MeArm Robotic Arm

Introduction

The MeArm is a small, cheap Robot Arm kit designed by Ben Gray of Phenoptix. It uses Servo Motors and laser cut acrylic.

This tutorial will show you how to wire it up, and will make it so that it can be controlled via an infra-red remote control.

You'll Need

• An Espruino Board wired up for Servo Motors
• An IR Reciever
• A TV remote control
• A MeArm: either a kit from Phenoptix, or laser cut your own from design files on Thingiverse

Wiring Up

Plug the Servo Motors into the servo header on the Espruino board as follows:

And plug the IR receiver directly onto the board, such that:

Software

Just copy and paste the following code into the right-hand side of the Web IDE, and click Send to Espruino.

We've explained in comments how everything works, but as you won't have the same TV remote control as us, when you press a button Espruino will print Unknown 12345789. Copy the number you get when you press each different button, and replace the numbers that we used with your numbers.

// Where the Servos are connected
var servos = {
  shoulder : B15,
  elbow : B14,
  base : C6,
  hand : C7,
};
// Add a variable that the 2 functions below will use
Pin.positions = {};
// A function that will set pulse width modulation up on the servo
Pin.prototype.goto = function(pos) { // -1 .. 1
  Pin.positions[this] = pos;
  console.log(this,pos);
  analogWrite(this, E.clip(0.5*pos+1.5,1,2)*0.05, {freq:50});
};
// This will just move one step in the direction that is given
Pin.prototype.move = function(dir) {
  Pin.positions[this] = E.clip(Pin.positions[this]+dir,-1,1);
  this.goto(Pin.positions[this]);
};

// Is the hand clenched?
var clenched;
// These are the values for how much the servo should move if the hand is clenched or not. The values you need will depend a lot on how you assemble your MeArm
var hand_on = 0.15;
var hand_off = -0.2;

// On initialisation, set the MeArm up to it's default positions
function onInit() {
  servos.shoulder.goto(-0.5);
  servos.elbow.goto(0);
  servos.base.goto(0);
  clenched = false;
  servos.hand.goto(hand_off);
}

/* This is a simple scripted sequence of movements. The base is left alone, but the arm:
 *  * Moves to it's starting position with an open gripper
 *  * Reaches out
 *  * Closes the gripper
 *  * Returns to its original position
 *  * Releases the gripper
 */
function doGrab() {
  servos.shoulder.goto(-0.5);
  servos.elbow.goto(0);
  servos.hand.goto(hand_off);
  setTimeout(function() {
    servos.shoulder.goto(-1);
    servos.elbow.goto(-0.4);
    setTimeout(function() {
      servos.hand.goto(hand_on);
      setTimeout(function() {
        servos.shoulder.goto(-0.5);
        servos.elbow.goto(0);
        setTimeout(function() {
          servos.hand.goto(hand_off);
        }, 800);
      }, 500);
    }, 800);
  }, 800);
}

/* As we're plugging the IR receiver right into 3 GPIO pins,
   we have to set them up to give it the power it needs */
A1.write(1);
A0.write(0);

/* Set up the IR receiver. The codes used here are the codes that we got
from our remote control, so you'll have to use the codes shown by the 
`print("Unknown "+code)` statement below. */
require("IRReceiver").connect(C3, function(code) {
  switch (code) {
    case "111100000111000000100000010111111": onInit(); break; // power
    case "111100000111000000101100010100111": doGrab(); break; // menu - start 'grab' sequence
    case "111100000111000000100100010110111": servos.elbow.move(0.1); break; // ch +
    case "111100000111000000000100011110111": servos.elbow.move(-0.1); break; // ch -
    case "111100000111000001010011001011001": servos.base.move(-0.1); break; // left
    case "111100000111000000100011010111001": servos.base.move(0.1); break; // right
    case "111100000111000000000011011111001": servos.shoulder.move(0.1); break; // up
    case "111100000111000001000011001111001": servos.shoulder.move(-0.1); break; // down
    case "111100000111000000001011011101001": clenched = !clenched; 
                     servos.hand.goto(clenched ? hand_on : hand_off); break; // ok
    default: print("Unknown "+code);
  }
});

// Finally, initialise
onInit();