PId

PID stands for proportional–integral–derivative controller in control engineering. It is a closed-loop feedback controller that seeks to minimize the error between a desired setpoint and a measured process variable by adjusting a control signal. The controller computes a signal u(t) as a weighted sum of three terms: proportional to the current error e(t), proportional to the integral of the error, and proportional to the derivative of the error. The gains Kp, Ki, and Kd determine the influence of each term. In discrete implementations, the equation is often written as u[k] = Kp e[k] + Ki Ts ∑ e[i] + Kd (e[k]-e[k-1])/Ts. Tuning aims to achieve fast response with minimal overshoot and steady-state error while avoiding instability and integral windup. Common methods include manual tuning, the Ziegler–Nichols rules, and model-based approaches. Applications include temperature control, motor speed, and process industries. Anti-windup measures and proper discretization are important for practical performance.

PID also refers to a process ID, the numeric identifier assigned by an operating system to each active process. PIDs are used by system calls, process management tools, and daemons to refer to processes and to signal or terminate them. PIDs are typically positive integers within a namespace; some systems reserve small values (for example, PID 1 is the init process on many Unix-like systems). PIDs may be recycled after a process ends, and containers or namespace isolation can create separate PID namespaces. Common tools such as ps, top, and kill display or operate on PIDs, and the file /proc/[pid] exposes information about the process. Security considerations include race conditions between obtaining a PID and acting on it, and the need to verify ownership and permissions.

Because PID is an acronym with multiple meanings, the context usually indicates whether it refers to control theory or to process management.