Inverse Kinematic Library for Arduino

Calculates the inverse kinematics for a 3 links arm with a rotating base. Tested with Arduino UNO and Teensy 3.6, with the robotic arm Arduino Braccio.

How it works?

A Link is a straight line from one joint to another. We usually need the length of links to calculate the inverse kinematics. For the braccio robot arm, we have 4 links: base that connects a0 with a1 with a length of 0mm, upperarm that connects a1 and a2 with a length of 200mm, forearm that connects a2 with a3 with a length of 200mm, and hand that connects a3 with the end effector with a length of 270mm.

Here, a0, a1, a2, and a3 correspond to each servomotor of the arm.

To simplify calculations, we assume that the origin point starts at a1 and a0 shares the same point with a1 (link length of 0mm).

Declaring Links

We can setup a link by declaring a variable of type Link:

Link myLink;

Then, we initialize the Link with the following values:

myLink.init(_length_, _min_angle_, _max_angle_);

Each angle is defined in radians and the length is defined in mm.

Finally, we attach each Link to the inverse kinematic solver:

InverseK.attach(base, upperarm, forearm, hand)

To get the inverse kinematic, first we need to define 4 float variables:

float a0, a1, a2, a3;

These variables are passed to the solver as pointers and their values are modified by the solver when we call the following function:

InverseK.solve(_x_, _y_, _z_, a0, a1, a2, a3) // this returns TRUE or FALSE

If a specific approach angle phi is necessary (e.g. you need to approach a piece from above), you can use:

InverseK.solve(_x_, _y_, _z_, a0, a1, a2, a3, phi) // this returns TRUE or FALSE

Here, x, y, and z and defined in mm and follow the system:

The solver will return TRUE if a solution is found and FALSE if there are no solutions. In case of a FALSE return, the values of a0, a1, a2, and a3 are not guaranteed.

What is the math behind the solver?

This solver basically uses the Rule of Cosines to find the angles that lead to a specific coordinate in space.

To calculate the rotation angle of the base, the cartesian coordinate is transformed to polar coordinates.

Practical Example

// Include the library InverseK.h
#include <InverseK.h>

void setup() {
  // Setup the lengths and rotation limits for each link
  Link base, upperarm, forearm, hand;

  base.init(0, b2a(0.0), b2a(180.0));
  upperarm.init(200, b2a(15.0), b2a(165.0));
  forearm.init(200, b2a(0.0), b2a(180.0));
  hand.init(270, b2a(0.0), b2a(180.0));

  // Attach the links to the inverse kinematic model
  InverseK.attach(base, upperarm, forearm, hand);

  float a0, a1, a2, a3;

  // InverseK.solve() return true if it could find a solution and false if not.

  // Calculates the angles without considering a specific approach angle
  // InverseK.solve(x, y, z, a0, a1, a2, a3)
  if(InverseK.solve(550, 0, 50, a0, a1, a2, a3)) {
    Serial.print(a2b(a0)); Serial.print(',');
    Serial.print(a2b(a1)); Serial.print(',');
    Serial.print(a2b(a2)); Serial.print(',');
    Serial.println(a2b(a3));
  } else {
    Serial.println("No solution found!");
  }

  // Calculates the angles considering a specific approach angle
  // InverseK.solve(x, y, z, a0, a1, a2, a3, phi)
  if(InverseK.solve(550, 0, 50, a0, a1, a2, a3, b2a(90.0))) {
    Serial.print(a2b(a0)); Serial.print(',');
    Serial.print(a2b(a1)); Serial.print(',');
    Serial.print(a2b(a2)); Serial.print(',');
    Serial.println(a2b(a3));
  } else {
    Serial.println("No solution found!");
  }
}

void loop() {

}

// Quick conversion from the Braccio angle system to radians
float b2a(float b){
  return b / 180.0 * PI - HALF_PI;
}

// Quick conversion from radians to the Braccio angle system
float a2b(float a) {
  return (a + HALF_PI) * 180 / PI;
}

How to install?

1. Download the project as a .zip file.
2. Open Arduino IDE
3. Select Sketch->Include Library->Add .ZIP Library...
4. Locate the .zip file you just downloaded (Should be named CGx-InverseK-master.zip)
5. Select the file and click Open
6. Ready!

You can check the example sketch at: File->Examples->CGx InverseK->CGx_InverseK_Example

If you want to control the arm using Gcode, check: CGx-Gcode-RobotArm

Found any issues or questions?

Please post a new issue with your question or suggestion.

Pull requests are also welcomed.