Grasping Virtual Objects with Multi-�oint Haptics

Quan-Zen Ang, Ben Horan, Zoran Najdovski, Saeid Nahavandi

Centre for Intelligent Systems Research, Deakin University, Australia

ABSTRACT The majority of commercially available haptic devices offer a single point of haptic interaction. These devices are limited when it is desirable to grasp with multiple fingers in applications including virtual training, telesurgery and telemanipulation. Multi-point haptic devices serve to facilitate a greater range of interactions. This paper presents a gripper attachment to enable multi-point haptic grasping in virtual environments. The approach employs two Phantom Omni haptic devices to independently render forces to the user’s thumb and other fingers. Compared with more complex approaches to multi-point haptics, this approach provides a number of advantages including low-cost, reliability and ease of programming. The ability of the integrated multi-point haptic platform to interact within a CHAI 3D virtual environment is also presented. KEYWORDS: haptic grasp, haptic gripper, multi-point haptic, object manipulation, virtual reality, CHAI 3D. INDEX TERMS: H.5.2 [User Interfaces]: Haptic I/O

1 INTRODUCTION Multi-point haptic devices [1] facilitate interactions which are not possible with a single point of interaction, for example object grasping [2] and size discrimination [3]. Despite this limitation, commercially available multi-point haptic devices are scarce. Prototype devices [4-7] represent one alternative however are often very complex, cost prohibitive or do not satisfy the requirements of haptic grasping. This paper presents a multi-point haptic platform consisting of a gripper attachment and two low-cost commercially available desktop haptic devices. Forces can be independently rendered to each finger and accurate control over the rotation of the grasped object can be achieved. The approach enables accurate and intuitive grasping and manipulation of objects in virtual environments and offers low-cost, reliability and ease of programming.

2 METHOD As the basis to present this approach to multi-point haptic grasping, this work considers the various contact models for a haptic interaction point in a virtual environment. Three contact models (frictionless, frictional and soft) [8] were considered, and the soft contact model was chosen because it only requires two such points to impose a force closure on an object. A force closure grasp immobilises the grasped object and is able to resist disturbances in any direction. The soft contact models were implemented in the virtual environment allowing the user to grasp and orientate the virtual objects.

Two Phantom Omni haptic devices are coupled together by the

introduced gripper attachment which resembles a Sarrus linkage (Figure 1). This constrains the relative motion of each finger so that the fingertips can only move toward or away from one another along a single axis (the grasp axis). This constraint eliminates the relative rotation of the fingers about the grasp axis. Relative rotations are undesirable in typical grasping tasks as they can cause uncertainty in the rotation of the grasped object. As a result of the constraint, the rotation of the object about the grasp axis can be accurately controlled. Utilising two haptic devices enables forces to be displayed at each finger independently. It is suggested that this provides an advantage over devices which apply equal and opposite forces to the two fingers [8].

The multi-point haptic platform (Figure 2) is operated by inserting the thumb and a finger into the thimble-like openings of the gripper attachment. Interchangeable finger holders allow for different thumb-finger pairing, size variations, as well as ambidextrous use of the platform. A base secures the two Phantom Omni haptic devices side-by-side at 170 mm spacing. Finger separation of up to 100 mm can be achieved with the gripper attachment. The gripper attachment weighs 43 grams, slightly less than the total 50 grams of removed stylus and connection jacks of both devices.

Figure 1. Gripper attachment

Figure 2. Multi-point haptic platform

qa@deakin.edu.au, ben.horan@deakin.edu.au, zoran.najdovski@deakin.edu.au, saeid.nahavandi@deakin.edu.au

189

IEEE Virtual Reality 201119 - 23 March, Singapore978-1-4577-0038-5/11/$26.00 ©2011 IEEE

3 RESULTS In order to evaluate the workspace of the multi-point haptic platform, the workspace of a single Phantom Omni haptic device was first considered. This served as the basis to compare the workspace with that of the multi-point haptic platform. While the determination of the Phantom Omni’s workspace is straightforward, the workspace of the gripper attachment (two coupled devices) depends on its orientation in the global reference frame. A simplified analysis where the orientation of the gripper attachment is considered as constant is presented. A point in the centre of the two finger interaction points is used as a reference. Figure 3 depicts the right-half of the approximate volumetric workspace of the reference point. To aid in clarity, given that the workspace is symmetrical about the Y-Z plane, only half of the workspace is depicted. Subject to the limitation that the fingers must be less than 60 mm apart and there is no change in the orientation, the shown workspace representation is useful to indicate the volume in which a virtual object can be grasped. The relative uniformity of the volume is a desirable attribute. It was also observed that the workspace of the gripper attachment subject to the above discussed conditions is not significantly smaller than that of a single Phantom Omni.

Figure 3. Contour plot of the right half of the collective workspace

of the multi-point haptic platform

The virtual environment utilises the CHAI 3D open-source library [9]. It was executed on a Windows XP PC with 2 GB of RAM and running on a 3 GHz Intel Core2 Duo CPU. Figure 4 is a screenshot of the CHAI 3D cube example. The library and example was adapted to run two Phantom Omnis haptic devices concurrently with the necessary position and force offsets. To enable physics rendering, the virtual environment utilises an Open Dynamics Engine (ODE).

The screenshot demonstrates a virtual cube stacking task. The introduction of a second point of interaction enabled the user to intuitively grasp the virtual cubes, lift and orientate them, and stack them on top of one another. In contrast, the original cube example featured a virtual spring attachment method that made stacking difficult as the cubes would swing and bounce around uncontrollably. These observations support the importance of multi-point haptic grasping for certain tasks and applications.

Figure 4. Grasping and stacking virtual cubes with CHAI 3D [9]

4 CONCLUSION A new approach enabling multi-point haptic grasping was presented in this paper. The approach utilised two low-cost commercially available Phantom Omni haptic devices to independently display forces to each of the user’s fingers. A gripper attachment facilitated accurate control over the rotation of the grasped object about the grasp axis. Compared to more complex approaches to multi-point haptics, this approach is low-cost, reliable, and easy to program. The multi-point haptic platform was demonstrated to be simple and intuitive to use in a virtual environment utilising CHAI 3D.

REFERENCES [1] Z. Najdovski and S. Nahavandi, "Extending Haptic Device

Capability for 3D Virtual Grasping," in Haptics: Perception, Devices and Scenarios, ed, 2008, pp. 494-503.

[2] Z. Najdovski and S. Nahavandi, "Characterising a novel interface for event-based haptic grasping," in Robot and Human Interactive Communication, 2009. RO-MAN 2009. The 18th IEEE International Symposium on, 2009, pp. 992-997.

[3] S. McKnight, N. Melder, A. L. Barrow, W. S. Harwin, and J. P. Wann, "Psychophysical size discrimination using multi-fingered haptic interfaces," in Proceedings of Eurohaptics, 2004.

[4] A. F. Abate, M. Guida, P. Leoncini, M. Nappi, and S. Ricciardi, "A haptic-based approach to virtual training for aerospace industry," Journal of Visual Languages & Computing, vol. 20, pp. 318-325, 2009.

[5] S. A. Wall and W. S. Harwin, "Design of a multiple contact point haptic interface," in Proc Eurohaptics, 2001.

[6] S. Walairacht, K. Yamada, S. Hasegawa, Y. Koike, and M. Sato, "Two-handed multiple-finger virtual object manipulation environment with haptic cues," Electronics and Communications in Japan (Part II: Electronics), vol. 87, pp. 65-73, 2004.

[7] H. Kawasaki, T. Mouri, M. O. Alhalabi, Y. Sugihashi, Y. Ohtuka, S. Ikenohata, K. Kigaku, V. Daniulaitis, K. Hamada, and T. Suzuki, "Development of five-fingered haptic interface: HIRO-II," in Proceedings of the 2005 international conference on Augmented tele-existence, Christchurch, New Zealand, 2005, pp. 209-214.

[8] F. Barbagli, K. Salisbury, and R. Devengenzo, "Toward virtual manipulation: from one point of contact to four," Sensor Review, vol. 24, pp. 51-59, 2004.

[9] F. Conti, F. Barbagli, D. Morris, and C. Sewell, "CHAI 3D: An Open-Source Library for the Rapid Development of Haptic Scenes," in IEEE World Haptics, Pisa, Italy, 2005.

190