Spark-Initiated Mini-Explosions Pop Tiny Braille-Reader Dots

What you’ll learn:

The problems associated with, and possible solutions to, creating miniature pop-up dots as a tactile indicator surface for Braille readers.
How a system using elastomeric bumps that are “popped” by spark-initiated, methane-based mini-explosions may be a viable solution.
How such a system was constructed and evaluated as a small array, as well as the associated multidisciplinary scientific analysis.

Devising a fast-response tactile Braille display for vision-impaired users, in which the dots rise and then return flush (or nearly so) to the surface, is an electronic-motion challenge. Among the issues is that a usable Braille-text display needs a large number of tiny dots (1-3 mm diameter) with very tight pitch (about 5 mm). A full-page Braille display (25 lines, 40 cells per line) has 6,000 dots, clearly not a setting for conventional electromechanically actuated solutions.

While soft pneumatic-based actuators initially showed promise in small-resolution conformable tactile displays, they’re not practical since their control valves are bulkier than the actuators themselves.

Other alternatives also have limitations: thermal actuators usually take seconds to finish a heating/cooling “work loop” cycle in the absence of thermal-management accessories (because of heat-transport limitations); pulsed electromagnetic systems suffer from low actuation forces and interference between individual actuators (crosstalk) when shrunk down to the size of a Braille dot; and piezoelectric devices have large production costs at scale and need longer, cantilevered geometries that impede their ability to be arranged in a densely packed array.

Now, a collaboration of researchers at Cornell University and Technion-Israel Institute of Technology has devised and tested a radically different approach. They used tiny electrodes to ignite a microvolume of premixed methane and oxygen, thus causing a thin silicone membrane to inflate (Fig. 1).

1. Valveless microliter combustion repeatably and powerfully displaces hyperelastic membranes: (A) A rendering of the single-cylinder (5-mm diameter, 2-mm depth) actuator details the construction and intra-cylinder arrangement of gas ports and spark ignition electrodes. The liquid metal (LM) electrodes terminate at the cylinder edge (Inset); their exposed surfaces forming the spark gap. (B) High-speed photography captures fast dynamics of single-cylinder actuation strokes. (Scale bar: 3 mm) (C) Image analysis from high-speed videos shows that methane–oxygen combustion reaction rates increase sharply as gas-equivalence ratio Φ rises. (D) Maximum membrane stretch ratios estimated from the same video data used in C are above 400%. (E) Energy output, response time (stroke response frequency), and actuator volume metrics taken from literature capture the approximate power densities of other reported tactile display systems. (F) Continuous reactant replenishment enables high spark frequencies to trigger high-frequency actuations up to 1.2 kHz. Vertical lines depict spark frequency equal to the theoretical rate at which the gas flow refills the cylinder (shading shows the standard deviation of each measurement).

The controlled “explosion” can be directed via independent channels to raise specific dots for a highly tactile indication (note: this isn’t the same as haptics). A magnetic latching system gives these dots a post-ignition persistence, but they can be reset simply by pressing them down.

Sparking Traces

Key to the new system is an array of molded silicone and microfluidic liquid-metal traces that function as electrodes to produce sparks to ignite a lean methane-oxygen fuel mixture in a 5-mm-diameter, 2-mm-tall silicone cylinder. The exothermic reaction quickly pressurizes the cylinder, displacing the silicone membrane up to 6 mm in under 1 ms. At these small scales, the wall-quenching flame behavior allowed researchers to construct a 3 × 3 array of 3-mm diameter cylinders with 4-mm pitch (Fig. 2).

2. Design for individually addressable, high-density arrays of soft actuators. (A) Arrayed actuator design is compliant and biocompatible. (Scale bar: 15 mm) (B) An exploded view visualizes the layer-by-layer device architecture. (C) High-speed images show individual operation of different actuators (Movie S5). Two-dimensional diagram of high-voltage (HV) demultiplexing traces given in righthand corners. (Scale bar: 5 mm) (D) A tactile display application was demonstrated with an elementary magnetic latching pin array placed directly above membranes; the pins rest in a steel machined plate.

Further, the fluidic elastomer actuators cool quickly and very little fuel is required, so they maintain that a commercial version would be safe despite potential misgivings about methane from prospective users. Among the many non-obvious issues the team had to resolve was the risk of spark-initiated “flashback”—a well-known concern in larger piping. Here, they had to develop and install a small-scale, 1.5-mm-thick, sintered metal disk in the central junction of the fuel channels to prevent it.

They demultiplexed the voltages via high-voltage reed relays arranged on two small copper PCB prototyping plates, after testing for individual spark control (Figs. 3 and 4), crosstalk, and inductive-kick issues. Relay switching was controlled via Arduino Uno digital output ports. Although the researchers didn’t explicitly call out the voltage they used in their published paper, their isolated dc-dc supply is rated for a 2-kV, 250-µA output.

3. Array spark gap wiring detail: Each picture shows a unit cylinder without the membrane top to reveal the internal architecture. The left panel is a rendering of an assembled actuator, and the right panel is a picture taken from the eyepiece of a binocular stereo microscope (overhead illumination). Light in the center is an electric arc. Arrows point toward piercing ends of wires. Scale bars: 1.5 mm.

4. HV reed-relay circuit prototypes: (a) FR1 PCB copper-trace negatives were cut away with a desktop CNC milling machine to connect two sets of three HV reed relays to the device and common inputs. Relays were stacked on top of each other and separated with nylon standoffs at the corners. Scale bar: 50 mm. (b) A separate design was hand-routed with a Dremel tool and connected to 3-kV rated HV relays; one is visible as the red rectangle under the nearest PCBS. Scale bar: 100 mm.

Electromechanical vs. Combustion

“Having something that can change its shape in a way you can feel, like real objects, doesn’t exist right now. There’s this tradeoff between having small actuators, and size and weight and cost. It’s so difficult,” said Rob Shepherd, project head and associate professor of mechanical and aerospace engineering in the Cornell College of Engineering. “Everybody’s been trying electromechanical systems. So we said, well, what if we don’t do that at all and we use combustion. Small volumes of gas can create powerful outputs.”

The current system consists of nine fluidic elastomer actuators, but the researchers are hoping to scale that up and eventually create a full electronic tactile display. They also plan to convert the system to widely available butane, already used in countless pocket lighters.

The research was supported by the National Science Foundation, the Air Force Office of Scientific Research and the Sloan Foundation. Full details are in their paper “Valveless Microliter Combustion for Densely Packed Arrays of Powerful Soft Actuators” published in the Proceedings of the National Academy of Sciences. While that paper is behind a paywall, an open version is fortunately available here.

In addition, a Supporting Information File links to a 23-page report that may have more interest to engineers than the main paper. It contains details of the materials used, fabrication steps and techniques, test arrangements, a quantitative table with specifics comparing this approach with respect to various parameters versus other approaches, as well as relevant chemistry, fluid mechanics, and physics equations. It also includes links to six short videos.