The pneumatic artificial muscle (PAM) is typically a gas actuator developed using a bionic design to imitate biological muscles. McKibben artificial muscle is the most famous pneumatic artificial muscle, a soft hollow rubber tube.
Pumping incredibly compressed air into or out of the rubber tube to achieve the body’s radial expansion or contraction while being constrained by the coaxial woven sheath to generate axial force to drive the robot is the basic operating principle of pneumatic artificial muscle.
Pneumatic artificial muscles are becoming increasingly important in the industry and rehabilitation treatment owing to their inherent compliance and high force-mass ratio.
Due to its unique structure, a single pneumatic artificial muscle can only provide extension or contraction motion in a single linear direction. A single muscle cannot achieve multiple degrees of freedom because real robots typically need to accomplish complex and variable goals. As a result, scientists frequently use multiple artificial muscles to drive the robot’s joints to overcome various challenges.
PAMs have been applied mostly in bio-robotic applications or biomimetic robots. As these actuators resemble the characteristics of actual skeletal muscles, researchers have tried to emulate the “soft” compliant structure of organic muscle, bone, tendons, and skin by PAMs.
Pneumatic artificial muscle robots have the following advantages.
- Lightweight and small size. Pneumatic artificial muscle has a very light overall mass because rubber (including neoprene, acrylate rubber, etc.) and the metal connection made of aluminum alloy are its primary materials. For instance, Festo’s pneumatic muscle with a 5 mm inner diameter weighs only 27 g per meter.
- High gravimetric specific power. For pneumatic artificial muscles, the power-to-weight ratio is very high. For instance, a 20 mm inner diameter pneumatic artificial muscle can deliver 1500 N of pulling force, which can meet the load requirement in real-world applications.
- Easy installation and smooth motion. Pneumatic artificial muscles do not require the assembly of gearboxes and other transmission components, in contrast to conventional motors. They can be set up in the robot frame by simply coupling air hoses and sealed plugs. In addition, pneumatic muscles can be driven more smoothly than cylinders because they lack sliding components like pistons. It is possible to significantly increase the efficiency of pneumatic potential energy to mechanical energy conversion.
- Clean and safe. The movement of pneumatic artificial muscles does not pollute the environment and uses a clean, environmentally friendly energy source. In addition, the pneumatic artificial muscle has good flexibility, which helps the robot achieve flexible active/passive suppleness control in a variety of constrained working environments or on occasions with significant human-robot interaction, such as during surgery and gait rehabilitation training.
Due to their unique physical makeup, pneumatic artificial muscles have advantages and disadvantages. The pneumatic artificial muscle, influenced by internal compressed air, exhibits a complex nonlinear relationship between air pressure and contraction length. Its system parameters are frequently only discernible within a narrow range of operating air pressure. It is difficult to develop an accurate kinetic model across the operating air pressure range.
The pneumatic artificial muscle typically exhibits hysteresis and creep effect, which impacts the increase in output force. These effects are caused by friction between the braided mesh and rubber tube and elastic deformation of the rubber tube during contraction or diastole.
Among the advantages of the PAM is the ability to provide high power outputs with relatively light weights and possesses inherent compliance, thus, meeting the need for safety, simplicity, and lightness that human-robot interaction requires. Those characteristics, combined with the fact that PAM possesses similar properties to those of the human muscle, make it a promising actuator choice for therapeutic devices, which are designed for rehabilitation therapy of patients suffering from degenerative muscle diseases and extremity impairment or neurological injuries that affect their kinetic abilities.