Choppy motors?

#1

So I’m working on a scissor lift design and when I use a button to run the motors in one direction (scissor moves upwards) everything works fine and smooth. When reversing the motors(scissor moves downwards) it stutters and is choppy. I’ve tried messing with code for hours but could not figure anything out. Any help?

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#2

Without seeing your code it’s hard to make definitive suggestions, however, one common mistake is setting the motors in more than one place within a loop (such as the while loop in driver control), that would be the first thing to check. See this thread where we gave an explanation.

https://vexforum.com/t/how-to-use-shoulder-button-mobility/43516/1

and also this.
https://vexforum.com/t/robotc-bad-programming-habits/28357/1

If that’s not the problem, it may be a mechanical issue. Scissor lifts can be difficult to control, make sure that movement is smooth even without motors.

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#3

Wow, thank you so much for this! Our lead programmer graduated and left us so we are all kind of new to programming. This was a total lifesaver. Thank you again!

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#4

Please don’t revive a dead thread. If there is something that you need to know that wasn’t answered, just start a new thread.

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#5

Also, since scissor lift and choppy motors were mentioned … there is a potential mechanical reason that is very specific to the linear slides, that are frequently used with scissors.

If motor is mounted on the linear slide bracket, then when you apply the torque it tries to turn the bracket and jams the sides of the green slider against the linear rail:

This could be either going up or down and mostly depends if motor is mounted asymmetrically and on which side of the bracket.

Lubricating slider will help, but the proper solution would be to add some sort of the rollers:

On the software side, adding slew control will also help, if this was the root cause of the choppy movement.

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#6

On a different note, I don’t think I’ve ever seen someone use Sprockets and Chain as a linear motion system before. Do you not have rack gears?

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#7

Well, this was the case when you optimize for the minimum number of parts, their weight, and overall simplicity of the build…

If you need to move straight up around 10-15" with manipulator staying parallel to the ground, then linear slide and chain offer all of that.

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#8

Well yeah, but usually people use the parts that are made for that, like the rack gears.

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#9

It does look like it is more compact though. Could be a good substitute.

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#10

would like to see the rest of that

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#11

There wasn’t much of the robot beyond what you have already seen.
Standard vex claw on the front, and two drive motors and the battery on the back.

Minimal4motorClawbot

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#12

What I’m wondering is how you connect the chain to the slide rail…
I wouldn’t be confident in just sticking a screw through the chain, as that could break it. Looks really interesting, though.

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#13

and any mechanical stops.
Nice drawing!

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#14

All you need is a screw and the rubber band - works surprisingly well:

TopChainAttachment

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#15

Well that’s cool. Thank you!

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