Rotary testing of flat and round material offers special advantages: it enables dynamic testing for small damage with high spatial resolution and high sensitivity independent of the feed speed, which can also be very small - in extreme cases even " stagnant ". This ensures that the filter technology of dynamic testing is used to its full extent and most interference effects can be suppressed. Disc sensors are used for surface testing, hand rotors for smaller boreholes, internal rotors for larger boreholes and pipe inner surfaces and external rotors for the outside of the pipe. For both the inner and outer rotors, systems are available for the size range from 0.8 mm to 200 mm. Depending on the testing task, absolute, differential or multi-difference sensors are used.