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picoRing: battery-free rings for subtle thumb-to-index input

Ryo Takahashi, Eric Whitmire, Roger Boldu, Shiu Ng, Wolf Kienzle, Hrvoje Benko

Abstract

Smart rings for subtle, reliable finger input offer an attractive path for ubiquitous interaction with wearable computing platforms. However, compared to ordinary rings worn for cultural or fashion reasons, smart rings are much bulkier and less comfortable, largely due to the space required for a battery, which also limits the space available for sensors. This paper presents picoRing, a flexible sensing architecture that enables a variety of \textit{battery-free} smart rings paired with a wristband. By inductively connecting a wristband-based sensitive reader coil with a ring-based fully-passive sensor coil, picoRing enables the wristband to stably detect the passive response from the ring via a weak inductive coupling. We demonstrate four different rings that support thumb-to-finger interactions like pressing, sliding, or scrolling. When users perform these interactions, the corresponding ring converts each input into a unique passive response through a network of passive switches. Combining the coil-based sensitive readout with the fully-passive ring design enables a tiny ring that weighs as little as 1.5 g and achieves a 13 cm stable readout despite finger bending, and proximity to metal.

picoRing: battery-free rings for subtle thumb-to-index input

Abstract

Smart rings for subtle, reliable finger input offer an attractive path for ubiquitous interaction with wearable computing platforms. However, compared to ordinary rings worn for cultural or fashion reasons, smart rings are much bulkier and less comfortable, largely due to the space required for a battery, which also limits the space available for sensors. This paper presents picoRing, a flexible sensing architecture that enables a variety of \textit{battery-free} smart rings paired with a wristband. By inductively connecting a wristband-based sensitive reader coil with a ring-based fully-passive sensor coil, picoRing enables the wristband to stably detect the passive response from the ring via a weak inductive coupling. We demonstrate four different rings that support thumb-to-finger interactions like pressing, sliding, or scrolling. When users perform these interactions, the corresponding ring converts each input into a unique passive response through a network of passive switches. Combining the coil-based sensitive readout with the fully-passive ring design enables a tiny ring that weighs as little as 1.5 g and achieves a 13 cm stable readout despite finger bending, and proximity to metal.

Paper Structure

This paper contains 26 sections, 3 equations, 5 figures, 2 tables.

Table of Contents

  1. Introduction
  2. Related work
  3. Instrumented hand
  4. Ring-based input device
  5. Battery-free ring-based sensor
  6. Theory of operation
  7. System overview
  8. Sensitive passive inductive telemetry
  9. Resonance detection
  10. Design of battery-free ring coil
  11. Implementation
  12. Wristband-based reader coil
  13. Ring-based sensor coil
  14. Technical evaluation
  15. SNR for turn number
  16. ...and 11 more sections

Figures (5)

  • Figure 1: picoRing design. (a) The system illustration of picoRing. picoRing is based on passive inductive telemetry (PIT), in which the wristband coil wirelessly monitors the passive response of the ring coil caused by user inputs. (b) The circuit diagram and signal processing of picoRing which consists of a ring-based fully-passive sensor coil and a wristband-based reader coil connected to a reader board. (c) Overview of distributed capacitance arrangement (DCA) technique which can increase the passive response by enabling a high inductor at a high frequency.
  • Figure 2: Design of passive variable capacitor. Among three types, picoRing uses type 1.
  • Figure 3: Implementation of a wristband coil.
  • Figure 4: SNR evaluation of picoRing. (a) SNR for the resonant frequency of the ring coil. (b) SNR for the distance between the wristband and the ring. (c) SNR for different finger bending angles (d) SNR for proximity metallic items.
  • Figure 5: picoRing demonstration. Photograph, application example, and peak in the frequency response of picoRing (a) press, (b) slide, (c) joystick, and (d) scroll.