Piezoelectric force transducers: The principle is simple – the possibilities are endless

There are different types of force transducers, also known as force sensors. The most widespread models are based on strain gauges and have already been presented in another article on the HBM website. Here, we want to discuss another technology used for measuring forces, the piezoelectric effect. In the following article, Thomas Kleckers will explain how these piezoelectric sensors work.

Thomas is Product Manager for force transducers at HBK, and has a degree in Engineering Physics, so it shouldn’t come as a surprise that he especially likes the principle behind the piezoelectric transducer. To quote him: “The principle is simple – the possibilities are endless.”

The piezoelectric crystal

To understand how a piezoelectric force transducer works, we first have to take a look inside it. Here, at the heart of the sensor, we will always find a piezoelectric crystal, such as quartz. Piezoelectric materials are materials that produce an electric charge under mechanical stress. The principle is simple: the electric charge obtained is proportional to the applied mechanical stress. And the mechanical stress is linear connected to the force introduced. A charge amplifier can convert this charge into an easy-to-measure 0…10 V signal. In the end, the output voltage is proportional to the force.

Thomas Kleckers, HBM — Thomas Kleckers, Product Manager Force Sensors at HBK

molecular crystal lattice of piezoelectric crystal — On the left, we see the unstressed molecular crystal lattice (in the centre is the charge, which is balanced in this case). On the right, the crystal is subjected to mechanical stress: the centres of symmetry of the charges move apart and charge can be measured at the top and bottom of the crystal.

Piezoelectric force transducer design and method of operation

The relationship between the force applied to the crystal and the change in charge is proportional. In other words, the greater the force, the greater the stress, the greater the charge. This principle is exploited in piezoelectric force measurement technology. However, for the crystal to become a transducer it needs a bit more. “The output signal does not depend on the size of the sensor, and this is a particular advantage”, says Thomas Kleckers.

Most piezoelectric sensors contain two crystal elements. An electrode is located between these crystals and picks up the charge on the inward facing sides of the crystals. A cable connects the electrode to the charge amplifier. In addition, the crystal disks are housed in a metal enclosure. This doesn’t only protect the crystals, it also provides a second point of contact with them, as it is connected to the charge amplifier via the cable shield.

"It is essential to have a very good contact between the crystals and the electrode, and between the crystals and the enclosure. Therefore, the materials must be precise, of high-quality, and have even surfaces with a scarcely measurable roughness. A good transfer of the electric charge can only be achieved if the surfaces have an excellent contact" Thomas Kleckers

Design of a piezoelectric sensor. The quartz elements convert the applied stress into a charge that is picked up by the electrode between the sensors.

Properties of piezo sensors

Piezoelectric force transducers have the special property of covering large measuring ranges. In other words, the same sensor can be used for measuring very small and very large forces. Piezoelectric force transducers are, therefore, very flexible (in use) – and are available in miniature sizes of just a few millimetres. Their deformation under load is negligible due to their high rigidity. Consequently, the sensor has an exceptionally low influence over the structure in which it is integrated.

On the other hand, piezoelectric transducers are prone to drift: “The charge always finds one way or another to balance itself out”, says Thomas Kleckers. For this reason, the difference in charge required for measurement cannot be maintained indefinitely. It can be assumed that drift of 10 N/min maximum will take place. Once the measurement chain has been broken in, this figure gets much lower during service. However, it remains the same regardless of the force measured. This means that drift has more of an impact if you measure low forces over a long period of time than if you measure large forces, or if measurement times are short.

Effect of drift with small forces
Effect of drift with large forces

Effect of drift with small and large forces: When measuring 5000 N, a longer measurement time is possible; with smaller forces the effect of drift is significant. Therefore, piezoelectric force transducers must regularly be reset to zero.

Piezoelectric force transducer applications

Piezoelectric force transducers can either be pre-stressed or not, depending on the intended application. Pre-stressed sensors are calibrated and can be used immediately after installation. Force washers still have to be pre-stressed during their installation. This is generally done using screws or load pins. This produces the best possible contact between the different material surfaces, enabling the charge to be transferred. However, these additional components can alter the sensitivity of the measuring point, which therefore needs to be adjusted, or calibrated, after the pre-stressing process.

"Piezoelectric sensors, especially small ones, are often retrofitted in existing systems. While their dimensions probably won’t be a problem, calibration will always be necessary." Thomas Kleckers

It is important to make sure that the transducer delivers quantitatively correct results in its specific installation environment and in all the prevailing ambient conditions. Piezoelectric force transducers are especially beneficial in cyclic processes, as Thomas Kleckers explains. “One example is when two components are connected with a particular force, as in the case of riveting. The transducer and charge amplifier measure the force characteristic of the riveting process, enabling extremely effective quality control. After measurement, a reset is initiated, and the transducer returns to zero. Then comes the next rivet. Drift has no influence whatsoever on the result, as the measurement time is short.” Thomas Kleckers tells us that he finds the use of piezoelectric force transducers in presses particularly remarkable. “A press stamps with a force of 50 tons, that’s 500 kilonewtons. As the process plateaus, some fine readjustments need to be made. Here, we’re talking about roughly 100 newtons. A ‘RESET’ takes place between this first and second step, so that during step two the force can be measured at a high resolution. This way, the large measuring range of piezoelectric force transducers is exploited to the full.”