ArmIK

Arm Inverse Kinematics.

The Arm

The code here controls the arm, based on input from an Xbox controller. The arm can swivel clockwise and counter-clockwise, lift up and down, and extend in and out. The gripper can be separately lifted up and down, rotated clockwise and counter-clockwise and pinch open and closed. The following image should illustrate that.

Setup

There are two sketches in this repository: Arm/Arm.ino and Gripper/Gripper.ino for the arm and gripper boards, respectively. The sketches share a lot of common code, which is refactored away into libraries. Those libraries follow the naming conventions of BURT_arm_X, where X is the functionality offered by the library.

To compile the sketches, the Arduino IDE needs to know where those libraries are. Unfortunately, to avoid compiling the arm and gripper sketches together, we can't simply place them in the same folder. But the only other path Arduino recognizes is the user's sketchbook directory. (You can find the path to your sketchbook by going to preferences. The libraries are located in <sketchbook>/libraries.) There are two options to get the sketches to compile:

1. Preferred Create a hard link to the libraries in this repository. Each platform has a different way of doing this, but on Windows you can run the following command in the command prompt set to this repository:

for /d %i in (libraries\*) do (mklink /J %userprofile%\documents\arduino\%i %i)

2. Workaround Manually copy the directories in /libraries into your sketchbook. This would mean that changes to the libraries won't synchronize with the sketchbook and you'd need to re-copy the files every time. The hard link method avoids this by linking the libraries.

Code Details

The controls

ARM CONTROLS:
- arm swivel:       L (horizontal)
- arm lift:             L (vertical)
- arm extend:       Bumpers
- precise swivel:     D-pad (horizontal)
- precise lift:     D-pad (vertical)
- precise extend:   Y/A

GRIPPER CONTROLS:
- gripper open:     Triggers
- gripper rotate:   R (horizontal)
- gripper lift:     R (vertical)
- precise open:       X/B

MISC:
- calibrate:        START
- driving mode:     BACK

This way, you can easily rotate and lift the arm (resulting in X and Y movement) at the same time by using the left joystick, or do the same for the gripper by using the right joystick. Using a “precise” control will move minimally to allow for precision movement along one axis at a time.

The code

The Arduino script handles transforming the transmitted controller inputs into instructions for the motors. It does so by keeping track of the gripper’s position and updating it in small increments when the controls are pressed. When IK is involved, it calls IK::calculateAngles with the new position and passes the angles along to the motors. If needed, IK::calculatePosition can also calculate the position based on the angles.

There are a few safety measures in place:

• IK::calculateAngles uses IK::calculatePosition to double-check its work. If the difference falls out to be more than IK::tolerance, the operation fails and the arm doesn’t move.
• IK::calculateAngles checks its results against IK::bLimits and IK::cLimits which dictate how far the joints can physically move due to hardware limitations. If the result comes out to be more than that, the operation fails and the arm doesn’t move
• The Arduino scripts only move the gripper by a small distance per second
• The Arduino scripts check the result of IK::calculateAngles against the current angles. If the difference comes out to be too much to move in a short time, the operation fails and the arm doesn’t move.

The Motors

We're using TMC5160 stepper motors to control the arm, which means we need to use their libraries. You can download the library at their repository (make sure you're on the Release_v1 branch) and the documentation here. For more details on our usage, see the TMC_utils library.