SLAA891B April 2019 – February 2020 MSP430FR2512 , MSP430FR2512 , MSP430FR2522 , MSP430FR2522 , MSP430FR2532 , MSP430FR2532 , MSP430FR2533 , MSP430FR2533 , MSP430FR2632 , MSP430FR2632 , MSP430FR2633 , MSP430FR2633 , MSP430FR2672 , MSP430FR2672 , MSP430FR2673 , MSP430FR2673 , MSP430FR2675 , MSP430FR2675 , MSP430FR2676 , MSP430FR2676
Products with capacitive touch sensor interfaces are more common than ever and typically feature a few button sensors or maybe a collection of buttons, such as a keypad. The button sensors provide a simple "touch" or "no touch" user input and their typical round or square shape can be created easily with a PCB CAD design tool. As product interfaces evolve, interfaces are beginning to include more complex sensor types, such as a slider, wheel, or touchpad. Designing these types of capacitive touch sensors shapes with a PCB CAD design tool can require a considerable amount of time, even for the best engineers and PCB designers. And even after the design has been completed, changes in product requirements often force the designer to make modifications to the design or completely start the design over. Automating the design steps can all but eliminate this time consuming process.
This application report describes how to create scalable self-capacitive slider, wheel, and touchpad sensors with OpenSCAD, a free Solid 3D CAD modeling tool, and custom scripts created by Texas Instruments. Modifying the script parameters and running the script generates a new sensor pattern that can be exported to DXF format then easily imported into any PCB CAD tool. This flexibility and speed makes it now possible to create complex sensor shapes effortlessly.
The scripts described in this document can be downloaded from http://www.ti.com/lit/zip/slaa891.
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OpenSCAD is a free software application for creating solid 3D CAD objects. It is a script-only based modeler that uses its own description language; parts can be previewed but cannot be interactively selected or modified by the mouse in the 3D view. OpenSCAD is free and released under the General Public License version 2. OpenSCAD is available from http://www.openscad.org/. It is also available for macOS® and Linux® operating systems.
The version at the time of this writing is 2019-05. Although the primary strength of OpenSCAD is 3D rendering, its 2D capabilities are perfect for creating 2D capacitive electrode shapes that can be quickly manipulated by simple modifications to the script parameters that control the various attributes of the design.
The sensor design scripts are provided by Texas Instruments under a BSD license so you are free to modify them if needed. These scripts create and manipulate primitive objects that are assembled into complex slider and wheel sensor designs, using only these seven OpenSCAD commands:
square()
circle()
translate()
rotate()
hull()
difference()
intersection()
For information about these commands, refer to the OpenSCAD user guide.
This document does not cover the theory of capacitive touch or provide design guidelines for construction and layout of capacitive sensors on a PCB, but it does illustrate common slider, wheel and, touchpad sensor designs and their geometric features that relate to the corresponding script parameters.
It is assumed the reader has some familiarity with capacitive touch technology, slider or wheel sensors and sensor design principles. If new to capacitive touch, visit http://www.ti.com/captivate for information about MSP430™ capacitive touch sensing microcontrollers and http://www.ti.com/captivatetechguide for sensor design guidelines.
NOTE
The PCB examples provided in this document are demonstrated using Altium Designer 19. Refer to your PCB CAD tool documentation to perform the equivalent steps.
OpenSCAD.exe and the OpenSCAD scripts are supported in Windows®, macOS®, and Linux® operating systems. The optional batch files provided with this application report were written and tested on Windows 10. If using macOS or Linux, it is up to the user to create a similar shell script for that platform.