Penetration hardness testers are well-known in the art, and generally include a diamond- or ball-tip penetrator and means for applying minor and major loads of predetermined magnitude through the penetrator to a test specimen in successive load cycles. The minor load is first applied, and the penetrator position is noted. The major load is then applied and removed, with the minor load still applied, and penetrator position is again noted. Specimen hardness is then determined as a function of a difference in these minor-load penetrator positions.
It is important in performing such hardness tests that the penetrator load be fully applied or removed, and that the penetrator be allowed to settle in position, before noting penetrator position or advancing to the next load cycle. (It will be appreciated that the term "load cycle" is employed in the generic sense as encompassing both application and removal of a selected load.) In manual testers, this is typically accomplished by an operator by observing a dial indicator responsive to penetrator position. The U.S. Pat. No. 4,182,164 to Fohey issued Jan. 8, 1980 to the assignee hereof discloses a semi-automatic penetration hardness tester with digital readout which includes means for detecting proper application of minor and major loads, and for aborting the test sequence in the event of improper application or early removal.
Previous attempts to provide fully automated penetration hardness testing have incorporated delay circuits or timers for suspending operation for a predetermined time after a load has been applied or removed. This delay time is selected to be sufficiently long so that it can be assumed that penetrator motion has settled after each load cycle before a reading is taken of penetrator position. ASTM Specification E-18, for example, provides for a delay of several seconds after each load cycle, requiring on the order of ten seconds to complete a load sequence and obtain a hardness reading. Of course, penetrator settling time varies with material hardness, so that such automated schemes must be designed to accommodate worst-case situations, thereby necessitating substantial, often unnecessary delays. These delays present a major impediment to 100% test applications in production environments. In a laboratory environment, the requisite delays test operator patience, and often result in readjustment of the delay time and inaccurate test results
Another problem extant in the art of automated hardness testing lies in calibration of the test systems. Typically, this is accomplished by manual adjustment of variable resistors or the like while operating upon test specimens of known hardness. Such calibration techniques are prone to error, due in part to the need for operator intervention and judgment, among other factors.