PID balancer

The PID balancer is a baseline agent designed to test new Upkies with minimum dependencies. Try it out as follows:

./run_pid_balancer.sh

The agent balances the robot by PI feedback from torso-pitch and ground-position to wheel velocities (WheelController), keeping the legs' hip and knee joints straight (ServoController). The wheel controller adds a feedforward non-minimum phase trick for smoother transitions from standing to rolling.