Specifications

Related functions
Trim, calibrate, and adjust
www.digikey.com/maxim-industrial 143
Trim, calibrate, and adjust
Making industrial equipment
accurate, safe, and afford-
able with electronic
calibration
We demand safety in our factories.
Customers expect quality products,
which require accurate manufactur-
ing equipment. At the same time,
equipment must be affordable. How
can manufacturers deliver “perfect”
equipment at a reasonable price? In a
word, calibration. Electronic calibra-
tion enables the remote calibration
and testing of field devices such as
sensors, valves, and actuators. Because
field devices and programmable logic
controllers (PLCs) are size constrained,
they benefit from the small size of
electronic calibration devices.
All practical components, both
mechanical and electronic, have
manufacturing tolerances. The more
relaxed the tolerance, the more
affordable the component. When
components are assembled into a
system, the individual tolerances
sum to create a total system error
tolerance. Through the proper
design of trim, adjustment, and
calibration circuits, it is possible to
correct these system errors, thereby
making equipment safe, accurate,
and affordable.
Calibration can reduce cost in many
areas. It can be used to remove
manufacturing tolerances, specify
less-expensive components, reduce
test time, improve reliability, increase
customer satisfaction, reduce
customer returns, lower warranty costs,
and speed product delivery.
Digitally controlled calibration
devices and potentiometers (pots)
are replacing mechanical pots in
many factory settings. This digital
approach results in better reliability
and improved employee safety. This
increased dependability can reduce
product liability concerns. Another
advantage is reduced test time and
expense by removing human error.
Automatic test equipment (ATE) can
perform the test functions quickly
and precisely, time after time. In
addition, digital devices are insensi-
tive to dust, dirt, and moisture, which
can cause failure in mechanical pots.
Testing and calibration fall into three
broad areas: production-line final
testing, periodic self-testing, and
continuous monitoring and readju-
stment. Practical products may
use some or all of the above test
methods.
Compensating for component
tolerances using final-test
calibration
Final-test calibration corrects for
errors caused by the combined
tolerances of many components.
One or more adjustments may be
required to calibrate the device
under test (DUT) to meet a manufac-
turer’s specifications.
To provide a simple example, we
will say that this equipment uses
resistors with five percent tolerance
in several circuits. In design, we
simulate the circuits and perform
Monte Carlo testing. That is, we
randomly change the resistor values
within the tolerance limits to explore
their effects on the output signal.
The simulation results in a family
of curves that show the worst-case
errors that the resistor tolerances
cause. With this knowledge, the
designer decides to use the circuits
as-is and to simply adjust the offset
and span (gain) during final test to
meet system specifications. So, we
make measurements in the final
production test and have a human
set the span and offset using two
mechanical pots. Calibration is
complete, but have we solved the
problem, masked the problem, or
added a bigger unknown?
Experienced production engineers
know human error is a real issue.
Unintentional slips can ruin the
best of plans. Asking a human to
perform a boring, repetitive task is
asking for problems. A better way is
to automate such a task. Electrically
adjustable calibration devices enable
quick automatic testing, which
improves repeatability, reduces cost,
and enhances safety by removing the
human-error factor.
Improving reliability and
long-term stability by power-
on self-test and continuous/
periodic calibration
Manufacturing tolerances are
compensated for by calibration
during the final production test, and
that data is utilized when a system
is powered up. Environmental
parameters in the field also create
a need for test and calibration.
Such environmental factors include
temperature, humidity, and circuit
component aging (drift), which result
in signal span and offset errors. Some
circuits contain control or average
information, which can be periodi-
cally memorized. These factors are
accounted for with a combination of
self-test at power-up and periodic or
continuous testing. The field testing
may be as simple as sensing temper-
ature and compensating accordingly,
or it may be more complex.
Many products include an internal
microprocessor, which can aid
testing. For example, a weight scale
can compensate for the weight of the
product package, such as a plastic
bag or glass jar. Subtracting the
weight of the package (tare weight)