Green Smog: Robot Design Case Study

By Jeremy Diaz, Traig Born, James Tidwell

The design team: Dr. Andrew Wright (standing), Traig Born (left), James Tidwell (right)


Introduction: This is a summary of the design process that UALR’s competitive robotics team undertook in the development of its entry in the 2005 FIRST Robotics Competition. The UALR competitive robotics team consists of college professors, college students, and high school students. For the past six years the team has designed and built robots to compete in the FIRST Robotics Competition.

This robot was designed to compete in the game called “Triple Play”. In Triple Play 2 teams of 3 robots went head to head in a match. To win a match, teams had to score the most points by placing small tetrahedron frames, built from PVC pipe, on top of large tetrahedron goals.

Design Process: The initial step of the design process was a series of brainstorming sessions. The first session was open to all team members. “What could be done in the game?” was the subject of the brainstorming. An exhaustive list of possible strategies and tactics was created. After much deliberation, it was determined that the robot would be an offensive scoring machine designed to quickly place the small tetrahedron frames, called tetras, on the top of goals.

The tetras would be retrieved from either an automated loading station, a manned loading station, or the ground. Then the tetra needed to be raised 5 feet and placed onto a goal. The system needed to perform in a hostile, chaotic environment, where collisions with other robots or obstacles may occur. With this in mind the functional requirements were determined. The tetra delivery system was divided into four subsystems. Each subsystem had its own functional requirements:

Subsystems and functional requirements:

  1. Tetra Grabber
    a. Independent of tetra orientation
    b. Able to pick up from the ground, autoloader, and human loading stations
    c. Deliver to the next subsystem in under 2 seconds
  2. Tetra Storage Unit
    a. Store up to 6 tetras
    b. Provide a convenient interface
  3. Vertical Lift Module
    a. Provide 5 foot of vertical actuation in 5 seconds
    b. Lift 50 lbs
  4. Tetra Placement Module
    a. Place tetras on center or corner goals
    b. Actuate in 2 seconds

The second brainstorming session took place on the UALR Competitive Robotics Team’s message board. This was a closed brainstorming session, in which only the team engineers participated. The subject of this brainstorming was mechanisms to satisfy the functional requirements.

Early design concepts centered on the use of an articulated arm. Different components of the arm were used to satisfy the functional requirements of the subsystems. The end effector was the tetra grabber subsystem. The shoulder satisfied the functional requirements of the vertical lift module. However, an arm of the necessary length would tend to be unstable and difficult to control. It would also make the robot top heavy, causing it to flip. While the versatility of an articulated arm is an advantage in some cases, a more optimal solution exists.

Instead of a complex single device, such as a robot arm, we chose to use separate specialized mechanisms to satisfy the functional requirements. The subsystems were renamed and each one was assigned to a different design team. The vertical lift module was renamed the Liftetup. The functional requirements for the tetra placement module and the tetra storage module were combined and the device was dubbed the Droptetoff. The tetra grabber module was named the Picktetup.

Subsystems: The Liftetup is comprised of a scissor lift. The scissor is extended by a lead screw, which is actuated by a gear box. The gear box has a 1:100 reduction ratio and is driven by two DC motors. The drill motors from the 2005 FIRST robotics kit are used. The functional requirement which states, 5 feet of linear actuation, was reduced because the Droptetoff also provided some vertical travel.

Liftetup Specifications:

  • Range of Travel: 2 ft.
  • No Load Speed: 4” per second

The Droptetoff is an inclined plane which moves tetras from the Picktetup, up and across the vehicle, then onto the goal. An inclined plane is used to provide the remaining 3 foot of travel required to reach the top goal.

The tetras are allowed to rest midway through the plane providing a way of storing multiple tetras in the device. The tetras are placed with the apex of the lower tetra protruding into the next. This takes advantage of the hollow tetras and increases the storage capacity.

The preliminary design of the mechanism used overlapping conveyors to move tetras up the inclined plane. The conveyors used were chains hooks affixed to them. This was too heavy, due to the long lengths of chain required, and was scrapped.

The final design used a reciprocating shuttle and a series of one-way latched hooks. Each cycle of the shuttle raises a tetra onto the next set of hooks.

Droptetoff Specification:

  • Actuated with Lead screw
  • Driven by Van Door Motor

The Picktetup sub-system is made up of a pair of arms which sweep in from either side orienting the tetra and pulling it onto the shuttle. A problematic aspect of this device was the interface between it and the Droptetoff. This was solved by designing the Picktetup so that it could sit under the rails which made up the inclined plane portion of the Droptetoff. A single motor was placed in the middle of the device and used to rotate the arms inward.

The required timing between the Picktetup and the Droptetoff would be managed by software using sensors. This could have been solved using mechanical timing techniques but the use of sensors and software was more easily implemented.
This project done through support and funding provided by:

University of Arkansas at Little Rock
Hendrix College
IGUS Young Engineer’s Support Program (YES), and here
Controlled Automation