Intro: Introduction to Manipulators
Creating the right manipulator for a challenge is one of the hardest parts of the FIRST Robotics Competition (FRC). In my four years as a student, it was always my team's biggest failing point. Though the game challenge in FRC changes from year to year, there are often tasks that are similar to ones from previous years. For example, the 2012 game, Rebound Rumble, had clear elements of the 2001 game, Diabolical Dynamics, and of the 2006 game, Aim High. For this reason, it is beneficial to be familiar with basic manipulator designs used in previous games.
This tutorial will provide an overview of manipulators commonly used in the FIRST Robotics Competition (FRC). Each step will discuss a general manipulator type and provide examples of implementations of the manipulator.
This tutorial was made through the Autodesk FIRST High School Intern program.
A willingness to learn
Step 1: General Guidelines
Before I jump into the nuts and bolts of different manipulators, I wanted to provide some general guidelines that will help you choose and design a manipulator.
First, let strategy drive your manipulator design, not vice versa. What this means is that your manipulator should achieve the design requirements your team decided upon in forming a strategy, instead of forming a strategy based upon the manipulator you cobble together.
Second, design within your teams’ limits. If you know you just don’t have the resources to build the super-complicated manipulator that you think will dominate every aspect of the game, don’t do it! Go for the simpler one that you can build and will fulfill one role really well. However, also don’t be afraid to push your team to overcome your limits. For example, my team pushed ourselves to build a practice bot this past year, and it ended up being really beneficial.
Third, always have active control of the gamepiece. For example, if a ball needs to be transported through your robot, do it with a conveyor, not a ramp. If you don’t actively control the gamepiece, it will inevitably jam or fall out of your manipulator.
Finally, prototyping and iterative development are key to building a successful manipulator. Start out with a prototype, and then iteratively improve it until you are ready to build a final version. Even then, be looking for improvements that will make it better.
Photo Credit: http://www.chiefdelphi.com/media/photos/30826
Step 2: Arms
One common design used for arms is a 4 bar, or parallel, linkage. Such a linkage is shown in the third picture. The main feature of this design is that the end effector is held in a constant orientation.
Tips for arm design:
- Pay attention to weight – can cause the arm to be slow or even fail
- Use light materials such as circular or rectangular tube and sheet metal
- Use sensors such as limit switches and potentiometers to simplify controlling the arm
- Counterbalance the arm with springs, gas shocks, or weight to stabilize it and reduce load on motors
Step 3: Elevators
Like arms, elevators are used with an end effector to control the gamepiece. They are usually raised by winding cable on a drum. Though it is only necessary to pull the elevator up, it is wise to include a return cable that can pull the elevator down to prevent jamming. There are two main styles of routing the cable so that it raises the elevator: continuous rigging and cascading rigging.
Elevators with continuous rigging (shown in the second picture) have one continuous cable from the winch to its last stage. As the cable is drawn in, Stage 3 is the first to move up and the last to move down when the cable is released. Two advantages of this design are that the cable goes up the same speed it goes down, meaning that a return cable can be placed on the same drum, and that the tension in the cable is low. Its main disadvantage is that its middle sections are more susceptible to jamming.
Elevators with cascading rigging (shown in the third picture) have individual cables connecting each stage of the lift. This results in all stages rising simultaneously as the cable is drawn in. However, any return cable must have a different speed than the main winch, which can be handled by using drums of different diameters. While the middle sections of a cascading elevator are less susceptible to jamming, the tension on the lower stage cables is much higher than in an elevator with continuous rigging.
Though elevators and arms are similar, there are some important distinctions. Elevators tend to be more complicated and heavier than single jointed arms. In addition, elevators usually move vertically and are unable to reach outside of the robot’s perimeter. However, they do not change the robot’s center of gravity as they move, and their position can be precisely controlled with the proper use of sensors and programming. In essence, each has their own advantages and disadvantages, leaving the decision of which to use up to teams. One other option is to combine these two options by placing an arm on the last stage of an elevator, an example of which is shown in the fourth picture.
Step 4: Grippers
The following list of different grippers corresponds to the pictures above:
- Two finger pneumatic gripper
- Two finger linear pneumatic gripper
- Three finger linear pneumatic gripper
- Motorized gripper
- Pneumatic gripper
- Basic roller claw
- Hinged roller claw
Finally, several tips for gripper design:
- Ensure that your gripper applies enough force to hang on to the gamepiece
- Make your gripper grab onto and let go of objects quickly
- Make it easy to control by using sensors to automate basic operations
Step 5: Ball Collection and Transport
While grippers are useful for manipulating single objects that may be unusually shaped, often times FRC games involve a bunch of balls. Two capabilities that are commonly required in these games are collecting balls and transporting them within a robot.
The most effective method of collecting balls changes from year to year depending on the rules.
In the 2012 game, Rebound Rumble, teams were allowed to have appendages that extended beyond their robot. Many teams decided that having drop down ball collection systems would be advantageous, resulting in appendages that used rollers to funnel the balls into a single intake or over their bumpers and into their robot. Several examples of these robots are seen in pictures one through three.
In the 2009 game, Lunacy, teams were not allowed to have manipulators that extended beyond their frame perimeter. If they wanted to collect balls off of the floor, they had to have an opening in the front of their robot to do so. This also led to many wide-base robots because it allows for a greater opening for balls to enter. Some examples of these robots are seen in pictures four and five.
There are several possible ways to transport balls once they are collected by a robot, but the most common is using polyurethane belts. Polyurethane belts (also known as polycord) are adjustable length belts and are commonly used for conveyors and low-load power transmission. Every single one of the robots pictured above uses polycord to some degree. The final picture shows polycord in greater detail.
Step 6: Shooting
Getting a ball from a robot to an otherwise inaccessible location is another common task in FRC. This requires launching the ball, usually using a catapult or wheeled shooter similar to a baseball pitching machine.
The most common solution to this challenge is to compress the ball against a spinning wheel, which accelerates it enough to launch it a significant distance. The two main variations of this design are single and double wheeled shooters. Single wheeled shooters are simple and tend to put lots of backspin on the ball. The ball’s exit velocity is approximately equal to ½ of the wheel’s surface speed. Double wheeled shooters are more complicated mechanically, but can propel the ball farther. This is because the ball’s exit velocity is approximately equal to the wheel’s surface speed. The first two pictures show some examples of shooters.
As many teams learned in 2012, the key to building an accurate shooter is to tightly control as many of the variables involved as possible. These include controlling wheel speed, launch angle, velocity of balls entering the shooter, orientation of the shooter relative to its feeding system, and ball slippage against the wheel and hood surface.
Catapults are much less common in shooting games because they are unable to fire very quickly. However, their main advantage is that they can be more accurate than traditional shooters. Catapults are usually powered by pneumatics or springs. The final picture is of a team that used pneumatics to power a catapult this past year.
Step 7: Winches
Winches have multiple possible uses in FRC and therefore are found as elements of larger manipulators. Two of their most common uses are in storing energy for a larger mechanism and for lifting an entire robot. When used to load an energy storage device, winches are usually designed to only operate in one direction, with a release that allows it to spin freely, thus releasing the stored energy. A picture of a winch designed to do this is shown in the first picture. Another use for a winch is to lift a robot. In this case, it is usually not enough to have a separate gearbox dedicated to the task, causing teams to build a power take-off gearbox, which is capable of diverting power from the drivetrain to a separate mechanism. Though it is merely a way of driving a winch, I decided to show an example of one in the second picture because it is an interesting mechanism.
Step 8: Conclusion
As you have begun to see, there are many different possible manipulator designs that can be used in the FIRST Robotics Competition. With so many teams working to solve the challenges, each with their own backgrounds, this is of course bound to happen. Becoming aware of what has been done before can save you valuable time by using previous manipulators as baselines for both your team’s prototypes and final designs.
However, also be careful to not let previous designs limit your thinking. If upon receiving the challenge, you immediately choose an old design to use, you may be overlooking a better solution. In addition, sometimes the most creative, outlandish solutions that are specifically tailored to a challenge ultimately prevail. For example, the manipulator pictured was very different than most from the year it was used, but was highly successful. If you remember this and the general tips I suggested at the beginning, you will already be well on your way to creating a successful manipulator.
Thank you to Andy Baker of AndyMark for making his presentation on manipulators publicly available. Many of the pictures in this tutorial are from it.