Hot runner controllers are specialized temperature controllers that keep specialized injection-mold components at the design temperature in order to keep the plastic flowing. They use several different control techniques. Limit control establishes set points that, when reached, sends a signal from the hot runner control to stop or start a process variable. Linear control matches a variable input signal with a correspondingly variable control signal. Feedforward control does not require a temperature sensor and provides direct control-compensation from the reference signal. Proportional, integral and derivative (PID) control requires real-time system feedback. PID control also monitors the error between the desired variable value and the actual value. Fuzzy logic is a hot runner temperature control technique in which variables can have imprecise values (as in partial truth) rather than a binary status (completely true or completely false). Hot runner controllers that use advanced or non-linear controls such as neural networking, adaptive gain, or emerging algorithms are also available.