Seems like it's pretty common to use a tether (like the OpenROV) for yanking it back out in case of failures also - I assume any self-contained failsafe like dropping a weight could itself fail?
Yes, any failsafe could fail, but there are ways to reduce that risk. For example, if you use an “default safe” system where you have to actively keep a weight from falling, then when you lose signal or lose power, the weight drops, you know that in the “default” state, you’re safe. And example would be if you held a weight in place with an electro magnet. As soon as you lose power, the weight would drop, and the vessel would return to the surface.
This is the same idea behind semi truck brakes. The brakes are by default engaged. When the truck is running, there is a build up of pressure to disengage the brakes so that the truck can drive. If something goes wrong (loss of power on the truck), the brakes are engaged by default to stop the truck.
This was basically the first lesson in automation class, which made an impact at the time.
A safety switch should be normally closed, energizing a normally open relay, such that any broken cables or connections would be like hitting the switch, causing the relay to de-energize, subsequently cutting power to the main circuit.
Same in industrial control systems that are used to control machinery where a failure could lead to injury or loss of life (or property). The default is to stop the machine in a state where it is known to be safe, this can lead to some pretty difficult problems if you want to get out of that state to re-start the machinery but the alternative is to continue to move when you really shouldn't be.
Even something as simple as an emergency stop switch and the subsequent re-energizing of the system can be a very hard to solve problem from a control perspective. Especially with Servo systems with incremental rather than absolute encoders.
Common should probably give way to expected. The Navy uses tehtered control on its torpedos for the utmost in stealthy attacks. If the best the Navy and military budgets can come up with is tethered control, not likely something in the hobby world will be better.
