[waialua359]

Originally Posted by ksafin

The video can be found on their channel in the iR3 section (http://www.youtube.com/user/robotin3days?feature=watch)

The gearbox they're using is made by them, it's the Modulox Lunch Box gearbox (http://www.andymark.com/product-p/am-0846.htm) however they've modified it a bit by adding a cylinder to push a dog gear for the shifting on the final drive axle, and have an encoder at one point.

Yes, if the motor is off and the gearbox is not in neutral it will almost definitely move. To counteract that, the iR3 team basically has an encoder on the gearbox and uses a closed-loop algorithm to essentially hold the winch back until they're ready to release into neutral (to my understanding). AKA, theres an encoder, and in fraction of a second, if it notices the force of the catapult pulling up on the winch (and thus moving the axle), the program will compensate for this force on the axle by the catapult by turning on the motor until it reaches the position it should be at. This is done at such a quick pace that it's not even visible and seems that the winch is completely steady. When ready to launch, just shift into neutral.

The two noteworthy videos are:
http://www.youtube.com/watch?v=4WZ1t...EBsIGLxVsJdhL5
http://www.youtube.com/watch?v=eLzEN...EBsIGLxVsJdhL5

I'm no programmer and wonder if the algorithm can truly hold back the force necessary to keep it stationary. In the video shown, there is a reference to Anthony Lapp helping them with the shifting mechanism, using a spring. It sounded to me that there is a latch of some sort involved. In 2010, we used a ratchet gear setup to prevent backdriving.
If its just shifting into neutral by removing one of the stage gears, with some fancy programming, then this should be a cinch!