PneuAct-II: Hybrid Manufactured Electromagnetically Stealth Pneumatic Stepper Actuator

Additive manufacturing (AM) is one of the emerging production methodologies transforming the industrial landscape. However, application of the technology in fluidic power transmission and actuation is still limited. AM pneumatic stepper motors have been previously introduced to the field of image-guided surgical robotics, where their disposability and customizability are considered a significant advantage over conventional manufacturing. However, intrinsic dimensional limitations of AM parts and their poor surface quality affect mechanical performance. In this letter, a novel design, PneuAct-II, is presented combining AM, subtractive machining, and off-the-shelf components to achieve higher mechanical performance and resolution. Moreover, a motor identification setup has been built to automatically measure different aspects of the PneuAct motors, including wear, friction, leakage, and stall behavior at various boundary conditions. The effects of input pressure, stepping frequency, signal-width, and external torque on the stall behavior of motors with different clearances are studied. A maximum torque of 0.39 N$\cdot$m at an input pressure of 6.5 bar is achieved for a motor with a total volume of 90 cm$^3$, and a clearance of 156 $\mu$m. A nominal resolution of 2.25$^{\circ }$ at full-pitch and 1.125$^{\circ }$ at half-pitch is accomplished. Both resolution and mechanical performance (667 N$\cdot$m/bar $\cdot$ m$^3$) outperform the state-of-the-art (240 N$\cdot$m/bar $\cdot$ m$^3$ by PneuAct-I).