HaptEx: Investigating Haptic Notification Channels for Exoskeletons Across Different Levels of Actuation

Exoskeletons are increasingly deployed in real-world contexts, where communicating critical system states or unexpected events is important for effective interaction. Haptic feedback offers a direct communication channel, integrating naturally with the actuated body region. Yet, it remains unclear how well haptic feedback is perceived while the body is being actuated. In a controlled study (N=24) with a shoulder exoskeleton, we compare four common haptic notification channels (poking, proprioceptive, thermal, vibrotactile) under different levels of actuation. Results show that poking was detected fastest, while thermal and proprioceptive notifications were most accurate and noticeable. Actuation levels affected error rates and noticeability, but not response times. Participants reported that thermal notifications aligned best with the actuation levels, producing a distinct sensation that blended naturally with movement. In contrast, proprioceptive notifications conveyed the strongest sense of urgency. We discuss design implications for leveraging haptic notifications to support embodied communication with exoskeletons.