Watlow Optimizing Your Process System with the Series 988 Controller - Manual (Page 12)

Optimizing Your Process System with the WATLOW Series 988
2.2
Basic Control Strategies and Terms
Process Control
Process controls are of two basic types: open
loop and closed-loop. An open-loop control
device has no self-correcting feedback informa-
tion. The closed loop system uses feedback
information from a sensor to adjust the system.
As the process changes, the feedback loop
provides up-to-date status information that the
controlling device uses to make self-correcting
adjustments. The closed-loop control device
provides much better process control.
Control Modes
A variety of control modes offer various degrees
of controllability. The most common modes are
on-off and PID control. The PID control cat-
egory includes varying degrees of complexity
that provide accurate, stable control under a
variety of conditions.
ON/OFF Control
The operation of the ON/OFF control, as the
name implies, turns the output device full ON
or full OFF. Temperature or process sensitivity
(hysterisis) is designed into the control action
between the ON and OFF switching points to
prevent switching the output device ON and
OFF within a span that is too narrow. Switch-
ing repeatedly within such a narrow span will
create a condition of intermittent, rapid switch-
ing, known as output “chattering.” The process
is always controlled “about set point,” dictated
by the switching hysterisis of the ON/OFF
control. This form of control action further
dictates that there will always be a certain
amount of overshoot and undershoot. The
Control Strategies
Proportional
Band
Time Proportioning
Time
Temperature
Set
Point
Droop
Overshoot
Switching
Sensitivity
On - Off
Time
Temperature
Set
Point
degree of overshoot and undershoot will be
dependent on the characteristics of the entire
system.
Time Proportioning
Time-proportioning control provides more
precise control of a process. A time-proportion-
ing control operates in the same way as an ON/
OFF control, when the process is far enough
away from set point to be outside the propor-
tional band. When the process approaches set
point and enters the proportional band, the
output device is switched ON and OFF at the
established cycle time. At the lower limit of the
band, the ON time is considerably greater than
the off time. As the process more closely
approaches set point, the ratio of ON to OFF
time changes: the amount of ON time decreases
as the OFF time increases. This change in
effective power to the load provides a throttling-
back effect, resulting in less overshoot.
The ON and OFF cyclic action continues until a
stable relationship is achieved. At that time,
the system will be stabilized such that the
process is controlled at a point below set point.
The process stabilizes with a resultant droop.
This condition will remain providing there are
no work-load changes in the system.
Integral
If the droop cannot be tolerated, there are ways
to compensate for it. Integral (automatic reset)
is an automatic adjustment to compensate for
Profile developed by proportioning control
Profile produced by ON-OFF control