axis 14 the sensors which are inserted into the sensor apertures 136 138 enable a homing function to establish and then monitor the rotation state of the two planetary arms the sensors optically sense reflectors 140 142 shown in fig3 mounted respectively on tops of the counterweights 122 and 132 which are respectively angularly fixed with respect to the gearbox 120 and magnet arm 130 once the two arms have been homed the timing or relative phase of their rotations needs to be maintained in one embodiment for improving the synchronism a computer based motion controller 150 shown in fig1 is interposed between the system controller 88 and the motors 32 36 driving the rotary shafts 28 30 for example a devicenet dnet communication link 152 transfers commands from the system controller 88 to the motion controller 150 the dnet communication system is a well known industrial computerized control system demonstrating high reliability and ruggedness the motion controller 150 in turn controls two motor drives 154 156 over a communication link 158 for example based on the well known mechatrolink in general the motion controller 150 sends different sets of motion control signals to the two motor drives 154 156 indicating the respectively required motion of the two motors 32 36 the motor drives 154 156 respectively drive the two motors 32 36 with the required phase between the rotations of the motors 32 36 the system controller 88 sequentially polls the various elements under its control by.