A complete overview of the motion cockpit with all features is show below.
All the small additions help to make the simulation as real as possible.

Above setup was shown at FS Weekend 2008 in Lelystad Airport in the Netherlands,  Nov 1&2 2008.  http://www.fsweekend.com/ . The FSweekend page describes the demo's I gave. 
 

The basic idea shown below was already described in Motion Systems page.

The system needs three actuators, and together they will need to carry the weight of the platform.

To reduce the power in the servo motors, the actuator was build in such a way that most of the static platform lifting force is accomplished via bungee cords. Weight balance is absolutely key in achieving heave with relatively low power requirements. The drawing of one actuator is shown below.


The bungee cords need to be long to keep the pulling force equal over the actuator travel. For details see Mechanical components III

Platform basic setup:

The dimensions are such that there is sufficient space to tilt the Simpit in all directions. Heave range is about 40cm.
A later addition was the stabilizing rod in the center. Without that, the platform would make swinging motion after sudden pitch or roll motion, which was annoying and distracting. 

Construction:

Note that the 25x25mm square metal  used in some older pictures is not up to the job: The extreme forces during heave will bend the rear bar (see below). A new supporting frame was made from rack-shelve metal.

Since the 3 actuators are not in line anymore, they need to be build with more side-support. I have also increased the total height of the actuator, in order to increase the amount of travel. The V-belt is 68 inch outer diameter. The metal bars are connected to the V-belt with cable clamps.


The 3 actuators are placed in triangle. The orientation is such that there is space for the bungee cord of each actuator. In the middle, the stabilizing rod can be seen, that protrudes through the bottom-plate of the simpit.

The position pickup of each actuator is via bottom guiding wheel, that is gear coupled to the 10 turn potmeter. 

Update: The original position pickup via gear on the bottom pulley has a drawback: It has some slip; After a couple of flights, the mid position will shifted. An improved position pick-up makes use of rope coupled to spring-loaded gear drive to potmeter. See:  Improved position pick-up.

Because the platform is hanging above ground via 3 V-belts, there is some room for movement in horizontal directions. During tests, it was found that the platform will swing back and forth after sudden pitch and roll excursions, which can be annoying. I therefore added a mechanical stabilizing structure, that will dampen this rocking motion.

The stabilizing structure consists of a 30mm diameter metal pipe, that is welded onto a 5mm metal plate.
This metal pipe then protrudes through the floor of the simpit.  


Since the simpit floor moves up and down and also tilts with roll and pitch, I build a ball joint that can slide over the metal pipe. The ball joint is not completely  fixed to the simpit floor, but attached via a wooden plate that can slide in all horizontal directions. Without this freedom of horizontal movement, the simpit pitch, roll and heave movements feel too stiff and unnatural.

The ball-joint consists of a wooden sphere, that is mounted in a tight fitting enclosure. To make the enclosure match the sphere, I applied epoxy putty between wooden enclosure and sphere. 

I used petroleum jelly between the plate and the simpit floor to create friction.  In this way, there is sufficient horizontal movement, but the swinging motion is completely damped out.

The metal pipe will protrude through the simpit, but it will be in between your legs, and will just not touch the switch console. 

Mounted the simpit on the structure: There is some bending and squaking, but everything holds.
The pitch-up and pitch-down angles are much bigger than platform I & II. The ceiling will limit total heave to 2/3 of max travel. Need to be limited via software.

Also measured the weight pressure on each actuator: The rear actuators need 45kg of balance weight, (~ 8 strings of bungee cord),  the front needs 40kg (~7 strings of bungee cord, later changed to 8 strings as well)

The bungee cord mounting is shown above: 8 strings for the rear actuators, and 8 strings for front  (I found out by testing). With actuators in top position, the bungee is slightly stretched. This can be adjusted by choosing another hole in shelf-metal.


Stretching the bungee cord can be difficult, as you need to pull around 45kg. I made a stretch-tool that makes use of a lever and cable tightener to hook and unhook the bungee cord holder.

The other side of the cable is fastened to the V-belt. The cable loop passes underneath alongside the V-belt. Grooves are added to the pulley to keep the cable away form the V-belt.  

Getting the simpit on the platform is easy: move the metal support beams downward and block the actuator drive with a screwdriver or something. Then the simpit can be slid onto the support beams.

Without pilot, the front actuator already gets depressed due to the simpit weight.
With pilot, all actuators get depressed. Although on the picture they seem to go down all the way, but overall there is relatively good balance. This needs to be tuned to the pilot's weight. I found that without good balance, the servo system consumes a lot of steady state power, but after re-balancing, average power is around 60W, with heave peaks to 1500W.  

First flight tests have shown that heave is a great motion addition, although it is very dynamic and requires lots of tweaking to get it right. See Motion Software III and Video's III

Fooling your brain only works when you are sitting in a completely enclosed cockpit, so I installed the cockpit covering that I also used in the #2 platform. The big fresnel lens will result in a very wide and deep scenery view, which will help a lot in making the brain 'believe'.

Motion is a whole different experience now! You actually can reduce the pitch and roll excursions; Small motion cues will be more easily accepted by the brain when it goes together with a wide visual scenery view without other external clues.

One problem: With the cover installed, the rig will easily hit the low ceiling. Ouch!  I have now lowered the pilot seat and cockpit by about 10cm.

New enclosure construction is shown below.

View of wooden frame, which has the same width as the triple LCD structure

View of scenery display w/o fresnel lens.  Right, the fully covered structure

With the flat sides, a door is much easier to implement. The door adheres to the wooden frame with the help of magnet strips. There are two 50W spotlights at the rear, that can be dimmed. 

All things come to an end: platform IV development needs the space that platform III is occupying: After almost 3 years of good service, #3 has reached end of life..... Below pictures of the #3 dismantling.

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