Design Principles

Basic Theory

At its most basic, a McKibben actuator is not unlike a balloon:

Air expansion inside of a balloon (left) and pneumatic piston (right).

Air molecules are continuously in motion, and apply a very tiny force whenever they impact a surface. Pressure is the combined force of gas or liquid molecules spread across the area that the molecules impact. When air is blown into a balloon, the air will collide with the inner surface of the balloon, imparting pressure. The pressure thus causes the balloon to expand, as more air is blown into it.

Pressure always acts in every direction equally, meaning the force on the interior of the balloon is identical throughout, and the shape the balloon takes depends on how it is constructed. Because of this, it is also difficult to direct pressure into acting in a specific direction. Most pneumatic actuators, such as pistons, use rigid cylinders to contain the air. Pressure will push against all surfaces, but only expand a piston that is allowed to move. By its nature, this mode of action is restrictive, allowing only linear movement and force output.

A McKibben actuator hybridizes the non-rigid balloon with a rigid pneumatic piston, using a flexible woven sheath instead of a rigid cylinder to guide the expansion of the balloon into a specific form:

Actuation of a simplified McKibben actuator. Grey arrows indicate the directions in which stretching and contraction occurs.

This mechanism of action allows for the sheath to be designed in various ways to produce variable, flexible motion. In the most basic configuration, the sheath will contract in the direction parallel to the airflow, and will expand radially outwards. Both the expansion and contraction develop tension in the sheath, which manifests as a force that can pull loads applied on the end opposite the air flow.

Design Considerations

In designing a McKibben actuator, the primary considerations that affect the end performance of your actuator is its size and shape.

Size

The force output of a McKibben actuator is closely related to the amount of air inside of the balloon. Note, when we refer to amount, we refer to number of molecules.

A greater number of molecules within the same volume will generate a greater pressure, and greater pressure means greater force. Alternatively, a larger volume actuator can fit more molecules of a given pressure. This inverse relationship between pressure and volume is called Boyle's Law (which should be familiar for anyone who has taken a fluid mechanics or general chemistry course).

Translating this for our actuators, it is usually easier to upscale the size of the actuator to obtain a greater force output, than it is to modify the supply of pressurized air. Modifying the pressure, on the other hand, would require one to use an entirely different pump or compressed air supply, and acquire new tubing/valves that are rated for safe operation at your desired pressure.

Obviously, increasing the size of the McKibben actuator isn't an end-all solution. A larger actuator occupies more space and has more weight than a smaller one, and as a result, the power-to-weight ratio of your actuator will drop. In addition, a larger actuator may take longer to compress/relax, depending on the power of your compressed air supply. Larger air supplies will add weight to your design, so the ideal size of your design will vary on a case-by-case basis.

Shape

The shape of a McKibben actuator can have a direct bearing on how it deforms, and is one of the key advantages of pneumatic artificial muscles as a whole. The shape of the balloon can be curved in numerous ways, and the flexible nature of the woven sheath allows it to wrap around non-flat surfaces. You can take advantage of this to create designs that wrap and twist around objects, or act as joints that bend and straighten as the actuator relaxes and compresses.

References

Kang, Bong-Soo, et al. "Dynamic Modeling of Mckibben Pneumatic Artificial Muscles for Antagonistic Actuation." IEEE International Conference on Robotics and Automation, 2009, www.semanticscholar.org/paper/Dynamic-modeling-of-Mckibben-pneumatic-artificial-Kang-Kothera/368ad5ea12f58d7e3bcba1f7ac9b53f8e7e84e82.

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