more corrective signal is added until
there is no deviation between the measured variable and the setpoint. In industrial control, the adjustment parameter
for integral mode is “resets per minute”
where a reset is a correction equal to
the proportional correction. Dedicated
control engineers have told me that the
automotive cruise control from our earlier example may operate solely in integral mode. If the vehicle deviates from
the set speed, it just keeps marching in
more correction until you’re back on the
magic number.
Adjusting the controller for more
resets per minute makes the control
action more assertive, until the tuning is
too aggressive and the loop becomes
unstable.
Sometimes process deviations come
along suddenly and it is appropriate to
cause the valve to jump to a new position to begin correcting the upset before
the process has even completed its deviation. Derivative control sends a control
output proportional to the rate that the
deviation occurs. It’s as close
as possible to actually anticipating an upset. This is useful
for processes such as temperature control; if there is a quick
change in demand the valve
can get ahead of temperature
deviation and bring it back
toward setpoint before the temperature gets too far out of
limits.
Figure 3
Feedforward Control
Yet another option is feedfor-
ward control. For example, if
the customer needs exactly
67.42% of valve travel for a
particular process mode,
he/she can program the DCS
to output 14.79 milliamps
blindly to send the valve to
that position. This avoids the
possibility of various instabili-
ties encountered with feed-
back loops. There is some divergence to
be dealt with; if the valve does not, for
Spence Engineering Company
Introduces:
P3-PistonPower Pump™
any reason (positioner error, wear, pipe
scaling, etc.), deliver exactly the correct flow rate, the cumulative results
will deviate from the desired value progressively after time. It is customary to
install a parallel feedback control loop
for “trim,” allowing instantaneous stable response and accuracy for continuous duty.
The First and Only Spring-less Non Electric
Condensate Pump!
The P3 is the First non-electric pressure driven conden-
sate pump that features a revolutionary new mechanism
with a spring-less design that helps prevent stall and break
down.
LCV Linear Control Valve
Single-Seated, Unbalanced Valve with Cage
Retained Seat.
Unique Design Allows Maximum Inter-
changeability of Parts
QB Rotary Control Valve
Designed and Engineered to Handle Nearly
All Industrial Process Control Requirements!
www.spenceengineering.com
1.800.398.2493
A Misconception
There is a prevailing misconception I
have encountered in the industry, which
is that users do not understand digital
positioners have an internal PID feature
and are reluctant to use them for fear
the digital positioner’s PID will interfere with their distributed control system’s PID. However, the digital controller’s PID loop is only concerned with
how accurately the positioner/actuator/
valve work together in response to input
from the plant DCS. An appropriately
tuned digital controller only follows the
control signal more precisely and does
not compromise the settings of the DCS
control loop.
In the pneumatic days, instrument
technicians of the time practiced calibration and tuning procedures that were
more art than science. Now, it is much
