Université Libre de Bruxelles (ULB)
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- Offer Profile
- The ULB is a multicultural
institution which has 7 faculties and a range of schools and institutes and
is, at the same time, a comprehensive university providing academic tuition
in all disciplines and study cycles.
With its three Nobel Prize winners, a
Fields medal, three Wolf Prize for physics, two Marie Curie Prizes and 29%
of the Francqui prizes awarded, the university is also a major research
centre which is recognised by the academic community the world over.
Vibration Isolation and Damping
- Hard and soft Stewart platforms for payload isolation and steering
- Higher precision pointing performances are required by optical payloads for
high data rate inter-satellite optical links communications, for space borne
interferometer missions instruments, and in general for telescopes pointing
applications. Sensitive payload pointing performances are adversely affected
by the mechanical vibration environment generated on board by mechanically
noisy equipment like momentum wheels, cryo-coolers, solar array drives,
fluid pumps etc.
Frequently a solution for vibration reduction consists in implementing soft
passive isolation mounts (typically by means of polymeric materials or
spring / damper systems). The simplicity and cost effectiveness of this
approach is counterbalanced by the limited performances that can be achieved
by its implementation. Indeed the efficiency of this system in the low
frequency range and for low vibration levels is quite limited. Furthermore
additional issues specifically related to the launch induced loads and to
the space environment (need for a launch lock device, de-pointing after
release, ageing effects…) may make their use less attractive.
Our research activity was therefore initially focused on active solutions.
Active vibration isolation normally includes sensor components (typically
for sensing displacements or accelerations or forces at critical locations),
actuators (typically piezo-electric or electromagnetic), power and data
processing capabilities for the control function. Among the various
possibilities, the so-called Stewart Platform architecture is particularly
attractive; it is an hexapod construction, where the six legs provide the
required degrees of freedom to the payload.
- The active vibration isolator built at the ULB (2004)
- Active isolation isolator for space interferometry
This project started in 2000; the objective consisted in building a six-degree-of-freedom generic active damping interface.
The Active Damping & Steering Interface is designed to connect two arbitrary structures: it can be used as a microvibration damping device, or as a high precision pointing mechanism. It is made of six legs, in accordance with the Stewart Platform architecture; each leg consists of a linear piezoelectric actuator, a collocated force sensor and two flexible tips for the connection to the plates. The maximum axial stroke is 90µm and the maximum tilt is 4mrad.
The legs of the interface are controlled in a decentralized manner with a “sky-hook” controller implemented with an Integral Force Feedback (IFF) control law.
(picture on the left)
Active damping of structures with piezoelectric
transducers generally implies at least a sensitive signal amplifier (for the
sensor), a power amplifier (for the actuator) and an analog or digital
filter (for the controller). The use of all these electronic devices may be
impractical in many applications and has motivated the use of the so-called
shunt circuits, in which an electrical circuit is directly connected to the
piezoelectric transducer embedded in the structure. The transducer acts as
an energy converter: it transforms mechanical (vibrational) energy into
electrical energy, which is in turn dissipated in the shunt circuit. No
separate sensor is required, and only one, generally simple electronic
circuit is used. The stability of the shunted structure is guaranteed if the
electric circuit is passive, i.e., if it is made of passive components such
as resistors and inductors. (picture on the right)
- The first machine, MAX, consists of a small,
38cm long, 1.3kg rectangular hexapod that has been build for gait studies.
Each leg has two degrees of freedom, actuated by position servo-motors, and
a contact switch in the foot. A wide variety of regular gaits have been
implemented. An algorithm for autonomous pit avoidance has been
SILEX consists of a 13kg, 50cm high hexapod with
hexagonal architecture. Each leg consists of a closed loop mechanical
structure with three degrees of freedom provided with DC motors. The
kinematics has been designed to achieve gravitational decoupling. The
control architecture is decentralized: each leg has its own control board,
based on a microcontroller INTEL 87C196KC, which solves the inverse Jacobian
equations in real time. The six local controllers and a two channel
inclinometer are onboard the robot.
A hierarchical control scheme involving 3 levels has been implemented. Level
A concerns navigation and path planning; it is ensured by a human operator
who prescribes the desired speed for the vehicle with a joystick (3
components). Level B includes the gait control, the attitude and altitude
control and the calculation of a force distribution reference. A free gait
algorithm (gait control) which allows a smooth and stable motion for an
arbitrary velocity vector of the vehicle (including a rotation about the
vertical axis) has been developed. The level B is implemented in a central
computer (PC). Level C handles the leg trajectory and the servo control as
well as the force control of the leg (active suspension). The force feedback
is provided by force sensors based on straingages that are included in the
feet. Level C is implemented at the leg level (6 microcontroller board).
IOAN is a 1.2 kg, 40 cm long walking robot. The robot has six
legs; each of them with two-degree-of-freedom. The chassis consists of three
articulated bodies connected by servo-controlled universal joints equipped
with torque sensors (strain gauges). This particular device produces an
active suspension and improves considerably the agility of the walking
vehicle, by allowing the central body to follow the ground profile.
Furthermore, the vehicle can walk on both sides and can recover from
roll-over thanks to the actuated universal joints which allow an autonomous
transfer from one side to the other. The simplicity of the leg kinematics provides robustness
and makes Ioan extremely easy to control.
Pipe inspection robots
- HELI-PIPE family consist of four different types of
robots for in-pipe inspection. The robots has two parts articulated with a
universal joint. One part (the stator) is guided along the pipe by a set of
wheels moving parallel to the axis of the pipe, while the other part (the
rotor) is forced to follow an helical motion thanks to tilted wheels
rotating about the axis of the pipe. A single motor (with gear reducer
built-in) is placed between the two bodies to produce the motion (no direct
actuated wheels). All the wheels are mounted on a suspension to accommodate
for changing tube diameter and curves in the pipe. The robots are autonomous
and carries their own batteries and radio links.
D-170 is a robot for 170 mm pipe diameter with the rotor rigidly connected
to the axis of the motor (placed on the stator) designed for pipes of small
curvature (radius larger than 600 mm).
- D-70/1 HELI-PIPE robot
- D-70/2 HELI-PIPE robot
- D-170 HELI-PIPE robot
Portable Arm Exoskeleton
- In many teleoperated or virtual activities with force
feedback, the use of a fully portable haptic device would increase the
easiness and performances of the command task, compared to devices linked to
the ground or a table. Applications range from robotic arm teleoperation in
severe environments (space, nuclear reactors, deep waters…). to applications
in virtual reality either in the domain of virtual training in large volumes
(such as virtual assembly) by means of immersion caves or head mounted
displays or in the domain of stroke patients rehabilitation. As the
operator does not have to be linked to a fixed base, or in environments with
obstacles and as a multi-DOF portable device allows force feedback on
several contact points, the operator is more immersed in the environment
during the manipulation.
The Sensoric Arm Master (SAM) has been designed as a wearable haptic
interface with a serial kinematics, isomorphic to the human arm. SAM
contains 7 actuated DOF corresponding to the joints of the human arm
(shoulder, elbow and wrist flexion/extension, shoulder and wrist
adduction/abduction, arm and forearm pronation/supination) and 6 sliders
allowing morphological adaptation between active joints and human
articulations. That corresponds to a good compromise between operator
immersion capabilities (maximised workspace and no singularity) and
mechanical complexity. Each joint of the exoskeleton has a similar
conception with a local actuator, a position and torque sensor, allowing
several kinds of control strategies (impedance, admittance control). The
actuation has been selected with a compact system composed of a brushed DC
motor, a capstan and gearbox.
Sam joint design
- Space Telescopes and Terrestrial Extremely Large
- Large light-weight telescopes in space are considered
key elements enabling future Earth observation and space science.
The first large space telescope, “Hubble”, uses a monolithic primary mirror
of 2.4 m diameter. The Hubble Space Telescope primary mirror has an area
density of about 180 kg/m2. This monolithic approach cannot be used for much
larger telescopes due to mass and volume limitations imposed by today’s
launch capabilities. Thus the current generation space telescope, the James
Webb Space Telescope now under development, makes use of a segmented primary
mirror of 6.5 m diameter. The segments will be folded during launch and will
deploy once in orbit. The position and the alignment of the segments will be
actively controlled to correct deployment and fabrication errors as well as
thermal and gravity disturbances on-orbit. The area density will be below 20
However, continued demand for new science and observation from space will
require even larger telescopes and diameters in the order of 20 m need to be
achieved. Entirely new concepts will have to be envisaged for deployable
space telescopes with primary mirror area densities below 3 kg/m2.
The Active Structures Laboratory of ULB is developing a prototype of a very
lightweight telescope in which the segments in addition to being positioned
and oriented will also be deformable.
- Nanotechnology tries to develop new kind of materials
and tools to increase the performance of sensors, actuators, computers,… One
of the biggest challenges of this technology is the manipulation of
components with dimensions less than 100 nm and subjected to forces at
molecular level like Van der Waals, electrostatic, capillary and chemical
forces. A lot of applications can be found in several fields like
biotechnologies (ADN and protein study), data storage or material science (
nanotube or surface film characterization).
One way to achieve nanomanipulation is the use of a surface imaging tool
called AFM (Atomic Force Microscope). The nanometric objects are manipulated
by the AFM’s cantilever with a feedback loop on the exerted force. The
displacements are done by piezoelectric actuators and the interaction forces
are measured through the deflection of the cantilever.
As the dimensions are bellow the micrometer, it is impossible for an
operator to observe his manipulation through an optical microscope. To
achieve an effective work, an other kind of interface between the user and
the nanoworld is necessary. In a teleoperated manipulation, it is composed
of a 3D graphic virtual reality and a haptic device. This last device exerts
scaled forces from the AFM measurement to the operator and sends the scaled
position from the hand of the user to the microscope tip. This setup can
improve drastically the controllability and efficiency of nanomanipulations.
In our Laboratory, we use an AFM SMENA A from NT-MDT as nanomanipulator. On
the other side, we use two kinds of haptic device. First we have developed a
3 DOF haptic device with voice-coil actuators and potentiometer sensors.
Second, we use a 3 DOF desktop Phantom haptic (Senseable technologies). The
two main parts of this hardware are linked by high performance
microcontrollers and a modular real-time program in the MATLAB environment.
With our system, we realized manual mechanical lithography in CD sample and
we succeed to sense surface topography and surface forces (like capillary)
through the haptic device.
- Main structure of an AFM nanomanipulator
- DOF haptic device for teleoperated nanomanipulation
- Teleoperated nano-litography on CD surface