Chapter Three: Overview General Information
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Chapter Three: Overview
General Information
The Type 250 controller is a versatile controller capable of pressure, flow, or flow ratio control.
It is used for pressure control when a pressure transducer provides the feedback signal, and flow
control when a mass flow meter provides the feedback. If a second mass flow meter is used for
external set point, flow
ratio
is controlled.
The purpose of a controller is to compare the required pressure level (set point) with the actual
pressure level (feedback input) and make appropriate corrections in the pump speed (via
adjustments on the valve) until the actual pressure equals the required pressure.
Figure 3, page 18, shows a simple pressure control loop consisting of an input signal, controller
output valve, vacuum system, and pressure transducer. The pressure transducer converts pressure
to an electrical signal which is compared in the controller to the required signal (in most cases, a
precision pot whose voltage is changed by rotating the dial). Note that in a simple pressure loop,
the flow rate is not measured because it is unnecessary.
Any error between the actual (feedback) and required (set point) signals is amplified by the
controller and fed to the valve. For example, if the pressure is higher than the set point, the
controller reduces the current to the valve, which reduces the flow into the vacuum system,
subsequently reducing the pressure.
An additional requirement of the controller is to provide a stabilizing or tuning control. This
control (PHASE LEAD) allows you to compensate for the different delays, or lags, that occur in
various systems.
Multi-Gas Ratio Control
When controlling multiple gases, the 250 unit operates in a similar manner to the single gas
system, except that a 0 to 10 Volt Pressure Control Signal (PCS) from the 250 controller is used
to control the set point signals to multiple mass flow controllers (MFC).
As in single gas systems, if the pressure is below the set point, the controller increases the PCS
which increases all flows in a predetermined ratio. The exact ratio of the flow is determined
using a power supply/display unit such as the 247, 647, or multiple 246 units.
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