Battery Buggy

Battery Buggy is a Division B vehicle event for the 2018 season. It requires participants to design and build a moving, battery-powered vehicle capable of traveling a certain distance and stopping as close to the ending dot at that distance as possible. The device must meet several parameters regarding size, total power used, types of power used (power must be electrical), and safety restrictions.

Ways to Build
Chassis material is up to your own discretion (common choices are wood/plywood, plastic, or metal), but choose a rigid material that you can connect into a tight frame. Be cautious with thin pieces of wood due to warping, which could negatively affect the vehicle's trajectory, causing it to turn. Cars can be three or four wheeled, but four wheels are recommended. Here are some examples

Chassis
The chassis is the main body of the vehicle, on which the wheels, braking system, and electrical components are mounted. A simple rectangle made of thick wood or plywood (available at most home repair stores) typically functions as a good chassis. Any material could be used, but it's best to use the lightest material feasible. It should also be easy to drill holes for mounting components.

Erector sets also work fine for a chassis.

Wheels
Rear/Drive wheels appear to be "lite flight". These cost a little extra, but are considered the best by some. Many others that are less expensive (or even free if you recycle) are almost as good. A larger diameter wheel revolves fewer times to make the buggy go the specified distance. To run bigger wheels you need a higher geared motor. Bigger wheels make the buggy go faster, but are heavier. Smaller wheels revolve more to travel the distance, and should be geared lower. Smaller wheels are slower but lighter.

The wheels collectively must fit within a 30 cm by 60 cm space of any height throughout the entire run. As of 2018, there is a score bonus if the center tape remains completely within the widest wheelbase of the vehicle between the starting and ending points; thus, a wider space between the wheels could help earn this bonus. However, the wheels must be built such that they do not exceed that width at any point.

Distance Measuring
Classic threaded axle/wing nut system. You have to go quite a ways to better this arrangement. As the axle turns, a wing nut travels along its length. As the wing nut nears one end of its travel, it contacts a switch and turns off the motor to stop the vehicle.

Dowel
For 2018, a 1/4 inch or larger wooden dowel is required to be used for measurement. The dowel should be the leading part of the buggy, and should be placed at the front of the vehicle positioned approximately perpendicular to the floor, such that there is 1 cm or less between the bottom of the dowel and the floor. It should also be at least 20 cm long.

Adding the dowel to the existing buggy should not be difficult, but care must be taken to make sure that the dowel does not detach during runs; this would result in a Construction Violation, which results in significant penalties.

Electrical System
Battery Box/Holder: Selected to hold the size/number of batteries used. Battery holders can be as simple as a piece of wood with nails driven into it to hold the batteries in place and make contact with the terminals at each end. They are available from Radio Shack and others.

Batteries: Any size batteries can be used as long as they comply with the voltage and current requirements of the motor and the voltages specified in the rules. Larger batteries are heavier and require a sturdier chassis, stronger motor, more braking force. Normal dry cells (primary cells) can be fairly expensive over time. Rechargeable batteries (secondary cells) require a greater initial outlay, but can reduce costs significantly over time.

Switches: The vehicle circuit above uses separate start and stop switches. A common slide switch, located near the center of the chassis, is used to start the vehicle. This type of switch can require significant force to actuate and may cause the vehicle to move before it is actuated. It may pay to investigate other options that take less force. A lever switch (sometimes referred to as a micro switch) located near the wheel at one end of the threaded axle is used for the stop switch. As the wheels turn, the wing nut travels on the axle until it contacts the lever on the switch and depresses it to turn off the motor and stop the vehicle. Because of the way this switch is being used, it is useful to use a latching switch. These switches are available from Radio Shack and many other electronic/robotic suppliers. Make sure that the switch is also compliant with the rules.

Wires: Almost any electrical wire can be used. A common source is recycled 4-conductor telephone wire. It can be separated into individual wires by removing the outer cover. Solid wire (has only 1 large strand of wire inside) is stiffer and easier to work with, but breaks sooner when flexed. Stranded wire (has several thin strands of wire inside) is more difficult to work with, but doesn't break as easily when flexed.

Motor: Motors determine the voltage and current capacity needed from the batteries and must be mechanically compatible with the transmission/gearbox used. Motors are usually supplied as part of commercially available transmissions/gearboxes, such as Tamiya gearboxes. Another source for gearboxes/transmissions is old (or not so old) battery operated screwdrivers. The batteries may even be recycled if they are still good.

(Note: You might be able to recycle the entire rear axle with transmission and wheels from an inexpensive Radio Control vehicle.)

Dynamic Braking (Regenerative Braking): Uses the drive motor as a dynamo (generator)to slow the vehicle. When the electrical current is turned off, the motor/vehicle begins to coast and acts as a generator. The force required to turn the generator is proportional to the current (not voltage!) generated. When the generator is connected to an open circuit (infinite resistance), no current flows and the force needed to keep the generator rotating is only that needed to overcome friction. If the generator output terminals are connected together with a low resistance connection (short) the current in the circuit increases, which in turn requires more force to turn the generator. This acts as a brake to slow the vehicle.

Turning (Not necessary for 2018)
For some past years, the vehicle was required to have some way of turning. The easiest is to make the back axle at an angle to the front and simply turn the device at the starting point. The angle should be adjustable.

Using a sighting device is not absolutely necessary, but it can help you make a more precise turn. Also, if you use the angled axle turning system, you need some way of making a straight line with the end dot to measure against.

Braking systems
There are multiple ways to brake a Battery Buggy. The easiest way is to use a basic wing nut brake, which is depicted in the image below.



This does not, however, remove power from the drive motor, which you want to do to prevent the motor from stalling as this can melt wires and burn up motors. To do that, you need to have the wingnut hit (and lock against) a momentary switch. If the motor is in a Normally Closed circuit (as it is in the diagram below), when the wingnut pressed the switch, the motor will be turned off. You can also wire the circuit by adding the wire that runs between NO and the (-) side of the motor so that it acts as an eddy current brake and resists further rotation. These measures should provide an immediate and effective braking system that can be reliably calibrated.

For more information on braking, see Scrambler. Note that in the current rules, no part of the vehicle other than the wheels may contact the floor.

The Competition
At competitions, the event supervisors select a "target distance" within 9 and 12 meters. The distance will be in 50.0 cm intervals for regionals, 10.0 cm intervals for state and 1.0 cm intervals for national tournaments. The device must be capable of getting as close as possible to this "target distance". You must also try to have no side to side deviation from the ending dot as you will receive a center line bonus as long as your buggy does not leave the center tape. Participants place their buggy at the starting point, and announce to the judges that their run is about to commence, at which point they activate a switch with a pencil and let the buggy run. Teams aren't allowed to touch their buggy from that point until it stops moving. The team members may not move past the starting line and may not "follow" their buggy. They must wait until the event supervisor tells them to come and retrieve their buggy. Two runs are given, with the score of the better run being used for ranking. Obviously, the closer you are to the "target dot" and the faster you are, the more points you will receive. Because of the nature of this event, a great deal of testing is required before competition. Teams must practice traveling the various intervals and getting the right angle of turn while recording the time it takes to travel such distances, and should also test the buggy on many different surfaces (carpet, hardwood, tile flooring, etc.).

Scoring
Scoring is calculated by adding together the time score, distance score and center line bonus. The time score is the buggy's time in seconds from the .5m line to the 8.5 m line multiplied by 2. The distance score is the distance of the center of the dowel from the target point to the nearest millimeter multiplied by 4. The center line bonus is acheived by keeping the center line withing the buggy's widest wheel base and subtracts 25 points from a teams score. The better of a teams two runs is used, and the low score wins. Negative scores are possible. Failure to impound a practice log will incur a 500 point penalty, and an incomplete log will receive a 500 point penalty. Their are no tiers but penalties for violations. A competition violation has a 1000 point penalty, and a construction violation incurs a 5000 point penalty. Failure to cross the 8.5 m line after passing the .5 m line is considered a competition violation.

Miscellaneous
There are many ways of creating an adjustable system, including, but not limited to, through the use of a threaded rod/nut combination, the use of a limit switch, or simply charging rechargeable batteries to an ideal voltage. Innovation was slowing to a halt, however, so the event was removed in favor of Robo-Billiards back in the 2002-2003 season. Battery Buggy was finally brought back in the 2010 season.

In the 2011 season, the vehicle also had to maneuver around a bucket placed along the track.

Because of the many variables involved in running the car, much testing must be done to figure out exactly what to do, since theoretical calculations do not always match real life.

Links

 * Microswitches
 * Electric Vehicle pics
 * EV thread
 * Note: Electric Vehicle, which was a C event in 2008 and 2009, is similar to Battery Buggy
 * Cheese's Battery Buggy Guide