Rebound Rumble: To Drive or Not to Drive

February 16 2012 myGL 190


Dr. Gilbert Lai is a mentor for the FIRST Robotics team SWAT 771. He is helping an all girls team from grades 7-12 design a basketball-shooting robot for this year’s annual FIRST Robotics Competition. Dr. Lai is using MapleSim and Maple to help the team understand the principles involved and design their robot. This blog post is part of a series that chronicles the progress of the team.  Posts in the series include:

In my previous blog post, I mentioned there are a number of factors governing the trajectory of a projectile. For the purpose of the competition, each team is really looking at what combinations of the shooter launch speed and launch angle for a given location within the game arena would be best to maximize the scoring. Ideally, both of these factors are adjustable to provide the most flexibility in game strategy. Realistically, though, both of them would be limited by practical considerations, such as availability of suitable motors, accuracy of sensors and precision of controllers. Even if both factors can be adjusted arbitrarily and accurately, solving for them simultaneously for a given location in real-time would likely not be practical at all.

As such, a divide-and-conquer strategy, where one of the factors is assumed to be fixed and the remaining one is then being solved, is probably better suited for this competition. Following this chain of thought, my team has explored a number of options in our robot design: 

  1. A slider mechanism for adjusting the shooter launch angle (assuming fixed launch speed);
  2. An alternate pivoting mechanism with a threaded rod (also with fixed launch speed);
  3. 3. A fixed shooter launch angle but with varying launch speed.

Screenshots of the shooter (which is consisted of four flywheels) being driven through a slider mechanism (download the model from the Application Center), for adjusting the launch angle up and down are shown below. 

In this case, the launch angle is adjusted through a motor attached to the anchor pivot (located at the bottom of the red colored rods in the screenshots). As the support link (red) is rotated about the anchor pivot, the tip of the support link (red) will slide along the bottom of the shooter. Given a fixed length of the support link, the shooter angle will vary as the tip of the support link slides towards or away from the pivot point.

The potential advantage of this design is that the motor can be mounted lower than the pivot point. This will help lower the centre of gravity of the robot (high centre of gravity is a tipping hazard, especially when we are travelling on the bridge inside the game arena). Another potential advantage is that the torque requirement for adjusting the launch angle (pitch of the shooter) can be reduced (when compared to driving the launch angle directly at the pivot point) by varying the anchor pivot location relative to the centre of mass of the shooter, as well as the length of the support link. In terms of disadvantage, this mechanism represents a more complex design, which might cost more and longer to build.

An alternate mechanism for actively driving the shooter launch angle is through the use of a threaded rod (download the model from the Application Center). The idea here is that a support link will be extended from the shooter pivot and attached to the thread of a threaded rod. As the rod rotates, the support connection collar will slide up and down the rod with the threads. This will then pitches the shooter up and down about the pivot. Screenshots of such mechanism is shown below.

From the screenshots, the red rod represents the threaded rod, which would be driven by a motor to rotation about its axis. The green rod represents the support link extended from the shooter pivot (the shooter pivot axis is represented by the blue rod). A support collar would connect between the threaded rod (red) and the support link (green). The support collar will move the support link (green) up and down along the threaded rod (red) as the threaded rod is rotated. In addition, the support collar provides a pivot between the support link (green) and the threaded rod (red) so that as it moves up and down, the relative angles between the two rods can vary. A pivot at the base of the threaded rod allows it to pivot forward and backwards as well.

The pros and cons of the threaded rod design is very similar to the ones for the slider design. In fact, both designs are variations in the use of lever to reduce the amount of efforts in adjusting the shooter launch angle.

The third design my team has explored is to forgo having a variable shooter launch angle. The idea here is to mount the shooter at a fixed (not adjustable) angle with respect to the robot chassis. Scoring (shooting) at different location in this case will be achieved by adjusting the shooter launch speed (through varying the speed of the flywheel).

The advantage of this option is that the mechanical design of the shooting mechanism is much simpler. However, the potential disadvantage is that we might be limited to being only be able to score from a certain parts of the game arena, depending on the range of launch speed achievable from our motors.

In summary, each of these design options has its own advantages and disadvantages. The issues to be considered when selecting a particular design for the robot include its complexity (do we have the skills, tools, parts and time to realize the design?), as well as the robustness/reliability (would it break down easily? And if it breaks, can it be repaired quickly?)

So which option did we select at the end? Stay tuned to our team website and follow us to the competitions (FRC Waterloo Regional and FRC Greater Toronto West Regional) to find out :)

Dr. Gilbert Lai is an independent consultant. He is also a mentor for FIRST Robotics team SWAT 771 ( As a former employee of Maplesoft, he was team lead in the MapleSim simulation engine development team and was technical adviser on various MapleSim add-on toolboxes.

Trained as a Computer Engineer, Dr. Lai’s research interests include robotics control (force feedback teleoperation), aerospace and mechatronics applications (helicopter modelling and control). In his spare time, he enjoys computer games, Sci-Fi movies and quality time with his family.

Follow @gilbert_lai_phd on Twitter.

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