Section: New Results
Keywords : Haptic, Pseudo-Haptic, Control/Display ratio, Mass, Training, Milling, Under-Actuated.
Haptic interaction consists in providing the user of a Virtual Reality system with the sensations involved by touch (i.e. tactile and force feedback) during the manipulation of virtual objects. We describe hereafter our recent results in the field of haptic interaction which concern: (1) perception issues (the influence of Control/Display ratio on perception of manipulated objects), (2) interaction techniques with haptics (the "A4" technique and the "Bubble" technique), and (3) a vocational simulator (the "Virtual Technical Trainer").
Haptic interaction consists in providing the user of a Virtual Reality system with the sensations involved by touch (i.e. tactile and force feedback), mainly during the manipulation of virtual objects. Historically, the development of haptic interfaces originates from tele-operation. Indeed, the first force-feedback interfaces were developed for tele-operations within hazardous environments. But nowadays, a larger number of applications has been foreseen for haptic interaction in Virtual Reality. These applications belong to various fields: Medicine (chirurgical simulators, rehabilitation), Education (display of physical or mathematical phenomena), Industry (virtual prototyping, training, maintenance simulations), Entertainment (video games, theme parks), Arts and Creation (virtual sculpture, virtual instruments), etc. Thus, the field of "haptics" concerns an increasing number of researchers and companies specialized in Virtual Reality.
The integration of haptic feedback within a virtual environment raises many problems at different levels - including the hardware and software issues. Furthermore, a current major limitation for the design of haptic interfaces is our poor knowledge concerning human haptic perception. It is indeed fundamental to take into account the psychological and physiological issues of haptic perception when designing the technology and the use of virtual environments based on haptics. We therefore concentrated our work on both the perception issues and the implementation issues. We present hereafter our recent results in the field of haptic interaction in virtual reality:
the study of the influence of the Control/Display ratioon the perception of mass of manipulated objects in VR,
the Bubble technique: a novel interaction technique for large VE using haptic devices with limited workspace,
the A4 technique: a novel interaction paradigm for contact rendering when using under-actuated haptic devices,
the Virtual Technical Trainer: a virtual environment dedicated to the technical training of milling machines in VR.
Influence of Control/Display ratio on the perception of manipulated objects
In order to reach and manipulate virtual objects, VE generally provide the user with a virtual cursor (for instance a "virtual hand") which reproduces the movements of his/her real hand. The ratio between the amplitude of movements of the user's real hand and the amplitude of movements of the virtual cursor is called the Control/Display ratio (or C/D ratio). Our objective was here to study the influence of the Control/Display ratio on the perception of mass of manipulated objects in Virtual Environments (VE).
Thus, we have conducted a series of two experiments. In both experiments, a discrimination task was used in which participants were asked to identify the heavier object between two virtual balls (see Figure 18 ). Participants could weigh each ball via a haptic interface and look at its synthetic display on a computer screen. Unknown to the participants, two parameters varied between each trial: the difference of mass between the balls and the C/D ratio used in the visual display when weighing the comparison ball. The data collected demonstrated that the C/D ratio significantly influenced the result of the mass discrimination task and sometimes even reversed it. The absence of gravity force largely increased this effect.
These results suggest that if the visual motion of a manipulated virtual object is amplified when compared to the actual motion of the user's hand (i.e. if the C/D ratio used is smaller than 1), the user tends to feel that the mass of the object decreases. Thus, decreasing or amplifying the motions of the user in a VE can strongly modify the perception of haptic properties of objects that he/she manipulates. Designers of virtual environments could use these results for simplification considerations and also to avoid potential perceptual aberrations.
These results were published at IEEE Virtual Reality 2005  . This work was achieved as a collaboration with CPNI Lab., University of Angers.
The A4 technique
The objective of this work was twofold. First, it aimed at positioning the performance of under-actuated haptic devices as compared to fully-actuated haptic devices and unactuated devices (i.e. input devices) in virtual reality. Second, it proposed a technique - called "A4" (Automatic Alignment with the Actuated Axes of the haptic device) - to improve the perception of contacts when using an under-actuated haptic device in virtual reality.
The A4 technique focuses on point-based haptic exploration. When a contact occurs in the simulation, we suggest to rotate the virtual scene in order to align the contact normal with the direction of the actuated axis (or axes) of the haptic device (see Figure 19 ). With this technique, the virtual scene moves itself automatically to provide a more "realistic" sensation of contact.
An experimental evaluation showed first that the performance of under-actuated force-feedback was located between the no-haptic condition (worst performance) and the full-haptic condition (best performance). Second, the use of the A4 technique decreased strongly the "penetration" inside virtual objects, and thus globally improved the performance of the participants in situation of under-actuation.
These results were published at World Haptics Conference 2005  .
The Bubble technique
Haptic interfaces were shown to greatly enhance interaction with Virtual Environments (VE). Using such interfaces enables to touch, grasp and feel physical properties of virtual objects. However, in the case of grounded interfaces such as the VIRTUOSE force feedback arm, these devices allow a haptic interaction only inside their limited physical workspace. Therefore, the user can not reach and interact with virtual objects located outside this workspace easily.
The "Bubble" technique is thus a novel interaction technique to interact with large Virtual Environments (VE) using a haptic device with a limited workspace. It is based on a hybrid position/rate control which enables both accurate interaction and coarse positioning in a large VE (see Figure 20 ).
The haptic workspace is displayed visually using a semi-transparent sphere (looking like a bubble) that surrounds the manipulated cursor. When the cursor is located inside the bubble, its motion is position-controlled. When the cursor is outside, it is rate-controlled. The user may also "feel" the inner surface of the bubble, since the spherical workspace is "haptically" displayed by applying an elastic force-feedback when crossing the surface of the bubble.
This technique proved very useful to interact with a large VE using a haptic device with a limited workspace. The Bubble technique was presented to the Haption Company and it was much appreciated. It will be available in the next release of the VIRTUOSE API (the commercial haptic programming interface of Haption).
These results were published at World Haptics Conference 2005  . This work was achieved as a collaboration with CPNI Lab., University of Angers.
The Virtual Technical Trainer
Hundreds of people are trained to the use of milling machines in AFPA centers each year. Learning with a milling machine is a long and complex process. It is expensive since it requires a large amount of material and it implies maintenance costs.
Therefore, we have proposed a new system called the Virtual Technical Trainer (VTT). This system is dedicated to the technical training of milling in Virtual Reality. VTT simulates more specifically the milling activity. VTT provides milling trainees with a real haptic feedback using a PHANToM force-feedback arm. This force feedback is used to simulate resistances, when the tool mills the material.
We have also investigated the use of pseudo-haptic feedback to simulate force feedback within VTT. A pseudo-haptic feedback is incorporated in the VTT environment by using a passive input device - a SpaceMouse - which is associated with the visual motion of the tool on the screen. It is possible to simulate different sensations of resistance, by modifying appropriately the visual feedback of the tool's motion.
The last version of VTT can use a haptic device which was specifically designed for the purpose of our pedagogical application (see Figure 21 ). Furthermore, realistic audio feedback (which was recorded in real situations) and additional visual assistances could also be added, in order to increase the perception and understanding of the milling task.
A preliminary evaluation of VTT showed that this simulator could be used by vocational trainers successfully. It could help them to teach the basic principles of machining at the first stages of vocational training courses on numerically-controlled milling machines.
This work was published at VRIC 2004, EuroHaptics 2004, and more recently at IEEE Virtual Reality 2005  . This work was achieved as a collaboration with a consortium of industrial and academic partners : CLARTE (Centre Lavallois de Ressources Technologies), AFPA (Association Nationale pour la Formation Professionnelle des Adultes), and University of Paris 5. It was also related to the RNTL french platform for Virtual Reality "PERF-RV".