Continuously Variable Pneumatic Control
Application to 8421 Crane
by Mark J E Bellis, 14th November 2009

The crane 8421 is a good example of where Continuously Variable Pneumatics is useful.
The existing crane, built according to the instructions, has 2 pneumatic cylinders
end-to-end, to raise the jib.
This works fine when excess air is pumped into the cylinders and the crane job is at its full height.
However, when the crane jib is not fully raised, the jib height varies with the load on the hook.

Continuously Variable Pneumatics allows the jib to maintain its height with some variation in the load.
If the jib falls as the load increases, the valve positions will change, allowing more air into the
cylinders to compensate, bringing the jib back up to its commanded height.
If the jib rises as the load decreases, the valve positions change the other way, allowing more
air to escape, bringing the jib back down to its commanded height.

A heavy load application, such as the crane, might require an increase in air pressure from the
compressor.
Given that the air pressure is a multiplier in the loop gain of the system (see system diagram),
it is possible that the system might be less stable at higher pressures.
The lengths of the valve levers and gains of each input (jib height feedback, height command and dither)
can be varied to restore stability.

This is work in progress.

Other lifting applications include a ride-in flight simulator, which would use 3 or 4 CVP systems,
possibly inter-connected.

Mark J E Bellis