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DIY Motion Platform 2 - Page 1

DIY 3DOF Motion Platform 2

 3-DOF Platform 2 - Cockpit fittings and hood to be added

MOVIE CLIPS of Platform 2 in motion are on Page 2 along with some photos. Page 3 has dimensioned drawings.

JAN '08 After the experience of building the 1st motion cockpit I thought it would be interesting to see whether a more conventional 3-point support type platform might be designed which would be a bit less complex mechanically and so a bit easier to build. Keeping the cost down was still an objective off course as was maintaining a reasonable range of motion, including some decent heave capability - many lower-cost 3-DOF platforms I've seen seem to have very limited vertical stroke.

My intention was to use the DIY linear actuators I had worked on already to drive the movements (more detail about these here). Given their thrust capacity (about 50 kgf ea. maximum) this would require that the bulk of the weight of the platform and payload be carried by a separate load support mechanism. The role of the linear actuators is then primarily to accelerate and decelerate the moving mass. The load support mechanism is also used to constrain unwanted horizontal linear movements and yaw rotations. After considering the options I went for two actuators at the front and one at the rear - the center of mass of the payload is likely to be closer to the front given the likely weight of screens, instruments, controls etc. I've recessed the pilot's legs into the platform in order to reduce the overall height, to reduce the extent (and weight) of any superstructure required and to move the payload center of mass down closer to the physical roll and pitch pivot points.


These pivot points, with the load support mechanism, are positioned directly below the pilot and are probably a touch further to the rear than I would like - however the pilot's legs are where they are and clearance needs to be left for the pitch movement. All designs require some compromises. Any resulting pitch moment should not unduly challenge the actuator capacities however - the main thing is to balance the vertical force.



The load support mechanism is a spring loaded half-scissors mechanism. It has two legs in an inverted "V" arrangement pinned at the top and running in horizontal linear guides at their base. The base of each leg is drawn into the mid-line of the platform by bungee or shock cords which provide the spring effect. Downward load on the legs from the platform through the gimbal forces the base of the legs to separate and stretch the bungee cord which resists the force.


The trigonometry of the mechanism is interesting in that it inverts the normal load/extension characteristics of a spring, and with certain choices of spring stiffness produces a sufficiently flat load/displacement response to be of use in this application. The graph below shows the calculated spring load and support load variations over the 30 range of movement of the legs that match the 400 mm heave stroke length of the platform. These profiles are based on 10mm bungee - the total number of wraps determines the mean load carried by the mechanism.


The arrangement would appear to avoid the need for added counter balancing weights in the platform which should help to maximise the motion performance with the DIY actuators. And bungee cord is not expensive - so the problem of sourcing properly specified springs is eased for the DIY'er.


One of the main things that becomes apparent when studying the overall design is the more limited range of motion compared to the 1st motion cockpit with its independent DOF's. Platform 2 will do about +/- 26 pitch, +/- 23 roll and +/- 200 mm heave - but not simultaneously. The combined movement ranges are primarily limited by the actuator stroke length of about 400mm. The 1st motion cockpit does much better on the pitch and roll angles and all DOF's can move to their full limits simultaneously. So you pay your money and make your choice with these designs - more performance requires more complexity.


Most of the mechanical complexity on this design is contained within the DIY actuators - the mechanism on the platform contains mainly simple pivots and fairly straight forward linear guides in the load support mechanism. The legs of the support mechanism are pinned at both ends and consequently are in compression. The gimbal is a simple cross pinned setup and the remaining structures should be fairly straightforward to construct.



Click on the thumbnails below for some higher resolution images of the CAD model renderings.





Have a look at the movie clips on Platform 2 - Page 2 to see the platform running shortly after its first assembly - still work to do.

FEB '08 Update - build is in progress - 3 actuators now running - see movie clips - clip 1  clip 2   clip 3.

LATE FEB '08 Update - I've now managed to get the new platform running and am getting a feel for how it operates. So far so good and it generally seems to do what was expected. Pitch, Roll and Heave movement are all there. Platform 2 - page 2 shows some pictures and a few short movie clips.


April '08 Update - Basic dimensioned drawings of the parts are on Page 3. Provided free of charge for information purposes.




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