Teledyne hfc-302 ユーザーズマニュアル

  

Page 13 of 31 

 

variable directly.  This analog output signal could be 0-5 volts, 0-10 volts (or 4-20 ma for units with 4-
20 ma boards) or any value in between. 

On the controller card there is a jumper that sets whether the control loop controls mass flow or an 
external process variable.  See Figure 2.7.  If the jumper is over the top two pins, the loop controls 

mass flow.  If the jumper is over the bottom two pins, the loop controls an external process variable.  
This process variable signal must be supplied on pin 12 of the D connector (for U pin out units) of the 

measurement card.  When the controller is set for external variable control it will open or close the 
valve as necessary to make the external process variable signal match the command signal.  The 
command signal may be 0-5 volts, 0-10 volts (4-20 ma for 4-20 ma input/output cards) or any value in 

between.  If the process variable has a response time that is much faster or slower than the flow 
meter signal it may be necessary to adjust the gain potentiometer. 

The flow controller will operate normally with any command input signal between 0-5 volts (4-20 ma 
for units with 4-20 ma input/output cards)  If the command signal exceeds ±14 volts it may damage the 

circuit cards.  During normal operation the control loop will open or close the valve to bring the output 
of the flow meter signal to within ± 0.001 volts of the command signal.  The command signal will not 

match the flow signal if there is insufficient gas pressure to generate the desired flow.  If the 
command signal exceeds 5 volts the controller will continue to increase the flow until the output 

matches the command signal.  However, the flow output does not have any guaranteed accuracy 
values under these conditions. 

If the command signal is less than 2% of full scale (0.1 volts or 4.32 ma) the valve override control 

circuit will activate in the closed position.  This will force the valve completely closed regardless of 
the flow signal. 

The valve override control line provides a method to override the loop controller and open or close the 
valve regardless of the flow or command signals.  During normal operation this line must be allowed to 

float freely.  This will allow the loop control to open and close the valve as it requires.  If the valve 
override line is forced high (> +5 volts) the valve will be forced full open.  If the valve-override line is 

forced negative (< -5 volts) the valve will be forced closed. 

On the top left of inlet side of the flow controller there is a hole 

through which the gain potentiometer is accessible (Fig 2.3).  This 
gain potentiometer affects the gain of the closed loop controller.  

Normally this potentiometer will be set at the factory for good 
stable control.  It may be necessary to adjust this potentiometer 

in the field if the system varies widely from the conditions under 
which the controller was setup.  Turning this gain potentiometer 

clockwise will improve stability.  Turning the potentiometer 
counter-clockwise will speed up the valve reaction time to 

changes in the command signal. 

 

Fig. 2.7