DH Instruments PPCH-G, Руководство по эксплуатации и обслуживанию

Страница: 36 / 160

PPCH-G™ OPERATION AND MAINTENANCE MANUAL
© 2007 DH Instruments, a Fluke Company Page 26
PPCH-G includes a Ready/Not Ready feature that indicates when current
pressure control requirements have been met (see Section 3.2.3).
With static pressure control active a Ready condition occurs whenever:
•
NO control valve is operating.
•
The current measured pressure is at the target value within the hold limit.
•
The rate of change of pressure is less than the current stability limit.
Figure 10 illustrates Ready/Not Ready operation in Static control mode.
Figure 10.
Ready/Not Ready in static control mode
3.2.2.4 MONOTONIC CONTROL
Monotonic control
mode is designed to set pressure increments and take data
from a device under test while assuring the evolution of pressure is always in the
same direction (ascending for ascending increments, descending for descending
increments). In monotonic control mode the pressure is set near the target value
and then controlled at a low rate of change in the same direction as the target
pressure approach (see Figure 11). Measured pressure data is taken from the
PPCH-G and output from the device under test during the slow ramp near the
target pressure. Monotonic mode is also used to float the piston of a piston
gauge when PPCH-G is used as the automated pressure control component in a
piston gauge system.
The advantage of this control mode is that in a sequence of pressures,
ascending or descending increments are set without overshoot or oscillation as
is desirable when determining the hysteresis of a device under test (DUT). This
method also tends to achieve a ready condition more quickly than dynamic or
static control. However, the final pressure achieved is not equal to the target
value and for best results it is necessary to average alternating PPCH-G and
DUT readings to compensate for possible timing differences in the readings.
When setting a monotonic pressure point, PPCH-G attempts to set a low ramp
rate less than the rate limit at the near end of the pressure target limit and then
maintains the low ramp rate until a new pressure target value is entered or the
range “upper limit” (UL) is reached (see Section 3.4.4). If the ramp rate exceeds
the specified target rate or changes sign, a
Not Ready condition occurs. See
Table 8 for default pressure and ramp rate target limit values and Section 3.4.6
to customize the limits.