Effect of bladder wall thickness on miniature pneumatic artificial muscle performance

Abstract

Pneumatic artificial muscles (PAMs) are actuators known for their high power to weight ratio, natural compliance and light weight. Due to these advantages, PAMs have been used for orthotic devices and robotic limbs. Small scale PAMs have the same advantages, as well as requiring greatly reduced volumes with potential application to prostheses and small scale robotics. The bladder of a PAM affects common actuator performance metrics, specifically: blocked force, free contraction, hysteresis, and dead-band pressure. This paper investigates the effect that bladder thickness has on static actuation performance of small scale PAMs. Miniature PAMs were fabricated with a range of bladder thicknesses to quantify the change in common actuator performance metrics specifically: blocked force, free contraction, and dead-band pressure. These PAMs were then experimentally characterized in quasi-static conditions, where results showed that increasing bladder wall thickness decreases blocked force and free contraction, while dead-band pressure increases. A nonlinear model was then applied to determine the structure of the stress-strain relationship that enables accurate modeling and the minimum number of terms. Two nonlinear models are compared and the identified parameters are analyzed to study the effect of the bladder thickness on the model.

Bioinspired passive variable recruitment of fluidic artificial muscles

“…Tiwari et al (2012) and Meller et al (2014) included work comparing hydraulic and pneumatic actuation—the results indicate that while pneumatics are cleaner and often have a simpler fluid circuit, the compressibility of air causes pneumatic FAMs to be less efficient than their hydraulic counterparts. More recent studies have found more in-depth, predictive models of FAM output force and stroke during static and dynamic conditions (Ball and Garcia, 2016; Pillsbury et al, 2015; Sangian et al, 2015; Tondu, 2018). There have also been studies of how FAM design parameters impact the efficiency of an entire electro-hydraulic system in a self-contained mobile robot (Chapman et al, 2017) as well as multiple other applications of FAMs in robotic and rehabilitation applications (do Nascimento et al, 2008; Gordon et al, 2006; Pack et al, 1997; Tondu and Lopez, 1997).…”

“…This is regulated by the central nervous system by the motor neurons and their associated axons (Henneman et al, 1965b). An analogous behavior can be created in FAMs; those with thicker or stiffer elastomeric bladder materials tend to require larger applied pressures before output force or significant contraction are generated (Pillsbury et al, 2015).…”

“…It has been shown (Meller et al, 2014; Sangian et al, 2015) that there are bladder-induced nonlinearities in FAMs based on the stiffness of the bladder. These bladder stiffness effects result in a minimum pressure that must be applied to a FAM before it contracts (Pillsbury et al, 2015). Pillsbury et al (2015) refer to this minimum pressure as “deadband pressure” and Sangian et al (2015) refer to a similar phenomenon as “threshold pressure.” There is a relationship between the material thickness of the bladder as it compares to the volume of the whole FAM to this initial actuation pressure.…”

“…These bladder stiffness effects result in a minimum pressure that must be applied to a FAM before it contracts (Pillsbury et al, 2015). Pillsbury et al (2015) refer to this minimum pressure as “deadband pressure” and Sangian et al (2015) refer to a similar phenomenon as “threshold pressure.” There is a relationship between the material thickness of the bladder as it compares to the volume of the whole FAM to this initial actuation pressure. This minimum actuation pressure will be called threshold pressure P th in this study and will be described in more detail in section “FAM modeling.” This threshold pressure is typically considered a loss in the actuation of FAMs because it prevents force from being applied to the braid and therefore generating work output in the FAM until a larger pressure is applied.…”

The Effects of Manufacturing Parameters on Static Characteristics of Water Hydraulic Artificial Muscles

“…Moreover, the improved model and the measured data shown that the improved model can predict muscle static response more accurately. Pillsbury et al [19] fabricated and tested several PAMs with bladder thickness that varied between 0.397 mm and 0.794 mm. They verified that contraction force and free contraction both decrease with increasing bladder thickness.…”