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 common models are based on strain gages, which we already presented here. This article deals with another technology used for measuring forces: piezoelectric force transducers. For this article, Thomas Kleckers kindly explained how piezoelectric sensors work. He is Product Manager for force transducers at HBM, with a degree in Engineering Physics. So it's no wonder he especially likes the principle behind the piezoelectric transducer. To quote him: "The principle is simple – the possibilities are endless."
To understand how a piezoelectric force transducer works, we first have to take a look inside it. Here we will always find a piezoelectric crystal, such as quartz, at the heart of the sensor. Piezoelectric materials are materials that produce an electric charge under mechanical stress. The principle is really simple: the electric charge obtained is proportional to the applied mechanical stress. 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 mechanical stress.
The relationship between the mechanical stress applied to the crystal and the change in charge is proportional. In other words, the greater the stress, the greater the charge. This principle is exploited in piezoelectric force measurement technology. For the crystal to become a transducer, however, it needs something more. "The output signal does not depend on the size of the sensor, and this is a particular advantage", says Thomas Kleckers.
As a rule, a sensor contains two crystal elements. An electrode is situated between these crystals. This electrode 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 not only protects the crystals, but 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 have high-quality, precise and even surfaces, and roughness that is scarcely measurable. A good transfer of the electric charge can only be achieved if the surfaces have an excellent contact." Thomas Kleckers
The special property of piezoelectric force transducers is that they cover very large measuring ranges. In other words, the same sensor can be used for measuring both very small and very large forces. Piezoelectric force transducers are therefore very flexible – and are available in miniature size just a few millimeters thick. 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, 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. It remains the same regardless of the force measured, however. 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 use short measurement times.
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. The force washers still have to be pre-stressed during assembly. 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.
"Small piezoelectric sensors, in particular, are often retrofitted in existing systems. So the dimensions are mostly no problem, but there is no getting away from calibration." Thomas Kleckers
It is important to make sure 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 defined 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 their use 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."