The exoskeletal part drives the internal/external rotation of
the upper arm and the elbow joint, whereas horizontal and
vertical shoulder rotation is actuated by an end-effectorbased
part connecting the upper arm with the wall-mounted
axes 1 and 2.
The robot becomes statically determined only in combination
with the human arm. To prevent the robot from
falling down and to avoid unfavourable shear forces and
torques onto the human shoulder, the weight of the robot is
compensated by a passive counterweight. This is achieved
by a counterweight of 2.5 kg that is connected to the robot
via a cable guided by two pulleys.
This end-effector-type kinematics for the shoulder joint
has been selected as exoskeleton mechanics would make it
difficult to get the robot axes in alignment with the anatomical
axes of the human, and misalignments would
mechanically overstress and potentially harm the shoulder.
2.3 Mechanics and actuation
The vertically oriented linear motion module (axis 1)
drives vertical shoulder rotation (flexion/extension and
abduction/adduction) movements. Horizontal shoulder
rotation (horizontal flexion/extension and horizontal
abduction/adduction) is realized by a backlash-free and
back-drivable harmonic drive module attached to the slide
of the linear motion module (Table 2).
The interconnection module connects the horizontal
shoulder rotation drive with the upper arm rotary module
via two hinge bearings. Elbow flexion/extension is realized
by a harmonic drive rotary module.
Internal/external shoulder rotation is achieved by a
special custom-made upper arm rotary module that is
connected to the upper arm via an orthotic shell. For easy
access to the patient’s arm, the module is made out of two
half-cylinders. An inner half-cylinder (Fig. 2) is guided by
32 ball bearings fixed to the exterior wall. It is actuated by
three steel cables fixed to the two ends of the cylinder and
rolled around the extension of the motor shaft. This guidance
allows transfer of static loads in several DOF while
remaining backlash-free and enabling low friction circular
motion. Custom-made cuffs (Fig. 3) ensure a comfortable
fixation of the patient to the arm rotary module.
2.4 Adaptation to different body sizes
To apply the robot to patients of different sizes, the lengths
of exoskeleton segments need to be adjustable. Table 3 and
Fig. 4 show the ranges of all possible adaptations.
In order to fix the shoulder position of the patient under
the fulcrum at axis 2 (horizontal shoulder rotation), an
individually adjustable belt system was constructed
(Fig. 9). An optional hand support (Fig. 4) can be added to
allow comfortable hand posture.
2.5 Sensors and control hardware
The four exoskeleton actuators are equipped with optical
incremental sensors and redundant potentiometer-based
sensors for position measurements. A six-DOF force-torque
sensor beneath the horizontal arm rotation module
measures forces and torques of the shoulder joint (axis 1–
3). The elbow torque is measured by a separate torque
sensor (Fig. 1).
The controller runs on a Matlab/Simulink XPC target
(The Mathworks, Inc., USA) computer with 1 ms loop
time. Four analogue channels provide inputs for the current
amplifiers (Maxon 4-Q-DC Servoamplifier ADS 50/5;
Maxon Motor AG, Switzerland). The four-encoder signals,
the analogue signals from the redundant potentiometer-