5 - Measuring parameters

Magnetostrictive position sensor

Applying an axial magnetic field to a magnetostrictive wire current passing through, a twisting occurs at the location of the axial magnetic field. Magnetostrictive position sensors use a permanent magnet as position marker, to generate the axial magnetic field. The position of the marker can be determined by applying a current pulse to the waveguide and starting a timer. The current pulse causes an ultrasonic wave to generate at the location of the marker magnet. The wave travels along the waveguide toward the detector, which generates a voltage pulse stopping the timer. The elapsed time is proportional to the distance between position magnet and the detector. Source: Balluff.hu

Differential transformer

A linear variable differential transformer consists of three coaxial windings, the center winding is the primary and the others are the secondaries, which are connected together at one of their two terminals. When AC excitation is applied to the primary winding and the ferromagnetic core moves within the coil assembly the coupling between the primary and each of the two secondaries changes. As a result, the output voltage magnitude changes from the null, which occurs when the core is centered.

Potentiometric transducer

Potentiometric displacement transducers are rather simple devices in which a sliding contact (wiper) moves over a resistance element. The result is a resistance change between the wiper and one of the contacts of the resistor.

Foil-gage rosettes. Meander-shaped metal foils or thin metal films deposited onto polyimide or other flexible insulating substrate are used in strain gages. They are stuck onto the measured surface the deformation of which causes resistance changes due to the geometrical piezoresistive effect.

Piezoresistive Si pressure sensors

Piezoresistive silicon pressure sensors employ resistors as sensing elements, diffused or implanted into the surface region of a membrane made by anisotropic etching. A pressure difference between the both sides of the membrane causes its deformation. Due to the piezoresistive effect, the resistors follow the deformation with resistance changes; two resistors increase and two decrease, resulting in a multiplied effect in the Wheatstone-bridge configuration. The resistance changes, , can be expressed as follows: where G is the gauge factor; E is the Young's modulus of the membrane; a is its thickness, and K a is constant depending exclusively on the geometrical sizes. The output voltage of the bridge, U, related to the supply, U₀, is:

Piezoresistive thick-film pressure sensors

The sensing element of the polymer thick-film pressure sensor consists of a circular edge-clamped epoxy-glass (or flexible polymer) diaphragm on which four PTF resistors connected into a Wheatstone-bridge configuration are screened and cured. In order to maximize the loss of the bridge balance and the voltage output, two resistors must be positioned near the centre of the diaphragm and the other two resistors near the edges. The pressure to be measured induces a strain in the diaphragm and the resistors change their values, two resistances increase and two decrease, similarly to the silicon-based version. A very simple membrane structure can be used for relative pressure measurements (pressure differences related to the ambient pressure). Absolute pressure sensor needs a closed cavity with a constant pressure or vacuum in it. Conventional CERMET thick-film versions can also be fabricated using ceramic or enamelled steel substrates and resistor pastes.

Fibre-optic pressure sensors

Extrinsic fibre-optic pressure sensors contain a flexible membrane covered by reflective material, which can be deformed under pressure difference. The transmitted light intensity between the input and output fibres is modulated by the deformation of the membrane due to the conic angle changes of the reflected divergent light bundle.