With this year’s FTC challenge one of the biggest game elements is the cap ball and the mechanisms involved in lifting it. A lot of this article specifically is talking about the REV Robotics Linear Motion Kit and a few problems that can occur with it.
The most common problems with the lift we experienced were items such as racking, stages falling out, and cable coming off of the pulley bearings. One of the biggest we faced with our lift was racking. For our robot, we decided to make two separate lifts that were joined together by a horizontal crossbar for maximum stability and power. The problem with this design is that if the lifts aren’t symmetrical enough or the motors are mismatched, then when they get too far off from each other the lift will rack, or go up at an angle, making it so it can’t go up any more. The only ways to fix this problem is by either correcting the lifts to match, swapping the motors out for motors that are better paired, or programmatically trying to match the motors by restricting power on one of them.
Another major problem our lift had was too much friction in between the stages when bringing the lift down, when the stages don’t go up in order, or even when sliding stages around during repair or testing, some of the stages can fall out of the others leaving you with a big mess. This problem occurs because with the linear motion kit, the piece that stops the stages from going up too far is the slider inserts that act like a stop, but there is no stop in the kit to keep the stages from going down too far and falling out. The solution that our team uses is tapping the top or bottom holes of the rev material of the rev bars and putting a screw and small washer in so that the slider inserts hit that , and the washer acts like a stop. This problem can also be solved by putting a 3-D part or other stop in the same area.
The final common problem we noticed was the cables coming out of the pulleys on the lift. We tried to temporarily solve this problem using another pulley on top of the actual(used) pulley to keep the string in but we noticed that either the string could still come out or the pulleys would rub and create friction. Other flaws with this design include that it made stringing up the pulleys when the string came out much harder and that to adjust the gap between the pulleys at least 2 nuts had to be loosened or removed making it difficult to do these things, especially in between matches. A better solution would, which we eventually designed, is a 3-D part that goes around the pulleys to hold the string especially one with a slot rather than a hole to make fine tuning the gap easier as well.
The REV Linear Motion Kit is a good base for a cap ball lift or any type of linear slide, but we found that it was not a perfect solution. It took quite a bit of modification, but our take on the lift ended up very nicely.