Overview of Actuator Molecube Parts
This page describes how to obtain all mechanical and structural parts needed to build an Actuator Molecube. Please note that we describe the procedures for the manufacture and assembly of all actuator Molecube circuit boards on the following electronics design page. Actuators are the most complex of all Molecubes mechanically as each of them includes moving parts, a servo motor, and a motor controller to enable robotic movement. One you've put together an actuator, building other types of modules will be much easier.
To build one actuator Molecube, you will need the following parts (this list excludes the circuit boards):
|Part Description||Manufacturer||Manufacturer's P/N||Quantity||Cost, USD||Notes||Vendor(s)|
|Rapid-prototyped structural ABS parts||Custom||N/A||1 set||$50||Estimated material cost for 3D printing||Laser Reproductions, etc.|
|Dynamixel AX-12 or AX-12A Robot Actuator||ROBOTIS||902-0003-001||1||$45||cost estimate for purchases of unit quantities||Tribotix, Mouser, Robotshop inc|
|Thin section precision ball bearing||National Precision Bearing||KAA17XLO||1||$40||cost estimate for purchases of 50 pcs or more||Impact Bearing|
|Slip ring capsule electrical collector||Moog Inc||ac6373-12||1||$94||Cost estimate for purchases of 50 pcs or more||Blacksburg Operations|
|Main axis potentiometer||Piher||N-15||1||$3||Cost estimate for purchases of 50 pcs or more||Mouser Electronics|
|Flat flexible 50-conductor cable 20 mm long||Wavelink LLC||050-050-0022-aa||4||$10||Cost estimate for purchases for 50+ modules||Picasales, Digikey, etc|
3D-Printed Chassis Components
|Each actuator Molecube needs two identical robot shells with snap sockets, one internal half that holds the motor with a retainer bracket / top casing, and one internal gear holder with the internal gear itself. In our case, we 3D-printed all these parts from ABS filament using a Stratasys SST fused deposition machine. All Solidworks designs and corresponding STL files posted here have the dimensions and tolerances tuned to work well with Stratasys SST. You can print all plastic parts directly using the STL files from the gallery below. To modify the files, please use their Solidworks models.
There are two ways to print these parts: individually, or as a full set. If printed individually, different parts can be printed in different colors. Alternatively, all ABS parts for one actuator Molecube can be printed together at once using this STL model or Solidworks assembly file. If you do not happen to have a Dimension SST or another 3D printer handy, it should be fairly easy to get these parts printed at a nearby Makerspace or a job shop, as most of them now offer 3D printing services.
Dynamixel AX-12/+/A Servo Motor
|Dynamixel AX-12, AX-12+, and AX-12A actuators are compatible with the actuator Molecube design. The specs for AX-12A are here. Molecubes have been designed to support continuous module rotation, including arbitrary angles that can be less or greater than 360 degrees. Inside of the Molecube, the output shaft of ax-12 is geared down with the ratio of 1:3 in order to increase the output torque of a module. In this way, in order for an actuator Molecube to make a full 360 revolution, the servo has to make a 3 x 360 = 1080 degrees revolution.
Dynamixel AX-12 series actuators are available from several vendors, including the following:
Thin Section Precision Ball Bearing
|Molecubes are built with KAA17XLO thin section precision ball bearing 1.75" x 2.124" x 0.1875". This is an X-type ball bearing which means that its every ball has four points of contact with the bearing races: two points of contact with the inner race and two with the outer race. Such bearings can support combinations of axial, radial, and moment loads. This allows us to only use one X-type bearing in the design of an actuator Molecube to make it sufficiently rigid against various combinations of axial, radial, and moment loads that might occur during the robot operation. We bought our bearings from this vendor:
Continuous Rotation Electrical Collector
|AC6373-12 slip ring provides 12 electrical power and signal transmission lines across the continuous rotation joint between the two halves of the actuator Molecube. Slip ring (a.k.a. electrical collector) allows modular robots to revolve endlessly in any direction while transmitting electrical power and communication signals through all connected modules, including the continuously rotating actuators. Without slip rings, the opposite sides of the rotating actuator could only be interconnected with electric wires. However, such wires would twist and eventually tear in parts under continuous module rotation. For our project, we bought these slip rings from the following vendor:
Main Axle Angle of Rotation Sensor
Piher N-15 is a rotary potentiometer that measures the absolute angle of rotation of a molecube actuator. This angle of rotation sensor is mounted on the main actuator axle into a small clearance between the servo cap and the slip ring limiting surfaces highlighted with green on the right. Several characteristics of this device make it virtually irreplaceable. As the Solidworks model on the right shows, there is very little space available in the actuator Molecube for embedding an angle measurement device. The only place where a sensor can be joined with a rotating module shaft is between the cap of the servo (of the south module half) and the electrical collector (of the north half). The clearance between these two parts is only 7.47 mm. For reliable operation, an actuator module needs to sense its absolute angle anywhere within its full 360 degrees range of continuous rotation. The only way to provide such sensing capability without an encoder (which would not fit within the 7mm clearance and would double the overall module cost) is to use a potentiometer with a wide enough measurement angle so that its small insensitivity region could be covered by the measurement range of the internal servo sensor. We bought this unit from:
Internal Connection Flat Flex Cables
|Each Molecube is equipped with several miniature internal printed circuit boards (PCBs). These PCBs host Molecubes' internal micro-controller units (MCU), power conditioning circuits, sensor interfaces, motor controllers, RGB LED drivers, and custom inter-module power and communication interface connectors. The PCBs are integrated immediately beneath the Molecube shells, so that the inter-module electrical connectors protrude outwards sufficiently to ensure reliable contact with the neighboring modules.
For connecting the internal micro-controller unit (MCU), electronic interface, and motor controller boards comprising each individual modules with each other at a right angle we used flat 22 mm long 50-conductor cables with a conductor pitch of 0 5 mm most of 50 conductors are used to transfer power and ground signals and the rest are used for numerous sensor signal as well as communication vendor s contact info wavelink llc po box 4358 manchester nh 03108 phone 603-606-7489 or 603-216-5401 fax 603-626-7718 email cdobens temcotool com picasales
Next, proceed to Building Molecube Interface PCBs