Programmable soft valves for digital and analog control

Abstract

In soft devices, complex actuation sequences and precise force control typically require hard electronic valves and microcontrollers. Existing designs for entirely soft pneumatic control systems are capable of either digital or analog operation, but not both, and are limited by speed of actuation, range of pressure, time required for fabrication, or loss of power through pull-down resistors. Using the nonlinear mechanics intrinsic to structures composed of soft materials-in this case, by leveraging membrane inversion and tube kinking-two modular soft components are developed: a piston actuator and a bistable pneumatic switch. These two components combine to create valves capable of analog pressure regulation, simplified digital logic, controlled oscillation, nonvolatile memory storage, linear actuation, and interfacing with human users in both digital and analog formats. Three demonstrations showcase the capabilities of systems constructed from these valves: 1) a wearable glove capable of analog control of a soft artificial robotic hand based on input from a human user's fingers, 2) a human-controlled cushion matrix designed for use in medical care, and 3) an untethered robot which travels a distance dynamically programmed at the time of operation to retrieve an object. This work illustrates pathways for complementary digital and analog control of soft robots using a unified valve design.

Keywords: analog control; digital logic; nonlinear mechanics; programmable devices; untethered soft robotics.

Fig. 1.

Soft piston, switch, and valve. (A) Schematics and photos of the soft piston actuator. (B) Schematics and photos of a mechanically actuated pneumatic switch. (C) Piston actuator combined with the mechanical switch and an external elastic band, forming a pneumatic valve.

Fig. 2.

Tunable valve. (A) Micro-computed tomography (micro-CT) image of the bistable valve configured as a pneumatic switch. (B) Force-distance curves of different device components. In this case, the switch uses elastics with k = 110.7 ${\text{N} \text{m}}^{-1}$ and a piston pressurized to 80 kPa. (C) Snap-through and snap-back pressures of the pneumatic valve with different elastic stiffness constants (k, in units of ${\text{N} \text{m}}^{-1}$). Rapid changes in output pressure as a function of input pressure demonstrate the bistability of the valve. (D) We measure snap-through and snap-back pressures while varying elastic band stiffness. Experimentally measured valve snap-through and snap-back pressures for the valve (solid black diamonds) compared with predictions from the semiempirical model (red and blue curves) show good agreement.

Fig. 3.

Digital control using pneumatic logic gates. The bistable valve is arranged with different input connections to achieve logic gate behaviors. (A) A NOT gate is made by attaching supply pressure (Boolean 1) to the initially unkinked tube and atmospheric pressure (Boolean 0) to the initially kinked tube. (B) An AND gate is created by setting the piston chamber and initially kinked tube as inputs and connecting the initially unkinked tube to atmospheric pressure. (C) An OR gate is made by setting the piston chamber and initially unkinked tube as inputs while connecting the initially kinked tube to supply pressure. (D) SR latch schematic. The output is undefined when both inputs are 1. When both inputs are 0, the SR latch outputs the previous state, creating memory. (E) INHIB gate schematic. (F) Three-input XOR* gate schematic, along with the equivalent logic circuit. All gates were tested with a pneumatic 1 between 60 and 75 kPa.

Fig. 4.

Analog control using a soft pressure regulator. (A) Diagrams of the pressure regulator. If output pressure is too high, the valve restricts airflow. If output pressure is too low, the valve allows flow. (B) Regulator output pressure depending on input force. (C) Top view of the five-finger control glove and soft robotic hand with one soft actuator for each finger. The system is powered with a single constant pressure input. (D) Pressure regulator used as an analog human input with an analog control glove. Output pressure (which controls pneumatic devices, such as the soft robotic hand) varies as the string is pulled or released. (E) Holding the robotic hand between fully actuated and unactuated states. (F) Single actuator pressure measurements during device operation. (G) Angle of the analog finger actuator on the soft robotic hand tuned to correspond to the user’s finger angle.

Fig. 5.

Ring oscillator. (A) Schematic of the assembled ring oscillator logic circuit with a soft pressure regulator controlling input pressure. (B) Diagram of the regulator and oscillator circuit showing valve connections. (C) Pressure output from each gate with a constant pressure supply of 78 kPa. Maximum pressure output is 76 kPa, meaning only a 3% reduction in pressure occurs. (D) Period of oscillation at different supply pressures. (E) Schematic of the counter circuit. (F) Number of cycles before switching states at different capacitor volumes and regulator input pressures. (G) Diagram of untethered robot. (H) Robot retrieving an object at two different distances based on counter settings programmed dynamically at the time of deployment. The internal volume of the robot is 80 cm³, yielding 11 and 7 stepping cycles for the 39- and 24-kPa deployments, respectively, based on the cycle programming data shown in F.

Fig. 6.

Multifunctional pneumatic cushion. (A) Logical circuit used with the cushion matrix. Five switches, each with individual functions—left (L), right (R), feet (F), head (H), and oscillate (O)—are labeled. (B and C) Visualization of the rolling and lifting functions of the matrix, with schematics, pictures, and experimental pressure measurements. (D) Experimental pressure measurements of the cushion while the oscillation function is activated. The prototype was operated with an input pressure of 70 kPa. The cushion is restrained with a transparent acrylic sheet to simulate the weight of a user. Two-dimensional pressure maps are shown (with orange-red representing high-pressure outputs and blue-black representing low-pressure outputs). (E) Direct analog control of the pneumatic cushion. Red boxes show pouches that are being inflated.