Project : alcove
Section: Scientific Foundations
Piezoelectric actuators in haptic devices
Piezo-electric actuators are quite interesting for haptic devices. First, there are well suited for low speed-high force applications, so they do not need any reduction gear, avoiding backlash and control loss. Moreover, thanks to their energy conversion principle, such drawbacks as cogging torque or slot effects concerning electromagnetic actuators do not appear here. Besides, multi degrees of freedom are available on a few actuators, ( for instance 2D translator), although classical electromagnetic ones only allow one rotation or one linear moving. Last, most piezo-electric actuators are locked for no voltage feeding, which may be interesting for keeping the last position of the haptic device.
Identification and Control of a planar piezoelectric actuator
Within the aim of a multidegree of freedom actuator, a piezoelectric actuator is studied by Ph. D. student François Pigache . In order to establish the Force control, this one was the subject of a first causal non-linear model, in order to understand its behavior. Thereafter, the simplification of contact interface made it possible to define a simplified linear model. Its representation is based on the Causal Ordering Graph, that is useful for determining the control rules, by a simple inversion of the Graph. This model was validated by comparing simulations results and experimental caracteristics from a previous thesis.
Using commercial piezo-electric actuators in force-feedback devices
Travelling Wave Ultrasonic Motors are low speed-high torque motors, and thus may be well suited to force-feedback devices.
In that purpose, a simplified model of the motor is first established; we checked its accuracy by experimental tries. This model helped to improve our understanding of its behavior, and led to a new method for identifying the motor's parameters. We proposed a command scheme and applied it in a force feedback stick. The design is simple, but needs a torque measurement. So we carry on studies in order to achieve torque estimation and remove the torque sensor. Looking forward, fitting the DigiTracker with piezo-electric force-feedback would be possible.
In 3 dof workspaces, a heavy cognitive load resulting from the difficulty of representation of the environments decrease the users control abilities. Also, few devices allow to achieve pointing, tracking and selecting tasks in a precise, fast and intuitive way.
In June 2003, we proposed a new desktop USB device called "Digitracker"   for 3d. This miniature master arm with 3 pivots has a low apparent mass and is isotonic. The cognitive load is reduced from its use in absolute mode and its position control. The user's forearm and hand are laying on the desk and the end effector (switch) is held between the thumb and the forefinger. Possible applications are remote positioning tasks or CAD in simultaneous use with the Digihaptic. The addition of ultrasonic motors in order to provide force feedback is being studied.
Development and evaluation of a new haptic device
The DigiHaptic is a three degrees of freedom ground-based device that is comprised of three levers associated with the thumb, forefinger and ring finger (see 6.2). The DigiHaptic has been evaluated and compared to the SpaceMouse in 3D steering task. We found that users performed faster on the SpaceMouse but were less coordinated and accurate than on the DigiHaptic for the most complicated paths. The DigiHaptic is now going to be evaluated and compared to other devices in navigation tasks where it appears to be well suited.
During his student project, Sylvain Hénot  developped some demo applications to show the DigiHaptic performances in manipulation and navigation tasks with force feedback. A demo with an OBJ files loader has been developed to show the objects manipulation performances. Another demo concerns navigation in 3D worlds with force feedback where it possible to load the rich 3D worlds that can be found in games maps. Another application allows the DigiHaptic to be connected to any application such as Catia or 3D Studio Max where the SpaceMouse can be used.
Optimal Control applied to haptic devices
Interconnection between a haptic device and a virtual environment generally brings stability problems. So, a compromise between the compensation of the mechanic defects and the stability of the system is necessary. This compromise is given by perception of the user. The objective of Nicolas LEROY, Ph. D. student, is to find control laws, which explicitly optimised user perception. For the problem's formulation, it is necessary to know explicitly the user perception and to choose a well-defined framework of optimisation (like flat systems). Finally, an optimal control law gives force feedback.
Application of haptic feedback have been limited due to the lack of cutaneous sensing. Today, we want to create a user-centered design tactile actuator which will provide cutaneous information and fit into kinesthetic devices.
François MARTINOT, Phd student, starts this work in studying the shapes, roughness and temperature cutaneous perceptions in order to define technology requirements in relationship with a real touch mode.
In collaboration with IEMN laboratory (Philippe Pernod) we participate to the study of a dense pin array based on usage electrostrictive technology.
A travelling Wave tactile device
This device is made of a stator from a Travelling Wave piezo-electric motor. When the finger tip touches the surface, different friction sensations are felt according to the travelling wave's speed. The sensation can be improved if the finger tip's speed is measured and taken into account for the travelling wave's speed reference.