Absolute sensors

The name “absolute sensor” is derived from the fact that the absolute value of the voltage induced in the test coil is measured. The signal of a defect or a change in the properties of the test specimen is superimposed on the signal when the test specimen is free of defects or when the coil is empty. The test signal is therefore influenced not only by defects, but also by the material properties of the test piece and the environmental conditions. If, for example, the sensor heats up during operation, its electrical resistance changes and the absolute value of the induced voltage starts to “wander” (drift). This is the major disadvantage of these sensor systems. To suppress such and similar interferences, an additional compensation coil is often connected in between. This must be arranged in such a way that no interaction with the test specimen occurs but environmental influences are effectively suppressed.

If only the changes in the measured value caused by deviating sample properties are to be displayed, the absolute value must be compensated for this change. The simplest way to achieve this is through the so-called settlement procedure. Here, two absolute sensors, which are as identical as possible, are used, which are connected against each other (like two identical batteries, which are connected with their negative poles). The sum of the two partial voltages is zero if there are test specimens with identical properties in both coils. In this case, both coils provide equal measurement signals, which cancel each other out due to the electrical circuitry.

Such a coil system is called a difference system with arm’s length comparison. In it, the test object is compared with a sample piece of specified quality, the “test standard”. A display only occurs if the test object deviates in its properties from the test standard. A difference of 1% can fill the entire display scale only with suitable instrument sensitivity. In order to always obtain reproducible measurement results with a certain instrument setting, it is expedient to mark the sensors so that the same sensor is always used as a “reference sensor”.