Design

Steering Rack Design

The steering rack is the interface which allows the motor to turn the wheels. The motor rotates the pinion shaft which has the pinion gear on it. The pinion gear drives the rack gear, making the rotary motion of the pinion shaft into linear motion to push the tie rods in and out. The rack gear has 3/8-24 threads tapped at each end to allow for heim joints to attack to the rack gear. Our custom steering rack design allows for grease inside the case to lubricate the pinion and rack gears. Our design incorporates a grease shaft seal to keep the grease inside the case and the dirt and dust outside. The team incorporated linear bearings on each end of the case to support the rack gear. These linear bearings keep the rack gear aligned with the pinion gear while also supporting the rack gear from loads transferred into the rack from the suspension system.

Steering Column Design

The steering column is necessary to allow the driver to turn the wheel, which will be encoded by a potentiometer, which will be located on the bottom end of the shaft. The range of motion of the steering column must be controlled, to achieve the desired steering sensitivity, and to prevent damage to the potentiometer if the shaft is turned to the end of its rotation. To limit the motion while maintaining space on the shaft end opposite the steering wheel for the potentiometer, we have introduced a second shaft, we call the "layshaft," which is connected to the column with two spur gears. The gear ratio is 1:1, but the exact ratio is not critical. The layshaft spins on two ball bearings, while the column uses bronze bushings, and both shafts are prevented from axial movement by shaft collars. The layshaft is tapped on one end, and a bolt is threaded in which moves in a slot. The range of motion is limited when the bolt bottoms out in the slot. On the other end of the rotation, the bolt stops when it reaches another bolt, which is set as an adjustable stop. This bolt is threaded into the tapped end of the bolt slot, and there is a jam nut that can be tightened to prevent movement of the stopper bolt.

Electrical Design

Schematic

Microcontroller and Sensing

Motor Driving

Flowchart

The electrical design shall be responsible for converting the driver input into an analog signal read by a microcontroller to control a motor that drives the steering rack. A 24-volt battery shall be used to supply the motor with power, and a DC-to-DC converter will convert the 24 volts down to 5 volts for the microcontroller. An ATmega328p was selected as the microcontroller for the project.  

To sense the position of the steering wheel, a potentiometer will be placed at the end of the steering column shaft. A voltage will be applied to the trace, and the voltage of the wiper will be read by the microcontroller to determine the rotational position of the shaft. A second potentiometer shall be used to determine the linear position of the steering rack. The microcontroller shall then use linear interpolation to accurately control the motor based on the input on the steering wheel.

An H-bridge shall be used to control the speed and direction of the motor. Two digital output pins on the ATmega328p will swap the polarity of the motor, which would change the direction of the motor, and two PWM output pins on the microcontroller will vary the average voltage across the motor, which would change the speed of the motor.