Force Measurement Glossary: Characteristics of Force Transducers

Benefit from our practical on-line glossary to look up the most important technical terms in the field of force measurement:

Eingangswiderstand

Der Eingangswiderstand ist der Widerstand, der an den Anschlüssen des Kraftaufnehmers gemessen werden kann, an denen die Speisung angeschlossen wird. Da die Wheatstonesche Brückenschaltung mit weiteren Widerständen versehen wird, die dem Abgleich des Kraftaufnehmers dienen, können die Werte für Eingangs- und Ausgangswiderstand unterschiedlich sein.
Wenn Sie Kraftaufnehmer parallel schalten, beachten Sie bitte, dass der Widerstand der Gesamtschaltung sinkt. Dieser Gesamtwiderstand darf die in der Bedienungsanleitung des Messverstärkers angegebene Grenze nicht unterschreiten.

Eingangswiderstand

Der Eingangswiderstand ist der Widerstand, der an den Anschlüssen des Kraftaufnehmers gemessen werden kann, an denen die Speisung angeschlossen wird. Da die Wheatstonesche Brückenschaltung mit weiteren Widerständen versehen wird, die dem Abgleich des Kraftaufnehmers dienen, können die Werte für Eingangs- und Ausgangswiderstand unterschiedlich sein.
Wenn Sie Kraftaufnehmer parallel schalten, beachten Sie bitte, dass der Widerstand der Gesamtschaltung sinkt. Dieser Gesamtwiderstand darf die in der Bedienungsanleitung des Messverstärkers angegebene Grenze nicht unterschreiten.

Input resistance

The input resistance is the resistance that can be measured at the transducer connections where the excitation is connected. Due to the fact that additional resistors for balancing of the force transducer are connected to the Wheatstone bridge, the input and output resistance values can vary.
When connecting force transducers in parallel, please remember that the resistance of the circuit as a whole is reduced. This total resistance must not exceed the limit specified in the amplifier's operating manual.



Isolationswiderstand

Der Widerstand zwischen einer beliebigen Anschlussleitung und dem Federkörper ist der Isolationswiderstand. Der Isolationswiderstand muss dem im Datenblatt angegebenen Wert bei Raumtemperatur entsprechen, andernfalls ist der Aufnehmer auszutauschen, da die Kenndaten sonst nicht mehr stimmen.

Linearität

Die Linearität beschreibt die größtmögliche Abweichung der realen Kennlinie von der idealen Gerade.
Die Angabe erfolgt in Prozent relativ zur Nennkraft.
Bei Referenzkraftaufnehmern (U15, Top Class Aufnehmer, …) mit allerhöchsten Genauigkeiten wird oft kein Linearitätsfehler angegeben, da hier nicht mit einem Kennwert, sondern mit einem Polynom oder mit Stützstellen gearbeitet wird. In diesem Fall wird die so genannte Interpolationsabweichung angegeben, also die maximale Abweichung von der gefitteten Kurve.
Außerdem gilt bei Referenzkraftaufnehmern, dass angegeben wird, welche Messgenauigkeit der Kraftaufnehmer bei einer Kalibrierung mindestens erreichen wird. Da hierzu die Vorgaben der internationalen Richtlinie ISO376 herangezogen werden, ist die Angabe bei HBM in den technischen Datenblättern entsprechend. Das heißt, dass sich die Angabe zur Linearität auf den Ist-Wert bezieht, also relativ zum Messwert ist.
Kraftaufnehmer für den Einsatz im industriellen Bereich oder zur experimentellen Mechanik richten sich nach der VDI/VDE 2638. Hierbei ist die Linearität auf die Nennkraft bezogen.

Beispiel:


Ein Kraftaufnehmer weist bei 20 % der Nennkraft (1000 N) eine Linearitätsabweichung von 0,2 N auf. Relativ zum Endwert ist damit die Linearitätsabweichung 0,02 %. Relativ zum Ist-Wert ist die Abweichung auf Basis von 200 N zu berechnen und liegt bei 0,1 %. Dies ist der fünffache Wert.

Grundresonanzfrequenz

Wie jedes System aus Masse und Feder weisen auch Kraftaufnehmer eine Resonanzfrequenz auf. Diese lässt sich berechnen durch:

m ist hierbei die Masse, die schwingt, nicht zu verwechseln mit der Masse des Aufnehmers. Cax ist die Steifigkeit des Kraftaufnehmers. Die Resonanzfrequenz in den technischen Daten berücksichtigt nur den Kraftaufnehmer und nicht die notwendigen Einbauteile. Die relevante Resonanzfrequenz des gesamten Aufbaus ändert sich natürlich, wenn zusätzliche Massen an den Aufnehmer montiert werden, deshalb ist diese Angabe ein Richtwert, der für die dynamische Auslegung eines Aufbaus ist immer die Beachtung der Einbausituation erfordert.
Eine Faustregel ist, dass der Aufnehmer bis 20% der Resonanzfrequenz eingesetzt werden kann.


m ist hierbei die Masse, die schwingt, nicht zu verwechseln mit der Masse des Aufnehmers. Cax ist die Steifigkeit des Kraftaufnehmers. Die Resonanzfrequenz in den technischen Daten berücksichtigt nur den Kraftaufnehmer und nicht die notwendigen Einbauteile. Die relevante Resonanzfrequenz des gesamten Aufbaus ändert sich natürlich, wenn zusätzliche Massen an den Aufnehmer montiert werden, deshalb ist diese Angabe ein Richtwert, der für die dynamische Auslegung eines Aufbaus ist immer die Beachtung der Einbausituation erfordert.
Eine Faustregel ist, dass der Aufnehmer bis 20% der Resonanzfrequenz eingesetzt werden kann.

Grundresonanzfrequenz

Wie jedes System aus Masse und Feder weisen auch Kraftaufnehmer eine Resonanzfrequenz auf. Diese lässt sich berechnen durch:

In practical applications, the linearity error can be substantially reduced by competently selecting the transducer's calibration range. If a force transducer with a nominal (rated) force of 100 kN, e.g., is used at 50 kN, it can be calibrated in this range. This halves the error, since computation of the linearity error now can be based on the calibration range.


Nominal (rated) displacement

The force transducer is deformed when the force to be measured is applied. The nominal (rated) displacement specifies the deformation at nominal (rated) force. It is an important characteristic because, together with the nominal (rated) force, it determines a transducer's stiffness. The force transducer's stiffness is crucial to its resonance frequency. From the physical point of view it is absolutely permissible to compare a force transducer with a very stiff spring.


Nominal (rated) force

The nominal (rated) force is the force at which the transducer is 100 percent loaded. All transducer specifications are met in this force range. Please keep in mind that tare loads resulting, e.g., from the tare weight of mounting parts also need to be taken into account and that they use up part of the nominal (rated) force. With dynamic loading, it is essential to take into consideration the transducers' oscillation width.


Nominal (rated) temperature range

In the nominal (rated) temperature range the force transducer meets the values given in the specifications.


Operating range of the excitation voltage

The excitation voltage is the force transducer's supply voltage; in general a range is specified. It is essential not to exceed the maximum excitation voltage, since otherwise the permissible voltage at the strain gages will be exceeded. As a consequence, the electrical power will be too high and the strain gage will become too hot. This will result in a change in zero point (temperature effect on zero point) and a change in sensitivity (temperature effect on sensitivity).


Operating temperature range

In the noIn the operating temperature range the force transducer enables measurements to be taken, however, the measurement accuracy is limited.minal (rated) temperature range the force transducer meets the values given in the specifications.


Output resistance

This is the resistance present on the wires connected to the amplifier input. When connected in parallel, the output resistance's tolerance should not exceed 10 ohm; otherwise cross currents could affect the measurement result.


Reference excitation voltage

All measurements for determining the characteristics are taken at reference excitation voltage.


Reference temperature

In the noIn the operating temperature range the force transducer enables measurements to be taken, however, the measurement accuracy is limited.minal (rated) temperature range the force transducer meets the values given in the specifications.


Relative creep

All strain gage-based transducers show a small signal variation under constant loading which approximately has the form of an exponential function. This process is also called "creep upon loading". With the transducer being relieved of the force, the signal changes in the reverse direction inmore or less the same way. This process is also called "creep upon unloading".

The time period during which the value has been determined is specified in addition to the maximum value of the signal variation in percent.
It is essential to keep in mind that the creep influence must not be calculated relative to the nominal (rated) force but, instead, always needs to be calculated relative to the force applied. HBM specifies the creep value after 30 minutes; due to the typical form of an exponential function, this value can be assumed in good approximation as the creep maximum. This value must in no case be linearly extrapolated.


Relatice sensitivity error tension/compression

Force transducers for tensile and compressive loading often have a small characteristic-curve variation for mechanical reasons, depending on whether they are used with tensile or compressive forces.

This parameter describes the maximum difference.


(Relative) breaking force

Either an absolute value or a percent value is specified. The name of this characteristic indicates the possibility that the transducer may break.

Tip: Please refer to the safety instructions given in the transducer's mounting instructions.


(Relative) limit force

As with the operating force, a relative value based on the nominal (rated) force in % or the absolute operating force in N is commonly specified.
If the transducer's limit force is exceeded, the transducer is likely to be no longer suitable for measurement.
Often the force transducer is plastically deformed upon exceeding of the limit force and there is a significant zero point variation. The force transducer may no longer be used and has to be replaced, because the force transducer shows substantially changed specifications. In particular, there is the risk of mechanical limit values such as breaking force and dynamic oscillation width being reduced.


(Relative) maximal operating force

The maximal operating force is specified either as an absolute value in N or relative to the nominal (rated) force Fnom in %. The force transducer will not suffer damage up to the maximal operating force, provided that it is not used several times within this range. The relationship between the output signal and the applied force is repeatable, i.e. the measurement error increases, however, the force can still be assessed.
Force transducers need to be dimensioned such that the maximal operating force is not utilized.


(Relative) permissible oscillatory stress

The relative permissible oscillatory stress specifies the stress up to which the transducer is fatigue resistant. This value is usually specified as a relative quantity based on the nominal (rated) force.
In general, the permissible oscillatory stress is given as a peak-to-peak value, i.e. the difference between the maximum and the minimum force. Force transducers may be stressed with this amplitude both dynamically and alternatingly.
Example: A tensile and compressive force transducer has a nominal (rated) force of 200 kN, the permissible oscillation width is 100%. In this case, the transducer may be loaded between 0 and 200 kN as well as between -100 kN and 100 kN.


Relative reversibility error

In practical applications, the linearity error can be substantially reduced by competently selecting the transducer's calibration range. If a force transducer with a nominal (rated) force of 100 kN, e.g., is used at 50 kN, it can bThe relative reversibility error is the difference between the force transducer's characteristic curve with increasing and decreasing force. HBM specifies the maximum deviation. In addition, the data sheet specifies the force range in which the relative reversibility error has been determined. The value is specified in fractions of the nominal (rated) force (e.g., 0.4 Fnom = at 40% of the nominal (rated) force).
The relationship used for linearity applies analogously; with reference force transducers, the relative reversibility error is specified relative to the actual value.e calibrated in this range. This halves the error, since computation of the linearity error now can be based on the calibration range.


Sensitivity

The sensitivity indicates the output signal in mV/V which is produced when the transducer is 100 percent loaded, i.e. loaded with its nominal (rated) force. Any zero signal will be deducted. Example: A transducer shows a zero signal of -0.1 mV/V. The sensitivity is 2 mV/V; in this case, the output signal at nominal (rated) force is 1.9 mV/V. 2 mV/V is a very common output signal for force transducers. As described above, strain gage-based force transducers require a voltage supplied by the amplifiers (excitation voltage). A sensitivity of 2 mV/V means that a force transducer produces an output signal of 2 mV at nominal (rated) force, when supplied with one volt. With 5 V excitation voltage, the corresponding output voltage will be 10 mV. A force transducer's output voltage can be calculated as follows:

With U being the output voltage, U0 the excitation voltage, C the sensitivity, F the applied force and Fnom the transducer's nominal (rated) force. This formula assumes that the zero signal is ideally zero.

The transducer's behavior is similar when the force varies, i.e., the transducer from the example is loaded with half its nominal (rated) force to get 1 mV/V at the output. With an excitation voltage of 5 V this corresponds to 5 m V.

A nominal (rated) sensitivity is specified in the technical data sheets. This sensitivity is valid for all force transducers of a type and is therefore given with a tolerance, the so-called "sensitivity tolerance". For this reason, every HBM force transducer comes with a manufacturing certificate specifying the exact sensitivity for the respective transducer.

Tip: Always adjust the amplifier as specified in the manufacturing certificate or according to an existing calibration to ensure optimal measurement accuracy. In this case, the sensitivity tolerance does not affect the error computation.

The transducer can also be ordered with so-called TEDS. A small chip containing the manufacturing certificate's exact specifications is installed in the transducer or in the cable. Amplifiers supporting this feature can read these data and use it for automatic setup.

 


Stiffness

A transducer's stiffness is calculated from the nominal (rated) force Fnom and the nominal (rated) displacement Snom.

Stiffness is primarily determined by the transducer's design principle and its nominal (rated) force. In physics it corresponds to a spring constant. Stiffness is crucial to the computation of a transducer's resonance frequency.


Storage temperature range

In the noIn the operating temperature range the force transducer enables measurements to be taken, however, the measurement accuracy is limited.minal (rated) temperature range the force transducer meets the values given in the specifications.


Temperature coefficient of sensitivity

Strain gage-based sensors show only a minute change in sensitivity resulting from temperature variation. This is due to the fact that the spring element materials' module of elasticity decreases with increasing temperature - equal force results in higher strain and thus a bigger output signal. The strain gages' gage factor (the sensitivity), too, is dependent on temperature.
With many force transducers the sensitivity's resulting dependence on temperature is compensated for and is thus very small. With both creep and error computation, it is essential that the specified value is always related to the current measured value.


Temperature coefficient of the zero point

In addition to the sensitivity, the zero point slightly varies according to the temperature. The Wheatstone bridge largely compensates for the effect of the individual strain gages. The remaining error is explained by tolerances that cannot be avoided. This small error can be further reduced by appropriate wiring so that modern force transducers have a remaining error of less than 0.05%/10K.


The temperature coefficient of the zero point always needs to be related to the nominal (rated) force, irrespective of what force is measured. For this reason, we recommend using a force transducer with a particularly small TK0 when working under major temperature variations and/or in the partial load range.


Zero signal

The zero signal is the force transducer's output signal prior to installation. When you install the force transducer the signal changes due to pre-stress and the masses of mounting accessories.


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