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 of a 6-degrees of freedom hexapod — also called Stewart Platform — on the ESP32. 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. See hexapod_desktop_app/hexapod_desktop_app.cpp.

PlatformIO VERSION

The platformio.ini file is written for PlatformIO Core 4.0. If your PlatformIO version is older, you can upgrade with the command platformio upgrade. If you don’t want to upgrade, you can pick the old platformio.ini file here : https://github.com/NicHub/stewart-platform-esp32/blob/bc47243335e571190a0a5bb4c3ecf3de08c9b6ca/platformio.ini.

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.

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.

servo	ESP32 pin
0	13
1	15
2	27
3	14
4	33
5	25

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.

color	signal	ESP32 pin
white	SCL	IO22
NC
red	GND	GND
green	VCC	VCC
black	ATT	IO4 (Not required, gives VCC if Nunchuck is connected)
yellow	SDA	IO21

EXTERNAL LIBRARIES

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

I²C Addresses

Default address	New address	Device
0x40	0x41	PCA9685
0x70	=	PCA9685 (broadcast)
0x40	=	Nunchuck (accel)
0x52	=	Nunchuck (joystick)
0x68	=	GY-91 (MPU9250)
0x76	=	GY-91 (BMP280)

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

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