Patent GB-1361797-A (GB)
Title: Crack detector
IPC: G01N33/08
Filed: 1971-09-01
Abstract: 1361797 Acoustic inspection devices; switching and pulse discriminator circuits DIAMONL INTERNATIONAL CORP 1 Sept 1971 [28 Oct 1970] 40795/71 Headings H4D and H3T [Also in Division Al] An article is tested for cracks by tapping it with or on a transducer to impart vibrationa. energy to the article, measuring with the transducer vibrational energy returned from the article, and determining waveform or decay characteristics of the measured energy. In the Fig. 1 embodiment for testing eggs a row of transducer crystals 5 are mounted or discs 4 behind a stretched plastics sheet 3 through which project buttons 2 for impacting egg E as it rolls over them. Inertial masses 6 sheet 3 and damping material 7 provide acoustic isolation of the crystals. For each impact, the relevant crystal gives an output signal which may be a single pulse or a pulse train. A sound egg gives a sharper, narrower pulse than does a cracked egg, or alternatively a longer pulse train with narrower individual pulses than does a cracked egg. For detecting a cracked egg on the basis of pulse width the crystal outputs are amplified at 12, clipped at 15 and widthdiscriminated at 14. For ejecting the cracked egg at a point downstream of the testing station, the discriminator output is delayed, by a tape loop 18 with record head 15 and read-out head 16, before application to a reject gate. To detect a crack on the basis of pulse sharpness (Fig. 8 embodiment) the pulse is processed to give a voltage representing the slope of the trailing edge of the pulse and comparing this slope with a reference value representing the leading edge slope, the trailing edge slope being lower for a cracked egg than for a sound one. Pulse width discrimination (Fig. 2).-The crystal outputs are fed via voltage-follower op-amps 30 to op-amp 32 connected as a Schmidt trigger. The Schmidt output (54, Fig. 3B, not shown) is integrated on capacitor 43 to give a voltage proportional to the width of the crystal 5 output pulse (Fig. 3A, not shown) and this voltage is compared at 33 with a reference voltage to give an output signal if the reference voltage is reached (indicating a cracked egg). If crystal 5 output is in the form of a decaying pulse train, the Schmidt trigger responds only to the first pulse in the train because capacitor 36 and diode 37 cause the trigger threshold to be raised by the first pulse. The threshold is restored to its original level by the (relatively slow) discharge of capacitor 36 through resistors 34, 35. Pulse slope discrimination (Fig. 8).-The crystal 5 output pulse (Fig. 7A, not shown) is fed via voltage follower 3 to differentiating network 102, 103. A positive voltage (134, Fig. 7D, not shown) proportional to the leading edge slope of the pulse is obtained on capacitor 110 via non-inverting op-amp 100 and diode 127, while a positive voltage (135 for a sound egg, 136 for a cracked one, Fig. 7D, not shown) proportional to the trailing edge slope is obtained on capacitor 111 via inverting op-amp 101 and diode 128. These voltages are compared by op-amp 32 whose output is integrated on capacitor 43. If the trailing edge voltage is lower than the leading edge voltage, the integration voltage at 43 (Fig. 7E, not shown) reaches a reference voltage set in comparator 33 which then gives an output signal (indicating a cracked egg).  In various modifications crystals 5 may be replaced by magnetic or capacitive transducers, or vibration sensitive transistors, or fluidic devices. A cracked article may be detected by clipping and counting the pulses in a pulse train output from a transducer (there being fewer counted pulses than for a sound article).