Civil Engineering Reference
In-Depth Information
accuracy, the digital data provided by the actuators and the transducers (position,
force and strain). The number of acquisition channels depends on the machine
complexity. The channels concerning the actuators and the load cells have to be
multiplied by the number of actuators, with the consequent increase in the per-
formances required by the data acquisition devices. The acquisition of the ex-
tensometers signal requires one channel (two if the procedure requires an average
between two values) for each wave direction and, if required, one extra channel for
the diagonal extensometer. In order to reproduce the prescribed loading pro
le, the
data acquisition should be performed in real time at 12, 14 or 16 bit according to the
level of accuracy required.
By means of graphs the testing activity can be facilitated by the visualization of
crucial data which can highlight malfunctions and discrepancies between the real
and the prescribed values. The most signi
cant information is the correlation, both
in warp and
fill direction, between actuator displacement and time, stress and time,
strain and time, strain and stress. Other additional tools are the time indicator, the
calibration of stress and strain transducers, the initial stress and strain zeroing, the
acquisition frequency, the recording frequency, and the emergency interruption.
The data concerning time, ram position, commanded force, applied force and
strain, should be saved and exported through a
file containing the data subdivided in
warp and weft directions. In the presence of batteries of actuators and load cells,
both single values and mean values should be reported.
The control of the testing apparatus is generally based on a proportional-integral-
derivative controller (PID controller). It calculates the error value as the difference
between the setpoint and the real measured value of a variable. The key aspect of
this approach is that the controller is based on an algorithm which involves a
proportional, an integral and a derivative contribute.
1
A key feature of the con-
trolling system is the opportunity to change the PID parameters in order to modify
and optimise the response of the machine. An example of the results that can be
achieved is shown in Fig.
4.16
.
4.1.11 Calibration
The calibration procedure must be performed in accordance with the international
standard already available for the veri
cation of static uniaxial tension/compression
testing machines and the veri
cation and calibration of the force-measuring system
(EN ISO 7500-1:
2004
).
The procedure should contain a set of operations that establish, under speci
c
conditions, the relationship between values of quantities indicated by the biaxial
1
These values can be interpreted in terms of time: the proportional contribute depends on the
present error, the integrative contribute on the accumulation of past errors, and he derivative
contribute is a prediction of future errors.
