What is sensor calibration?
When using temperature sensors, you are actually measuring
a voltage, and relating that to what the operating temperature
of the sensor must be. If you can avoid errors in the
voltage measurements, and represent the relationship between
voltage and temperature more accurately, you can get better
temperature readings. How much effort is worthwhile
depends on the application's error tolerances.
There are four adjustments that a good calibration
All voltages are
measured with respect to a reference. All devices operate
at some operating voltage. Any displacements in these
voltages, or any consistent errors during measurement,
will produce consistent errors that affect all measurements.
Offset corrections make these errors as small as possible.
The voltage that you
measure is not really the voltage present on the sensor device.
Amplifiers and attenuation between the sensor and the digitizing
converter change the signal level. To recover the sensor
information, you must restore the data to the original level
accurately. Uncorrected gain errors tend to produce measurement
errors that change consistently across the operating range.
between measured voltage and sensed temperature is
in general nonlinear and dependent on the physical properties
of each sensor type. Over a limited range, a simple linear
function is often a sufficient approximation, but a more
complicated curve is necessary to describe the relationship
accurately. Generalized curves defined by standards help,
but they won't match any individual device perfectly. For best
accuracy, you need to calibrate, and adjust the coefficient values
of the conversion function.
After applying the offset, gain, and linearization
corrections, the results might not be in the most useful
form. A good follow-up step is the following:
Calibrating linearization curves
Calibration is a process of aligning what your formulas
say with what real devices actually do. This involves taking
some accurate measurements.
You can't control what a sensor does directly. It responds
based on its physical properties. But you can to some extent
control what your sensor measures. You can establish a set of
temperature levels that span the operating range, and measure
those levels with a laboratory-grade temperature standard. For
each of those measured temperature points, observe the
response level of the sensor. If you construct a curve that
passes through these points, you will have a very good
calibration specialized for the individual sensor.
Given the temperature vs. voltage data set, treat
the sensor readings as noisy input values, and the temperature
measurements as the corresponding output values to be produced.
Taking multiple measurements and averaging them helps to
obtain the best possible data quality for calibration.
From here, there are two ways that you can go.
Explicit function. Use a processing
command that applies a "calibrated conversion function" for each
measurement. This can be a very generic calculation like a DAPL
expression or a
GENPOLY command, or it can be a
specialized conversion such as the
Piece-wise linear approximation. Select a
set of representative points along the curve and approximate the
curve with straight lines "point to point." Code the "breakpoints"
as vectors and supply them to the DAPL command
INTERP, which can then locate the appropriate line section and
evaluate the function at intermediate points.
For more information about the linearization curves
most commonly used for temperature sensor calibration, check