I have students who are about finished with their “In the Zone” competition robot. But they keep having a problem where their lift works just fine for awhile and suddenly stops working after while. We thought it might be a power issue but they attached a power expander and plugged their motors in so that all of the motors on the left side of the robot are in ports 1-5 and the motors on the right are in ports 6-10. Does anyone have any ideas we could try? Has this happened to anyone else?
It’s PTC tripping. Look it up on google, there’s articles there on how to stop it. What ratio are you running, what type of lift, and how many motors?
They have a six bar lift with 4 motors and a roller intake on the end of the lift. What does PTC tripping mean? Their motors are connected to a motor controller and an extension cord - is there too much resistance?
PTC tripping: In short, your students are asking too much of their motors. You’re tripping the motor’s internal circuit-breaker (called a PTC). Even though it seems like 4 motors is a lot, a 6-bar nonetheless has a lot of metal in it, combined with whatever claw/grabber you have out at the end of it (plus perhaps a motor on the grabber?).
More details here in an article I wrote on Motor Overload.
Thank you, this is very helpful. I found a video that explained it as well. Our competition this weekend - I wish we had figured this out earlier! This is only my second year doing this so I’m learning a lot as I go. Any suggestions for quick fixes? I was planning to have them find ways to reduce rotational friction and increase the torque on their lift. Any suggestions for the driver during the competition to try and avoid this happening if we’re not able to fix it?
Also they’re using regular vex gears - would high strength gears help?
Adding rubber band to your lift will help reduce stress on your motors allowing it to work better without tripping the ptc
Building on @JuiceBox 's comment – Are you using rubber bands at all on the lift? If not, then that’s probably reason #1 that your motors are having to work so hard. I’ve never built a 6-bar, so I’m not exactly sure where the rubber bands should go (perhaps another commenter can tell you), but I recommend watching some youtube videos of robots with 6-bars and seeing what those teams are doing for placement. Use YouTube’s slo-mo on some reveal videos to see details. The rubber bands should be enough that you can manually move the lift to the middle of its height and let go, and the lift will just stay there on its own.
Building on what @RuizK1 said: One way to reduce friction is to use VEX high-strength gears with the round green idler inserts. See this other article I wrote about Idler Gears & Inserts. (I sound like a broken record on the Forum lately; I have indeed written many articles.) However, swapping out gears is rather time-consuming, so that may not help you for this weekend.
My team didn’t build a 6 bar lift so I don’t think I can help much, but I used this forum post to understand how to properly rubber band lifts. Maybe this will help: https://vexforum.com/t/uniformly-tensioned-rubber-band-system-analysis-guide/28402/1
While it may seem like rubber bands won’t help much, they can make a robot a lot more fluid and stable.
This is so helpful guys - thanks so much! I think due to the time constraint adding the rubber bands is the way to go. I looked over it and think I can understand where they should go. I’ll look at it a little more in depth tomorrow. I appreciate the advice and the resources!
rubberbanding on a 6-bar, you get the most out of going from one diagonal to the other as shown here,http://farm7.static.flickr.com/6112/6251919490_c248363337_b.jpg
Thank you so much!! We will try this and cross our fingers that it helps!
Also, it may seem counter-intuitive, you should also bring down the motor power. Rather than keeping it at full 127, keep it down somewhere around 100. At one point, the power of the motor remains more or less the same, and higher values just increase the power draw from the cortex, increasing the chances of burning out.
Okay, the rubber bands were a great fix and the lift is working. The only problem now is that, with the rubber bands on, the lift won’t go all the way down and it will exceed the size restriction. When we push it down it goes back to size but if you take your hand off it it lifts back up. Will this be a problem? Are they able to take the rubber bands off for inspection but still use them during competition? I’ve attached a picture so you can see how high it’s lifted.
No since you have to be in 18x18x18 at the start of every match it wouldn’t be legal to take them off during inspection and put them back for a match. One thing you can do is try getting rid of the set of rubber bands closer to the front of the bot and also try moving the point where you have the rubber bands closer together so it doesn’t bounce up as much.
Our team put standoffs on a bar, then used loose zipties to hold the lift down onto the standoffs. Once the lift is released, the standoffs get out of the way and the zip ties just hang.
Would you mind posting a picture of what your talking about, I’m having trouble visualizing this.
I’ll post a picture tomorrow (once I get back to school). You might also see it in the tease I’m putting together soon (not sure).
Ok cool thanks
@JuiceBox I’ve uploaded the video, you can see it from the side here (around 1:15/1:20 in the video)
here’s a screenshot with the standoff circled in blue.