Conventional pistons are made of a rigid chamber and a piston inside that can slide along the chamber’s inner wall while at the same time maintaining a tight seal. As a result, the piston divides two spaces that are filled with two fluids and connected to two exterior fluid sources. If the fluids have different pressures, the piston will slide into the direction with the lower pressure and can, at the same time, drive the movement of a shaft or other device to do physical work. This principle has been used to design many machines including piston engines, hydraulic lifters and cranes such as those used on construction sites, and power tools.

A prototype of the linear tension piston, rested (top) and pressurized (bottom). (Wyss Institute at Harvard University)

Conventional pistons suffer from several shortcomings; for example, the high friction between the moving piston and the chamber wall can lead to breakdown of the seal, leakage, and gradual or sudden malfunctions. In addition, especially in the lower pressure spectrum, energy efficiencies and response speeds often are limited.

A new way to design pistons replaces their conventional rigid elements with a mechanism using compressible structures inside a membrane made of soft materials. The resulting “tension pistons” generate more than three times the force of comparable conventional pistons, eliminate much of the friction, and at low pressures are up to 40 percent more energy-efficient.

The tension piston concept builds on the researchers’ fluid-driven origami-inspired artificial muscles (FOAMs) that use soft materials to give soft robots more power and motion control while maintaining their flexible architectures. FOAMs are made of a folded structure that is embedded within a fluid in a flexible and hermetically sealed skin. Changing the fluid pressure triggers the origami-like structure to unfold or collapse along a pre-configured geometrical path, which induces a shape-shift in the entire FOAM, allowing it to grasp or release objects or to perform other kinds of work. By using a flexible membrane bonded to a compressible skeletal structure inside and connecting it to one of the two fluid ports, a separate fluid compartment is created that exhibits the functionality of a piston.

In an object-crushing comparison between a conventional piston (air cylinder, left) and a tension piston (right), the tension piston can produce greater forces at the same air pressure. (Wyss Institute at Harvard University)

A rise in driving pressure in the second fluid reservoir surrounding the membrane in the chamber increases the tension forces in the membrane material that are directly transmitted to the bonded skeletal structure. By physically linking the skeleton with an actuating element that reaches out of the chamber, compression of the skeleton is coupled to a mechanical movement outside the piston.

The piston was tested against a conventional piston in an object-crushing task and showed that it broke objects like wooden pencils at much lower input pressures (pressures generated in the skin-surrounding fluid compartment). At the same input pressures, particularly in the lower pressure range, the tension pistons developed more than three times greater output forces and displayed more than 40 percent higher energy efficiency by harnessing the fluid-induced tension in the flexible skin materials. More than one tension piston can be incorporated into a single chamber or the surrounding chamber could be fabricated with a flexible material like an airtight nylon fabric.

Watch the piston test on Tech Briefs TV here. For more information, contact Benjamin Boettner at This email address is being protected from spambots. You need JavaScript enabled to view it.; 617-432-8232.

Motion Design Magazine

This article first appeared in the August, 2019 issue of Motion Design Magazine.

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