What is it?

RoFI Connection Mechanism (RoFICoM for short) is an open-hardware connection system designed for the RoFI Platform. RoFICoM focuses on reusability in order to make it easily integrable into any module for which RoFICoM provides communication to other modules. As the name suggests, its primary purpose is to (inter)connect modules. To achieve this, RoFICoM has a well-defined interface that can be divided into three parts:

  • mechanical structure,
  • connector-to-connector interface (communication between two RoFICoMs),
  • connector-to-module interface (communication with the integrated module).

Mechanical structure

RoFICoM is built from 3 main parts: body, clip and skirt.

The body is non-movable part that is mounted into the module. Clip and skirt are the most important parts for establishing strong mechanical connections between two RoFICoMs. The clip component consists of four latches attached to a ring. Those latches get attached while connecting with other RoFICoM. While expanding, the clip gets rotated and expanded upward by the motor. In the process, the latches on the mating side slide under each other, therefore forming a mechanical connection that prevents connectors from being pulled apart. However, the hooks themselves are insufficient for a mechanical connection, as a mutual rotation of the connectors releases the connection. This is why the skirt is needed. Two connected skirts stop the rotation against each other.

In order to help and/or ease the process of connection there are magnets placed in the holes of skirt. This helps when two connecting sides are slightly misaligned.

The motors that rotates the clip and the skirt are located at the bottom of the body. To rotate the clip and the skirt there is another component, the gearing, that is connected to the clip with a small steel pin that spans from the holes in the body up to the skirt. When the motor activates, it rotates the gearing with the two components and the steel pins move the interconnected components among the hole path in the body, elevating the components.


  • compact, flat layout (diameter 50 mm, thickness 17 mm)
  • data and power sharing between connected units
  • load capacity
    • normal direction: 110 N
    • shear direction: 50 N
  • actuation time: less than 0.6 s
  • genderless, 90-degree symmetrical with automatic orientation detection

Connector to Connector Interface

Connector to connector is done by small PCB inside skirt with spring-loaded pins for connection. This PCB provides both data and power sharing to the connected module. For data communication RoFICoM uses UART transferring frames with specified format and CRC. Those frames hold “blobs” - binary data that is transferred between modules.

Connector to Module Interface

RoFICoM provides SPI interface for communication between module and RoFICoM. The communication on the bus is performed in transactions. Transaction format consists of command, which is first 8 bits of transaction, followed by command data, then at least 8 bytes pause with connector response. Commands 0-127 are reserved for the base connector and are required to be supported by all connector types. For example some connector commands are: version command, status command, send and receive earlier mentioned blob.


RoFICoM utilizes many peripherals to accomplish its goals. The currently used peripherals are: SPI (Serial Peripheral Interface), UART (Universal asynchronous receiver-transmitter), I2C (Inter-Integrated Circuit), motor, power-sharing and lidar. Lidar is device used for measuring the distance from RoFICoM, and communication with lidar is utilized via the I2C protocol. The power-sharing provides connection between RoFICoM’s current module and mating side’s module powerlines and measuring the current and voltage on powerlines.

Motivation of RoFICoM

Since the RoFI platform is grid-based, it results in several requirements for both the platform and, more importantly for us, the docking system. In order to answer “Why do we need it?” let’s walk through the requirements, their cause and how RoFICoM implements them.

RoFICoM has a flat design with ability to expand and retract. This allows RoFICoM to connect over empty space between adjacent modules. As result modules don’t have to be physically touching to be able to connect.

RoFICoM has 90-degree symmetrical design, this is best seen in figure below on the skirt design. Therefore it enables RoFICoM to connect in four different orientations (0°, 90°, 180°, 270°) and makes the connector genderless.

RoFICoM forms strong mechanical connection using previously mentioned clip and skirt. This feature not only helps RoFICoM stay connected under pressure, but also remain connected without electricity consumption or to avoid limitations of magnetic force.

RoFICoM is designed . This design is accomplished by the clip and skirt, which enables connection between mating sides to be established independently of each other. Additionally this enables the RoFICoM to connect to passive docks.

Last but not least, the docking system must support data and power sharing, which is used to coordinate and charge modules. RoFICoM solves this with a connector-to-connector interface.