Feilsignalet

Feilsignalet is a term used in Scandinavian languages to denote the error signal in a dynamic system. The word combines feil, meaning error or fault, with signalet, meaning the signal, and is used in contexts ranging from control theory to fault diagnosis. The feilsignalet represents the discrepancy between a desired reference signal and the system’s actual output or measurement.

In continuous-time control theory, the error signal is commonly written as e(t) = r(t) - y(t), where r(t) is the reference (setpoint) and y(t) is the measured output. In discrete-time systems this becomes e[k] = r[k] - y[k]. Sign conventions vary; some texts use e(t) = y(t) - r(t).

The error signal is the primary input to feedback controllers, which aim to minimize the error by adjusting actuators. In a proportional–integral–derivative (PID) controller, for example, the control action is a function of e and its integral and derivative. In fault-detection applications, the feilsignalet can be monitored as a residual or innovation signal to identify deviations from expected behavior.

Applications of the concept span industrial automation, process control, robotics, aerospace, and electrical engineering. Beyond control, error signals are used in signal processing and diagnostics to characterize disturbances, drift, or sensor faults.

Related terms include the setpoint, reference signal, residual, and feedback loop.