Mechatronics Mobility Aid
Objective and Ideation
Our team was tasked with finding and solving an issue within our community by implementing an array of electronic components via a microcontroller.
Pittsburgh is a vibrant city with a diverse population. It has a rich history as a frontrunner in innovation, beginning with the boom of the steel industry in the 19th century. However, the expedited development of the city during that time has since slowed and we are facing an infrastructure crisis.
My team and I were crossing the street one day after class and one of us tripped over a curb. After she shook off the embarrassment, we scoffed at the poor road work and went about our day. This was a minor inconvenience for us, but for certain populations, this is a major hassle. We decided to address this by designing and constructing a mobility aid for children in wheelchairs living in Pittsburgh and beyond.
The broken, trash-ridden sidewalks and driveways of Pittsburgh’s South Oakland neighborhood, just off the University of Pittsburgh’s campus.
We wanted to create a portable ramp that could be placed on the ground and extend over a curb. This would be no small task, as most retractable ramps are large and clunky. We narrowed down the target users to children who use a wheelchair so that we could confine the model to a small scale.
Due to the tight timeline of the class, the scope of this project was geared toward the technical details, rather than human-centered market research. In other words, we were building the product before we knew that it was the right product to build. This was mostly a demonstration of proficiency with mechatronics elements. We decided on a mini rover that the child could hold with them, like a pet toy. The child could carefully drop it to the ground, where it would begin moving toward a curb. Then, the ramp would retract, and the child could ascend the incline.
CAD Model of the rover showing its underside with the scissor mechanism
3D Modelling and Electronic Components
After many iterations, we designed the final CAD Model you see here and began writing the C++ program for our ATMEGA328P microcontroller and its components. Here’s how it works:
There is a front and back chassis, each with their own motor to operate their respective set of wheels. The middle set of wheels is only for support. The user would press a button on the top plate when the rover is securely placed on the floor. This would cause both motors to move forward at the same rate until a curb interrupts its path. A button on the front face of the rover would be pressed upon encountering the curb. This would trigger the back motor and front motor to rotate in opposite directions, thus pulling out the ramp via the scissor mechanism. A distance sensor near the middle set of wheels measures the distance between the two chassis to convey how far the ramp has extended. The sensor tells the system to stop when the ramp is fully extended. The program then enters a long delay to allow the user to safely utilize the ramp before it closes.
CAD Model of the rover in its extended mode
CAD Model of the rover in its compact mode
The rover fully extended
Gear transmission and mounted front motor
We worked on the electrical components of the rover simultaneously with the mechanical components to ensure smooth sailing at each step. The system used three breadboards, three 9V batteries, one ATMEGA328P microcontroller, two H-bridge chips, two pushbutton switches, and a multitude of other simple circuit elements. The first pushbutton switch was mounted on the front face of the front chassis to detect when the rover was coming in contact with a curb. The second pushbutton switch was mounted on top of the ramp plate to detect that someone was on top of the ramp.
Distance sensor mounted to the back of the front chassis
Next, we ran some speed and torque calculations to approximate the specifications needed for both motors to meet our design parameters. This guided our selection for the DC motors we installed. To properly situate the motors and transfer the torque, we laser-cut two gears (1:1 ratio) from vinyl for each motor. One gear was attached to the motor shaft while the other was attached to the axel. After many roadblocks to 3D printing a motor mount, we settled on screwing the motors down.
The rover fully compacted
Side view of the scissor mechanism
The distance sensor was mounted to the end of the front chassis to measure how far the scissor mechanism was pulling out. This was done via a white piece of paper placed on the back chassis in line with the sensor. When the sensor and white paper were at a specified distance from each other, the ramp was fully extended and both motors would stop.
Bringing it to Life
Our first step when transitioning from CAD to a physical prototype was laser-cutting the chassis, ramp components, wheels, and scissor mechanism elements from 1/4 in. plywood. We utilized puzzle piece edges so that the side walls could slot in supported by wood glue. The scissor elements were connected with bolts and nuts. We carefully adjusted the torque on the bolts to ensure that the scissor mechanism could pull out smoothly without wobbling from side to side.
Internal circuitry
Once the rover was fully assembled, we added some final embellishments. Rubber bands were wrapped around the wheels for non-slip grip. The face contributed to the marketing of the product as a mobility aid for kids disguised as a toy.
The team performed a successful live demo for our professor that received a perfect score. We also provided documentation for our code, materials used, motor selection analysis, and wiring diagrams. As a whole, the project received an A+.