Thanks Walter, but I make no claims for the idea. I found the info through a patent search, and constructed my own version to see if, or how well it would fit our requirements.
I must admit that yesterday I was disappointed that I coudn't report an amazing result.
As they say - "No pain, no gain" - so I'm still thinking about it, both possible ways to improve the design, and looking at much simpler methods.
One that occurs to me is to drop a fine stream of sand down between to sheets of glass, separated by only a few mms, then have a horizontal jet of air from a compressor knocking them sideways.
It's the same principle as my attempt above, but much simpler to build.
John
PS As they say over here - "Massive, man."
It's like doing jigsaw puzzles in the dark.
Enjoy today's problems, for tomorrow's may be worse.