Introduction to Robots

Introduction to Robots

Welcome to the Intro to Robots. This guide is designed as a resource for FIRST Robotics teams to aid in the design of effective robots for the FIRST Robotics Competition. Designing top robots for FIRST takes experience and a certain insider knowhow. There are distinct strategies that make it different than designing robots in general.

This guide will address what goes into a successful FIRST robot, strategies for brainstorming and tips for working around strict FIRST guidelines.

• Drive Train
• Arms and Lifts
• Tips and Tricks
• More Resources

Drive Train

The drive train is arguably the the most important part of any FIRST robot. A robot with a weak drive train, no matter how well engineered, will be ineffective on the playing field. No matter what the game strategy is, there will always be a need to push other robots around. Your goal is to be on the winning side of any pushing match.

Drive Train Selection

The type of drive train you choose for your robot should depend primarily on two things: The nature of the game and the machining capabilities of the team. Try and decide if the game requires a fast and highly maneuverable robot or a strong robot that can push its way around. It is also important that you choose a drive system that can be built both in a timely manner and to the tolerances required for the complexity. You will want your drive train to be finished in five weeks at the very least in order to give your base driver practice driving.

Types of Drive Trains

Four Wheel Drive

This is the most common and one of the most effective drive systems in FIRST. If consists of four powered wheels where the left and right sides are powered independently of each other. Most often this is achieved with an output shaft on each side of the robot coming out of a gearbox. Each shaft has two sprockets which run to the two wheels on each side.

Two Wheel Drive

This is a system in which there are to powered wheels and either two unpowered wheels or two casters. Two wheel drives are significantly less effective than four wheel and are not marginally easier to build. If you do use a two wheel system for some reason, the most important thing to remember is to put as much of the robot weight over the driven wheels as possible. Weight over nondriven wheels reduces pushing power.

Crab Drive

The idea behind crab drive is that each of the wheels are swiveled about a vertical axis as well as being driven by a drive motor. This allows the robot to travel in two dimensions without changing the direction it is facing. The wheel turrets can be all chained together and driven with one motor, they can be chained in two groups of two or they can be independently driven.

The Power Train

The power train is the part of the drive train consisting of the driving motors and their respective gear reductions. The most powerful motors in the kit should be reserved for use here. A good power train uses anywhere from four to six motors. Save your four Chiphua motors for this purpose and maybe even your two Fisher Price motors of you are willing to build a gearbox to match the speeds.

A Note on the IFI Gearbox

The gearbox provided in the kit is distributed by Innovation First, Inc. and is a good way for rookie teams to get an easy and effective drive train up and running. This transmission is far beyond anything FIRST has ever offered in the kit before and has the versatility to accept either one or two CIM motors. You would have to have a pretty good reason to justify using something other than this provided transmission.

Wheels and Gearing for Slipping

The ultimate trick for choosing a gear reduction is optimizing power without wasting any. The faster you gear your drive train, the less torque or "pushing power" it will have. The higher the reduction, the slower you go and consequently the more torque you have. What a lot of people don't realize is that there is a maximum amount of weight your robot can push, regardless of gearing. This magic number is equal to the weight of your robot multiplied by the coefficient of static friction of your wheels. More specifically, it is the amount of load over all driven wheels multiplied by the coefficient of friction of those wheels.

So the idea is to get the most possible pushing power out of your robot without slowing your robot down to a crawl. If you want the most amount of pushing possible then gear the robot to the point at which the wheels would barely start slipping if the robot were to drive up against a wall. Any reduction higher than this wastes power and compromises speed. You can find this ratio either by calculation or by experimentation. If you plan to experiment in order to find it, be sure to have a roller chain as your second stage reduction (beyond the gearbox) in order to switch out different size sprockets. You can gear faster than the slipping condition, but I wouldn't deviate too far.

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Arms and Lifts

Arms and lifts are the part of the robot that usually requires the most engineering. They need to be robust and reliable. Not all robots have arms or lifts and there are certainly varying degrees of complexity. Arms are usually defined by the number of axes it has. Each axis represents one degree of motion. For example, your elbow is a one axis joint because it has one degree of freedom. Your shoulder in comparison, is a two axis joint because is can cause motion on two separate planes. So if you were to make an arm for your robot with an 'elbow' and a 'shoulder' it would be a three axis arm. usually arms require one motor or pneumatic piston for each axis. Sometimes pistons are only considered 'half axes' because they only have two real positions. A lift is kind of like an arm with one axis, up and down.

Arm or Lift?

The best way to approach arm and lift design is to first think about what you want the robot to do. Picture the motion you want it to make without thinking about any specific hardware. Then try to break down the motion into separate simple linear and rotational motions. Things to consider are whether the robot needs to merely raise something higher into the air and dump it or if it needs to specifically place an object or manipulate it.

Types of Arms and Lifts

Top-Mounted Arm

This is the most common arm you will see in FIRST. It is just a rigid arm that pivots about a joint located at the top of a mast. This is a popular design because it takes full advantage of the five foot height limit. A simple arm in this configuration could theoretically reach from the floor to as many as ten feet. If the arm is extendable, it could reach even farther.

Telescoping Lift

When it comes to vertical lifts, there is nothing as robust or reliable as a telescoping lift.

Four Bar Linkage Arm

A four bar linkage is yet another type of arm. It is illustrated by the image at the right.

Motors and Counterbalancing

Just as there are motors suited for use on the drive train, there are also motors suited well for arms. The most popular motor used on arms and lifts is the van door motor. This motor is strong and already has a very low free-speed. Also, it is more resistant to backdriving than most other motors. For a top-mounted arm, you will want about a 5:1 reduction off the van door. Even stronger than the van door motor is the Fisher Price motor with black gearbox. If the Fisher Price is not in your drive train (remember, drive train gets first priority) then consider using it as an arm joint or turret. Another popular motor for lighter capacity arm applications is the window motor. The window motor is the weakest of the three but is the most resistant to backdriving.

Even if you are using a powerful motor on your arm, you may want to consider counterbalancing. Basically this entails reducing the load on the motor placing weight or a spring to counteract the initial weight of the arm. This means the motor only has to be strong enough to lift whatever object it is picking up. My favorite way to counterbalance arms are with constant-force gas springs.

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Tips and Tricks

Designing FIRST robots takes practice. Over the years you will formulate your own little collection of tricks. Here are some things I have learned through my experience.

Carl's Tips

Don't design beyond your means

When starting out, it is easy to design lavish and complex robots that play the game perfectly. One mistake I often see rookies make is designing something beyond their ability to manufacture it. Six weeks is an extremely short amount of time to design and build a robot. Choose a design only as complex as you feel comfortable building. A good design is one that has different fallbacks in case certain mechanisms don't work out.

Get a moving chassis as soon as possible.

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More Resources

One thing you will find out about FIRST is that there is no shortage of people willing to help you out. There are resources all over the internet, you just need to learn where to look.

Chief Delphi

Chief Delphi is the premiere online FIRST community. Here there are hundreds of people waiting to help you out with your FIRST-related problems. Here there are white papers on a variety of robot related topics and a showcase of other teams' robots.

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This tutorial was written by a 461 advisor, Carl Agnew.

Illustrations were drawn by 461 member, Elaine L.