Fig. 1: Piezoelectric measuring chain

Galvanic separation in piezoelectric measuring chains

From charge amplifier to accessories - the 'Paceline' series of piezoelectric products provides a special feature that significantly facilitates its use in practice and, in particular, mounting:  galvanic separation of the summing box or charge amplifier's enclosure from the cable shield of the coaxial cable. The technical implementation of this feature involves shielding of the coaxial cable isolated from the enclosure. Suitable shielding designs are required to ensure that the measuring chain complies with existing EMC guidelines.

Requirements for the measuring chain

A measuring chain comprises at least a charge amplifier and a sensor - this can be a calibrated transducer (CFT series) or a force washer (CFW, CLP) as well as a piezoelectric strain sensor (CST). Connecting the piezoelectric sensors in parallel is relatively simple using the above mentioned summing box. The cables too need to meet specific requirements. These include a very high insulation resistance, freedom from noise even under motion and mechanical strength. Special coaxial cables comply with these conditions. When mounting the sensor it is necessary to ensure that it is normally mounted onto a metal part. Hence the sensor's enclosure is also electrically connected to the object on which the force sensor is mounted.

Fig. 1 shows a measuring chain comprising four sensors connected to a charge amplifier by a summing box. The sensors are mechanically connected. Since force application parts from hardened steels are used in force measurement technology, the enclosures too have the same electric potential.

Fig. 2: Piezoelectric force washer

Avoiding interference effects

In practice, it is often the case that the summing box or charge amplifier are mounted such that they have a different electric potential. With the summing box and charge amplifier enclosures being connected to the coaxial cable's shielding and thus to the sensor's enclosure, there are compensating currents flowing through the cable shield when the electric potentials differ. These compensating currents may possibly interfere with the measurement.

Using plastics parts for electrically separating summing boxes or charge amplifiers from the objects onto which they are mounted is state-of-the-art technology. The circuit is interrupted, no compensating currents can flow. However, there are two disadvantages: On the one hand, the technician must remember to order insulating washers. On the other hand, mounting is complicated.

This is not necessary with Paceline products: Metal (charge amplifier, summing box) can be screwed onto metal (control cabinet, industry-standard rail), since there is no electrical connection between cable shield and enclosure. This facilitates mounting.

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