4 Degrees of Freedom

[AndyT]

Hello all,

My name is Andy Tulenko and I have been working on a way to make a very inexpensive motion platform for a while now. I came across Jims website where he details using steering gear boxes from cars and inspiration struck. I now would like to share my design for 4DOF. These are;

A. Pitch
B. Roll
C. UNLIMITED Yaw
D Heave

I tried to link to a picture but it wont do it, I think it has something to do with my server so, hopefully this link will work.

http://geocities.com/andytulenko/images/4dof.bmp

Heres the list from the numbers in picture 1.

1. The cockpit platform (GREEN)
2. The pitch and roll drive systems as designed by Jim (RED)
3. The yaw drive system (RED)
4&5. A very large bearing set with a big hole in the middle (Lt BLUE & Dk BLUE)
6. Load bearing springs of the type you see on big trucks (PURPLE)
7. A giant spring loaded shock absorber type device (YELLOW)
8. The base that sits on the ground (BLACK)
Not numbered. Heave drive system (RED next to #7)

Pitch and roll Jim has already covered.
Yaw is accomplished by using the giant bearing set with the hole. IMPORTANT: The dark blue half of the bearing set is connected to the fat part of the yellow shock. The light blue section spins freely. This gives you unlimited yaw in both directions. The drive here is a direct drive from the motor to the bottom of the giant bearing set using a gear system. One cog to another sideways kind of thing.

For heave, the giant yellow shock is powered by the drive system next to the number 7 in the diagram. I forgot to draw the second giant bearing set which the heave drive is mounted on. It has to spin freely to allow yaw. The mount is between the base #8 and the unit pictured. The purple springs are there to reduce the weight load on the heave motor so that it can bounce the load with little trouble. This reduces the strength of the motor needed.

As I type this, I see a problem with mounting the heave drive below and so I think it should be mounted opposite the roll motor ON TOP of the bearing set with an access slot to the inside of the yellow shock. That should solve the spinning problem.

I have not done any measurements on this since this is a prototype diagram, so I can not do any calculations for you, but you can do those easily enough on your own.

Another change from Jim's design is the drive systems all use a 'Train Wheel System'. Have you ever looked at the main drive wheel of a locomotive? The lever slides in and out as the wheel turns. With this design you should be able to use a stepper motor and have not only faster response, but just as great a range of motion.

And there you have it folks. I hope that you like it and I hope you will comment on it. I plan on building it for my sim. It should be pretty inexpensive, but we will see.....


RATZ! I see another problem. How to steady the lt blue half of the bearing so that the yaw drive can spin the thing.... Well.... I thought I had it all worked out... I hope nobody minds me thinking out loud in the forum here....

[AndyT]

Ok, Picture 1 has problems. Picture 2 solves them.
Let me describe the new parts.

The ORANGE bearing set and the DK RED motor make one assembly that works just like the collective on a helicopter rotor hub. When its activated, it lifts the fat part of the yellow shock up and still allows it to spin without restriction.

The DARK GREEN plate is attached to the skinny part of the yellow shock, or it could be mounted to the base, it does not matter as long as the yaw motor can get to the gear teeth on its edge.

Sorry for creating this here in the forum but I actually do some of my best designs while in the middle of explaining them to someone. and, since my wife has a negative zero intrest level in this... you folks are elected.

It is still actually a pretty simple design and if you use strong components I see no reason why you could not build a full 747 cockpit on this thing and have it bounce you and your flight crew around quite easily.

Now while picture 2 fixes the problems, I reccomend picture 3 because it simplfies the wiring issue. The biggest problem with having unlimited yaw is the wires are going to get all wound up around the center yellow shock mount. There is a device that fixes this however. I beleive its called a 'slip ring' and it works just like the universal on your car. This device is made specifically for this kind of application. They sell them with up to 8 wires I think, but if you use picture 3 then you only need 4 conductors. The yaw motor is mounted on a non-moving part and so you dont have to worry about its wires.

Here is a link to a company that makes the better version of these connectors. I really reccomend this particular brand because they do not have the problems most slip rings have. Most have corrosion, electrical noise, variable resistance and other problems this brand does not. They have a new design using liquid metal that just makes it all so much better. they are kind of costly but, you only need one so go for it!

http://www.mercotac.com/html/430.html

I dont know if any of you have seen this page but check this out for motor control systems;
http://www.classicflight.co.uk/DriveChain.htm

[AndyT]

Hmmm... 80 reads and no comments?

[ekezz]

Hi Andy,
Your post is very interesting stuff and the reason I had not commented yet is that you are way ahead of me. I think you and Jim are telling us about uncharted territories, and this reads very well. Unfortunately I have no experiences with motion platforms and don't have the welding skills to get everything sorted out. And my overhead needs to be finished first. So please keep on posting as it is great reading material. And I think some pictures will help putting the text into context as my mind works on a visual level.
ps where do you get your yellow shocks?

[Roland]

Hi Andy,

Indeed some food for thought. You're quite right, the easiest way to achieve multiple motion axis on a platform is to make a staggered construction, where some of the actuators are moving together with some other motions. Your idea brings to mind the system that is used on helicopters, not sure what it was called, but it could provide motion info via two rings, to lower ring being controlled by two actuators.

In your comment : " If you use strong components I see no reason why you could not build a full 747 cockpit on this thing and have it bounce you and your flight crew around quite easily. "
The components definitely have to be VERY strong. Note that the forces for "bouncing" a cockpit with occupants can be very large. The actuators (like Jim's) are probably not up to that job, although they are just fine for slower moving platforms. Like Jim mentioned on his site, the key to motion platforms is finding suitable actuators. What is "suitable" is very much depending on your platform weight, size and and motion requirements.

I'm currently trying to figure out how high actuator drive forces can get with certain platform size, weight and acceleration requirements. As soon as I got some results and measurement data I'll describe them on my site.

Roland
www.simprojects.nl

[Jim NZ]

Hi Andy, The only thing I can see wrong with 3 is where the heave motor is... you might get "pinching" with the drive being so far away from the splin. Where it is in 1 is actually better as its pulling/pushing very near the spline, but all this is being done by only one motor, irrespective of the help it gets from the spring.

As Roland says though, to include full heave, brings in tremendous forces especially with some of them having to be fast.
The method used be www.inmotionsimulation.com (and others) is probably the easiest to replicate but still, heavy drive units are needed. All you would need to do is plonk all that on a turntable of sorts.

The steering box units I used would certainly not be strong enough.

A big thankyou though Andy .... The slipring unit ... marvelous .. I was trying to track something like that down when you made this posting and whallaa .. end of problem. Yes they are a good unit, and yes they are bloody expensive but probably worth it compared to other alternatives.

Looking forward to your developments Andy, but be warned, once you start building,,, its like a desease ... your hooked !!

Cheers in the meantime ... Jim NZ
www.jimspage.co.nz