Watlow Optimizing Your Process System with the Series 988 Controller - Manual (Page 18)

Optimizing Your Process System with the WATLOW Series 988
2.8
Basic Control Strategies and Terms
Glossary
annunciator — a device that uses pilot lamps to
indicate the former or existing condition of a
system being monitored.
ANSI — American National Standards Institute.
burst fire — output that switches full AC cycles
ON and OFF repeatedly. Zero-cross burst fire
switches only at the zero-voltage point of the
AC sine wave. Variable-time-base burst fire
would switch ON and OFF 30 times a second to
achieve a 50-percent power level with a 60-
cycle AC power supply. Also see “zero switch-
ing.”
calibration offset — adjustment to the actual
process input and to the process values the
Series 988 uses for display and control.
cascade — control algorithm in which the output
of an outer control loop is the set point for an
inner loop. The inner loop, in turn, determines
the control action.
closed loop — control system that uses a sens-
ing device for process variable feedback.
cold junction — point of connection between
thermocouple metals and the electronic instru-
ment.
cold junction compensation — electronic
means to compensate for the effective tempera-
ture at the cold junction.
current transformer — a transformer, designed
for measuring electrical current, with its
primary winding connected in series with a
circuit carrying the current to be measured.
dead band — The dead band setting determines
the amount of interaction between heat (reverse
acting) and cool (direct acting) control outputs.
default parameters — the parameters, or pro-
grammed instructions, permanently stored in
the microprocessor software.
derivative — anticipatory action that senses the
rate of change of the process, and compensates
to minimize overshoot and undershoot. Also
see “rate.”
differential control — With enhanced software,
the Series 988 controller can control one
process in relation to the difference of a second
process. A set point is added to the measured
value of the second process. This sum is used
as the set point for the input 1 process value.
DIN — Deutsche Industrial Norms, a widely
recognized German standard for engineering
units.
droop — difference between the set point and
stabilized process value.
duplex control — With enhanced software,
duplex control splits a single process output
into two individual outputs. For example, a 4-
20mA output is split into a 4-12mA direct
acting (cooling) output and a 12-20mA reverse
acting (heating) output, thus allowing one
control output to function as two.
duty cycle — percentage of load-ON time relative
to total-cycle time.
external transmitter power supply — a DC
voltage source of power for external devices.
filter — as applied to the Series 988, a low-pass
filter designed to minimize display or process
input fluctuations.
form A — single-pole, single-throw relay that
only utilizes the normally open (N.O.) and
common contacts. These contacts close when
the relay coil is energized. The contacts open
when power is removed from the coil.
form B — single-pole, single-throw relay that
only utilizes the normally closed (N.C.) and
common contacts. These contacts will open
when the relay coil is energized. The contacts
will close when power is removed from the coil.