ME 380 Design Project
In my 3B term at Waterloo, there was a group design project to prepare students for the fourth-year design project. Due to my experience with Arduino and desire to expand my skills, I chose to do the electrical portion of the project. This meant primarily working with mechatronic components such as the motors. I also contributed by 3D-printing a lot of the parts found in the final product since I also enjoy mechanical design and additive manufacturing.
The Problem
We had to work in groups of 5 to build a device that could move an object 30cm to two separate drop-off locations. This seems very open-ended and that was the point as to not limit the solution space, but we did have to choose two design objectives. The first was strength to cost, meaning we had to move a 3kg object within a $300 budget. We also chose the compliance objective such that the device has to move any shape or textured object that can fit within a 7cm cubed volume.
Our Solution
We created three systems to deliver the object from the pick-up platform to the two drop-off locations. Our solution was inspired by material haulers or transportation devices found in industrial settings.
1. Pull Sweep
We called this mechanism the pull sweep which is an arm that is attached to the elevator and translates horizontally using a steel laser-cut rack and pinion. The rack and pinion is driven using a motor and the gears on either side are attached using a rod. The pull sweep is able to pull an object off of the pick-up platform and move it onto the elevator platform. Then at the top of the elevator, the pull-sweep continues its motion pushing the object onto a slide.
2. Elevator
The elevator does exactly what it sounds like. It raises the object and pull-sweep from the pick-up platform up to the top of the slide. This elevator uses a cable that winds around a spool to raise and lower the object. The platform that the elevator raises, has a carriage underneath with rollers. These rollers are constrained by the two vertical extrusion support structures that hold the elevator motor at the top. This allows the elevator to have a smooth vertical motion and supports the entire assembly.
3. Slide
The last subsystem of the design is a slide that transports the object from the top of the elevator down to the two drop-off locations. The slide splits into two paths about half-way down which is where the diverter arm sits. The diverter arm is attached to a servo motor below the slide that rotates 90 degrees. Depending on the position of the diverter arm, the object will go to one of the two drop-off locations.
My Contributions
Motor Selection
None of the subsystems would work without the three motors for the pull sweep, elevator, and slide. For the pull sweep and elevator, NEMA 17 stepper motors were used. These motors have high torque and low rpm which suits our application. They can also micro-step meaning you can accurately stop and start the motor so that the position of the elevator and pull sweep is extremely precise and reliable. Torque calculations were performed to ensure the motors could lift the 3kg object. These motors required a power supply and motor drivers to operate. The diverter arm used a servo motor because it didn't need much torque or speed but rather accuracy when rotating 90 degrees. They were also a good way to save on cost since they only rotate 180 degrees which is all we needed.
Circuit Design and Programming
I got to continue working with Arduino using it as the logic controller for this project. The process for the Arduino was as follows. Take user input from a pushbutton. Turn on an LED to show which drop-off location it was set to. Move the servo motor on the slide to the correct location. It would then lower the elevator, reach the pull sweep out, pull the object in, raise the elevator, and push the object onto the slide. One issue I had was how to ensure the motor stops and starts at the required locations. As I mentioned, we were counting the steps of the motor and micro-stepping to have an accuracy of 0.1mm. However, there are often mechanical inconsistencies such as gears skipping or the cable being caught on the spool for a brief moment, and this would cause the pull sweep and elevator to end up in the wrong location. I added limit switches to the elevator and pull sweep so that it gets contacted at the end of the desired motion. This made our design far more reliable.
3D-Printing
Since a lot of the components for the pull-sweep, and elevator platform required high precision, we chose to 3D-print these parts. I was in charge of the slicing and printing process while also designing a few parts myself. Due to the strength requirement, the parts needed to have some strong mechanical properties. I increased the strength of the parts by raising the infill% and having more perimeter walls. This was more than sufficient to hold 3kg and the motors. I designed the three motor mounts myself in SolidWorks that attach to the pull sweep, structural extrusions, and slide.
Takeaways
Through this project I got to improve my electrical circuit design skills. I also got to work with stepper motors and limit switches for the first time. I got to do some mechanical design and learned how to improve the strength of 3D-printed parts. I also spent a bit of time in the machine shop, making the sheet metal components that can be found on the slide and elevator. We did very well on this project and did extensive reliability testing. Our design lifted a max of 5kg and delivered the object to the final location on 99/100 tests.