GB1427751A - Non-contacting gauging methods and apparatus - Google Patents

Abstract

1427751 Automatic calibration of thickness gauges WESTON INSTRUMENTS Inc 15 Feb 1973 [15 Feb 1972] 7403/73 Heading G1A In a thickness gauge for measuring thickness deviation of a strip material "S" from a pre-set standard thickness by measuring the attenuation of radiation (X-rays or radio-active) transmitted through it, a detector system is employed which initially calibrates itself to provide a zero output reading (on a thickness deviation meter 20) for a selected standard thickness and which automatically adjusts the gain of its detector output so that the maximum expected deviation of the thickness from the standard can come within the range of its output meter. After calibration the standards are removed and the strip measurement can commence. Selecting the required comparison standard. The selected standard is formed and inserted into the radiation beam by a standards magazine containing a number of coded discs different parts of which are pushed in the beam path by solenoids in response to electrical control signals. A control circuit, Fig. 2, (not shown) incorporating a pulse generator and two counters with associated logic circuitry is disclosed. The relative counting rates of the counters are adjustable so that when the output of one of the counters reaches a fixed value the output of the other 48 (Xa) is proportional to the selected standard thickness including any compensation necessary due to the strip being of a different material of the standard. The value of the counter 48 (Xa) is fed via a buffer storage register 61 to actuate the standards magazine 12. Nulling the water to the nominal thickness. Once the standard(s) is in the beam the meter 20 is nulled to the nominal thickness of the strip (e.g. made to read zero) by circuitry incorporating a feedback loop 71 with a digital to analogue converter 70 supplied with a series of digital trains, and the number of pulses in each train represents the current difference between the detector output and the desired output (e.g. zero). Each pulse train causes the feedback signal to be modified (so several consecutive will cause the detector output to oscillate in a "servo" fashion with a decreasing amplitude about the desired level), till the detector output is substantially at the desired level. The feedback signal controls the source intensity or the detector amplification. The feedback system includes counters 80, 82 to detect when the meter is substantially nulled and to initiate a further calibration process (see below). Adjusting the meter range to the maximum thickness deviation. After nulling the meter the control circuit Fig. 2 (not shown) of the standards magazine 12 causes other standards corresponding to the maximum expected thickness deviation to be added into the radiation path. The gain of amplifier 18 is then adjusted by circuitry associated with feedback loop 99 until the maximum deflection of the meter substantially corresponds to the expected maximum thickness deviation, in this process a series of digital trains are produced whose values (formed by subtracting the digital values proportional to the actual meter output from the digital values corresponding to the maximum thickness deviation) are supplied via a D/A converter and the feedback loop 99 to amplifier 18 to vary the gain, in a similar iterative fashion to the feedback loop 71, until the meter range corresponds to the thickness variation. The apparatus is then ready to measure the strip once the standards are removed. The description also discloses a control circuit Fig. 3 (not shown) comprising a series of interconnected flip-flops controlling the sequential actuation of the various stages of the calibration process.