Building Robot Hands with Compliant Under-Actuated Fingers

Techniques for building compliant robot hands and fingers.

Building capable robot end effectors, particularly high-complexity hands, can be a daunting challenge.  In this article, we will examine the fabrication of a robot hand with compliant, under-actuated fingers that is rugged enough to bounce back from twisting, end-on and side impacts, falls, collisions, and even severe back-bending.  The specific fabrication process explored is akin to shape deposition manufacturing using materials such as resins (epoxy / Delrin) and urethanes (a "rubbery" substance) of various durometer (hardness).  This particular technique was used to build early hand prototypes for MIT's Nexi (or MDS) robot from the Personal Robotics Group, and further refinements resulted in the Meka Robotics H2 Compliant Hands, as seen on the Simon robot.    Read on for details and pictures -- this should be of interest to robotics hobbyists and professionals alike.

There has been a lot of interest by researchers in compliant hands.  The motivations for simple, compliant robotic hands for manipulation are numerous, and well covered in a recent MIT Tech Review article.  We will examine a design of just such a type of finger, a technique that was ultimately refined to produce the latest Meka Robotics H2 Compliant Hands:

Meka Robotics H2 Compliant Robot Hands  Meka Robotics H2 Compliant Robot Hands

As I mentioned, an early prototype of this hand appeared on the Nexi robot from MIT:

MIT Nexi MDS Robot with Meka Robotics Hands  MIT Nexi MDS Robot with Meka Robotics Hands

Apparently Carl Pisaturo collaborated with the Meka Robotics folks on this early design, and he was gracious enough to put some photos and design notes online.  The specific technique is similar to shape deposition manufacturing (SDM) where various structural and mechanical elements are simultaneously fabricated and assembled through an alternating cycle of deposition and shaping.  In essence, the process uses a series of molds with various material types to build a complete structure -- a good overview of this process can be found in this paper:  "Biomimetic Robotic Mechanisms via Shape Deposition Manufacturing."

The notes on Carl's page are not step-by-step, so I'll try to piece together the steps from images.

Compliant robot finger fabrication  Compliant robot finger fabrication

Compliant robot finger fabrication


Compliant robot finger fabrication

To start building a finger, they begin by building the urethane benders (black flexible pieces between the hard blue pieces on the fingers above).  A master mold pattern is built from machined Delrin, teflon tubes, and metal wires.  From the master, numerous silicone molds can be made.  The silicone mold is filled with urethane, with steel cables and teflon tubes embedded within.

Central to this design is the cable-reinforced urethane bender, 3 different types of which form the "hinge joints" of a finger.  Cast-in tunnels for wiring run down the center, and the dual "X" cables provide torsional rigidity.  Physical keying and cable stubs keep the benders in place within epoxy "bones".

Urethane thermoset elastomers such as this are very rugged, with excellent tear and abrasion resistance.


A master mold for the entire finger assembly is constructed in a similar manner.

Compliant robot finger fabrication   Compliant robot finger fabrication

The urethane bender is placed into the silicone mold, the remainder of which is then filled with a hard epoxy resin.  A more complete overview of this process is shown below (which assumes the black urethane bender has already been cast and hardened). 

Compliant Robot Finger Fabrication

Holes are made in the epoxy either by machining or by placing tubes in the mold.  These holes support the addition of the Delrin rollers used as cable guides.

Compliant robot finger fabrication  Compliant robot finger fabrication

Finally, pads made of a lower durometer (softer) urethane are cast for the finger pads and finger tips.

Compliant robot finger fabrication  Compliant robot finger fabrication

Threading the rollers with a cable that is actuated by motors inside the hand or forearm yields an under-actuated, cable driven finger.

Compliant robot finger fabrication

Adapt the design, add some force-sensing series elastic actuators in the palm, and apply some good control electronics, and you go from a simple prototype to the new Meka Robotics H2 Compliant Hands.

Compliant robot finger fabrication  Meka Robotics Hand

Cool stuff!

I should also note that in addition to this cool info on robot fingers, Carl Pisaturo also has a treasure trove of tips about working with urethane, silicone, and epoxy that are worth checking out.