Biomedical Engineering Reference
In-Depth Information
these different settings that mimic real life activities. The second unique feature
of RUPERT is its unidirectional actuation. Each DOF of RUPERT is actuated by a
single PMA, thus resulting in unidirectional actuation in the different DOFs. The
actuation in the other direction is achieved either through gravitational pull on
the different segments of the robot or through the patient's existing motor control.
This particular design of the robot was based on two factors -
(1) Most stroke patients have difficulty moving in one direction (for example,
difficulty in extending the elbow) because of a weak agonist or hyperactive
antagonist. In a majority of stroke patients the stroke affected UE assumes a
posewithflexedandsupinatedelbow,alongwithflexedwristandfingers.
based on this pattern of motor disability seen in a large section of the stroke
population.
(2) When a patient wears the robot, it is basically mounted on the patient's torso
through a patient fixation system. Thus, a light weight exoskeleton is crucial
for the comfortable use of the device. The use of single PMA to actuate
each DOF, instead of two, helps reduce the weight and size of the robot.
Additionally, this design also reduces the other required components of the
system (such as air regulators, pressure sensors, and analog and digital IO
channels on the computer interface board) by half, which can lower the overall
cost of the device.
However, in some of the DOFs such as the elbow flexion/extension and elbow
supination/pronation a passive elastic element (bungee cord) was introduced
in order to provide some assistive force in elbow flexion and elbow pronation
respectively. This was done mainly for subjects who have a weakness in directions
such as elbow flexion and pronation, when their corresponding PMAs are not
activated. Although these elastic elements will help assist the subject in elbow
flexion and pronation, their inclusion does require additional force from the PMA
to overcome its resistance in addition to the force required for providing active
assistance in elbow extension extension.
The under-actuated design of RUPERT is an important issue to consider from
an engineering point-of-view. Such designs are not very common in engineering
systems because of the lack of controllability that results from these designs. This
lack of controllability could also be therapeutically disadvantageous because of
limited repertoire of sensorimotor stimulation that can possibly be provided by
the robot.
Additionally, the use of unidirectional actuation will also encourage a greater
level of volutnary participation from the subject while using the robot for training
assisted therapy tasks. Although this might not be a good approach for subjects
with very weak muscle control, the under-actuated design can potentially keep the
subject engaged in therapy and reduce “slacking behavior” when the subject lets
the robot take over and do the work for them (Wolbrecht
et al.
(2008)).
