BRIX5: Modular Vibrotactile Wearable Toolkit (2020)
Few open and reproducible tools exist for prototyping wearable sensor-actor systems with state-of-the art vibrotactile haptics, e.g., for augmented feedback applications or novel human-computer interfaces. As part of my thesis work and project NeuroCommTrainer, I developed BRIX5, a modular platform for prototyping feature-rich wearables. Based on the interconnect specifications Qwiic and Feather, BRIX5 is compatible with a rapidly growing open ecosystem. Key contribution are an adaptable base module and compact extensions for motion sensors, arrays of both narrow-, and wide-band voice coil tactile actuators. Filling the gap between proprietary, off-the-shelf wearables and custom prototypes, we plan to release BRIX5 as open hardware, as a step towards a common platform for shared vibrotactile wearable research and teaching. BRIX5 provided the basis for the recent belt and NeuroCommTrainer prototypes, and was developed in collaboration with Sebastian Zehe (hardware design and assembly) at the AmI Lab.
Topics
Methods
- Modular Systems Design
- PCB Design + Manufacturing
- Embedded Development (Arduino)
- Scientific Writing
A major barrier to entry in wearables creation is the lack of common rapid prototyping tools. Previously proposed toolkits have remained isolated solutions not easily reproduced at scale nor interoperable with a growing variety of breakout boards. These on their own require substantial integration engineering to be suitable for use in wearables. For wearable vibrotactile (VT) haptics and the study of rich, anatomy-adapted stimuli to eventually leverage the distributed nature of the sense of touch, even fewer options exist. As control of tactors arrays and embedded synthesis for wide-band tactors remains difficult, most research to date still relies on eccentric rotating mass motors (ERMs) as tactors (tactile actuators).
We developed the BRIX5 toolkit for the development of such multi-channel VT wearables and the iterative design process from technology probes to functional prototypes. Instead of creating an isolated toolkit, BRIX5 is an open concept focussing on compatiblity and an intentionally minimal modular design. Based on the Feather and Qwiic specifications, it is compatible with hundreds of existing extensions.
Building on established, well-documented and supported platforms lowers the bar to entry and allows for knowledge generalization. Modularity allows adaptation to use cases and users, and can support an iterative development approach through (re-)use and extension as prototypes and available technology evolve. Iteration and adaptability are key in rapidly determining sensor/actuator choice and placement location of on the body. Compatibility and minimal custom electronics reduce development efforts and increases repairability, especially important in wearable development, where prototypes are subject to significant wear-and-tear. A toolkit such as BRIX5 can at best remain a work-in-progress, a flexible mediator between established and emerging technologies. Our design prioritizes an increase in flexibility and reduced maintenance over the highest-possible integration, minimizing lock-in to specific technologies, such as microcontrollers or wireless standards.
To bootstrap vibrotactile wearable development, we developed an adaptable base controller, extensions for motion sensors, and for control of multi-channel narrow- and wide-band VT haptics.
BRIX5 Base Controller
Base module with extension examples
Base module: Exploded view
The base module can be extended via a top-mounted Feather wing and through four Qwiic headers on the side, for which a bus power reset circuit allows automated recovery from hangups. Qwiic cables up to 500 mm are readily and cheaply available and can easily be extended with coiled or shielded wire if robustness or length is required, I2C-over-RJ485 and signal-reconditioning modules exist to cover very long distances.
Motion / Orientation Sensor: Mini/MicroBNO Extensions
Our miniaturized BNO extensions solve the problem of compact, low-drift motion/ orientation sensors, commonly required in wearable projects.
The chosen Bosch BNO055, a 9-axis magnetic, angular rate, and gravity sensor, provides raw and filtered data as well as best-in-class sensor-fusion for absolute orientation estimation. While the base module includes one BNO sensor, we designed miniaturized extensions in two sizes, miniBNO and microBNO for applications requiring additional sensors. The mini-size is suitable for strap-mounting and exposes more chip configuration options. The micro-size is approximately fingernail-sized and meant to be integrated into garments, e.g., glove fingertips, or affixed to the body directly with adhesive tape. For new setups – though our PCB designs accommodates all variants – we use more recent CEVA BNO080/085 variants. These provide orientation output rates up to 400 Hz, allow flexible configuration of input sensors, and exhibit less drift. Through I2C-multiplexers, we have used up to 16 BNOs simultaneously.
Multichannel LRA and ERM: QuadDRV
Our QuadDRV extension enables closed-loop control of NBT arrays. To date, most VT wearables to date drive ERMs in an open-loop fashion, which results in slow ramp-up and ramp-down of vibration intensity. The QuadDRV module drives four ERMs or LRAs, and is built around four TI DRV2605L haptic drivers multiplexed by a I2C-bus switch. For ERMs, the DRV2605L allows active braking and overdrive, improving ERM pulse sharpness significantly to a level comparable to that of voice coils. For LRAs, it provides automatic resonance tuning, necessary to derive optimal output amplitude despite resonance frequency varying between tactor samples and with integration. The DRV2605L has a built-in effect library and sequencer and can also operate in streaming mode for external control. Tactors are connected through a 8-pin Molex Picoblade (1.25 mm pitch) plug, chosen for its compact size and the availability of manufacturer-direct pre-made cables.
Wide-band Tactors: Audio-400, Lofelt L5 and Amp-200, Adapters
The Audio-400 extension provides multi-channel embedded audio/haptic synthesis for WBTs. A highly integrated form of a dual Teensy Audio Adapter it provides four channels of high-fidelity synthesis to drive wide-band actuators or headphones. It is based on a Teensy 3.2 microcontroller controlling two stereo digital-audio-converters (NXP STGL5000) and is implemented as a board-on-board PCB. The Teensy Audio Library performs synthesis with help of controller’s DSP extension. DSP routing is readily pre-configured through a web-based graphical editor. Our Audio-400 module can operate stand-alone, or be controlled via Qwiic.
Our primary tactor choice are the Lofelt L5, compactly housed together with the Amp-200 module, a highly integrated DG-Class audio amplifier (Maxim MAX98307, 3.3W). Audio-400 and Amp-200 connect via the same JST SH headers Qwiic uses, thus requiring only a single cable type for all interconnects in the BRIX5 system.
Power and Audio Adaptors
We designed a set of audio adapter boards. These allow rapid switching between tethered (external signal source, e.g., for prototyping) and untethered operation, optionally allowing simple extension by routing four channels of audio/haptic signals through off-the-shelf RJ45 ethernet cabling. A combination of audio adapter, power, and Amp-200 modules provides a modular, functionally equivalent to the Syntacts system hardware.
Power supply specification varies with application and number of tactors driven simultaneously. The BRIX5 base module’s Feathers usually supply up to 600 mA; for applications without base module, we designed a standalone power module (up to 800 mA output power. We have also successfully used the Smart Prototyping ZIO Battery LiPo Battery Manager (up to 1.35A).
Housing
We 3D-printed housings for mounting our modules on 1" webbing straps (on an Ultimaker UM3). A shared parameterized CAD model (Siemens PLM SolidEdge) facilitates rapid adaptation to new modules. A two-way hook on the top cover allows to affix zip ties for strain relief.