Last year was super busy with two projects, one of which was building and understanding shape customizable flex sensor designs. We presented and demo’d the work at ACM TEI 2017 in March at Yokohama.
A brief walk through of the work, for all those who missed the talk and the demos:
Motivation: Flexible and deformable user interfaces have been of great interest to the HCI research and interaction design communities. An important question for realizing interactive prototypes is how to sense deformations.
A few approaches have used commercially available flex sensors and strain gauges. These being easy to interface, result in slightly thicker prototypes and are shape restricted. Slimmer thin film flex sensors have been demonstrated by screen printing piezoelectric ink material on various substrates. Such sensors involve multi-material,multi-step fabrication and are not easy to customise for various shapes. Research on printed electronics has showcased building various sensing modalities such as touch and pressure sensing through techniques such as conductive inkjet printing to instantly realize digitally designed sensors. However, continuous bend sensing has not been investigated so far, using such instant fabrication techniques for unique sensor shapes.
Basic Principle of Flex sensing Our work builds on the basic principle of resistive flex sensing. Conductivity of certain materials changes on flexion. Resistance of the material increases when bent in one direction and decreases when bent in the opposite direction. We utilize one such conductive material, silver nanoparticle ink, to print fabricate flex sensors.
Standard design and design primitives The most basic of a flex sensor would just be a single line whose resistance is measured from its two ends. For obtaining a higher change in resistance to bending, a bunch of closely spaced lines can be laid out in parallel, whose resistance is measured from the two ends.
We identify 6 primitives within the design:
- Line width
- Line length
- Line spacing
- Number of sensing lines
- Connecting traces
We can customise each of these primitives to obtain flex sensors of highly varied geometries.
Digital design and instant inkjet fabrication The sensors can be designed in any vector graphics application such as Inkscape or Illustrator. They are then fabricated in a single step through conductive inkjet printing. In our case, we fill conductive silver ink in an off-the-shelf inkjet printer. (Alternately, one could directly pattern the designs on a substrate by drawing with a conductive pen.)
Voltage divider interfacing Once the design is fabricated, we can quickly interface it to a microcontroller using a breadboard, a known value resistor and a few cables. The known value resistor and the printed flex sensor form a voltage divider and we read out the output voltage on a microcontroller such as Arduino and find out the resistance of the printed design in different flexing scenarios.
Deformation Sensing We characterize the sensor response for two types of deformations: static and dynamic. Static deformations are those where the sensor is deformed from state to the other slowly. On the other hand, dynamic deformations are faster and the sensor does not have time to settle at a particular state while being deformed.
We printed 3 sensor samples each for 2 different sizes and flexed them across cylinders of various diameters for finding their static deformation behavior. When the sensor was bent with printed side up, the sensor resistance increased from its flat state value and vice versa. Normalized response for both the sensor sizes was quite similar indicating that uniform scaling did not affect the sensor response for the two sizes.
We then mounted a sensor on a motor assembly to flex it repetitively. A large sized sensor was flexed from flat state to flexion of 3cm radius 50 times at a speed of ~0.25Hz. For our test, the sensor response was highly repetitive and the sensor did not require re-calibration.
Applications A wide variety of end user applications can be built using such custom deformation sensors. We illustrate a few examples:
Animating virtual entities through direct physical input We can directly animate virtual entities through physical deformation input. Using a two sensor linear array, we can animate an emoticon to various moods.
Multimodal IO Sheet We can integrate various other sensing modalities such as touch sensing and output capabilities such as LEDs along with flex sensing on a single printed sheet.
The sample bow shaped sheet has two flex sensors on the side and a touch sensor in between. Each flex sensor is accompanied by a resistor to form a voltage divider. Each of the sensors also have a corresponding led that glows when the sensor is interacted with.
Making an origami model involves a one-time usage of a particular paper surface. A crease on the paper surface due to a fold would result in permanent increase in the sensor resistance.
We can leverage our bend sensors to detect fold sensing for such single usage scenarios. These can act as an affordable guidance tool for novice users to learn the correct sequence of folds. We can alternatively use plain graphite, drawn with a pencil, to pattern it directly on thinner origami paper.
For more details, please refer to the paper. It is an interesting read.
TEI ’17 Proceedings of the Eleventh International Conference on Tangible, Embedded, and Embodied Interaction, 2017
Many thanks to my adviser for guiding this work through and also the reviewers at ACM TEI 2017 for their valuable feedback.
For any questions or comments, just send a mail. 🙂