MKS MF1 - Flow Path; Measurement Technique; Control Circuitry

Chapter 7: Theory of Operation
99
Flow Path
Upon entering the flow controller, the gas stream passes first through the metering section of the
instrument for its mass flow to be measured. The gas moves on through the control valve for its
rate of flow to be regulated according to the given set point, and then exits the instrument at the
established rate of flow.
The metering section consists of one of the following:
• A sensor tube for ranges <
10 sccm (N
2
equivalent)
• A sensor tube and parallel bypass for ranges > 10 sccm (N
2
equivalent)
The geometry of the sensor tube, in conjunction with the specified full scale flow rate, ensures fully
developed laminar flow in the sensing region. The bypass elements, in those instruments
containing them, are specifically matched to the characteristics of the sensor tube to achieve a
laminar flow splitting ratio which remains constant throughout each range.
Measurement Technique
The flow measurement is based on differential heat transfer between temperature sensing heater
elements, which are attached symmetrically to the sensor tube. This senses the thermal mass
movement, which is converted to mass flow via the specific heat, C
p
, of the gas. The resulting
signal is then amplified, digitalized and linearized. The corrected digital signal is then transferred to
the control section (controllers only) and also converted into a 0 – 5 V analog signal (Default
setting).
Analog versions of the units described herein provide the analog flow signal and via the USB Setup
Interface the digital information, Profibus, RS485 or USB versions provide just the digital
information without the analog signal.
The measurement principle of keeping temperatures constant results in much shorter response
time than conventional principles.
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 settle time is 0.8 s, for faster tuning contact MKS.
The control valve is closed when no power is applied (Normally Closed, N.C.). 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.