Calculating current required for powering multiple robots in parallel
From the textual description, this seems to be the configuration planned:







simulate this circuit &ndash; Schematic created using CircuitLab

The Magician controller documentation states:


  Built in dual FET "H" bridge with current limiting ...
  
  ... each output has been internally limited to 910mA to prevent damage from accidental short circuits.


So the motor driving FETs provide the required current limiting and short-circuit protection on the motor side, nothing additional needs to be done.

Current Calculation: 

The above tells us that no matter which DC motors are used, the maximum current in worst-case conditions would be of the order of 2 Amperes per robot with 2 motors, both stalled.

In order to provide for 3 robots, the floor grid must thus be able to supply up to 6 Amperes. Add a bit for headroom, say 8 Amperes total.

Voltage calculation: 

The Magician boards suggest that supply voltage should be greater than 5.5 Volts, and not more than 9 Volts. The bridge rectifier output will need to be smoothed by capacitors, to achieve the desired 6 to 9 Volt DC. 

A mains-to-6 Volt transformer will provide a peak DC voltage of 6 * 1.4142 = ~8.5 Volts minus 2x diode drop, so say around 7.5 Volts. (Transformers are rated at their RMS voltage. Peak voltage is obtained by multiplying by sqrt(2))

This is comfortably within the desired operating range, so you're all set, other than increasing the reservoir capacitor C1 until the system works reliably, and then adding a few thousand microfarads on top. The 4700 uF indicated in the diagram will possibly be too low at full motor load, leading to unacceptable ripple on the supply line - start with 4700, keep increasing this if the controller resets on acceleration or stall.