3D printed digital pneumatic logic for the control of soft robotic actuators

Soft robots are paving their way to catch up with the application range of metal-based machines and to occupy fields which are challenging for traditional machines. Pneumatic actuators play an important role in this development, allowing the construction of bioinspired motion systems. Pneumatic logic gates provide a powerful alternative for controlling pressure-activated soft robots, which are often controlled by metallic valves and electric circuits. Many existing approaches for fully compliant pneumatic control logic suffer from high manual effort and low pressure tolerance. In our work, we invented 3D printable, pneumatic logic gates that perform Boolean operations and imitate electric circuits. Within 7 hours, an FDM printer is able to produce a module that serves as either an OR, AND or NOT gate; the logic function is defined by the assigned input signals. The gate contains two alternately acting pneumatic valves, whose work principle is based on the interaction of pressurized chambers and a 3D printed 1 mm tube inside. The gate design does not require any kind of support material for its hollow parts, which makes the modules ready to use directly after printing. Depending on the chosen material, the modules can operate on a pressure supply between 80 and over 750 kPa. The capabilities of the invented gates were verified by implementing an electronics-free drink dispenser based on a pneumatic ring oscillator and a 1-bit memory. Their high compliance is demonstrated by driving a car over a fully flexible, 3D printed robotic walker controlled by an integrated circuit.

Methods
The described pneumatic elements were fabricated on a 3D printing platform as detailed in Conrad et al. (https://doi.org/10.1007/978-3-030-64313-3_6). The flexible parts, like valves and logic gates, described in this work have been printed in one continuous process with “Recreus FilaFlex TPU A60” (shore hardness A 63), “Recreus FilaFlex TPU A70”  and “Recreus FilaFlex TPU A82” with fitting print parameters (please see supplements).

In all pneumatic experiments, the pressurization of chambers and channels was controlled by a custom test stand. This setup contained six pneumatic proportional valves driven by a LabView program. The software supports the creation of linear patterns and repeating sequences, which simplified the systematic characterization by applying rising and falling pressure ramps to the logic elements. For the characterization of a single valve, unused inlets were sealed and the gate output socket was connected to atmospheric pressure via a flow sensor to measure the volumetric stream. After changing the state on the input connectors, the software waited four seconds for the module to reach a new static internal flow and then recorded this value. To test the behavior of the complete logic gate, a pressure sensor was connected to the output. Here, the applied pressure was measured continuously to characterize the dynamic behavior. For deformation experiments we used a mechanical testing machine equipped for compression tests up to 10 kN.

The data was processed using RStudio 2023.03.1+446 "Cherry Blossom" Release (6e31ffc3ef2a1f81d377eeccab71ddc11cfbd29e, 2023-05-09) for windows. The same software was used to create figures.