A new study has demonstrated the advantages and potential applications of an innovative multi-chambered soft pneumatic actuator. Researchers described how the actuator generates cyclical motion and characterized its trajectory and force output in an article published in Soft Robotics.
Coauthors Alar Ainla, Mohit Verma, Dian Yang, and George Whitesides of Harvard University detailed the actuator’s construction and control in the article “Soft, Rotating Pneumatic Actuator.” And they presented two sample applications of the actuators stirring fluid and providing locomotion.
The actuators, called cyclical vacuum-actuated machines (cVAMs), consist of three, four or five soft pneumatic air chambers arranged around the circumference of a circle surrounding a central rod. A typical cVAM contains four chambers that can pull the rod in different directions by applying vacuum to one or more chambers. For instance, a cVAM unit where two neighboring chambers are actuated at the same time tilts the rod in the middle. Actuation angle is a function of the material and pressure. Sequential actuation of a four-chamber device using reduced pressure moves the central rod cyclically in an approximately square path.
The cVAMs were manufactured by replica molding. Molds were fabricated in acrylonitrile butadiene styrene (ABS) plastic using a Stratasys 3D printer. Then a silicone-based Ecoflex elastomer was molded and cured to produce the soft cVAM chambers.
To control the actuators, a microcontroller operated a miniature valve bank containing 12 V, 100 mA SMC solenoid valves. Valve flow conductance was 0.08 l/s-bar (0.016 Cv) with response time <5 msec. Lengths of 0.76 mm (0.03 in.) ID polyethylene tubing for pneumatic actuation were attached to the elastomer structure using a needle to puncture holes. The elastomer formed a pneumatic seal around the tubing upon insertion.
From a central vacuum line in the laboratory, the researchers varied supplied negative pressure over a range of 0 to −90 kPa using an electronic controller. The team operated the cVAMs at frequencies from 7.5 to 750 rpm by changing the hold time between each of the steps that make up one complete revolution.
The researchers sought to characterize the actuator trajectory and its exerted force as they varied the material used for fabrication, the number of chambers, and actuator size. They also demonstrated two applications of the actuator.
One entailed simultaneously delivering fluid while stirring using a cVAM. Here, the team replaced the central rod with a stainless-steel needle connected to a flexible tube for delivering fluids. Then a solution of fluorescein dye in water was delivered at the rate of 10 ml/min into a 150 ml bath of water while the cVAM operated at 375 rpm. Because cVAMs produce a cyclical motion without rotating the needle, it is possible to deliver fluids without entangling the tubes. Qualitatively, the cVAMs achieved a homogeneous solution within 10 sec.
The researchers also looked at using the actuators for locomotion in a four-legged “walker” with four cVAMs to move its legs. Actuating the ring of pneumatic chambers in clockwise or counterclockwise sequence causes the tip of the central rod to move in a cyclical manner while following an approximately square trajectory. They took advantage of this motion to build a quadrupedal soft robot. In designing a gait, they mimicked the walking gait of a reptile. A reptile moves its diagonal limbs simultaneously and so does the soft robot.
Vacuum pulses were applied to four chambers on each leg in a cyclic sequence. The front and back legs of each side had a phase difference of 180°, whereas two diagonal pairs of legs each had the same phase. The “walker” moved at 0.5 cm/sec (or 0.12 body lengths per second) when vacuum states changed every 0.1 sec.
According to the Harvard team, cVAMs offer a new method to generate cyclical motion using a soft actuator. Among the advantages, they cannot burst at actuation pressures more than their operating specifications because they operate under partial vacuum rather than positive pressure. Rotating speed is controlled by the hold time between steps while negative pressure in the chambers controls the shape of the trajectory. This relationship permitted speeds to around 750 rpm without altering the trajectory.
Also, cVAMs contract rather than expand in use, making them suitable in space-constrained environments. Elastomers experience less strain when operated using reduced pressure, compared to operation under positive pressure, and thus may have a longer life. And because the actuators are pneumatic, cVAMs can be used in settings sensitive to electromagnetic fields or with flammable liquids. In contrast, unshielded electric motors could cause interference or explosion.
The authors also noted a few limitations. Under ambient conditions, pressure difference is limited to about 100 kPa because cVAMs actuate using vacuum. This governs the force that cVAMs can apply, although the limitation could become an advantage in hyperbaric conditions such as in deep-sea applications.
Operation of a cVAM involves many cycles of low amplitude stretching and compressing of the elastomer “hub,” and failure in this region is a possibility—although the researchers feel this is unlikely for at least one million cycles. Finally, cVAMs are not directly suitable for powering an axle and wheel because the central rod does not actually rotate. Instead, it follows an approximately square trajectory.