An underactuated tendon hand is a robotic gripper that uses fewer motors than the number of joints it controls. This matters because it can grasp many different objects without needing a separate motor and sensor for every knuckle. A single motor can pull a tendon cable through several joints, letting the fingers wrap around curved, uneven, or fragile shapes.
This design is common in lightweight prosthetic hands, soft robotics, and warehouse grippers.
The key idea is mechanical intelligence: the hand’s structure helps solve part of the control problem. When a tendon is pulled, joints begin to rotate, but the joints that touch the object stop first while the remaining joints continue moving. Springs, elastic bands, or joint compliance then help the finger return to its open position when tension is released.
By tuning pulley radii, tendon paths, spring stiffness, and joint friction, engineers control how grip force and finger shape are shared across the hand.
Key Facts
- Underactuated means the robot has fewer actuators than degrees of freedom.
- A tendon transmits pulling force only, so a return spring or elastic element is often needed to reopen the finger.
- Motor torque and tendon tension are related by τmotor = T r, where r is the motor spool radius.
- Joint torque from a tendon is approximately τjoint = T R, where R is the tendon moment arm around the joint.
- If a tendon pulls through several joints, the total tendon displacement is approximately ΔL = R1θ1 + R2θ2 + R3θ3.
- Contact with an object redistributes motion: blocked joints stop rotating while free joints keep curling until the grasp stabilizes.
Vocabulary
- Underactuation
- Underactuation is a design condition where a mechanism has fewer motors or actuators than independently movable joints.
- Tendon drive
- A tendon drive is a cable or flexible line that transmits pulling force from a motor to one or more joints.
- Degree of freedom
- A degree of freedom is one independent way a system can move, such as rotation at a finger joint.
- Compliance
- Compliance is the ability of a mechanical part to bend, stretch, or yield under force instead of staying perfectly rigid.
- Moment arm
- A moment arm is the perpendicular distance from a force line to a rotation axis, which determines how much torque the force creates.
Common Mistakes to Avoid
- Assuming one motor means one joint moves only is wrong because a tendon can route across several joints and create torque at each one.
- Ignoring tendon moment arm is wrong because the same cable tension can produce different joint torques if pulley radii or routing distances are different.
- Treating tendon length change as equal to one joint’s arc motion is wrong because a multi-joint tendon displacement is the sum of contributions from all joints it crosses.
- Forgetting the return mechanism is wrong because tendons usually pull but do not push, so springs, elastic materials, or another tendon must open the finger.
Practice Questions
- 1 A motor spool has radius 0.012 m and produces torque 0.30 N m. What tendon tension does it create if friction is ignored?
- 2 A tendon with tension 25 N passes around a finger joint with a moment arm of 0.008 m. What torque acts on the joint?
- 3 A three-joint tendon finger touches an object at the fingertip first, then the middle link, then the base link. Explain how underactuation helps the finger conform to the object without a separate motor at each joint.