  Parallel robots provide the potential to be leveraged for human-robot
collaboration (HRC) due to low collision energies even at high speeds resulting
from their reduced moving masses. However, the risk of unintended contact with
the leg chains increases compared to the structure of serial robots. As a first
step towards HRC, contact cases on the whole parallel robot structure are
investigated and a disturbance observer based on generalized momenta and
measurements of motor current is applied. In addition, a Kalman filter and a
second-order sliding-mode observer based on generalized momenta are compared in
terms of error and detection time. Gearless direct drives with low friction
improve external force estimation and enable low impedance. The experimental
validation is performed with two force-torque sensors and a kinetostatic model.
This allows a new identification method of the motor torque constant of an
assembled parallel robot to estimate external forces from the motor current and
via a dynamics model. A Cartesian impedance control scheme for compliant
robot-environmental dynamics with stiffness from 0.1-2N/mm and the force
observation for low forces over the entire structure are validated. The
observers are used for collisions and clamping at velocities of 0.4-0.9m/s for
detection within 9-58ms and a reaction in the form of a zero-g mode.
