in case of EMERGENCY, is a loop of NOPs an option?

[JesseK]

Easy partner... hardware timers are ONE OF THE best and most accurate ways of timing when code executes, or upon what interval. A must for true, high performance PID/LeadLie/StateSpace loops.

I'm not trying to be rude; it's just that I've seen this in control systems projects in college, at my current job, and in FIRST robots who dead-reckon their autonomous.

Your controller generally has the same voltage inputs and has a low power consumption. Therefore, on a FIRST robot it can output the required PWM signal to the Victor regardless of the battery voltage. However, if the battery isn't always at the same exact voltage the actual amount the motor turns will be different every time you run autonomous.

In the original poster's case, nOPs are good in an emergency situation, however it is very dependant upon the battery manager of the team ensuring the robot has a juiced up battery every time. You'd be surprised how many times you start a match with "charged" 10-11v battery simply because there wasn't enough time to charge it between the last match and your current one or someone mixed the batteries up. It also takes several hours to go from 12v to the max 13.2v. The point is, over the course of a day you'll probably almost never get consistent voltages. For mechatronical timing to work as suggested, the power source MUST be constant. It works in a pinch, but it's generally bad engineering to do in the long-term.

I believe the easiest-to-design sensor in this case would be a black mark on the shaft with a light sensor/line detector in conjunction with a state machine in the code in order to determine when the shaft is at the correct position. This will consistently get you to the correct spot every time. You can reference some of the photos in FIRST: Behind the design, team 111's feedback sensors for the turret. If the shaft must do more than one full rotation, you can either try to account for that in the code or use a shaft encoder instead.