The stiffness of the inner bladder can be changed by varying the material, using materials with different durometers and elasticities, or by varying the geometry, i.e. changing the wall thickness. Using the testing methods described here, the following effects of stiffness on actuator behavior were observed:
Isometric testing shows that the force-pressure relationship is independent of stiffness. As actuators were not allowed to contract, little energy goes into stretching the bladder material, and so the stiffness of the bladder material does not have a significant effect. |
Stiffer actuators are slower to reach a given amount of force. |
If pressure is held constant, actuators which are less stiff can reach higher contraction amounts and apply more force. |
There is a wide variety of braided meshes available from manufacturers, with different diameters, materials, fiber sizes, etc. One important characteristic of braided meshes to consider during mesh selection is the initial braid angle.
The braid angle is the angle formed between the longitudinal axis of the mesh and the mesh fibers. |
The braid angle can be measured by examining the mesh under a microscope:
In general, lower initial braid angles are preferable. There is a neutral/"magic" angle of 54°, which can be derived analytically, at which no more axial contraction or radial expansion of the mesh can occur. Mesh braid angles typically start below this neutral angle, and move toward it as the actuator contracts. Therefore, meshes that start at a lower angle will have a longer "stroke length" and contraction percentage, with corresponding higher forces.
Varying braid angle has the following effects:
In dynamic tests, actuators with lower braid angles achieved higher forces, and reached those maximum forces more quickly. |
In quasistatic isometric tests, actuators with lower braid angles achieved higher force for a given pressure. |
In constant pressure tests, actuators with lower braid angles achieved higher forces and contraction amounts. |