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1.3 Requirements
1. Serial: The Supervisor supports direct set point control by the host computer
using an RS422/RS485 serial communications interface. The RS422/RS485
serial interface uses an ASCII text command message format. The host com-
puter can access all of the Supervisor's menu parameters including the pres-
sure set point, the measured pressure and all of the control parameters. For a
more information about RS422/RS485 serial communications, refer to the
“SENTRY Supervisor Operation Manual and Serial Communications
Programmer’s Guide.”
2. Analog: An alternate interface uses a 0-5 V analog signal for set point control.
The process tool controller sends a set point signal that corresponds to the ex-
haust pressure set point in the process recipe. The exhaust set point from 0-5
volts is directly proportional to the full scale of the SENTRY 1510 pressure
transducer or 0-50.8 mm of water column. The SENTRY 1510 will control the
process pressure in the range of 5-46 mm of water column, which corresponds
to an analog set point control voltage range of 0.295 V to 4.528 V. When the
recipe entry for exhaust level changes, the process tool controller changes the
set point signal. The Supervisor receives the set point signal at an A/D port.
The Supervisor interprets the digitized set point value as a pressure between 0-
50.8 mm of water column and commands the stepper motor to adjust the
spring tension.
In any control mode, the set point is compared to the measured pressure. If the
set point value has changed, the Supervisor's closed-loop PID algorithm will
compute a change in the stepper motor position to change the spring tension. The
stepper motor will be pulsed and the spring position will be changed until the
measured pressure equals the set point.
The Supervisor's closed-loop algorithm executes at a rate of 4 loops per second.
Pressure measurements and graphic display updates are posted at that rate. In
general, the stepper motor position will remain fixed unless a set point change is
made by a change in the set point signal. Once the SENTRY 1510 has reached
the new set point, only small occasional updates are made to the spring, as fluc-
tuations are compensated for mechanically by the piston regulator. However,
significant changes in process gas flow rates or major changes in facility exhaust
can cause the closed-loop control to update the spring position.
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