DR4B going down issues

Our robot is having problems with hitting the sides of the stationary towers it is mounted on when going down. Those of you with experience with a DR4B, would you suggest mechanically stabilizing the lift even more or programming a PID loop to make both sides of the lift go down at the same speed more accurately. I feel as though having a PID loop would slow down the lift significantly. Or does anyone have any other ideas?

You mean that the lift wobbles from side to side as it goes down? I would recommend first mechanically stabilizing the lift, then testing PID control.

I would first check to see if every pivot point on your lift is equally “pressured” (basically meaning check for unnecessary friction). After that i would probably go ahead and cross brace the lift as much as possible in places it wont get in the way of other parts of the bot. c- channels will probably be the strongest but making a bar with standoffs and axle collars will be just fine along with a ziptie in the middle. Mine works fine without the PID but if necessary after everything I suggested before, just use pots on each side and make a PID.

The thing is that it never has a problem hitting the sides while going up and putting any more mechanical constraints on the bottom is hard to do due to there being the mobile goal lifter in the back. Also, is there any specific thing I could do to check for equal friction on each side?

could u post a picture of your dr4b
A picture would help find the source


Making the towers and axle length as short as possible should help, especially on the second stage because that will make it stronger and reduce the chances of wobble. We were just having this problem, so we rebuilt it, making it much thinner and stronger, so there is much less wobble now.

Yeah but then I can’t fit the mobile goal in the middle and more importantly the cones in the middle of the lift.

I think that you’re misunderstanding or maybe I’m misunderstanding how your lift is set up. Making the towers thinner should only increase the space that you have for the cones and mobile goal. Even though connecting an axle to both sides may help like others have suggested, if yours is like ours, that isn’t possible and that’s not what I mean. I could help more if I had a picture of your lift.

Hi there! There are a few easy solutions I would try, and at least 1 should work for you.

1.) Use PID. PID shouldn’t slow down the lift if executed properly, and it’s very simple logic. You essentially want to set the error to the difference between the 2 sides, so if the left side is too low, the left side will slow down and the right side will speed up. This is the most elegant solution because it doesn’t require any changes to the physical robot.

2.) Try reinforcing the lift. The main issue seems to be racking, and there are a few effective ways to eliminate racking. You can add diagonal Xs across the lift, put metal on the top and bottom of c channels and standoff them together, or physically connect the gearboxes so the gears are synchronized. Another easy thing to try is making the lift thinner, since more distance between the 2 halves of the lift results in more racking.

3.) Make the topmost 4 bar more narrow. If the bottom 4 bar is 17" across and the top one is just 12", there’s no way for the top 4 bar to hit the lift towers (unless you really build it badly :P). Making a slightly thicker gearbox and making the top 4 bar slightly more narrow can work wonders.

4.) If all else fails, add diagonal “skis.” This is not a very good solution, but it’s easily implemented and is functional. Basically, you want to add lexan guides above the lift towers so the lift is forced into the center of the towers. I’ve only used this solution once before (and regretted it once too :slight_smile: ), but if nothing else works, this is always a possibility.

Well, hopefully you can figure it out! Best of luck on your lift!