Predictive maintenance and quality
September 1st 2008 The line between predictive
maintenance and quality
assurance monitoring is becoming
increasingly blurred as similar
techniques are being used for both
applications. Typical are car door
latches that must be 100%
inspected for product liability
protection with all the test data
being stored on a computer hard
disk. The same data can also be
analysed to flag up potential or
actual maintenance problems
using variations in product
specification. Although these
variations may be within
acceptable specification
tolerances, trend analysis can be
used as a vector to production
machine condition.
To some extent, the increase in
both continuous predictive
maintenance and 100% QA
monitoring is due to the availability
of high reliability, precision sensors
that can be expected to last almost
indefinitely under even the most
arduous conditions. Although a
number of techniques can be used
to measure acceleration, pressure
and force, the three most useful
parameters, piezoelectric and
piezoresitive sensors offer very
specific advantages in that they are
largely a 'fit and forget' solution.
These quartz based sensors do not
exhibit significant drift in response
to changes in ambient conditions
and have an almost unlimited life
without noticeable deterioration.
An indication of the long term
stability of these sensors is
provided by a Kistler pressure
sensor which, after fifty years of
use including being rescued from a
rubbish skip, was still within
original specification and able to
be calibrated as easily today as
fifty years ago.
In the past, such high precision
sensors were used almost
exclusively in research and
development laboratories but work
done by Kistler Instruments in
recent years has produced a range
of piezoelectric and piezoresitive
sensors suitable for industrial
applications ranging from
measuring oil pipeline and
wellhead pressure to monitoring
component performance in
automobile and aerospace
vehicles. An example of the new
breed of robust sensors from
Kistler is a miniature tri-axial
accelerometer designed for long
term use in ambient temperatures
from 54°C to plus
165°C. Weighing only
6.4g, the
accelerometer has a
measuring range of
±250g with a
sensitivity of 20mV/g
over the frequency
range of 1Hz to 9kHz
with only a 5%
deviation, making it
ideal for a wide range
of vibration
application.
One of the more arduous
monitoring tasks for which
piezoelectric sensors are ideal is
the maintenance of resistance
welding tongs, especially the
robotic versions widely used in the
automobile manufacturing industry.
Electrode cap shape, welding
current and clamping force are
interdependent and require regular
monitoring, if weld quality and
productivity are not to be
jeopardised. The Kistler clamping
force tester that can be integrated
with electrode cap machining and
weld current checking provides an
efficient 'service station' to optimise
performance and minimise
maintenance. The key to this tester
is the robust, long life piezoelectric
force transmitter that measures
clamping force accurately and
reliably at minimum electrode
spacing. Other methods are
available but none combine long
life, precision with almost no
maintenance needs. More articles from Kistler Instruments Limited: | 
