Glossary of Control Engineering Terms - E

Eigenvalues: The state-space equivalent of the poles of a transfer function. They determine the stability and dynamics of a system.

Embedded System: A combination of computer hardware and software, and perhaps additional mechanical or other parts, designed to perform a dedicated function. In some cases, embedded systems are part of a larger system or product, as is the case of an anti-lock braking system in a car. Contrast with general-purpose computer.

Encoder: A measurement device which converts mechanical motion into electronic signals. Usually an encoder is a rotary device that outputs digital pulses which correspond to incremental angular motion. The encoder consists of a glass or metal wheel with alternating clear and opaque stripes that are detected by optical sensors to produce the digital outputs.

Equilibrium: A steady-state operating condition.

Equivalent Dead Time: To a controller, a process may appear to have more dead time than is actually present. More accurately, the equivalent dead time consists of pure dead time together with process components contributing more than 180 degrees of phase shift. The phase shift of dead time increases proportionally with frequency. Any process having more than 180 degrees phase shift has equivalent dead time. Controllers cannot be tuned to make the process variable respond appreciably before an equivalent dead time without going unstable.

Error: In closed loop control, error = setpoint - process variable (PV), or setpoint - feedback measurement. The controller uses the error in its calculation to generate the signal that will reduce the error to zero.

Ethernet: The standard for local communications networks developed jointly by Digital Equipment Corp., Xerox, and Intel. Ethernet baseband coaxial cable transmits data at speeds up to 10 megabits per second. Ethernet is used as the underlying transport vehicle by several upper-level protocols, including TCP/IP.