An electropermanent magnet valve for the onboard control of multi-degree of freedom pneumatic soft robots

Abstract

To achieve coordinated functions, fluidic soft robots typically rely on multiple input lines for the independent inflation and deflation of each actuator. Fluidic actuators are controlled by rigid electronic pneumatic valves, restricting the mobility and compliance of the soft robot. Recent developments in soft valve designs have shown the potential to achieve a more integrated robotic system, but are limited by high energy consumption and slow response time. In this work, we present an electropermanent magnet (EPM) valve for electronic control of pneumatic soft actuators that is activated through microsecond electronic pulses. The valve incorporates a thin channel made from thermoplastic films. The proposed valve (3 × 3 × 0.8 cm, 2.9 g) can block pressure up to 146 kPa and negative pressures up to -100 kPa with a response time of less than 1 s. Using the EPM valves, we demonstrate the ability to switch between multiple operation sequences in real time through the control of a six-DoF robot capable of grasping and hopping with a single pressure input. Our proposed onboard control strategy simplifies the operation of multi-pressure systems, enabling the development of dynamically programmable soft fluid-driven robots that are versatile in responding to different tasks.

Fig. 1. Design, operating principle, and fabrication of EPM valves.

A Picture of the EPM valve. B Qualitative demonstration of the flexibility of the integrated EPM valves on a thermoplastic structure. C The EPM valve uses a pinching effect to obstruct fluid flow: the magnetic force between the magnetized EPM end cap and the keeper plate results in the collapse of the TPU channel. C-i A side view of the valve in its closed state with a graph of the corresponding pressure over the length of the valve, and C-ii A side view of the valve in its open state with a graph of the corresponding pressure over the length of the valve. D Fabrication process of an EPM valve involving (1) thermal bonding of stacked layers of thermoplastic films to create the channel, (2) insertion of tubing to facilitate pneumatic connections, (3) integration of the EPM assembly, and (4) placement of the EPM into the alignment slot.

Fig. 2. Modeling the electromagnetic behavior of EPMs.

A The dimensions of the EPM viewed isometrically and from a cross-sectional perspective. The coil modeled does not show the full 150 turns of the actual EPMs. B Force vs. air gap for the EPM as determined via the analytical model, via simulation in COMSOL, and experimentally. C Cross-sectional view of the EPM assembly, highlighting the elements contacting to induce the pinching effect of the TPU channel.

Fig. 3. EPM valve characterization.

A 3D model of the EPM TPE valve showing a view of the valve in its closed state. B-i Responses of the EPM valve to a constant pressure of  ≈150 kPa over 15 minutes. B-ii Switching of the EPM valves when supplied with a constant pressure of  ≈ 50 kPa to demonstrate its performance over cyclic conditions for 15 min. C Schematic of the experimental setup used to obtain the flow response of the EPM valve. D Responses of the EPM Valve to incremental pressure step inputs with the EPM valve initially in the closed position D-i and with the EPM valve initially in the open position D-i The shaded error bars of each test show the mean and standard deviation for three trials. E A  − 100 kPa step input with (left) the valve open and (right) with the valve closed starting from an initial pressure of 30 kPa. F The transient response of downstream pressure under a ramp-up staircase waveform pressure input. The experimental curves show the average magnetic flux with one standard deviation.

Fig. 4. Timed sequential actuation of SBAs facilitated by three-DoF EPM valve.

A A schematic representation of the EPM valve architecture used to control a three-DoF SBA. B A time sequence of the SBA's movement in which the tip rotated through 360° without returning to the rest state between each command. C A schematic representation illustrating the EPMs as a switching mechanism to either pressurize or exhaust the actuator. D A time sequence of the SBA's movement in which each combination of chamber inflation states was reached.

Fig. 5. Various modalities achieved using six-DoF EPM valve.

A Electronically controlled pneumatic soft robot showing SBAs, TPE fluidic channels, and EPM valves mounted on a flexible acrylic plate with a single tube for positive and negative constant pressure supply. A side view of the multi-function six-DoF robot during its gait depicts the use of the SBAs for legged locomotion. B A schematic representation of the EPM valve architecture used to control two three-DoF SBAs. C The multi-function robot used as a grasper to pick up (from left to right) a pencil, a cup, a dried apricot, and a rubber duck. The top and bottom rows show the grasper in its OFF and ON states, respectively. D A time sequence showing the multi-function six-DoF robot locomoting across a table. E A time sequence of the two segment SBA arm showing a selection of dual-curvature positions.

Fig. 6. Two sequences demonstrating the two operation modes of the six-DoF robot.

A With a constant positive pressure supplied to the robot’s inlet, the valves open in sequence, allowing the actuators to inflate. Once all actuators have inflated, the inlet switches to negative pressure, the valves close, and the sequence repeats indefinitely until a keyboard command is sent. B With the valves states fixed, the inlet pressure alternates between positive and negative pressure, inflating and deflating the chosen actuators in sequence for a preset duration. In both A, B, the schemes show, from top to bottom, the valve states, the input pressure, images from the experiment, and schematic representations of the robot state. The time axes are not shown to scale.