  Citation: Low,K.; Berg, D.R.; Li, P.Y. A Novel 3D Ring-Based Flapper Valve for Soft Robotic Applications. Robotics 2022, 11, 2. https://doi.org/ 10.3390/robotics11010002 Academic Editor: Guangjun Liu Received: 30 November 2021 Accepted: 18 December 2021 Published: 22 December 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). robotics Article A Novel 3D Ring-Based Flapper Valve for Soft Robotic Applications Kelly Low 1 , Devin R. Berg 2, * and Perry Y. Li 1 1 Department of Mechanical Engineering, University of Minnesota, Minneapolis, MN 55455, USA; low00004@umn.edu (K.L.); lixxx099@umn.edu (P.Y.L.) 2 Engineering and Technology Department, University of Wisconsin-Stout, Menomonie, WI 54751, USA * Correspondence: bergdev@uwstout.edu Abstract: In this paper, the design and testing of a novel valve for the intuitive spatial control of soft or continuum manipulators are presented. The design of the valve is based on the style of a hydraulic flapper valve, but with simultaneous control of three pressure feed points, which can be used to drive three antagonistically arranged hydraulic actuators for positioning soft robots. The variable control orifices are arranged in a rotationally symmetric radial pattern to allow for an inline mounting configuration of the valve within the body of a manipulator. Positioning the valve ring at various 3D configurations results in different pressurizations of the actuators and corresponding spatial configurations of the manipulator. The design of the valve is suitable for miniaturization and use in applications with size constraints such as small soft manipulators and surgical robotics. Experimental validation showed that the performance of the valve can be reasonably modeled and can effectively drive an antagonistic arrangement of three actuators for soft manipulator control. Keywords: hydraulic valve; soft robotics; flapper valve; actuation; control 1. Introduction The use of soft robotic manipulators has been explored for many applications where the inherently compliant nature of the device provides improved functionality. Some par- ticularly interesting examples include graspers and minimally invasive surgical tools [1,2], where a common theme emerges in the need for the delicate handling of breakable objects or human tissues. When it comes to the control of soft manipulators, there are a wide variety of techniques, with electroactive polymers or pneumatic power being common and, to a lesser extent, emerging devices based on hydraulic power [3–7]. The use of hydraulic power enables robotic devices with a higher power-to-weight ratio and thus a greater payload capacity. However, there exists a gap between conventional fluidics at the meso-scale (mm to cm) and the techniques of traditional microfluidics at the micro-scale (μm to mm) that includes the type of high pressure–low flow rate fluid power components that would be necessary for the application of hydraulic power to use in soft robotics [8–10]. This is particularly true in surgical robotics where anatomical size constraints can restrict devices to the millimeter scale [11–13]. This paper describes the design of a flow control valve targeted at miniature surgical applications and specifically a natural orifice surgical robot described in Berg, 2013 [11]. With this potential valve application in mind, the scale of the present device is such that the overall diameter is less than 25 mm and the valve orifices range from less than 1 mm to 1.5 mm. The ability to control the flow of the fluid is essential to any hydraulic system. Typically, this is achieved through the use of valves which can control whether the flow is ON or OFF and possibly the rate at which the fluid is flowing. The method by which flow is stopped can include rotational motion such as in a rotary valve, linear motion such as in spool or poppet valves, or with a flapper-nozzle valve that acts parallel to the flow direction. Additionally, proportional control can be achieved through varying the size of the valve opening or by control of an ON/OFF valve. Either method carries advantages and Robotics 2022, 11, 2. https://doi.org/10.3390/robotics11010002 https://www.mdpi.com/journal/robotics