Single-Cable Actuation Overview

• Single-cable actuation is a method that uses one cable to drive one or more degrees of freedom via direct cable-pulley, mechanical multiplexing, or continuum routing techniques.
• It reduces complexity, weight, and friction by eliminating multiple actuators, achieving high torque transparency and energy recuperation in dynamic robotic applications.
• Applications span legged robots, robotic hands, continuum manipulators, and deployable structures, while challenges include high-voltage requirements, engagement delays, and precise tensioning.

Single-cable actuation refers to the class of mechanical architectures, analytical models, and control strategies in which a single mechanical cable or transmission input actuates one or multiple output degrees of freedom (DoFs). The term encompasses technologies ranging from cable-pulley transmissions with high torque transparency in dynamical robots, to mechanical multiplexing using switching or clutching devices, to energy-based modeling in continuum robots and large-scale flexible-structure control with a single cable input. Key applications include legged robotics, hand/gripper actuation, shape control in flexible structures, and continuum manipulator modeling.

1. Fundamental Principles of Single-Cable Transmission

Single-cable actuation can exploit direct cable-pulley transmissions, cable-based mechanical multiplexing, or distributed cable routing for the actuation of lightweight, highly dynamic systems. The defining mechanical principle is that transmission of force or motion is mediated by a single cable that interfaces—sometimes simultaneously—with one or multiple outputs. Representative implementations include:

• Single-stage cable–pulley drives: A synthetic-fiber cable is helically wrapped in an antagonistic pair around drive and load pulleys, providing a direct, backlash-free torque path with minimal friction (efficiency η ≳ 0.99 in static tests) and high torque transparency, as demonstrated in highly dynamic robotic limbs (Hwangbo et al., 2018).
• Mechanical multiplexers: A single powered shaft distributes motion via clutching elements (e.g., electrostatic capstan clutches), enabling any subset of outputs to be selected and actuated in either SISO or SIMO modes, with independent or simultaneous control (Amish et al., 14 Jan 2025).
• Continuum robot cable routing: A single cable is routed at an offset along a continuum backbone, producing actuation via curvature control, both for force- and displacement-driven strategies (Wu et al., 4 Sep 2025).
• Large flexible structures: Shape control is realized by tensioning a single actuation cable attached to a deformable beam or boom, as in the CABLESSail solar sail concept (Lee et al., 23 Jan 2025).

Essential to all methods is elimination or drastic reduction in the number of independent actuators, reducing mass, friction, inertia, and power requirements, and simplifying the architecture.

2. Cable-Based Mechanical Multiplexing

Recent advances in mechanical multiplexing leverage single-cable architectures for multi-DoF robotic actuation with minimal hardware overhead. Notably, electrostatic capstan clutch-based multiplexers enable a single shaft, powered by one motor, to control numerous parallel outputs (Amish et al., 14 Jan 2025).

• System architecture: A central shaft (Ø 25.4 mm), rotated bidirectionally via a DC motor and idler gear train, is lined with N multiplexer units. Each unit controls an output DoF via a pair of electrostatic clutches (for CW/CCW coupling), a gear, a leadscrew, and a slider carrying a tendon.
• Operating principle: High voltage (∼900 V) is applied to selected clutch pairs, engaging frictional coupling via the Johnsen–Rahbek effect and the capstan law; output torque is distributed based on friction limits.
• Modes of operation:
  • SISO (time-division): Only one clutch pair is engaged at a time; the motor sequentially drives each DoF.
  • SIMO (simultaneous): Any combination of units can be engaged concurrently, distributing torque proportionally and enabling coordinated grasping or holding.
• Performance metrics: Maximum output force per unit is 22.24 N for stroke lengths up to 5 cm. Engagement times scale with load (∼98–424 ms). Transmission efficiency is high at low loads (η ≈ 88%) and degrades with increased force (η ≈ 13% at 22 N), commonly due to surface roughness. Zero-power hold is accomplished by self-locking leadscrews.
• Scalability: The number of outputs N scales with shaft length; each additional unit adds ∼50 g. Integration into a 4-DoF robotic hand demonstrated both sequential and simultaneous finger motion (2.27 kg grasp).

Primary limitations include the high-voltage requirement for clutch operation, switching speeds limited by electrostatic engagement times, and efficiency losses at high normal forces due to surface imperfections (Amish et al., 14 Jan 2025).

3. Analytical and Control Models of Single-Cable-Actuated Systems

The analytical treatment of single-cable actuation critically depends on the underlying system: rigid-body with cable-pulley transmission, continuum manipulator, or flexible structure.

• Cable-pulley torque transmission: The joint output torque is τⱼ = τₘ ⋅ rₚ, with negligible reflected inertia from the motor side (rₚ²Jₘ ≪ J_L), and nearly perfect energy transparency due to the absence of gear friction or backlash (Hwangbo et al., 2018).
• Continuum robots: The Lightweight Actuation Space Energy Modeling (LASEM) framework formulates the total system potential energy as

$$\mathcal{E}[u,\kappa] = \int_{0}^{L}\left(\frac{1}{2}EI\kappa^2 + \frac{1}{2}EAu^2\right)ds - F\int_{0}^{L}(u + e\kappa)ds.$$

The closed-form analytical model provides both kinematic ($\Delta l$) and static (F) maps, supporting fast forward/inverse computation and scalable discretized optimization (Wu et al., 4 Sep 2025).

• Passivity-based control for flexible booms: Control is formulated as a second-order ODE with passivity properties. A proportional-derivative (PD) controller with time-varying feedforward achieves robust shape regulation under uncertain dynamics, using

$$u(t) = u_{ff}(t) - k_p(\theta(t)-\theta_d(t)) - k_d(\dot\theta(t)-\dot\theta_d(t))$$

and

$$u_{ff}(t) = m\,\ddot\theta_d(t) + b\,\dot\theta_d(t) + k\,\theta_d(t),$$

ensuring convergence despite 20% parameter uncertainty in system properties (Lee et al., 23 Jan 2025).

Simulation and hardware validation confirm sub-millimeter-tracking error and sub-millisecond computational overhead in continuum robots, as well as robust, low-overshoot shape control for flexible structures.

4. System Performance and Experimental Results

Single-cable actuation architectures have demonstrated superior performance in multiple settings:

• Highly dynamic robotic limbs (Capler-Leg):
  • Transmission efficiency η ≳ 0.99 (static), energy recuperation rate 96.5% in repetitive hopping, peak output torque 52 Nm at 35 rad/s, and near-frictionless backdrivability (Hwangbo et al., 2018).
  • Cable-pulley doubles as a thermal path, enabling higher continuous power and improved motor cooling.
• Multiplexed robotic hand:
  • A single-motor, multi-output electrostatic clutch array enabled independent and simultaneous finger actuation, supporting both precise SISO manipulation and high-force, SIMO grasps (2.27 kg kettlebell), with position holding at near-zero power (Amish et al., 14 Jan 2025).
• Continuum manipulators: Single-cable LASEM achieves <0.3% forward kinematics error compared to full Cosserat-rod solvers; real-time control is feasible at kilohertz rates with low computational load (Wu et al., 4 Sep 2025).
• Flexible solar sail booms: Passivity-based single-cable control achieves <3% overshoot, ∼10–50 s settling, and stable tracking under uncertainty in both simulation and scaled-experiment regimes (Lee et al., 23 Jan 2025).

5. Applications, Scalability, and Limitations

Single-cable actuation is optimal for systems demanding lightweight, high-transparency, or low-power solutions, and where actuator count, inertia, or routing is constrained.

Major application domains:

• High-DoF manipulators (robotic hands, snake robots, continuum arms)
• Legged robots and high-performance robotic limbs
• Underactuated or variable-stiffness hands and exoskeletons
• Flexible/deployable structures (solar sails, trusses, masts)

Scalability and extensibility:

• Mechanical multiplexers: N simply scales with additional clutch-leadscrew units; efficiency primarily limited by transmission losses and available shaft length (Amish et al., 14 Jan 2025).
• Cable-pulley systems: No major loss of performance across cycles (verified over 0.5 M operations), but depend on precise assembly/tensioning (Hwangbo et al., 2018).
• Continuum models: Computational scaling is linear with DoF discretization, facilitating real-time optimization in segmented structures (Wu et al., 4 Sep 2025).

Limitations:

• Electrostatic clutch systems require high voltage (∼900 V) and have engagement delays, limiting fast actuation (Amish et al., 14 Jan 2025).
• Cable-pulley transmissions depend on the thermal properties and creep resilience of the chosen synthetic cable (e.g., DM20 softening at >80 °C).
• All cable-based systems require rigorous termination/tensioning strategies to avoid slippage, backlash, or creep over time.
• Single-cable continuum and flexible structure actuation is inherently limited to a single movement or bending direction, unless additional cables or antagonistic actuation are introduced (Wu et al., 4 Sep 2025); thus, spatial manipulability may be reduced compared to multi-cable systems.

6. Future Directions and Generalization

Recent research generalizes single-cable principles to additional domains:

• Integration into wearable haptic devices and exoskeletons seeking silent, low-power, and low-mass braking/holding mechanisms (Amish et al., 14 Jan 2025).
• Generalization of energy-based single-cable control, passivity-based feedback, and efficient numerical optimization to arbitrary flexible or continuum structures (Lee et al., 23 Jan 2025, Wu et al., 4 Sep 2025).
• Exploration of cable-multiplexed actuation in deployed aerospace structures and field robotics, with focus on autonomous diagnosis of cable health, thermal management, and fault-tolerant operation.

A plausible implication is that future robotic platforms requiring numerous lightweight or highly coordinated DoFs will increasingly employ multiplexed or energy-based single-cable schemes, especially where actuator density or distributed sensing imposes constraints that favor centralized actuation and energy recovery architectures.

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Key References:

• Electrostatic clutch-based multiplexed actuation in robotic hands (Amish et al., 14 Jan 2025)
• Capler single-stage cable–pulley transmission for dynamic robots (Hwangbo et al., 2018)
• Analytical actuation-space energy modeling for continuum robots (Wu et al., 4 Sep 2025)
• Passivity-based shape control for flexible cable-actuated booms (Lee et al., 23 Jan 2025)