System Design Elements
Chapter Three: Overview
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System Design Elements
This section discusses, in a general way, how the 146 instrument can be used to control a
pressure/vacuum system. A Control board must be installed in the 146 instrument for the
control capabilities to function. The 146 instrument utilizes three parameters of control (gain,
lead, and integral), and these are discussed in some detail. This section also provides basic
information about pressure control, and explains how to recognize and correct pump choking.
General Control Theory
The 146 instrument can control a gas system via mass flow control with an MFC. The more
typical source of control is with pressure sensors. Only one sensor can control the gas system at
any one time. That is, the pressure reading from only one sensor is compared to a set point and
used to create a valve drive signal. To change which sensor is being used to control the system,
you change the channel designated as the control channel. The control parameters for each
Control board can be stored in a “control recipe.” The 146 instrument supports up to four control
recipes. These recipes can be edited in Tuning Mode. Each control recipe can be associated with
a different channel by selecting a different control channel in Setup Mode.
The control system for a 146 instrument typically consists of six parts:
•
the sensor(s)
•
a gas supply
•
a control valve (gas inlet or exhaust throttle valve)
•
a process chamber
•
a pumping system
•
the 146 instrument
The system may also include one or more ratio flow controllers, or mass flow controllers if
controlling via mass flow.
Normally, gas pressure is controlled by altering the pumping speed of a pump that removes gas
from the system (less frequently, it is regulated by controlling gas flow into the system via a
control valve). The 146 instrument can regulate the control valve, be it a downstream throttle
valve or an upstream solenoid valve, or control one or more ratio flow controllers. If a Control
board is not installed in the 146 instrument, a separate controller must be used to control gas
pressure.
When controlling gas pressure, the 146 unit obtains a pressure reading from a sensor operating on
the control channel, and compares the reading with a set point. It then sends an external control
signal to position a downstream throttle valve, an upstream solenoid valve, or to regulate one or
more ratio flow controllers. The gas system is continuously regulated to ensure that the actual
pressure reaches, and is maintained at, the set point pressure.
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