Scion Instruments 436-GC Service Manual page 77

436-GC/456-GC
Flow Controller.
The Flow Controller is a closed loop Proportional Integral type (P1) controller.
A flow sensor is formed from a differential pressure transducer sensing the pressure drop across a flow
restrictor. The Pressure transducer appears as a Wheatstone bridge and is connected to the PWA via J3.
The differential outputs of the bridge are connected to J3,4&2, which are then amplified by AR4. AR4 is a
low drift instrumentation amplifier. [Gain = 49.4K/(R30) +1 = 80.8] The top of transducer bridge is
connected to J3,5 and is driven from the output of opamp AR7,7. The bottom of the bridge is connected to
J3,3 and J3,25. This connection also connects a laser trimmed current sense resistor located on the
transducer assembly to the bottom of the transducer bridge.
The remaining side of the current sense resistor connects to analog ground at J3,24.
The excitation current flowing through the bridge is sensed by the input to opamp AR7,6 and compared to
the 2.5 Volt reference at AR7,5. In this way the bridge excitation current is kept constant with reference to
the 2.5 Volt reference.U4 is a digital to analog converter with a voltage output that varies between 0.0 Volts
and - VREF. VREF is the +5.00 Volt reference voltage that connects to U4,2&7.
The Flow DAC output connects from U4,3 to R3,3. R3 is a precision low drift resistor network. Part of this
network is used to form a summing node at R3,13 and R3,14. This node sums the output of the AR4 and
U4,3 together. Under normal conditions the output of AR4 is positive and the output of U4,3 is negative.
When the controller is at a controlled setpoint the junction of the summing node approaches zero.
The summing node junction connects to an integrator formed by AR6,C33 and R10, and an inverting
amplifier formed by AR5,R4 and R37. The outputs of the integrator and the inverting amplifier are summed
together at the node where R31 and R51 join together. In this way the integral and proportional terms of
the controller are added together. The summed Integral and Proportional terms provide a driving voltage to
AR3,3.The coil of an analog proportional valve is connected to J2,13&14. Q3,R12,R56 and 1/4 of AR3 form
a current sink driver for the proportional valve.A voltage at AR3,3 translates into a current scaled by sense
resistor R12.In this way a control loop error occurring at the summing node of R3,13 and R3,14 causes a
change to the valve coil current in a direction that corrects the error.R55,C38,R56 and R57 keep the
current sink stable.R27 and R48 provide a nominal current drive to the proportional valve when the
Integrator output is zero volts. Changing the impedance of R27 and R48 provides a means of adjusting the
forward gain of the controller.An offset voltage of 1.25 Volts is connected to AR4,5 ensures a positive offset
voltage above zero. For a DAC voltage of zero the controller is guaranteed to turn the proportional valve
hard OFF.Partial immunity to ground noise is achieved by referencing the bottom of R48 and AR5,10 to the
+14VGND.
Notes:
AR6 is a high gain extremely low input bias current precision opamp. It was selected for it's high gain, and
the low input bias current permits easy use of high resistor values and low capacitor values in both the flow
controller and pressure controller integrators.
R3 is a thin film resistor pack with a 5ppm/C ratiometric drift spec. It is used for it's thermal stability.
Pressure Controller.
The Pressure Controller is a closed loop Proportional, Integral, Derivative type (PID) controller.
A Pressure transducer is connected to J3 appears as a Wheatstone bridge. The differential outputs of the
bridge are connected to J3,12&10, which are then amplified by AR2. AR2 is a low drift instrumentation
amplifier. [Gain = 49.4K/(R59) +1 = 10.9]
The top of transducer bridge is connected to J3,12 and is driven from the output of opamp AR7,14. The
bottom of the bridge is connected to J3,11 and J3,17. This connection also connects a laser trimmed
current sense resistor located on the transducer assembly to the bottom of the transducer bridge.
The remaining side of the current sense resistor connects to analog ground at J3,16.
The excitation current flowing through the bridge is sensed by the input to opamp AR7,13 and compared to
the 2.5 Volt reference at AR7,12. In this way the bridge excitation current is kept constant with reference to
the 2.5 Volt reference.
U4 is a digital to analog converter with a voltage output that varies between 0.0 Volts and - VREF. VREF is
the +5.00 Volt reference voltage that connects to U4,2&7.
The Pressure DAC output connects from U4,6 to R3,5. R3 is a precision low drift resistor network. Part of
this network is used to form a summing node at R3,11 and R3,12.
This node sums the output of the AR2 and U4,6 together. Under normal conditions the output of AR2 is
positive and the output of U4,6 is negative. When the controller is at a controlled setpoint the junction of the
summing node approaches zero.
SCION Instruments
436-GC/456-GC Service Manual Revision B February 2019
610 Hardware description
Page: 76