Dynamometer

By measuring force acting between two bodies force is determined. The devices or technical equipment to hot, if it is weight forces, scales or general dynamometer, load cell or force sensor. From the 19th century, the name still occasionally used dynamometer derived (from the Greek: dynamis - force), which in the English language is common today.

This article provides an overview of the active principles. Individual sensor types are linked. For basics of sensors see the meter.

Arrangements

The force is a vector quantity that can act in all three directions of space. Therefore, it may be:

Most force sensors measure the force in only one direction. The power flow direction and the direction of measurement of the sensor must match, so train or pressure is measured.

The force is measured in two or three dimensions.

Depending on the installation, a distinction:

The sensor connects two parts and takes over the entire power flow. It must therefore satisfy the requirements for the strength and rigidity of the connection. The measuring range of the sensor must be greater than the force to be measured.

The sensor is integrated into a component and only covers part of the measured force ( force shunt measurement). The advantage is lower demands on the sensor. The disadvantage is that a calibration when installed is necessary to determine the sensitivity of the measurement setup.

With a known lever arm, a torque can be easily recognized as a force.

If one of the two bodies is an integrated into the meter test mass, we speak of an acceleration sensor.

Elastic deformation with displacement measurement

Most sensors use the elastic deformation of metal and transform the force into a linear way around. Hooke's law describes the relationship; simplifies the following applies:

In the resting state (static force measurement ), the spring force is equal to the force to be measured. The path difference is determined with the methods of distance measurement for very small channels and shown as motor. These include:

• Dynamometer often used in school lessons, ring dynamometer, torsion balance, gravimeter
• Inductive sensor: displacement measurement with plunger
• Capacitive sensor: The deformation of the distance between two capacitor plates changes. The capacitance change non-linearly to the force. These sensors are suitable for temperatures up to 700 ° C, especially for long -term measurements over several years.
• Optical scanning in atomic force microscope
• Interference optical power sensors: a laser interferometer measures the distance to the deformation body. At the measuring point no electronics, no adhesive is necessary, required optical quantization without A / D converter. The Ilmenau University of Technology has developed such sensors in 1984.

Obsolete:

• Mirror - fine extensometer according to Martens: The deformation body moves over lever a mirror. A light pointer indicates the measured value.
• Measuring the path difference with a precision measuring microscope on a graduated scale.
• Proving Ring: A ring converts the force into a change in length. Inside the ring a leaf spring is mounted which is brought manually to vibrate. With a micrometer, they will once again steamed without and with force. The force is read from the micrometer as a difference between the two settings.

Elastic deformation with resistance measurement

• Strain gauge ( DMS ) expand their electrical resistance when stretched. They are glued to the body deformation or directly to the measuring object and electrically evaluated. Frequencies up to 8 MHz are possible, they require temperature compensation, long-term performance is not as good because of creep. It is divided into: Metal strain gauges: The length of elongation and contraction increases the transverse electrical resistance.
• Semiconductor strain gauges: The electrical resistivity increases with strain due to internal forces ( piezoresistive effect). This effect brings a much greater sensitivity to metallic DMS.

Obsolete:

• Tasimeter: resistance change of coal by pressure.

Magneto-elastic effect

Through the influence of the force, the magnetic permeability μ is changed (inverse magnetostriction ). Thus, the transferred from the primary coil to the secondary coil magnetic flux changes.

The advantages of this mode of action are highly robust against overloads and high sensitivity. The ABB company marketed under the name Pressductor ® This patented measuring principle since 1954.

Piezoelectricity

Main article: piezoelectricity

A piezoelectric ceramic element is formed by force a charge distribution Q which is proportional to the force.

Piezoelectric force transducers can be designed to be very stiff and highly dynamic measure forces ( up to 100 kHz). By the flow of the charge no good results are obtained by static and quasi-static loads. By overlaying layers of 3 discs with different measuring direction and multi- component force sensor are possible.

Electromagnetic compensation

They work like an electrodynamic loudspeaker. In a magnetic field, there is a moveable coil. The current through the coil is proportional to the force on the spool when the deflection is compensated for. A position control with the proximity sensor keeps the coil at a fixed position. From the general formula of the Lorentz force results:

Such power sensors can measure very small forces ( max. 20 ... 30 N ) and are used eg in precision balances.

Vibrating wire principle

The period of a tensioned string or a belt is dependent on the clamping force.

• Generally vibrating wire transducer

• The belt tension of drive belt can be adjusted by optical frequency measurement of the vibration.

Other procedures

• Waveguide force microscopy