Pitot And Static Controllers; Operation - GE Druck ADTS 405 Service Manual

Druck ADTS 405 Service Manual

Pitot and Static Controllers

11. The controller PCB is a self-contained pressure measurement control system which takes the
analogue set-point from the data converter and compares it with the voltage from the transducer. A
difference (error) is used to generate the pulse width modulated drive voltages for the 'apply' and
'release' solenoid valves which determine the pressure in the manifolds. Both controllers operate in a
similar way with additional configuration held in EEPROM.
12. The controllers receive pressure and pressure rate commands over the internal communications
ring and convert them into controlled pressure and pressure rates at the output using the rough
vacuum and pressure supplied by the pump unit. They accurately measure the pressure and pressure
rate at the output and transmit it over the communications ring the control computer.
13. Other commands can be used to open and close auxiliary solenoid valves, switch the controller
on and off (measure mode), hold the changing control pressure and oscillate around the set-point. Any
errors detected during internal self-test and during normal operations will be coded and transmitted on
the internal communications ring.

Operation

14. If the controller is on, a pressure and pressure rate demand received is processed by the
microcomputer and output via the set-point generator (digital to analogue convertor) as an analogue
voltage which forms the inner loop set-point. The inner loop is a high bandwidth analogue closed loop
pressure control system. The inner loop maintains a pressure in the manifold which at all times closely
follows the inner loop set-point.
Inner loop
15. The pressure in the manifold is measured by the control transducer which is a silicon strain-
gauge pressure transducer giving a voltage output proportional to the pressure applied. This voltage is
amplified by the signal conditioning circuits, given further gain depending on the mode of operation by
the gain switching circuits and then subtracted from the inner loop set-point and amplified in the error
amplifier. The resulting error signal is frequency compensated to keep the inner control loop stable,
then applied to a pulse generator circuit that produces a pulse width modulated output, through a
power driver circuit, to both the vacuum and pressure solenoids.
Outer loop
16. As the pressure changes in the manifold the pressure approaches the set-point the outer loop
starts operating. The pressure in the manifold is first measured by the main transducer. This is a
vibrating cylinder pressure transducer producing a high stability, high accuracy frequency (square
wave) proportional to pressure and an analogue voltage proportional to temperature.
17. The pressure signal feeds a 24 bit period counter which counts a 20 MHz reference oscillator for
a number of cycles of the incoming square wave representing pressure. This enables the incoming
square wave to be measured to a very high resolution by the microcomputer. The microcomputer
software uses the pressure and temperature data to calculate the exact pressure being applied to the
main transducer. Further corrections are made by the microcomputer using the stored zero and span
constants from the controller module and the stored constants for the humidity sensor.
18. The resulting accurate pressure reading is amplified and frequency compensated by the
microcomputer and, after subtracting the entered aim point, the resulting value is sent to the set-point
generator (DAC) to generate the inner loop set-point. This causes the manifold pressure to change
until the pressure measured by the main transducer is exactly the same as the pressure demand sent
by the hand terminal.
K171 Issue No. 1 Page 1-9