Instrumentation & Measurement Magazine 24-9 - 42

The strips are clamped
to the support with the
help of two M1.5 screws,
and the strain gauges installed
in the strips were
HBM®
LC11-6/350 with a
grid length of 6 mm with
their centers located at 35
mm from the free extremity
of the sensor in order to
maximize the sensitivity
of the sensor while allowing
sufficient space for the
clamping screws.
According to the performed
calculations, a
sensor made from copperberyllium
alloy with a
stainless-steel measuring
probe would have been
subjected to a magnetic
force of 0.65 N/mm, or
30 N if integrated along the
whole length of the sensor.
This would compromise
Fig. 3. Schematic drawing of the top right sensor of the array under calibration. The displacement is imposed with the
crosshead of the universal testing machine.
the behavior of the sensor for the application under discussion,
and for this reason other materials with poorer electrical conductivity
were considered in the framework of the study. The
best alternative was found to be Titanium grade 5, which presented
still favorable mechanical properties, in combination
with brass M1 screws as measuring probes, and the magnetic
forces were calculated to be 0.02 N/mm, or 0.92 N. Considering
the boundary conditions of the strips with the sensor in
service, cantilevered in one extremity and simply supported
by the object to be measured in the other one, and the position
of the strain gauges, the effect of the magnetic fields would be
contained to less than 50 μm, judged sufficient for the application
under consideration.
The previous paragraphs describe the design of a single
sensor. Nevertheless, in some cases the individual sensors
need to be mounted in arrays to accomplish their mission. This
is the case for our application which is described in the next
sections of this paper. For this reason, when mentioning to " the
array of sensors " in the calibration and dynamic characterization
sections, the we refer to the probe used for this application,
discussed in the section Application: Q1-type HL-LHC BeamScreen
Testing Campaign.
Calibration
The sensors were calibrated according to ASTM F2537 [11]
standard for a range of 1 mm. The displacement sensors
were mounted in a calibration apparatus so that they could
be subjected to a precise and known displacement. The displacement
was applied in steps over a range of 1 mm. The
acquisition system collected the displacement and the electrical
readings. The voltage and displacement readings were
42
taken as the average of 120 readings over 0.1 s. The voltage
versus displacement curve was plotted in order to determine
the calibration factor.
The calibration apparatus was a universal testing machine
from Zwick Roell Z400 [12] that was calibrated according to
ISO standard. The array of sensors was mounted so that the tip
of the sensor under calibration was normal to the crosshead of
the universal testing machine (Fig. 3). The machine was programmed
to do three cycles of loading and unloading with
controlled displacement. The machine held the displacement
every 0.1 mm for 1 s at steps defined over a range of 1 mm. The
displacement steps were equal for both deflection directions
and for all the cycles. The DAQ system was set up for an acquisition
frequency of 1200 Hz. A HBM®
WI LVDT [13] with a
nominal range of 5 mm, and a maximum linearity deviation of
±0.2% was used as reference of the displacement. Both LVDT
and linear displacement sensors were carefully aligned and
positioned with the help of a laser level. All of the electronic
equipment was turned on at least an hour before the calibration.
The calibration was performed at a temperature of 22 °C
±0.5 °C. The electrical voltage output and the displacement
measurements for a period of 0.1 s in the middle of the 1 s steps
were taken for analysis. The repeating cycles aimed to verify
whether the data was reproducible.
ASTM F2537 standard requires that the linear displacement
sensor must be within the following criteria: the coefficient of
determination (R2
) value must be greater than 0.95; the standard
deviation of the voltage output of any calibration point
must not be greater than 0.010 V/V full scale; and the percent
errors at each calibration point must be evaluated together

Instrumentation & Measurement Magazine 24-9

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