Symmetrical force sensors with strain gauge measurement

S-Beam Load Cells

  • Measuring range 0 to 50kN
  • Integrated or external amplifier
  • Tension and compression
  • Robust construction
  • Description
  • Good to know
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S-Beam load cells – reliable and robust, tension and compression measuring

The technology of the strain gauge measuring principle, proven for decades, guarantees a long service life of our force transducers. They are robustly designed and available in various versions. Very reliable as well as combinable with an external measuring amplifier, our S-Beam force sensors are also characterized by the fact that they are available with protection class IP54 or IP65.

Operating principle of force transducers
The measuring chain of force transducers consists of a mechanical deformation body, a mechanical-electrical transducer (sensor element) and a subsequent electrical amplifier for processing the measured signal. The operating principle is basically the same for all force sensors and, pictorially speaking, a bending spring (deformation body) best illustrates the principle: A force is applied to a deformation body on which areas of compression and tension are created. Sensor elements are attached to this body, which detect these changes in shape, convert them into electrical signals and pass them on for processing. To obtain an equally accurate result, the deformation paths must be kept as small as possible by design. Depending on the application and the forces to be measured, deformation bodies in the constructive designs of bending beams, S-beams or load cells are used. The design and the material significantly determine the properties of a force sensor and, above all, the nominal force.

Strain gauge sensor technology
In a force transducer, the measuring body is decisive for the quality in the measuring chain, since it experiences compression and tension as a result of the force application. But basically, only a small part of a force transducer is the actual force sensor. To detect the forces, sensor elements, so-called foil strain gauges or strain gages for short, are only attached at certain points on the deformation body. These strain gages (mechanical - electrical transducers) convert mechanical strain and compression in the deformation body into electrically measurable signals. The strain gages consist of an ultra-thin metallic resistance grid and are bonded to the measuring body with an insulating carrier foil in the Wheatstone bridge circuit (four strain gage elements) at the calculated strain and compression zones: two elements detect the tension, two the compression that occurs. The strain gage elements are supplied by means of a supply voltage. If the measuring body experiences strain/compression, the resistance in the strain gauge grid and thus the output voltage changes.
Since the resistance changes are in a few mV/V ranges, the voltage signal is conditioned in the subsequent amplifier circuit for further processing.

Advantages of strain gauge technology:
Very high accuracy
Highly robust
Excellent for dynamic load changes
Extremely high long-term stability
Proven technology

Wheatstone bridge circuit
For an unloaded measuring body with a balanced strain gauge bridge circuit, the output voltage is zero volts. Since a measuring body reacts, for example, to temperature changes with tension and compression, 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 each of the two strain gauge pairs (one pair for compression / one pair for strain) experiences an almost identical change in resistance with respect to magnitude and direction, there is almost no change in the output signal in total. Also, undesirable mechanical bending moment or shear force influences diagonal to the measuring direction are compensated to a certain degree by the symmetry of the strain gauge bridge circuit.

Measuring amplifier (integrated or external) and calibration.
Particularly noteworthy are our KT versions. 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 the force sensor and the measuring amplifier as well as time-consuming tuning work between the sensor and the 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 factory calibrated in Newton in the desired force direction. According to the specified mounting position (upright or suspended) or the preferred load direction (compression or tension), the zero point and characteristic value are adjusted.
For sensors without integrated measuring amplifier, external measuring amplifiers such as our IMA2DMS are therefore offered.

Installation instructions
Some important basic rules must be observed for the error-free and safe installation of force transducers.

Force application: Always apply and dissipate forces perpendicularly and precisely.
The measuring force must act as precisely as possible in the measuring direction of the force transducer. In this way, the applied load and the force transducer form a continuous line of force action. In the case of composite load forces, the actual line of action of the force (resulting force vector) must be determined and the transducer aligned accordingly. Components acting 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 as a result. Deformations or mechanical adjustments (e.g. drilling holes in the measuring body) must be avoided at all costs!

Crash, overload, breakage protection
When designing devices for force measurement, the specified nominal force must be strictly adhered to in order to exclude measurement uncertainties or, in the limit case, destruction due to irreversible deformation of the force transducer. If an overload occurring in the limit case cannot be excluded by the mechanical design of a force measuring device, appropriate devices must be fitted to protect the force sensor. In the case of compressive forces, for example, supports can limit the strain range of the deformation body. Particular attention must be paid in the case of a suspended installation position of a force transducer subjected to tensile force. In the case of suspended or hovering loads, precautions must be taken to secure the load (e.g. by chains or suspension ropes mounted parallel to the load cell). Otherwise, there is a possibility that the measuring body "breaks / tears" in case of overload and the load falls down.

Surface properties: Stable and solid support surface
For load cells in particular, the measuring body must be installed on a solid platform, in accordance with the mounting instructions. Deflection of the base plate must be avoided at all costs. The substructure provided for mounting should be sufficiently large and have a mounting surface as flat as possible.

Creating a structurally stable measuring chain
The load carrier, force application components and the force transducer must be rigidly connected, i.e. free of play. In the case of movable mounting positions, in particular also in the case of suspended mounting in the direction of traction, rod ends, or ring nuts must be used for force application. In the case of multi-axis degrees of freedom, a cardanic mounting should be used to avoid measurement uncertainties as well as destruction of the transducer by impermissible transverse and lateral forces.

Tensile force and/or compressive force
Preferably, force transducers are operated in one load direction, i.e. either in tension or compression. Only S-Beam force sensors are suitable for alternating loads. Shear beams and load cells can usually only be loaded in compression (observe force application in the data sheet!). Force sensors with integrated electronics are calibrated in one direction only, in tension or compression.

Avoid shock and vibration
Shock and vibration signals influence the measurement result of a force transducer
(force F = mass "m" x acceleration "b"), e.g. by superposition during static measurements. The dynamic forces that occur must be considered 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 lie well below this resonant frequency in its frequency range.

Article No. Nominal load Direction of force Amplifier Protection class Featurel/Option Images 3D Data sheet Order Wishlist
KM1403 0 to 1kN Tension and compression Without IP54 For industrial scales Order
KT1403 0 to 1kN Tension and compression Integrated IP30 With signal converter Order
KM1503 0 to 10kN Tension and compression Without IP54 For batching systems Order
KT1503 0 to 10kN Tension and compression Integrated IP40 With signal converter Order
KM1506 0 to 20kN Tension and compression Without IP65 Compact and robust Order
KM1603 0 to 50kN Tension and compression Without IP67 High measuring range Order
KM500 0 to 500N Tension and compression Without IP54 Very compact Order