In this article, Thomas Kleckers, Product Manager Force Sensors at HBM, explains how to select the right piezoelectric sensor for your application.
Piezoelectric sensors: Which is the right one for my application?
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 a piezoelectric crystal that produce a charge when subjected to compressive force. In general, two such slices are used, with an electrode inserted in between. It absorbs the resulting charges. The surrounding housing also serves as an electrode. The requirements placed on the surface quality of the crystal and the housing are very high and crucial in determining the quality (linearity, response behavior) of the force sensors.
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 high initial load
- Wide measuring range
- Measurement at very high temperatures
- Extreme overload stability
- High dynamics
When the decision to use a piezoelectric transducer 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 and provide you with a selection guide for choosing the right sensor.
Application conditions indicating that piezoelectric sensors should be used:
1. Confined space for sensor installation
Piezoelectric force sensors can be very compact — e.g. the CLP series with heights of 3 to 5 mm (depending on the capacity of the force washer). Such sensors are therefore perfectly suited for integration with existing structures. The sensors come with an integrated cable, since they cannot accommodate plugs, due to their 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.
2. Measurement of small forces with high initial load
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.
3. Wide measuring range
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.
4. Exceptionally high temperatures
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.
5. Extreme overload stability
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 measurement of small forces.
6. High dynamics
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 dynamic properties. The stiffness of the attachments and subsequent electronics need to be taken into account as well. Piezoelectric measurement chains in general are perfectly suited to 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.
Which sensor is the right one?
If one of the above-mentioned aspects is true for your application, you need a piezoelectric transducer for your force measurement. However, which sensor is the right one? Here is the selection guide we have put together based on typical use cases.
“Piezoelectric force washers are widely used. Can I use such a sensor in my application as well?”
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. In addition, calibration is required, since the sensor installation is also decisive for the sensitivity of the measuring point. This means that, after mounting, the sensitivity of the measuring point needs to be determined by calibration to ensure high-quality results.
“I cannot perform a calibration; however, I would like to use piezoelectric sensors.”
Our recommendation: Type CFT force sensors
Sensors that have already been pre-stressed and calibrated are available for such applications, for example, type CFT force sensors that are immediately ready for measurement. The scope of delivery includes a test record that specifies their sensitivity.
“I want a sensor that can be easily integrated; however, my application requires a plug connection.”/“Point loads may occur.”/“I need to take into account very high forces.”
The height of construction of CFW force washers is relatively high, 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 and are thus, very flexible. Different cables can be connected such as the KAB145 robust 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!
“I need to measure force at extraordinarily high temperatures.”
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.
“Space is a constraint in my application, the height of construction is the decisive factor."
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. In addition, 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.
“I cannot accommodate a force washer, and a sensor based on strain gauges or a CFT transducer also require too much space.”
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. The measurement accuracy depends particularly on the material onto which the sensor is mounted.
We will be happy to advise you
Do you have a specific application and wonder whether a piezoelectric sensor is the right choice? Just contact us - we will be happy to advise you.