Mid-Air Thermo-Tactile Fire: Ultrasound Haptic Display for VR
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: 10.1145/3489849.3489889
- First paper characterizing mid-air thermo-tactile thresholds; foundational reference for the lab’s subsequent wearable thermal haptics work
