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.
Understanding Robotics: Underactuated Tendon Hand
A tendon hand works because the cable creates a linked motion rather than a set of independent motions. Pulling the cable shortens the path available to every joint on that finger. At first, several joints may bend together.
Once one fingertip or finger segment meets an object, that part cannot move much farther. The cable tension then causes motion at joints that still have room to bend.
This is why a simple finger can conform to a cup, a tool handle, or a piece of fruit. The object itself becomes part of the mechanism because its shape decides where motion stops.
The geometry of the cable strongly affects the result. A cable wrapped farther from a joint pivot has more leverage, so the same pull creates more turning effect at that joint. It also uses more cable length for each degree of bending.
Engineers must balance strength against range of motion. The spool on the motor creates a similar tradeoff. A small spool gives a stronger cable pull for a given motor effort, but it must turn more times to close the hand.
A larger spool closes the hand faster, but provides less pulling force. These choices affect motor size, battery use, closing speed, and the maximum object weight the hand can hold.
Real tendon systems are not perfectly rigid or frictionless. Cables can stretch slightly, especially if they are long or made from soft material. They can rub against guides and pulleys, which means some motor effort is lost as friction.
A hand may close differently while gripping than while opening because friction resists motion in both directions. This effect is called hysteresis. Springs can help reopen a finger, but a spring that is too stiff wastes motor energy during closing.
A spring that is too weak may leave the finger partly closed. Good designs use smooth cable routes, low-friction guides, and enough compliance to avoid crushing delicate objects.
Students can see the same ideas in bicycle brake cables, puppet strings, fishing reels, and the tendons in their own fingers. Human hands have many muscles and tendons, yet even people rely on passive effects from joint shape, skin contact, and tissue stiffness. When studying robotic hands, track three things carefully.
Follow where the cable runs. Notice which joint touches the object first. Then consider where the remaining cable motion goes after contact.
This sequence explains why a gripper can hold varied shapes without calculating the exact angle of every joint. It also shows the limit of the design. The hand adapts well, but it cannot place each finger joint at an arbitrary position with the precision of a fully motorized 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.