In order to make the platform move, each axis needs some type of actuator. These actuators can be of the rotational kind, where the actuator directly drives the pitch or roll rotation point, or the actuator is a linear travel type, that pushes the edges of the platform. The linear actuators’ travel needs to be long enough to make the desired axis movement, and its force needs to be sufficient to push the platform weight with some speed. Quick response, accuracy, reliability and safety need to be considered. The type of drive control is also important for interfacing the whole thing to the PC.
Hydraulic valves are mostly used where high forces are needed. They are normally not the choice for home-build platforms, since they require complex oil pump and driving systems.
Pneumatic valves are easier to build, and can be successfully used for driving low loads. It is however more difficult to control their thrust and position accurately.
Electrical linear actuators are becoming popular for driving motion
platforms. They can be used for low to medium loads, can be accurately
controlled, and interfacing is relatively easy, by using an analog electric
driving system with positional feedback via a potmeter (servo control
system).
There are several options
for home-build electric linear actuators:
Above a sketch of an actuator that consists of an electric motor that drives
theaded steel shaft, over which a counter threaded cylinder moves in and out. These type
or actuators are often used in CNC machines. The threads need to be specially
made for this type of application, and normally cannot drive high forces, as
they easily wear out. Industrial actuators of this kind often use
ball-bearings in the thread (rollerscrew). The picture shows two types of
actuators that (I think) are used in hospital beds. They are strong, but
move too slow for our purpose.
An electro motor driving a gear box with lever is often used in small motion
platforms. The gearbox and lever need to be very sturdy, as the forces on
the lever and end gears are very high. On
http://www.jimspage.co.nz/intro.html Jim has made a great
description on how his platform works. With a 12V electric motor driving a
car steering box, sufficient force can be generated for moving a small
cockpit on tilting platform. It also has the advantage that there no problem
with end-stops: The platform's travel is automatically limited.
The pulley driven cable method is shown above: It's relatively simple to
build yourself, and can exert quite some force. Some gearing may be
needed, but winding a cable on the motor shaft can already be sufficient.
Cable tensioning needs to be assured for this system to work well. I have
used this system for Force feedback
on the flight yoke , the DIY motion platform
I, DIY motion platform II
and DIY Motion Platform III.
Ian Hopper has designed a neat linear actuator for home-build motion
platforms. For more info see
http://buggies.builtforfun.co.uk/Sim/linear-act.html