Velocity PID control and setpoint ramping

[Oblarg]

One thing I'm still rather clueless about is how little ramping we could get away with while still reducing the impact on the gears. We haven't yet timed how fast the robot is decelerating under the current control loop, and we probably won't be able to between now and our next district.

If we knew both that and by what factor we need to reduce the loading on the drive to reduce the chance of future damage to acceptable levels (I'd say just preventing gear failure would be acceptable), then we could calculate a rate to start with. Since we're not immediately stripping this out, I don't think we're too far above such a threshold, so perhaps halving the acceleration would be a good place to start...

Alternatively, if we knew the max rated load of the gears we could calculate the maximum acceptable acceleration from the robot weight and gear diameter, but I don't see any such figure on VexPro (and I wouldn't be surprised if gear failure is more complicated than simply a "maximum load").

Anyone have any guesses?


EDIT: Looking at an online gear strength calculator (http://www.botlanta.org/converters/dale-calc/gear.html), it seems I should be aiming for under 100 lbs of force on the gear.

The gear that broke (14t) was meshing with the gear on the output shaft (60t). Both of these gears are experiencing the same load. The load on this set of gears is clearly going to be higher than that on the input gear that meshes with the CIMs. So, I reason thus:

The maximum permissible torque on the output shaft is 100 lbf * 3 inches (pitch diameter of 60t gear).

The robot mass is about 130 lbm with bumpers and battery, one half of which is loaded on any given side of the drive. The wheels are 8-inch diameter.

Thus, the maximum permissible acceleration of the robot is (100 lbf * 3 inches) / (130/2 lbm * 8 inches) = 5/8 g = ~20 ft/s^2

Our robot moves about 10 feet/second at top speed, so we should be decelerating from full speed to zero in no more than half a second.

Does this seem right to everyone else? Any idea how close to this we should be willing to stray?

[tr6scott]

So, in labview...
lineal ramp...
We use this.

Believe it was written by Killer Bees, we have just been dragging it along for years.

[Oblarg]

We toyed with an input ramp today, and found that the values we had been considering (roughly a half-second from 0 to max) were way too slow for effective driving.

We're going to move forward with the ramp disabled for now, and hope that switching the 14t gear to a steel version will prevent further catastrophic drive failure (we observed no major damage to the 60t gear meshing with the 40t gear upon taking apart the gearboxes yesterday, so we think we'll probably be OK).

Thanks for all the help!