An 18 DOF Hexapod project using LinkIt Smart 7688 + PCA9685

Youtube video1

Youtube video2

Hardware

• Body: 3D printed, go to STL folder
• LinkIt Smart 7688
• Adafruit 16-channel PWM driver (PCA9685)
• SG-90 mini servo x 18
• DC/DC Buck Voltage regulator (Mini 360 from Taobao/Amazon)
• 2-cell LiPo battery

Electronics

There are 18 servo controlled by 18 PWM channel, 2 of them are from 7688 itself (GPIO18 & GPIO19), the rest are from the PWM driver connected to 7688 via I2C interface

And 4 DC/DC buck voltage regulator are used because servo draw lots of amount of current (500mA each, 9A total). Each of them provide 3A current and serve 6 servos (2 legs) Voltage regulator 1~3 serves all 18 servos Voltage regulator 4 serves 7688 and PCA9685 (PWM driver IC)

How to Use/Start

• ssh to 7688 as root
  • as root, do npm install ws
  • put hexapod_core.js, hexapod_srv.js and settings.js to /root
  • put index.html to /www/hexapod/
  • edit /etc/rc.local, add command node /root/hexapod_srv.js > /root/log 2>&1 & before exit 0
• reboot 7688
• open url http://mylinkit.local/hexapod on chrome browser
  • make sure client is in the same network with 7688
  • make sure the 7688's device name is mylinkit.local. If not, modify the URL

Software

hexapod_core.js

The file contains:

• Logic to control 18 servo via I2C or PWM0/1 (check function writePwm)
• IK (inverse kinematics) algorithm (check function loc2angle)
• Movement path table (check table _mode)
  • There are 9 path now:
    1. d1:forward/backward waving (youtube video)
    2. d2:left/right waving (youtube video)
    3. d3:circle waving (youtube video)
    4. fwd:move forward
    5. bwd:move backward
    6. sl:shift left
    7. sr:shift right
    8. tl:turn left
    9. tr:turn right
  • The path are pre-generated by anohter JS, described later
• The main loop (check function _next)

hexapod_srv.js

The file contains:

• websocket server, receive command from browser (client side)

index.html

The file contains:

• websocket client, running on browser (client side)

setting.js

The file contains:

• default settings

How 7688 control Adafruit PWM driver (PCA9685) via MRAA

First, create an I2C object

var mraa = require("mraa")
var pwms = new mraa.I2c(0)

Second, setup I2C address (the default is 0x40, check adafruit's page for how to set a different address)

pwms.address(0x40)

Setup PRE_SCALE so that PWM frequency is 61Hz, the period will be 16.37ms, PCA9685 is 12-bit precesion PWM, it has 4096 unit, So it will be 4us per unit (16.37 / 4096) According to datasheet, "Writes to PRE_SCALE register are blocked when SLEEP bit is logic 0 (MODE 1)", so a modify/restore MODE0 is required before/after modify PRE_SCALE.

pwms.writeReg(0, 0x10)   // SLEEP bit = 1
pwms.writeReg(0xFE, 110) // 61Hz, 16.37ms, ~4us per unit
pwms.writeReg(0, 0x0)    // SLEEP bit = 0

According to datasheet, the first 2 registers of each pin are LED_ON, and the other 2 registers are LED_OFF. But I found out writing to LED_ON does NOT work. Instead it is inversed, only writing to last 2 register (LED_OFF) works. Normal servo operates from 1000us to 2000us: 1500us correspond to center position and 1000/2000us correspond to both end. The value should be divided by 4 because each unit is 4us.

value = value / 4        // us to unit
pwms.writeReg(6+pin*4, 0);
pwms.writeReg(7+pin*4, 0);
pwms.writeReg(8+pin*4, value & 0xFF);
pwms.writeReg(9+pin*4, (value>>8) & 0xFF);