ator coupled to the throttle plate. The
throttle plate is no more than the automotive equivalent of the butterfly valve.
If you rest your foot lightly on the accelerator, you can feel the control system
modulating the position of the throttle
in response to the rise and fall of the
road. When you start uphill and the system senses the car slowing, it opens the
throttle. When you start down the other
side of the hill and the car gains speed,
the system closes the throttle. When it
all works properly, the speed variance is
too small to notice (see Figure 2, page
16).
Figure 1
cruise control in your vehicle. The
process sensor is usually a magnetic
sensor that infers vehicle speed by the
frequency of gear teeth passing a magnetic pickup. This frequency signal goes
to a black box and is compared to the
remembered frequency from when you
hit the “set” button. The black box
(controller) outputs to a pair of solenoid
valves to drive a vacuum-powered actu-
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All the components of a control loop can
be scattered around the plant, or they
can be combined into one device. A
pressure regulator measures the pressure by the force against the bottom of
the diaphragm and compares it to the
setpoint, which is the spring force
against the top of the diaphragm. This
modulates the valve disc proportionally
in response to any force imbalance
between the diaphragm and the spring
(see Figure 3, page 18).
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Proportional-only Control
Regulators can control pressure, as seen
in Figure 3, and almost any other
process variable such as flow, level or
temperature. But judging from the proliferation of control valves and control
systems, regulators are not perfect. The
spring and outlet pressure in a regulator
are only balanced at one position of the
disc. The disc can be repositioned only
in proportion to how much the actual
process pressure deviates from the setpoint.
Assume the Figure 3 diaphragm in
the regulator has an effective area of 10
square inches and the unbalanced area
of the disc is 1 square inch. Full stroke
of the regulator is arbitrarily assigned
at 0.2 inches, and the spring selected
has a spring constant K of 500 pounds
per inch. If the set (cracking) pressure
of the regulator is 50 psi, the force on
the regulator will be 50 psi x 10 in2 or
500 pounds. For the spring to balance
that it must be compressed X=F/K or 1
