14.1 Project Proposal Car
Introduction
A programmable car is a type of mechanism where the steering/direction of the car is controlled automatically through mechanisms instead of user input. We were originally inspired by Da Vinci’s car, which was a self-propelled mechanism capable of moving along a preprogrammed path. We wanted to improve upon this design and be able to change the preprogrammed paths. This made us think of other programmable physical mechanisms like the player piano, which plays music automatically by following along the indentations and holes of paper while blowing air through them.
Problem Statement:
Our problem statement is to design standardized, removable 3D cams to allow for complex path following in a programmable car with no user input, while running off a single motor. The final design of our car will require complex position analysis as well as linkage systems to translate the 3D motion of our cam and follower into 2D motion for the wheel turns which simple joints can not do.
Mechanism
What we propose is to design a programmable car that uses one motor to power rear wheel drive (RWD) and a 3D Cam and Follower system that turns the steering wheel. This would give the car the ability to drive and create complex shapes as it turns. In order to achieve the desired motions with a single motor we will need an off-the-shelf differential drive for our RWD to allow our car to make turns properly, a geartrain from the motor to the cam to control torque and speed, and a linkage system that controls our steering from the movement of a Cam and Follower. We will have two designs as our preliminary ideas will be for our prototype, which will just be a 2D Cam and Follower system, following which we will then attempt to transition to a 3D Cam and Follower system, allowing us to create complex paths for the car to follow.
Geartrain from wheels to 3D Cam: For the geartrain from the wheels to the cam, we will need to figure out the velocity and torque needed to turn our steering mechanism as well as how long it takes to make each path. Once we decide the torque and velocity needed, we will be able to derive the gear ratio needed and attach it to a belt and pulley that spins the cam from underneath.
Cam and Follower + slider crank (Prototype): For our 2D design of the Cam and Follower, we will have the follower act as the slider linkage of a slider crank mechanism that will turn our front wheel, guiding the direction of our car. We will need to ensure the mechanism is non-grashof and unable to make a full rotation, which would cause our steering to break, and our car will be unable to turn properly.
3D Follower Steering Mechanism (Final): For our 3D design of the Cam and Follower we will be drawing inspiration from past teams and follow a helical screw model to give our steering the complex shape we want. We will have the Follower following along the helical edges of the screw to trace out its path while also compensating for the increasing length of the follower linkage as it goes down the 3D Cam. The Cam will also need to be “standardized” in the sense that it is able to be swapped out with another 3D Cam that is programmed differently, essentially allowing for path customization for each CAM.
Differential Drive: The main focus of our project is the steering and pre-programmed path it will be able to follow, which is why we are buying an off-the-shelf differential to minimize as many variables as possible when prototyping our steering mechanism. We want to give the Cam and Follower steering system as much focus as possible, while not having to worry about whether the differential we made was stopping us from turning properly.
Proposed Scope
Problems to solve (step by step):
Slider-steering angle analysis
Find out the mathematical relationship between the input slider position and the angle of the steering wheel
3D cam design
As a prototype, design a 2D cam to demonstrate that we could control the steering wheel with the cam and follower mechanism
In the final project, the goal is to design a 3D spiral cam that could record a more complicated path
Stretch Goal: make a program that generates the cam design for an arbitrary input path.
Power train design
Design the transmission gear train for cam motion. Calculate the gear ratio required to simultaneously power both the drive wheel and the cam.
Design and source the power chain components (differentiator, gearbox) to ensure robust performance of the final product.
Manufacturing
The 3d spiral cam and most of the structural components of the project will be 3d printed
Moving parts in the drive train, including the differentiator, gearbox, shaft, wheels, and bearings, will be purchased.
Preliminary Design Ideas
We are starting with an analysis of the steering mechanism. The preliminary design is for the cam - the program, which stores the path the car will take in the profile of its perimeter.
The average diameter, neutral position for the cam, will be 15cm in diameter, or 7.5cm in radius from the pivot point. The radius will vary between 5cm and 10cm, allowing a total linear displacement of the follower of +/-2.5cm. At +2.5cm from neutral (or a radius if 10cm), the steering column will be rotated 45 degrees to the right. At -2.5cm from neutral (or a radius of 5cm), the steering column will be rotated 45 degrees to the left. This translation will be achieved by a rocker-slider 4-bar mechanism, with elastic or spring components to keep the follower in contact with the cam.
Future iterations will include complications to allow for 3D cams, allowing for longer programs.
Grashof- The steering subsystem is a cam-driven rocker-slider mechanism. The rockers oscillation is bounded to +/- 45 degrees by the cam profile, which serves as the “programmable” input constraining the steering angle. The rocker-slider mechanism must be a non-Grashof 4-bar - no portion of it will rotate 360 degrees.
Grubler: The fundamental goal of the steering mechanism is to translate linear displacement into constrained rotational displacement. Therefore, we will design the steering mechanism to be constrained, M=1.