Syncing rd4b lift with potentiometers

I built a reverse double four bar lift for my robot. However I realized that the two would not go at the same speed. There were discussions about a master lift, and a slave lift though… how could I be able to sync two lifts using potentiometers?

There are two ways to do this.

If it’s not synced during driver control, that’s because of bad build quality. You need much better mechanical support between the two sides, as you can’t really use PID algorithms and the like to stabilize it in driver control.

If it’s during autonomous that you want it synced, use PID. George Gillard made a great guide to PID with VEX in mind.

Assuming you aren’t driving the two sides separately (that is, one button for the left, one for the right), why would it be synced in driver but not auto?

You generally need more accuracy in autonomous because you can’t correct it if it’s not synced. I don’t know how you didn’t realize that.

Additionally, it’s really not that hard to make a double reverse 4 bar mechanically synced.

Why not? We used a PID on our DR4B for opcontrol to hit the desired values on both sides. While it didn’t technically sync them, the effect was the same…

It worked pretty well for us after we spent a few practices tuning constants. I mean I do agree that build quality > PID but its perfectly viable to use PID in opcontrol.

No, I realize both of those things, but I’m not sure how you’d sync it in driver that you couldn’t also do in auto - unless you were to control both sides separately. I suppose you could lower the lift all the way, so both sides hit the mechanical stop, but I still don’t see how a lift consistently synced in auto would have problems in driver.

I know, but because of the chain bar and mobile goal lift in the middle of the rd4b lift, I can’t manage to put enough stabilizers between the lifts. It was common for people to have two separated claws last year in starstruck, and they had it on the same angle with potentiometer/encoder. I want to do something like that

You can’t correct your lift for synchronization in driver control, either, unless you have separate buttons for the different sides (which would be preposterous).

Neither @puzzler7 nor I “realize” that, because what you are saying does not make much sense. In driver control, you either set a height or a power for the whole lift and if it is mechanically strong, both sides will be in sync (in my experience). In autonomous, you set a height and it goes there with PID. If the lift is mechanically strong and neither side is broken, both sides should be in sync and get there.

oh… ok

I did it.

You absolutely CAN use PID in driver control.

We are considering doing this, is it really hard or anything? Also is the best way to do it with a master/slave function? Also is it necessary?

no its easy. We just use P

@Daniel Shin
Ideally you want to solve for as many issues mechanically. As @Aponthis has said. Be mindful of how you link each side of the lift. We usually use 2 axles and link them in the middle with a shaft coupler. Also, you have to make sure the powered gears at linked together and not the driven ones.
Linked Mogo lift:

Take note of how we linked the two motors of the mogo lift. (the tops of the motors had been taken off)
Also, this picture is from October 26, some has changed, and some has stayed the same. :wink:

EDIT: posted the same post twice for some reason.

You cannot use pid realistically to sync the lift since you would not be able to move it to custom heights without an iterator, with which you’d still have only a few set heights, though you can hold the lift at a position using pid (set Target to current place).

Just mechanically sync your lift.

You can use PID and still have full manual control. You simply use your usual up/down buttons to modify the PID target value instead of directly setting the motor speed. With a good lift control task (i.e. one that changes the target value by a fixed amount on every timed button check iteration), you can easily get manual control, presets and side-to-side balancing.


I would be pretty surprised if it were impossible, since one of my students programmed it successfully on her own.

Bingo. This is how it’s done.