Virtual Reality Aided Assembly with Directional Vibro-Tactile Feedback Holger Regenbrecht∗ University of Otago Dunedin, New Zealand
Joerg Hauber† University of Canterbury Christchurch, New Zealand
Ralph Schoenfelder‡ Aremes Consulting Ulm, Germany
Andreas Maegerlein§ Design Tech Ammerbuch, Germany
Abstract
running systems in the industry that are well down the road towards achieving that goal.
We introduce different approaches to user interface devices that provide directed tactile feedback to the user’s hand. The basic idea is to enhance the user’s six degrees of freedom of interaction within virtual or augmented environments by offering an additional three-dimensional tactile feedback as an immediate, directed response from the virtual world. We also describe the prototype systems TactilePointer and TACTool, which utilize vibro-motors, alarm buzzers, and piezo bend elements as actuators in combination with magnetic and optical tracking. The prototypes have been informally tested within collision sensitive virtual environments.
If one wants to simulate a packaging task using VR technology, one has to consider more senses than just vision. In this context, haptic and acoustic feedbacks are of particular interest. Because of its importance we decided to further investigate the haptic factor (see [Salisbury et al. 2004], [Burdea 2000] for basic concepts). How can one supply the sensation of ”feeling/touching” to an assembly task? More precisely, how can one feel the physical contact of a virtual battery with its surrounding engine environment while the battery is being built in?
CR Categories: H.5.1.b [Information Interfaces and Presentation]: Multimedia Information Systems—Artificial, augmented, and virtual realities; I.3.6 [Computer Graphics]: Methodologies and Techniques—Interaction techniques; B.4.2 [Input/Output and Data Communications]: Input/Output Devices—Haptics Keywords: virtual reality, haptics, vibro-tactile feedback, interaction
1
Introduction
Virtual Reality (VR) technology is on its way to become an every day tool in certain fields. Although most of the promises made a few decades ago are still not reality, VR is widely used in areas such as design reviews, assembly and packaging tasks, military simulation, and chemistry. We argue that visual feedback only is insufficient especially when the task demands for precise spatial placement of objects relative to each other (see [Jayaram et al. 1999], [Bloomfield et al. 2003]). This applies particularly to packaging tasks of digital mock-ups in the automotive and other engineering industries. Packaging refers to the simulated assembly and disassembly procedures on a physical or virtual object and takes place at an early stage of the product’s development process. To drastically shorten development cycle times and to cut costs, many manufacturers are going to apply Virtual Reality technology in a broad way to this part of the process [Sauer 2001]. The ultimate goal, the achievement of a comprehensive, 100 percent digital mock-up (DMU) development process, is far from reality; but today there exist some successfully ∗ e-mail:
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The most obvious solution to this kind of problem would be to use force feedback (FFB) devices, like a SensAble Phantom device (http://www.sensable.com) in the VR setting. Some promising research was undertaken e.g. by [Buttolo et al. 2002] or [Salisbury and Srinivasan 1997]. While very good results can be expected regarding the sensation of feel, the instrumentation of the real environment is enormous in terms of space, infrastructure, maintenance, and costs. In addition, the simulated working space is limited to the operational space of the FFB devices. A second approach is to use physical mock-up components in combination with Augmented Reality (AR) or VR. The basic idea and concept was introduced by [Ishii and Ullmer 1997]. In our case, the battery and its installation space would have to be physically produced using perhaps rapid prototyping technology, then it would have be tracked within the real environment and finally represented within the virtual environment. This approach already saves some effort in the creation of physical mock-ups, as only relevant parts need to be moulded, and can be considered as a step towards the final DMU goal. Still it lacks comprehensiveness. A third approach utilizes tactile feedback (TFB) instead of FFB. The human tactile sense is the part of the human haptic system, which is associated with the sensation of touch perceived as stimulation on the skin’s surface. Although this kind of sensation is limited to low forces only, it can be applied much more easily in a VR setup than FFB. Early research in this field combined TFB actuators with data glove technology (http://www.immersion.com). However, the concept of data gloves turned out to be rarely accepted and thus this approach is not very promising. The integration of TFB into more commonly used input devices such as mice (like [Hughes and Forrest 1996] or Logitech iFeel MouseMan, www.logitech.com) or trackpoints (like [Campbell et al. 1999]) is more advantageous as the users are already familiar and efficient with these devices. These devices typically generate a simple on-off-type of vibration to address the tactile channel, but unfortunately no directional feedback is given with this approach. In the example of the virtual battery assembly, the collision of the battery could be transmitted using a vibrating mouse for example, but the direction of collision could not. TFB can be generated using different technologies including electrical impulses, air pressure, and mechanical forces. Out of these approaches the application of vibro-tactile feedback proved to be one of the most promising when regarding effort and effect (see http://www.vibrotactile.org/references.html for a comprehensive bibliography on vibro-tactile feedback).
[Kontarinis and Howe 1995] have shown that the use of low-cost, VT actuators in assembly tasks can increase performance and reduce errors.
annoying factor. Therefore we rejected the use of miniature loudspeakers for our approach.
The approach presented here is unique due to the integration of directional vibro-tactile feedback into a tracked interaction device (handheld). We will present two prototypes that were built to support VR aided assembly tasks during the last two years. The prototypes implement different form factor approaches as well as different technologies. We will discuss these issues in detail and outline the results of informal user testing in different setups.
2.1.2
2
Vibrotactile feedback in the user’s hand
Electromagnetic Alarm Buzzers
These devices are mainly used for indication of certain system states in low-budget customer devices. The alternating field of a coil causes a permanent magnet, which is attached to a flat spring and a foil membrane, to oscillate. This results in noise created by the vibration. When touching the device with the foil removed, a noticeable vibration can be felt with best results achieved at amplitudes of +/- 10 Volts at a frequency of 150 - 200 Hz. The main shortcomings of this technology are the disturbing magnetic fields (interferes with magnetic tracking) and the very limited contact area in relation to the size of the device.
In order to create a vibro-tactile stimulation, vibro-actuators are brought into contact with the skin. This triggers the mechanoreceptors in the stimulated area via vibrations. The important factors to be considered for the perception of this stimulation are: vibration frequency, vibration amplitude, the density of the receptors, and the adaptation behaviour. [Kaczmarek et al. 1991]
2.1
VT approaches
There are multiple technical approaches for vibro-tactile stimulation on the market and in research. One has to choose the appropriate technical solution according to: the stimulus to be achieved, acceptance, prize, electrical and mechanical environment of the actuator to be built in, and availability of the technology. In general, two different approaches can be distinguished: (1) the application of a physical force to the skin or (2) electrical stimulation. We will focus on the mechanical solution for acceptance and availability reasons. In principle, one could use virtually any device that transforms electrical energy into small mechanical movements in the order of a micrometer or a millimetre to create vibro-tactile stimulation. For example, in a study by [Deguara et al. 1999] a 3x3 stimulator array of loudspeakers was built into a vest; [Lee and Kwon 2001] use an electro-magnet with a kernel, and the Cutaneous Communication Laboratory at Princeton University utilizes bimorphic piezo bend stripes. One device dedicated to VT stimulation is the Shape Memory Alloy (SMA) [Mascaro and Asada 2003]. However, we chose to investigate the use of customizable off the shelf (COTS) components that met our requirements. In the following, we review a selection of components that we regard to be suitable for VT-prototype-design and briefly set out their advantages and disadvantages.
2.1.1
Figure 1: Electromagnetic alarm buzzers in test
2.1.3
Cell Phone Vibro-Motors
Cell phone vibrators consist of an electro-motor with an eccentric mass attached to it (see figure 2). When the motor starts to spin the mass generates the vibration. The higher the voltage (typically amplitudes of 2-3 Volts) the higher the vibration perceived. Controlling the frequency is much more complicated because it depends mainly on the eccentric mass. It is easy to couple the vibro-motor unit to a housing, which is desirable to achieve an overall vibration signal. When a more focussed stimulation is required (like to the fingertips) one has to ensure an appropriate decoupling. Cell phone vibro-motors are easy to apply, but complicated to control in terms of the amplitude and frequency of the vibration. Preferably, they would be controlled with a (direct current) square-wave signal.
Miniature Loudspeakers
We have tested three different COTS loudspeakers that could probably fit into a handheld device. Amplitude and force are perceived clearly when the speakers were controlled by a voltage of +/- 5V and a frequency of about 200-250 Hz. The membrane already present serves as an ideal carrier for mechanical oscillations when touched lightly. Unfortunately, there is a direct correlation between force and size of the moving coil, which make the loudspeaker solution almost impossible considering the size of the desired device. Additionally the noise generated by the devices (starting at about 50 Hz) is an
Figure 2: Cell phone vibro-motors
2.1.4
Piezoelectric Bending Actuators
Piezo benders consist of two material layers: a ceramic piezo layer and a metal layer. When passing a current between these two lay-
ers the piezo element shortens in length (piezo-electric effect) and bends the bi-material element. This effect can be used to provide a vibro-tactile stimulus. E.g. with a voltage of +/- 70 V and a (resonance) frequency of about 300 Hz one can achieve a mechanical displacement of +/- 200 micro meters at a blocking force of 0.15 N, which would be easily felt by the recipient. The main advantages of piezo elements (figure 3) are: (1) no magnetic ”pollution”, (2) simple (maintenance free) design, and (3) flat and small form factor.
the hand to represent the direction ”down”, (2) the index finger for ”left”, and (3) the thumb for ”right”. To represent the direction ”up”, both, the index finger and the thumb, were simultaneously stimulated. In order to signify the directions ”front” and ”back”, a different approach was chosen. Placing additional actuators at relevant skin parts would have required an adjustment of the grip’s shape. We discovered that if an actuator is firmly attached on one end of a long, stiff object such as the plastic housing of TactilePointer One, users could easily perceive on which end the source of a vibration was located. We used this ability and placed two cellphone motors on each end of the plastic housing to represent the directions ”front” and ”back”. The cell-phone motors were picked because they provided a stronger stimulus compared to the buzzer types used for the other directions. The latter actuators in turn allowed a direct physical contact to the skin, while the rotating parts of the motors seemed to prevent an application in such a way.
Figure 3: Piezo bend bars
2.2
Tracking
For virtual assembly tasks the user’s hand or the interaction device needs to be tracked (position and orientation in the real environment). There are several tracking technologies used in such VR environments: mechanic, acoustic, magnetic, optic, inertial, or combinations of these. Magnetic tracking systems (AC or DC) are most commonly used, because they are easy to apply and have been available on the market for over a decade. The main disadvantage is their sensitivity to metals or electro-magnetic fields within the environment, which contribute to significant errors in measurement. [Specht et al. 2000] Optical tracking methods are more expensive and require some effort for setup, calibration, and a guaranteed line-of-sight. They do deliver very precise tracking results however. When trying to haptically support assembly tasks in an industrial development environment one cannot only choose the best tracking method for the sake of haptics, but rather has to select from the systems already present in the environment. In our case, the options were limited to either magnetic tracking (Ascension MotionStar (http://www.ascension-tech.com) or Polhemus Fastrak (http://www.polhemus.com)) or optical tracking (A.R.T. Dtrack (http://www.ar-tracking.com)).
3 3.1
Prototype I (”TactilePointer”) TactilePointer One
Based on our review of ”off-the-shelf” vibro-actuators, we developed our first prototype of a handheld interaction device with multiple spatially separated vibro-actuators [Hauber 2002]. Three VT buzzers and two cell-phone motors were applied. As can be seen in figure 4, the buzzers were placed to stimulate (1) the palm of
Figure 4: TactilePointer One: Principle The arrangement of our actuators assumes that the pointer is held in the way shown in Figure 4 with the back of the hand pointing downwards. In this posture, the relative position of the stimulus to the coordinate frame of the device is in accordance with the orientation of the user in his environment. That is, if the user feels a vibration on his thumb, which is located on the right side of the device, then he or she can easily map this stimulus to the direction ”right”. The actuators are controlled by a professional grade D/A PC card (Meilhaus ME-3000). A small amplifier circuit board was built to provide the required power. Also integrated into the plastic tube was an Ascension MotionStar sensor. The API libraries for the D/A controller and the tracking sensor were integrated into an OpenInventor based Virtual Reality software package (DBView, [Sauer 2001]). The control of the three alarm buzzers followed a simple angular interpolation scheme, which maps the possible 360 degrees to the three actuators. The data of the tracking sensor were mapped 1:1 to the virtual environment.
3.2
Usability
In the first usability evaluation, we were especially interested in whether users could ”understand” the direction of the vibration stimulus they perceived and if this information could help to improve a spatial navigation task within a virtual environment. The task we chose was to navigate a virtual red ball along a threedimensional spiral curve as can be seen in Figure 6. The goals were (a) to keep the ball on track as well as possible and (b) to complete the task as fast as possible. Participants wore a tracked, head mounted display (monoscopic) and were immersed in a Virtual Environment where a virtual spiral
In total 27 subjects participated in this study. After a short introduction, they were able to try out the device to get an idea of what the vibrations felt like. Afterwards, every participant fulfilled the task in every condition. The order of conditions was randomised to avoid learning effects. During each trial, the distance between the position of the ball and each occurrence of feedback were constantly logged. Based on this data, we were able to determine how often the feedback resulted in successful corrective action taken by the user. We considered the feedback to be ”successful” if the user clearly reduced the distance between the ball and the curve directly after the feedback occurred. Figure 5: TactilePointer One: Actual device with controller unit
was displayed right in front of them. The position of the red ball was determined by the tracking unit located in the TactilePointer. Therefore, to navigate the ball in the virtual space, participants simply had to move the tactile-pointer in the real space.
Figure 6: Screenshot of test task If the position of the red ball deviated from the given spiral curve for more than a certain limit, we provided feedback to help the participant get back on the curve. Besides the tactile feedback, we provided visual feedback in the form of flashing arrow cones located around the ball (see figure 7).
In general, participants quickly picked up the concept of the directional vibro-tactile feedback and reliably named the correct directions of the feedback during the initial warm-up test phase. Initially some participants were a little startled when the device suddenly started to vibrate in their hands. However, after a short period, they got used to the stimuli; they were able to focus more on the direction that each stimulus signified. Overall, the participants enjoyed the task and all of them were able to successfully guide the ball to the end of the spiral. Concerning the individual actuators, the following success-rates were achieved:
Figure 8: Successful tactile feedbacks by direction stimulus for TactilePointer One This result suggested that our approach of mounting the cell-phone motors on the front and on the back of the housing tube compared well with the VT-buzzers that directly stimulate the skin of the hand for guiding users corrective actions. Surprisingly for us was the poor success rate for the ”down” stimulus on the palm and ”left” stimulus to the index finger tip. This indicated a possible cognitive mapping problem. The following results were found in the three conditions.
Figure 7: Screenshot of test task The feedback direction was pointing away from the spiral suggesting a collision with an imaginary outer tube that occurred when the ball lost the track of the spiral. This meant for example that if the ball was located too far to the right side of the spiral, the feedback ”right” was given to guide the user to move the ball further to the left and get the ball back on the spiral curve. The single conditions were: 1. Tactile feedback only 2. Visual feedback only 3. Tactile and visual feedback combined
Figure 9: Relative successful tactile feedback and average completion time by condition The visual feedback only condition achieved the fastest average completion time. However, it also reached the worst feedback success ratio. In the tactile only condition a much higher success rate could be achieved. Apparently, it took subjects a little longer to interpret the tactile stimulus. However, once they understood it, they used the given information for effective corrective action reasonably well. The big benefit of tactile feedback over visual feedback was that it could always be perceived and was not sensitive to occlusion like the visual feedback.
Figure 12: TACTool: Scheme Figure 10: TactilePointer Two: Photo of device pants had some previous experience with the task and with the handling of TactilePointer One. Unfortunately, the new placement for ”left” and ”right” (applied to first and third segment) on the index finger still suggested some cognitive mapping problems. TactilePointer One and TactilePointer Two demonstrated that the use of directional vibro-tactile feedback is a promising concept for navigation tasks in virtual environments.
Figure 11: Successful tactile feedbacks by direction stimulus for TactilePointer Two
Surprisingly, users in the visual and tactile feedback mode did not perform as well as in the tactile feedback mode only. This indicates a sensorial overload for inexperienced users when they get offered information to the visual and the tactile channel simultaneously. Apparently, untrained users preferred to concentrate on one channel only. Also surprisingly, our unorthodox approach of mounting the VT motors for stimulation directly to the housing (tube) lead to a very good perception of ”front” and ”back”.
4
Prototype II (”TACTool”)
Starting with the findings and experiences gained from the first prototype (”TactilePointer”) an informal survey with potential users from automotive development departments was undertaken. Besides the pros and cons that were identified earlier, some additional features and shortcomings arose. This lead to the following major requirements for the new prototype: • Improvement of ergonomics and handling • Improvement of tracking (especially accuracy and drop-outs)
The major shortcomings turned out to be: the awkward handling, the unusual form and design of the device, the weak mechanical amplitude of the alarm buzzers, the relative lack of sensitivity of the palm for perceiving the feedback, and the difference in perceived amplitude of the (strong) VT cell phone motors in comparison to the weak alarm buzzer signals.
• Standardised types of actuators (irritation arose from the use different stimulation forms with vibro-motors and piezo benders)
3.3
• Improved aesthetics
TactilePointer Two
Based on these findings a second, revised version of the device was developed (see figure 10). Aside from some form factor improvements the main modification was the replacement of the alarm buzzers with piezo bend bars. The principle of attaching the VT motors to the tip and end of the tube was retained because of the good perceptual results. The three alarm buzzers were substituted by four piezo benders, which had been embedded into rubber foam and covered by thin plastic strips. Two buttons for system control were integrated as well. The test scenario was slightly modified (introduction of a cone in the inner part of the spiral as a ”view barrier”) and informally evaluated with seven of the previous 27 subjects from the first user study. The vast majority of users assessed the new version as generally better regarding handling and form. The general ratio of successful feedback was also higher than in TactilePointer One, especially for the direction ”down”, as can be seen in figure 11. However, this might have been influenced by the fact that partici-
• Increased power of stimulation (weakness of piezo’s) • Improved robustness (TactilePointer prototypes broke frequently when used by public users, especially piezo elements)
• Evaluation using a more realistic test environment and task • Improvement of directional fidelity of stimulation After considering practical application scenarios in packaging tasks [Maegerlein 2003] and performing a couple of design iterations, we favoured a very different form factor for the device. We decided against a pointer metaphor, as the main interaction is with (in the real world) tangible engine parts (e.g. generators, screws, filters, batteries, mufflers). The natural way of handling these objects would be grasping: the user’s hand clasps around the object; with respect to a virtual environment setting the stimulation should have its source within the object grasped. The main design idea is to have a handle or prop, which serves as a proxy for the actual object. This leads to a design illustrated in figure 12. The device (”TACTool”) is held in a vertical orientation initially. The user’s hand grasps the device as if grasping the actual object. The mediation of the vibro-tactile feedback is done through the device itself. The directions are encoded in a Cartesian coordinate system.
For the sake of standardization, only vibro-tactile cell phone motors were used. These motors are strong enough to give a comprehensive feeling of collision. The motors are embedded into a rubber material and covered with elastic plastic stripes. Built into the device is a push-button, which is triggered when the user presses the middle cylinder. This gives the impression of grasping an object. The trigger signal is used to ”clutch” the virtual object to the real device. Figure 13 shows the TACTool.
conducted at a ”Responsive Workbench” [Krueger and Froehlich 1994] with a three camera Dtrack system mounted to it. The users were wearing tracked shutter glasses. The generator scenario served as an informal test to get the system up and running and to prepare for a more formal test. The second test (”battery”) involved 24 subjects (20 with valid data). Although no statistical significance could be achieved, some general findings can be drawn:
Figure 15: Application scenario 2: Battery placement
Figure 13: TACTool: Photo of device The VT motors are precisely controlled by the TactaBoard (http://www.vibrotactile.org/tactaboard). The TactaBoard is developed especially for controlling VT cell phone motors. It allows for precise control of all parameters needed for our purpose in a convenient and reliable way. To obtain higher precision and robustness we decided to use an optical tracking system in combination with TACTool. The A.R.T. Dtrack system uses 2 or more cameras to track retro-reflective markers within the real environment. The level of precision is in the order of about 1mm and 1 degree. The markers (small reflective balls) have been integrated into the design concept of the device (upper part of the TACTool in figure 13).
The overall handling of the device is more satisfying to the users than the first prototype. The same can be said about the aesthetics, robustness, tracking fidelity, and stimulation power properties. The majority of subjects reported that the tactile feedback forced them to change their movement according to the direction of the feedback. On the other hand, most participants were unable to report after the experiment, whether they could assign the feedback to the appropriate side of the battery. In general, the time for task completion and the number of collisions has been measured higher with TFB compared to the nonTFB condition. This does not necessarily lead to the conclusion of an increased number of errors, rather it can be assumed that the subjects accepted collisions with the virtual surroundings instead of making extra effort to avoid them.
5
Conclusion
We have presented three prototypes establishing a new generation of interaction devices that enable directed vibro-tactile feedback in virtual environments. Further we reported on informal usability studies we conducted with those devices. Although more empirical data needs to be obtained especially for the packaging tasks the TACTool already demonstrates benefits. Encouraged by positive user feedback and our own observations we believe that our approach could be extended to a variety of application fields.
Figure 14: Application scenario 1: Generator placement The TACTool was informally tested in two realistic packaging scenarios: (1) generator placement in an engine and (2) battery placement in the engine cavity (see figures 14 and 15). The tests were
6
Acknowledgments
The authors like to thank Silvan Thiele, Joe Baur, Graham Copson, Brendon Woodford, Robert Lindeman, and the participants in the various experiments. This work was supported in part by a grant
from the German Ministry of Education and Research (”VRIB” project, grant 01 IR A05 A).
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