2.5. Effect of magnetic field strength on locomotion
To investigate how magnetic field strength influences locomotion, we examine the locomotion data represented as total actuation distance (da, Figure \ref{402200}B) with respect to step number (Figure \ref{402200}C-D). Representing robot locomotion as da facilitates comparison between robots tested at different ya offsets because robots experience the same time-varying magnetic field strength at the same da. We note that both the strength and direction of the magnetic field vary during actuation due to the rotation of the actuator magnet (magnetic field strength plots in Figure S1, Supporting Information).
The data in Figure \ref{402200} show that, in general, smaller ya caused an increase in speed due to the higher magnetic field strength and thus stronger actuation at smaller ya until an upper limit where the high magnetic field strength lifts the robot’s feet from the bottom of the lumen for the entire step. As expected from this observation, increasing the robot mass can increase the upper magnetic field strength limit, as shown by the data for MR-LF 4.43 g, which performed the fastest locomotion at ya = 9 cm.
As the field strength approaches the upper limit, a slight perturbation or changes in conditions at this field strength (e.g., inherent variation of motion) can result in a significant variation of the speed as the robot will either produce the largest speed due to the highest field strength, or a near zero speed due to the loss of contact with the bottom of the lumen. The magnetic field strength at ya = 10 cm (10.32 mT, xa = 0, θa = 90°) appears to be near the upper limit for three robots (MR-DF, MR-LF 2.55 g, and MR-LF 2.87 g). Indeed, at ya = 10 cm with the same experimental condition, each of these three robots exhibited effective gait with high speeds in some trials but were lifted and resulted in near zero speed in other trials due to inherent variations in conditions and motion. This significant variation in speeds, which is evident in the ya = 10 data in Figure \ref{402200} and S3, caused the large error in the average speed at ya = 10.