I had a very interesting technical visit to the National Theatre last Tuesday morning, including a visit to the grid 27.5m above the Olivier stage.
The Olivier is an open stage in an ampitheatre style and the fly tower is more-or-less hexagonal in shape. It was recognised back in the early 1970s that standard flying bars parallel to the proscenium was not appropriate for a venue without a proscenium, so instead, a very flexible point hoist system was installed. There were 170 hooks which can be moved left or right from their home position, connected via a switching matrix to 36 variable frequency controller units known as cyco-converters. It was all controlled via a PDP 11 Mini computer and a custom control desk. The system was clever enough to move any of the hooks at any individual speed in either direction, up to the limit of 35 moving at any one moment (One was a a spare).
The system worked very well for the first three decades but accelerated component failures in recent years now mean that only six controllers work and lots of the winches are unservicable. On that basis, the old system is now relegated to rigging and static flying. Looking at an article written in 1979, it seems that the current control desk is not the original one so there has been an interface refresh at some point.
The theatre has now installed a phase 1 upgrade consisting of 30+ new hauling units, more about the new stuff next time.