Watlow Optimizing Your Process System with the Series 988 Controller - Manual (Page 19)

Optimizing Your Process System with the WATLOW Series 988
Basic Control Strategies and Terms
2.9
form C — single-pole, double-throw relay that
utilizes the normally open (N.O.), normally
closed (N.C.) and common contacts. The user
has the option of wiring for a form A or form B
contact. Refer to the form A and form B above
for more information.
hunting — oscillation or fluctuation of the
process between the set point and process
variable.
hysteresis — in ON/OFF control, the process
change necessary to change the output from
full on to full off.
input linearization — For thermocouple and
RTD inputs, the process variable is automati-
cally linearized. Certain flow transmitters
generate a non-linear signal corresponding to
the flow being measured. The square root of the
signal is calculated to generate a linear signal.
integral — control action that automatically
eliminates the offset, or “droop,” between the
set point and actual process value. Also see
“reset.”
isolation — Electrical separation of sensor from
high voltage circuitry. Allows use of grounded
or ungrounded sensing element.
JIS — Joint Industrial Standards. Also Japanese
Industrial Standards Committee (JISC). Estab-
lishes standards for equipment and compo-
nents.
linearization — the extraction of a linear signal
from the non-linear signal of a flow transmitter.
Also see “input linearization.”
NEMA 4X — a front-panel rating that certifies
the control as washdown capable and corrosion
resistant.
ON/OFF control — control of a process by
turning the output full ON below set point and
full off above set point.
open loop — control system without sensory
feedback.
output — action in response to difference be-
tween the set point and process variable.
overshoot — condition in which the process
exceeds set point due to initial power up or
process changes.
P control — proportioning control.
PD control — proportioning control with deriva-
tive (rate) action.
PDR control — proportional derivative control
with manual reset is used in fast responding
systems where the reset causes instabilities.
With PDR control, an operator can enter a
manual reset value that will eliminate any
droop in the system. The option requires
enhanced software.
PI control — proportioning control with integral
(auto-reset) action.
PID control — proportioning control with inte-
gral (auto-reset) and derivative (rate) action.
process variable — regulated system parameter,
such as time, temperature, flow, humidity, etc.
proportional band — span of the process from
the set point within which time proportional
control action takes place.
proportioning control — See "time proportion-
ing control."
rate band — a thermal control band that defines
where the rate (derivative) function begins. The
Series 988 rate band centers on the set point
and is twice the width of the proportional band.
ratio — application in which the flow of an
uncontrolled stream is measured and used to
maintain the flow of a controlled stream at a
ratio to the uncontrolled stream.