Once you have some commands created, you can set one of them to be the default command for a subsystem. Default commands run automatically when nothing else is running that requires that subsystem. A good example is having a drive train subsystem with a default command that reads joysticks. That way, whenever the robot program isn't running other commands to operate the drive train under program control, it will operate with joysticks.

The RobotDrive class is designed to simplify the operation of the drive motors based on a model of the drive train configuration. The program describes the layout of the motors. Then the class can generate all the speed values to operate the motors for different configurations. For cases that fit the model, it provides a significant simplification to standard driving code. For more complex cases that aren’t directly supported by the RobotDrive class it may be subclassed to add additional features or not used at all.

An analog trigger is a way to convert an analog signal into a digital signal using resources built into the FPGA. The resulting digital signal can then be used directly or fed into other digital components of the FPGA such as the counter or encoder modules. The analog trigger module works by comparing analog signals to a voltage range set by the code. The specific return types and meanings depend on the analog trigger mode in use.

Ultrasonic sensors are a common way to find the distance from a robot to the nearest surface

Gyros typically in the FIRST kit of parts are provided by Analog Devices, and are actually angular rate sensors. The output voltage is proportional to the rate of rotation of the axis perpendicular to the top package surface of the gyro chip. The value is expressed in mV/°/second (degrees/second or rotation expressed as a voltage). By integrating (summing) the rate output over time, the system can derive the relative heading of the robot.

Another important specification for the gyro is its full-scale range. Gyros with high full-scale ranges can measure fast rotation without “pinning” the output. The scale is much larger so faster rotation rates can be read, but there is less resolution due to a much larger range of values spread over the same number of bits of digital to analog input. In selecting a gyro, you would ideally pick the one that had a full-scale range that matched the fastest rate of rotation your robot would experience. This would yield the highest accuracy possible, provided the robot never exceeded that range.