  This paper discusses the design of a novel compliant in-pipe climbing modular
robot for small diameter pipes. The robot consists of a kinematic chain of 3
OmniCrawler modules with a link connected in between 2 adjacent modules via
compliant joints. While the tank-like crawler mechanism provides good traction
on low friction surfaces, its circular cross-section makes it holonomic. The
holonomic motion assists it to re-align in a direction to avoid obstacles
during motion as well as overcome turns with a minimal energy posture.
Additionally, the modularity enables it to negotiate T-junction without motion
singularity. The compliance is realized using 4 torsion springs incorporated in
joints joining 3 modules with 2 links. For a desirable pipe diameter (\text{\O}
75mm), the springs' stiffness values are obtained by formulating a constraint
optimization problem which has been simulated in ADAMS MSC and further
validated on a real robot prototype. In order to negotiate smooth vertical
bends and friction coefficient variations in pipes, the design was later
modified by replacing springs with series elastic actuators (SEA) at 2 of the 4
joints.
