Interference Effects - HBM WA Series Mounting Instructions

Interference effects

8 Interference effects
Inductive displacement transducers are relatively insensitive to external electrical
and magnetic fields. The ferromagnetic enclosure of the transducer forms an
effective magnetic back‐circuit, so that external fields are shielded and
measurement is not affected. The carrier‐frequency method eliminates
superposed signals such as mains interference.
The alternating magnetic field inside the transducer coils is concentrated in the
core. The magnetic return flow is through the transducer enclosure. In doing so
the magnetic flow must bridge the air gap between the core and the forward end
of the enclosure. Here a stray magnetic field is formed, and is effective up to a few
centimeters in the surrounding environment. Normally this constant 4800‐Hz field
does not interfere.
Even if solid iron components in the immediate neighborhood modify this field, the
effect on the measured value remains very small.
The effect should remain significantly below 0.2 % of the full scale value.
If two or more transducers are arranged closer together than around 5 cm,
superposed stray fields can have an effect on the measured value. If a transducer
core moves, the stray field from the other transducers changes. In this special
case the effect can be up to 0.2% of full scale value in an extreme case. The effect
depends on the distance between the transducers and is greater for the shorter
displacement transducers.
The remedy in such cases is to shield the transducers from one another. The
following options are available:
1. shielding with field attenuators
an electrically sound, conducting shielding the stray fields induce eddy cur­
rents which neutralize the fields. This method is highly effective: an alu­
minum sleeve with walls 1 mm thick over the transducer housing attenuates
the effect of the neighboring transducer to as little as 1/10.
2. An aluminum sheet 1 mm thick between the measuring points likewise pro­
vides an effective shield.
3. Shielding with ferromagnetic metal sheets.
The stray fields are restricted to the transducer enclosure causing them.
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A00546_11_YI0_01 HBM: public
WA...