Battery Buggy

Overview
Battery Buggy is an event that requires participants to design and build a moving, battery-powered vehicle capable of traveling a certain distance and stopping as close to that distance as possible. These devices need to meet many parameters, such as size, total power used, types of power used (which has to be electric), and safety restrictions. The "buggy" must be designed with an adjustable mechanism of some sort to allow for different distances to be traveled. Battery Buggy was revived for the 2010 season after many years of inactivity or being an occasional trial event.

Ways to Build
Chassis material is up to your own discretion (common choices are wood, plastic, or metal), but choose a rigid material that you can connect into a tight, square frame. The closer the frame is to having 90 degree joints and the less bending occurs during travel, the straighter the vehicle will travel. Ensuring that your wheels and axles are uniform, straight, and identical is also important. Using a carpenters square is the easiest way to ensure your axle placements are exactly 90 degrees. Here are some examples

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.



Needs of a Buggy
Chassis: a simple rectangle of 1/4 in thick wood, available from most home repair stores. Any material could be used, but it's best to use the lightest material you can. It's also easy to drill holes for mounting components. Erector set also works fine.

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 heaver. Smaller wheels revolve more to travel the distance, and should be geared lower. Smaller wheels are slower but lighter.

Drive axle/Transmission: One of several Tamiya gearboxes available. Assembled straight out of the box without modification.

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

Motor: The gearbox/transmission usually dictates what motor can be used. Motors are supplied with the Tamiya gearboxes.

Battery Box/Holder: This vehicle uses a common battery holder that allows for the batteries to be seen. They are available from Radio Shack and others.

Steering: This is pretty basic, an articulated chassis with a vertical bolt to hold the two parts together and act as a pivot point. A longer wheelbase is easier to make go straight. Use a wing nut for easier adjustment.

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.

Sighting Device: Not absolutely necessary. However, it can help your buggy go straighter.

Brakes: None. As long as speeds are relatively slow, brakes are unnecessary. It would be fairly simple to add "Dynamic Braking" to this vehicle.

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.

Batteries: Any size batteries can be used as long as they comply with the voltage and current requirements of the motor. 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. These switches are available from Radio Shack and many other electronic/robotic suppliers.

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. 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.

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.

At the Competition
At competitions, the event coordinators select a "target distance" within a pre-determined range (check the rules), and your device must be capable of getting as close as possible to this "target distance" while maintaining a straight line of motion (no side-to-side deviation). The possible distances are rather spaced at regional competition (0.5 meter intervals when I did it) but shrink gradually as you move on to nationals (0.01 meter intervals when I did it). Participants place their buggy at the starting line, and announce to the judges that their run is about to commence, at which point they 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 distance" 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 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.).

Misc
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.

Links
Microswitches

Electric Vehicle pics

EV thread
 * Note Electric Vehicle was a C event in 2008 and 2009, it is similar to Baterry Buggy