Soft upper body robot

The upper body, including the arms, pelvis, chest, and back has soft, 3D printed air-filled modules connected to pressure sensors to sense contact and provide protection to the child and robot while interacting. These soft skin modules cover the underlying actuators and the rigid 3D printed frame with printed bearings.

Each arm has four joints: two in the shoulder,one elbow joint, and one wrist joint. The torso has four serial vertebral joints with one end connected to the middle of thepelvis and the other end to the shoulder joints.  The arm has a 3 DOF shoulder joint and a 1 DOF elbow joint, which can perform grasping and hugging motions. The torso has a 2 DOF pitch-yaw joint to make motions like crouching or arm swinging. 

Soft skin

The robot has a 3D printed soft skin module  which has a flexible, contact-sensing air-filled cavity. This helps to absorb unexpected impacts, reducing the likelihood of human injury and actuator damage. It also provides contact force feedback via a pressure sensor connected to the air-filled cavity. These modules give the robot the ability to sense contact forces on its various links. 

The robot’s upper body contains eight soft skin modules: one at each hand, one  on each upper arm, two on the chest, one at the waist, and one on the back. These softskin modules each include a soft, air-filled  cavity. Each module’s cavity is enclosed by a 1.5mm thick membrane of rubber-like material. Each module consists of an inner rigid frame which provides structural support and servo mounting points. Inside of the cavity is a rigid stick with a rubber-like ball on the end which prevents the module from being deflated to a shape from which it can not return. This module attaches directly to the end of  a Dynamixel MX-28.  

Frame

Underneath the soft skin, a rigid frame links the servos together. This frame also supports servo output shafts under loads experienced during motion and physical contact.  Modular 3D printed bearings are used to constrain motion and distribute loads.

Manufacturing

The modules are 3D printed using a Stratasys Objet 260 Connex multimaterial 3D printer, which can print a single part with both rigid and flexible features. The rigid materials used are VeroWhitePlus and VeroClear. The flexible rubber-like material is TangoPlus.  

The method used to 3D print the soft skin modules is PolyJet printin. This method relies on a gel-like material to support overhanging part geometry as it prints each layer. This support material is UV-cured while printing, but can be broken up and removed post-print using a pressure washer. Because of that, cavities, must be printed with an opening through which this support material can be removed. A corresponding cap for each module is also printed to seal the cavity once clean.