A. Actuation Stress/Strain analysis
In order to find the best-suited actuator for joints in serpentine robots we performed a detailed analysis mostly based on the comparison of performance indices of mechanical actuators introduced by Huber et al. [1997] and complemented by our own investigations.
The original paper did not include electric motors. It also included only select types of pneumatic actuators. We made some reasonable assumptions about the transformation of rotary motion to linear motion and calculated the performance indices for some electric motors with a ball screw transmission mechanism that produces reasonable linear speed and force. We also calculated the performance indices for a few pneumatic bellows and artificial pneumatic muscles and added those results in Figure 3.
Actuators that are closest to the top right corner of Figure 3 are naturally suited to lifting weights and propelling masses in the orders of magnitude required for serpentine robots.
The superior characteristics of hydraulics (compared to pneumatics) are diminished once actuation stress is related to actuator density. Furthermore, hydraulics also becomes less desirable over electric motors once efficiency is considered, as was shown in Table I. One should also note that Huber’s analysis considers the actuator only, but without the volume (or weight) of the compressor, manifolds, valves, fittings, and pipes. It is difficult to calculate the performance indices for the whole actuation system with precision because the volume and weight strongly depends on the application. In general these weight factors work in favor of electric systems. Once these considerations are taken into account, in addition to the actuation stress/strain analysis illustrated by Figure 3, it appears evident that there is some advantage to electric motors. However, the fact remains that actuation strain of most cylinder-type actuators is limited to 1.0 and only pneumatic bellows produce the largest value (reaching 4) without any external mechanisms. Also overload ratio of pneumatic actuators is significantly higher then competitors.
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