Towards Reliable Tactile Mid-Air Interfaces: Analysis of Influencing Factors of the Perception of Tactile Mid-Air Feedback
Authors: Bernhagen Max , C Bullinger Angelika 
Published: July 2022

Human-machine interfaces require an efficient and reliable interaction under various con-ditions. Especially under conditions with high cognitive workload which require rapid situa-tion assessment, interfaces should reliably support the human perception. Here, research addresses gesture-based interfaces as a possible interface to enable intuitive interactions based on ingrained daily routines. Using spatial commands executed by the bare hand, one can recreate real world interactions like pushing a knob, turning a controller, or using a ges-ture as an input command. In comparison to real input devices gesture-based interfaces lack haptic feedback for the user. Tactile feedback is important as it indicates for example interaction borders (e. g. edge of the mouse pad) or provides feedback of a successful in-teraction (e. g. sensation of the pushed button). This helps to increase the usability of mid-air gestural systems. For the realisation of mid-air tactile feedback two technologies can be considered. Vortex-generators and ultrasound-based feedback utilise the bare hand and need no device attached to the hand. However, they provide weak feedback which can be influenced by airflow, hand posture, clothing, workload or other factors. To achieve the aforementioned benefits, users have to reliably perceive the tactile feedback – and to do so, perception of tactile mid-air feedback needs to be researched in more detail.We present a method for the analysis of influencing factors on the perception. A driving simulator was the basis for a standardised apparatus in which tactile feedback was pre-sented via a vortex-generator. For each influencing factor, participants were asked to do a driving task (Lane Change Task - LCT) and detect tactile stimuli in parallel. By the help of the method of constant stimuli, a psychometric function for each influencing factor was derived. On this function detection, thresholds of 50%, 90%, 95% and 99% were chosen to represent the most important values in terms of human-machine interaction. In compari-son to widely used methods like staircase procedures, this approach promises to give a better insight into the effects of the influencing factors as the whole psychometric func-tion can be analysed instead of one distinct value.Two experiments (N= 80; 31) were conducted to apply the approach and analyse the influ-ence of workload by a variation of speed and secondary task type. Also, one experiment (N= 16) investigated the influence of hand temperature on the perception of the feedback. Results show that increased speed and the addition of a secondary tasks significantly in-crease the perceived workload. Regarding the perception of tactile stimuli, slight differ-ences for different workload conditions and a cut-off for high workload conditions were found. Furthermore, the effect of temperature on the perception on tactile feedback could be shown. Based on the studies, advantages and disadvantages of the proposed approach are dis-cussed. Also, the impact of workload and temperature in terms of design recommenda-tions for human-machine interaction are examined. The presented approach suggests a promising method to investigate the impact of influencing factors on specific design ele-ments for human-computer interaction. Further studies should investigate the eligibility for other modalities and applications.