Yu Jiang Tham

Dream it. Build it.

Tag: arm

How to Build a Robotic Arm that Tracks Your Hand Movements (Part 2)

A few days ago I wrote part 1 of this tutorial on building a robotic arm that you can control with your hand movements in air.  That tutorial dealt with the hardware side, so now I’ll talk about the software and how it all works together.  In case you missed the first part, here’s the robot arm again:



I used node.js with the leapjs and johnny-five packages to get data from the Leap Motion device and send it to the Arduino.  The code itself is on GitHub here:



To run it, we’ll start by including the leapjs and johnny-five packages.  You should install the packages first into the directory that you’ve unzipped everything to (npm install johnny-five && npm install leapjs).  Next, in your Arduino software, you will need to go to hook up your board and then go to Files > Examples > Firmata > StandardFirmata (this is required for johnny-five).  Then hit Upload.

Screen Shot 2013-07-29 at 2.05.08 PM

To start the software:

node robotarm.js


The javascript library for Leap, called leapjs, works by waiting for a Leap ‘frame’ to be emitted to the listener, which then calls an anonymous function.  We use this function to set our variables for later use with johnny-five. Here’s the basic form of it:

var controller = new Leap.Controller();
controller.on('frame', function(frame) {
  <get our Leap Motion data here>

The johnny-five package basically requires you to initialize a board and whatever else you are going to use inside that.

board = new five.Board();
board.on('ready', function() {
  servoBase = new five.Servo(3);
  servoShoulder = new five.Servo(9);
  servoElbow = new five.Servo(10); 
  servoClaw = new five.Servo(6);


There are four servos in the arm, and three separate main controls.  The base rotation is controlled by the x position of the hand.  The end effector is controlled by the distance between two fingers.  Finally, the shoulder and elbow joint angles are calculated by the inverse kinematics equation with inputs of the y and z coordinates.

function calculateBaseAngle(x) {
  var n = 100*normalize;
  x = 1.5+2*x/n;
  var angle = 90+Math.cos(x)*90;
  return angle;

While the Leap Motion device measures coordinates linearly, the base rotation is circular.  I wanted to feed the x value through a cosine function so that it would not be too sensitive in midranged values. In order to calculate the base angle, I needed to get its range of values that would jibe well with a cosine function, and since the Leap Motion’s vales are in the range of -300 to 300 in Leap Space, I divided by 100 times the normalize value, which I will go into more detail in later paragraphs.

Since the servo rotation values are between 0 and 180, I wanted 90 to be the value when the arm was pointing forward, so modified the values so that they will range from 0 to 180 with 90 being in the center.


For the shoulder and elbow joint angles, the servos are placed in such a way as to allow for a large range of motion and “negative” values for the elbow.  See the diagram below; the triangles show the pivot points of the servos.


In order to calculate the servo angles, we use an inverse kinematics equation. In physics, forward kinematics is used to calculate the position of an end effector, given its joint angles.  However, with the Leap Motion device, we want to translate our hand’s position information into joint angles, thus we have to use inverse kinematics:

function calculateInverseKinematics(x,y,z) {
  z = -z;
  var t1 = Math.acos((square(z)+square(y)-square(l1)-square(l2))/(2*l1*l2));
  var t2 = Math.asin(((l1+l2*Math.cos(t1))*y-l2*Math.sin(t1)*z)/(square(l1)+square(l2)+2*l1*l2*Math.cos(t1)));
  return {
    theta1: t1,
    theta2: t2

I invert the z value so that reaching towards the Leap Motion device in front of me will make the arm go forward as intended.  The other theta1 and theta2 equations are simply the inverse kinematics equations from the following website:


The one thing to note is that we have to calculate the length (variables l1 and l2) based on the distance in terms of the Leap Motion device coordinates.  The easiest way to find the length is to output your hand’s y-position to the console.

Place your arm section over the Leap Motion device and place your hand at the bottom of the section.  Make note of the y-coordinate.  Move your hand to the top of the arm section and make a note of the y-coordinate.  Subtract the two numbers and the absolute value is the length in Leap space, which is what you will use for your length in the inverse kinematics equation.  Do the same for the second arm section.  These values will obviously be different for different arm lengths, so be sure to change this:

var l1 = 40*normalize;
var l2 = 40*normalize;

The normalize value is set to 3 in my app, but you can tweak it to change the sensitivity of the system as well as the range of motion.  Normalize is also used in the input for the inverse kinematics function:

if(frame.hands.length > 0) {
  handPosition = frame.hands[0].palmPosition;
  angles = calculateInverseKinematics(0,-10+handPosition[1]/normalize,handPosition[2]/normalize);
  moveBase = 180-calculateBaseAngle(handPosition[0]/2);
  moveShoulder = toDegrees(angles.theta1);
  moveElbow = 45+toDegrees(angles.theta2);

Since we are operating the shoulder and elbow joints on only the yz plane, we feed a zero into the x position (the x axis is calculated separately by the base rotation angle).  I decreased the y input by 10 so that my hand would not have to be as high above the Leap Motion device, but you can play with this value and your mileage may vary.  Another thing to note is that the elbow has an offset of 45 degrees.  This is intentional so as to allow for “negative” elbow values.


The last part that needed to be done was to calculate the angle of the end effector, which would basically open and close the end effector (gripper).  This was simply done in the Leap loop while calculating the finger distance:

if(frame.pointables.length > 1) {
  f1 = frame.pointables[0];
  f2 = frame.pointables[1];
  fingerDistance = distance(f1.tipPosition[0],f1.tipPosition[1],f1.tipPosition[2],f2.tipPosition[0],f2.tipPosition[1],f2.tipPosition[2]);
  moveClaw = (fingerDistance/1.5) - minimumClawDistance;

What I’m doing here is basically getting the first two fingers (and ignoring the rest) and finding the distance in free space between them using a distance function which I have defined (which would be the same as any other distance function).  The Leap motion does have one annoying bug, which is that it will no longer detect your two fingers if you put them together.  Thus, I wanted to make the claw close without having to have my two fingers touch, which is the minimumClawDistance.

All of the global variables are read by the Johnny-Five board loop and the output is sent to the servos here:

this.loop(40, function() {
  if(!isNaN(moveShoulder) && !isNaN(moveElbow)) {
  if(moveClaw >= 0 && moveClaw <= 100) {
  console.log("Base: " + Math.floor(moveBase) + "\tShoulder: " + Math.floor(moveShoulder) + "\tElbow: " + Math.floor(moveElbow) + "\tClaw: " + Math.floor(moveClaw));

The main things to note are the if statements; they basically protect the system from some unintended values (and will protect your arm from crashing down into the table if it his a NaN value).  The loop reads values every 40ms, but you can play around with this value if you’d like.


Well, that’s all for now.  I hope you enjoyed this post!  Let me know if you have any questions.

How to Build a Robotic Arm that Tracks Your Hand Movements (Part 1)

This past week, I made a robotic arm that tracks your hand  in free space using a Leap Motion, Arduino, and node.js.  It tracks my hand very well.  It is able to pick up stuff and set it down with relatively good precision, although there is definitely some room for improvement.  I can’t pick up anything too heavy with it (mainly due to the gripper limitations), but it’s a good first try.  See the video below:

I’ve spit this tutorial into two parts.  This part 1 (you are here) deals with building the hardware.  Part 2 deals with the software implementation.  Let’s get started!



I built all of this in the kitchen of my studio apartment in San Francisco.  Unfortunately, I did not have access to any sort of workshop, so I had to use whatever I had lying around.  I ended up ordering a few things from Amazon.  The tools I used are as follows:

Additionally, the materials/components I had to buy to get this thing working are:

I bought a sheet of aluminum (0.025″ thick) and used the ruler and sharpie to figure out the dimensions of the arm.  I then set about cutting the aluminum with the aluminum scissors (be sure to use gloves and eye protection since the aluminum is sharp).  After the cut, The aluminum can be bent by hand, since it’s relatively thin.  Push any corners that aren’t straight against your table to flatten them.  We want to get the piece as flat as possible.


After that, we want to bend the piece of aluminum so that we can have some semblance of an arm section.  We’ll bend the piece twice to get that in place.  To make a sharper bend, use the ruler. Press the aluminum against the table and pull it up against the edge of the ruler.


Align the servo arm holes to the metal and mark the spots where you want to drill holes for the servo arm screw to attach to the aluminum arm section.  Drill holes there.


Before coming up with a design for the servo base attachment plate, you need to see what your servo’s range of motion is.  We want to make sure that at the 90 degree point, the servo arm points up.  The design I came up with is sketched on this piece of paper:


After that, creating the next arm section is similar, except we want to make it shorter and thinner  for two reasons.  One, less weight.  Two, the end effector (gripper) will have some length as well, and we don’t want to have an out-of-proportion arm.  That would look ugly.

The end effector is a relatively simple design that mimics the following design I saw on youtube, except I used aluminum for the joints and paperclips to keep everything in place.  Here’s the youtube video:

I tried using wood, but the pieces were too thin and I split them when I drilled the holes.  Here’s the final end effector that I came up with.  I used plastic spoons because I didn’t have anything else lying around.  Here’s what it looks like after I put it together.


And then the rest of the arm just comes together after some additional drilling of holes and aligning of pieces!  Here’s the final arm (minus the wood board that I used to attach to the base).



That’s that for the hardware! Continue to part 2 where I will go into detail about the software and how to link it to your Leap Motion device.

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