Project Objective: Develop power-saving solutions for aerial drone surveillance of small UAVs by inventing a robotic tail that allows the drone to perch on branch-like objects without continuous battery use. Our team designed a tail capable of wrapping around a tree branch, solidifying, and supporting the drone's weight without significantly impacting its limited battery power.
Project Requirements:
Support 5kg Drone
Does not interfere with the drones flying conditions
Tail should have a heating/cooling response time < 5 seconds
Operate within a normal ambient temperature range
Remote control of System
The Design
This project began as an extension of some university researching dealing with continuum robotic manipulators using a low melting point alloy (LMPA) for medical devices. This design utilizes LMPA metal joints in a hyper-redundant under actuated tail like manipulator. Each joint acts as a point of virtual actuation in which they can be individual locked and unlocked (through the process of heating) and actuated independently using one of two motors. This system was designed to be strong and require low power usage, to preserve the limited battery power of a drone. Most systems that grasp typically are required to be active while doing so, where as this system is locked in its passive state. This implies that the system only needs to be powered while it is moving, which is highly desirable for this and possibly other applications.
In order to utilize this style joint while still developing a functional product, a structure had to be designed around it. The design presented here is how this manifested. The links that house the joints are electro-mechanical components that both serve as structural and logic elements. This keeps the system compact and modular while still maintain strength and functionality. Each joint has a heater which is monitored by a thermistor which is how the system is able to lock and unlock joints in a controlled manner. The individual links each can rotate 120 degrees. Control of this rotation is handed by two servo motors mounted at the base using a pretensioned cable system.
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The Heating System
This is the most important component I had the pleasure to design. Its a liquid metal clutch system that utilizes the LMPA and its surface tension to wet two concentric brass tubes. This provides a very strong temperatures solder joint that at relatively low temperatures can be melted releasing two concentric brass elements relative to each other while acting as a very low friction surface. When not powered the system cools to a frozen state locking the two concentric elements to one another.
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Housing
In order to solve the problem of running long cables to every joint which would be incredibly cumbersome the idea to make the housing modular and self contained was developed. The idea to serially chain and make every joint individually addressable was developed and instead of designing a mechanical joint and adding electronics after the fact the PCBs themselves where designed to both provide the mechanical strength and electrical logic for each joint in the system. This way every going could be wired to the one below it and still be individually controlled. This also (in theory) would allow for heating of several joints on a duty cycle (similar to PWM control) so more complex configurations could be accessed more quickly. With correct turning the system in theory could be brought to a warmed state on the edge of melting to be activated more quickly.
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The Joint
The final product of combining the housing and the heater is a single joint member capable of controlling its own heating element and communicating with neighboring joints.
LMPA Surface Wetting
The key to the design is in the actuator, the biggest break through was the ability to wet the concentric tube with the LMPA. I discovered it was possible through capillary effects to flow the LMPA into a slip fit gap between two brass tubes which acts as a temporary solder joint that can be released at relatively low temperatures. This provides a ridged connection between the tubes when solid, and a low friction connection when liquid. Since the tubes where brass I was also able to solder them into the PCB housing with relative ease. It was an extremely satisfying discovery and really made this project possible.
Project Conclusion
Despite our best efforts, the project did not fully meet all its goals within the available timeline, and we were unable to deliver a final working solution. However, we achieved each individual goal in isolation. The joints proved to be robust, successfully supporting torsional loads from a 5kg robot in various worst-case orientations. The tail was compact and did not interfere with the drone's flight. In lab settings, joint actuation was achieved in under 5 seconds, though this may not be replicable in real-world scenarios. The modular design was promising, but we had to hardwire every joint due to trace line overheating and subsequent failures.
Lessons Learned
When designing complex and small systems, start with a larger prototype to ensure functionality before miniaturizing. I spent too much time dealing with tiny wires and machining delicate ceramic parts that frequently broke. Test electronics early and often; even simple circuits can be complex and time-consuming. Always test your circuits before powering them. Integrating mechanical systems, electronics, and software for the first time often leads to failures. Embrace failure as a learning opportunity to find what works effectively.