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Control Mode Pneumatics
Pressure Supply Port
The Pressure Supply Port connects the user's regulated gas supply to the Pneumatics
Module. Please refer to Appendix A for gas specifications and supply pressure limits.
The supply pressure is connected to the right-hand port of the rear chassis. This regulated
pressure should be set to the full-scale Pt range + 10 psi (0.7 bar). Internally, this pressure
is regulated down further to supply the Ps control manifold.
Vacuum Supply (Exhaust) Port
The system can be operated with a single vacuum pump. Select a vacuum pump with the
test port volumes and system slew rates in consideration. A high capacity vacuum pump
is required to allow the unit to be zeroed. The minimum requirements are noted in
Appendix A. The vacuum pump must be fitted with an auto-vent solenoid valve that
vents the vacuum to atmosphere if the power to the pump is interrupted.
Pressure Control
Pressure control is performed by a dual-loop control system. The inner loop is a digital
loop using a high frequency response silicon strain gauge sensor, a digital controller, and
two pulse-width modulated solenoids. The two solenoids either add gas to the test port or
remove it. The outer loop provides closed-loop control, based on feedback from the
quartz Bourdon tube and an analog output setting which provides the desired set-point for
the inner loop.
Control Strategy
Inner vs. Outer loops
The inner loop accepts a signal from the outer loop and uses this signal as the pressure set
point. The inner loop uses a high frequency response strain gauge sensor as its reference.
The frequency response of this transducer allows for high-speed modulation of the
solenoids. This sensor is aligned to the high accuracy quartz sensor when the system is
Auto-tuned.
The outer loop is a lower frequency response, analog-digital loop. The outer loop uses a
calibrated, forced-balanced sensor. This sensor is used to monitor the pressure of the
system. The results are used for the displayed pressure. The outer loop is responsible for
adjusting the signal sent to the inner loop to compensate for temperature and time drifts
associated with the inner loop sensor.
Transducer Module
Quartz Bourdon Tube Sensor
The quartz Bourdon tube sensor is mounted in a machined aluminum housing. The sensor
consists of a helical quartz tube with a mirror affixed to one end, as shown in Figure 2-4.
A rigid beam is attached transverse to the axis of the helical tube. Attached to both ends
of this beam are electromagnetic coils. Mounted beneath the coils are permanent magnets.
A lamp assembly directs light through a quartz window onto the mirror affixed to the
helical tube, as shown in Figure 2-4. The mirror reflects the light back through the
window and strikes two matching photodiodes. When there is zero pressure differential
across the helical tube, the photodiode assembly is mechanically adjusted so that the light
spot is centered between each photocell. In this "zero position," the outputs of the two
photodiodes provide energy used to maintain the quartz assembly in its zero position;
thus, a force balance is created.
Theory of Operation
Control Strategy
2
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