Two principles dominate in force measurement: strain gauge–based force transducers and piezoelectric sensors. Strain gauge based force transducers comprise a spring element to which force is applied. This force causes minimal deformation of the spring element.
Strain gauges installed at the right points are extended and hence show a change in resistance. Minimum four strain gauges are connected to a Wheatstone bridge circuit. When a voltage is fed to this measuring bridge, the resulting output voltage is proportional to the applied force as shown in Figure 1.
Figure 1. Principle of operation of a strain gauge sensor. The size of the spring element determines the measuring range.
Piezoelectric sensors include two crystal disks with an electrode foil mounted in between. When force is applied, an electric charge is obtained that can be measured with a charge amplifier. The charge is proportional to the applied force. The piezoelectric sensor design is shown in Figure 2.
Figure 2. Design of a piezoelectric sensor. The crystal disks (green) convert applied force into a charge that is picked up by the electrode between the sensors.
Which Sensor Principle is Suited for which Application?
Static Monitoring Tasks
Strain gauge based sensors show no drift and are especially suited for long-term monitoring tasks. Modern sensors from HBM for instance S2M achieve creep values of below 200ppm relative to the measured value, a negligible error in several applications.
Due to their operating principle, piezoelectric sensors have a drift, which is an estimated 1N/min when running the measuring chain. As this value remains unchanged, irrespective of the measured force, the relative measurement error resulting from the drift is especially unfavorable when small forces are being measured over a long period of time. Figure 3 shows the effect of the drift with small and large forces.
Figure 3. Effect of the drift with small and large forces: When measuring 5000N, a longer period of measurement is possible; with smaller forces the effect of the drift is significant. The following becomes obvious: The period of measurement depends on the required accuracy and the force to be measured.
Dynamic Force Measurements
There is a very small deformation shown in piezoelectric sensors when a force is applied, very high stiffness is offered. This results in a high resonance frequency, which, in principle, is highly favorable in dynamic applications.
However, the entire measuring chain is critical to the dynamic properties. It is significant that the mounting parts used to install the sensor have additional mass which has an impact on the overall mass of the system and also the cutoff frequency. Also, a number of charge amplifiers have a bandwidth dependent on the charge and thus on the force that is measured. Large forces result in high electrical charges which in turn restricts the bandwidth.
The circuit used for connecting the strain gauges enables a number of error effects to be compensated for. Along with the temperature effects on the zero point and sensitivity, the linearity of transducers or the impact of the bending movement is included. Strain gauge sensors enable accurate static calibration. Spring elements can be designed to achieve optimal repeatability. Consequently strain gauge based force transducers exclusively are used in the field of reference force measurement.
High Initial Load
On application of a force, piezoelectric sensors produce an electrical charge that can be short-circuited, if required. The status of the charge amplifier input and the status at zero force are equal. Consequently the charge amplifier’s input range is not impacted even by high initial loads. Piezoelectric transducer technology enables measurements at maximum resolution even in unfavorable conditions.
Application in Challenging Environments
Certain strain gauge based transducers offer an IP68 degree of protection (S9M, U10M with cable option). The sensitive strain gauges are protected by hermetically sealed enclosures. This permits use of these transducers in unfavorable environments. Charge cables for piezoelectric transducers are available using a special seal to ensure that the connection to the sensor housing is hermetically sealed and thus guarantee high operational safety.
High Accuracy Requirements
Modern force transducers achieve considerable accuracy, this refers especially to strain gauge based transducers that provide excellent individual errors of 200ppm. This applies for industrial standard products; force transducers for calibration tasks (e.g. HBM TOP-Transfer) consistently achieve smaller individual errors.
Piezoelectric sensors have a slightly higher linearity error, in general 0.5% relative to full scale. They are also limited by their high drift. Calibration in the force range in which measurements are to be taken later on enables significantly higher accuracy to be achieved.
Application where Space is a Constraint
Piezoelectric force sensors are highly space-efficient, for e.g. the CLP series with heights of less than 4mm. Such sensors are the optimal solution when integration in existing systems is required.
HBM offers products and services for an extensive range of measurement applications in many industries. HBM's product range covers sensors, transducers, strain gages, amplifiers and data acquisition systems as well as software for structural durability investigations, tests and analysis.
This information has been sourced, reviewed and adapted from materials provided by HBM, Inc.
For more information on this source, please visit HBM, Inc.