Piezoelectric sensors generate an electrical charge when a force is applied. The diagram above illustrates the operating principle.
Charges are generated proportionally to the force. The unit of measure for the charge is pC (10-12 Coulombs, which is equivalent to a charge of 3.12 * 10-6 elementary charges).
Sensors that use quartz as the piezoelectric sensor material exhibit a sensitivity of about 4.3 pC/N. This means that if a force of one Newton is applied to the sensor, a charge of 4.3 pC is produced. Sensors like the new CFT/25kN use gallium phosphate as the piezoelectric crystal. The advantage is that twice the sensitivity can be achieved in this way. Twice as much charge is produced by the same force. The charges are directed to a charge amplifier, which converts the signal into a 0...10 V signal.
The advantage of this technology is that the sensitivity of the sensor remains the same regardless of the nominal (rated) force, assuming the same material. A very large sensor can also be used to measure a very small force. Another reason to prefer this technology is that charges can be physically set to zero. A short circuit can be used to produce a charge of zero pC on the input when the sensor is loaded by a force (in our example pre-stress).
In this situation the charge amplifier can be set to a higher sensitivity so that the measuring range corresponds to the force being measured. The pre-stress is not relevant. It makes no difference for the resolution and accuracy of the measurement whether a piezoelectric sensor is operated under initial load or with none at all. It is always possible to bring the input of the charge amplifier to zero by using the RESET function.
Piezoelectric sensor working under an initial load: After the initial load is applied the measuring chain is brought to zero by a reset. Now the charge amplifier can be operated in a very well adjusted (smaller) measuring range.
A force washer is installed under a bolt. The object is to measure a tensile force acting on the screw connection. First the pre-stress force is applied. The pre-stress force can also be determined by measuring it with the force washer itself. A zero balance for the measuring chain can be performed by triggering a RESET on the charge amplifier. Then there is no charge on the input. Now the charge amplifier can be set to any measuring range. Even very small forces can now be measured reliably.
- This measurement is especially simple with modern digital charge amplifiers such as the CMD600, which can be set to any measuring range.
- In the example shown here the measurement is in the force shunt. A calibration in the installation situation is required before forces can be measured quantitatively. For more information read the article 'Installation of force transducers'.
- Piezoelectric sensors are always subject to drift. This makes it necessary to bring them to zero cyclically or use a high-pass filter. If neither option is available, strain gauge-based sensors must be used.