Payload Delivery Drone
THE PROJECT
For our first of two senior design classes we were instucted to create a delivery drone that brought a payload from one location to another. I ended doing mechanical design for the entire project. The rendering you see to the right is an iteration of my first article where the premise was to integrate all necessary payload mechanisms into the frame itself, creating a integrated, repeatable structure.
Our chosen payload was a water bottle or soda bottle that would be delivered during sports games or events where access to refreshments might be limited.
My initial design approach was to develop a system of compliant prongs that would mechanically mate to the existing frame. As you see in the rendering, these 4 prongs stick out underneath the frame, accepting a majority of water bottle cap diameters by using the lip underneath of the cap of a water bottle to attach. The way this payload would be delivered in theory is that the customer/reciever would grab the water bottle middair as the drone is flying, at which point the drone turns off. The customer pulls the water bottle out of the attachment mechanism and sets the drone on the palm of their hand to reengage it into flight.
4 prongs proved to introduce an issue with necessary compliance which I solved by removing a prong and modifying the rest as you will see explained later on.
DESIGN 2 FINAL ITERATION
In the rendering above and the photo to the left you can see the 3 pronged mechanism and the final iteration of this specific design. Removing one of the prongs allowed the water bottle to slide laterally into the prong pocket. Due to the nature of 3d printed layers, the force on the prongs by the water bottle was acting on the weak point of bonding between layers rather than the material itself. Therefore, this mechanism and implemenation didn't allow for as much compliance as I had imagined. By removing one of the prongs, reinforcing two of the leftover prongs with the wall of the drone frame, and slightly increasing the width at the press point before the water bottle slots into place, I ended up with a working mechanism that held a water bottle in place very well. Removing and reinserting the water bottle was very easy and this frame design was complete.
FINAL DESIGN AND COMPETITION
Above and to the right you'll see renderings as well as a photo of my final design, the one that ended up being used in our final showcase. The story behind this is interesting actually, we had purchased a linear actuator early on to integrate into our drone as a means for electronic release of the payload. A couple days before the final showcase we had a working design (D2O prototype 2) but decided to try and integrate the electronic actuation which was talked about in our ideation phase. Another member of the group was working on a modular design while I was designing the integrated compliant version. His design didn't work out but we as a team decided to take the modular idea and incorporate it into a final version of the drone. I took the role of designing a completely new platform while a team member spent time designing a mount for the linear actuator. When trying to use the linear actuator with our transmitter however we realized that it didn't use the right connecter.
So, two days before the competion we decided to repurpose a servo that was laying around from an another group's project. As soon as we got this servo working with our transmitter I began designing to incorporate the servo into the new design. You can see this in the lower black plate in the rendering directly to the right. The rectangular hole is where the servo slotted into and the two holes were used to mount it in place. The floating circular structure you see just below that is where the water bottle was guided into place while the servo with its attachment rotated into place just under the lip of the water bottle, holding it steady. The servo was mated to a potentiometer like knob on our transmitter, which when turned also turned the servo. By turning this knob mid flight the water bottle was released to the 'customer' in our demo and the drown was flown back to the starting point.
As for general assembly, I used the parts we had available in the short time frame we were given. Having had extra standoffs from the battery mount I used these to act as a threaded hole for screws which entered into the arm from both side through both the upper and lower plate (shown below), holding the entire drone together. For the arms to slot in correctly I used small uniform tolerance which maintained contact at the pointed triangular portions of the arm but had clearance at the inward cut. Multiple points of contact and retention made for a robust end product.
The version you see to the right is a first iteration that worked very well in practice. The whole design was developed in a couple hours and the parts were 3d printed overnight, assembled the next day, and calibrated to completion. Highest rating from the professor was recieved.
FEA Analysis of the modular arm in iteration 2
The vertical ribs are taking most of the stress as shown above, which is ideal. A failure point I came across post-competition was at the end of the arm right before the motor mount location
Failure point at the end of arm that occured during a post-competition test flight. Design modification on top of this would be to extend vertical rib to the end
Servo mounting solution
