Displacements

Displacements  play a role in all civil engineering structures. As long as the structures cannot move freely, these constraints, stress and strains are created, which is why these have to be limited for most structures.

A displacement sensor measures the distance between an object and its reference point. This allows the length changes to be determined over a defined period of time. The distance changes are registered via voltage fluctuations and transmitted in the form of a standard signal to the data logger, which further processes the input data. 

LVDT sensors for path measurement
LVDT sensors for path measurement
Datalogger for recording the displacements
Datalogger for recording the displacements

Systems for measuring displacements

A number of different optical and inductive sensors are available for distance measurement: 

  • Laser optical distance measuring systems
  • Fiber optic position measuring systems
  • Capacitive displacement measuring systems
  • Inductive sensors
  • Displacement sensors and transducers
  • LVDT displacement transducer and LVDT travel sensor  

Transducer (LVDT)

Most inductive displacement transducers work according to the principle of differential choke (inductive (half bridges). They are based on the fact that the inductance of a coil changes when the magnetic permeability of the material in the coil varies. An inductive displacement transducer essentially consists of two coils connected to each other, which are cast into a metal cylinder in a sealed and vibration-proof manner. The longitudinal axis of the metal cylinder and the direction of movement of the object to be measured must be parallel to each other or the movement must be transformed into a movement parallel to the axis of the cylinder by an appropriate mechanism. A ram with a core is guided through the cylinder and connected as rigidly as possible to the moving body.

Schematic structure of an inductive displacement transducer
Schematic structure of an inductive displacement transducer

The inductance of a long cylindrical coil depends only on the coil geometry and the magnetic permeability of the surrounding material. The sensor consists of two coils. The core of the plunger can not be in both coils. If the core is e.g. in coil 1, coil 2 is empty. As the plunger moves, the inductance of one coil decreases while that of the other coil increases. The movement of the plunger to be detected is converted via inductance changes into a varying voltage that is easy to measure.

The metal cylinder and the core usually consist of a material with very high magnetic permeability in order to achieve the greatest possible inductance change even at low deflection.