1.4 Patient cooperative arm therapy and specific
research aims
Since the therapy progress depends on training intensity
and training duration, the motivation of the patient turns
out to be a key factor for an efficient rehabilitation [14].
The patient needs to get motivated to contribute actively to
the movement, which may enhance recovery. In addition,
to make longer training duration and more training sessions
possible, it is crucial that the patient enjoys the therapy,
stays concentrated, and does not get bored.
The so called ‘‘cooperative arm therapy’’ discussed in
this paper takes into account the following key aspects that
motivate the patient: (1) the device stimulates the three
most important sensory modalities of the patient, i.e. the
haptic, visual and auditory senses and (2) the robot and
the patient cooperate and interact, i.e. the robot assists the
patient just as much as needed to perform a particular
movement task.
While many clinical studies (see [26] for review) have
been conducted with end-effector-based robots with limited
possibilities to control position and orientation of the
human arm in the three-dimensional space, not much
clinical evidence has been reported from work with actuated
arm-exoskeleton robots. The key aspects of this project
are that the ARMin device allows precise joint
actuation and 3D movement of the arm, that the device
allows to work in ‘‘patient cooperative’’ control modes and
that the device includes a comprehensive audiovisual user
interface.
2 Methods
2.1 Specifications
Training of ADL includes tasks like eating, drinking,
combing hair, etc. For most of these ADL tasks, the hand
has to reach a point in space, grasp an object, and then
control position and orientation of the object until the task
is completed. Therefore, the robot must be able to support
movements of the shoulder, the elbow, and the wrist.
Approximating the shoulder by a three-DOF ball-andsocket
joint, and allowing elbow flexion/extension, pro/
supination of the lower arm and wrist flexion/extension,
results in a device with at least six active DOF. To simplify
the task, our first prototype was built only with four active
DOF supporting the movements of the shoulder joint and
elbow flexion/extension.
The range of motion (ROM) must match as close as
possible the ROM of the human arm [35]. In order to obtain
a satisfactory control performance of patient-cooperative
control strategies, which are based on impedance and
admittance architectures, the robot must have low inertia,
low friction and negligible backlash. Furthermore, the
motor/gear unit needs to be back drivable. Back-driveability
is required for good performance of the impedance
control [10–12] and it is advantageous for the safety of
exoskeleton robots (cf. 2.10).
The required velocities and accelerations have been
determined by measuring the movements of a healthy
subject during two ADL tasks (eating soup and manipulating
a coffee cup). These values served as inputs for a
simple dynamic model applied to estimate the required
joint torques. In order to ensure that the robot will be strong
enough to overcome resistance from the human against
movements due to spasms and other complications that are
difficult to model, rather high values have been selected
(Table 1). The required end-point payload is 1 kg and endpoint
position repeatability is 10 mm. These values allow
manipulation of objects like a coffee cup.
Furthermore, it is required that the robot is easy to
handle and that safety is always guaranteed for both patient
and therapist.
2.2 Kinematics
A semi-exoskeleton solution has been selected for the
mechanical structure of the robot called ARMin (Fig. 1).
The robot is fixed via an aluminium frame at the wall with
the patient sitting in a wheelchair, placed beneath. The
patient’s torso is fixed to the wheelchair with straps and
bands (Fig. 8). ARMin comprises four active and two
passive DOF in order to enable elbow flexion/extension
and spatial shoulder movements [24]. The distal part is
characterized by an exoskeleton structure, with the
patient’s lower and upper arm placed inside orthotic shells.