Test heads of the scanning type have been long known in different forms in the field of nondestructive testing of materials, in particular for testing the surfaces of semifinished products. U.S. Pat. No. 3,281,667, shows an arrangement in which a test probe mounted at the top end of a lever is reciprocated as a result of the other end of the centrally mounted lever being connected to a cam. Also, U.S. Pat. No. 3,469,182 describes a probe bar with a number of test probes to which an oscillating movement is imparted by cam action at its two ends.
A first disadvantage of the arrangements described in the referenced patents, is to be seen in the curved movement paths of the probes, which can be avoided in a relatively simple manner by the use of corresponding guides. However, a more serious disadvantage is that the speed of the oscillating movement exhibits a sinusoidal variation between zero and the maximum value, and has only a very small constant zone. It has been found that this variability of oscillating speed proves particularly disadvantageous for electrical processing, i.e., the filtering of the signal voltage of the probe. Due to the varying speed of the probes, an excessively large bandwidth for the filter is required, which, in turn, has a detrimental effect on the signal-to-noise ratio of the signal voltage.
From U.S. Pat No. 3,311,819 an arrangement for testing billets is known which permits a scanning movement of essentially constant speed over a predetermined area, i.e., the billet width. In this case, the oscillating movement is imparted to the test probe by the piston of a hydraulic jack which changes its direction of movement when a sensing element signals that the billet edge has been reached by the probe support. A disadvantage of this arrangement lies in the high cost, the necessity of a hydraulic supply line, and the relatively large dimensions of such an arrangement.