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1.2 General Description
General Description
The SENTRY 1510 consists of two parts:
1. Mechanical pressure regulation based on the "floating piston" concept
2. Electrical set point control, consisting of the SENTRY Supervisor, a pressure
transducer and a stepper motor.
A new pressure set point is provided to the SENTRY Supervisor through the
process tool controller,or it can be changed manually. When the set point is
changed, the Supervisor applies a PID algorithm to the difference between the
new set point and the current process pressure. Based on the results of the PID
Algorithm, the Supervisor commands the stepper motor to increase or decrease
the force on the piston to match the new set point. Once the new set point is
reached, the stepper motor becomes inactive and the "floating piston" takes over
dynamic control.
The "Floating Piston"
The SENTRY 1510 uses the concept of a "floating piston" for its dynamic control.
Figure 2.1 shows a cross section of the SENTRY 1510 controller. There are
several forces acting on the piston:
1. Gravity acting on the mass of the piston, pulling it down.
2. The spring, moving the piston up or down.
3. Clean room pressure, forcing down on the inside of the piston
4. Process pressure on the bottom of the piston
The piston's vertical position forms a gap that changes rapidly in response to fluc-
tuations in facility vacuum, reference port static pressure (clean room air pressure),
or process flows. With constant process flows and exhaust pressure, the piston
will stabilize at a certain height. When a fluctuation occurs, the net force on the
piston will no longer be balanced. The piston will move based on the direction of
the new net force until the forces are once again balanced. Changes in any of
these forces cause the piston to react as follows:
1. The weight of the piston cannot change
1
2. The spring can be moved up or down with a stepper motor (this can be thought
of as increasing or decreasing the effective weight of the piston). This changes
the pressure set point (or the pressure to which the valve is controlling) within a
wide house exhaust pressure range. When the house exhaust pressure moves
out of this range, the stepper motor can adjust the spring to maintain the set
point.
1
For those familiar with the SENTRY 1000, the movement of the spring changes the effective weight of
the piston, acting like weights added to the piston in the SENTRY 1000.
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