I have been thinking about this and perhaps the best way to do it is to halve the transformer. Install the primary coil in the charging station and the secondary on the robot. Kind of like the rechargeable toothbrush. You will lose eficiency, but will not electrocute anyone.
Another way would be to separate a switching power supply because it allready has 2 circuits connected through optical couplers, if I remember corectly.
Or, just use a transformer with a high enough voltage in the secondary coil, make contact plates, then regulate different voltages onboard the robot.