Up the Ramp
 

Hey guys I just got back from the St Louis regional where our team faired ok only due to alot of help from other teams. But one problem tht came up with us many times was the fact that our robot would only make it over the ramp in "auto" mode and would never make it up or over in normal driver mode. Why is that, it's stumped us for the past 48 hours.

Our team (940) ran 2 Chippy's with a 10 tooth gear on the motor going to a 60 tooth gear on the front wheel. The tires are the "Skyway Tuff wheels" 8.5 inch on the front and small casters on the back.

Any idea why only auto will get us up the ramp?????????

we had some issues with our driver because he was going full reverse to full forward and blowing the 40 amp breakers running our drill motors.

So it could be due to driver error and drive styles.

Make sure that when you move the joystick all the way, the appropriate speed controller maxes out (check the LED). If it doesn't then the problem is somewhere in your code.

-Joel

could it be that our alignment was off a little from left to right. it seemed to always want to go to the right. But I almost got rid of all the pull by changing the caser and camber, so Im still lost. Also could it be that auto mode can manage the bot better than a human?

I'm betting that auton mode sends the speed controllers a max signal but the joysticks do not. Anything mechanical would affect both auton and driver controlled performance.

-Joel

one last thing it seemed like a 6 to 1 reduction wasnt good enough at the camp we were super fast and had no torque, what is a good gear ratio for a chippy to get good torque?

we had alot of trouble with tripping breakers when we ran the chippys at 1:16. we're at 1:32 now and we're blazing fast, but we can't push well without tripping the breakers again. i'd guess 1:64 would prolly get ya about 4 to 6 feet per sec and decent pushing power.

good luck with the reductions =-\

edit: our wheels are about the same size as yours, too

I am guessing on this one but I would first ask if you calibrated the speed controllers. The calibration procedure sets the maximum speed forward and reverse as sent by the joysticks. In "auto mode" you were likely sending a higher value to the speed controllers than in joystick/human mode. This is a simple fix.
As to the chippies (I prefer "chalupa" myself.) these motors are designed to give max output power at a very high RPM, so if you want to get the max out of the motor, you have to design your gear ratios appropriately. This is not something to guess at and the size of you tires makes a big difference in the design. Choose a gear ratio that is too low and you will stall, too high and you will not run fast but you will have lot's of power. Joe Johnson and many others have spent a lot of time writing about this subject, see the White papers section on this site.
http://www.chiefdelphi.com/forums/pa...C&pagenumber=3

we run our chippy drive with an 80:1 reduction as the average gear reduction, we can vary it up and down with different output sprockets, but it has more than enough power to get itself over the ramp and move around some robots while doing so

Just use the drill motors, they work great :D

As Al S pointed out, if this IS the case, then you might calibrate your Victors.

Some Victors (883's and a 4) in "factory calibration" (default setting, cal button pushed at power on) which I tested recently gave maximum output (no pulses, steady 12V output) for PWM numbers of 40 and less and for 229 and above, leaving you lots of leeway for your joysticks to not give 0 and 254 (they won't, especially if you've trimmed them).

In the same test, they stopped giving pulsed output in the range from 124 to 139. If this is test was in fact valid, it explains why we have such a hard time "zeroing" the joysticks: The default program allows you to set the stick to 127 both ways, but the Victor gives output if the x and the y are both off by 1 or 2, resulting in a 123 being sent to the Victor.

I wonder if this could be the cause of the turning - the motors are going in opposite directions, and neutral seems displaced.

The test involved feeding a count-up or -down number to the selected PWM output, and looking at the o/p of the subject Victor on a 'scope. The Victor was loaded with a smallish 12V turn-signal lamp, about 1.5 cm diameter, about 3.5 cm long incl moulded glass base (can't locate the package, & I don't know the number offhand). Full output was declared to be a steady 12V output, with no pulses. Neutral was declared to be the state of no-pulse 0 V output.

Lloyd, did you guys happen to document the results by any chance? We have never gone to this extent and it would be interesting to see the relationships between digital in and actual output. Just wondering...

My GUESS is that you are runnig too high of a gear ratio. If it is a speed controller not giving you full power, (and the fact that you are using a gear ration of 6 to 1) your design is marginal as to whether it has enough torque to get you over the ramp.

Some quick calculations 8.5*PI()/12*5500/6/60 show you would be going 34 ft/sec at the motor's free speed. I'm guessing you are always running at half that or below. No matter what your speed controllers are doing, increasing your gear ratio will help. Even if you double your gear ratio to 12:1 you should still travel at speeds in excess of 15 ft/sec (crazy fast) and still have twice as much torque.

My true recommendation would be to get your gear ratio in the neighborhood of 25:1 at a minimum if you are not shifting gears.

Just a note to add to the earlier comment about the joysticks.
Autonomous mode will send exactly the max PWM you tell it to, but we've found variations from joystick to joystick even discounting offsets due to trim wheel. One set of our joysticks when trimmed perfectly send values between 23 and 239 as the max/min, so human drivers would always be under powered.
The simple check is to measure each of your joystick outputs with DEBUG statements and the slop can be easily corrected for in the code.

I think we can attest to this fact.

We used the drill motors for our two wheel drive and when using direct drive had problems with the breaker flipping constantly because of too much torque on them. After we converted to direct drive, we almost got up there, but the breakers would flip and our front castors would send us crashing down to the bottom of the ramp again.

I think the drill motors this year were one of the biggest problems that teams had. If it wasn't one thing, it was another!