Research ArticleSOFT ROBOTS

Electronics-free pneumatic circuits for controlling soft-legged robots

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Science Robotics  17 Feb 2021:
Vol. 6, Issue 51, eaay2627
DOI: 10.1126/scirobotics.aay2627
  • Fig. 1 Soft-legged untethered quadruped robot with a bioinspired gait pattern controlled with an electronics-free pneumatic actuation system.

    (A to D) African sideneck turtle exhibiting a diagonal couplet walking gait. (E) Image of the untethered quadruped robot with onboard soft valves powered by a pressure-regulated CO2 canister; key components are labeled, as are the directions of leg motions for forward walking. Pneumatic oscillators are used to control the motions of each diagonal leg pair for forward walking. Each leg of the robot was 173 mm long from base to foot in its neutral state. (F to I) Sequence of images from a video of the robot walking using only the pressurized CO2 canister as a source of energy, with two pneumatic oscillator circuits generating rhythmic leg actuation. (J) Pneumatic logic circuit for rhythmic leg motion. A constant positive pressure source (P+) applied to three inverter components causes a high-pressure state to propagate around the circuit, with a delay at each inverter. While the input to one inverter is high, the attached actuator (i.e., A1, A2, or A3) is inflated. This sequence of high-pressure states causes each pair of legs of the robot to rotate in a direction determined by the pneumatic connections. (K) By reversing the sequence of activation of the pneumatic oscillator circuit, the attached actuators inflate in a new sequence (A1, A3, and A2), causing (L) the legs of the robot to rotate in reverse. (M) Schematic bottom view of the robot with the directions of leg motions indicated for forward walking.

  • Fig. 2 Soft ring oscillator concept.

    (A) Each of the valves acts as an inverter by switching the normally closed half (top) to open and the normally open half (bottom) to closed. (B and C) The soft ring oscillator is designed to sequence inflation and deflation of different chambers on the robot (e.g., oscillator A: A1, A2, and A3). The soft ring oscillator actuates the chambers in sequence, which results in the limbs rotating in a circle. These schematics depict the moments immediately before (B) and after (C) the pressure in chamber A3 increases beyond the valve snap-through pressure (Pst = 35 kPa). In (B), both A2 and A3 are inflating. In (C), once the pressure in A3 > Pst, the exhaust for A2 opens and A2 begins deflating as depicted. (D) Representative plot of the pressure at the three nodes of the oscillator when actuated at 150 kPa. The moments depicted in (B) and (C) are labeled on this plot as inflating and deflating, respectively.

  • Fig. 3 Simple circuits for generating a diagonal couplet gait.

    (A) The dual-purpose three-valve ring oscillator circuit (labeled as circuit 1) controlled the pressure in 12 chambers with four chambers (two mirrored pairs) connected to each inverter for a phase offset of 0°, 120°, or 240° between the rotation of each diagonal leg pair (depending on the positions of the second pair of chambers relative to the first). (B) Schematic of the quadruped robot controlled by oscillator A and oscillator B when oscillator B has a phase offset of φʹ. (C) Box plot depicting the velocity of the robot for four different phase offsets. The 0°, 120°, and 240° phase offsets were controlled with circuit 1, and the 180° phase offset was controlled by circuit 2. (D) The parallel oscillator circuit (circuit 2) consisted of two oscillators controlling the two leg pairs in parallel. One of the oscillators was delayed by a phase controller (pictured here), and the other was not (not pictured), resulting in a one-time delay of one oscillator with respect to the other upon activation of both circuits. (E) The delay was adjusted by changing the resistance of an element R (i.e., the length and inner diameter of a section of tubing) connected to an inverter so that it only delayed the circuit once at the initiation of oscillation. (F) Representative measurement of the pressure in the three chambers of the delayed oscillator. The time offset delayed the initiation of the second oscillator (shaded region), effectively controlling the phase between two oscillators.

  • Fig. 4 Soft bistable 4/2 valve for switching gaits.

    (A) Operation of the 4/2 valve acting as a latching DPDT switch. The valve switches the direction of rotation of the limbs between (B) counterclockwise and (C) clockwise. (D) Representative plot of the pressure in the three chambers A1, A2, and A3 before and after the controlling 4/2 valve is switched (vertical line at 10 s).

  • Fig. 5 Omnidirectional control of the walking direction of the robot with 4/2 valves.

    (A) The output sequences of a three-valve ring oscillator circuit are controlled by the states of the two 4/2 valves. The states of the valves control the direction of rotation of the two diagonal leg pairs. We connected the 4/2 valve between two of the chambers to switch the chamber order for the soft ring oscillator sequence. The robot can translate in four directions based on the states of the two 4/2 valves. (B) By adding a 4/2 valve to the outputs corresponding to each leg, the robot can rotate clockwise or counterclockwise in addition to translating in any direction.

  • Fig. 6 Tethered control of the quadruped robot.

    (A) Schematic of the robot, with a single pressure supply line, tethered to a manual controller. (B) Image from demonstration of the robot walking forward, left, backward, and then right in sequence, with commanded directions indicated. (C) X-Y position of a single point on the robot body during the experiment (see movie S2) tracked from the video with motion tracking software. (D to F) Images of demonstration of manually controlled obstacle avoidance by first walking diagonally using one diagonal pair of legs and then walking forward with both diagonal pairs, with associated schematics indicating the actuation of the legs.

  • Fig. 7 Sensor input for autonomous gait reversal.

    Contact with the wall switches the walking direction of the robot by switching the state of a soft sensor that is connected to a 4/2 bistable valve. (A) The pneumatic control circuit powered by a constant pressure source actuated one diagonal pair of legs causing the robot to walk toward a wall. (B) When the soft sensor contacted the wall, the soft sensor switched state, and (C) the robot began walking in the opposite direction.

  • Movie 1. Electronics-free air-powered circuits to control the rhythmic movement of a soft legged robot.

Supplementary Materials

  • robotics.sciencemag.org/cgi/content/full/6/51/eaay2627/DC1

    Supplementary Text

    Fig. S1. Consistency of the soft ring oscillator.

    Fig. S2. Motion of the foot of the robot when the pressure in the pneumatic controller is perturbed.

    Fig. S3. Comparison of the speed of the robot while carrying the small (left) and large (right) CO2 canisters and regulators.

    Table S1. List of .stl files.

    Data S1. Archive of .stl files (.zip format).

    Movie S1. Video of the African sideneck turtle exhibiting a diagonal couplet walking gait.

    Movie S2. Video of omnidirectional locomotion, controlled by the four states of two DPDT soft pneumatic switches.

    Movie S3. Video of robot walking untethered, powered by a pressurized CO2 canister fitted with a pressure regulator.

    Movie S4. Demonstration of the robot navigating around an obstacle, with manual switching from a single leg pair diagonal gait to a diagonal couplet walking gait, controlled by a dual-purpose three-valve ring oscillator, accelerated 4×.

    Movie S5. Video of the electronics-free robot autonomously avoiding an obstacle.

  • Supplementary Materials

    The PDF file includes:

    • Supplementary Text
    • Fig. S1. Consistency of the soft ring oscillator.
    • Fig. S2. Motion of the foot of the robot when the pressure in the pneumatic controller is perturbed.
    • Fig. S3. Comparison of the speed of the robot while carrying the small (left) and large (right) CO2 canisters and regulators.
    • Table S1. List of .stl files.

    Download PDF

    Other Supplementary Material for this manuscript includes the following:

    • Data S1. Archive of .stl files (.zip format).
    • Movie S1 (.mp4 format). Video of the African sideneck turtle exhibiting a diagonal couplet walking gait.
    • Movie S2 (.mp4 format). Video of omnidirectional locomotion, controlled by the four states of two DPDT soft pneumatic switches.
    • Movie S3 (.mp4 format). Video of robot walking untethered, powered by a pressurized CO2 canister fitted with a pressure regulator.
    • Movie S4 (.mp4 format). Demonstration of the robot navigating around an obstacle, with manual switching from a single leg pair diagonal gait to a diagonal couplet walking gait, controlled by a dual-purpose three-valve ring oscillator, accelerated 4×.
    • Movie S5 (.mp4 format). Video of the electronics-free robot autonomously avoiding an obstacle.

    Files in this Data Supplement:

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