Cooperative Work in Virtual Environment with Force Display

We have developed cooperative work system.


Introduction

There are many advantages in cooperative work, for example sharing work, instruction and so on. There have been several systems proposed to support cooperative work in computer generated 3D space. Takemura and Kishino built a cooperative work environment using virtual environment by combining head tracking stereoscopic displays and DataGloves[1]. Loeffler et al. have developed a system called "Networked Virtual Art Museum" which is shared with distributed users via telephone lines[2]. Sato and Ishii have developed a networked 3D virtual environment using force feedback device called "SPIDAR". It enables users to communicate face-to-face and hand over virtual object[3]. When we practice penmanship or sports like tennis, trainers take trainee's hand and teach how to move it. Such trainer-trainee task is one of unique task for cooperative work as against for individual work. In order to teach something by taking trainee's hand in virtual environment, it is necessary to indicate reaction force from virtual environment. Most of virtual reality system, however, has no force feedback mechanism. In this paper, we describe about a method of teaching task in virtual environment with force feedback. We have developed a software tool for programmers of haptic virtual environment called VECS(Virtual Environment Construction System). Using the system, we developed a program that enables two users to work simultaneously in virtual environment with force feedback. The program provides force to bind two user's hands. This function assists teaching task in common virtual environment. VECS has a network capability by using Ethernet. Recently, it is easy to get a large quantity of information via network such as Internet. If we use such network, there is large time delay among users. The time delay imposed by limits on the speed of light(radio transmission) and computer processing at sending and receiving signals. It is unacceptable to feed resolved force continuously back to the same hand that is operating the control. This is because the delayed feed back imposes an unexpected disturbance on the hand that the operator can't ignore and which, in turn, forces an instability on the process. We propose a haptic interface that enables user to teach skill even thought there is time delay.

System Configuration

2.1 Hardware

Force feedback for each user is realized by a 6 degree-of-freedom master manipulator. Visual information is displayed by visual display device. The one site of the system employs two computers: a graphics computer for real-time monocular image of virtual space, and an I/O computer that supervises sensors and actuators. The I/O computer is equipped with analog-to-digital(A/D) converters and a parallel input/output unit. The graphics and I/O computers are connected by a serial(RS-232C) communication line. The graphics computer are IRIS Indigo2 and IRIS IndigoXS; the I/O computer are NEC PC-9801. Those sites communicate each other via TCP/IP socket connection. Hand position data and a flag which indicates grasping object are transmitted to the other site. At the same time each graphic workstations updates the database of virtual world. In this system, net transmission delay via ethernet is about 0.1 sec.

(1)Desktop force display

A 6 degree-of-freedom manipulator was developed as a force display. The manipulator applies reaction forces to the fingers of the operator. The manipulator employs parallel mechanism. The typical design feature of parallel manipulators is an octahedron called "Stewart platform." In this mechanism, a top triangle and a base triangle are connected by six length-controllable cylinders. This compact hardware has the ability to carry a large payload. The structure, however, has some practical disadvantages in its small working volume and its lack of backdrivability (reduction of friction) of the mechanism. In our system, three sets of parallelogram linkages(pantograph) are employed instead of linear actuators. Each pantograph is driven by three DC motors. Each motor is powered by a PWM(Pulse Width Modulation) amplifier. The top end of pantograph is connected with a vertex of the top platform by a spherical joint. This mechanical configuration has the same advantages as an octahedron mechanism has. The pantograph mechanism improves the working volume and backdrivability of the parallel manipulator. The inertia of motion parts of the manipulator is so small that compensation is not needed. The working space of the center of the top platform is a spherical volume whose diameter is approximately 40 cm. Each joint angle of the manipulator is measured by potentiometers. Linearity of the potentiometers is 0.5%. The maximum payload of the manipulator is more than 700gf, which is more than a typical hand.

(2) Graphic computer

Image of the virtual space is generated by graphics work stations, IRIS Indigo2 Extreme and IRIS Indigo XS. The CPU are R4000(100MHz) and R3000(33MHz), which manage virtual space and haptic representation.

Deform virtual object

mpeg 961kbyte Cooperative work

In usual virtual reality system using DataGlobes, the users can't grasp virtual object simultaneously. Using force display, we can grasp virtual object simultaneously.

Grasp another user's hand

User of VECS can grasp another user's hand. When user grasp another user's hand, he/she feel the force pulling toward the another user.

649Kbyte mpeg : Grasp hand

Method of Cooperative work in time delay.

We selected a trainer-trainee task in which an instructor teaches a beginner how to move virtual object. In such case, trainer takes trainee's hand. Our system provides force to bind two user's hand (Figure 6). In case there is time delay between them, the task is fairly difficult. The trainer's hand is pulled toward delayed trainee's hand and the trainee's hand is pulled toward delayed trainer's hand. That causes instability. The most important point of teaching task is to teach trainer's movement to trainee. We propose following force presentation for compensation of time delay.

(1) For the trainee, pulling force toward delayed trainer's hand is presented. Trainee feels like his/her hand is bound to the trainer's hand and is pulled by trainer.

(2) For the trainer, We present pulling force toward delayed his/her own hand. Trainer can check trainee's movement by visual feedback. However the trainer must keep in mind that there is time delay. If the trainer forgets the time delay and he/she reacts quickly to trainee's delayed movement, they are confused and that causes instability. These force plays role like viscosity or friction force to the trainer. The trainer can't move the hand so quickly.

Performance test of cooperative work in time delay.

The environment of experiment

As a usability test of this method, we examined repositioning task of virtual objects. The interface of this test is shown above. We put three targets at similar distance from the start point. There is a virtual ball at the start point. Users can only move it when both trainer and trainee grasp it. The ball is located at the middle point between trainer's hand and trainee's hand during it is grasped. Through the experiment, predictor display[6] is used. It displays current user's hand as semitransparent and delayed hand as solid. We can recognize what will be doing by predictor display. In this experiment, we use virtual balls in stead of flat surface. The reason why we don't use the same environment as previous experiment is that subjects can easily recognize whether they grasp the object or not. The experiment was conducted to estimate this technique under two conditions: with and without force feedback, through 0-s time delay, 1-s and 3-s respectively. Each condition contains 2 trials. We took 6 volunteer subjects from the student of our university. We examined accuracy of tracing the trainer's trajectory. Mean distance between trainer's hand and trainee's is calculated at each program cycle.

The avobe graph shows mean distance of the each condition. Horizontal axis indicates time delay. The mean distances between the trainer and trainee are indicated by bar chart. The data includes error bars which indicates standard deviation. Each value of condition with force feedback is 50% smaller than without force feedback in the same time delay (t=5.9882155, t=6.5586435, t=5.7867037 respectively, and critical value= 2.20098627).

Discussion

We can not find significant difference between two conditions from average completion time. Force feedback actually assisted the task, although resistance applied to the trainers required time to complete the task. However, we can find qualitative differences from mean distance. All subjects reported that force feedback is very useful. The trainee could feel his hand was pulled by the trainer. The trainer could feel the viscosity and reported that they could easily realize the distance between trainer's current hand and trainer's delayed hand. It suggests the trainer doesn't have to keep in mind the time delay.

Conclusion

We have developed the system which enables us cooperative work in virtual environment with force feedback. Through the experiment of deforming task, the effectiveness of teaching task with force feedback was demonstrated. In case that there is time delay between users, we propose a technique in which pulling force toward trainer's hand is presented for the trainee. And pulling force toward trainer's delayed hand is presented for the trainer. Through experiment of repositioning task, the effectiveness of the technique was demonstrated. In our current system, only the instructor can control the trainee. In order to support more interactive communication, however, the system should support bi-directional communication. Future work of our research will be development of bi-directional communication method.

References

[1] H.Takemura and F.Kishino. "Cooperative Work Environment Using Virtual Workspace" CSCW '92 proceedings (1992)

[2]Carl Loeffler. "Distributed Virtual Reality:Applications for Education, Entertainment and Industry" ICAT'93 Proceedings (1993)

[3] M.Ishii,M.Nakata and M.Sato. "Networked SPIDER" PRESENCE, Vol.3,No.4 (1994)

[4] H.Yano and H.Iwata. "Collaboration in Virtual Environment with Force Feedback" JPSJ SIG Notes 94-CG-69(1994)

[5] H.Yano and H.Iwata. "Cooperative Work in Virtual Environment with Autonomous Free-form Surface" The Transaction of The Institute of Electrical Engineers of Japan Vol.115-C,No. 2 (1995)

[6] T.B.Sheridan. "Space Teleoperation Through Time Delay" IEEE TRANSACTION ON ROBOTICS AND AUTOMATION,VOL.9,NO 5 (1993)


Other reference page: Groupware lab( KUZUOKA Lab), Univ.of.Tsukuba

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