Mid-Air Thermo-Tactile Fire: Ultrasound Haptic Display for VR

Wed, 01 Sep 2021 00:00:00 +0000

Overview

Imagine reaching toward a virtual campfire and actually feeling the heat wash over your hands — no gloves, no controllers, nothing on your skin. Mid-Air Thermo-Tactile Fire is a proof-of-concept system that delivers both thermal warmth and vibrotactile pressure to a free hand hovering above a custom device, using a combination of heated airflow channels and a 40 kHz ultrasound haptic array.

Published at ACM VRST 2021 (ACM Symposium on Virtual Reality Software and Technology), this was the first system to simultaneously characterize thermo-tactile mid-air feedback thresholds and demonstrate them in a VR fire interaction scenario.

The Problem

Mid-air haptics (ultrasound) had proven that focused pressure can be delivered without contact. Thermal mid-air feedback existed in industrial settings (heat lamps). But simultaneously combining both — localized, controllable, synchronized — for real-time VR had not been demonstrated.

Key unknowns at project start:

What temperature range can be achieved mid-air at realistic interaction distances (15–25 cm)?
Does the ultrasonic pressure signal interfere with thermal perception (or vice versa)?
What warm detection threshold (WDT) and heat-pain threshold (HPDT) apply to mid-air vs. contact thermal stimulation?

System Design

Hardware Architecture

Ultrasound display: 16×16 transducer array (256 elements), 40 kHz carrier, capable of focusing pressure at 10–25 cm above surface
Thermal channel: open-top acrylic chamber with 4 heating coils; a low-speed centrifugal fan directs warm air up through the focus zone
Temperature control: PID loop via Arduino — thermocouple at the focal plane feeds back to heater PWM, ±1°C stability
Integration: ultrasound focus point and warm airflow column co-aligned within ±5 mm

Measured System Specs

Parameter	Value
Peak achievable temperature at focal plane	54.2°C
Ultrasound pressure at focus	3.43 mN (100 Hz, 12 mm radius)
Temperature stability (mean error)	0.25% over 10 min
Interaction distance range	12–22 cm

Unity VR Integration

Unity 2020 LTS with SteamVR / OpenVR SDK (HTC Vive)
Custom C# bridge communicates over USB serial to Arduino controller
VR scene: virtual campfire with particle system; hand proximity triggers thermal ramp (further = cooler, closer = warmer) while fire flicker drives vibrotactile modulation at 4–12 Hz
Thermal latency from Unity event to onset at skin: ~120 ms (dominated by airflow thermal inertia)

User Evaluation

Threshold Study — WDT and HPDT

N = 14 participants
Protocol: method of limits (ascending/descending); 5 trials per direction, 3 interleaved staircases
Conditions: mid-air thermal only (no ultrasound) vs. mid-air thermal + ultrasound (thermo-tactile)
Measures: WDT (°C), HPDT (°C), response time to first detection

Haptic Pattern Recognition Study

N = 14 participants (same cohort, separate session)
Task: identify 4 spatial haptic patterns (dot, ring, horizontal bar, vertical bar) presented mid-air
Conditions: non-thermal (room temp) vs. thermal-on (heated airflow active)
Measure: identification accuracy, confusion matrix

VR Experience Study

N = 10 participants
Task: 5-minute campfire scene; ratings on warmth realism, presence, comfort

Results & Key Findings

WDT: mean 32.8°C (SD=1.12) — consistent with contact-based thermal WDT literature (validates mid-air stimulation as perceptually equivalent)
HPDT: mean 44.6°C (SD=1.64) — also matches contact norms; no elevated pain threshold from airflow delivery
Pattern accuracy: 98.1% (non-thermal) vs. 97.2% (thermal) — no significant degradation (p=.38); thermal channel does not interfere with tactile perception
Thermo-tactile condition received significantly higher VR realism ratings than tactile-only (p<.05)

Lessons & Evolution

This project established the core technical finding that underpins the entire MI Lab thermal haptics research line: thermal and tactile cues can coexist mid-air without masking each other, enabling richer multi-modal VR experiences. Every subsequent project (Snow, Fabric Thermal Display, Fiery Hands) built on these baseline thresholds and the dual-channel architecture proven here.

Impact

📄 Published: ACM VRST 2021 — Proceedings of the 27th ACM Symposium on Virtual Reality Software and Technology
DOI:
First paper characterizing mid-air thermo-tactile thresholds; foundational reference for the lab’s subsequent wearable thermal haptics work

ACM VRST | Haokun Wang