inch. If the flow rate increases so that
the disc needs to move to 50% of travel
(0.1 inch), then the force on the spring
relaxes by 50 pounds (Kx0.1), and the
pressure would be controlled at 45
pounds. Extrapolating, the outlet pressure would have to go to 40 psi for the
regulator shown in Figure 3 to go to
full-open 0.2 inch stroke. This is the
effect of proportional-only control. Control texts call this “proportional offset”
while regulator-specific literature calls
it “droop.”
Figure 2
Supply Pressure Effect
Another effect seen in regulators is the
variation in outlet pressure caused by
changes in inlet pressure. If the inlet
pressure for the Figure 3 regulator
increases by 100 psi, the disc effective
area is 1 inch, so the additional 100 psi
pressure causes 100 pounds of additional opening force. With the 10 in2
diaphragm, the effective set pressure
would then drop by 10 psi for the sum of
forces to balance again. Regulator literature either calls this “supply pressure
effect” or “inverse sympathetic ratio.”
Improved regulator designs use a balanced disc or a control pilot to increase
precision.
We are concerned here with control.
Some processes need more than just a
proportional control strategy to regulate
the process values within acceptable
limits. There are additional strategies
developed by engineers for better control. In the pneumatic days, the first was
called “reset.” Now we call it “
integral.” Additionally the industry has
developed “derivative” (formerly
“rate”), so we usually have three control strategies combined: proportional,
integral and derivative, or PID. It is
astounding to imagine that engineers
were designing these features into all-pneumatic controllers in the 1950s,
because now the most mundane electronic single-loop controller has all
three strategies in its programming as a
matter of course.
Integral control is better described as
reset. Whenever the measured variable
deviates from the set pressure, the controller resets its output so more and
