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Sensors, switches and transducers
T
he new concept in pressure transmitters removes the
constraints that are placed on the measuring task by user
requirements and opens up new degrees of freedom. The
company which the author represents was already in the process
of investigating digital temperature compensation concepts when
microprocessor technology was still in its infancy, and was one of
the first companies on the market to implement these concepts when
it manufactured diving computers and digital pressure gauges. This
concept was realised almost 10 years ago with faster measuring with
the use of analogue outputs instead of displays, and a considerably
wider temperature range.
Floating sensor
At the same time, electronic measuring procedures that were becom-
ing increasingly accurate demanded the continuous development
of oil-filled pressure measuring cells, because every mechanical
connection between the silicon pressure sensor and some kind of
housing was transmitting externally-induced force to the sensor and
causing phantom measuring signals. This applies to the extent that
spanner torque can cause the zero point of unfavourably designed
pressure transducers to change during installation. If the materials of
the pressure connection and the connecting nozzles differ from that
of the measuring point, process-dependent force from temperature
changes occurs during operation due to differing material expansion.
Even with a floating silicon sensor, it still demonstrates the typical
temperature dependency of the TCN (TC zero point) and TCE (TC sen-
sitivity) output signals, but this is now considered as a characteristic
property of the individual sensor and reproducible as such. Ideally,
the resistance measuring bridge of the pressure sensor itself should
be used as a temperature sensor. Its overall resistance is individual
and easy tomeasure in relation to the temperature. That which caused
considerable problems when silicon pressure sensors were initially
developed about 30 years ago can now be used in a beneficial way.
The non-linearity typical for the individual sensor at a given
temperature can be reproduced extremely accurately with the lat-
est micro-mechanical sensors. However, the effects are extremely
temperature dependent. For this reason, pressure transmitters have
to be calibrated at several temperatures, and possible measurement
deviations are easiest to show using an area that is spanned by the
pressure and temperature coordinates.
Reproducibility as the basis
Reproducible temperature and pressure data is the basis of the
polynomic compensation that can be calculated using a microproces-
sor. Each measurement that is output by the pressure transmitter is
mathematically calculated with reference to the support point data
which has been determined during calibration in the factory. It has
been shown that sensor-dependent and temperature-dependent de-
viations are easiest to correct using the following 3
rd
order polynom:
P(S,T) = A(T)∙S
0
+ B(T)∙S
1
+ C(T)∙S
2
+ D(T)∙S
3
The temperature-dependent coefficients A(T) to D(T) are also calcu-
lated froma 3
rd
order polynom. During operation, the analogue signals
from the pressure and temperature sensor are made available to the
microprocessor via a 16-bit A/D converter. The microprocessor then
calculates the relevant values for the compensation coefficients using
the calibration data matrix stored in an EEPROM, and then the exact
pressure values using the above-mentioned equation. A measuring
time of approximately 2 ms means that these calculations are per-
formed at least 400 times per second.
The analogue output signal belonging to each measurement
is generated in a completely flexible way in accordance with the
parameterisation performed by the user using D/A converters and
subsequent output amplifiers. 0 to 10 V or 4 to 20 mA (2-wire tech-
nology) is available as a standard output, which is protected from
When precision is required
By B Vetterli, Keller AG, Switzerland
The more important it becomes to have measuring accuracy, the more focused the pressure transmitters available on the market are on certain
applications. Digitally compensated pressure transmitters show the limits of what is possible.
T
ake note
• All sensors introduce errors.
• Errors can be understood and quantified.
• Digital compensation can correct for errors and provides exceptional
precision when needed.
Electricity+Control
September ‘12
42