The Use of A Sonic Digitiser and Mini-Computer in Particle Size and Shape Characterisation

Abstract

Large and expensive image-analysing computers are available, but their cost is in the region of tens of thousands of pounds. The system displayed here can provide most of the measurement Support that a particle technologist might expect of a larger system, but at a cost of L4,OOO or less, because advantage is taken of recent advances in microelectronics. The computational capability is in fact kreater than that of the larger conventional image-analysers. A perspex plate, 60cm square, has a linear microphonic array along two of its adjoining edges. A hand-operated spark emitter can be positioned precisely at any point on the square, when the microphones will measure the time taken for the noise of the spark to travel to the two edges. These times are digitised into the x and y co-ordinates of the selected point by an interfacing circuit and fed to the expansion port of a Commodore YET 3 0 3 2 uini-computer. If a large number of points along a traceable outline are to be digitised, the emitter can be set to repetitive operation at a convenient rate to avoid having to keep on pressing the button. What the apparatus can do after digitisation is governed by the programming of the computer. The equipment demonstrated here allows an origin for co-ordinates and a scaling factor to be built in. The perimeter .>nd the area of any digitised outlines can be calculated, and 2articles can be counted and grouped by any selected criterib,, typically size and/or shape. Closed outlines can be Fourier-analysed, either by the conventional method, the "rotating vector", in which the co-ordinates are converted to their polar form and r, the radius, is analysed as a function of 0, the angle made by the radius with the datum direction, or alternatively, where the particle outline is re-entrant So that this method fails, by the ZaPn i ; Roskies (1972) rr,ethod.