor cable drives have commonly been used in present upper-limb
exoskeleton robots. Gear drives do not create slip as in the case of some cable drives. Also bevel gear drives can be used to transmit power between non-parallel axes. Therefore,
compact joints can be designed for the upper-limb exoskeleton robot. Compact joints are important to the upper-limb exoskeleton robots used in daily motion assist. Backlash is
inherent in gear drives. Also it is difficult to obtain precise back-drivability with gears. Therefore, gear drives should be carefully designed for the upper-limb exoskeleton robots.
Since gear drives cannot be used to transmit power over relatively long distances, motors should be fixed near the actuated axis when gear drives are used for power transmission. [4]
Figure 2.5 Joint axis drive system. [5]
2.1.6 Speed Control by Using PWM
