HOME
Flexible Joint Finger
An exploration into the practicality of a flexible jointed, as opposed to traditional hinge jointed, finger for robotics and prosthetics applications.

Design emphasis: durability and safety in real world interactions   

Key Feature: Structural Compliance
It is a hard truth of robot arm design that as one works outward from the torso to the fingertips, parts become smaller, more numerous, and more delicate. This is why robot hands have tended to be delicate and expensive. 

Yet it is this most delicate part of the robot - the hand - that must physically interact with the real world.  And these interactions, bumpy in the best of times, can be violent during the long process of software development.  A bad line of software can crash a hand, resulting in major repair costs and delays.

Clearly, for AI software development in the area of manipulation to proceed apace, as well as for robotic and prosthetic hand usage in gereral, a robustness-centric approach to hands and fingers is key. 

One approach to achieve robustness is structural compliance (e.g. a finger with rubber parts that give). Another is high strength (e.g. titanium hinge joints).  How these various approaches perform in the harsh test of reality can only be known by building and testing.
Novel fabrication techniques were a big part of this project, which is more akin to SDM (shape deposition manufacturing) than traditional methods.

Various molding and casting operations, as well as some machining, were involved in the fabrication of these fingers.

To the right is shown an early silicone finger mold being made around a delrin and teflon tube pattern.
There's a big difference between a nice design and a nice design that lasts.

Repetitive and overstress testing are essential when dealing with novel material arrangements like these - there are no roadmaps.  Realistic tests quickly illuminate misconceptions, strengths and weaknesses in a design, and form the basis for design evolution.  

A 2 axis tendon pulling machine with counters was built which allowed unattended repetitive tests of tendons, joints and whole fingers. Various loads and ranges of motion could be tried.  100K reps was deemed an acceptable longevity.
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. 

The downside to any rubber joint strategy, however, is the force required to bend it.  These rubbers also do not immediately return all the way.
The final finger has soft urethane rubber pads which, together with the compliant nature of the whole finger structure, give excellent grasping performance. 

The finger underactuated, meaning it has more joints than actuators - and it "goes where it needs to go" in response to the shape of the object it's gripping. It moves by means of tendons  (7 x 19 stainless steel cable with nylon coat) along its palm side.  The finger itself is not powered - the motors and capstans which pull the tendons are located off-hand in the forearm.
This project explores the compliance approach, and starts a lineage with flexible urethane rubber bender joints like the prototype shown at right.  These rubber joints can bounce back from twisting, end-on and side impacts, and can also "collapse" in severe back-bend events. 
at a glance...

About Human Size
3 Degrees of Freedom
1 or 2 Actuators
Reasonable Grasp Strength
Excellent Abuse Tolerance
Excellent Longevity
Reasonable Torsional Rigidity
Wiring Tunnels to Each Segment
Lightweight - 35 grams
Flexible Joint Finger  2008
FlexiBone Robot Finger 2008

Research and Development by Carl Pisaturo
in association with MEKA Robotics

Aprox. Human Size: 5" long x .75" high x .6" wide
Urethane Rubber, Stainless Steel , Delrin, Teflon Tubing, Teflon knuckle disks.
Flexible Joint Robot Finger 2006-7

Research and Development by Carl Pisaturo
in association with Jeff Weber, MEKA Robotics

Aprox. Human Size: 5" long x .75" high x .6" wide
Urethane Rubber, Stainless Steel Cable, Epoxy, Delin rollers, Teflon Tubing.
In contrast to the flexi-joint finger above, the flexi-bone design (prototype shown at left) uses a more traditional pivot design, and puts the flexibility in the "bone" portions of the finger. 

The finger is underactuated - that is, a single tendon operates all 3 of the joints at once, allowing it to conform to the posture required of the gripped object. 

A hard urethane rubber engages the steel arms and gives the bulk of the "bone" excellent strength, and a bonded soft urethane on the palm side gives good grip on a wide variety of objects.

When the tendon relaxes, an elastomer band on the dorsal side opens the finger.

The finished fingers have excellent robustness, being able to bounce back after severe pulling, twisting and side loading.
Three of these fingers plus a thumb variant are incorporated into the MEKA Robotics H2 compliant hand shown at left.
works of  Carl C Pisaturo