Many hours of operation with adjusted PD controller
parameters without a patient did not yield instable behaviour.
Only high-frequency and high-amplitude external
perturbations (e.g. induced by the patient) cannot be
compensated by the controller and can cause vibrations
(Fig. 9, right).
However, assuming that the bandwidth of human arm
movement is limited to 10 Hz [30], such high-frequency
and high-amplitude perturbation cannot be induced by the
human.
This is not a strict stability proof, but both, the simulation
and the experimental results, support the hypothesis
that the robot stays stable when it interacts with the human
arm.
4.2 Specifications of the device
Most of the specifications for the robot (Table 1) could be
fulfilled (Table 5). The ROM of the vertical shoulder
rotation is limited by the mechanical construction of the
robot.
4.3 Pilot study
The teach-and-repeat mode allows the therapist to easily
define an appropriate movement and deliver smooth
movements. While the patient is passive during the mobilisation
therapy, he or she is active during the ball-gamesupported
therapy. The results of the questionnaire suggest
that the patients like both, the ball game and the mobilisation.
The patients rated their fixation in the device with
the average mark 4.6, which is significantly lower than the
other marks. As the elbow fixation did not cause any
problems, this relatively bad mark seems to be related to an
uncomfortable shoulder fixation.
In fact, the shoulder actuation module is the most critical
component (Fig. 1). It includes 3 actuated DOF and 2
passive DOF, which is advantageous in that the human
shoulder is not constrained by the robot fixation. This
allows movements of the shoulder in 3 rotary DOF with
large ROM. Although the weight of the robot has been
compensated by a passive counterbalance system, additional
subject-dependent weight and active joint torques
produced by the robot will be transferred to the human
shoulder as the human arm is closing the open kinematic
chain of the robot. During standstill, the distal part of the
robot and the human arm beyond the passive axis 5 (Fig. 1)
is in a static equilibrium about axis 5. Therefore, the
resulting vertical force acting on the shoulder depends on
the robot and arm position and the weight of the mechanical
components and the human arm. The weight of the
distal part of the robot alone induces a vertical shoulder
force of approximately 8.5 N (robot arm in a horizontal
position, elbow extended). Furthermore, the drives of axes
1–3 produce reaction forces that are transmitted through the
shoulder joint to the environment, which further increases
the load at the shoulder.
Therefore, patients with instable shoulder joints, e.g.
resulting from shoulder subluxations, could not be treated
by the robot. Thus, our shoulder actuation approach is
acceptable for healthy subjects but problematic for patients
with shoulder problems. As many stroke patients suffer
from shoulder problems, it has been suggested to modify
the shoulder actuation module.
4.4 Prototypes
All simulations and measurements presented in this paper
have been performed with the presented 4 DOF version of
ARMin (Fig. 8). Derived from the results of this work, a
new version with 6 DOF has been set up (Fig. 12). The
robot is statically determined and equipped with a new
shoulder actuation principle. A system of linkages moves
the centre of rotation of the shoulder in the vertical plane,
when the arm is lifted. This function is required to provide
an anatomically correct shoulder movement and to avoid
the shoulder getting mechanically overstressed due to
misalignment of the technical and anatomical joint axes
when lifting the upper arm above face level. The device is
not evaluated yet