HAPTIC BRACELET: A mechanistic understanding of a wrist-worn haptic device
We all would agree that technology has created an immersive world of sounds and sights from the comfort of our own homes, but something is missing: TOUCH. Imagine yourself consoling a loved one miles apart from you and suffering from an ailment, not just by verbal and visual interactions, but also through touch, passing a pat on their arm to assure them we are all together in this. What if this could be achieved by wearing specialized haptic devices on the arm/shoulder of both the users that recognize touch gestures and transfer that perception between the users!
Tactile sensation has gained paramount importance in perceiving reality for humans. The sense of touch feels most realistic when we simulate what happens in the real world: our fingertips deform as we touch or squeeze a stiff object. Since devices can be designed in several ways, we are not constrained to simply mimic what happens, but also create touch from virtual objects.
Most conventional haptic devices are designed to provide a sense of touch to the user during virtual reality interactions, in which persons wear head-mounted displays that isolate them from the real world. However, with augmented reality (AR), a person can see both the real world and the virtual world and needs to switch between real and virtual world interactions. Prof. Mine Sarac of Kadir Has University described this major challenge for design of haptic devices to be used in AR. Fingertip devices would limit grasping and touching in the real world. An alternative design is needed to enable persons to perform real interactions occasionally and still take advantage of the sense of touch during virtual interactions.
Designing haptic devices for situations in which fingertips should be left free for real-world interaction has challenges. The miniaturized actuator cost and reliability in the output forces limit the design capabilities and practical uses. Thimble-like haptic devices worn on the fingertips show robust performance and perceived realism when users are not touching or grasping physical objects. To explore solutions for design challenges of haptic devices that leave fingers free, researchers from Allison Okumura’s CHARM (Collaborative Haptics and Robotics in Medicine) LAB at Stanford have developed a haptic device worn on the wrist or forearm that relocates the delivery of stimulation from the accustomed fingertip to the arm.
Even though the haptic feedback on the arm cannot provide perfectly realistic feedback, the authors postulate that haptic signals that hint at the real properties of an object are sufficient to create “believable” interactions. This novel haptic feedback conveys information about the grasp forces on the forearm and the performance of actions without increasing cognitive or attentional demand from the user thereby qualitatively adding to the user’s experience.
“One straightforward way of providing this sense of touch is by applying different magnitudes of force while poking and stretching the forearm. The funny thing is that when the same numerical value of force say, one newton (N) poke and one newton(N) stretch, is applied to the arm, it feels different. What we want to understand is how people perceive these inputs and generate the same level of this perceived intensity. We believe future wearable devices will be an array of normal and shear actuators. Hence, this information about the forces becomes particularly important while designing wearable haptics and will help us in fine-tuning and controlling the experience of the user,” says Hojung Choi, a Ph.D. candidate and author of the report[1], published in IEEE Robotics and Automation Letters.
The authors performed two experiments to compare the user’s perception with stimuli from the haptic bracelet. The first study (the Stiffness Discrimination experiment) was a complete result-oriented extension of the researchers’ earlier work [2] about haptic feedback on the wrist without considering the virtual stiffness value of objects. The second study investigated the Point of Subjective Equality (PSE), the conditions where a normal and shear force feels the same. Twelve right-handed participants carried out both experiments.
The goal of the first study was to investigate and understand the effect of force direction, and location of applied force needed for believable perception during virtual interaction. The participants wore the haptic bracelet and sat in front of a computer screen that showed two shapes that looked identical but differed in stiffness. They also had a fingertip sensor on their index finger for the tracking system. The users moved their index fingers to press and drag the stiffer object into a target zone. In return, they received haptic feedback in the direction of normal and shear forces, on the top, the underside, or both sides of their wrist. The level of displacements was kept the same for both normal and shear directions.
It was found that the best location for haptic feedback in the wrist was very subjective and was quite irrelevant in terms of perception. Participants performed better with haptic forces perpendicular to their skin instead of shear forces, which was evident from the task accuracy. Stimuli perpendicular to the skin enabled participants to differentiate stiffness levels better than for skin stretch, especially when the difference in stiffness of the object was larger. The subjective comments from most of the users that normal force was the “easiest to notice” matched with the results. Interestingly, this finding contradicts previous studies conducted on the sensitivity and ease of noticing normal versus shear forces on the skin.
The PSE experiment elucidated the difference between the perceived intensity of stimuli normal (perpendicular to the skin) and shear (stretching the skin). For both the studies, two sets of a position-controlled linear actuator were used. The study showed differences in the perception of participants given different actuator displacements and applied forces. The PSE results showed that normal and shear stimuli were perceived as having the same intensity even when the shear displacement was larger than the normal displacement. This result was the same for all participants, irrespective of reference stimulus type and intensity, or which wrist was used. When the stimulus was analyzed in terms of interaction forces, normal stimuli resulted in significantly greater perceived forces than the shear stimulus needed to create a similar perceived intensity.
In this research the investigators take a black box approach to model human perception and skin properties. “We highlight the importance of correct modeling while comparing various stimuli. We then try to model the behavior of human skin under the conditions that would be necessary to conduct a comparative experiment,” explained first author Mine Sarac.
When asked about the underlying motivation and application of this work to future research, author Hojung Choi said, “This work is a more fundamental approach to understanding the distribution of the reaction forces when thinking and designing future haptic devices for different body locations. This study would open horizons to understand and explore the design and perception of wearable haptic devices on arms and other body locations.”
The mechanistic understanding of wrist-worn wearable haptics from this work provides a promising future for designing and developing haptic devices for different body parts and understanding how the movement of the part of the body wearing the haptic device affects user perception.
Mine Sarac is now an Assistant Professor in the Department of Mechatronics Engineering at Kadir Has University in Istanbul, Turkey.
References:
- Sarac, M, Huh, TM, Choi, H, Cutkosky, M, Di Luca, M & Okamura, AM 2022, “Perceived intensities of normal and shear skin stimuli using a wearable haptic bracelet”, IEEE Robotics and Automation Letters. https://doi.org/10.1109/LRA.2021.3140132
- Sarac, M, Okamura, AM, Di Luca, M 2019, “Haptic sketches on the arm for manipulation in virtual reality,” ArXIV, in IEEE World Haptics Conference. https://arxiv.org/abs/1911.08528
The eWEAR-TCCI awards for science writing is a project commissioned by the Wearable Electronics Initiative (eWEAR) at Stanford University and made possible by funding through eWEAR industrial affiliates program member Shanda Group and the Tianqiao and Chrissy Chen Institute (TCCI®).