Return the native speed measurement required to achieve desired rotations-per-minute

def to_native_units(self, motor):
        """
        Return the native speed measurement required to achieve desired rotations-per-minute
        """
        assert abs(self.rotations_per_minute) <= motor.max_rpm,\
            "invalid rotations-per-minute: {} max RPM is {}, {} was requested".format(
            motor, motor.max_rpm, self.rotations_per_minute)
        return self.rotations_per_minute/motor.max_rpm * motor.max_speed