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Full Motion Flight Simulation Platform

The extension measurement devices ("extension meters") use a commercially available angle encoder to convert a length value into a number of impulses corresponding to the current extension of the pneumatic pistons.

The idea is to have a little box that is mounted to the pneumatic cylinder, and which has a thread extending from the upper side. This thread is attached to the upper end of the moving rod and gets pulled out as the rod extends. The problem here is to have a propper retracting force which pulls the thread back into the box when the rod moves back into the cylinder.

Here's a tool that apparently must contain exactly what's necessary to fulfill this task - and it's not expensive and available in every hardware store:

The steel spring is held in place between two teflon disks.

This spring is inserted into a spring holder made of plexi glass, as shown in the left picture.

Each unit consists of an L-shaped piece of aluminum, as shown in the right picture. You can see the angle encoder and a screw that will later redirect the thread.

A special aluminum spool is attached to the axis of the angle encoder. This spool will hold the thread and allow the spring to be attached to the angle encoder's axis.

One of the two teflon disks is pushed onto the axis and the plexi glass spring holder is mounted on the unit. The spring is then hooked into the slit of the spool's axis.

After pushing the second teflon disk onto the axis and securing it with a screw the spool is given about 10 rotations to tighten the spring. Then the thread is inserted through a hole in the spool (the end of the thread is secured with a knot, so that it won't slide through the hole).

The spring's force will then wind the thread onto the spool, and the thread can then be lead through the screw and the hole in the spring holder.

The units are inserted into a piece of rectangular aluminum tube and held in place by two screws. Outside the tube you can see the mounting bracket which will be used to connect the extension meters to the pneumatic cylinders.

Finally the cable and plug has to be attached, and top and bottom of the extension meters are closed with commercially available plastic end caps.

Here you can see two completely assembled extension meters. A little o-ring has been knotted into the thread to prevent it from being pulled into the unit too far. This o-ring will also be used to hook the thread onto the rod.

The extension meters are mounted to the pneumatic cylinders, and the thread is hooked into the upper end of the rod.

Status

All six extension meters have been built and are functioning.
The angle encoder together with the HCT-2016 Quadrature Decoder chip provides 512 impulses per revolution, which results in a length resolution of under 0.15mm. This is certainly good enough for this type of application.