tecsis GmbH - PRODUCT NEWS
    
Customised Force Sensors.

The tecsis GmbH based in Frankfurt/Main, has
set as its aim to supplement the hitherto range of force
transmitters with a new kind of force transmitter, which
as a result of extremely favourable production cost
combines industrial applicable accuracy with a bright
variety of signals.

A thin-film sensor with a resistance bridge, which has proved
itself millions of times over in pressure measuring technology,
has been selected as a basis for this development. Through
the skilful integration of the sensor in a deformation body adapted
to the greatest extent to the given ambient conditions, the expansion
can be transferred to the measuring membrane so that a signal
proportional to the force is given. The accuracy will be better
than 1 %. The wheatstone bridge signal at the force load will be
identical to the one given under the pressure load.

This allows to use all existing amplification circuit boards already
standard in the tecsis pressure sensor line. All the today standard
analogue transformer signals (e.g. 4..20 mA, 0..10 V) can be
made available in this way. A variety of digital signals and bus
technology such as HART (RS232), CAN-Bus and the Profibus
DP are available.

In the past a lot of force measurement applications could not be
established because expensive force transmitters were not
affordable.

With the tecsis 'thin-film-technology' it is possible to offer
customised solutions at a good price-performance ratio.

Measuring principle
Thin-film sensors have been used in pressure transducers for many
years at tecsis. The sensor itself comprises a high-grade stainless
steel pot, the bottom of which takes the form of a membrane. In a 
thin-film process a bridge resistor is fitted to the outer edge of this
membrane and is designed in a way that if the membrane arches
under pressure a pair of resistors register the surface expansion
in the middle and the other resistors record the upsetting in the
rim zone.

Moreover, at the time of developing the layout the aim was to keep
the interconnection track system as short and symmetrical as
possible. The result has been an arrangement in which the direction
of sensitivity of the two pairs of resistors under 90º to each other.
This special setup permits definite measuring of plane strain status
in the membrane.


To measure forces with this sensor a deformation body has to be designed in a way that a plane strain status is given within the region of the membrane. Now two different loads are feasible - upsetting or shearing at membrane plane.

In the first stage tecsis focused on the shearing load. As far as this was concerned it was necessary to design a suitable deformation body but, on the other hand, it was necessary to transfer the tension to the measuring membrane. Offering itself as basic element in this case was a round bolt which is particularly versatile in use. It is used as standard component in countless fastening elements such as forked bearings or chain bolts but also deflection roller axles. If the bolt is weakened by machining between the bearing and force discharge point leaving a thin bridge at symmetrical level, a cross section will be left which is very close to a double-T section. Fixing the bolt on one end and the other end is loaded with a vertical force at bridge level to the bolt axis, no deformation worthy of note will occur in the region of the round section. However, shearing will be predominant in the vertical bridge in the region of the double-T section. Applying the thin-film sensor to this bridge placing the measurement resistors at an angle of 45º to the bolt axis, a relatively symmetrical deflection of the bridges proportional to the shearing force is obtained. The applications of these force sensors are nearly unlimited due to the flexibility of the deformation body. We implemented these load cells in the lifting technology e.g. cranes, fork lift trucks and all kinds of special hoisting devices. _____________________________________________________ tecsis GmbH
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October 2001
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