Code 17x: How To Set Up the MFC
Chapter Seven: Operation in Setup Mode
242
Set Point Mode of Operation
When using the Set Point mode of operation, the 146 unit sends out a voltage corresponding to a
user-defined set point (in sccm). The set point voltage takes into account any zero offset which
may have resulted from zeroing the MFC, to ensure that the MFC controls to the desired flow
rate. The 146 unit can operate up to four MFCs. Each MFC operating in Set Point mode has a
set point value assigned to it.
Totaling Mode of Operation
When using the Totaling mode of operation, two MFCs operate in conjunction with each other.
One MFC (defined as the co-channel), operates according to a fixed set point. The other MFC,
defined as the totaling MFC, adjusts its output voltage to maintain a user-defined total flow rate
(set point) for the system. For example, you want to maintain a flow rate of 100 sccm. Enter a
set point of 100 sccm and set the co-channel to a fixed flow rate of 20 sccm. The totaling MFC
will subtract 20 (flow rate of the co-channel) from 100 (the set point) and adjust its voltage to
produce a flow rate of 80 sccm. Therefore, the total flow rate will be 100 (20 + 80). As with the
Set Point mode of operation, the 146 accounts for any zero offset.
Ratio Mode of Operation
In the Ratio mode of operation, the 146 instrument is capable of controlling up to four MFCs
simultaneously. The 146 uses a pressure control signal (PCS), or a total flow signal, as feedback.
The Ratio mode uses a logical control channel to execute the PID control algorithm and achieve
the desired gas ratio. The logical control channel (control channel 0) is present even if no
Control board is installed. The 146 unit calculates the control signal to correct the gas ratio and
sends the signal to all MFCs configured for Ratio control.
Two possible control methods exist within Ratio mode: control with a pressure measuring
control channel, and control with a flow measuring (MFC) control channel.
The ratio controllers maintain the gas ratio, and the 146 instrument maintains the absolute
pressure by controlling the total flow.
Essentially, the Ratio mode of operation scales and proportions the 0 to 10 Volt control signal
among the MFCs configured for Ratio mode, based on their chosen ratio set points. The set
points can range from 0 to 200%. Since most MFCs accept a 0 to 5 Volt signal, the control
signal for a given MFC is always scaled by ½, then multiplied by the ratio set point percent:
MFC control signal = ½ (control signal)(ratio set point %/100)
Example 1: Assume you have two MFCs configured for ratio control and you change the
control set point to 100%. Each MFC has a set point of 50%. The PID control algorithm
determines that full scale flow is required to meet the new set point, so the equation becomes:
MFC control signal = ½ (10 Volts)(50%/100) = 2.5 Volts
Each MFC would receive a 2.5 Volt control signal.
Artisan Technology Group - Quality Instrumentation ... Guaranteed | (888) 88-SOURCE | www.artisantg.com
