The piezoelectric principle enables force sensors to be designed which have distinctly different characteristics than sensors based on strain gauges. Piezoelectric force sensors consist of slices of monocrystalline crystal that creates an electric charge when subjected to a compressive force. In general, two such slices are used, with an electrode inserted in between. The entire structure is enclosed in a housing. The charge is absorbed by the electrode and the housing and is transmitted to a charge amplifier by a coaxial charge cable.
The figure shows the typical layout of a piezoelectric force transducer:
2. Piezoelectric crystal
Piezoelectric sensors are dependent on good contact between the crystal and the housing. This requires, on the one hand, precise processing of the crystal and component surfaces that come into contact with the crystal, and, on the other hand, the use of pre-stressed sensors. In practice, at least 10 % pre-stress is used to ensure reliable contact – higher pre-stress enhances the metrological properties. The pre-stress must, of course, not overload neither the pre-stressing elements nor the sensor.
Applying a force to a piezoelectric crystal results in an output signal in the form of a charge Q, measured in pC (10-12 C). The charge can be calculated using the equation:
Q = qxy*F
Where F is the force and qxy the piezoelectric constant. The latter depends on the type of crystal used and the crystal direction that is being loaded. The most commonly used material is quartz, with a sensitivity of 4.3 pC/N and the temperature limit of 200 °C. HBM uses gallium phosphate as well. Its sensitivity is about twice as high as that of quartz (approximately 8 pC/N). Its temperature limit is 850 °C, which, however, cannot be fully utilized with force transducers, since these have a limit of 300 °C due to thermal stress.
The decision of whether to use piezoelectric force sensors or sensors based on strain gauges depends on the application. Piezoelectric sensors are preferable, particularly when the application has one of the following requirements:
- Confined space for sensor installation
- Measurement of small forces with a high initial load
- Wide measuring range
- Measurement at extraordinarily high temperatures (up to 300 °C)
- Extreme overload stability
- High dynamics
Strain gauge-based sensors, too, offer advantages over piezoelectric force sensors; for instance, they enable tensile force to be measured and often are more economical. In addition, they provide better accuracy, drift-free measurement, and static calibration. When it comes to reference measurements, there is no alternative to strain gauge measurement technology.
We recommend checking, in any case, which transducer meets the requirements of the measuring task at hand in the best and most cost-effective way. When the decision to use a piezoelectric sensor has been made, there remains the question of which force transducer is the right one. In the following, we will take a closer look at the typical fields of application to enable you to make the right choice.
Background – Application conditions indicating that piezoelectric sensors should be used:
Piezoelectric force sensors can be very compact—e.g. the CLP series with heights of 3 to 5 mm (depending on the size of the force washer). Such sensors are therefore perfectly suited for integration with existing structures.
Piezoelectric sensors produce an electrical charge when a force is applied. However, the sensor is subjected to forces that go beyond the actual force measurement, for example, during installation. The resulting charge can be short-circuited, which sets the signal at the charge amplifier input to zero. This enables the measuring range to be adjusted in line with the actual force to be measured. High measurement resolution is thus guaranteed, even if the ratio of initial load to force to be measured is extremely unfavorable. Cutting-edge charge amplifiers such as the CMD600 allow for virtually continuously variable adjustment of the measuring range and thus, support such applications.
Piezoelectric force transducers show their strengths in multi-stage processes as well. Imagine a multi-stage pressing process; first, high forces are applied in the actual pressing process. The piezoelectric measurement chain is adjusted accordingly. The second stage involves the tracking of the force, i.e. the measurement of minor force variations. In this example too, we benefit from the special feature of piezoelectric sensors that involves physical elimination of the signal at the charge amplifier input. The charge amplifier input is set to zero again and the measuring range can be adjusted to ensure a high resolution.
Some applications require force to be measured at very high temperatures. In these applications, force transducers based on strain gauges reach their physical limits. Piezoelectric force washers of the CHW series, however, have been designed precisely for such applications and can be used for measurement up to 300 degrees C.
All piezoelectric sensors, with a few exceptions, have the same sensitivity. This, in turn, means that the output signal of a force sensor with 20 kN capacity at a given force equals the output signal of a sensor with 700 kN capacity. Therefore, in terms of resolution and accuracy, it does not matter which of the two sensors is used. The measurement chain can be set up for maximum force and yet enables the measurement of small forces.
Piezoelectric sensors have very small displacements and provide correspondingly high stiffness—this makes them the perfect choice for use in dynamic applications. However, the entire measuring chain has an influence on the dynamic properties. The stiffness of the attachments and subsequent electronics need to be considered as well. Piezoelectric measurement chains, in general, are perfectly suited to the highly dynamic measurement of small forces. Force transducers based on strain gauges, on the other hand, are the first choice when it comes to dynamic measurement of large forces.
If one of these aspects is true for your application, you need a piezoelectric transducer for your force measurement. However, which sensor is the right one?
Excursus: Calibrating a piezoelectric force measurement chain – the reference sensor
We recommend clarifying the requirements to be met by the sensor, before investing in a force sensor that is to be used as a reference for calibrating force measurement chains. A general requirement is that such a force transducer is traceable to a national standard. This means that the reference force transducer must have been calibrated in a DIN EN ISO/IEC 17025 accredited laboratory. These laboratories can prove traceability to the respective national metrology institute (Physikalisch Technische Bundesanstalt, in Germany) and are subject to verifiable guidelines on the methods used and the training of staff.
The video uses the example of the CFW force washer to show how a piezoelectric force measurement chain should be calibrated.
Alongside the suitable transducers, HBM also offers calibration services.
Most users prefer piezoelectric force washers because they can be integrated with the measurement object or machinery without having to make major mechanical changes. However, these sensors always require mounting with pre-stressing, i.e. an initial load to be applied using screws or pre-stressing sets to prevent damage and ensure sufficient bending moment stability. Please make sure to observe the load-carrying capacity to avoid overloading the sensor or the pre-stressing element (screw). Moreover, calibration is required, since the sensor installation is a decisive factor for the measuring point’s sensitivity. This means that, after mounting, the sensitivity of the measuring point needs to be determined by calibration to enable the force to be measured in Newtons.
Our recommendation: Type CFT+ force sensors
CFT+ sensors have already been pre-stressed and calibrated at the factory. The mechanical connection is made by flanges. In line with the large measuring range, the force sensors’ sensitivity is specified in three measuring ranges: At nominal (rated) force/capacity, at 10 % of its capacity, and at one percent of its capacity.
The CFT+/25KN is a special version that uses gallium phosphate. This sensor version achieves twice the sensitivity (compared to quartz) and is especially suitable for measuring extremely small forces.
Our recommendation: Type CFT force sensors
Like the CFT+, the CFT force sensors have been pre-stressed and calibrated, which makes them ready to use without prior calibration. They are mounted via thread connections and have standard mechanical connections.
Type CFT sensors are small and use gallium phosphate as well. They are ideal for applications in the range of a few Newtons such as the testing of miniature components.
The height of construction of CFW force washers is slightly higher than with the CLP series, which means that there is more material between the force application part and the measuring element. The CFW/700 KN is the largest force washer of the series, with an internal diameter of 36 mm. As a result, it is less affected by unfavorable mounting conditions. Series CFW force washers come with a plug connection. Different cables can be connected such as the robust KAB145 charge cable, which features a connection to the sensor housing that is sealed with an O-ring. It is ideal for use in harsh environments!
Our recommendation: Force washers of the PACEline CHW series
The force washers of the CHW series have been designed for use at extremely high temperatures. The CHW-2 model can be used at temperatures of up to 200 degrees C, CHW-3 even at temperatures of up to 300 degrees C. These force washers, too, need to be calibrated. Their low temperature sensitivity allows calibration at room temperature.
Our tip: CLP miniature force washers
The CLP series is perfectly suited to such applications, because the sensors’ height of construction is only 3 to 5 mm, depending on the size of the force washer. Moreover, the sensors come with an integrated cable, since connectors cannot be accommodated due to the very low height of construction. Sensors are available for all thread sizes, from M3 to M14. The low height of construction requires that the force on the sensor surface be distributed as uniformly as possible.
The CSW force washers measure forces that are applied in parallel to the force washer. The sensors have the same compact dimensions as the CLP series. The miniature force washers need to be calibrated to enable quantitative results to be determined.
Please note: A two-component force sensor can easily be built up using a CLP and a CSW sensor. Typical applications include machine monitoring in milling or turning processes.
The right sensor: Piezoelectric strain transducers of the CST series
Strain transducers of the CST series are very small and can be fastened with a screw. They function according to the following principle: When force is applied to a structure, this results in a deformation (strain) which often is proportional to the applied force. The sensor measures strain. CST can be mounted, for example, onto welding guns or press tools. It reliably measures strain in these components. These transducers, too, need to be calibrated. Series CST strain transducers have a very high sensitivity and can thus, also be used with very stiff structures, i.e. with very low strain levels. The measurement accuracy depends particularly on the material onto which the sensor is mounted.
Product Manager for force sensors at HBM
More technical articles on force measurement can be found in our Force Tips & Tricks