Scion Instruments 436-GC Service Manual page 78

436-GC/456-GC
The summing node junction connects to an integrator formed by AR6, C31 and R9, and an inverting
amplifier formed by AR5,R5 and R36.
A derivative term is formed by AR5, R7, R8, C25, C26, R34 and R35. The input to the derivative term is
connected to output of AR2.
The outputs of the integrator and the inverting amplifier as well as the derivative circuit are summed
together by a summing amplifier formed by AR6, R32, R33, R42, and R6. In this way the Integral
Proportional and Derivative terms of the controller are added together.
The summed terms at the output of AR6, 14 provide a driving voltage to AR3,5.
The coil of an analog proportional valve is connected to J2, 11&12. Q2, R13, R53 and 1/4 of AR3 form a
current sink driver for the proportional valve.
A voltage at AR3,5 translates into a current scaled by sense resistor R13.
In this way a control loop error occurring at the summing node of R3, 11 and R3,12 causes a change to the
valve coil current in a direction that corrects the error.
R53,C35, R54 and R49 keep the current sink stable.
An offset voltage of 1.25 Volts is connected to AR2,5 ensures a positive offset voltage above zero. For a
DAC voltage of zero the controller is guaranteed to turn the proportional valve hard OFF.
Immunity to ground noise on the +14VGND is improved by the 3.16K and 1K resistor network connected to
AR6,12.
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.
The derivative term input is connected to the output of AR2 instead of the loop error to prevent a change in
set point from causing a derivative "kick".
Temperature Controller.
The function of the temperature controller is to provide improved stability and calibration accuracy, by
keeping the flow and pressure transducers at a near constant temperature.
The controller has a machined aluminum block which surrounds the two pressure transducers that are
plugged into J3. The flow sensor flow restrictor is pressed into this same block.
Q4 functions as a heater and is bolted to the block. RT1 is a thermistor that is located inside the transducer
cavity of the block. R30 and R59 are in close proximity to the thermistor.
Under normal use a jumper is installed between J2, 15 & 16 connecting one side of R18 to RT1 to AR1,1.
The output of AR1 provides a negative voltage to the bottom of resistor divider R20 and R18. Under
conditions where the controller is at set point and controlling the resistance of RT1 will equal the resistance
of R18 and the bridge formed by R18, R19, R20 and RT1 will be in balance with the voltage at AR1,5
approaching zero.
A deviation in temperature from the set point value will cause the output of AR1,1 to change and result in a
correcting voltage at AR1,5. The output of AR1,7 drives the Integrator formed by AR1,C22,R47 and R2. As
the Integrator output changes it causes a change to the heater drive circuit formed by AR5, R43, R14 and
Q4. A voltage at AR5,12 causes a current to flow in Q4, the current sense resistor R14 returns a voltage to
AR5,13 forming a current sink circuit. R70 and C42 roll off the amplifier for stability.
A 499 ohm resistor between the base and emitter of Q4 serves to reduce the loop gain of the circuit and
help make it stable.
(The minimum current gain of Q4 is approximately 1000.)
As current in Q4 changes it's heat dissipation is also changed. As the control loop needs more heat the
drive is increased and for less heat reduced. In this way a closed loop temperature controller with a fixed
set point is formed. R18 is the set point value, and can be changed to be any resistance within the range of
the thermistor temperature curve.
Zero compensation is included for stability. A capacitance multiplier is formed by AR1, R1, R26 and C21.
This circuit behaves as if it were a capacitor across R23.
The value of capacitance is given by C21*(R26/R1) = 30uF.
The combination of this capacitance and R23 provides a compensating zero which stabilizes the circuit.
R28 and R46 form a resistor divider that sets the maximum current drive as well as changing the forward
gain of the controller.
SCION Instruments
436-GC/456-GC Service Manual Revision B February 2019
610 Hardware description
Page: 77