5 Rules

1. Is an initial stress required?

The CFW force washers are significantly flatter in design and are not pre-stressed. Pre-stressing is required when mounting. These transducers must be calibrated when in a mounting condition. The pre-stressing of the force washer is necessary to ensure the linea­rity and structural durability of the transducer. This pre-stressing means that another mechanical element is mounted parallel to the force transducer. We recommend that the force washer is pre-stressed to at least 10% of its nominal (rated) force. The force washer itself can be used to determine the initial stress. A part of the measuring force is now shunted through the pre-stressing element. Pre-stress and force shunt are determined by the installation situation. It is therefore necessary to calibrate the force washers after mounting is complete, i.e. to compare the output signal of the transducer with a known force. The accuracy of the measure­ment results depends here mainly on the accuracy of the calibration.

Block diagram of a charge amplifier

2. Increase the quality of your output signal

The charge output by a piezoelectric transducer is converted into a directly proportional voltage by means of a charge amplifier.

Piezoelectric sensors are ideally suited for dynamic, i.e. non-zero point-related measurements. The drift generated by piezoelectric measure­ment chains is so low that it does not affect accuracy even with high requirements.

The drift is an effect of the charge amplifier. The transducers themsel­ves do not display drift if mounting and connection are implemented correctly. The maximum drift of a measurement chain is 0.1 pC/s or 25 mN/s if quartz is used as sensor material and 13 mN/s for the sensor material gallium phosphate. In order to achieve a lower drift, please note the following two points:

1. Running-in behavior of the charge amplifier

The charge amplifier should run in at least a hour before measurements start.

2. Cleanliness of the connections

If the insulation resistance of the cable between transducer and charge amplifier is too low, the measurement chain will drift as charge will discharge via the too low insulation resistance. In order to keep the piezoelectric measurement chain drift low, all plugs and sockets must be kept clean at all times. Please ensure that you do not touch open contact surfaces with the hand as this reduces the required insulation resistance. We also recommend that protective caps (scope of delivery) should be left on the sockets of the sensors and charge amplifiers until the sensor or charge amplifier is connected. When the connections are disconnec­ted, the protective caps should be screwed back on again. Piezoelectric transducers must be connected with a charge amplifier with a high quality coaxial cable, HBM offers the cable 1-KAB143/3 for this purpose. Such a cable cannot be repaired, it must be replaced if damaged. If the measurement chain is always operated with a connected cable and the sensors always stored with protective caps, then contaminated contact surfaces do not generally occur.

5. Note the load limits

In piezoelectric force transducers, the crystal lies in the direct force flow. The measuring elements (quartz or GaPO4) are designed for each transducer at the maximum occurring normal force. Applied bending moments can lead to a overload of the transducer as the crystal is more strongly loaded on one side and, in contrast, the load is relieved on the other side. The maximum mechanical stress is calculated from the addition of the stresses caused by the bending moment in the crystal and the load stresses of the axial forces to be measured. The maximum permitted surface pressure must not be exceeded under any circumstances. As the output signal is not dependent on the nominal (rated) force of the sensor in piezoelectric transducers, it is possible to select transdu­cers with a higher nominal force in order to avoid an overload in such cases. The following diagram shows the maximum permissible bending moment, dependent on the process force. The highest permissible bending moment can be accepted by the force washer with 50% load stress. If the bending moment is generated by a force from the side, a lateral force is also produced reducing the maximum values. The measurement error caused by the bending moment is low, as the higher material stresses on the one side of the crystal are compensated for by the lower stress on the other side. If a piezoelectric force washer (1-CFW/50kN) is loaded with a bending moment of 100 Nm, this produces an output signal of -2.3N. Please note the maximum lateral force as per data sheet.