Drivetrain

AboutDrive trains are the systems that make the robot move, and often form the backbone of the chassis. The GRT drivetrain group is well-known within the FIRST Robotics community for our student-designed and -built gearboxes, intricate design work, and precision manufacturing. Since the need for robot movement is universal to any game design, the drive train group does significant work each fall semester to study gearbox designs, practice designing modified versions of past gearboxes, and develop precision machining skills. Redesign considerations range from power use to manufacturability. New designs are fabricated by student machinists in our shop, who create high precision parts on our mills and lathes. One of DT's most important recent achievements is the implementation of "swerve" drive.

"Swerve" Drive Gearbox"Swerve" drive allows a robot to be omnidirectional: it can move from side to side without changing the way the robot faces. Other omnidirectional drivetrains such as Mecanum often sacrifice grip to make this happen, but "swerve" drive maintains traction by using a normal wheel housed in a rotating fork. GRT first started using "swerve" drive in the 2017 FIRST game, Steamworks. Since then, we have been improving the design, and making the manufacturing process more efficient.

Historically, GRT has manufactured our own gearbox modules using a variety of machines. For our 2019 gearbox, we used a laser cutter to make custom gears and pulleys, a CNC mill to make gearbox plates, a 3D printer to make "swerve" gearbox forks, and a lathe to turn all the axles and modify sprockets.

To power the "swerve" gearbox, we use a Rev Robotics Neo motor with a series of sprocket and pulley ratios to achieve a final drive ratio of 6.3:1, giving the drivetrain a theoretical top speed of about 12 ft/sec. For the rotation of the fork, we use a VEX Robotics BAG motor connected to a VexPro planetary stack with a 70:1 reduction. At the end of the planetary stack, each "swerve" has a 26-tooth gear driving a custom laser-cut 100-tooth gear with a bearing race.

2-Speed Drive GearboxAn earlier design of the GRT gearbox was our 2-speed, ball-shifting gearbox. This version is a 3-stage reduction, which allows us to use 6 inch or larger wheels (a larger wheel would otherwise gear up the transmission). Our gearboxes are designed to minimize the space requirement, with a footprint extending from the transmissions only 2.7 inches. This comes courtesy of several innovations. Most notable are the inverted shifters: these are the pneumatics that drive the shifting cluster, turned around and set within the gearbox itself, cutting about 2 inches out of the width. Using this linkage to pull the shifter from afar, the cluster performs better than an off-the-shelf equivalent, holding a gear and shifting down to 25 PSI of pressure. Other innovations include the first reduction of the gearbox, accomplished through a belt drive on the face of the box. This further reduces profile, and allows the motors to be turned backwards, with most of their volume outside of the robot base area.

Belt-in-Tube TransmissionIn tandem with the 2-speed drive gearbox, we have used a transmission that transfers power to the wheels via a series of GT3 belts, laid inside box-beam transmission tubes. These box-beam tubes serve as effective housing and as core components of the robot's structure. In response to the terrain obstacles of FIRST Stronghold (2016), we adapted this arrangement in a bent-tube design, maintaining the characteristics of our traditional design, while lifting the large front and back wheels off the ground to help us tackle the worst of the obstacles.

TrainingTraining of new members of the drivetrain group is crucial to our success so it begins as soon as general shop training for the team has completed. The foundation for our designers and machinists alike begins on the lathes, where students learn to manufacture complex, high precision parts. The technique of try-until-you-succeed works very well, and in the process, the students develop a very keen understanding of how the parts will be interacting in the systems they will go on to design. During the later stages of build season, lead drivetrain designer(s) for the year work with a small group of students on the intricacies of gearbox and transmission design. The final competition robot usually includes at least one system designed by these trainees, along the lines of a winch or flywheel gearbox. After competition seasons end, the drivetrain design training begins in earnest. Here the designers that have just gone through a year of development, building, and competition help their successors through the problems that they faced. This training carries on into the summer, where much of the formality of the earlier lessons is lost, and the mentorship slowly morphs into the new team's research and development for the following year. The importance of the effectiveness of this process should be clearly evident. On student design teams such as this, where the turnover of head designers is almost 100% from year to year, it is critical that this knowledge gets passed down, so that we can continue to improve, and further our legacy of stunning student design.

DT Candid
DT Candid
gearbox
Gearbox