Optical Wheel (Chopper)
The Optical Wheel can be seen in the picture above on the right hand side of the lower motor. The black plastic disk has 8 slots in it and must fit within a U shaped piece of plastic which contains a infrared light source and collector.
The controller monitors the speed of one of the motors (the one with a optical wheel "chopper" attached to an elongated motor shaft). If the speed of this motor is less than 3mph, no power will be provided to the motors. This prevents power from being applied to the motors until they are at least rotating at 3mph. This saves significant amounts of battery power and prevents the motorboard from starting accidentally.
The motor makes approximately 12.75 turns for every turn of the wheel. Since there are eight optical slots on the chopper per turn, this means 100 optical pulses per turn of the wheel. One turn of the wheel is at best 100 inches (39.26cm), so there is approximately 1 optical pulse per inch of travel. 3mph translates to 53 optical pulses per second, less than this amount, power will be cut from the motors. The Roth motorboards "maximum" speed of 18mph translates to 317 optical pulses per second.
Because there is no indication that the motor is moving less than 3mph, if the optical wheel fails for any reason the motorboard will assume that 3mph has not yet been reached. The motorboard will not function and there will be no indication as to the cause of failure.