Stuart platform vs rectangular pt-actuator

Looking to the experts on this site for the differences and benefits of the 2 configurations? The latter can use extruded aluminum to build the cockpit and mounting hardware is much easier. Any other advantages in regards to motion etc...

A stewart platorm has 6 dof - the sim can move in all directions

A sim that is rectangular and uses an actuator on each corner has only 3 dof

This design is unstable and would actually work better with only 3 actuators !

So, rectangular sims that have 4 actuators on the corners and 2 for sway and traction loss advertise as a 6 DOF are not truly 6 DOF?

Traction loss is not a traditional dof to be accurate

If it has sway its a 4dof

That would be :
Heave
Pitch
roll
Sway

Surge is a useful dof on a driving sim in my opinion

Also using 4 actuators, one on each corner makes ot unstable. The 4th actuator could be used fpr something else

4x drivers for 3DoF design to me is a "faulty design". Searching through all published and research papers and robotics & motion platform industries just could not find a single such 4x for 3DoF design. Similarly, for 6DoF, if one adds a 7th actuator, then will not work with 6DoF. It constrains the motion, and no longer be able to produce 6 degree freedom motion. Today, all motion platforms designed as n x drivers for nDoF config. I never have seen such n+1 drivers for n DOF (except for D-Box). I found only the most expensive sim NADS/Lexus/Toyota uses 4x actuators on the corners, but their purpose is for vibrations only, not for motion. The max vibration amplitude limits under < 0.2 inch though:

https://www.researchgate.net/public...TY_OF_IOWA_A_TOOL_FOR_DRIVING_SAFETY_RESEARCH

Sadly, most people don't realize such fact, thinking because there are 4 tires on a car therefore 4x must be right and better. No, it is not. It is still 3DoF nothing more, no matter what you try, the chassis is a rigid body in this case still does 3DoF. By archiving so, the 3x config will offer correct 3DoF motion while 4x config dose not. To me it is worst to think of putting 4 actuators on 4 corners. If I turn up the motion intensity I will get floating feel and cannot feel roll/pitch coherently while high speed cornering. If I turn down the motion all the way, then I got "tactile" effect only. I do recommend others to go with 3x or 2x.

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3dof vs 4dof


4 actuators -- https://l.facebook.com/l.php?u=https://www.youtube.com/watch?v=TrQANt0KqW4&fbclid=IwAR1lINotsdc9UmPl2lwjmjNrXgy7y9dckXaZk6f_jlINXJwHLB1lzrZ6l6I&h=AT1NX6Uiw17DowG9AjsgZExc9ut0Ptqd6a25ZZURC5hsA-5KG5o4B3GOQiom2AanUoL_17fyxBInZNogXXRDFvl4aepZkfyAnz_tproxV0Rj2MXK9X5aT9iUlDhe-RMivio7MF1Na4I9&__tn__=R]-R&c[0]=AT1FOa0H3ZaG6W8o2J-anqhNzT2P9NeqC_Qn3guM1nCCljuMmXIX1cgmZhG5mz9lucL8D3DbmOtGFwjHhanZW_8NVhUU7-45rJgTtKFIYfhm7dT0FA92Gwqw9haRrEkGe6RcEBTa9nOKR0IISQ6Sw1DFgnPT2aMANjM


3 actuators -- https://l.facebook.com/l.php?u=https://www.youtube.com/watch?v=qjHhm-c-7hQ&feature=emb_logo&fbclid=IwAR1PO0WN2m8_pA7MDw2rTamE4x6q-t7RHlf12iEMwYnFvyBloStDwUGEcKM&h=AT3m-LIhNy0gnuZQGu9UEGdWxklMCDnGQU05_WQojhUnmIpCLmoCX-iIUij9Z0XyNtt-GEFrJ3HBSe0nIe5JTPUWFvRX77UDRQhY8vwfoRYLsw0GsyhIyvpn3IgyLMoO&__tn__=R]-R&c[0]=AT1FOa0H3ZaG6W8o2J-anqhNzT2P9NeqC_Qn3guM1nCCljuMmXIX1cgmZhG5mz9lucL8D3DbmOtGFwjHhanZW_8NVhUU7-45rJgTtKFIYfhm7dT0FA92Gwqw9haRrEkGe6RcEBTa9nOKR0IISQ6Sw1DFgnPT2aMANjM

When you need to bolt together the aluminum frames in rectangular shape to prevent bending or twisting, there is only certain places you can mount the actuators to prevent it from tipping.

Between a three point structure and a 4 point structure, the 3 point one has dramatically more chances to tip over...

A 3 legged stool is more stable than a 4 legged stool !

http://theohiobeacon.com/articles/3-legged-stool/

Yes, but unfortunately you have to place the actuators vertically, not on angle as the stool example. And the stool legs are not retracting dynamically moving the center of gravity as the motion rig does...

The angle of the legs is not important

The number of them is

These videos show the problem

4 actuators



3 actuators



It is not possible to have one leg in the air with 3 actuators

Hahaha, nice of you to find video of the only system that had this issue with legs lifting up uncoordinated, the original leonardo connected SFX-100

The videos are not mine, i found them when researching different sim designs


Am I right to assume you have a sim with 4 linear actuators ?

I understand people like to build static sims with aluminium beams and slowly they evolve into motion one upgrade after another.

Lots of people do not understand that 4 actuators does not mean 4dof but 3dof

As an engineer I stand by the logic that only 3 actuators are needed - its more stable and actually cheaper to build it that way

As an mechatronics engineer, I can tell you three legs are static stable, not dynamic if your center of gravity is high and you have inertia involved.

I'm all for 6DOF Stewart platforms, but they can take lots of space and not easy for everyone to have in home environment.

The 4 actuators (3DOF) is small footprint and low profile, the rig can be mounted very low.

There is definitely a time when 4 peg leg actuators are better than three.

Stability, as its strangely being argued against here is one of them (to prevent from tipping over). I have tipped over, blind to the world in vr, in my 3 actuator rig (I like violent motion, what can I say).

Another is load assist. I built a fairly heavy gseat and most of the weight is on the back of the rig where I chose to have 1 of my 3 actuators. There is a compelling argument in my case to put 4 actuators on the rig to get more load carrying capacity on the back (one could also beef up the actuator and motor for the rear).

Another is vibration (especially in the case where you can lock a push pull actuator to a base in order to twist the chassis) although this is probably better suited to tactile units for our purposes.

The real argument you are making is that with a 3 or 4 peg leg actuator design you still only get heave pitch and roll. Of course pitch and roll have a center of rotation on the floor so its really more like heave, surge, and sway.
I wouldn't be so quick to say 4 actuator are a bad design. There are lots of people who use 4 actuators who don't suffer from one leg lifting (the instability you're describing) because they have good mathematical models driving the motion and motors that can keep up with the demanded position.

Also traction loss is definitely a degree of freedom. Its yaw, just not at the center of rotation that most people think of when they hear yaw. Its also very limited in travel but the same is true of stewart platform yaw.

In my opinion I think the stewart platform is the better motion platform because you get true surge, true sway, and pitch and roll have a center of rotation of your center of mass (if you choose to built it that way of course).
I am surprised how immersive it is in my new rig to have pitch and roll of the actual car 1:1 on the rig with the center of rotation located at your center of mass. The hills and banks of the Nürburgring are a great example. Super immersive.


That being said the stewart platform is way more money, work, space, tuning, etc. So you're looking at an intermediate (pegleg) vs advanced (stewart) journey.

In the end Stuart is less expensive.
When you enter this world it never stops.
You start with 2 dof, and want 3dof
You have 3dof and want TL
You have 4 dof and want 6
Then you want full roll (where I am right now)
And then in the end a g seat over the rig.... (where I am also)
Go with the hexapod right away, it will be less expensive

By the way, with real 6dof, we can do good motion cueing and use at least the classical motion cue algorithm.
With a good software and fast actuators, audio transducers can be replaced by them.
I know, a transducer in the pedals could be good, but road surface and engine vibration is already on the rig.

Or if you're a racer, full yaw.

Having built most of the designs out there, bang for buck a 3DOF (heave, Roll, Pitch) motion rig + G systems for generating continuous surge/heave and sway is hard to beat.