Theory Application

Theory Application

Soft Robotic Actuation Principle

Pneumatic Actuation:  

Pneumatic actuators are devices that use the energy in the form of a fluid (typically compressed air) to create mechanical motion [1]. The most commonly consist of pistons, cylinders and valves. A widely used pneumatic actuator is the McKibben muscle, patented by Gaylord, Richard H. [2].The muscle simply contracts due to an increase in pressure causing a linear contraction. A diagram of the muscle's contraction process can be seen in the diagram.

A diagram of how the McKibben muscle contracts in relation to pressure increase

Figure has been copied [3]

Diagram of how the McKibben muscles contract linearly in relation to increase of internal pressure in stages (a) - (c). Relative pressure is noted at each stage of the illustration.

 

Explanation of concept(s)/underlying theory:

In order to simplify the creature design and lighten the weight, locomotion with a single muscle would be greatly preferred. However, there’s an issue that arises with this. How do we take simple, one dimensional motion to something that can move a creature forward? The answer comes with the forces involved. If a muscle can put more force going one direction than the other, then we can use simple motion to move the creature. There are many ways of accomplishing this, but a simple and reliable method is shown below.

A diagram of a leg mechanism with numbered and labeled parts. The leg consists of 2 linkages and it attached to McKibben muscle that is attached to a base.

diagram of the force diagram of the leg mechanism in the open position diagarm of the force diagram of the leg mechanism in open position

Force Diagram of Leg mechanism while in its opened and closed position

This basic leg mechanism operated through relatively simple principles. The force of the muscle and the force of the return rubber band each provide a torque on the leg, which as a result produces a force on the foot. The force from the foot is backwards and downwards when the muscle is actuated, causing the foot to increase grip and push the creature forward. When the muscle is released, the force on the foot is upwards and forward, causing the leg to lose grip and slip forward. This combined motion propels the creature successfully. To analyze the forces, the basic torque equation can be used.

T = F * d

For this equation, d is the distance at which the force acts at from the center pivot. This is not the distance at from the point of application, as shown in the figure below.

Diagram demonstrating relationship between force and distance

When the foot is stationary, the leg can be analyzed by the following equation. This equation is a combination of the general torque equation, accounting for all three forces on the leg.

T = Ff  * df + Fb * db + Fm * dm

If Fb and Fm are found, and Ff can then be calculated. Fm, the force of the muscle, can be found by testing. This can be as simple as attaching a scale to the end of the muscle and then actuating it. The rubber band acts like a spring, so the general spring equation (Hooke’s law) can be used. The variable x in this is the elongation of the rubber band from the free length, and k is the spring constant. This will have to be empirically tested in order to be obtained.

F = -k * x

 

 

References

[1] “What Is a Pneumatic Actuator and How Do They Work?” Process Industry Forum, www.processindustryforum.com/article/what-is-a-pneumatic-actuator. Accessed 15 Mar. 2024.

[2] R. H. Gaylord, “Fluid actuated motor system and stroking device,” 2844126, Jul. 22, 1958 [Online]. Available: https://patents.google.com/patent/US2844126A/en

[3] J. Midiri and K. Trieu, “BYOE: McKibben Creature: A Low-Cost Robotic Simulation of a Biological Environment,” in Experimentation and Laboratory-Oriented Studies Division (DELOS) of the 2024 ASEE Annual Conference & Exposition, ASEE 2024, Portland, OR, USA, June 23-26, 2024 [Online]