# Tension problems

Have problem lifting my arm and it is falling down b/c lack of tension. I am using the rubber tubing but it still isn’t enough. Any suggestions?

Some easy things to try would be:

2. Make it tighter
3. Mount it differently

Tried tightening. Haven’t tried adding more… MIght have to mount it different… I have it in the back just tied down… But ugh… I don’t know what to do.

post a picture

You have the double-tread arm, correct? I don’t see how much elastic can do with this design, but for keeping the arm up, you should use your motors on partial power to hold the arm up. We used the scissor lift because it stays up without the motors having to hold it.

Think of arm as a point mass at the end of a lever.
The Gravitational Potential Energy when arm is up is
GPE = mgh (mass * gravity * height)

Think of rubberbands or elastic as a spring, with Spring Potential Energy
SPE = kxx/2
where k is strength of spring, and x is amount it is stretched,
and force of spring at any point is kx.

For a 4-bar arm, the easiest spring configuration is from

• hand end of the low bar
• to the shoulder end of the high bar

For a 1-bar arm, the usual problem is wrong radius for the spring to act on to provide torque.

The rubber tubing does tend to stretch out if stressed over time. Some quick fixes:

1. Change to fresh tubing. Before you attach it, lift the arm to its maximum height (or the position where the tubing is at its shortest). Store the robot with the tubing in its shortest position. Change to fresh tubing the morning of the event. If you don’t have any more/enough tubing, untie the old piece and re-tie it, making it shorter than before. Warning: untying old tubing sometimes creates holes/cracks that will weaken the tubing.
2. Attach 2 or 3 layers/loops of tubing. Tie each loop separately, so the loops work independently if one breaks.

A programming fix takes longer, but it is sometimes needed. This thread has several solutions, but some are rather complicated. One of the simplest ones is post # 7 (one of the China teams). Our pseudocode modification of that solution is posted below (sorry, no Easy C v4 on this computer).

The program (in operator control) takes inputs normally from all channels when the arm is driven up or down.

When the arm is not driven, it checks to see if the arm is in the “middle”, where it needs a “boost”-a low level of power to hold up the arm. The variable “boost” can be tweaked to suit the weight of your arm, but we just tried a couple values, and it amazingly, they worked, so we picked the lowest value that could raise the arm and our maximum load of objects.

When the arm is at the far top and far bottom, it again takes inputs normally.

Use buttons on ch 6 to lift and lower the arm, and pot1 (input from the potentiometer) to give the level of the arm.

while(1)

[INDENT]pot1 = GetAnalogInput(1) //How high is arm?
Button1State = GetJoystickDigital(channel 6, button 1) // Is drop button pushed?
Button2State = GetJoystickDigital(channel 6, button 2) //Is lift button pushed?

If (Button1State==1)
[INDENT]connect all channels to motors normally[/INDENT]
Else if (Button2State==1)
[INDENT]connect all channels to motors normally[/INDENT]
Else
[INDENT]connect driving base channels to motors normally
If (Pot1>upperlevel) // protect gears by not raising arm too high
[INDENT]SetMotor (lift motors = 0)[/INDENT]
Else If (Pot<lowerlevel)//protect gears by not pushing arm too low
[INDENT]SetMotor (lift motors = 0)[/INDENT]
Else
[INDENT]SetMotor (left lift motor = boost) // arm is driven by 2 motors, opp directions
SetMotor (right lift motor = -1*boost)[/INDENT]
end else[/INDENT]
end else[/INDENT]
end while

The limits of this solution are that when the arm is above “upperlevel” it will drop to that level, but if you set “upperlevel” is too high, you risk crunching the gears because “boost” will push the arm up when there’s not enough downward torque to balance it. I’m still working on more sophisticated “holding up arm” solutions, and plan to share when they’re “prettier.”

A 1lb bag of the official size #32 rubber bands goes for about \$8 at your local office supply store. I think our robot has about 40-50 of these on now as they tend to stretch out over time and get weaker. We could replace them but adding them on is easier.

Sometimes the black surgical/pneumatic tubes offers an easier atatchment than hooking a slew of rubber bands via stand offs/shafts.

A picture would help give much better direction and help to you though.

How do you have yours tied? I don’t have a camera right now.

Lol I suggest you start putting rubber bands in as many places as possible. We put nearly 100 rubber bands on our robot and it worked quite well.

There certain sizes we must have? Thickness? Length? Ect…

Size #32 like Team80_Giraffes said. They are available nearly everywhere. Then start putting them everywhere that will help until your arm will stay up by itself without any power.

We usually put some from the back of arm to the bottom of our towers, but there are other helpful places.

Just make sure the elastic doesn’t overpower the resistance to turning given by the unpowered motors. You don’t want your elastic to pull the arm up before a match and violate your size limits.

Here is my set up from the back. I have 5 rubber bands on each side… Now this lifts my front off the ground which isn’t good…But it still isn’t holding like I would like during autonomous… I think I can move my anchors up a little to keep it from lifting the front of my intake from the ground. The question is, how do you find that “sweet” spot… That keeps the front intake where it should be, but put enough tension to keep it from falling during autonomous…

Well first of all I suggest using a small standoff instead of an axle because that will give stronger support and will be easier to put on/take off rubber bands. Second I suggest using at least 8-11 rubber bands on each side. Try experimenting until you find that sweet spot.

Well we had 10 on each side but it’s lifting the front intake off ground. We were doing standoff but it was a pain when adding more to unscrew stand off. We use shaft and collars and we can just loosen one side slide over and add more bands quicker.

why do you need to unscrew the standoff? you can just lift the arm all the way and put on and take off rubber bands. If 10 is too much then try 8.

Maybe I am confused… I had the stand off screwed on both sides so I would have to unscrew then slide bands on. Are you saying just screw the standoff on one side and leave a gap on the other so you can just slide them on and off?

We found that angling our tension helped a lot with the problem of having to balance “the amount of tension to lift the arm” and “the amount of tension to keep the arm on the ground”. To do this, we bisected the angle formed by the arm (a six bar) and the vertical. Our reasoning behind this tension was

1. When tension is parallel to the arm, it effectively does nothing at the lowest point (does not raise the arm off the ground). This tension is most effective at 90 degrees + the number of degrees you offset the tension from the vertical.

2. When the tension is perfectly perpendicular to the ground, it is most effective when the arm is at raised 90 degree, but it will act upon the arm at its lowest position.

Thus, we found the midpoint to be the most helpful. Someone correct me if I’m extremely wrong – this is purely based on our own experimentation. Also, we use a PI loop just to iron things out, prevent motors from breaking or stressing out and go to certain setpoints.

Here’s a link illustrating these ideas.

The picture didn’t show… Would LOVE to see a picture of it… Thanks for the help guys.