Early PolyBot G3 Prototype.

The third generation of PolyBot is currently under construction. Much of the mechanical hardware for 180 segments and 20 nodes has been built. Electronics have been designed and prototyped and software is in development. The goals for this large batch of modules are to demonstrate locomotion with large configurations and reconfiguration between configurations. 

PolyBot G3 Module Design

The target form factor for G3 was a 5 centimeter cube. The G3 module is actually a bit smaller at 50 x 50 x 45mm The main drive was custom designed and built. It uses a modified Maxon 32mm diameter brushless pancake motor as the source with a 3.75:1 planetary gear stage between the motor and the size 8 100:1 harmonic output stage. This new main drive weighs only 70 grams compared to G2's 300 grams bringing the total module weight down from 450 grams to about 200 grams. The G3 drive should deliver 1 Nm of torque and the machined aluminum frame has a range of motion of +90 to -90 degrees. In addition, an actuated roller ratchet will provide 10-15 Nm of braking in either direction.

The two connection plates on either side of the module are identical, hermaphroditic and have a 4 way rotational symmetry. That is, any two connection plates may be attached together at 90 degree increments. To connect the plates four grooved pins enter four holes on the opposing plate and are grabbed by a latching mechanism that can be later released by a shape memory alloy actuator. Each face has 4 times redundant custom made hermaphroditic electric connectors to enable power and communications to be passed from module to module. Each face also has four IR LEDS and sensors for face to face docking during reconfiguration and rudimentary module-to-module communication. This communication is used during initialization by the robot to discover its configuration.

Each module contains a Motorola PowerPC 555 embedded processor with 1 megabyte of external RAM. This is a relatively powerful processor to have on every module and its full processing power has not yet been utilized. The final goal of full autonomy may require the use of these processors and memory. Each module communicates over a local bus within chains of segments using the (controller area network) CANbus standard. The six sided nodes will have switching and routing capability to pass messages from segment chain to segment chain.

In G3, the sensing includes the hall-effect sensors built into the brushless DC motors serving both for commutation as well as joint position with a resolution of 0.04 degrees, an absolute joint angle sensor (custom potentiometer),  four accelerometers (one redundant, but can frequently be used for joint angle sensing) for measuring orientation relative to gravity and potentially contact bumps, contact whiskers, and low resolution force sensors on the interface pins. In addition, due to the placement of the IR components on the G3 interface plates, these components may also be used for proximity sensing.

G3 Demonstrations

IR tracking demonstration

• Prototype light tracking