- The flipper worked, but it could definitely be better. Should be more powerful and easier to control
- Maintain designing for repairs by using connectors, making things easy to replace, and build spares
- Make 3D printed frame more durable by changing print orientation and making walls thicker, especially up front
- Make it easier to get opponents on to the flipper/wedge for better control
With these goals in mind, I started my redesign. I finally had access to Solidworks again, so I was planning to move the old assembly from OnShape to Solidworks, but with so many things changing, it was easier just to start from scratch with the learnings I had in mind.
As with the first version of Drop Test, I started the design with a general layout sketch. The overall flipper design worked well in its first competition, but needed some tweaks to help improve. First, I wanted to shorten the overall robot to make it more compact so that I could increase the durability of the 3D printed frame. This meant increasing the angle of the wedge slightly to compensate. This meant moving things around slightly to maintain the same overall working principle. As you can see in the picture below, as a result of this re-shuffling of components, the trigger mechanism (below left, right side) is fully contained within the bot instead of sticking out of the rear of the bot like it did in the first version. This shortening also meant that the leaf spring was shorter and had a greater tip deflection when the flipper was fully loaded. Without going into the details too much, this should create a higher flipping force than the original design as well. Finally, I realized that the original layout of the trigger mechanism was highly inefficient mechanically. The main issue was that the trigger had a roller on it that acted as a pseudo cam follower to ride along the backside of the flipper arm to load the mechanism. I realized that if I switched the follower to the flipper arm side it would be more efficient and closer to a traditional snail cam design. Essentially, if you imagine the pair of rotating arms as gears instead, with the roller on the trigger, the driving gear had a fixed, large diameter that would result in a "gearing down" scenario which produces more speed and less torque. Instead what I wanted was a "gearing up" scenario where the roller, now on the flipper arm, contacts the trigger arm near the pivot point, such that there is a much smaller torque arm, similar to a small driving gear. This change should allow me to use a lower gear reduction on the flipper motor to allow for a quicker reset time overall. For more information on the math behind all this go check out the original post.
Wedge Design
Drive Train
When I started this process, I realized, looking back at my previous bots, that I had never made a 4WD robot before. Whether it was due to the style of robots I had chosen, or my design style in general, I never really had weight or room for 4 wheels in my robots up until now. I knew I was going to be tight on weight again, especially considering I was trying to make the robot more durable, so 4 motors to drive 4 wheels was out of the question. I might revisit the idea in the future though as the N20 gearmotors are about half the weight of my current drive motors, although less durable.
My solution for this was belt drive. The rear wheels would be connected directly to the drive motors, while the front wheels would be spinning on a dead shaft. The two sets of wheels would be connected by an O ring acting as a round belt. This required me to design my own hubs that incorporated round belt pulleys into them. Given that I was already 3D printing lots of parts for this project, it seemed easy enough to design and print my own hubs specifically for the task. Both hubs would use the Fingertech foam tires that I used in the previous version. The rear hub is based off of the original Dale's Hubs that were sold by RobotMarketplace years ago, where the main hub mounted to the motor shaft through a setscrew and an additional plate screwed into the end to secure the wheel to the hub. The front hub would ride on a shoulder bolt that would act as a dead shaft for the hub to spin on. For this hub, since I can't screw on an additional plate to secure the wheel, I intend to simply glue the wheel to the hub and treat them both as disposable since they are cheap enough. Both hubs include a pulley feature to hold a 3/32" O ring that is stretched 15% past nominal to tension the belt to the point that it will drive the front wheels. I'm not entirely confident this system will work as intended, but for a first go at a custom belt driven wheel solution I think it's good enough to build to test out.
Flipper Upgrades
To fix the flipping force issue, as I mentioned before, I decreased the length of the spring while increasing the deflection. Looking at the equation below we can see that this should increase the force at the end of the spring, leading to a greater flipping power.
Now, having a programmable servo on hand that I could take apart to use for my needs, and having the motor setup already established, it would be much easier to incorporate. In this version of the bot, I wanted to change the trigger drivetrain from metal gears to timing belts to save weight and allow me to re-organize the internals of the robot. As you can see below, I had already designed the trigger to be 3D printed with an integrated timing pulley. This allowed me lots of flexibility in the design and positioning of my servo feedback system. Because the trigger shaft was now a dead shaft, the easiest way for me to connect the potentiometer to the trigger was through a pair of equal sized gears. One integrated into the trigger, and one attached to the potentiometer. This was the servo control board that I salvaged from a servo would know the direct position of the output arm as intended.