Part

Tendon drive

You might say: cable drive, wire drive, tendon actuation, cable-actuated arm, pull-pull drive, Bowden cable

A tendon drive sends tension members from motors or winches, often mounted at the base, through guides to anchors along a link chain or compliant backbone. Pulling a tendon changes its length and creates a bending moment; because a tendon pulls rather than pushes, bidirectional bending normally uses an antagonistic pair or another return force.

What it does

It moves distal shape while keeping actuator mass away from the moving arm, at the cost of guide friction, elastic stretch, slack recovery and backlash.

Ask

Where is each tendon anchored, how is it guided, what pretension keeps it taut, and how will tension or slack be noticed?

Typical

One tendon actively bends a planar section in one direction; an antagonistic pair controls both planar directions; three tendons or two antagonistic pairs can provide spatial bending. A continuous backbone still uses a finite set of tendon and actuator channels; adding sections can require more motors.

Trouble

A slack cable gives dead travel and backlash; curved guides consume force, and elastic stretch makes commanded spool motion differ from backbone shape.

Catalogue: Tendon routing

• Spacer-disk guides: A slender flexible backbone carries a finite stack of disks; holes in each disk guide tendons through straight subsegments and the termination locations define sections.
• Continuous-lumen backbone: A larger backbone contains internal lumens that guide tendons continuously, removing separate spacer disks while making the continuous tendon path part of the backbone.
• Sheathed / Bowden routing: A flexible outer housing routes a cable from a remote winch to the moving mechanism; it can simplify packaging, but bends and liner friction consume force before the tendon reaches its anchor.

Connections

• See also: Electric motor
• See also: Gearbox
• See also: Motor driver
• See also: Robot dynamics
• See also: Virtual stiffness and damping

Sources

1. Jessica Burgner-Kahrs, Tendon-driven Continuum Robots, Tendon-driven Continuum Robots: introductionIn so called tendon-driven continuum robots (TDCR) several tendons are routed alongside the robot’s flexible backbone and fixed at different locations.
2. Continuum Robotics Laboratory, Tendon Driven Continuum Robots, Tendon Driven Continuum Robots: general structureTendon actuation is an extrinsic actuation principle, as no actuators are located within the robot’s structure.
3. Jessica Burgner-Kahrs, Tendon-driven Continuum Robots, Tendon-driven Continuum Robots: motion capabilitiesSince tendons cannot be pushed, using one tendon results in a bending degree of freedom which can only be controller actively in one direction. A pair of antagonistic tendons, i.e., located on opposite sides of the backbone, is required for control of both directions for in-plane bending.
4. Jessica Burgner-Kahrs, Tendon-driven Continuum Robots, Tendon-driven Continuum Robots: remarksAnother property of tendons is that they can only support tension and tend to get slack in the presence of compression. When additional forces are applied, a portion of these forces are used to recover from the state of slack. This recovery leads to what is known as actuator backlash
5. Jessica Burgner-Kahrs, Tendon-driven Continuum Robots, Tendon-driven Continuum Robots: remarksHowever, any increase of the number of segments requires an additional set of motors to actuate it, resulting in a bulky actuator system.
6. Ytec 3D, Animatronic Tentacle, How it worksEach section has 4 cables. When 1 cable contracts, and the opposite cable expands, the section bends evenly. In the base are 6 winches that contract and expand each set of cables.
7. Ytec 3D, Animatronic Tentacle, Improvements left: cable guide tubesI have found that every degree you bend takes force away from the mechanism. This problem worsens with the square of the angle.

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