Automated Material Transfer Mechanism
In my 3B term at Waterloo, there was a group design project to build some type of object transportation device. 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 motors. I also contributed by 3D-Printing a lot of the parts found in the final product since I am also trying to specialize in 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. However, we did have to choose two objectives for our design. The first was optimizing for 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 subsystems to deliver the object from the pick-up platform to the two drop-off locations. Our solution was inspired by material transfer mechanisms commonly found in industrial settings.
See here for a video of our design!
1. Pull Sweep
We called this mechanism the pull sweep which is an arm that translates horizontally. It is made of a steel laser-cut rack and pinion that is driven using a NEMA 17 stepper motor. The purpose is 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. The carriage of the pull sweep uses rollers and an extrusion to keep a smooth horizontal translational motion. There are limit switches placed at either end of the extrusion that trigger the pull-sweep's motion to stop.
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 winch mechanism with a cable that winds around a spool to raise and lower the object. The winch is operated by a NEMA 23 stepper motor. The elevator has its own 3D printed carriage underneath with rollers. These rollers are constrained by the two vertical extrusion supports that hold the elevator motor at the top. This allows the elevator to have a smooth vertical motion and supports the entire assembly. Lastly, limit switches are placed at the top and bottom of the vertical extrusions to stop the elevator at either position.
3. Slide
The last subsystem of the design is a slide that transports the object from the top of the elevator down to 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 mounted below the slide. Depending on the position of the servo motor, the diverter arm will direct the object falling down the slide to either 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 and NEMA 23 stepper motors were used respectively. 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 as it provides the logic 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 the device is set to
- Set the diverter arm to the correct position using servo motor
- Lower the elevator
- Reach pull sweep out
- Pull the object onto the elevator from the pick-up platform using pull sweep
- Raise the elavtor to the top of the slide
- Push the object onto the slide
- Let gravity do the rest
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. 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 3kg strength requirement, the parts needed to have relatively 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 stepper 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 did some mechanical design for parts that integrate with electrical components 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. Resulted in a 93% in the ME 380 course.