Question Research/Planning for a 6DOF Build.

Hi All!

Long time lurker who has been planning a build for far too long and am hoping I could get some input/advice so I can finally pull the trigger and put my dreams into a reality.

After seeing the price of the 6DOF MotionSystems.eu platform (20k+ EUR), I realised DIY was going to be my only option. I was also considering the DOFReality 6DOF unit but appreciate I'd be mainly paying for the convenience and a relatively quick and easy setup(?). Not to bash what they're doing at all, but I feel like the specs potentially leave a bit to be desired and I can get more out of a cusom build(?) as I don't necessarily mind the extra work and tweaking that will be necessary in setting up my own solution.

I'm especially inspired by Peacemaker Motions's setup but also Departed Reality's and Christopher Knauf's approach (all on Youtube but I've seen their build posts here also).

For a bit of context, I'm a mixed simmer (Racing and Flight mainly, but don't want to limit myself, so would like the ability to try out whatever else may randomly tickle the tastebuds such as trains, boats or whatever else I can experience with motion) and I exclusively play in VR. This pretty much solidifies the choice for the rig being 6DOF.

My first purchase was a Next Level Racing simrig (GTTrack), although this was not really with motion simulation in mind. I was aware they sell a motion platform bolt on, however was always of the thinking that I would just build and bolt it onto my own DIY motion build when the time came so I could continue to enjoy my racing and flying with my existing setup. I also appreciate that may complicate things due to the dimensions and weight of the existing rig and am not against the idea of ditching/modifying the existing rig if I have to. I understand it may not be as simple as bolting on also, so I won't force it to work if I simply need to start from the ground up.

I have a few (or many) questions which I've been struggling to find answers to and would grately appreciate your expertise. I'm so glad a community like this exists!

Motors motion type:

Threaded rods vs arms

I've noticed there are mainly 2 types of movement used for the actuators, arms/levers and threaded rods. What are the pros/cons of each approach?

Arms:

• I've noticed the arms can have a smaller footprint but does this also limit the range of motion?

• How does the length of the arm affect the overall range of motion?

• Is a longer arm "weaker" and slower?

• Does this in any way affect the motor choice, i.e. not needing a motor as 'powerful' as a threaded rod linear actuator approach?

Threaded Rod:

• Is the rod length essentially the length of motion/travel?

• How does this affect the dimensions of the Stewart/hexapod Platform? i.e. does the footprint of the platform affect the amount/range of motion possible or is there like a 'golden rule'?

• What is the significance of the thread pitch i.e. does translate to slower/faster motion or increased/decreased 'power' requirement?

• Is the pitch the same as the lead size?

P.S. I know there are also belt systems/approaches but would prefer a direct force for increased precision/reliability - belts have backlash, lag, wear over time, etc... I just feel the other 2 approaches would be better for longevity and accuracy.

Motors:

• I've seen a large variation in motors used, however from my research it seems the 80/90ST Servo Motors are the defacto - however I appreciate this may also be very dated information?

• Are there newer/modern alternatives that are perhaps smaller/more power effecient/more powerful, etc? I see SFX-100, ODrive and other such words thrown around but must admit, I don't quite understand what their differences are or whether they're just variations of the 80/90ST motors?

• I'm assuming the motor determines the compatible drivers/controllers? I.E. If I'm to use a Thanos Controller, it is only compatible with certain motors?

Torque:

• How does the motor torque affect the motion? Is this about the speed/strength of the motion or does it more appropriately translate to weight/load limit?

• Does one motion type require more torque over the other (arm vs threaded rod)?

• How do I know the requirements in order to be able to handle a certain load?

• Does this affect the speed of motion or is it more about 'strength' (weight/load limit)?

Motion:

• How much motion is too much motion? I understand that the software cleverly translates motion not into direct movement of the platform, but instead to the force the motion in sim would exert (if that makes sense). With that said, if for example I'm 20 degrees pitch up in flight sim, would I need the same or more motion to also compensate for the additional G factor?

• Does the size of the rig determine the available range of motion/required motor strength?

Part Strength:

• I do have a 3d printer but question the reliability/strength of some parts as quite a lot of load of force will be exerted on certain parts. Is this a concern?

Apologies for the absolute wall of text but I really want to get this right and fully understand what I'm doing. That's all the questions for now as I don't want to get ahead of myself and subsequent questions (if any) will depend on the answers here.

I did ask ChatGPT for some input and it gave me the following response which may save some time/effort for responses, please let me know if there are any additional considerations to the answer or anything simply wrong.

Thank you for your time and patience, hope this is not 'too much'. (links removed to mentioned products/parties as it was flagged as potential spam), please let me know if there is anything I should amend in the post/any rules I'm breaking, really sorry if that's the case as is not my intention.

Wow, these are really a lot of questions. I can't answer them all at once but step by step.

I think what you call "threaded rods" are usually referred to as "ball screws". They are widely used for high precision positioning applications like CNC machines. High precision and low backlash is not really important for sim rigs. So most DIY builders use the cheapest possible ball screws available. Basically all work but they have a disadvantage: the rolling balls make a rumbling sound especially when rotating fast. So a good advice is NOT to use the most commonly used pitch of 5mm but instead choose 10 or even 20mm pitch and add a reduction gear (timing belt) instead of coupling the motor directly to the screw. This way the screw spins slower making less noise. It also results in much less inertial load to the motor and less vibration due to runout/imbalance.

Lever arm actuators or rotational actuators (compared to linear actuators with ball screws) are generally easier to build because the don't require specially machined or expensive parts like linear bearings. They are also cheaper if (and only if) the (always required) reduction gearbox is already part of the motor. Many use wheelchair or windscreen wiper motors. If you have to buy seperate planetary gearboxes for large servo motors the price of the gearbox can be higher than the motor itself so the price benefit is gone.

In theory you could make the arm as long as you want. But longer arms need more torque so you have to select the gear reduction factor accordingly.

So before commiting to an actuator type, arm length or thread pitch my advice is to calculate the required motor power, first. Ball screws and rotational actuators using toothwheels both have good efficiency. So the required power depends only on the weight of the rig and the desired max speed. Worm gears have a lower efficiency and require more motor power for the same output.

Lifting 75kg at a climb rate of 1m/s requires one horsepower. So if rig and pilot weight 150kg and you want to move 0.5m/s it's the same 1HP. The movements of the rig are generally not streight up at constant speed but very complex. So as a rule of thump you can assume that only half of your motors (3 of 6) have to handle the full weight of the rig. So 300W per motor should theoretically be enough. Many use 750W motors which is plenty.

Next you need to decide how much stroke you need. More is always better but if you don't have a barn or hangar but have to put your rig in the living room or garage the ceiling height limits your max heave and pitch.

Thank you very much for your reply!

I'm mainly heading in the direction of linear actuators with ball screws. The main reason for this is a nice balance of speed, precision and motor 'friendliness'.

I'm looking at ballscrews with an 8mm lead for a nice balance of my mixed playstyle - 10mm/20mm as you have mentioned is fine and fast for racing, but I don't want to lose some of the finer detail I'd benefit from in flight sim for example or add complications of reducers/gearboxes, etc. You have stated the high precision and low backlash not being important for simrigs, but it's the accuracy that I'm actually desiring. Is it really that negligible? - I want to feel as much detailed feedback/motion as reasonably possible.

5mm is very detailed but not as 'fast' for when I'll need it in intense applications such as racing, this is how I've gotten to 8mm as a nice balance. With that said, I have not considered any reductions and this seems unnecessarily over complicated if the motor and ballscrews are appropriate in the first instance? Or is it more for the noise as you have mentioned? My preference is to couple directly to the motor although I don't have a specific reason for this currently other than purely from a simplicity and 'specs' standpoint. I don't want any additional complication/point of failure or to sacrafice resolution/detail and coupling directly seems the most effective and accurate option for that?

Said motors I'm leaning towards are the 80ST's with ~2.3-2.9nm of Torque (750w motor) which I've calculated should be fine for ~1k KG force/load at a 10mm lead, so 8mm will be slightly more(?) (overkill, but possible and playing it safe to have some overhead?). To clarify, my load will likely be about a quarter or half of that max.

But I also don't know about the others such as sfx-100/odrive setups and their pros/cons/considerations.

Am I on the right track so far or am I missing some important considerations?

Precision and "detail" as you say are no issue at all. If you have a 3D printer with stepper motors you see a big difference of microsteps vs. full step and the pitch also matters. But all modern servo drives have at least 1000 steps/revolution, many have 8000 or 10000. 8mm vs. 10mm pitch would mean 1µm vs 1.25µm for 8000 steps/rev. That's ridiculous, you won't feel any difference.

What's more important is the feeling, that means you want low delay and smooth motion. Backlash can be felt if it's more than 0.1mm as it creates small bumps on direction reversal. But due to the bias load of the weight you hardly ever run into it. Even with aggressive car racing you should never encounter negative gs except when crashing and then it doesn't matter.

Arm actuators are non-linear, of course. At both ends of the stroke the "effective pitch" (linear motion per rotation angle) decreases. This makes calculation and tuning a bit more complicated. But the software does everything for you, so no worries.

The speed you need depends on the distance of your pivot points where the actuators are attached to the rig. For small 2DOF or 3DOF rigs where the actuators are directly under the seat you don't need much speed so 5mm pitch is OK. But for a big rig where the joints are far out like this

(thanks @Klaus Schmidinger and @Dirty )
you want something like at least 300mm/s. With 5mm pitch this would require 3600RPM which is insane for a long spindle. For 10mm pitch it's 1800RPM which is reasonable but maybe still loud. 16 or 20mm pitch would be even better but requires a reduction gear.

2.5Nm with no reduction will give you around 1.3kN force at 10mm pitch for each actuator. At an angle of 60° (each pair of actuators forming an equilateral triangle) this results in 6.7kN total lift force. More than you'll ever need. The servo motors can also be overloaded for short periods of time by a factor of 2 or 3. So peak forces for acceleration and decelleration is also no problem.

So if you have the money I'd go for AC servos. I don't know much about the SFX or Odrives I have to admit. But those "universal" motor drivers usually need a lot more tuning and extra tricks for protection. So yes, you can save some money but a ready-to-go combination of drivers and motors is much more convenient.

BTW, the low friction of ball screws has a disadvantage: you need to account for the case of power failure. Worm gears are self locking and rotary actuators automatically provide a "soft stop" at the bottom. With ball screws and high pitch the rig almost free falls down in the case of a failure. So you'd need one of

1. motors with electromecanical brakes
2. hydraulic end stop dampers
3. brake resistors for the drivers
4. gas springs as counter weight balance

Really appreciate the feedback and patience!

To clarify I'm understanding correctly, in reality, the difference between 5, 8, 10, 16 and even 20mm lead rods is practically microscopic in relation to motion simulation?
So instead these really matter more for precision applications such as robotics and CNC for example?

I cannot find the specific steps for the 80ST-M02430 motors (and I'm guessing it may vary per manufacturer), but assuming worst case scenario in your example of 1,000 micro steps, it would still essentially be less than a 1mm difference between the rods? Meaning for the application of motion simulation, the human body is highly unlikely to tell the difference in 'detail' between them, meaning realistically, no decernable accuracy is lost by moving to a larger lead size?

In that case, if the lead doesn't practically matter, am I right to assume the lead choice would be more down to the desired lift force and/or desired speed of motion and spacing of each actuator?
Ofcourse this is all with the assumption that the motor is adequate for the choice of lead screw and the torque requirements that would accompany.

As for the power failure issue, I'm planning on using a UPS which I could probably set to send the park command in the event of a power failure. I did consider motors with brakes, but I don't know enough about the practicality of this application as I believe the controller would also need to be compatible with the brake function or some additional functionality would need to be implemented to cater for the brake?

Well, yes and no. First, a screw with 10mm pitch and direct drive would result in the same speed range and resolution as a screw with 20mm and 1:2 reduction gear. But the 10mm screw has to rotate twice as fast and has four times the inertia (that scales with speed squared). Inertia matters for long and heavy screws and it affects servo tuning. But for 16mm diameter and <1m it doesn't really matter. Lower speed means less wear for the dust wiperrs and less noise.

You don't feel position. In fact you can't tell any difference at all if you wear VR goggles. You feel acceleration. Human senses are quite inaccurate. I'd say +/-10% acceleration makes no difference for large strokes. But you can feel vibration with very little amplitudes.

Yes exactly. Make sure the motors are powerful enough (750W surely are). Then choose arm length or screw pitch according to your requirements and finally select the gear ratio so you have enough torque for the selected lever length or pitch.

An UPS can protect against an actual power failure. But there are other cases like software bugs that can cause a servo drive to quit (overload due to excessive acceleration). Motors with electromecanical brakes are the easiest. You can power the brake magnet from a relay connected to the "status OK" output of the drive. In any case of failure power to the brake is cut and it engages. But this is also the most expensive solution.

3D printed parts can be very handy. Have a look at @Dirty 's linear actuator design. He made quite clever use of 3D printed parts and even 3D-printed bearings out of special low friction plastic from IGUS. This is totally OK if you make sure that the tension load stays within the limits and a failure/breaking of the parts dont have catastrophic consequences. For example, if a bushing inside the actuator fails then the pushrod rubs against the housing but it doesn't fall apart.

However, if one of theese 3D printed joints fail the whole rig can flip over which could cause serious injuries.

Are there any resources for the dimensions/layout of the base/actuators and how the length of the rods relate to this and affect the motion/footprint?

You can use FlyPT Mover to visualize and calculate everything, it's the easiest way

Just add 6Dof linear hexapod and edit its dimensions, than run 3dviewer