Watlow 8LS Controller - Manual (Page 136)

Glossary
128 8LS User’s Guide
O
Offset
The difference in temperature between the set-
point and the actual process temperature. Offset
is the error in the process variable that is typical
of proportional-only control.
On/Off Control
A method of control that turns the output full on
until setpoint is reached, and then off until the
process error exceeds the hysteresis.
Open Loop
A control system with no sensory feedback.
Operator Menus
The menus accessible from the front panel of a
controller. These menus allow operators to set or
change various control actions or features.
Optical Isolation
Two electronic networks that are connected
through an LED (Light Emitting Diode) and a
photoelectric receiver. There is no electrical con-
tinuity between the two networks.
Output
Control signal action in response to the difference
between setpoint and process variable.
Output Type
The form of PID control output, such as Time
Proportioning, Distributed Zero Crossing,
SDAC, or Analog. Also the description of the
electrical hardware that makes up the output.
Overshoot
The amount by which a process variable exceeds
the setpoint before it stabilizes.
P
Panel Lock
A feature that prevents operation of the front
panel by unauthorized people.
PID
Proportional, Integral, Derivative. A control
mode with three functions:
Proportional action dampens the system
response, Integral corrects for droops, and Deriv-
ative prevents overshoot and undershoot.
Polarity
The electrical quality of having two opposite
poles, one positive and one negative. Polarity
determines the direction in which a current tends
to flow.
Process Variable
The parameter that is controlled or measured.
Typical examples are temperature, relative
humidity, pressure, flow, fluid level, events, etc.
The high process variable is the highest value of
the process range, expressed in engineering units.
The low process variable is the lowest value of
the process range.
Proportional (P)
Output effort proportional to the error from set-
point. For example, if the proportional band is
20
º and the process is 10 º below the setpoint, the
heat proportioned effort is 50%. The lower the
PB value, the higher the gain.
Proportional Band (PB)
A range in which the proportioning function of
the control is active. Expressed in units, degrees
or percent of span.
See PID.
Proportional Control
A control using only the P (proportional) value of
PID control.
Pulse Input
Digital pulse signals from devices, such as opti-
cal encoders.
R
Ramp
A programmed increase in the temperature of a
setpoint system.