What is a PID controller in espresso machines?

A PID (Proportional-Integral-Derivative) controller is a digital temperature regulation system that monitors boiler temperature and makes rapid adjustments to maintain a precise setpoint. Without PID, machines heat up, overshoot, cool down, and cycle — causing shot temperature to vary. With PID, temperature is maintained within ±0.5°C. Most mid-range and prosumer machines include PID; entry-level machines use simpler thermostats with much wider temperature swings.