Actuator Molecube Design Overview

From molecubes
Revision as of 17:08, 11 September 2019 by Victor Zykov (talk | contribs) (→‎Mechanical Design)
(diff) ← Older revision | Latest revision (diff) | Newer revision → (diff)
Jump to navigation Jump to search
Molecube actuator design specifications.
Mechanical
Dimensions (L x W x H) 66 x 66 x 66 mm
Weight 200 grams
Torque, max 49.5 kg-cm
Rotation Continuous 360°
No-load speed, max 100 deg/sec (17 rpm)
Structural material Printed ABS plastic
Swivel axis bearing Single, thin section, X-type
Actuator type Robotis AX-12 or AX-12+
Drive type Spur gearbox
Overall reduction ratio 1:762
Back-drivable Yes
Robot Interfaces
Mechanical interface 4-way sym. Pin-to-Socket
Module retention method Interference fit
Insertion / separation axial
force for interference fit
~200 N
Reconfigurability Manual, external
Continuous current capacity
of the power supply bus
8A DC
Power
External DC power supply 12...24 VDC
Current consumption at max torque 1 A
Sensing
Module orientation relative
to attached modules
Yes, 4 orientations
Servo position Yes, ~1.5 degrees
Servo temperature Yes
Controllers
North half controller ATMega16
South half controller ATMega16
Servo controller
(embedded in AX-12)
ATMega8
Communications
Internal serial protocol Half Duplex Asynchronous,
8 bit, 1 stop, no parity
Internal serial link RS-232
Internal serial link speed up to 1 Mbps
Inter-module comms protocol RS-232
Inter-module comms speed up to 1 Mbps

The design of the actuator Molecube module includes mechanical, electrical / electronic, and software design components. The pages in the "Actuator Molecube" section of this wiki (see sidebar menu on the left) describe all these design components and provide associated building, testing, and programming instructions. Microprocessor software source code and documentation are available in the supporting GitHub repository.


Mechanical Design

The design of an actuator module[1] is inspired by our earlier successful design that was used to demonstrate physical robotic self-replication[2][3][4]. The original Molecube design of 2004 has been miniaturized, simplified, and ruggedized.

Figure 3. Servo case cutout exposes its internal gear for torque transfer. (Servo cover removed.)
Figure 4. Actuator gear meshing.
Figure 7. Actuator Molecube inter- and intra-module communications diagram.

Each robot, as shown in Fig. 1, has a shape of a cube with rounded corners and comprises two triangular pyramidal halves connected with their bases so that their main axes are coincident. These cube halves are rotated by the robot motor about a common axis relative to each other. Each of the six faces of a robot is equipped with an electromechanical connector that can be used to join two modules together. Symmetric connector design allows 4 possible relative orientations of the two interconnected module interfaces, each resulting in different robot kinematics. The major mechanical parts of the module are shown in Fig. 2.

Each Molecube has six major structural parts that are shown translucent in Fig. 2 and can all be manufactured from ABS plastic using the available design files and a fused deposition rapid prototyping machine, for example, Dimension SST or similar.

The remaining major components are standard stock parts: the AX-12 servo, thin section X-type bearing and the electrical collector to enable continuous module rotation. The AX-12 servo occupies the largest volume inside of the module. Its body dimensions limit the smallest possible dimensions of the Molecube. To make the module as small as possible, and improve the torque transmission from the servo to the robot halves, the servo was modified as shown in Fig. 3. The front wall of the servo is removed, and the output internal gear of the Molecube is meshed directly with the output spur gear of the robot that is located inside of the servo body. This type of meshing allows the servo output shaft to be supported in the bearings positioned on both sides relative to the shaft's torque transmission point, thus reducing the gear deflection, wear, and the chance of gear disengagement under load.

Figure 5. Molecube electromechanical interface.
Figure 6. Molecube 4-way symmetrical electrical interface.

Connector Interface

Both mechanical and electrical interfaces are pin-and socket type as shown in Fig. 5. High mechanical retention force between two joined modules is provided by the interference fit of the eight interlocking pairs of ABS pins and sockets. 16 pairs of electrical pins and sockets provide 8 redundant channels of electrical connection for passing the ground, power, and communication signals, as well as dedicated processor inputs and outputs for robot orientation sensing relative to its neighbor modules, as shown in Fig. 6.

Control and Communications

Each of the two halves of every robotic module is equipped with one Atmel Mega16 microprocessor. Both microprocessors are connected to the global TTL level half duplex RS232 bus, to which all other joined actuators, controller, and other add-on robotic modules are connected. Inside of the robot, there is another communication line of the same type, to which both processors and AX-12 servo are connected. Any of the two microprocessors can be used to control the servo motor, however both are necessary for correct and complete sensing of the module orientation relative to its neighbors. The schematic of inter- and intra-module communications is given in Fig. 7. For both internal and external half-duplex RS-232 buses, the speed of communication is programmable up to 1 MBps.

References:

  1. Molecubes: An Open-Source Modular Robotics Kit by Victor Zykov, Andrew Chan, and Hod Lipson, in proceedings of Self-Reconfigurable Robots workshop at IROS-2007
  2. Robotics: Self-reproducing machines Nature 435 (7039), 163
  3. Cornell New Service: "Simple but seminal: Cornell researchers build a robot that can reproduce"
  4. Evolved and Designed Self-Reproducing Modular Robotics by V. Zykov, S. Mytilinaios, M. Desnoyer, H. Lipson, IEEE Transactions on Robotics, vol. 23, pp. 308-319, 2007.