Continuously Variable Pneumatic Control
by Mark J E Bellis, 10th October 2009

Pneumatic cylinders are usually 2-state devices.  They usually just go end to end.
A few years ago some AFOLs achieved mid stop pneumatics, making cylinders stop in the middle.
Its most obvious application is in vehicle steering, where a stable middle position is required.
My version is here: http://www.brickshelf.com/cgi-bin/gallery.cgi?f=147931
It includes cascaded stages.

Contunuously Variable Pneumatics takes the concept a stage further.
This new system allows a pneumatic cylinder to stop anywhere within its range on command.
Stopping anywhere in the range is usually the preserve of a Linear Actuator.
Linear Actuators have been used in many simulated hydraulic applications in sets.
It's time pneumatics caught up, especially with the advent of their return for 2010.
Thanks TLG for listening to AFOLs and producing set 8049 with pneumatics!

The Continuously Variable Pneumatic Control system in the pictures works like this:
There are 3 inputs:
- A PF battery box to power a motor
- A continuous air supply of about 20-30psi to power the pneumatics
- A variable position of a sliding beam - the red one at the top
There is one output:
- The pneumatic cylinder position (which may be any number of cylinders in series or parallel).

The input that commands the cylinder position is the sliding position of the red beam.
The motor adds a "dither" to the input, via the crank and yellow Half Beams.
The dither overcomes the hysteresis caused by stiction in the valve switches.
The result is the position of the two purple levers that control the valve switches.

The two switch valves are connected in parallel.  The valve switch lever angles are deliberately offset.
This removes most of the "dead" section length between one direction and the other in a single valve.
Therefore a small movement of the levers is enough to change the air commands to the cylinder from
one direction to the other.

The pump provides the air supply.  I sometimes use a car tyre air compressor, set to 20-30psi.
If you intend to get one, make sure it has a pressure setting, not just a meter.  Significant overpressure
would damage LEGO pneumatic parts.

The output cylinder is driven from the switch valve outputs.
The turquoise lever takes a proportion of the output movement and feeds it back to the switch valves.
The blue sliding beam alters the position of the valve bodies relative to the tops of the levers.
This in turn alters the air command to the cylinder.
Since the dead section between one direction and the other is so small, the cylinder may hunt for a
position that keeps it stable in one place.  The dither ensures that it soon picks up a change in the
position of either the input (red) beam or the feedback (blue) beam, which could move in response to
a change in the load.

If the load varies, it may be enough to move the stable position of the cylinder, but it is possible to
feed back a proportion of the output position to the device that commands the red beam position,
enabling a system to compensate for load changes.

The system bears some resemblance to a Class C electronic amplifier.
A Class C amplifier has each of two switch devices conducting for less than half the cycle time.
This system has each of two switch valves passing air flow for less than half the cycle time.

Expanding the system:
This system may be driven by an NXT motor (which has a position sensor).  The NXT would be able to
know the position of the cylinder just by knowing the position of the motor shaft, unless load changes
are too great.
The device driving the red beam position should be capable of absorbing the motor crank load without
moving.  A worm drive would do, or perhaps a linear actuator.  A rack might not withstand the crank load.
If a model requires multiple devices to be driven with Continuously Variable Pneumatic controls, a
single PF motor may provide the dither for all systems, as long as the motor speed remains high
enough.  The motor speed is about 240rpm with used batteries in this one.
Stages may be directly cascaded.  The input range is about 16mm slide of the red beam, so a
reduction of 2:1 from a pneumatic cylinder should be about right.  If a ratio of 5:2 is used, the input
to the next stage may be taken from the blue beam of this stage.

This system uses no more pneumatic parts than those that will be in set 8049.  That set plus a PF
motor set 8293 should provide most of the parts to make this system.  Enjoy!

Mark J E Bellis