Mobile devices with touchscreens (smartphones, tablets) are increasingly used to control remotely controlled toys, vehicles, drones, etc. or other devices.
In several cases an emergency stop functionality must exist. With older control systems, there was usually a physical big red button in a corner somewhere. In case of a smartphone/tablet, there are several issues with placing the emergency stop on the screen:
it takes up valuable space, especially as no other control elements should be placed near it (as the user is likely to be watching the remotely controlled vehicle instead of concentrating on the screen, so the accuracy on the screen might become severely limited). It is not a good thing to hit the emergency stop / emergency shutdown by mistake.
besides taking care to avoid triggering it by mistake, it also has to be accessible in an emergency. For example, if the vehicle is heading towards a cliff (or staircase) or a wall at high speed, the user doesn't have time to fumble through menus to find the emergency stop.
I'm thinking about using the physical buttons which are outside the touch-screen. Exiting the app by pressing the back key or the menu button might be considered as a choice (I'll just catch the onClose or onStop signals and emit the emergency stop command before closing the app), but it has a disadvantage for the case where several users can control the same device. In that case, closing the app might mean releasing control so others might continue controlling it.
There might be other physical buttons on the side of the mobile device (the power/wakeup button, the sound volume control etc.) but not all of them might be available on all types of mobile devices.
Is there an established practice for the emergency stop option for remotely controlled vehicles? If it's placed as a button on the screen, how to avoid triggering it by mistake while still making it quickly accessible in an emergency?