transportlag

Transport lag, also called dead time, is the time delay between a change at a system input and the observable effect at the output. It arises because material, energy, or information must travel a finite distance through a channel, pipe, or network. This phenomenon is common in chemical plants, pipelines, heating systems, and data networks, where disturbances or commands propagate before they can influence measurements or actuators.

In control engineering, transport lag is typically modeled as a pure time delay. In transfer-function form it is represented as e^{-sL}, where L is the lag time. When combined with dynamics the system may be described as a lag plus a dynamic, such as a first-order process with dead time (FOPDT) model characterized by gain K, time constant T, and delay L. The lag reduces phase margin and responsiveness, complicating controller design.

Implications for control include limited bandwidth, potential oscillations, and degraded disturbance rejection. L can be estimated from step tests or process knowledge and is a key parameter for controller tuning, particularly in predictive control. Techniques such as the Smith predictor compensate known dead time by using a model of the delay. Practical mitigation includes shortening transit paths, parallel channels, or employing feedforward or model-predictive control to anticipate outputs.

See also: dead time, time delay, Smith predictor, FOPDT model, process control.