Non-contact measurement — General

Non-contact dimensional measurements, whether of displacement, distance or shape, cover a wide range of methods, scales and media. A wave or field, whether electromagnetic or acoustic, is very generally emitted and received by the sensor. There is no such thing as a "one-size-fits-all" method, but for each specific case there is the most appropriate method, and the techniques available offer a wide choice.

Our choice will therefore be guided by these specifications, the most important of which are :

• the measurement range and its distance from the sensor ;

• the measured extent of the room surface, the spatial response ;

• type of measurement: absolute or relative ;

• environmental conditions: mainly temperature and pressure, but also humidity, dust, fog, etc;

• the medium through which the measurement is to be taken: air, vacuum, liquid, electrically conductive solid or insulator;

• the nature and characteristics of the part to be measured: electrical conductor or insulator, magnetic permeability, permittivity, ferromagnetic properties, magnetization, visual appearance (matte, glossy, white, black, opaque, translucent, transparent, radiant or luminous), degree of homogeneity;

• the direction of displacement: is it normal or parallel to the direction of measurement?

• bandwidth, rise time ;

• noise signal level ;

• resolution, accuracy ;

• acceptable drifts with temperature, pressure ;

• sensitivity ;

• output: voltage, current or digital, on/off.

The fields of measurement range from roughness with a resolution of the order of a tenth of a nanometer, which corresponds to the atomic scale, to geodesy with measurement distances that can reach several tens of kilometers, and even, at the limit, astronomy. We are particularly interested in the measurement domains that correspond to those of mechanics (i.e. from the order of 0.01 µm to a few tens of meters), which are still very broad.

This article opens a series dedicated to non-contact dimensional measurement:

• Non-contact measurement. General ;

• Non-contact measurement. Magnetic and capacitive methods ;

• Non-contact measurement. Optical methods (part 1) ;

• Non-contact...