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.

Motor Cogging
Pick up a bare motor (no gearboxes or gearheads) and turn the shaft. Do you feel how the shaft tries to "cog" to certain positions? The same thing will happen on the vehicle, and the better and more powerful your motor is, the more prounounced the cogging action is. The distance traveled by your vehicle is quantized (separated into discrete values) by this cogging action. However, gear reduction and small wheels will minimize these effects. You can calculate the how much each "cog" of the motor will affect the distance traveled by the vehicle with this formula:
 * $$D = \frac{w\times\pi}{n{\times}r}$$

Where:
 * D = distance traveled with each "cog"
 * w = wheel diameter
 * n = number of "cogs" in each revolution of the motor shaft
 * r = gearing (or pulley) ratio between the motor and the wheel

As you can see, this effect is only significant for those who are have bought bare motors and built their own (simple) low-ratio geardown system.

Some ways to minimize this:
 * Use a higher geardown ratio
 * Use smaller wheels
 * Use a motor with lots more cogging angles

Braking
Since EV is more forgiving than Scrambler in terms of what you can use, you have many more options in choosing braking mechanisms.

Mechanical Braking
As mentioned later, mechanical braking mechanisms like wingnut systems and others carried over from Scrambler. These, for the most part, are used almost exclusively when the timing mechanism is mechanical as well.

Electric & Electromagnetic Braking
Even if your vehicle uses a low-tech timer, you can still benefit from the rules by using an electrically triggered brake.
 * Microswitches You can cut power to your vehicle using a simple circuit with a microswitch in series with the motor. In the wingnut system, the wingnut would not hit the frame as it would on a Scrambler, but instead hit the microswitch to stop the vehicle.
 * Eddy current brakes Lenz's law states that current generated in a loop by flux will oppose the flux that generated it. A simple application of Lenz's law can be seen in the eddy current brake. Small, flat magnets are mounted to the edge of a plastic disc, with the poles alternating (see this; the magnets would be on the outside of the disk, however). This disk rotates fixed to the drive shaft of the vehicle. Coils of magnet wire would be mounted fixed to the frame of the vehicle, facing the edge of the magnet disc, but not touching. The coil is a single strand of magnet wire wound up into a coil. The two ends are not connected.
 * Since the disc and coil do not touch, the vehicle can move freely. No current is induced yet in the coil of magnet wire. However, once the loop is closed (the two ends of the coil are connected), current is generated within the coil. This current will generate, by Lenz's law, a force opposing the motion of the magnets on the disc, and slow the vehicle down. This loop can be closed mechanically, using a switch or electronically using a power transistor or relay.


 * Motor braking Magnets and coils of wire... what does that remind you of? That's right, an electric motor. You can apply the same principle above to a motor. Simply connect the two terminals of a DC motor, and it begins to resist rotation. In general, larger, more power, and more expensive motors tend to brake better this way. If you are using a motor controller to control your motor, look in the documentation to see if it supports braking. Most that do will automatically connect the motor terminal together when a neutral signal is received. In general, using the motor to brake is better than building a separate eddy current brake unless your motor is severely underpowered compared to your vehicle or if you vehicle has a lot of inertia.
 * Keep in mind that electromagnetic brakes described here are designed to convert motion into electric current, which is then immediately converted to heat. Do not be surprised if your motor or braking mechanism becomes very hot.

To summarize: an EV's motor can effectively serve as its braking system with a bit of circuitry. You do not need to build a separate braking system unless your vehicle is too heavy. Electromagnetic brakes can stop a vehicle without any mechanical parts touching, and generates more braking force for vehicles traveling faster.
 * Motor reversing Simply reversing the motor for about 100ms and then shutting off power will quickly stop a vehicle. This can then be combined with the motor braking to hold position.

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.

Time
The contestants must predict the time the vehicle will take to travel the given distance. The best way to know how long it will take is to run the vehicle at every distance multiple times and record the times.

Stopwatch Method
The simplest way to get the time of each run is to use a stopwatch. There are a couple of things to note about reaction time.
 * 1) Say your reaction time is about 0.3s consistently. Adding or subtracting three tenths of a second from you run's time won't make a huge difference in your score.
 * 2) The effect your reaction time has on your score is decreased even further by the fact that it should take you nearly the same amount of time to react to the car stopping. Say your 0.3s reaction time causes you to start the timer 0.3s after the car has started. It would also take you about 0.3s to stop the timer after the car has stopped. The time is just shifted 0.3s over, it starts later and ends later, but the time elapsed is still the same as the time it took for the car to travel the distance.

Microcontroller Timer Method
Another option, if you're using a microcontroller is to have a timer start right after the motor(s) are started and stop right after the motor(s) are stopped. Once the timer stops, the microcontroller would display the time on a display. tehkubix used a 7-segment LED to flash each digit in the time down to the hundredths of a second. If you can't add a 7-segment LED for whatever reason, you could use a single or multiple individual LEDs. This method requires more attention when reading the display because you may miss a flash. If using just one LED, the controller could flash the light once for each second elapsed in the time. Say the time was 16 seconds and 23ms. The light would flash once, pause for a few seconds, flash 6 times, pause, flash 2 times, pause and then 3 times.

Voltage Regulation
The 2008 rules put a restriction on the maximum voltage of the impounded batteries.

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

Some microcontroller kits (Lego, Vex) require 6 batteries or more to run properly. The rules are very specific about what you can impound, but you can step up or down the voltage outputted by the batteries without violating any part of the rules.

You can get a step up converter for free if you order samples from TI.

To use this system, you would input 6v from the batteries and step it up to whatever your microcontroller and motors require.