Interesting facts about force transducers, installation, design and measuring principle
Guide for Load Cells
Load cells convert acting forces into electrical signals. Depending on the direction of the force, a metal spring body undergoes expansion or compression, which is detected electrically by sensor elements and transmitted for signal processing. Our force transducers all work on the basis of proven strain gauge full bridge technology and provide an analogue output signal in mV/V.
The forces acting on the measuring body must not exceed the maximal nominal force for which a load cell is designed. The nominal force is determined by the rigidity of the measuring body. The design as well as the material defines the stiffness of the measuring body.
We offer a wide range of standard load cells for small loads up to several hundred kilo-Newton. For the implementation of special measuring tasks, our many years of experience and our broad application know-how are available to you for the optimal design-in; and this already starting from small quantities.
Force transducers consist in their measuring chain for force application of a mechanical deformation body, a mechanical-electrical converter (sensor element) and a subsequent electrical amplifier for processing the measured signal.
The mode of action is basically the same for all force transducers and, metaphorically speaking, a bending spring (deformation body) best illustrates the principle: A force is exerted on a deformation body, on which areas of compression and tension are created. Sensor elements are mounted on this body which detect these changes in shape, convert them into electrical signals and transmit them for processing. In order to obtain a virtually exact result, the deformation paths must be kept as small as possible by design.
Depending on the application and the forces to be measured, the following deformation bodies are used in the constructive designs
- Shear beams
- Load cells
The design and the material determine the properties of a force sensor and, above all, the nominal force.
In a force transducer, the measuring body is decisive for the quality in the measuring chain, since this body is the one subjected to compression and tension due to the application of force. But basically only a small part of a force transducer is the actual force sensor. In order to detect the forces, sensor elements, so-called foil strain gauges are only attached on certain points of the deformation body. These strain gauges (mechanical-electrical transducers) convert mechanical tensions and compressions in the deformation body into electrically measurable signals. The strain gauges consist of a wafer-thin metallic resistance grid and an insulating carrier foil is adhered to the measuring body in the Wheatstone bridge circuit (four strain gauge elements) at the calculated tension and compression zones: two elements detect the tension, two the compression that occurs. The strain gauge elements are supplied by a supply voltage. If the measuring body experiences a tension / compression, the resistance in the strain gauge grid and consequently the output voltage change.
Since the resistance changes are in a few mV/V ranges, the voltage signal is processed in the following amplifier circuit for further processing.
Advantages of the strain gauge technology:
- Very high accuracy
- Very high robustness
- Very well suited for dynamic load changes
- Very high long-term stability
- Proven technology
With an unloaded measuring body with a balanced strain gauge bridge circuit, the output voltage is zero volts. Since a measuring body reacts to temperature changes with tension and compression, for example, the Wheatstone bridge circuit suppresses these temperature influences to a very good degree. The so-called apparent strain has hardly any influence on the zero point in this type of circuit. Since the magnitude and direction of each of the two pairs of strain gauges (one pair for compression / one pair for tension) undergo an almost identical change in resistance, there is almost no change in the resulting output signal. Also, undesirable mechanical bending moment or shear force influences diagonal to the measuring direction compensate the symmetry of the strain gauge bridge circuit to a certain degree.
The output voltage Ua is zero if the resistance ratio of both bridge branches is equal. If these resistance ratios exist, then the bridge is called balanced. Ua is of the order of a few millivolts [mV], therefore the electrical signals are expressed in the unit millivolts per volt [mV/V].
Our KT versions are particularly worthy of mention. These force transducers are offered with an integrated measuring amplifier in the sensor housing and are calibrated at the factory. This eliminates wiring work between force sensor and measuring amplifier as well as time-consuming tuning work between sensor and amplifier. A stable signal is obtained in one unit, which is otherwise in the mV/V range.
All force transducers with built-in electronics or with standardized output are calibrated in the factory in the desired force direction in Newton. According to the specified mounting position (standing or hanging) or the load direction (compression or tension), the zero point and characteristic value are adjusted.
External measuring amplifiers such as our IMA2DMS are therefore offered for sensors without an integrated measuring amplifier.
KT Types with amplifier
Some important basic rules must be observed for the correct and safe installation of force transducers.
The measuring force must act as accurately as possible in the measuring direction of the force transducer. Thus, the applied load and the force transducer form a continuous line of force action. For composite load forces, the actual line of action of the force (resulting force vector) shall be determined and the transducer shall be aligned accordingly. Components that act differently, such as eccentric loads, transverse forces or torsional moments are disturbance variables and falsify the measurement signal. In addition, the spring body can be irreversibly deformed. Deformations or mechanical adjustments (e.g. drilling of the measuring body by yourself) must not be carried out!
When designing devices for force measurement, the specified nominal force must be strictly adhered to in order to exclude measurement uncertainties or, in critical cases, avoid destruction by irreversible deformation of the force transducer.
If the mechanical design of a force measuring device cannot exclude the possibility of an overload occurring in a critical case, appropriate devices shall be installed to protect the force sensor. Supports, for example, may limit the strain range of the deformation body in the case of compressive forces.
Special attention should be paid to the suspended mounting position of a force transducer that is subject to tensile force. In the case of hanging suspended loads, precautions must be taken to secure the load (e.g. by chains or suspension ropes mounted parallel to the force transducer). Otherwise, there is a possibility that the measuring body "breaks / tears" in the event of overload and the load falls down.
Especially for load cells, the measuring body must be installed on a solid platform in accordance with the mounting instructions. It is essential to avoid bending the base plate. The substructure provided for the mounting should be sufficiently large and have a mounting surface that is as flat as possible.
The load carrier, force application components and the force transducer must be connected rigidly, i.e. without play. In the case of movable mounting positions, in particular also in the case of suspension mountings in the direction of traction, rod ends or ring nuts must be used for force application. In the case of multi-axial degrees of freedom, a cardanic mounting should be used to avoid measurement uncertainties and destruction of the transducer by impermissible transverse and lateral forces.
Preferably, force transducers are operated in one loading direction, i.e. either in tension or in compression. Only S-Beam force sensors are suitable for alternating loads. Shear beams and load cells can usually only be loaded in compression (note force application in the data sheet). Force sensors with integrated electronics are only calibrated in one direction, either in tension or in compression.
Shock and vibration influence the measurement result of a force transducer (force F = mass "m" x acceleration "b") e.g. by superimposing them in static measurements. The dynamic forces occurring in this process must be taken into account when designing measuring ranges and overloading of the transducer due to dynamic load changes must be avoided. The resonant frequency of the various deformation bodies depends, among other things, on the mass and the stiffness (mechanical impedance). A vibration load must be significantly below this resonance frequency in its frequency range.