Classic feedback control loop with set-point, error, controller, plant, sensor and disturbance.
For controls / robotics / process-engineering courses and papers.
Re-label all signals with sampling index k (r[k], e[k], u[k], y[k]) and replace transfer-function labels with z-domain expressions (Kp + Ki * Ts / (1 - z^-1) + Kd / Ts * (1 - z^-1)).