MODULE M2 · 10.0 HOURS

Tendon-Driven Mechanism Design

0% complete

Prototype safety boundary

Low-voltage educational prototype, not a certified machine safety system
Do not use stop circuits created by learners as safety functions or emergency stops
Perform normal stops by software torque release; after planned stops, physically disconnect the 3 insulated power adapters and verify the de-energized state by measurement before maintenance or access
Do not place hands in the operating range while power is applied; test from no-load in a fixture
Wear impact-resistant work goggles during cutting, drilling, and tendon tension testing
In case of abnormal heat, odor, or smoke, do not approach; cut the power supply to the 3 adapters via a pre-designated building panel breaker outside the hazard zone or a certified upstream master disconnect and evacuate. If no upstream disconnect is available outside the hazard zone, do not energize the system. Torque release is not a substitute for power disconnection
Before introducing to environments with human access, a dual-channel, manual reset, EDM, feedback-capable dual contactor must be separately designed and verified under the review of a qualified safety expert

LEARNING OBJECTIVES

Module objectives

Understand the basic mechanical structure of tendon-driven mechanisms and the principles of joint simulation.
Learn the characteristics of tendon material (Dyneema SK78) for sophisticated robotic hands.
Learn tendon tension transmission paths and methods to prevent friction and wear in capstan design.
Calculate the stall torque of actuators and mechanical gain during tendon driving.

Fundamentals of Tendon-Driven Mechanisms

A tendon-driven system is a method of driving by transmitting tension from remotely located actuators to joints via tendons (cables) [S9]. By mimicking the tendon structure of biological fingers and moving actuators to the palm or forearm, the mass of the fingers themselves is reduced, enabling sophisticated movements [S10].

1. Selection and Tension Transmission of Tendons

Dyneema SK78, a high-strength, low-elongation fiber, is used in this design [S16]. This material has a breaking load of 230 daN (approx. 230 kgf) at a diameter of 1.5 mm, with an operating elongation of less than 1%, making it suitable for precision position control [S16].

2. Mechanical Gain and Actuator Selection

The XM430-W350-T smart actuator provides a stall torque of 4.1 N·m [S11]. Since the tendon transforms force through the capstan radius from the axis of rotation, the actuator’s torque output is replaced by tendon tension. The entire system uses 11 actuators, and the peak current total can reach approximately 25.3 A [S11]. Comparing fuse and load/power ratings alone does not guarantee safety or operating sequences. Review the fuse manufacturer’s time-current curves and power supply OCP characteristics together to verify protection coordination [S15, S24, S25].

3. Safety and Protection Design

Each 12 V power branch is operated through an independent fuse [S15, S24]. The 3 power adapters are each rated at 11.5 A, with a combined current capacity reaching 34.5 A, which sufficiently accommodates the system peak current of 25.3 A [S11, S15]. Design such that the sum of branch ratings exceeds the total actuator peak current to ensure operational safety.

WORKED EXAMPLES

Worked examples

Example 1: Calculating tendon tension during driving When actuator torque (τ) is 1 N·m and capstan radius (r) is 0.01 m, tendon tension (T) is T = τ/r = 1/0.01 = 100 N. Design considering the safety factor against Dyneema SK78's breaking load of 230 daN (approx. 2300 N) [S16].
Example 2: Power branch distribution and protection The sum of stall currents for all 11 actuators is 25.3 A [S11]. If distributed into 3 branches with 4, 4, and 3 units, the maximum load for each branch is 9.2 A, 9.2 A, and 6.9 A respectively. Comparing fuse and load/power ratings alone does not guarantee safety or operating sequences. Review the fuse manufacturer's time-current curves and power supply OCP characteristics together to verify protection coordination. Review the fuse manufacturer's time-current curves and adapter OCP characteristics together to verify protection coordination [S24, S25].

LAB PROTOCOL

Tendon Tension and Joint Friction Measurement Practice

1
Assemble the finger joint model using the provided links and bearings.
2
Connect the tendons and set initial tension using the tensioner.
3
Set the multimeter to DC voltage mode and physically verify the 12 V power adapter output for each branch.
4
Manually measure and record the rotational friction of the joint before energizing.

Safety check

Before maintenance/access, physically disconnect the 3 insulated power adapters and verify DC voltage of less than 1 V using a multimeter.
Never approach the operating range of the fingers while power is applied.
Always wear impact-resistant work goggles.

Lab deliverables

Tendon tension measurement data according to joint rotation angle
Friction analysis report
Final safety measurement records

ASSIGNMENT

5-Finger Robotic Hand Tendon Path Design

Deliverables

CAD drawings of robotic finger tendon pathsTendon friction and loss calculation sheetBranch-specific power load distribution and fuse protection design

Rubric

Is the tendon path designed to minimize friction at bends?
Are the physical characteristics of Dyneema SK78 considered?
Does the load distribution of the 3 power branches appropriately reflect actuator stall current?
Do the fuse and power short-circuit protection designs comply with BOM specifications?

KNOWLEDGE CHECK

Knowledge check

1What is the main advantage of using Dyneema SK78 tendons?Shock absorption due to high elongationVery low operating elongation and high breaking loadLighter weight and lower tensile strength than metalElectrical conductivity

2What is the appropriate reason for using 3 power adapters (11.5 A each) with 12 V?To drive all actuators with a single power sourceTo boost voltage to 36 V to increase torqueTo distribute and accommodate total peak current of actuators and protect with individual branch fusesTo eliminate power noise

FIELD CHECK

Completion criteria

All lab data and drawings must be included in the final report.
Must prove via measurement that the DC voltage of 3 branches is less than 1 V after physical power disconnection.
Tendon path design must include analysis considering capstan friction.

MODULE SOURCES

Module sources

Ruka-v2: Tendon Driven Open-Source Dexterous Hand with Wristarxiv.org · paper
US11325264B1 - Tendon-driven robotic handpatents.google.com · patent
D-Pro 1.5 mmliros.com · technical_documentation
XM430-W350 e-Manualemanual.robotis.com · datasheet
GST160A12-R7B Enclosed Desktop Power Adaptermeanwell.com · datasheet
ATO AFH Series In-Line Fuse Holderlittelfuse.com · datasheet
ATOF 287 Series 10 A Blade Fuselittelfuse.com · datasheet