Electric Vehicle

The goal of Electric Vehicle is to build and test a vehicle that can travel a given distance in a predictable amount of time while staying in a straight line.

2008 Rules

 * Wheel base (distance between front and rear axles): 40cm +/- 3cm
 * Vehicle width (distance between outermost sides of the tires on the widest axle): max 25cm
 * Batteries: At most 4 cells up to 1.5V (alkaline) or a battery pack up to 4.8V (4-cell NiCd or NiMH pack) can be on the vehicle at once, but two sets (or two packs) can be impounded
 * Distance given will be from 5 to 10 meters, with different intervals at different levels of competition
 * A pointer that is ahead of every other point on the vehicle, that is 1cm or less off the ground, used to determine score

Additional Rules

 * Can not start the vehicle by touching it: Must activate a switch with a "pencil, pen or wooden dowel"
 * Electronic and drive components can be used on vehicles, and can be made by contestants or bought
 * At the national level, the vehicle time prediction will be made by the Event Supervisor, not the competitor

The definitive rules are located in the official rule book. Know your rules front to back. You can never read the rule book too many times.

Competitive scores for EV come within fractions of a point to the full 200 point score.

Scoring
Vehicles are scored based on the following:
 * 1) Distance to the target line from the pointer (up to 100)
 * 2) Discrepancy between target and measured run time (up to 50)
 * 3) Distance to the intersection of target line and center line from the pointer (up to 40)

A bonus of 10 points is awarded if the center line remains between the wheels during the run.

Design
The basic chassis of an EV is fairly rigidly defined in the rules, and is actually fairly similar to Scrambler.

Like Scrambler, it is beneficial to use a vehicle track that is as wide as the rules allow. The further apart the wheels are, the finer adjustments you can make to your turning. In addition, a wider base can score the centerline bonus more easily. After all, even with the max track width and the minimum target distance, the vehicle can not stray more than 1.43 degrees in either direction to score the bonus.

Speed
Unlike Scrambler, however, you don't have to worry about speed. Precision (repeatable performance) and accuracy (close results) are what a good vehicle aim for.

To maximize your time score, you'll want a vehicle that is as slow as possible but still can go the full distance in less than 45 seconds (the max you can predict). This is because the time score is essentially based on percent error (except it is not per cent), and so deviation at a longer distance from the target time will result in a lesser error than the same deviation at a shorter distance.

To achieve a low speed, you'll want two things:
 * 1) Heavy geardown of your motor(s)
 * 2) Small wheels

Electric motors inherently run at very high speeds with little torque. In addition, the greater the load placed on a motor, the slower it will run. So, without gearing, a vehicle would at best slowly accelerate up to a very high speed, and at worst, the motor will not have enough torque to get the vehicle moving.

Smaller, wider wheels are just a matter of convenience. A thin, large, flat wheel is more difficult to secure to a shaft and may cause wobbling.

gh likes to use [| pulleys from Small Parts] with o-rings pulled over them. This kind of wheel is strong, very stable, and mounts to a a commonly used shaft size easily. In addition, the "tires" contact the ground in a single spot, allowing the lower sideways slip benefits of a thin wheel (as opposed to, say, [| fat foam wheels]).

Adjustment
Getting the vehicle to go straight is very important. For most people, this means building in a mechanism that allows them to change the angle between the axles, which changes the direction that the vehicle will curve towards.

Others choose a more complex setup -- their left and right wheels use independent motors, which are ran at different speeds to counteract any imperfections in the frame of the vehicle. This kind of correction might be a bit more susceptible to tilted floors, because it relies on a small amount of wheel slip to enact the turning.

Distance
The vehicle has to account for distance traveled in order to stop. There are three basic ways to do this.

Mechanical
See Scrambler braking systems. However, instead of just locking the wheels, you might consider using actuating a switch that cuts power to the motor(s) and maybe activates a braking system. See Dark Sabre's posts on microswitches.

Timer-based
This is fairly simple. The motor is turned on for an adjustable amount of time and then stopped. This can be done with a microcontroller or a one-shot 555 timer. With a microcontroller, this time can be very exact, down to the milli- or nanosecond by using a delay loop. The 555 timer can be adjusted by using a potentiometer for the R in the one-shot circuit.

Sensor-based
Because using sensors that sense the environment is not allowed by the rules, you are really limited to what types of sensors you can use. A shaft encoder counts revolutions of a shaft, in this case one of the axles of your car. By using a shaft encoder, you can figure out how far the car has traveled and based on that, stop the motors.