Discussion: Issues with 6DOF

Those graphics though! D= And 2D!

I would sure love to have a huge sway/surge table like that though.. plus at least 50 feet of heave and 360 pitch, roll, and yaw. =D Sadly I'll have to settle for the "poor man's" solution of a Stewart platform plus G-seat.... poor me! =P

Look on the FAQ's page under g-seat to get some links. Basically there's two types utilizing either paddles or air bags.

But this site is not the same as "Instructables" so you're not likely to find step by step instructions.

Maybe I overstated my case. =P

I don't need my hand held 100%, but I'd like to be able to put together a complete list of electrical, electronic, and mechanical parts at the outset, as well as being directed to the software needed and photos of the assemblies. I'll check the FAQ's page, I didn't know there was that kind of stuff there, thanks!

I think we understand @apointner . We are just being difficult. =P

Well folks, I'm not ready to show my modifications to the world yet but thanks to hexpod helping me out and promptly responding to my emails I've got it moving pretty nice so I thought I'd give you guys an early look. @Dirty @pmvcda @hexpod

This requires an explanation for sure, I know it's far from an ideal setup and there are a few other odd things too! Fortunately Hexpod's software allowed me to get correct kinematics even with my bizarre servo ranges! I very highly recommend his interface for 6DOF sims. I'll be trying out FlyPT Mover very soon too, but haven't yet.

1) Electrically actuated, hydraulically damped!:

Backlash has been a HUGE problem for me since I got my rig initially built. I would not recommend these gearboxes to anyone who wants to use the rig as a flight sim. For racing they are OK, but for flying I just could not achieve smoothness. In particular large downward heave motions caused a bone jarring shudder that was terrible. I basically had to use almost no heave in my profiles to avoid the violent shake! Pitch was also particularly bad. The VW Type 1 shock absorbers I used are also gas shocks, so they act as a spring and keep some tension against the backlash too.

Now the down side is that I had to give up quite a bit of displacement and angle in order to do this. These shocks have 150mm travel which is pretty good. Since the shocks are on a 160mm radius and the motion sim actuator rods (legs) are on 190mm radius this theoretically gives me about 178 mm of actuator travel. That's pretty decent, but sadly that's not all usable. First off, I have to use less so that small overshoots and such don't hit the mechanical limits. Second, because my angles are so strange I get a bit less actual sim travel out of the actuator travel. As of right now I only wound up with 130mm of travel, but I think I'll be able to get a couple more degrees out of it.

I could put the shocks on a shorter radius, but that would make them a bit less effective, and wouldn't help much because I can't use much additional travel anyway without the actuator rods (legs) interfering with the upper shock mounts.

The saving grace here is that it's pretty much universally agreed that you use much less angle with VR than with screens. More heave would sure be nice though! Roll is the main thing that suffered, but it feels better than it looks and I intend to add a G-Seat anyway. =) I think it was Seattime that recently posted that with a G-seat you will wind up using smaller movements from the 6DOF so it should all work out well ultimately.

2) Platform pitch up when sim is "neutral"

Well, one of the complaints I've had (previously using Ian's BFF) was that if I tuned the sim for a satisfying feeling of acceleration (pitch up) in race cars and on takeoff runs, hitting the brakes felt like I was standing on my nose.

My (terrible I know) solution which I've found surprisingly satisfactory was to shorten the back actuator rods (legs) a bit so that the sim is normally tilted back. I don't even notice with the VR headset on, and now my accelerations feel much more satisfying, and my decelerations no longer feel super exaggerated. Mind you, with SimTools/heXpod the difference in available travel can be easily adjusted out, but still if you do that you are failing to take advantage of the opportunity for bigger forward acceleration cues. When I have to move my sim, I will almost certainly rebuild it so it's oriented the standard direction to get more pitch up travel out of it.

3) The Start position is higher than it should be.

This is very soon to be solved. I had to request that Hexpod increase the range of a couple sliders in his software to accommodate the compromises I made to get the hydraulic dampers on the sim. As soon as I get my numbers all punched into the new version of heXpod and readjust my servo sensors to match I'll be able to achieve the ideal "home" position within my (oddball) range, and I'll also get just a touch more total travel added at the bottom.

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Try to ignore the mess, it's been heavy construction on this thing for months and there's a ways to go yet! The sim can move a good bit quicker than this, but any faster and the motors start having bigger and bigger sync issues. Here you will see small errors, I'll run it as fast as I can without getting sync errors after smoothing filters. That might be a bit faster than this video, but it might be slower too, I'm not sure how it's going to work out.



I would say to let me know what you think, but you might make me cry!

What a shame your rig looked awesome didn't know you were having all these issues and I have no idea what to suggest but I can feel your pain

Things are looking pretty promising. Between the dampers, heXpod software, and upcoming SimTools 2.4 with much improved filters I think I'm going to be finally getting the smooth ride I want!

Just to be clear, my sim has always been good for racing, it's the harsh bumps in the air that were a problem and for me flight is the higher priority.

Quick update guys: Tonight I got my first little flight in. There's still lots of cosmetic work to finish and I haven't even begun tuning the motion yet, and a million other things but I got my first ride and it felt like a HUGE improvement. It's a little too soon to say for sure because the profile is nowhere remotely close to where it should be, but first impression is that this mod was a major success.

I saw so many users facing this issue. (Lever arms rumbling like hell while traveling down EVEN at low speed)

I can’t believe it’s related only to the backlash on worm gearboxes !

It has to be a conjunction of other parameters which gives you such a shitty result.

It’s looks more like a friction issue.

Is it possible to move them freely up / down by hand?

The last one was @Gabor Pittner on the AMC/sabertooth setup.

To bad we can’t find on the forum a serious mechanician which could finally find the real source of the problem.

@Thanos , maybe a second PID loop inside the board ?

Even if it would add a small delay, maybe that’s the solution.

Servos have their own internal Pid loop and works quite well with the Pid of the AMC.

Why not have a feature which allow to run it twice on the controller ?

Maybe i am wrong, it’s only my intuition, but unless someone provide a reasonable proof why the backlash on some specific types of worms is an issue impossible to overcome, I will continue to believe that it can be fixed on the software level.

Well, I might take a look, but I think it might have a lot to do with the geometry of the rig also.
In your rig you have the load evenly distributed. The low center of the actuators make an hexagon.
In their rigs, that distribution is wrong in my opinion. Most follow a design which I find is wrong.
Also gearboxes don't help...

Good point @pmvcda

It make me think... @Trip Rodriguez , did you tried to run your platform with inverted arms (pointing outside ?)

The leg length should be increased.

No idea if it could help, just a simple question if you tried it.

Worm drives can not be moved by turning the output shaft, that is one of the main properties of worm drive. I can very easily move the gearbox by turning the input shaft when it is exposed.

I have not tried it, I would need to invest in much longer legs to do the experiment, which I can not afford to do.

Hexpod, if we want to look for causes of this other than gearbox backlash the only other thing I can think of on the hardware side is the sensor setup. Maybe we should compare notes and see if everyone with the problem is using the same type of sensor. I have these: https://www.digikey.com/product-detail/en/6127V1A180L.5FS/987-1392-ND/2620661 I have always just accepted what others have said, that it's the backlash and I still think that is probably correct but I think it's worth looking at every possibility.

As far as my rig, I am very satisfied with the way it is moving now. The only thing is I need is for the actuators to continue moving when one reaches the end of travel. With this feature, I tune until the out of range cues are not excessive and it feels great.

You can only tune 180 degrees of your arm movement with those Hall sensors wouldn't you of been better with 240 degree ones ?

I just bought what Thanos recommended to me at the time. I always figured the useful range was only 180 degrees anyway, from arm parallel to the leg at the top, to arm parallel to the leg at the bottom. Now that you said this I can see where more might be useful because the leg is not always at the same angle.

At this point though, I only use sixty degrees of arm rotation anyway. Arms are now 160mm though so the travel isn't bad as long as cues are allowed to clip at times. My actuator travel currently is 160mm, minus a bit so overshoots don't hit the limits.

We use less angle and displacement with VR anyway, and I'm told the same is true when you add a G-seat which I still plan on.

Back to the backlash/shaking topic: I've seen others with exactly the same rig as mine who didn't complain at all, and most of my friends say I'm being far too picky.

When it comes down to it though, the main person we need to satisfy with our motion rigs is ourselves (except for commercial use of course).

Ahh ok I misunderstood I was just saying that as you said you wanted it to go passed its limits but if your only using 120 degrees the 180's should be good, I used them on my worm gears they were great non issues at all but if was only 12v setup

Self locking? Back driving?

http://dpk3n3gg92jwt.cloudfront.net/winsmith3d/Self-Locking_and_Back-Driving.pdf

hmmmm.

If somebody could explain...

What more do you need?

Its both. Natural friction in worm gearing causes the gears to stick until enough power is applied to break them loose. This can then cause a surge. Backlash allows space for this surge to surge. At this point an oscillation can start and your bucking bronco ride begins.

I do want to replace them with digital units eventually. I'm actually only using 60° total travel. +/- 30° but as I said, that still gives me ~160mm travel which for VR is adequate IMO as long as clipping is allowed in extreme conditions.


Yaw: +/- 13.38°----------------------(26.76° total range)
Pitch: +11.17° / -12.63°------------(23.8° total range)
Roll: +/- 12.87°-----------------------(25.74° total range)
Sway: +/- 92mm---------------------(184 mm total range)
Surge: +109mm / -116.2mm------(225.2 mm total range)
Heave: +92mm / -81.3mm--------(173.3 mm total range)

Typically worm drive gearboxes can not be turned by applying torque to the output shaft. This is a standard property of worm drives that in many cases is the determining factor in selecting worm drive vs an alternative. It is referred to "self locking" because without the need for any separate mechanism you can turn off the drive and know that the load will just stop, not come down from gravity.

A back-driving worm drive is specifically designated as an exception to the rule.

yes cool, I dont have enough imagination and still doesn't have a precise information from where the micro jamming could come.
why the movement down is not smooth.

Backlash, friction ok, but where ? faulty bearing ? lack of lubrification ? design misconception ? wrong application for the type ?

https://www.quora.com/Is-a-worm-dri...ward-moving-only-or-can-it-go-reverse-as-well

I do believe that my hydraulic dampers/gas springs solved that problem 100%. Sudden reversal or acceleration of an actuator still generates a single bump as the loading of the arm switches direction but I think the stutter is completely gone based on the initial test.