PrintPut: Resistive and Capacitive Input Widgets for Interactive 3D Prints
We introduce PrintPut, a method for 3D printing that embeds interactivity directly into printed objects. PrintPut uses conductive filament to offer an assortment of sensors that an industrial designer can easily incorporate into their 3D designs, including buttons, pressure sensors, sliders, touchpads, and flex sensors. PrintPut combines physical and interactive sketching into the same process: seamlessly integrating sensors onto the surfaces of 3D objects, without the need for external sensor hardware.
Keywords3D printing Rapid prototyping Printed sensors
As computation becomes more ingrained into everyday objects, designers have a new responsibility to also consider how people interact with an object’s digital aspects. As a result, it is increasingly important for industrial designers to start considering digital interactivity earlier in their design process.
When developing new artifacts, designers create prototypes to guide their design process about how an object should look, feel, and behave. These early designs might be digital, in the form of illustrations or rendered mock-ups, or they may be physical, for example, sculptures or 3D prints. On the one hand, physical prototyping is extremely useful for informing the aesthetic and ergonomic qualities of a product. But unlike their digital counterparts, the results of these prototyping methods are typically non-interactive.
To address this, there is an active research area surrounding augmenting physical prototypes with touch sensors [1, 2, 8, 9]. These solutions usually have a tradeoff between ease of use, resolution, and customizability. More importantly, designers must use these solutions after making a physical prototype. Wrapping a physical object with external sensors may disrupt the crafted shape that a designer is exploring, or may not be possible on complex geometries.
We introduce PrintPut, a process to create input sensors on 3D printed objects. With PrintPut, designing shape and interaction occurs together. In this paper, we present a collection of sensors that an industrial designer can easily incorporate into their 3D designs, including buttons, pressure sensors, sliders, touchpads, and flex sensors. PrintPut enables a new category of objects with intrinsic touch sensing capabilities.
2 Related Work
A number of papers have explored the use of conductive inks to print flexible electronics. Rendl et al.  created PyzoFlex, a flexible sensor that detects touch, pen input, and hand proximity. Kawahara et al.  presented a method to create Instant Inkjet Circuits by modifying a consumer inkjet printer to print silver ink onto paper. They also demonstrated several capacitive sensors printed with this method. Olberding et al.  designed multi-layer topologies for flexible and cuttable multi-touch sensors. Gong et al.  extended this work with a printed sensor sheet that detects multi-touch and hand proximity with capacitive circuits, and folding and pressure with resistive circuits. With Resigraphs , Holman et al. demonstrated how designers can use resistive materials and paints to add touch sensing to non-planar objects.
There have also been a number of explorations using 3D printers for interactive prototyping and designing unique input and output methods. Ishiguro and Poupyrev presented methods to seamlessly integrate speakers into 3D printed objects . Savage et al. demonstrated Sauron, a rapid-prototyping platform for physical user interfaces . Sauron modifies 3D models such that, when printed, an internal camera can detect the motion of widgets such as push buttons, joysticks, and sliders. Leigh et al.  outlined the process they used to create custom conductive filaments. They also showed how the filament could be used to 3D print basic capacitive buttons and flex sensors. We extend their work by introducing a wider variety of sensors that can be directly integrated into objects and an automated workflow to do so.
3 Application Scenarios
Sound Wave Slider.
Multiple Sensor Toy.
4 PrintPut Implementation
The critical component of PrintPut is a conductive ABS filament. Commercial solutions are now available and inexpensive: we used Maker Geeks Conductive ABS . The filament has a high resistance and in our experience, a typical circuit has a baseline resistance in the MΩ range. PrintPut requires an ABS supported 3D printer with two extruders, such as a Makerbot 2X or Leapfrog Creatr. After a designer makes an object with sensor geometry, they import it into their 3D printer’s build manager and assign the base and conductive geometry to standard and conductive filaments, respectively. Once the object is printed, sensor values can be easily read by connecting it to an Arduino or other microcontroller with alligator clips. For a lower profile, a designer can instead attach thin wires with conductive adhesives (e.g. copper tape or 3M Z-Axis conductive tape).
5 Sensing Methods
Capacitive sensors are made from a single terminal. They detect touch by repeatedly charging the terminal and recording the discharge time. When a finger is introduced, the sensor detects a larger capacitance.
Digital Resistive Sensing.
Digital resistive sensors require two terminals, separated by a physical gap. The first terminal is connected to positive voltage (emitter), while the second connects to an input pin (receiver). Bridging the gap with a conductive material, such as a finger, allows current to flow, completing the circuit. A pull-down resistor is required at the emitter’s pin.
Analog Resistive Sensing.
Analog resistive sensors have a similar layout as digital sensors. In addition to detecting the presence of a bridge, the resistance of the bridge can modulate how much current is permitted.
6 Sensor Types
One of the basic button types is a capacitive button, as originally presented by Leigh et al. . They are made from a single pad of conductive filament, in any desired shape. Each capacitive button is directly connected to a unique input pin on the microcontroller.
Another possible button type is a resistive button, which acts as a digital resistive sensor. To make a resistive button, a designer places a pad of conductive filament on an object, split down the middle (Fig. 5). When a user places their finger on a resistive button, it bridges the gap and completes the circuit. We recommend a 1 mm space between the two pads: close enough that a finger can create a bridge, but far enough to avoid unintended touch signals.
Resistive X-Y Touch Pad.
GSR Pressure Sensor.
PrintPut provides two methods to create pressure sensors. The first is made in the same way as a resistive button (Fig. 5), but treating it as an analog resistive sensor. A finger’s galvanic skin response connects the two terminals by allowing current to pass through. Applying additional pressure lowers the resistance of the connection. In our experience, several other factors also contribute to the strength of the connection, including the size of the finger, sweat levels, ambient temperature, etc. . As such, this type of sensor is more suited to detect relative changes in pressure.
Spring Pressure Sensor.
We found that when dealing with a large amount of adjacent touch points, one should space them at least 3 mm apart. This recommendation ultimately limits PrintPut’s touch resolution. One could overcome this with additional hardware or improved signal processing (such as with the touch pad), but such detailed resolution is likely unnecessary for the demands of a disposable prototype. In addition, the material properties of conductive filament and the process of printer extrusion results in slightly inconsistent conductivity for seemingly identical prints. Simply baselining sensor values or adjusting pull-down resistors can help resolve this behavior. Lastly, printing very large 3D models may be slower than desired for rapid-prototyping purposes (i.e. several hours). This could be addressed with techniques that print structures as wireframes with high-resolution sections, such as WirePrint .
In this paper, we introduced PrintPut, a category of 3D printed objects with a new dimension of interactivity. We presented a collection of interactive widgets that a designer can integrate into their models to help inform decisions when prototyping, including buttons, pressure sensors, sliders, touch pads, and flex sensors. PrintPut combines physical and interactive sketching into the same process, allowing sensors to be printed seamlessly onto 3D objects without external sensor hardware.
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