GB1312452A - Apparatus for measuring thickness - Google Patents

Abstract

1312452 Ultrasonic measuring; ultrasonic couplings; transistor switching and pulse counting circuits UNITED KINGDOM ATOMIC ENERGY AUTHORITY 16 July 1970 [21 July 1969 16 Oct 1969] 36657/69 and 50983/69 Headings H3T H4D and H4J A thickness measuring device includes means coupling a pulse source and a detector to a specimen capable of carrying elastic waves wherein the time between the transmitter pulse and the reflection from the specimen near surface is at least five times greater than the time between successive echoes from the specimen rear surface, and a zero crossing detector for establishing a time reference for each echo. A device for measuring the thickness of a steel plate 12, Fig. 4, includes an ultrasonic transducer 11 coupled thereto by a polystyrene block 13. The transducer is shock excited by a pulse generator 14, which also gates rectifier 33 to prevent detection of the transmitter pulse. The first echo after the transmitter pulse is from the plate surface and sets bi-stable 43 to open gate 44. The next received pulse from the rear surface thus sets bi-stable 42, the pulse timing being accurately determined by zero-crossing detector 39. The received pulses also enter counter 45 which on reacting a specified count n, resets bistable 43, closing gate 44 and opening gate 46. The next received pulse then resets bi-stable 42 whose output is thus a pulse of length equal to the flight time of n echoes from the plate rear surface. This output pulse passes through a time expander 47 whose output opens an AND gate allowing 10 MHz clock pulses to enter a counter and digital display 49. Alternatively if the device is used to gauge a plate being machined, the output of a thumb-wheel switch may set a time period which is compared to the expander output, the difference being displayed on a centre-zero meter. The input channel includes an amplifier whose gain is controlled by the counter 38 and the pulse amplitude as detected by high and low level threshold detectors 34, 35. Transistor circuits.-In zero crossing detector 39, Fig. 6, T3 biases tunnel diode 41 at A on its characteristic. Fig. 7, as the input signal goes negative, the bias drops to B and switches to C, driving T1 into conduction. As the input rises the bias rises to D and switches to A as the input crosses zero. Time expander 47, Fig. 8, normally has T5 cut-off and C charges up, forwardbiasing the shunt diode causing the discriminator 48 output to rise due to equal inputs. When the input pulse arrives, T5 turns on, discharging C at a fixed rate, until the pulse is removed when C charges up through T4 to its original voltage. The discriminator 48 output is thus a pulse of length equal to the discharging plus charging time. When the input pulse is subtracted therefrom the result is a pulse whose width is a multiple of the input pulse in the ratio of the discharging rate to the charging rate (about 60). (For Figures see next page.)