Soft Pneumatic Gripper

This project explores a pneumatic gripper design that has its own on-board pressure source for actuation using a CO2 cartridge. This enables the gripper to be used in a wide range of applications where minimizing size and weight are critical, such as a gripper attachment for a quadrotor drone. The design of the silicone fingers replicates the “Hybrid PneuNets” concept to minimize the amount of gas required per actuation and optimize gripping performance.

I created a 3D-printed three-part mold to fabricate each of the "Hybrid PneuNets" fingers. These are the steps of the silicone molding process:

The two halves of the negative mold (transparent) were screwed together along with the positive mold (blue), forming the shape of the finger’s internal chambers. The Dragonflex 10A was poured through the rectangular holes of the assembled mold until all the holes were overfilled. The mold was immediately placed in a vacuum chamber for several minutes to extract all the bubbles before and fill in all the small features inside the mold.
The machine screws embedded in the mold were loosened, forcing the two halves of the mold to separate.
The cured silicone chambers was carefully peeled away from the positive blue mold.
The silicone chambers were joined with the inner surface that contains the inextensible paper layer. A 3 mm steel shaft was used maintain the shape of the internal air channel when joining the two silicone halves together.
The completed silicone finger was peeled off the mold surface and the 3D-printed rigid elements were loosely attached between the finger chambers using a kevlar threads to avoid impeding the finger from bending.
A 3D-printed gripper hub with barbed fittings is used to join the three fingers together with silicone sealant.

Improved gripping performance

The inextensible paper layer embedded in the gripper constrains the length of the finger, resulting in the finger bending when pressurized. The 3D-printed rigid elements between each silicone chamber and the concave shape of the chambers increases longitudinal strain. These design changes result in a 50% increase in deflection angle, improving gripping performance for the same amount of air.

Pneumatic actuation testing

Rapid inflation was achieved by first pressurizing a 120 mL air bottle using a small compressor motor and quickly releasing the pressure by actuating a solenoid valve open. An additional solenoid was used to rapidly deflate the gripper.

On-board pressure source

The first prototype utilized an on-board compressor motor only allowed for 3 actuation cycles before the air bottles needed to be repressurized, requiring an additional ~30 s delay. The revised system utilizes a compact 16 g CO2 cartridge and an adjustable pressure regulator, which can be tuned by the operator to output pressures ranging from 0-60 psi. The pressurized CO2 enables ~500 actuations cycles without any delay needed to repressurize.

Sensor integration

A pressure sensor monitors the pressure inside the gripper hub while actuating the solenoids, to consistently inflate to a 2.5 psi target pressure. All electronic components were soldered to a compact prototype with a gap in the middle for clearance of the CO2 cartridge. Three additional IR receivers were arranged pointing radially outward to receive signals from an IR remote, allowing for a human operator to control the gripper from any angle.

Automatic detection & gripping

An IR sensor embedded in the center of the gripper is used to automatically detect objects. The object detection range can be adjusted via a potentiometer screw to any setting between 2-30 cm from the bottom of the gripper. Whenever an object enters the detection zone and is within the range setting, the sensor sends a digital signal to the Arduino to inflate the gripper, closing the claw. The sensor PCB has a status LED that is on when the object is detected. A button was also added to allow the operator to enable/disable automatic gripping.

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