The unstable X lift

I have a Scissor lift. It Tips a lot, and no programming or support seems to help.
Yet, in this video the lifts are extremely stable. Am I missing something here, or should I trash the lift?

The lift has 4 sections, the bottom section is powered. I have rubber bands on every section. I am using four normal speed motors on a 35:1 gear ratio. All of the motors have IMEs. I am using teflon spacers in between all sections to reduce friction.

Yet the lift tips to one side or the other almost every single time it goes up.

What is wrong with my lift?:confused:

The lift in that video has encoders on it; it probably has some stability programming to keep it balanced. Your problem is most likely that one set of motors is turning just a little faster than the other; when the other set starts to fall behind, it jams and falls even further behind, destabilizing the lift. By using encoders, you can monitor the speed of each side of the lift and keep one from going faster than the other. This should fix your stability issues.

Scissor Lift. :slight_smile:

Make sure both sides have the same amount of elastics, same internal gearing on the motors, and same amount of friction.

Also, do you have some bars going across the lift and connecting both sides in each section? Perhaps some pictures would help.

I am using the encoders in my programming. My PID programming should have kept it straight, but it doesn’t.

I do have bars going across my lift to keep it stable (at least one per section), and not any of those 25x1 metal parts. I use the aluminum c-channels and aluminum angles.

The Chinese bots have an axle linking motors on both sides. Pictures would be great.

Here are some pictures:
motors and gears
the whole thing

In general, the build quality looks very good, and quite stable.

I think your main issue might actually be that your lift is too powerful/slow. With such a high gear ratio, the slightest bit of variance in the movement will add up. I know this sounds a bit silly, but some lifts also partially rely on getting up to the right height quickly, almost “shooting” up, and then maintaining it, this way the lift will quickly power through any inconsistencies in height.

So, here is what I would do:

  1. Add Latex Tubing or Elastics on each stage, making sure they are the exact same on both sides.
  2. Reduce the gear ratio to make it faster. 1:15 (Gears: 12t -> 36t, 12t -> 60t) should work no problem, perhaps even 1:7 (12t -> 84t)
  3. You might also want to consider spreading out your motors, i.e. 2 on the 1st stage and 2 on the 3rd stage. Or at least move them up to the 2nd stage, to reduce the slack through the stages they need to fight.

I don’t see any issues that stand out from the picture. You can try replacing the old slides on the bottom with the newer ones, because they usually have less friction and might be contributing to the issue if they are binding up. How bad is the tilt?

1:35 is a really high gear ratio, but I don’t think that’s part of the issue. If anything, having a high gear ratio should make the lift more stable. I wouldn’t recommend speeding up the lift unless you find out why the lift is not stable.

From personal experience, connecting the two axles (the ones connected to the lift motor on each side) together helps tremendously with any instability caused by differences in motor rotation. As long as you back the connection with a c-channel connecting both sides of the lift, it shouldn’t cause a major issue with friction. Also, put a bearing on the 12t gear… you’re putting a lot of strain on the motor without it.

Make sure you have enough elastics. Instability is often a problem with insufficient motor torque (although I’m not sure why a 1:35 lift will have problems with motor torque). Also, make sure none of your motors are weak.

I hope this helps- good luck!

Thanks for all of your help! We have linked the axles of both sides together at three locations and it’s working a heck of a lot better than it used to. Of course, there still is a few kinks, but nothing that my team and I can’t take care of.

Even though you’ve solved the issue, it should be noted that a traditional PID controller won’t keep motors at the same speed. The PID controller will ensure that both sides of the lift reach the same height at the end of the day, but won’t ensure that the motors are going the same true speed relative to each other. You could use a PID controller to help maintain that the motors go the same true speed, but that’s not a typical use-case scenario for VEX.

You could run a second P controller to keep the scissor at the same height. A new KP would be needed and the error rather than being (goal-current) where the goal is the top location you want to go, goal would be the other side of the lifts location. The original PID loop would ensure both sides get where the want to go while the second P loop would work to keep them balanced. These values would be added together to get the motor value to send to the motors.

Correct. I was working under the assumption that a standard/traditional PID controller was being used in my original post.