patient in order to increase or decrease the difficulty of the
game.
The control strategy (Fig. 7) is based on an impedance
controller, where the assisting force Fxref is determined,
based on the relative distance between the ball and the
human hand position. The force in horizontal direction
Fxref, that the robot exerts onto the patient’s arm to push
him towards the ball position is:
Fxref ¼
0
KðxballxhandÞ  ð1yballÞBdxhand
dt
 if yball[0:5
if yball0:5
ð6Þ
K is a constant value that can be adjusted by the therapist
and B is a damping factor. Typical values are K = 10 N/m
and B = 0.01 Ns/m.
During the first part of the rolling-down sequence
(yball > 0.5), the force Fxref is always zero. This kind of
delay gives the patient time to try to bring his hand to
the ball position without robotic support. In the second
part of the sequence (yball  0.5), the force Fxref is
proportional to the distance between the hand and the
ball (xball–xhand) and to the factor (1–yball), and therefore
zero if the patient was able to bring his hand to the ball
position during the first sequence. If not, then the supporting
force rises when the ball approaches the zero
level (yball ? 0).
2.10 Passive and active safety
Safety was a main issue during the design of the robot.
Passive safety features (no sharp edges, mechanical end
stops to guarantee that no joint can exceed the anatomical
range of motion, etc.) are combined with active safety
features. Four redundant absolute position-sensing potentiometers—
one for each joint—allow detecting malfunction
of a position sensor. Several surveillance routines are
implemented in the software. These include current and
speed monitoring, a robot self-collision detection algorithm
and several watchdog systems.
Whenever an abnormal event is detected, the safety
circuit immediately cuts the power of the motor drives. As
the robot is designed with a passive weight compensation
system (pulley, rope, and counterweight, see Fig. 8) it does
not collapse after power loss. Since all drives are back
drivable, the robot can easily be moved manually by a
therapist in order to release the patient from a potentially
uncomfortable or dangerous posture.
Last but not least, the physiotherapist always observes
the training holding a deadman button in his hand.
Releasing the button interrupts the motor power and stops
the robot immediately. This can also be achieved by
pressing one of the three emergency stop buttons. It is
expected that future robots will not require a permanent
supervision and that the deadman button could be omitted.
Beside patient safety, also safety of the therapist needs
to be considered. As the robot does not know the position
of the therapist, it is important that the therapist is aware of
the danger of collisions with the robot. Nevertheless, the