A cable-driven joint, often called a tendon drive, uses flexible cables to transmit force from motors to a robotic joint. This design is inspired by how muscles pull on tendons in human fingers and arms. It matters because the motors can be placed away from the moving joint, which can make the joint lighter, faster, and easier to fit into small robotic hands.
Tendon drives are common in dexterous grippers, prosthetic hands, surgical robots, and lightweight robot arms.
In a typical tendon-driven joint, cables run over pulleys or through curved guides and attach to a moving link. Pulling one cable creates torque that bends the joint, while an opposing cable or spring pulls it back. Antagonistic cable pairs allow controlled motion in both directions and can also adjust stiffness by changing cable tension.
The main engineering challenges are cable stretch, friction, backlash, tension control, and keeping cables routed so they do not slip off pulleys.
Understanding Robotics: Cable-Driven Joint (Tendon Drive)
A tendon drive is really a force path. The motor turns a spool, the spool changes cable length, and the cable pulls a link around its pivot. The position of the cable relative to the pivot matters greatly.
A larger pulley at the joint gives the cable more leverage, so a given pull can resist a larger load. The tradeoff is that the motor must pull more cable to make the same bend.
A smaller pulley makes motion quicker for a given cable movement, but provides less leverage. Designers choose this size by considering whether the robot needs strength, speed, or a balance of both.
Cables are not perfectly rigid parts. Under load, they lengthen slightly, then recover when the load falls. Some materials slowly change length after many repeated pulls.
This is called creep. A controller that assumes the cable never changes can place a finger or tool in the wrong position. Friction creates another important effect.
Cable tension is often different at the motor end and at the joint end, especially when a cable passes around several bends. When motion reverses, the cable may stick briefly before sliding. This produces hysteresis, meaning the same motor position can give a slightly different joint position depending on which direction it approached from.
Good tendon systems use feedback rather than trusting motor rotation alone. An encoder can measure motor movement. A second sensor near the joint can measure the actual bend angle.
Tension sensors can detect whether a cable is slack, overloaded, or broken. The control program compares the desired movement with these measurements and adjusts the motor pull. Pretension is useful because it removes slack before motion begins.
Too little pretension causes a loose, delayed response. Too much pretension wastes energy, increases wear, and can overload small parts.
In joints with two opposing tendons, changing both tensions together can make the joint feel firmer without changing its angle much. This is useful when a robot must hold an object gently yet resist an unexpected push.
Students can notice similar ideas in bicycle brake cables, puppet strings, fishing reels, and the cables inside some exercise machines. In each case, bends, slack, stretching, and friction change how force reaches the final part. A prosthetic hand shows why these details matter.
The hand must close reliably around objects of different shapes while staying light enough for comfortable use. Surgical tools need precise motion through narrow paths, where even small friction errors matter. When studying tendon drives, track three quantities separately.
These are cable length, cable tension, and joint angle. They influence one another, but they are not the same measurement. Keeping them separate makes it easier to understand why a robot can have a motor that turns correctly while the joint still moves inaccurately.
Key Facts
- Joint torque from a cable is τ = rF, where r is pulley radius and F is cable tension.
- Cable speed and joint angular speed are related by v = rω for a cable wrapped on a pulley.
- Antagonistic tendon pairs use one cable to flex the joint and another cable to extend it.
- Moving motors away from the joint lowers joint inertia, which can improve acceleration and safety.
- Cable stretch can be estimated by ΔL = FL/(AE), where A is cross-sectional area and E is Young's modulus.
- Friction in pulleys and guides reduces transmitted tension, so actual joint torque may be less than motor output suggests.
Vocabulary
- Tendon drive
- A transmission system that uses cables or tendons to pull on a robotic joint from a motor located elsewhere.
- Antagonistic pair
- Two tendons arranged so that one pulls the joint in one direction while the other pulls it in the opposite direction.
- Joint inertia
- A measure of how strongly a joint resists changes in rotational motion due to the mass distribution of its moving parts.
- Pulley radius
- The distance from the center of a pulley to the cable contact path, which sets the moment arm for tendon force.
- Backlash
- Unwanted looseness or delay in a mechanism that causes motion at the motor not to immediately produce motion at the joint.
Common Mistakes to Avoid
- Ignoring the pulley radius, because cable tension alone does not determine joint torque. The torque depends on both force and moment arm using τ = rF.
- Assuming cables can push, because tendons only transmit tension. A return spring, gravity, or an antagonistic cable is needed for motion in the opposite direction.
- Placing heavy motors on the joint in a design meant to reduce inertia, because the main benefit of tendon drives is often moving mass away from the moving link.
- Forgetting cable stretch and slack, because real cables deform under load. Stretch can reduce position accuracy and slack can cause delayed or uneven motion.
Practice Questions
- 1 A tendon pulls on a pulley with radius 0.020 m using a tension of 75 N. What torque does it apply to the joint?
- 2 A motor reels in a cable at 0.12 m/s around a joint pulley of radius 0.030 m. What is the joint angular speed in rad/s?
- 3 A robotic finger uses two antagonistic tendons instead of one tendon and a passive spring. Explain one advantage of the antagonistic design for controlling motion or stiffness.