[techhelpbb]

A dead man's brake doesn't need to actually be part of the drive assembly.

One can arrest the momentum of a wheeled cart just by creating friction between the surface on which the cart rolls say by dropping a rubber covered shoe down on that surface to create drag (might want to tapper the edges on that shoe so it doesn't stop quite so violently that the robot might come off the cart). It might be much easier to make a brake like this.