Johnsen-Rahbek Capstan Clutch: A High Torque Electrostatic Clutch

TL;DR

This work introduces the Johnsen-Rahbek effect driven capstan clutch (JRCC), a high-torque, low-power electrostatic clutch for SWaP-constrained robotics. By integrating capstan-based exponential tension amplification with JR/Coulombic electroadhesion, the authors derive a unified model and validate it experimentally using two PBI-based dielectric bands wrapped around a steel shaft, achieving up to $7.1$ N·m of holding torque and a peak specific shear stress of $31.3$ N/cm$^2$ at $500$ V with $2.5$ mW/cm$^2$. The results demonstrate that wrap angle and band geometry dramatically boost performance relative to planar designs, and the approach provides a framework for higher-torque, low-power electrostatic clutches. This work broadens the applicability of electroadhesion in robotics and points to future directions in material choices and multi-directional clutch architectures.

Abstract

In many robotic systems, the holding state consumes power, limits operating time, and increases operating costs. Electrostatic clutches have the potential to improve robotic performance by generating holding torques with low power consumption. A key limitation of electrostatic clutches has been their low specific shear stresses which restrict generated holding torque, limiting many applications. Here we show how combining the Johnsen-Rahbek (JR) effect with the exponential tension scaling capstan effect can produce clutches with the highest specific shear stress in the literature. Our system generated 31.3 N/cm^2 sheer stress and a total holding torque of 7.1 Nm while consuming only 2.5 mW/cm^2 at 500 V. We demonstrate a theoretical model of an electrostatic adhesive capstan clutch and demonstrate how large angle (theta > 2pi) designs increase efficiency over planar or small angle (theta < pi) clutch designs. We also report the first unfilled polymeric material, polybenzimidazole (PBI), to exhibit the JR-effect.

Figures (8)

• Figure 1: Comparison of JRCC against other reported electrostatic clutches (= planar and = curved). The highlighted region is the maximum observed for various wrap angles of a JRCC with a 25.4 µm band. The star point is for the JRCC with a 76.2 µm band demonstrating the highest recorded specific shear stress. The dashed line is a comparison to an equivalent planar clutch.
• Figure 2: a) The electrostatic capstan effect adds an electrically driven force component to the Normal forces in a standard capstan drive. Unlike the holding forces of a standard capstan, the electroactive force is a function of area. b) Using a PBI material, both JR and Coulombic electroadhesion is present.
• Figure 3: Design for a multi-wrap JR-effect driven capstan clutch (JRCC). The design consists of a stainless steel band wrapped around a PBI dielectric on a 25.4 mm diameter stainless steel shaft. This design can generate up to 7.1 N·m of holding torque.
• Figure 4: Data was fit to the derived JR-capstan equation assuming constant COF of $\mu$=0.20 with varying gap distance. Gap distance for 0.5, 1, 1.5, 2, 2.5, and 3 wraps were fitted to be 2.3 µm, 2.3 µm, 2.9 µm, 2.9 µm, 3.6 µm, and 4.1 µm, respectively, and 1.9 µm for 2.25 wraps polished band.
• Figure 5: Effect of pre-tension on holding torque. Theoretical fit to model assuming equal gap of 4.1 µm and only varying input holding force $T_{hold}$. Pretension increases the holding torque but will also increase rolling friction (0 V operating point).