ESS
The ESS (Experimental Servicing Satellite) –
Study and Lab Demonstrator
Immediately
after ROTEX, we started to build up laboratory experiments for studying
the dynamical behaviour and the rendezvous and docking capabilities of a
free-flying servicing satellite, consisting of a robot arm mounted on a
conventional chaser. A free-flying tele-robot ESS (Experimental Servicing
Satellite) was supposed to approach, inspect, and repair a malfunctioning
satellite. We emphasise that the movements of a robot arm, mounted on the
body of the servicing satellite, cause feedback to the carrier system,
which is non-negligible and might cause collision with the target
satellite. Therefore the effects of the manipulator’s motion onto the
carrier satellite have to be simulated, interpreted, and perhaps corrected
on-ground. We had to consider two satellite configurations, expressed by
their kinematic chains:
- ESS & mounted manipulator,
- ESS & mounted manipulator & docked target satellite.
The manipulator of ESS, equipped with a capturing tool, must follow the
residual movements of a selected object on the target (e.g. the main
thruster) by means of an image processing system whose data are passed
through an extended Kalman filtering process.
To simulate the dynamic behavior of the chaser during robot motions, we
have arranged two KUKA robots as shown below. Robot B is used to carry out
the capturing task, Robot A emulates the entire dynamic relation between
the chaser and the target satellite, where the dynamic coupling with the
AOCS is included.
After capturing the target satellite, the ensemble is stabilised and
reoriented. To free the manipulator for servicing activities and to
provide a stiff mechanical coupling, the target satellite is grasped by
means of a docking mechanism.
An interface to
the simulation of the AOCS (Attitude and Orientation Control System)
emulated the behaviour of the entire ESS under space conditions. A
special, in-house-developed capture tool, containing 6 laser range
finders, a wrist-mounted force-torque sensor, and a stereo camera pair,
allowed, in combination with the dynamics behaviour prediction, the fully
autonomous servoing, insertion, and capturing of the apogee motor nozzle,
which is typical for any geostationary satellite. We simulated the visual
servoing phase in the lab by the above-drafted two-robot system. One robot
carries a satellite mockup with an original-sized apogee nozzle, performs
a typical tumbling motion of a rigid body under zero gravity, and
superimposes the computed dynamic interactions of the service robot with
its (possibly) free-floating base. Thus in the lab the service robot (a
second robot) is inertially fixed and tracks the satellite in order to
capture it by inserting the capture tool into the apogee motor. The
approach of the target satellite is controlled by real-time, model-based
machine vision. Once contact between capture tool and apogee motor has
been established, force/torque-sensing takes over. In both phases all
Cartesian degrees of freedom are being controlled.