Beckhoff EP3356-0022, Документация

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Basic principles of strain gauge technology
EP3356-0022 17 Version: 1.4
Example:
In relation to the maximum output voltage U
out-max
of a bridge with 2 mV/V and Us = 5 V, this corresponds to
U
out-max
= 5 V * 2 mV/V = 10 mV. (For the nominal load) this results in a maximum resolution of
10 mV/67.3 nV = 148588 digits. Converted into bit resolution: ln(148588)/ln(2) = 17 bits. Interpretation: a
higher digital measuring resolution than 17 bits is thus inappropriate for such an analog signal in the first
step. If a higher measuring resolution is used, then additional measures may need to be taken in the
evaluation chain in order to obtain the higher information content from the signal, e.g. hardware low-pass
filter or software algorithms.
This resolution applies alone to the measuring bridge without any further interferences. The resolution of the
measuring signal can be increased by reducing the bandwidth of the measuring unit.
If the strain gauge is glued to a carrier (load cell) and wired up, both external electrical disturbances (e.g.
thermovoltage at connection points) and mechanical vibrations in the vicinity (machines, drives, transformers
(mechanical and audible 50 Hz vibration due to magnetostriction etc.)) can additionally impair the result of
measurement.
Creep
Under a constant load, spring materials can further deform in the load direction. This process is reversible,
but it generates a slowly changing measured value during the static measurement. In an ideal case the error
can be compensated by constructive measures (geometry, adhesives).
Hysteresis
If even elongation and compression of the load cell take place, then the output voltage does not follow
exactly the same curve, since the deformation of the strain gauge and the carrier may be different due to the
adhesive and its layer thickness.
Temperature drift (inherent heating, ambient temperature)
Relatively large currents can flow in strain gauge applications, e.g. I = U
S
/R
0
= 10 V / 350 Ω = 26 mA. The
power dissipation at the sensor is thus P
V
= U*I = 10 V * 26 mA = 260 mW. Depending on application/carrier
material (= cooling) and ambient temperature, a not insignificant error can arise that is termed apparent
elongation. The sensor manufacturers integrate suitable compensation elements in their strain gauges.
Inadequate circuit technology
As already shown, a full bridge may be able (due to the system) to fully compensate non-linearity, creep and
temperature drift. Wiring-related measuring errors are avoided by the 6-conductor connection.
References
Some organizations are listed below that provide the specifications or documents for the technological field
of weighing technology:
• OIML (ORGANISATION INTERNATIONALE DE MÉTROLOGIE LÉGALE) www.oiml.org/en
• PTB - Physikalisch-Technischen Bundesanstalt www.ptb.de/cms/
• www.eichamt.de
• WELMEC - European cooperation in legal metrology www.welmec.org
• DAkkS – Deutsche Akkreditierungsstelle www.dakks.de
• Fachgemeinschaft Waagen (AWA) im Verband Deutscher Maschinen- und Anlagenbau VDMA
www.vdma.org