MKS 1579A, Instruction Manual

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Chapter 7: Theory of Operation 1579A/579A
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Control Circuitry
In the digital control section the flow rate is compared to the setpoint value and a control signal
(digital) is generated.
The digital control signal is then conditioned by a PID-algorithm, optimized for fastest controlling
and finally fed into the control circuitry which steers the solenoid control valve. The digital control
reduces overshoots to a minimum and for completely regulating the flow until the difference from
the setpoint is zero. Typical settling time is 1 s, for faster tuning contact MKS.
The control valve is closed when no power is applied (Normally Closed, N.O.). Controlling flow is
done by levitating the valve plug from the valve orifice. The plug is mounted at the front end of the
solenoid armature.
Control Valve
The control valve is a specially designed solenoid driven valve. The armature is suspended by
springs. This design provides frictionless movement and thus precise control. Mounted at the front
end of the cylindrical armature is the valve plug which incorporates the seal disc. Forced by the
above mentioned springs the seal disc moves, when the solenoid is not activated, against the valve
orifice, closing it’s flow channel. Therefore the valve is closed when not activated. It is a „Normally
Closed“ (N.C.) valve.
The inside diameter of the orifice determines the conductance. Each flow controller incorporates a
valve orifice with a conductance in accordance to the full scale range. The valve orifice in standard
units is sized that with a pressure difference between inlet and outlet fitting of typical 1,75 bar to
2,75 bar the specified full scale flow rate will be achieved. (related to air or nitrogen). For more
information refer to
Appendix A, Specifications .
The mass flow monitor 579A/B has no valve!