project specification

Gummi Arm

A robot arm that provides a unique approach to lifting and pulling actions.


The GummiArm is a 7+1 DOF robot arm. The structure of the arm consists of plastic parts connected to digital servos. The servos are joined by PLA-based plastic parts that can be printed on hobby-grade 3D printers.

A key element is the agonist-antagonist joints driven by accurate digital servos, and with viscoelastic composite tendons. The inherent damping and the ability to adjust stiffness in real-time helps simplify joint control.


The arm structure is made of PLA plastic, and is printable on hobby-grade 3D printers. The first prototype was printed on a BQ (Madrid, Spain) Prusa i3 Hephestos printer with 20% infill.

The Dynamixel servos are controlled using the USB2Dynamixel device, and the corresponding ROS package. The arm is actuated by Dynamixel servos from Robotis Inc (Irvine, CA, USA). There are 5 agonist-antagonist joints, each with 2 Dynamixel servos pulling on tendons that work against each other. For example the biceps and triceps actuating on the elbow joint. 2 joints are directly driven by servos, and 1 servo is included in the forearm to drive a hand.

Elbow Design Process

A quasi-static loading setup was created for the elbow joint. The upper arm was locked in place, while the lower arm was replaced with a rigid beam with multiple attachment points for weights, from 70 mm to 200 mm from the joint axis, and at 10 mm intervals.

The actuator was commanded to a passive

horizontal pose. Three different weights (0.1 kg, 0.5 kg, and 1.5 kg) were then attached at different distances from the joint axis to generate a set of torques up to almost 3 Nm. The passive deflection of the joint was then recorded with the AX-12A encoder. This process was repeated three times for the 3 weights and the 14 distances. The same procedure was repeated for 5 different stiffness levels,

from 0% to 100%.


  • Structure is 3D printable
  • Various attachments available for customization
  • Modular build for size increase or reduction according to the users' needs
  • Twisted tendon-driven design