MANUAL: 55308
REV: A
ECO: 49582
13
6.1.3 PCB CALIBRATION
PROCEDURE
Numerous precautions are taken at PCB to insure
accurate and repeatable results. This section
provides a brief overview of the primary areas of
concern.
Since the Back-to-Back Calibration technique relies
on each sensor experiencing an identical
acceleration level, proper mounting of the test
sensor to the reference standard is imperative.
Sensors with mounting holes are attached directly
to the reference standard with a stud tightened to
the recommended mounting torque. A shouldered
mounting stud is typically used to prevent the stud
from “bottoming out” in the hole.
Both mounting surfaces are precision-machined
and lapped to provide a smooth, flat interface
according to the manufacturer’s specification. A
thin layer of silicone grease is placed between the
mating surfaces to fill any imperfections and
increase the mounting stiffness. The cables are
stress-relieved by first routing them to the shaker
head, securing them with tape or cable ties, then
routing them to a nearby stationary location. This
reduces cable motion, which is especially important
when testing charge output sensors and helps to
prevent extraneous noise or stresses from being
imparted into the system. A typical set-up is shown
in Figure 4.
Figure 4.
Typical Calibration Set-Up
Adhesively mounted sensors use similar practices.
However, in this case, a small portion of quick-
bonding gel or similar temporary adhesive is used
to attach the test sensor to a reference standard
designed with a smooth, flat mounting surface.
In addition to mounting, the selection of the proper
equipment is critical. Some of the more important
considerations include: 1) the reference standard
must be specified and previously calibrated over the
frequency and/or amplitude range of interest; 2) the
shaker should be selected to provide minimal
transverse (lateral) motion and minimal distortion;
and 3) the quality of the meters, signal generator,
and other devices should be selected so as to
operate within the limits of permissible error.
7.4 COMMON MISTAKES
Most calibration errors are caused by simply
overlooking some of the fundamental principals of
dynamics. This section attempts to address some
of the more common concerns.
For stud-mount sensors, always mount the
accelerometer directly to the reference standard.
Ensure that the mounting surfaces are smooth, flat,
and free of any burrs. Always use a coupling fluid,
such as silicone grease, in the mounting interface to
maintain a high mounting stiffness. Mount the
sensor
according
to
the
manufacturer’s
recommended mounting torque. DO NOT use any
intermediate mounting adaptors, as the mounted
resonant frequency may be reduced and thereby
compromise the high-frequency performance. If
necessary, use adaptor studs.
