Research ArticleSOFT ROBOTS

A soft, bistable valve for autonomous control of soft actuators

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Science Robotics  21 Mar 2018:
Vol. 3, Issue 16, eaar7986
DOI: 10.1126/scirobotics.aar7986
  • Fig. 1 Details of the soft, bistable valve.

    (A) Schematic showing the components of the valve. The valve consists of a hemispherical, elastomeric membrane separating two chambers. Control pressures in the bottom (P+) and top (P) chambers deform the membrane. When the membrane is in the downward position (state 1), it blocks air flow through a tube leading through the bottom chamber by kinking the tube. When the membrane is in the upward position (state 2), it blocks air flow through the top tube. (B) Photographs of the valve in both states. (C) When the pressure difference, ΔP, between the two chambers reaches a critical value, ΔP1, the membrane snaps to the upward position. When the pressure difference decreases below ΔP2, the membrane snaps back to the downward position. (D) The tubing kinks (and un-kinks) during the snapping process. The states of the bottom tubing (Q) and the top tubing (Embedded Image) are binary (i.e., open or closed) and hysteretic (movie S1).

  • Fig. 2 Measurements of the critical pressures.

    (A) Schematic of the apparatus used to measure ΔP1 and ΔP2 for different geometries. (B) Critical pressures, ΔP1 and ΔP2, as a function of H. (C) Critical pressures, ΔP1 and ΔP2, as a function of θ. (D) ΔP2 plotted against ΔP1 for valves with different H and θ values. The boundary of accessible critical pressures is defined by ΔP2 = ΔP1, and the values of ΔP for a valve with θ = 90°, and various H. Valves with critical switching pressures within this boundary are obtained when θ < 90°.

  • Fig. 3 Soft, bistable valve acting as a pneumatic switch.

    (A) The bottom tubing is connected to an air supply of constant pressure PS. The top tubing and the top chamber are connected to the atmosphere. The top and the bottom tubing are joined together behind the valve to form the output P of the pneumatic switch. The pressure in the bottom chamber is controlled by a variable pressure controller (P+). When the membrane bends downward, it kinks the bottom tubing; when it is bent upward, it kinks the top tubing. (B) Critical pressures ΔP1 and ΔP2 as a function of PS. (C) Output of the valve for different PS values and rectangular pulses as control input (P+ = 11 kPa). (D) Response of the valve to two rectangular pulses (P+ = 11 kPa) as the control input. A sinusoidal wave (frequency, 0.5 Hz; amplitude, 5 kPa) is superposed to the second pulse. H = 3 mm, θ = 87.5°.

  • Fig. 4 Gripper that grasps autonomously.

    (A) The gripper consists of five bending actuators, connected to a ring-shaped channel, around a soft, bistable valve. When the membrane in the valve is in its downward position, the pressure supply to the ring channel (PS) is blocked, and it is connected to the atmosphere. A second pressure supply (P+) leads to the bottom chamber of the valve and out through the contact sensor at the palm of the hand. The top chamber can be connected through an external valve to the atmosphere or the pressure supply PS. (B) Equivalent electrical circuit that represents the pneumatic control in the autonomous gripper. (C to H) Photographs of the gripper and schematics of the valve autonomously (C to E) closing around a tennis ball and (F to H) releasing the ball (movies S2 and S3).

  • Fig. 5 Pneumatic oscillator driven by an air source of constant pressure.

    (A) When the membrane is downward, air flows from the pressure source PS into a jar of volume V, but the tubing between the jar and the atmosphere is blocked. When the pressure P in the bottom chamber exceeds ΔP1, the membrane snaps upward and blocks air flow from the pressure source PS, and the jar vents to the environment. When P decreases below ΔP2, the membrane snaps downward, and the jar pressurizes again (movie S4). (B) Equivalent electrical circuit that represents the pneumatic feedback control. (C) Oscillations in the jar at PS = 11 kPa. (D) Rise time (tR) as a function of PS, with different V values. (E) Fall time (tF) as a function of PS, with different V values. Error bars in (D) and (E) show the SD of the mean over a 60-s measurement interval. H = 3 mm and θ = 87.5°.

  • Fig. 6 Autonomous soft robot with earthwork-like locomotion using an air source of constant pressure.

    (A) The earthworm consists of a linear bellows actuator with cylindrical sleeve as a restoring spring and a soft, bistable valve, integrated into the rear of the actuator. The design of the valve is the same as that for the pneumatic oscillator, with the bottom chamber of the valve connected to the bellows actuator. The bellows actuator bends upward during inflation and downward during deflation, which causes asymmetric contact between the feet and the ground, leading to asymmetric friction and directional movement. (B) Photographs of the moving earthworm at three points in time (movie S7). (C) Pressure inside the robot and positions of front end, rear end, and center as a function of time for PS = 17 kPa. The red dots indicate the times when the photographs in (B) were taken.

Supplementary Materials

  • robotics.sciencemag.org/cgi/content/full/3/16/eaar7986/DC1

    Materials and Methods

    Fig. S1. Kinking of tubing.

    Fig. S2. Geometry of devices for measuring the critical pressures.

    Fig. S3. Critical pressures as functions of wall thickness and scale.

    Fig. S4. Critical pressures as a function of the shear modulus.

    Fig. S5. Material characterization.

    Fig. S6. Influence of large input noise on the output.

    Fig. S7. Gripper with a valve without a top chamber.

    Fig. S8. Oscillator in intermediate state.

    Fig. S9. Oscillations at large input pressures with an additional pneumatic resistance.

    Fig. S10. Characterization of soft oscillator after 105 cycles.

    Fig. S11. Alternative designs.

    Fig. S12. Molds for the devices for measuring the critical pressures.

    Fig. S13. Assembly of the devices for measuring the critical pressures.

    Fig. S14. Molds for the tubing used inside the chambers of the valves.

    Fig. S15. Assembly of the tubing used inside the chambers of the valve.

    Fig. S16. Molds for the transparent valve.

    Fig. S17. Assembly of the transparent valve.

    Fig. S18. Molds for the pneumatic switch.

    Fig. S19. Assembly of the pneumatic switch.

    Fig. S20. Molds for the autonomous gripper.

    Fig. S21. Assembly of the autonomous gripper.

    Fig. S22. Molds for the oscillator.

    Fig. S23. Assembly of the oscillator.

    Fig. S24. Molds for the earthworm-like walker.

    Fig. S25. Assembly of the earthworm-like walker.

    Movie S1. Switching with the soft, bistable valve.

    Movie S2. Autonomous grasping with the soft autonomous gripper.

    Movie S3. Soft autonomous gripper without a top chamber.

    Movie S4. Soft oscillator.

    Movie S5. Soft oscillator equilibrating in intermediate state.

    Movie S6. Soft oscillator restarts after crushing.

    Movie S7. Autonomous earthworm-like walker.

    Data file S1. Molds for the devices to measure the critical pressures.

    Data file S2. Molds for the tubing used inside of the chambers of the valve.

    Data file S3. Molds for the transparent valve.

    Data file S4. Molds for the pneumatic switch.

    Data file S5. Molds for the autonomous gripper.

    Data file S6. Molds for the oscillator.

    Data file S7. Molds for the earthworm-like walker.

    Reference (45)

  • Supplementary Materials

    Supplementary Material for:

    A soft, bistable valve for autonomous control of soft actuators

    Philipp Rothemund, Alar Ainla, Lee Belding, Daniel J. Preston, Sarah Kurihara, Zhigang Suo, George M. Whitesides*

    *Corresponding author. Email: gwhitesides{at}gmwgroup.harvard.edu

    Published 21 March 2018, Sci. Robot. 3, eaar7986 (2018)
    DOI: 10.1126/scirobotics.aar7986

    This PDF file includes:

    • Materials and Methods
    • Fig. S1. Kinking of tubing.
    • Fig. S2. Geometry of devices for measuring the critical pressures.
    • Fig. S3. Critical pressures as functions of wall thickness and scale.
    • Fig. S4. Critical pressures as a function of the shear modulus.
    • Fig. S5. Material characterization.
    • Fig. S6. Influence of large input noise on the output.
    • Fig. S7. Gripper with a valve without a top chamber.
    • Fig. S8. Oscillator in intermediate state.
    • Fig. S9. Oscillations at large input pressures with an additional pneumatic resistance.
    • Fig. S10. Characterization of soft oscillator after 105 cycles.
    • Fig. S11. Alternative designs.
    • Fig. S12. Molds for the devices for measuring the critical pressures.
    • Fig. S13. Assembly of the devices for measuring the critical pressures.
    • Fig. S14. Molds for the tubing used inside the chambers of the valves.
    • Fig. S15. Assembly of the tubing used inside the chambers of the valve.
    • Fig. S16. Molds for the transparent valve.
    • Fig. S17. Assembly of the transparent valve.
    • Fig. S18. Molds for the pneumatic switch.
    • Fig. S19. Assembly of the pneumatic switch.
    • Fig. S20. Molds for the autonomous gripper.
    • Fig. S21. Assembly of the autonomous gripper.
    • Fig. S22. Molds for the oscillator.
    • Fig. S23. Assembly of the oscillator.
    • Fig. S24. Molds for the earthworm-like walker.
    • Fig. S25. Assembly of the earthworm-like walker.
    • Reference (45)

    Download PDF

    Other Supplementary Material for this manuscript includes the following:

    • Movie S1 (.mov format). Switching with the soft, bistable valve.
    • Movie S2 (.mov format). Autonomous grasping with the soft autonomous gripper.
    • Movie S3 (.mov format). Soft autonomous gripper without a top chamber.
    • Movie S4 (.mov format). Soft oscillator.
    • Movie S5 (.mov format). Soft oscillator equilibrating in intermediate state.
    • Movie S6 (.mov format). Soft oscillator restarts after crushing.
    • Movie S7 (.mov format). Autonomous earthworm-like walker.
    • Data file S1. Molds for the devices to measure the critical pressures.
    • Data file S2. Molds for the tubing used inside of the chambers of the valve.
    • Data file S3. Molds for the transparent valve.
    • Data file S4. Molds for the pneumatic switch.
    • Data file S5. Molds for the autonomous gripper.
    • Data file S6. Molds for the oscillator.
    • Data file S7. Molds for the earthworm-like walker.

    Files in this Data Supplement:

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