DriveTrain TankDrive Control

[mikets]

Originally Posted by mikets

We are actually doing 3. The built-in PID control of CANTalon is nice but it is doing a per motor PID control which is fine if we are controlling a mechanism with only one motor (e.g. an arm or an elevator). In a drive base where you have at least 2, 4 or even 6 motors, individual motor PID control doesn't address the need for the drive base to run straight. For example, if you try to have the robot go forward 10 feet and say you have calculated 10 feet is equivalent to 10,000 encoder counts. If you command each motor to go 10000 encoder counts, what if the left motor is slightly faster than the right motor. The robot will curve to the right but the left motor will achieve the 10000 count earlier than the right motor, so it will stop while the right motor will try to catch up. This means the robot will now curve left. It will get there but the robot may be traveling in a slight 'S'. In a multi-motor PID control scenario, you need an overall PID control, not individual motor PID control. You need the ability to look at the overall drive base movement and be able to shift different power to different wheels in order to keep the robot on target whether it is the heading or the distance. So in our library, we implemented a PIDDrive class that takes two PID controllers. One PID controller is controlling the distance and the other PID controller is controlling the heading. My analogy to this is one is controlling the gas pedal and the other controlling the steering wheel. The PIDDrive class will combine all sensor data (e.g. 4 encoders and 1 gyro) to come up with "distance travelled" and "heading" data. Then it applies the distance data to the distance PID controller to calculate the base power for all four wheels. It also applies the heading data to the "Turn PID controller" to calculate the differential power to the left and the right side of the wheels. So it can add/subtract a delta power from the base power between the left and right side of the wheels (i.e. shift power from the left to the right and vice versa). This overall PID control is a lot better than individual motor PID control.
BTW, if you want to look at our library, you can find it here:
https://github.com/trc492/Frc2016Fir...cPidDrive.java

Just realize I have moved the file to a different package and so link is out-dated. Here is a new link:
https://github.com/trc492/Frc2016Fir...cPidDrive.java