MDTmag.com 14 / March 2014
readings or continuous force readings for
force vs. time or force vs. displacement tabulation and plotting. Statistical calculations,
exporting, and reporting tools are commonly available in basic software packages. In
simpler applications, a printer can be used to
print the results and basic statistics. Some test
stands can be controlled by PC software to
help reduce operator influence, increase testing throughput, and improve data collection.
In OEM type applications, force measurement can be a viable alternative to analog
data acquisition systems. A force gauge can
become an integral component by interfacing
the gauge’s set point, digital, footswitch, and
analog inputs/outputs with a programmable
logic controller (PLC). One example involves
a manufacturer of sutures requiring frequent
testing of samples from the production line.
The test requires a pull to failure. A force
gauge is integrated into a semi-automated
custom machine, and pass/fail limits determine whether particular production lots are
cleared for onward shipment. Data is recorded for statistical analysis and reporting.
Test stand-based systems can also be in-
tegrated into complete systems, for example,
as a station in a pacemaker production line.
Serial communications between the test
stand, force gauge, and PLC can automati-
cally run the stand, collect data, and stop the
production line if a non-conforming sample
has been detected.
Calibration can be a simple process. Typically, the only part of a force measurement
system requiring calibration is the force gauge
itself. When it is due, it is removed from the
test stand and transported to the laboratory,
where deadweights or a master load cell is
used to calibrate to full scale and several points
in between. It is, therefore, not necessary for
the calibration technician to travel to the test
station to calibrate it.
When Does Force Measurement Take a
Throughout these examples, force measuring
instruments were used in cases where, in the
past, a more expensive alternative may have
been the default choice. However, despite
technological advancements, materials testing
takes over where force measurement leaves
off. Materials testing uncovers a wealth of
scientific information about the mechani-
cal properties of materials used in medical
devices, including various metals, plastics,
elastomers, ceramics, and others. Through a
combination of sophisticated software and
measurement of very small deflections, mate-
rial testers have earned their place in quality
control and research laboratories of raw
materials suppliers, medical device manufac-
turers, and educational and research facilities.
Force measurement systems’ deflection
measurement accuracy and resolution are
typically too coarse for reliable measurements, which means that applications such
as tensile testing of steel and compression
testing of ceramics become challenging.
In such cases, a materials tester fitted with
an extensometer and analysis software is
essential. Such software can capture a stress
vs. strain curve to help characterize materials
and automatically identify important features
such as Young’s modulus and yield strength.
For more specialized applications requiring the calculation of application-specific
coefficients, materials testing software is often
capable of providing such results.
With sophistication comes a learning
curve, which is why materials testers have
traditionally been used by trained professionals in a laboratory type environment. Force
measurement equipment cannot replace a
materials tester, but typically, have simpler
interfaces, reflecting their more focused
capabilities. For many companies, there is
a need for force measurement equipment
and materials testing equipment to coexist –
often a materials tester to test incoming raw
materials, and a force measurement system to
measure a finished component or assembly.
The Bottom Line
In today’s competitive medical device
manufacturing environment, it has become
more important to carefully analyze test
applications and the capabilities of available
instrumentation. Educating oneself about the
capabilities and limitations of force measurement equipment can better guide procurement decisions. Optimizing such decisions
can translate into decreased equipment and
maintenance costs, reduced operator training
time, and faster throughput. MDT
A pull test on a needle cover
helps to ensure that it is neither
too easy nor too difficult to
remove in the field. A typical peel force measurement
system consists of a manual or
motorized test stand, force gauge of
appropriate capacity, grips, and data
A tensile test is performed
on an elastomer sample.
Throughout these examples,
force measuring instruments
were used in cases where, in
the past, a more expensive
alternative may have been
the default choice.