Turning Trouble!

[Cadetdan]

Our robot is having a really hard time making simple turns. What is happening is that for about 30 seconds it will work fine, but then it starts making jerky turns. Basicly it starts and stops the turn even when the sticks are in the full turn position. Its almost like it has a mind of it own on turns, we dont have this trouble going forward though! Im really lost at this point any help would be great.

We are using the Cim's as motors and have a 27:1 reduction with 4WD.

Dan B

[ngreen]

You have to much traction. Many other teams are having this problem. What is happening is you are drawing to much current when you try to overcome the friction between the sliding wheels and carpet and your 40 amp breakers are switching making you jerky. I am assuming you are using a AWD setup. The best option is to make a set of the tires slide easier from side to side and keep your forward and backwards traction. If you are using the kit wheels. Many have solve part of this by attaching zip ties completely around a set of the wheels. If you have pnuematic tires, air them all the way up in most cases, some have complained about the bouncyness. Other team, with more time than you have, have created omniwheels which slide on rollers side to side and drive forward. There are other thread addressing these problems if you search under turning or boncy.

This is only during turning? If it does this driving straight you have a whole different range of problems.

Good luck and ask any more questions you need answered.

[KyleGilbert45]

Please search...

http://www.chiefdelphi.com/forums/sh...ighlight=turns

[Cadetdan]

I did search for the pre=posted thread, but I didnt think it was an over-grip problem. We have been running on a shop floor made of concrete, and our tire actualy lay rubber when we put the hammer down. So you guys are all thinking this is to much grip, and that we are tripping the 40 Amp breakers? Thanks for the help

[ngreen]

If you are burning rubber going straight and tripping breakers that way it most likely is the wrong gearing. What motors were you using? If it was the drill motors are you using the transmission in high or low gear? What kind of reduction do you have either from the the CIMs or from the drill motors? What size of wheels are you using?

From the information you have posted I see it as overgearing the motors. You'll have to provide more information before I could tell you.

[Cadetdan]

Were running the Cim's with a 27:1 reduction and using the Innova tires from the KOP. And were not tripping the breakers on forward or reverse, but we can lay some nice rubber.

[ngreen]

That doesn't seem too high..Ok, I took it as tripping the breaker going forward and backwards too. It shouldn't be the gear ratio, It doesn't seem like it would be friction in the system if it work normal forwards and backward (unless the drive shifts in someway that makes it bind when you turn). I would still stick with too much traction then. Usually it is better on concrete but if it is only while you are turning and your are using AWD tank steering traction and sliding always are a huge factor.

[the_short1]

jerkyness can also be caused be a WEAK batery!! ! ! !

push the 'select button' on the Operator interface until it shows the batery voltage.. *using a multimeter at this time is not the solution, it may read 12V, but while driving is a different story..

while driving and making turns, abserve how the voltage meter changes..(should fluctuate) if your voltage drops below about 7 (even momentarily) thats a sign your batery is getting low, you will start having those problems turning!... this is partially due to the friction on your wheels as well, as that will increase the amp draw on all the motors, hence making your batery pay the price..

hope that helps.

[scali]

We've noticed the same problem, but it only happens when the battery is starting to die. In fact, that's how we know to change the battery in the middle of practice =P

[slickguy2007]

We had the same problem with turning as you have and we found a way to fix it. We decided to scrap the 4WD system and just ended up going with 2WD. This fixed the problem with the jerky turning and now it has even gotten faster at turning.


GO 1403!!!

[Warren Boudreau]

A low battery can cause this problem. Also, check to see if your backup battery is fully charged. And is plugged in.

It might be a software glitch since you say it doesn't occur until after 30 seconds or so.

Do you have any sensors on the robot? Hall effect sensors? Encoders? These could be causing problems.

Also, check which PWMs you are using to drive the speed controllers. Avoid PWM 13 through PWM 16 if you can. They are handled differently than the lower 12.

[Ken Patton]

Make sure your battery connections and high current connections are all really good.

Make sure you don't have misalignment or binding in the drivetrain when the wheels are side-loaded (which is exactly what happens when you are turning). If you can pull the motor only out of the drive system, and feel the friction as you just rotate the wheel, and then feel the friction as you sideload (dont be gentle) and rotate the wheel, you might identify a misalignment or binding problem.

If you still have the problem, consider moving weight around on the robot to take weight off the wheels you want to slip the most (take care to not do anything that makes you super-tippy of course). Or, as suggested earlier in the thread, make the wheel slip more by reducing its traction.

Ken

[Mark Pettit]

The Dukes (991) had the same problem. We found that there are a number of fixes that work for this. The aforementioned cable ties or else vacuum hose cable-tied onto the wheels work but they don't look very attractive.
If you have the option to gear down, then that will help.
Those were our 2004 solutions.
This year, we were having the problem with bouncing through turns, but when we started testing auto mode the robot was turning as smoothly as silk. This points to a software problem.
In RC mode, we're tank drive, so it's hard to get the same exact power output to each side while turning. To remedy that, we programmed a button on each joystick to turn the robot in place by sending equal (but opposite for each side) power to the motors. That worked for us!
BTW- we're using the 8" Skyway wheels.

[Crebb]

Another thing that can cause that is if you have radio interference. Make sure the radio is up pretty high and away from electronics and wires. Hope this helps.

[Al Skierkiewicz]

Originally Posted by Cadetdan

Our robot is having a really hard time making simple turns. What is happening is that for about 30 seconds it will work fine, but then it starts making jerky turns. Basicly it starts and stops the turn even when the sticks are in the full turn position. Its almost like it has a mind of it own on turns, we dont have this trouble going forward though! Im really lost at this point any help would be great.

We are using the Cim's as motors and have a 27:1 reduction with 4WD.

Dan B

Dan,
You have a lot of experience outlined above, believe it. There are a several problems I see here that point back to one effect. After you have run the battery down a little, (30 sec) the RC can no longer stay up. The RC goes into a sleep mode when the main battery goes below about 8 volts. The backup battery keeps the program running but no PWM outputs are enabled. That is why the robot appears to have a mind of it's own. If you check that battery status on the OI as outlined above you will see that the battery intermittently falls well below 9 or 10 volts. (you may not see 8 appear in the window but it is likely happening.) The fact that your robot "lays rubber" on a concrete surface is a good indication that the final gearing is a little high. It seems that 8-10 feet/sec is a good speed for battery life and control. A good rule of thumb is that a battery should last 3-4 matches. This gives a good indication that the current demand is not so high to cause the RC to sleep or to trip breakers. BTW tripping breakers make them very hot, so don't touch them right after a match.
The cause for all of this is documented (over the years in many threads) and is fairly easy to understand. When a robot uses four high friction wheels (or more) drive without steering, the output of the motors can lock during turns. (Four stalled chalupa motors exceed the maximum current available from a fully charged battery.) At best, the side loads on the wheel bearings produce incredible loads on the drive motors putting them in near stall conditions. The tighter and faster the turn, the worse the problem. You can decrease the effect in software (when turning) but the easiest fix is to eliminate side friction on one set of wheels by modifying the wheel design or adding different wheels. Omni wheels work great for this but do require some added practice for stability.
As far as radio interference goes, if your modem appears to be compromised it also may be due to the battery being pulled down by the intense currents generated by the drive during the turn. Anything that affects the battery can affect the entire robot.