Force transducers

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With a force transducer , a (also called load cell, load cell, load cell or load cell) force measured, acting on the sensor. Usually, both tensile and compressive forces can be measured by elastic deformation.

Pull / push rod spring body
Diaphragm spring body
Spring body with strain gauges from a bathroom scale

In addition to force measurement , applications are also weighing (see also load cell ) and determining torques .

Spring force transducer

The spring body of the transducer is elastically deformed due to the force . The force absorption must take place in the prescribed direction. The deformation of the spring body (mostly made of metal ) is converted into a change in electrical voltage by means of strain gauges , the electrical resistance of which changes with the expansion . The electrical voltage and thus the change in strain is registered via a measuring amplifier . Due to the elastic properties of the metal, this can be converted into a force measurement value in which the transducer is calibrated .

Another method to record the deformation is with capacitive sensors .

For the purpose of tracing the unit force according to its physical definition, the calibration of force transducers up to a force of 16.5 MN is carried out in the Physikalisch-Technische Bundesanstalt in Braunschweig. Other accredited laboratories connected to the PTB can also trace the force unit back by using the reference method. With the methods used, these laboratories do not achieve the measurement accuracy of the force standards of PTB. In safety-relevant areas, the calibration is monitored and carried out by the MPA , TÜV , other certified institutes or certified calibration services.

Measuring range

Spring force transducers are available in measuring ranges from 0.5 N to a few tens of MN. The largest force transducers include so-called force transducer build-up systems, which are made up of several spring element force transducers. They reach nominal loads of up to 50 MN.

The smaller the nominal load, the more sensitive the force transducer is. Transducers with nominal loads in the range of a few tens of Newtons can be damaged or destroyed by the forces that are applied during handling. If a force is applied to the transducer that is well above its nominal load, the mechanical components may be loaded beyond their elastic deformation limit. First of all, this means that the measured values ​​of the sensor can no longer be reproduced. In addition, the material of the sensor and the material of the measuring grid of the measuring strips can be loaded up to the breaking load, as a result of which elastic properties are permanently lost. The relationship between the force applied and the path of deformation of the sensor is then no longer reproducible.

Types

There are numerous types of spring force transducers, some of which are listed below:

  • Bending beam
Bending beam for measuring the reaction torque on a water vortex brake WX-40 VEB Spezialfahrzeugwerk Berlin
In the case of a beam, the strain gauges are attached to a beam (lever). The lever is attached on one side and the force is applied on the opposite side. The lever is elastically bent and this also changes the elongation of the strain gauges attached lengthways on the top and bottom. This type of construction is mostly used for high-precision measurements. It can be subjected to both compression and tension.
Additionally or exclusively, the shear can also be measured (shear beam).
  • Ring torsion spring
The principle of the ring torsion spring is used very frequently in commercial force transducer construction. A toroidal body is subjected to force on the inside, the counter bearing is on the outside. Due to the load, the torsion spring experiences a constant torsion, which compresses the top and stretching the bottom. The strain gauges are accordingly applied to the top and bottom of the spring body.
A particular advantage of this design is that these force transducers react to a transverse force with an elliptical deformation, which practically does not change the overall deformation of the respective measuring strips. This makes force transducers of this type particularly stable against parasitic force influences such as bending moments and transverse forces.
  • S-shaped spring body
The spring body is shaped like an S. The force is applied vertically. The bending measurement is on the middle bar. Load cells of this type are characterized by their compact design, high reliability and accuracy, they are less sensitive to lateral forces when compared to bending beams.
  • Expansion cylinder
A cylindrical body is loaded in the direction of its first main axis. This results in an expansion in the first main axis direction and a transverse contraction in the direction of the second main axis. Expansion cylinders are easy to manufacture and very robust. Their disadvantage is a very high sensitivity to manufacturing tolerances in the force introduction points and to parasitic force influences. Therefore, these designs are preferred in the meganewton range, where higher measurement uncertainties are tolerated.
  • Hollow body expansion cylinder
Like an expanding cylinder, but designed as a hollow body. Such sensors are particularly suitable for very high forces in the high meganewton range; these devices are currently used up to 30 MN.
The strain gauges determine the deformations to be achieved under full load, which is why the radii are also given for a solid body. At very high loads, however, these parameters are often too small and no longer meet the geometric or kinematic boundary conditions of the respective application. The solution is to design a hollow body that can have a nominal diameter that is variable within certain limits with the same cross section and thus the same first main expansion.
Large-scale pressure transducers are often manufactured according to this principle.
  • Diaphragm spring body
The diaphragm body is related to the ring torsion spring. A membrane structure is implemented between load introduction and load discharge in the force transducer, which experiences a bending load when the clamping torque is blocked on both sides. As a result, the flexible membrane exhibits the S-sleep typical for this load case under load. This fact helps the application of the strain gauges and their connection to the Wheatstone measuring bridge in such a way that there are always two areas with negative and two areas with positive expansion.

Piezo force transducer

In a piezoceramic element, the effect of force creates a charge distribution that is proportional to the force. If this charge is measured with a charge amplifier , there are no insulation problems due to the short-circuit operation of the sensor element. If, on the other hand, you use a simple conversion into a voltage through the parallel capacitance, you will not achieve good results due to the rapid discharge of the charge with static and quasi-static load.

Depending on the type of crystalline structure of the piezo element, pressure or shear forces can be measured. Tensile forces can only be measured with preload. Piezoelectric force transducers can be designed to be very rigid and can also measure highly dynamic forces (up to 60 kHz depending on the version).

Force transducer with vibrating elements

These force transducers are only used in special cases.

For example, the resonance frequency of a string that is tensioned by the force can be measured.

In the atomic force microscope , the oscillation frequency of the carrier of the scanning needle is measured to measure the force. When it approaches the sample, the damping and resonance frequency change due to the van der Waals forces .

Electrodynamic force transducers

They work similarly to an electrodynamic loudspeaker ; the current through a coil in a magnetic field is proportional to the force if it compensates for the deflection; H. holds the bobbin in a fixed position. A position sensor and current control are required for this.

Detailed: An electric current flows through a moving coil , which electromagnetically compensates for the carrying force . This is done via an electrical control circuit that keeps the position of the coil constant. A capacitive or optoelectronic position sensor measures the position of the coil. If a force is exerted on the coil, it sinks into the pot magnet. The position sensor recognizes this and sends the control difference to a controller. This increases the coil current so much until the original "zero position" is reached again. The resulting increase in current is proportional to the force; the position is the same in the loaded and unloaded state.

Such force sensors can also measure very small forces precisely and are therefore u. a. Used in precision balances , fine balances , analytical balances and micro balances up to a resolution of 0.1 µg.

Resistive force transducers

These sensors change their electrical resistance when a force is applied . One example of this are so-called FSR sensors . These consist of two thin foils and conductive structures embedded in them. At a maximum pressure of 100 N / cm² the electrical resistance drops from a few megohms to around 1 kilohm.

Areas of application

Force transducers are u. a. in scales , press-fit devices , in cranes and excavators for monitoring the load , but also in testing technology, e.g. B. used in universal testing machines to measure tensile force or compressive force . Another area of ​​application is the measurement of the reaction torque of water vortex brakes , eddy current brakes and pendulum machines via a lever arm .

See also

Web links

Individual evidence

  1. http://www.ebe-gmbh.de/resources/News/PC_und_Industrie_S1_und_S138.pdf Capacitive force and torque sensors with capaTEC in Buying Guide Measurement & Sensor Technology 2015 , page 138, accessed on May 9, 2018