S T E W A R T    P L A T F O R M    O N    E S P 3 2

ABSTRACT

This is an implementation on the ESP32 of a 6-degrees of freedom hexapod — also called Stewart Platform, Gough-Stewart Platform or Parallel manipulator. The actuators are PWM rotary servo motors and the program is written in Arduino C for PlatformIO Core 4.0. This project is done in the frame of the P19 project at the Microclub.

Currently the platform can be operated:

• with a Nunchuck
• with Arduino C code (see Hexapod_Demo.cpp)
• with G-Code through serial (see python/gcode2serial.py)

The kinematics calculation is done in Hexapod_Kinematics.cpp and can be also be compiled in a desktop C++ program to cross check calculations (i.e. without actuating the servos). I managed to compile it with g++ on macOS Mojave and macOS Monterey. See hexapod_desktop_app/hexapod_desktop_app.cpp.

HOME PAGE OF THE PROJECT

The home page is a work in progress, but the video shows what the platform can do. The original implementation used an analog joystick, but the current version uses a Wii Nunchuck.

https://ouilogique.com/plateforme-de-stewart-esp32/

GEOMETRY SETTINGS

Geometry settings are defined in Hexapod_Config_1.h. The meaning of the parameters is also explained in doc/hexapod-parameters.pdf.

REQUIREMENTS

Create a file called src/WifiSettings.h containing:

#pragma once

const char *ssid = "";            // SSID of your WiFi router.
const char *password = "";        // Password of your WiFi router.
const char *ap_ssid = "STEW32-";  // SSID of the ESP32 WiFi network in soft-AP mode (15 char max).
const char *ap_password = "";     // Password of the ESP32 WiFi network in soft-AP mode.
                                  // Must be 8 char min or empty for no password.

COMPONANTS & WIRING

ESP32

WeMos ESP32 WROOM https://www.banggood.com/fr/WeMos-ESP32-WiFi-Bluetooth-Development-Board-Ultra-Low-Power-Consumption-Dual-Core-ESP-32-ESP-32S-p-1175488.html

PCA9685

https://www.mouser.ch/ProductDetail/adafruit/3416/?qs=F5EMLAvA7ICYzX4Av%252bhRHw==

External power supply

I currently use a 5 V / 10 A power supply, but 5 V is not enough. I need to upgrade to 7 V. https://aliexpress.com/af/32810906485.html

Rods

M3x100mm (140mm total) https://aliexpress.com/af/32775630549.html

Servo horn arm

Tritanium color https://aliexpress.com/af/32843432977.html

Servos

• I currently use clones of the Tower Pro MG996R servos, but they are bad and I don’t recommend them. https://fr.aliexpress.com/item//32636102294.html
• I formerly used clones of the Tower Pro MG90s Servos, but they were also bad and too small for this application. https://www.banggood.com/6X-Towerpro-MG90S-Metal-Gear-RC-Micro-Servo-p-1072260.html
• In the future, I will probalbly use Parallax 900-00005 servos. It seems that these are the one used by fullmotiondynamics in their videos. https://www.parallax.com/product/900-00005

Pins are defined in Hexapod_Config_1.h.

Nunchuck

• Nunchuck https://fr.aliexpress.com/item//32880983134.html
• Cable extension https://fr.aliexpress.com/item//32841281892.html

The Nunchuck library uses Wire.h and standard I²C connections internaly.

Pins are seen from left to right and top to bottom looking at the Nunchuck connector.

Cut the cable extension to connect to the ESP32.

EXTERNAL LIBRARIES

The external libraries are installed automatically during the first build because they are declared under lib_deps in platformio.ini.

I²C Addresses

CREDITS

Primary source of inspiration

• San-José State University / Full Motion Dynamics:
  • https://www.youtube.com/watch?v=j4OmVLc_oDw
  • http://fullmotiondynamics.com

Kinematics

• Hexapod kinematics of this project was originaly based on 6dof-stewduino, by Philippe Desrosiers, althought I reworked it in depth:
  • https://github.com/xoxota99/stewy
• He derived his implementation from the work of Daniel Waters:
  • https://www.youtube.com/watch?v=1jrP3_1ML9M
• Kinematics calculation is also explained in this document by an unknown author from the Wokingham U3A Math Group:
  • https://web.archive.org/web/20130506134518/http://www.wokinghamu3a.org.uk/Maths%20of%20the%20Stewart%20Platform%20v5.pdf
  • http://www.wokinghamu3a.org.uk/groups/mathematics/
• The project memememememememe was also an excellent source of inspiration. They share the code for RPi and a simulator in the Processing langage:
  • https://memememememememe.me/post/stewart-platform-math/
  • https://github.com/thiagohersan/memememe

Serial buffer for G-Code

• Derived from MarginallyClever:
  • https://github.com/MarginallyClever/GcodeCNCDemo/tree/master/GcodeCNCDemo4AxisCNCShield

FURTHER READING

• PID Control System Analysis and Design, By YUN LI, KIAM HEONG ANG, and GREGORY C.Y. CHONG
  • See Table 1, p.33 http://eprints.gla.ac.uk/3815/1/IEEE_CS_PID_01580152.pdf
• Understanding PID Control, Part 1: What is PID Control?
  • https://www.youtube.com/watch?v=wkfEZmsQqiA
• Modern Robotics, Chapter 7: Kinematics of Closed Chains
  • https://youtu.be/5wCK6XGC3ig