Anyone who has ever moved around wearing a VR headset knows that running into a virtual wall or piece of furniture does not stop your movement. You simply walk through it. Only walls in real space are obstacles that can hold you back in virtual space. However, with an omnidirectional treadmill, it is possible to move infinitely far in virtual space because the small rollers continually bring you back to the center.
Omnidirectional treadmill for unlimited movement
On an omnidirectional treadmill, you can move in any direction—forward, backward, sideways, or diagonally—and turn freely. The treadmill consists of 16 trapezoidal elements arranged in a circle with a diameter of five meters. Each element has numerous small aluminum rollers, which continually guide you back to the center. In real space, you are practically standing still, but in virtual space you can move around freely without ever encountering a wall.
The University of Stuttgart is the only research institution in southern Germany with an omnidirectional treadmill. It was financed by the Cluster of Excellence Data-Integrated Simulation Science (SimTech). Investing in this state-of-the-art virtual reality platform offers scientists a unique opportunity to explore innovative immersive technologies and interact with the virtual world. With its Project Network 7 (Adaptive Simulation and Interaction), SimTech is a pioneer in this field of research and uses the omnidirectional treadmill to experimentally evaluate the concepts it has developed.
If you can move around without limits, nothing can stop you in virtual space. But this lack of physical resistance can feel disorienting for users of VR headsets. That’s why Alexander Achberger, a postdoctoral researcher at the Visualization Research Center at the University of Stuttgart, is researching a solution to this problem on the omnidirectional treadmill. “I want to develop a haptic feedback system so that you are physically stopped by a wall or object in virtual space. You feel the collision with the obstacle and can no longer move forward.”
To this end, he has developed and built the first haptic feedback systems himself. These systems consist of several boxes equipped with an intelligent cable-and-pulley system. These boxes are mounted to a frame that is securely anchored to the floor. The cable is attached to the part of the body where the resistance is meant to be felt. The feedback systems are connected to the VR software via an interface.
When you approach an obstacle in the virtual environment, a mechanism engages the cable as soon as you would collide with it. You simply reach a point where you can’t move forward. “So you don't get any bruises,” says Achberger. “And the special thing about this system is that you can attach it not only to your hand but anywhere you want—for example to your hip.”
Flexible haptic feedback systems
The static system is suitable for many applications that do not require much movement such as simulating product assembly to check whether fitters can reach a screw that is difficult to access with their screwdriver. Thanks to the system, you can determine whether users would bump their heads or actually reach the screw. However, if you move around a lot in virtual space, the cable can get tangled.
That’s why Achberger is developing a new, flexible system. “It’s a ring that fits around your hips like a hula hoop.” Instead of being attached directly to your hips, the cables are fastened to the ring and move with it when you turn. You can move freely and will be stopped if there is an obstacle in your way.
Achberger and his team have already built and tested a prototype. For example, if a table appears, you feel it in your hip when you bump into it. You can always feel the resistance where the ring is located. Theoretically, you could also attach such a system to your head. This would alert you if you bumped your head. However, the hip ring is not yet suitable for the head because it is too large.
The haptic feedback system with its anchors fixed in the floor would also be unsuitable for the omnidirectional treadmill because the cable does not move with the person. The ring, on the other hand, allows rotation in all directions. Another idea is to attach the ring not to the person but rather around the outside of the treadmill. That would be a rotating ring with a diameter of five meters. “This means that when the user turns to the right, the entire ring turns with them, thereby keeping the cables properly aligned,” says Achberger. This is better for users because they don’t have to carry anything, and it solves the problem with the head. But Achberger says that’s a job for the future.
The omnidirectional treadmill is part of the Immersion Lab at the University of Stuttgart. It was set up as a joint laboratory by SimTech and the Institute for Visualization and Interactive Systems to conduct studies for simulations in virtual space. In addition to Alexander Achberger’s project, other research initiatives in SimTech are being pursued in the Immersion Lab — for example, within the PN7‑1(II) “PerSiVal” project, which focuses on biomechanical simulations combined with haptic feedback, or within the PN6‑8 project, which investigates immersive visualizations for human – AI collaboration.
Communication between systems is a challenge
The biggest challenge now is to coordinate communication between the omnidirectional treadmill and the haptic feedback system. “To give you an idea: if I’m moving forward and an obstacle appears, the haptic feedback systems may stop me, but I could still keep walking at the same speed,” explains Achberger.
This means that the omnidirectional treadmill must also be stopped. But the question is: at what point? And at what point does the haptic feedback system need to stop the movement? “The big challenge is to do it well so that it feels natural. It depends on how fast you are moving and where you are,” explains Achberger. The next question would then be: “How do you restart the omnidirectional treadmill?”
These challenges are examples of issues addressed by the SimTech Cluster of Excellence: How can physically realistic feedback and adaptive simulations be efficiently modeled, controlled, and evaluated in virtual environments? Achberger’s work makes an important contribution to this. Together with Michael Sedlmair, a participating researcher at SimTech, he has published innovative concepts on haptic feedback.
Achberger builds all haptic feedback systems himself and programs the communication between the feedback systems and the omnidirectional treadmill. The computer scientist taught himself a lot in the process. “In our university studies, we had a little bit of electrical engineering. We also learned how to work independently and teach ourselves new skills. There are plenty of tutorials on the internet, and I learn a great deal from colleagues,” says Achberger.
Test products virtually
Achberger has also founded a start-up, Haptive GmbH, to support companies in their digitalization efforts with haptic feedback systems. “For example, in vehicle development, VR is now used extensively to test how the trunk lid should be designed so that you don’t bump your head when loading and unloading,” he explains, “or to check whether certain vehicle parts can be installed without causing collisions.” You could also use virtual tools to check whether a desk is ergonomically designed or, with a virtual kitchen planner, whether the upper cabinets are still within reach.
“Another scenario I can imagine is a virtual cave expedition where you don’t know whether you’ll make it through the narrow passages. That would be much more realistic if you actually bumped into the walls or obstacles, especially when it gets tight and you have trouble squeezing through,” says Achberger. In virtual training courses, users could learn how to safely navigate caves. “It would be much more realistic with haptic VR.” The feedback systems are set to be launched on the market before the end of this year.
In a new version, the haptic feedback systems have an active motor. “Until now, they could only stop,” says Achberger. “With the new version, the systems can also apply force, meaning they can simulate resistance and active forces.” This allows you to feel the resistance of water when you immerse your hand in it. “Or if an object is magnetic, it pulls you toward it.” Achberger is delighted that the feedback systems have already generated strong interest in industry. Nevertheless, he prefers to stay in research and develop products that may be sold in a few years and help solve real problems.
Manuela Mild | SimTech Science Communication
Read more
Achberger, P. Gebhardt, and M. Sedlmair, “An Exploratory Expert-Study for Multi-Type Haptic Feedback for Automotive Virtual Reality Tasks,” IEEE Transactions on Visualization and Computer Graphics, vol. 30, Art. no. 11, Sep. 2024, doi: 10.1109/tvcg.2024.3456203
Achberger, “Moving haptics research into practice: four case studies from automotive engineering,” 2023. [Online]. Available: https://elib.uni-stuttgart.de/handle/11682/13922
Achberger, A., Arulrajah, P., Sedlmair, M. and Vidackovic, K., "STROE: An ungrounded string-based weight simulation device." IEEE Conference on Virtual Reality and 3D User Interfaces (VR). IEEE, 2022.
Achberger, A., Aust, F., Pohlandt, D., Vidackovic, K. and Sedlmair, M.,"STRIVE: String-based force feedback for automotive engineering." ACM Symposium on User Interface Software and Technology (UIST). 2021.
Achberger, A., Heyen, F., Vidakovic, K. and Sedlmair, M., “PropellerHand: A hand-mounted, propeller-based force feedback device.” International Symposium on Visual Information Communication and Interaction. 2021.
About the scientist
Alexander Achberger studied at the University of Stuttgart and holds a bachelor’s and master’s degree in computer science. He earned his doctorate at the Mercedes-Benz Virtual Reality Center, where VR is used mainly for vehicle development. Haptic feedback in virtual reality was also the topic of his doctoral thesis in which he explored how to make interactions or collisions with vehicles physically perceptible and therefore more realistic.
For the past two years, he has been further developing haptic feedback systems at the Visualization Institute of the University of Stuttgart in the research group led by SimTech Participating Researcher Michael Sedlmair. He also founded the start-up Haptive GmbH. He is deeply committed to research, but it is important to him that it has real-world impact. That’s why combining research and practice is ideal for him.
