Fig. 1: The ModLock connector with the female connector (top) and compliant male connector plate assembly (bottom) Fig. 3: Fabricated male connector plate assembly (left) and female connector pattern (right) The ModLock mechanism utilizes a female-male-female connection principle. Female connectors are placed on all system modules. To connect two modules a male connector plate assembly is used to connect the two female patterns together. Effectively, it operates as two gendered connections where any system module holds a female connector and a male connector couples the female patterned modules together. Each male connector has four flat head screws embedded into the plate. The screws are not turned, rather the head geometry acts as hooks. To connect two modules together, the screws are aligned to fit into four custom shaped screw insertion holes, (Fig. 1). The plates are pressed together so the heads go through the screw insertion holes (Fig. 2a). The faces are twisted 12° to align and tighten the male connector plate assembly with the female pattern. A stainless steel locking dowel on a compliant lever snaps into a lock groove to prevent the faces from untwisting. This is shown in Fig. 2b. Once the faces are tightened with respect to each other and the locking dowel locates in the lock groove, the connection between the male connector plate and the female connector is complete. A remaining female connector is secured to the other side of male connector plate to join two female connectors together. (a) Male connector plate assembly and female a pattern are pushed together To disconnect modules a user pushes on opposing compli- ant levers to disengage the locking dowels. He or she then twists the male connector plate assembly in either direction to disengage the screw heads. Modules can then be effortlessly pulled apart. Patterned features in the female connector are typically laser cut steel plates. The male connector plate body is 6mm Acrylonitrile Butadiene Styrene (ABS) which includes the compliant lever. A male connector plate’s compliant levers are pushed with a minimum of 40.N of force to unlock the locking dowel pin. This value was obtained using CAD software and built in Finite Element Analysis simulation (Fig. 4). Fig. 4: Complaint Lever (ABS) with Locking Dowel Pin Fig. 5: Early hermaphroditic style connector (V1) (left). ModLock with single direction lock and unlock (V2) (right). A male connector plate assembly measures 60mm on each side. The minimum size the matching female connector can be is also a 60mm sided square. Fig. 6: Tensile (left) and bend (right) testing of the female- male-female ModLock element with Instron 4206 Fig. 7: Plot of two runs of tensile testing lightweight version of the ModLock connector was developed for use on flying vehicles. The compliant portion of the connector is identical to the standard connector, but is only 1.6mm thick. These connectors were built into the design of a laser cut ABS reinforced foam core frame. A lightweight CR module with three ModLock faces was also built. The lightweight CR modules used high-torque, brushless, DC motors and custom controllers. A sensor was added to the motors, allowing for bi-directional rotation and low speed operation. Four to eight of these lightweight CR modules were attached to the frame in various configurations including a quadrotor, an octocopter, and a wheeled vehicle. A working quadrotor with four lightweight CR modules is pictured in Fig. 10b. We did not implement a ModLock connector on the output of the lightweight CR modules due to difficulties in mounting the connector to a propeller. The lightweight connectors experienced no failures from the > 5N tension forces and > 100Nmm of axial torque; however, the locking dowel pins fatigued their press fit hole in the thinner ABS and fell out after a small number of connect-disconnect cycles. B. Backward Compatibility Fig. 8: CKBot Modules with ModLock (a) Hexapad (HP), Linear Actuator (LA), and Angled Framing (UF) Fig. 9: CKBot passive attachments fitted with ModLocks (a) Hexapods built from CKBot modules fitted with the ModLock Fig. 11: Bi-bender male connector plate and application (b) Modular quadrotor with one disconnected lightweight CR module
