COLLABORATORS

Sampada Acharya, Felipe Borja, Tuo Wang, Stella Shen

MY CONTRIBUTION

Actuation mechanism + electronics

Project Description

Pillbugs are small crustaceans that can change their shapes according to various stimuli such as the variation of outside temperature, the activity of predators, or the intensity of light. Thanks to their modular cover shells, they can quickly morph into spherical shapes, which we call "conglobation". When the environment changes, they can uncurl back to a relatively flat shape. Here we report a design, fabrication, and modeling method of a robotic cover shell that can perform similar conglobation behavior. With a design inspired by the pillbug, the cover shell is modularized and actuated by a micro servo motor. We also integrate a Kamigami walking robot into the cover shell to demonstrate its potential for locomotion. This robot may help roboticists better understand the conglobation mechanism of roly-polys and its potential applications in robot locomotion and environmental adaptation.

Design Requirements

• As we use an existing robot (about 20cm x10cm x 10cm), our exoskeleton design should be compatible with our base robot’s dimensions.
• The exoskeleton should be able to expand to a spherical shape in two degrees of freedom.
• The resulting spherical robot should be able to curl and uncurl if actuation is added.

Working Principle

The robot is composed of seven major parts: A micro servo motor with customized, 3D printed spool; ten 3D printed hooks; four tension springs with identical length and stiffness; an Arduino microcontroller; ten fully 3D printed shell plate; commercially available thin fishing line; and a Kamigami robot. The Kamigami robot at the center of our design weighs 95.57 g while the actuating shell and the rest of its components weigh 363.33 g combined.

Our design goal is to control the robot curling and uncurling with the Kamigami robot inside the shell. A micro-servo motor that drives the fishing line generates an internal contraction force to actuate the shell. The servo motor is programmed by the Arduino microcontroller, so the users can control the robot curling and uncurling by applying two opposite actions on a joystick connected to the Arduino.

The spring is also a part of the curling and uncurling mechanism. When the servo motor applies tension to the fishing line, the shell extends during the curling process, and the spring extends, exerts contraction force, and stores potential energy. When the servo motor release the fishing line, the spring release the potential energy, exerts an expansion force, and the shell uncurls. This process can be described by F_s = kΔx and E_s = 1/2 k Δx²

Actuation

Selecting a proper location for the servo motor is critical to actuate the shell successfully. Firstly, the servo motor should not block the shell curling process. Secondly, the servo motor with the tethered fish line must provide sufficient torque. Equation τ = γ F sinθ describes the calculation of torque. Where τ is torque, F is the force provided by the servo motor and fish line, γ is the radius from the location of the force to the location of the shell plate, θ is the angle between the force and the level arm. After considering the two constraints, the group mounted the servo motor on the head of the robot.

Fishing line has the benefit of being strong yet lightweight. A fishing line is threaded through cover shell hooks and winded over the spool. The cover shell will curl when pulled by the fishing line. In order to pull the thread, the spool is attached to a continuous servo motor. The motor will automatically stop when the robot closes and it cannot pull the thread anymore. Springs were attached to the exterior of the cover shell. When the robot curls, the springs will store elastic energy for the uncurl motion later. To uncurl the robot, the spool will rotate in the other direction to loosen the thread. The elastic energy stored by the springs will allow the springs to return to their rest position, making the robot straight and uncurled again. We tested several springs with different stiffness. If the spring is too stiff, our motor will not be able to actuate the spool; while if the spring is too soft, there will be insufficient elastic energy to uncurl the robot.

In this prototype, we used a joystick to control the spinning direction of the motor for flexibility in control. All the electronics are powered by Arduino Nano. 

Results

Strengths and Limitations

Overall, we designed and implemented a functional prototype of a roly-poly inspired robot. Our robot has the following strengths and weaknesses. The cover shell is capable of protecting the robot's interior including electronics and any potential storage or payload. The actuation mechanism we chose only involves one motor, which is simple yet effective and makes the whole actuation mechanism lightweight. Since our current implementation leaves extra interior space, it has the potential of integrating more functions.

As a prototype, this work also has limitations. The segmented cover shell is relatively heavy. We could reduce the weight by replacing unnecessarily solid parts of the shells with hollow structures or lighter materials such as cloth, cardboard, or paper. Although the robot achieves curling and uncurling, the motions can be made easier and smoother by reducing the friction between contact joints.

Future Work

There are many possible modifications we are considering to supplement the robot's operational capabilities and enhance its performance. One main priority is reducing the weight and volume of the robot. This can be accomplished by modifying the shell pieces to be thinner or incorporating strategically-placed cutaways to reduce volume. To plug the gaps produced in this manner, we could stretch plastic film or fabric over the cutaways. This would cut down on the weight of a fully-solid PLA shell. Our current iteration of the robot is tethered. We aim to enable untethered operation by adding a battery and moving all processing components inside the shell. We could also add a WiFi or Bluetooth module to the microcontroller to achieve remote control rather than using a tethered joystick. Lastly, we would like to better integrate the walking module within the cover shell to allow the robot to walk with the shell. One method we explored but did not have time to develop was the design of custom, servo-actuated leg modules akin to "spider" robots that are \href{https://www.instructables.com/DIY-Spider-RobotQuad-robot-Quadruped/}{popular DIY robot platforms}. This design would remove the design constraint of incorporating a premade robot within the shell and would also greatly aid in ensuring our locomotion method has contact with the ground at all times.