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10 Appendix
The illustration (left) shows a typical arrangement of attenuating element, sensor and shaft.
How the system works:
Every inductive sensor forms an electric field at its active surface, from which the contact gap
(nominal contact gap "Sn”) can be derived as a function of sensor size. The table lists several
typical sensors with their characteristics. The effective contact gap is 0.8 x nominal contact
gap in the case of structural steel. An additional correction factor that depends on the material
must be included for other materials. These factors are specified in the sensor manufacturer's
data-sheets.
The installation distance between the sensor and the attenuating element's upper edge should
be Sn/2 (half the nominal contact gap). The sensor's diameter depends on the required nom-
inal contact gap and can be derived from the table. The attenuating element's diameter should
be equal to or greater than the sensor's diameter (for frequencies < 20% of the sensor's max-
imum switching frequency).
The height of the attenuating element should be at least 3/4 of the sn (nominal contact gap).
If this height is not assumed, the sensor can detect the shaft or it will be attenuate by the shaft
until the secured r.pm. recognition is not assumed any longer.
Under certain circumstances, this fault can be first recognised though higher r.p.m.
Fig. 10-13 Position r.p.m. sensor to the attenuating element
1 = r.p.m. sensor
Fig. 10-14 Dimensional drawing attenuating element for our exam-
ple
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