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Preamble

The 2-Stage Grab and Lift Mechanism has been a key weapon in the arsenal of World Robot Olympiad (WRO) teams. It makes use of the principle that given the freedom to move in 2 axes, the mechanism will execute these movements sequentially, with the less energy demanding action, the grabbing motion, done before the more energy demanding action, the lifting action.

Using a single motor to perform 2 actions could either be a strategic decision, freeing up the second manipulator motor, or a forced solution given the constraints of the mission. For WRO 2022, the Grab and Lift Mechanism was used by a few teams in the Junior High (Secondary) Category, and by the vast majority of Senior High (Tertiary) Category teams. In the Junior High category, using a Grab and Lift reduced the time spent placing the Marking Blocks as the robot could store a block behind the claw, avoiding any awkward maneuvers. In the Senior High category, the Grab and Lift was a popular choice as the Water Bottle and Laundry Blocks had to both translate in the XY plane and be lifted in the Z axis and put on top of the table and into the Washing Machines respectively.

As teams get more adept at solving competition missions, sharing solutions online and lowering the barrier to achieving a good score, organizers have been increasing the difficulty of these missions. This is evidenced by the fact that only 6 out of ~230 teams were awarded Gold across the RoboMission age groups at WRO 2022. Exacerbated by aging EV3 inventories and diminishing global supply forcing teams to move to the 6 port SPIKE or Robot Inventor system, 3-Stage mechanisms may soon become an essential tool for teams looking to do maximise efficiency.

The Grab and Lift and Dispense Mechanism

The 3-Stage Grab and Lift and Dispense Mechanism shown here is an example of a possible solution to the WRO 2022 Senior High Mission. An EV3 Medium Motor drives a Differential Gear (which we have also used in this fun build eons ago). In the first 2 stages of operation, the Differential remains stationary, with power transmitted to the knob gear driving a standard Grab and Lift Mechanism via the 3 12-tooth Bevel Gears. When the Grab and Lift has reached the lifting limit, the Differential starts to turn, rotating the driveshaft and the Dispense Mechanism.

While the basic principle of this mechanism is simple enough, there are some nuances to be highlighted in this design.

In order to prevent the Dispense Mechanism from rotating prematurely, it has to require more force to actuate than both the grabbing and lifting actions. While this may work out when the Grab and Lift is unloaded, the energy requirement changes depending on the object being lifted. Since the weight of the object may not be known beforehand due to the Surprise Mission or otherwise, the resistance must be easily configurable. In this build, this is done by meshing an additional 20 Tooth Double Bevel gear with the differential. Resistance is added by incrementing the number of 8 Tooth gears connected to the frame via Connector Pegs with Friction.

You would notice from the video that the lowering of the Water Bottle looks to decelerate. This is done in hardware rather than in software (the Medium Motor is running at maximum speed). In order to achieve this effect, a long axle is used for the Driveshaft, acting as a torsion spring as it is not perfectly rigid. The Dispense Mechanism also has an additional 8 Tooth output and is chained with 8 Tooth Gears connected via Connector Pegs with Friction. This turns the Dispense Mechanism into a Spring-Mass-Damper system which can be easily tuned. The Spring Constant can be increased by using a shorter axle, while the Damping Constant can be increased by increasing the number of 8 Tooth Gears.

Conclusion

The next few years will definitely be interesting for LEGO-based robotics competitions. While we have buffered ourselves against the EV3 supply crunch for now, our inventory is definitely aging, with failing MicroSD card slots being the chief issue for now. We understand that many teams that are using EV3s from their inception are having far greater issues with failing ports, screens, buttons, and other components critical to competition usability, and may not have the luxury of stocking up on these components with sky-rocketing prices. At the moment, the SPIKE and Robot Inventor systems are not real options due to the lack of ports and flaky build quality. For teams without EV3s, it may be that the surprisingly robust NXTs are a better solution head-to-head as having an IMU do not pose a significant enough advantage with the current rulesets compared to the presence of a screen and proper buttons for debugging.

Perhaps WRO will follow in the footsteps of competitions like the FIRST LEGO League (FLL) in designing simpler missions with tighter space constraints that favour the use of the IMU and the smaller footprint of electronics in the SPIKE system. In my opinion, this is suboptimal as WRO aims for an older audience with the Senior High Category (make open platform?) and has software complexity as a key differentiating factor compared to FLL, which is more hardware oriented.

While it remains to be seen how competition organisers will react, what we can do as Coaches and Participants is to keep learning and innovating to overcome the constraints of any system we may have to adopt.

Footnote

I have taken some photos and videos of teams in action at the WRO International Finals 2022!

If you have taken any photos or videos of our teams from Singapore in the Junior and Senior High Categories, I would love to have them! Do post a comment here or drop me an email at kenneth@coresg.tech 🙂

If you have been entertained by this article, do also check out our other WRO related content, including our Lift and Rotate Stepping Mechanism.