Mechanism Design
The key mechanism of Tether is a stylus attached to a self winding spring that passes through a joystick. Tether knows the precise location of the stylus in 3D space by measuring the length that the Tether string is reeled out and the angle which it comes through the joystick.
A potentiometer mounted on the string's spool measures how much the reel has spun and two potentiometers mounted on the joystick gimbal give the angle of the gimbal.
In order to keep Tether compact, before the string exits the joystick it takes a 90 degree turn over a pulley. In the final version of Tether, this pulley was 3D printed as a freely rotating cylinder to minimize the friction on the string.
The stylus has 2 buttons that provide increased functionality with apps. Tether's string is actually a wire that connects the microprocessor to these buttons. The spool transmits the electric signal during rotation via 2 brushes that contact a fixed plate underneath the spool.
Product Design
For our final design we had the goal of creating a sleek product that is compact and can easily attach to a variety of mobile device. It features a leaf-spring clip with padded tips that can snap onto almost any tablet or phone without blocking the display or damaging the screen.
The entire frame and part of the internal mechanism was 3D printed on a Multi-Jet Modeler.
The entire frame and part of the internal mechanism was 3D printed on a Multi-Jet Modeler.
Prototyping of tether
Tether went through countless iterations of design from early prototyping of the mechanism to various ergonomic variations. Some of the major steps are outlined below:
First Tether prototype with no sensing.
First functional prototype.
A more robust functional prototype. We began to explore the differences between using encoders and potentiometers for sensing of the length that tether was unwound.
A very robust prototype (though without buttons on the stylus yet). The clip we used was from a cup holder.
The design of our stylus itself went through many iterations. The final design was 3D printed on a FDM modeler.
Tether's SLA printed body was designed using component dimensions in Solidworks; however, wiring thicknesses, elasticity of build material, and friction between surfaces prevented a successful hardware integration
Spacing for PCB's, as well as holes for USB connectivity and charging were considered
Components were placed to optimize spacing and minimize the overall footprint, but one of the fatal flaws int eh design was the vertical, or "Z stack" of the potentiometer, wheel, and case; it proved difficult to retain concentricity of the potentiometer pin with the reel when the device was in use.
Tether's conductive string mechanism was based off this retractable cell phone car charger.
Testing the iteration of the hardware and software using our 3-D painting app