Fluke RUSKA 7750i, Руководство пользователя, Страница: 27 / 106

Theory of Operation
Software
2
2-9
Sensor Board
The Sensor Board monitor the quartz Bourdon tube pressure sensor, reference vacuum
sensor (Ps channel), and oven temperature sensor. The vacuum and temperature sensors
are aligned at the factory, and are utilized by the firmware.
The sensor board is also used to control and maintain the sensor housing at 50 °C.
Linearization Term
As described in the previous section, the relationship between the pressure being
measured and the current required to keep the quartz Bourdon tube in its zero position is
the main principle behind the operation of the ADTS’s sensing element.
Ideally this pressure-current relationship would be a linear equation of the form
, kP I
=
where
I
is current,
k
is a constant of proportionality, and
P
is pressure. However, due to
certain mechanical characteristics of the helical tube and its supporting structure, this
pressure-current relationship is slightly nonlinear. The nonlinear portion of this
pressure-current relationship closely follows the form of a second order polynomial, or
,
2
c bP aP I
+ + =
where again P is pressure and a, b, and c are coefficients generated during the calibration
procedure as discussed below.
When the user performs a three-point calibration, the ADTS’s software creates the three
coefficients based on the user’s zero, mid-point, and full-scale adjustments. From then on,
the nonlinear term given above is subtracted from the total pressure-current curve to
achieve the desired linear pressure-current relationship.
On the RUSKA 7750i, multiple quartz sensor ranges are used in order to provide the
percent of reading specification. In these instruments, a total of three points per range
with one point shared between two ranges is required. Therefore, a 5-point calibration is
performed to fully characterize the quartz sensors.
Software
The ADTS is a digital, software-based instrument. The controlling software uses a PID
algorithm and allows for user-definable units and onboard programming. The software
control loop is a digital outer loop with a high-speed inner loop.
Software Safety Controls
Preventing Operator Errors
The operator is required to verify a change to Control mode or Vent mode by pressing the
Enter key. Additionally, the control set-point is set to zero on power-up or when any of
the limit errors occur.
Pneumatic Errors
The ADTS continually monitors the pressure for low, high and slew (rate of change)
limits. When these limits are exceeded, the ADTS is returned to Measure mode, shuts off
the apply and exhaust control valves, and an error message is generated. Additionally a
vent limit may be set. The ADTS will go to Vent mode when the pressure exceeds this
limit.
Shut Offs
Measure mode can be entered at any time by pressing the Measure key on the main
keypad with no further acknowledgment required. The ADTS will turn off the controller.