How to hold the position when I lift up something?

I use sessors structure to lift cube, but i can’t hold the position, when I lift up some thing, it will descend gradually. Why?:confused:

How can I insert image? I don’t know how to use URL?

Can you re-post your image? The image is not coming through for me.

Without seeing your image, I’m going to guess you probably want to add some rubber bands or elastics: ideally you might want to make the elastic forces cancel out much of the scissor’s weight so that you take a lot of load off your motors.

Electrically, if you don’t have some sort of sensor system detecting the changing position of your lift, then the Cortex can’t control the motors and keep the scissor at a specified position.

Can you attach an image? Try clicking on the little paper clip icon to the right of the white smiley face (and just above the little envelope icon). This will open up a window in which you can upload your own image. In that little popup window, see the button below: “Upload File from your Computer.”

You might also be able to upload your own pictures to your Gallery. Near the top of the page, click on Gallery. This opens up a window in which you’ll see a red link to My Stuff. Click on My Stuff>>My Images, and then I think you will see a window with a link for Upload.

Then you can use that URL to link for your own images.

3 words: PID control loop.

Edit: Also, you can upload your image to imgur and then copy & paste the “message boards” link into your post.

The reason why a lift will gradually descend is because there isn’t enough power keeping your lift up.

If you really want to tune it mechanically, you can use rubberbands and play around with the angles and how far you stretch them. By doing this, you’ll use the force of rubberbands to keep the lift in the air.

If you want to tune it through programming, you can do two things. One, easier way, you can set the motors to equal a small power value (usually around 10-30) when no buttons are pressed.

if(button is pressed)
//raise lift

//give a bit of motor power

From there, you can add additional conditions and commands to make it more useful

The second, more complicated way is to put a sensor on it. You can have it record it’s current value, then if the value changes, you can feed it a little more or less motor power.

Typically however, if you had a sensor, you would create preset heights through the debugger window/experimenting. Then make it run through occasional check ups and adjustments to make sure the lift is at the designated height.

when i click “insert image” icon, it says “please enter the URL of your image” and I should fill in with “http://” , i don’t know how to do.

I just use a 393 motor and 12-tooth-gear , 84-tooth-gear to drive the sessors’ stucture. When i lift something, the motor will rotate reverse, and can’t hold the position.

That means you have to put the website of where the image is from. You have to upload the image to another website before you can show us.

I can upload image, thanks.
732012113 620.png
sossors 620.jpg

Is it possible that your motor is stalling and then overheating? These motors have special devices in them (known as PTCs) that will dramatically reduce the current to the motor if the PTC senses that the motor is getting too hot. If the motor is overloaded, it can get too hot, then the current effectively shuts off and the motor can no longer provide enough torque to support the forces on the scissor lift. Would adding another motor perhaps help with your problem?

Two ways to do this.
First, make sure you utilize as much elastic power as possible. Scissor lift is very convenient for rubber bands or tubings. Try different angles and tune it. Search for TVEX 9090C for elastic inspiration. They are the coolest and most efficient elastic assist I have ever seen.
Second, give your motor a little bit of power to counteract the force exerted on the lift. You might wanna be careful not to overheal the motor PTCs as you are doing so.

And one additional thing: I see that you are not using any bearing flats for your 12 tooth gear. Try using some just in case the gear clicks and breaks under great pressure. I know it’s hard to fit, but that’s what a scissors is all about. Reliability.

Have fun building!

A one motor lift is going to be marginal at best. At least add a second motor.

I’m going to make this simple.
4 ways to fix this:

1.add more rubber bands
2.power the motors at value 10/127 when you arent pressing the lift up button on the controller (if you’d like we can help you with how to program that)
3.Increase the gear ratio
4.Add more motors

These are in order of most important from 1 being the most important to 4 being last resort

Right now it looks like you should change to a 4 motor lift

Typically you would brake the motors, however I don’t think the VEX motor controllers support that feature.

Generally, find to amount of motor power it takes to support the arm, and set that to be your arm motors’ “zero point” so that values above it make the arm actually go up, and values below it (even if they are still positive) make the arm actually go down.


I would not resolve your issue with this solution. The reason being when and if you PTC’s trip,(which they eventually will) your PTC’s must go into a cooling down period with “0” call for power to the motor. With the dialed in values at rest your PTC’s will never reset. Rendering your lift useless for the rest of the match. As many of us know, you must let your PTC’s completely rest/cool in order for them to reset.
We learned this the hard way, cost us another tournament championship in a second round final last year.

Rubberband resistance is the easiest solution.
Extra motors are the best solution.

Idk about you, but 10/127 is perfectly fine. I’m going to disagree with you on this one. I have a team member putting it on 20/127 and didnt use rubber bands, it didnt trip the motors. (this is with vex toss up, and I have to admit 20 is cutting it close) My worlds toss up robot always idled at 10/127 and I hanged with the arm at the end of the match too. I had about 10 rubber bands on the lift in total. Never overheated and tripped the PTC, in fact I did not feel as though the motors were heating up that much. In fact this is pretty much what servo motors do, fight against the load constantly, thats why servos tend to heat up a lot over time(vex servos especially).

10/127 is just so little current even though its technically stalling, not enough to trip the PTC. When you said you learned this the hard way, did you ever idle your arm motors?

Rubber bands is first step, the arm idle is extremely useful, and I feel is a must for lifts. The most common method of placing rubber bands provides non linear tensioning, meaning that at the upper range of extension of the lift, the rubber bands do not assist as much as when the lift is down. Arm idling on the other hand provides linear assist in the entire range of the lift.

In sack attack, some teams put tons of rubber bands on the lift, to the point where the four bar would sag upwards. They then idled the motors down(-10/±127) when the last button pressed was lift down, and idled the motors up(10/±127) when the last button pressed was lift up.

In addition, the motors do not have to cool down at value 0. A trick when your drivetrain starts stalling is to drive VERY slowly, pushing the sticks as little as possible, this lets you move around a bit, while cooling down/resetting the PTC.

Some teams even apply the motor idle in their drivetrains. This prevents teams from pushing you while maintaning a static position. It is extremely effective even against teams with stronger drivetrains than you. However in drivetrains it tends to cause PTC trip, unlike lift motor idling.:frowning:

I think what TEAM LEGEND is saying is that once a PTC trips you have to leave the motor at a 0 PWM value otherwise it will never reset. If you use a holding value of 10 and your PTC never trips it shouldn’t be a problem. However, If you have a constant holding value of 10 and your PTC trips for whatever reason it may not be able to reset.

That’s exactly what I meant.

I see, I’ve never thought of power 10 affecting the cooldown of the PTC trip since it hasnt happened before.
Have you had arm idling on your robot and tripped the PTC’s before? I’m wondering if it would really affect PTC reset. I’m guessing it would still be able to reset but just take much longer.

This is my guess also. PTCs are essentially resistors with a positive temperature coefficient. As long as the current is kept low enough so that the PTC temperature is declining it should reset eventually.

My experience is that they never reset. I Guess something to test, if they do reset it takes a long (> 5 minutes) time.