Glossary of Control Engineering Terms - D

Dead Time: Dead time is the amount of time that it takes for your process variable to start changing after your valve changes. If you were taking a shower, the dead time is the amount of time it would take for you (the controller) to feel a change in temperature after you have adjusted the hot or cold water. Pure dead time processes are usually found in plug flow or solids transportation loops. Examples are paper machine and conveyor belt loops. Dead time is also called delay. A controller cannot make the process variable respond before the process dead time. To a controller, a process may appear to have more dead time than what it actually has. That is, the controller cannot be tuned tight enough (without going unstable) to make the process variable respond appreciably before an equivalent dead time. More accurately, the characteristic time of the loop is determined by equivalent dead time. Equivalent dead time consists of pure dead time plus process components contributing more than 180 degrees of phase lag. The phase of dead time increases proportionally with frequency. Any process having more than 180 degrees phase lag has equivalent dead time.

Delay: This term is often used in place of dead time.

Decoupler: Part of a controller that is designed to make a multivariable plant equivalent to a collection of single loops such that single loop controllers can be designed. The decoupler is often the inverse of the plant steady-state matrix gain.

Derivative: The "D" part of PID controllers. With derivative action, the controller output is proportional to the rate of change of the process variable or error. Derivative action can compensate for a changing process variable. Derivative is the "icing on the cake" in PID control, and is often not used. It can make the controller output jittery on a noisy loop and should be used with care. Normally a filtered derivative is used instead of a pure derivative.

DSP (Digital Signal Processor): An integrated circuit designed for high-speed data manipulations. Used in audio, communications, image manipulation, and other data-acquisition and data-control applications.

Discrete Logic: Refers to digital or "on or off" logic.

Discrete I/O: Senses or sends either "on or off" signals to the field. For example a discrete input would sense the position of a switch. A discrete output would turn on a pump or light.

Dominant Dead Time Process: If the dead time is larger than the lag time the process is a dominant dead time process.

Dominant Lag Process: Most processes consist of both dead time and lag. If the lag time is larger than the dead time, the process is a dominant lag process. Most process plant loops are dominant lag types. This includes most temperature, level, flow and pressure loops.