ACM SIGCHI Conference on Human Factors in Computing Systems
(ACM CHI 2018)

RFIBricks: Interactive Building Blocks Based on RFID
Meng-Ju Hsieh1 Rong-Hao Liang2 Da-Yuan Huang3 Jheng-You Ke1 Bing-Yu Chen1
1National Taiwan University
2Eindhoven University of Technology
3National Taiwan University of Science and Technology
To appear in ACM CHI 2018 Papers
"RFIBricks is an interactive building block system based on passive UHF RFID sensing. The system determines the identity and the built 3D geometry of passive building blocks and tracks in-hand assembly."
Video Preview
Abstract
We present RFIBricks, an interactive building block system based on ultrahigh frequency radio-frequency identification (RFID) sensing. The system enables geometry resolution based on a simple yet highly generalizable mechanism: an RFID contact switch, which is made by cutting each RFID tag into two parts, namely antenna and chip. A magnetic connector is then coupled with each part. When the antenna and chip connect, an interaction event with an ID is transmitted to the reader. On the basis of our design of RFID contact switch patterns, we present a system of interactive physical building blocks that resolves the stacking order and orientation when one block is stacked upon another, determines a three-dimensional (3D) geometry built on a two-dimensional base plate, and detects user inputs by incorporating elec- tromechanical sensors. Because it is calibration-free and does not require batteries in each block, it facilitates straightforward maintenance when deployed at scale. Compared with other approaches, this RFID-based system resolves several critical challenges in human-computer interaction, such as 1) determining the identity and the built 3D geometry of passive building blocks, 2) enabling stackable token+constraint interaction on a tabletop, and 3) tracking in-hand assembly.
Keywords: RFID; Tangible User Interface; Building Blocks; Constructive Assembly.
Paper
Video
ACM Portal (10 pages)
Youtube - Complete version (2m36s)
Youtube - Preview version (30s)

Introduction
Physical building block systems have been demonstrated as promising tangible user interfaces (TUIs) for interactive construction. The physical affordances and tactile feedback allow users to intuitively assemble the blocks into the desired physical form. The versatile and seemingly end- less possibilities allow users to develop their manual function, spatial recognition, and creativity.

Developing a reliable and easy-to-maintain building block system remains an open challenge. Regarding reliability, studies have proposed the use of building blocks with embedded electronic circuits and sensors to detect the assembly. Although the physical connections are reliable, the embedded active components require additional maintenance efforts, such as charging or battery replacement, which may limit the scalability and sustainability of the solution. Re- garding ease of maintenance, studies have proposed passive building block systems based on computer vision tracking, capacitive tracking and magnetic tracking. However, these solutions do not satisfy applications requiring full three-dimensional (3D) model construction.

RFID sensing techniques have been widely used for detecting the location and movements of passive tagged objects using using real-time classification of the received signal strength indication (RSSI) and phase angles, or incorporating with an occlusion-sensitive camera. However, they requires training and calibration and are insufficiently precise and responsive for interactive building block applications. Another method for achieving responsive user interaction is to modify the RFID tags and integrate electromechanical sensors (e.g. switches) into the circuitry con- nections between the RFID chip and antennas, so that the clear presence of or signal change in ID information can be utilized to indicate an interaction event. Although these techniques are reliable and calibration-free, they have not been extended to interactive building block applications

Figure 1. RFIBricks is a reliable and easy-to-maintain interactive building block system based on UHF RFID sensing. (a) Interactive physical modeling. (b) Tabletop tangible gaming. (c) Tangible programming. (d) Modular input device.
In this paper, we present RFIBricks (Figure 1) an interactive building block system based on passive ultrahigh frequency (UHF) radio-frequency identification (RFID) sensing. The system is mainly realized through a simple yet effective hardware solution: modifying UHF RFID tags as rich-ID contact switches (Figure 2a) to realize a lightweight, calibration-free building block system based on UHF RFID sensing. Each UHF RFID tag is first disassembled into a pair of compo- nents, namely a chip and an antenna, using a vinyl cutter.

Subsequently, we attach a magnet to each terminal as a low-ohmic magnetic connector that forces alignment. When the two parts snap to each other, the copresence of a pair of IDs is considered a unique interaction event.

Figure 2. RFID Contact Switch. (a) Modifying a UHF RFID tag as an RFID contact switch. (b) Spatial RFID contact switch pattern is applied to a basic 1×1 block.
A spatial RFID contact switch pattern is applied on the top and bottom of each building block (Figure 2b); also, four RFID contact switches are formed as a base plate, which is then deployed as a two-dimensional (2D) grid (Figure 3a). Each RFID switch on each block is registered once with the corresponding spatial properties. Thereafter, the spatial de- sign allows the system to not only recognize these blocks but also determine the position and orientation of a building block placed on the sensor grid by using Cartesian coordinates. For instance, the one chip placed on the bottom of block #47 represented the block’s identity, and the four chips on the base plate are registered with their actual spatial location (x,y,z = 0) and the represented orientation θ = {0,π/2,π,−π/2}. When the block is placed the plate, the registered information pertaining to the pair of IDs indicates that a block is stacked upon the plate in the stacked orientation. For instance, the event of block #47 stacked at (x=3,y=2,z=1,θ =π)is used to update the detected geometry. Moreover, it allows the system to resolve the stacking order of the building blocks (Figure 3b), irrespective of whether the blocks are stacked in the users hands or on a plate. By combining these features, we realized a lightweight, reliable, and interactive 3D building block system based on UHF RFID sensing.

Figure 3. Principle of geometry resolution. (a) A grid of RFID contact switches used as a base plate for localization. Unconnected blocks activate no tags. Connected blocks activate two unique ID simultaneously; thus, the ID and the orientation of the stack can be determined. (b) The stacking order of the building blocks is resolved in the same method.
In addition to supporting the resolution of discrete stacking location and the resulting geometry, we generalize the RFID contact switch design to sense interactivity in the 3D physical model. We demonstrate the generalization by interfacing with conventional electromechanical sensing elements, such as a push button and a switch. Moreover, to demonstrate the uses and design implications of this system, we present several example applications, including interactive physical modeling, tabletop tangible gaming, tangible programming, and modular input device creation. Furthermore, we detail the system implementation. Finally, the empirical study results are presented to validate the performance of this system and to provide directions for future optimization.

The proposed RFID-based interactive building block system overcomes several critical challenges in established problem spaces in human-computer interaction, such as 1) resolving 3D building block geometry and the ID of each block in a constructive TUI, 2) enabling stackable token+constraint interaction on a tabletop computer, 3) tracking in-hand assembly and 4) realizing a wireless and battery-free modular/stackable input device. The major advantage of the proposed approach is that it is simple and does not require batteries in each block, facilitating straightforward maintenance when deployed at scale.

Basic block. (a) Results. (b) Component overview.
Compound blocks. (a) 1×2 block. (b) 1×3 block.
Widget blocks. (a) Overview. (b) Schematics.
Fabrication process of a basic block. (a) Separating the antenna and chip of each RFID tag by using a vinyl cutter. (b) Results. (c) Mounting the parts on the corresponding positions of contact surfaces. (d) Mounting the surfaces on a block with magnetic connectors.
Tangible Minecraft. A user (a) stages the characters by placing them on the plate, (b) builds the environment by stacking the blocks, and (c) places a character on top of the built mountain. (d) Results.
Tower Defense. (a) Transparent tiles were fixed at the desired location on the tabletop. Weapon blocks were distributed to the players. A player (b) sets a weapon by placing a block, (c) upgrades a weapon by stacking, and (d) ignites a bomb by pressing the button.
Tangible programming. A user (a) selects a desired functional block, rotates it to the orientation of the proper parameter, attaches the block, and (b) triggers the action by pressing the button.
Modular input device. (a) A user combines the desired input modality by stacking the widget block together. Subsequently, the ac- tions of (b) pressing a push button, (c) turning a switch on or off, and (d) tilting the entire stack are detected correctly.
Experimental apparatus and pilot session results. Six layers of blocks stacked on a 3×3 tile were measured on a 5 mmthick acrylic supporting surface by (a) 1 antenna or (b) a grid of 2×2 antennas. (c) Experiment parameters. (d) Pilot session results in frame rate vs. tag amount.
Experimental results on the capability of sensing 1D stacking. (a) Results on Large blocks and (b) small blocks on different angles and offsets d. (c) Results on different base area sizes at d = 0, which refers to different stacking volume.
Acknowledgements
This research was supported in part by the Ministry of Science and Technology of Taiwan (MOST106-3114-E-002-010), National Taiwan University (NTU106R891206), and MediaTek Inc.
BibTex
@inproceedings{Hsieh:2018:RFIBricks, author = {Hsieh, Meng-Ju and Liang, Rong-Hao and Huang, Da-Yuan and Ke, Jheng-You and Chen, Bing-Yu}, title = {RFIBricks: Interactive Building Blocks Based on RFID}, booktitle = {Proceedings of the 2018 CHI Conference on Human Factors in Computing Systems}, series = {CHI '18}, year = {2018}, isbn = {978-1-4503-3362-7}, location = {Montreal, Canada}, pages = {to appear}, numpages = {10}, url = {}, doi = {https://doi.org/10.1145/3173574.3173763}, acmid = {2858527}, publisher = {ACM}, address = {New York, NY, USA}, keywords = {RFID; Tangible User Interface; Building Blocks; Constructive Assembly}, }