Design of a compact flat fabric pneumatic artificial muscle for soft wearable robotic devices

Abstract

Pneumatic Artificial Muscle (PAM) is a soft actuator driven by injecting air into a flexible, inflatable membrane. Many researchers have attempted to apply PAM to wearable robots or rehabilitation devices because of their high force-to-weight ratio and their compliance and hazard-free characteristics, allowing safe physical Human-Robot Interaction (pHRI). However, previous PAMs have shown limitations in practical wearable applications due to their cylinder structure and the main material rubber’s viscoelastic property. The cylindrical shape, which occupies a large volume of whether the air is injected or not, led to low wearability, and the viscoelastic property produced a slow response and a significant hysteresis error. This study proposes a flat fabric pneumatic artificial muscle (ffPAM), achieving high compactness, rapid response, and low hysteresis error. The flat structure using the commercially available rip-stop nylon fabric and 3D printed end fittings allows a cost-effective and straightforward manufacturing process. The aspect ratio of the ffPAM determined through the design parameter study was 1 : 1.79. Using the thin fabric of 0.4g as a base material, the actuator’s total weight is only 34.8g. The ffPAM exerts a maximum force of 118 N at 172 kPa, and the average maximum contraction ratio is about 24 %. By reducing the internal friction between the fiber or braid and the rubber material, the ffPAM achieved a low hysteresis error of 6.2 % and is reduced by more than half compared to other flat-type PAMs. Based on dynamic response experiments, the ffPAM was verified to have a response time of 0.032 s for contraction, and the bandwidth of more than 4 Hz, regarding the contraction length with respect to input pressure. Moreover, a maximum contraction length was maintained during 10,000 cycles. To emphasize the compactness of the ffPAM in practical wearable applications, the actuator was placed between the legs, and its compactness was compared with that of previous PAMs. Additionally, the ffPAM has a fabric-based soft contractile sensor for the measurement of the contraction. Based on the flat surface of the ffPAM, a direct embedding of a soft sensor by simply stacking a planar soft sensor over the actuator’s flat surface was introduced. The capacitance of the embedded capacitive contractile sensor has a linear relationship with the contraction length at constant pressures. Position control of the ffPAM using the embedded contractile sensor exhibited a remarkably rapid rise time of 0.25 s under the contraction length of 10 mm.