full Paper - Nguyen Dang Binh

2.1. The GRAB Haptic Interface
From a technical point of view, the GRAB interface is a new
high £delity, two-£ngers, 3D force-feedback Haptic Interface.
The interface, completely designed and developed by
PERCRO, is capable of replicating two independent force
vectors on contact thimbles attached to user’s £ngers (the
thumb and the index or both index £ngers). The interface
consists of two identical robotic arms (£g. 3); each arm has
six degrees of mobility and a workspace which covers a large
portion of the desktop (600mm wide, 400mm height, and
400mm depth). The £rst three DOF are actuated by DC motors
and allow to display an arbitrary force vector on the relative
thimble; the other three degrees of mobility, reproducing
a spherical joint, allow any rotation of the £ngers. In such a
way the user can orient his £ngers in any direction of the
space while experiencing the force feedback generated by
software application. In order to improve the transparency
of use of the device, a number of design guidelines have
been adopted. Remote localization of the motors, selection
of motors with high torque to mass ratio and light materials
adopted for the construction of the moving links allow to reduce
perceived inertia. For reducing friction and guarantee
zero backlash, all transmission systems between motors and
mechanical joints are achieved by metallic in tension tendons
routed on capstans and pulleys and no geared reducer
have been used. Furthermore, the actuation of shoulder is obtained
by a differential transmission system which allows to
achieve high isotropic control of the end-effector and which
helps to cancel spurious and misleading Coriolis effects at
high thimbles speed.
Figure 3: The new 3D force feedback Haptic Interface.
The interface is provided with a novel and open control
architecture which allow to reduce development costs and
facilitate upgrading of the device. The architecture is based
on a mix of commercial and custom components: the most
important are the motors power supply, which provides the
required current to the motors, and the standard SBC which
provides the required real time support for running a Real
c○ The Eurographics Association 2005.
Massimo Bergamasco / Future trends and Applications, Medicine
54
time operating system. The control system kernel will be
based on Linux and a real-time micro-kernel which ensures
high performances capabilities and allows the integration of
sophisticated control procedure to high level control software
(such as physical based modelling and force rendering).
Connection with the HGM is achieved by means of an
ECP parallel port interface which allow high data throughput
(>=500 Kbyte/sec), bi-directional communication, low CPU
resource request and no latencies. Furthermore the control
system identi£es, describes and compensates a wide set of
non linear features such as the arm kinematics and dynamics,
the unbalanced weight effects, the transmission model
of the elastic cable, the distributed friction effects, the electrical
features of the power supply, the errors due to structural
de¤ection under load conditions. In order to maximize
the precision in tracking £nger position and ensure coherence
between arms, two different calibration procedures are
implemented. This feature allows to obtain £ngers position
expressed respect to a user de£ned reference frame. Finally,
a set of higher level effects (embedded friction, textures, ³E)
have been implemented in control software.
2.2. The GRAB Haptic Interface
The new HGM is a C++ object-oriented toolkit, developed
by LABEIN, with all the algorithms that allow the user to interact
with any 3D virtual object providing: the haptic stimuli
to be rendered by the new haptic interface and sound aids
and speech recognition capabilities to improve the interaction.
The new HGM is based on DATum: an object oriented
variational non-manifold geometric modeller developed previously
by LABEIN. Its main role is to analyse the position
of the Usert’s £ngers (provided by the control systems of
Haptic interface) taking into account the action required by
the user (for example to make zoom or to get a speci£c help
audio) in order to get the corresponding audio messages and
calculate the values of the forces to be replicated by the haptic
interface on the Usert’s £ngers. The main functionality
provided by the HGM is the following one:
• Creation and rendering of virtual scenes
• Simulation of different types of forces in order to: represent
contact with an object, modi£ed by properties such as
stiffness, texture and stickiness; constrain the user to the
boundary of any virtual object; constrain the user to slide
along a trajectory or path; attract or repel a user to or from
an object; help the user to £nd an unexplored object; represent
the spring of a virtual button; simulate weight when
an object is grasped; detect when objects collide with another
one
• Audio feedback (both speech and non-speech) to provide:
– Static information: general information about the virtual
object/environment de£ned when the environment
was designed (for example the name of the object, ³E)
– Dynamic information: information related to the user’s