|Apachi V 1.0|
|Year Of Creation||2019-2020|
|Current Version||V 1.0|
|Information and Statistics|
|Weapon Class||Bent Bar/Overhead Bar|
|Weapon Speed||Around 800?|
|Other||Won the 12lb rumble|
Apachi is the first iteration of a 18 lb Bent Bar Shuffler. The purpose of this guide is to explain all of the reasoning behind the design decisions during the creation of Apachi. This guide will go in depth on why certain designs were chosen as well as explaining some of the calculations used to support those decisions.
- 1 Competitions
- 2 Apachi V1.0
- 2.1 Chassis
- 2.2 Drive Assembly
- 2.3 Weapon Assembly
- 2.4 Electronics
- 3 See also
- 4 Notes:
Created by: Afshan Chandani, Cory Stine, Dylan Adriano, Dylan Fife, Hank Hellstrom, Eli Tiemann, Marland Micket
|Drive Motors||Turnigy 540L V-Spec Inrunner 810kv|
|Drive Motor Controllers||HobbyKing Brushless Car ESC 2S-4S|
|Weapon Motor||Scorpion SII-4025-520KV|
|Weapon Motor Controllers||YEP 100A (2~6S) SBEC Brushless Speed Controller|
|Receiver||Hobby King 2.4Ghz Receiver 6Ch V2|
|Remote Control||Hobby King 2.4Ghz 6Ch Tx and Rx V2|
|Battery||2x Turnigy 2200mAh 4S 30C Lipo Pack|
The chassis for Apachi had to solve a couple of problems. First, it had to be contained within the bounding circle made by the bent bar weapon. Second, the chassis needed to securely hold the weapon due to the high loads from our large moment arm. Third, we wanted to have easy access to the foot modules in order to troubleshoot problems.
- For the first problem, the geometry of our feet had a large impact on our chassis shape. our feet were long straight lines that we wanted as far as we could get to the outside. We considered making a plus shaped chassis to maximize our space, but were concerned it would increase our wobble which would be especially bad because of our weapon hitting the ground. We chose a rectangular design.
- In order to secure the weapon, we added two steel supporting bars to hold the weapon shaft. We don't have much evidence that this worked because our weapon snapped every time we hit something full force, but these supports did not fail.
- In order to access our feet modules, we considered two options. The first option was to create a modular bounding box that could easily connect or be replaced. This would require a more complex interface between the module and the internal drive motor and geartrain. The main benefit of this is the plug-and-play nature leading to easy swaps. The second option, which we opted for, was to primarily focus on the side plates being easily removable and the feet accessible from there. While this meant the robot would be down whenever we needed to access the feet, it simplified the rest of the design and number of parts.
Some main chassis takeaways from our experience were:
- Pre-planning electronics could have saved a lot of pain
- Integrating a center bar with a shuffler heavily constrains design
- Using nonstandard dimensions to save a tiny amount of weight is annoying during machining
- It is worth double checking electronics hole dimensions
Apachi’s Drive system was a shuffler drive. This consisted of 2 symmetrical shuffler modules. Each shuffler module consisted of 6 HDPE feet with bearings pressed into them, and a hex shaft with aluminum cams clocked at 60 degree intervals. The feet were pressed onto the cams to create a cascading motion when the camshaft was rotated. The camshaft was driven by a spur gear and a pinion attached to an inrunner motor. Both sides were symmetrical such that there were always 2 feet on the ground at any given point in time.
The decision to make a shuffler system was in order for Battlebots as a whole to gain more knowledge of this new drive and take advantage of the 50% weight bonus, along with the traction advantage of more ground surface area. The decision to make 6 feet per side was due to the thought that producing identical cams with hex profiles would be the easiest way to manufacture the system. The feet were made of HDPE, the cams were made of aluminum, and the shaft was made of steel. The cams were designed to be a failure point, the thought being that their hex profiles would be rounded by the turning of the shaft. Because of this, the shuffler modules were designed to be easy to assemble. By taking off the outer side plate, the shuffler module is accessible as a stack of feet/spacers on the 2 shafts.
Link to spreadsheet Our calculations showed that a 4 foot design would be very slightly better, but we opted for the 6 foot design because we thought that the hex shaft would be easier to machine
- The shuffler drive worked a lot better than expected. Its mobility and controllability were much better than initially thought, and our weapon choice in hindsight did not need to be so driven by our perceived lack of mobility. The cam manufacturing was harder than expected. Due to the thickness of the cams and the necessary straightness of the hex cutouts, the Invention Studio’s 5 axis waterjet was required, including testing and cutting with an angle to compensate for the taper. In future designs, looking into EDMing the cams would be a good idea. The manufacturing of the hex shafts was also difficult, due to the rotating hex the tools took a lot of stress until the points were turned away. Looking back, a design with 4 feet per side could have also been used. While complicating parts, its weight savings, bearing savings, and potential easing of manufacturing would potentially be beneficial. Overall, the design exceeded expectations, while the manufacturing could have been optimized.
- As for performance in competition, the vibration caused by the drive system during competition caused screws to come loose very often. Systems using shufflers in the future should attempt to minimize screws for other fastening methods when possible.
- When adding additional weight on top of Apachi, it was determined that Apachi could drive with up to a total of 45-50 pounds.
Apachi utilized an overhead bent bar weapon, which is a bar situated above the robot with bent sections that would descend toward the floor to a level where it could strike other robots. Advantages of this style of weapon are a high moment arm, 360 degree strike area, and potential to act as armor in the event of a hit. Disadvantages are a high mass arm requiring reinforcement at the point of rotation, non invertibility of the weapon, and a tall system. The weapon transmission consisted of an outrunner motor with a V belt and pulley, which interfaced with a pulley on a central shaft held by a key and set screw. The central shaft attached to the weapon through another key.
The selection of an overhead bar was due to the unknown mobility of our shuffler module. With our ability to move and have our weapon facing the opponent in question, it was decided to make up for the potential vulnerability with a weapon that was dangerous from all sides. A ring was discarded due to the rectangular nature of shuffle modules, and an overhead bar was selected as a weapon. The lack of invertibility was not considered a problem. The weapon transmission system was largely designed around space concerns, with the outrunner motor having its pulley mounted on top and using keys and set-screws instead of profiles for more robust connections.
Get angry at Cory and make him fill this in
Multiple weaknesses made it into the design that ultimately resulted in catastrophic failure at multiple points. Specific design failures were selection of a weapon that required stable movement, as the overhead bar (even with a half inch of clearance) would impact the ground due to how a shuffler moves, and massive underestimation of the forces involved with such a high moment weapon. The main drive shaft proved to be too thin to handle the impact of the weapon, and there wasn't enough reinforcement to the connection point between the shaft and weapon to absorb the impact, resulting in the bending of the exposed section of shaft (roughly 0.5”) and the fracture of the weapon. Another point of failure was the level of hardening to the weapon; the bar was over hardened to withstand the subjected forces, resulting in fractures originating in the keyway of the shaft. The key of the shaft itself surprisingly survived the impacts, as it was believed that would be the point of failure. The weapon transmission also presented some weaknesses, with the double set screw attachment on the weapon motor a decision made out of necessity as opposed to being a robust connection. Overall the weapon system had some major design flaws that were not caught or excused due to the design constraints already in place. In a new iteration a new weapon design would be recommended.
When a flail becomes a viable alternative, you have made a
mistake beautiful robot
Apachi’s electrical system consisted of two 4 cell lithium polymer batteries, two brushless electronic speed controllers (ESCs) designed for RC cars which controlled the drive motors, and a different brushless ESC which controlled the weapon motor. The circuits for the weapon system and the drive system were isolated with each having its own battery and power switch. There was also a receiver which sent pwm signals to the speed controllers.
Multiple batteries were used in order to most efficiently utilize the space inside of the robot while providing the large current required by the motors. RC car ESCs were selected for the drive motors due to their ability to run motors in reverse which is not a standard feature of brushless ESCs. The ability to reverse was not necessary for the weapon so a standard ESC was chosen. The specific voltage, capacity, and maximum current for the batteries were determined based on the drive and weapon calculation spreadsheets discussed earlier. The ESC specifications were determined based on the voltage of the batteries and the maximum current draw of the motors.
See the other calculation sections
Based on the voltage level of the batteries after running for an entire match, they seem to be appropriately specced. More in depth testing was going to be done to determine whether battery specs, and therefore size and weight, could be decreased but these tests were unable to be completed. Additionally there were concerns about the weapon being underpowered due to the inability to spin such a large moment to high speeds. This could potentially be solved by some creative wiring solutions that were going to be tested but again were not. The wiring of this robot was somewhat tricky due to there being many spinning things inside the robot that the wires needed to avoid.
- [(link to other bot made this year) (Name of other bot made this year)]
(random joke, optional)
Naming Inspiration: It kinda looks like a helicopter
Apachi used the side plates of Spooki as weapons of the flail