Page 293 - Electrical Engineering Dictionary

fold-over distortion the names given to force manipulability ellipsoid a model
the phenomena associated with the genera- that characterizes the end-effector forces
tion of an erroneously created signal in the and/or torques, F, that can be generated
sampling process are called “fold-over” dis- with the given set of joint forces and/or
tortion or aliasing. Sampling a signal ex- torques, τ, with the manipulator in a given
hibits many of the same properties that a posture. Mathematically assuming that the
“mixer” circuit in RF communications pos- joint forces and/or torques, satisfy the equa-
T
sesses. The mathematical relationship for a tion τ τ = 1, the ellipsoid (taking into ac-
mixer circuit and a sampling circuit is ex- count kineto-static duality) can be written as
T
T
pressed by the trigonometric identity follows: F (JJ )F = 1. Here, J denote
geometric Jacobian of the manipulator. See
sin A·sin B = .5 cos(A−B)−.5 cos(A+B) also kineto-static duality.
forced commutation the use of external
From the equation it is evident that if the
circuitry to artificially force a current zero,
A radian frequency is not twice the B radian
thus allowing a diode or thryistor to turn off.
frequency, then the A − B term will produce
a signal whose radian frequency is less than
forced interruption an interruption in
B’s. This created signal will appear within
electric supply caused by human error, inap-
the original frequency bandwidth. These er-
propriate equipment operation, or resulting
ror signals can be minimized by incorporat-
from situations in which a device is quickly
ing an anti-aliasing filter (i.e., a low-pass fil-
taken out of service by automatic or manual
ter) on the input to the sample-and-hold cir-
switching operations.
cuit. The anti-aliasing filter bandlimits the
input frequencies so that fold-over distortion
or aliasing is minimized. forced outage the unscheduled interrup-
tion of electric power to a portion of a power
folded sideband an FM method used fre- system due to equipment failure, weather
quently in video tape recording (VTR) sys- conditions, or other mishaps.
tems. The term refers to the fact that there is
no corresponding upper sideband to the one forced outage rate a measure of perfor-
existing sideband. mance usually applied to generation units. It
is the ratio of equipment down-time vs. the
folding the technique of mapping many total time that the unit is available for opera-
tasks to a single processor. tion.

foldover a form of distortion in a digital forced system a dynamic system is said to
communications system which occurs when be forced if it is excited by a nonzero external
the minimum sampling rate is less that two source.
times the analog input signal frequency.
foreground (1) a process that is currently
footing impedance the electrical imped- theprocessinteractingwiththeuserinashell,
ance between a steel tower and distant earth. as opposed to background processes, which
get suspended when they require input from
foot-candle the unit of illuminance when the user.
the foot is taken as the unit of length. It is the (2)atermdescribingaUnixcommandthat
illuminance on a surface one square foot in is using your terminal for input and output,
area on which there is a uniformly distributed so that you will not get another Unix prompt
flux of one lumen. until it is finished.

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2000 by CRC Press LLC