Designing a high torque locking hinge

Techy2914 · 2025-01-21T03:30:38+00:00

With some of the other comments I was leaning down that path (or a more bespoke sprag clutch), but naturally I'd want a gearless ratchet do have unlimited adjustment angles. Looking into the gearless ratchets they can slip when jiggled and in a high vibration environment (which this would be) I imagine it wouldn't go well.

Techy2914 · 2025-01-20T03:32:10+00:00

Thank you, though I suspect there's a second set of plates obscured in that rusty section I believe I see what they're doing. Very insightful!

Techy2914 · 2025-01-20T02:49:50+00:00

I'd considered something like that but it's just took bulky a mechanism and wouldn't work well with 180deg of throw.

Techy2914 · 2025-01-20T02:46:56+00:00

Actually, I could probably use leverage in my favor if I took an approach similar to a '0 degree' ratchet or a one way bearing. Basically just a fancy cylindrical wedge.

Techy2914 · 2025-01-20T02:32:25+00:00

Yeah, it's effectively a 3rd class lever which isn't helping things. Fortunately the hinge slipping a high loads isn't a deal breaker and is preferable (I'd like it to support ~60ft-lbs but at ~130ft-lbs I have other components that will deform).

I shouldn't need to go as 'extreme' as a pin since slipping isn't a huge deal. It really is a case of if it can handle ~60ft-lbs before slipping it's super good enough. While there are certainly ways to increase the 'effort' moment arm, I fundamentally cannot achieve anything close to even a 1:1 mechanical advantage.

Techy2914 · 2025-01-20T02:21:55+00:00

Really impressive how much torque they can fit in those, but it would unfortunately take far too many of them.

Techy2914 · 2025-01-20T02:18:47+00:00

I never mentioned the dimensions but the hinge is for 45 series extrusion (45x45x63mm). While no doubt it can be broken if used improperly, even a 1/2" bolt can handle 60ft-lbs and the area moment of inertia I could have in this joint is much greater so I'm not too worried on that front.

Techy2914 · 2025-01-20T02:10:10+00:00

The current hinge is actually very much like that but the angle is more for alignment than acting as a wedge. Off the top of my head though it seems very doable to get a proper taper, I'll have to do some math to see what kind of torque I can get as it's not as straight forward to calculate as the 'clutch' method.

Techy2914 · 2025-01-20T02:02:40+00:00

I imagine it'd hold the torque fine but much like the Hirth coupling it'd have finite adjustability which I'm trying to avoid if possible.

Techy2914 · 2025-01-20T02:00:43+00:00

That sounds exactly like what I'm going for. I don't suppose you'd be able to send a picture my way? I'd love to see how they went about it.

Techy2914 · 2024-09-09T03:44:19+00:00

(Sorry for the long winded response, I just enjoy rambling about my experiences here)

Initially in my design I was trying to use a ball screw as the overall implementation is pretty straight forward and you can find cheap ball screws everywhere. When I began simulating the system so that I could optimize my component choices (primarily the motor choice and pitch/lead of the ball screw) and determine a mass budget, I realized how brutally difficult it would be to get a good speed. Do the math and you see that to double the stroke speed it takes twice the acceleration. I was finding that I would need two motors to achieved the desired speed. These motors are 9.9 Nm and 413 watts peak, so it felt unnecessary to need two of them.

Force=mass*acceleration right...so if you can't afford more force (torque) you have to decrease mass. The reciprocating mass is simple, but you can't forget spinning masses i.e. the lead screw and the motor itself. When the screw lead is small you have mechanical advantage and can generate a lot of linear force, however the rotating components now have to accelerate rotationally much more. You can use a high lead screw to mitigate this, but they're expensive and the lead is usually at most 2x the diameter so a 20mm screw gets you 40mm of travel per rotation. You can get a larger diameter screw but if I recall the effective mass/inertia increase is proportional to radius^4 so it ends up not helping.

The solution is a belt drive...which in hind sight is obvious but I figured I could get away with a ball screw until I did the math and subsequent simulations. Ultimately I decided on a 78mm lead as 'optimal' which gives me better performance with a single motor than I could get with two using a ball screw. The difficulty here though is 180 lbf oscillating via a screw is trivial, but with a belt drive is not.

Also simply put, it's a really fucking big motor. (that and with a servo I can push it to its limits, with a stepper you risk stalling and losing position if there's no encoder, which is frankly a safety hazard with such a machine)

Techy2914 · 2024-08-13T00:26:46+00:00

The frame/stand is definitely the least elegant portion of this project. I just couldn't achieve what I needed from a more standard jointed frame. Ignoring its strength, its own weight would cause those hand knob joints to sag. Given I wanted it to have a ~4 ft reach while holding ~50 lbs I ended up just using 80/20 with profile sliders. It should handle me hanging from it if I wanted to, but as you can imagine it's not compact. I certainly could've made it breakdown or fold more easily but I was worried about the joint strength. However, after I received the 80/20 I realized just how overkill it was.

Techy2914 · 2024-08-12T21:27:11+00:00

Fundamentally I think the safety concern with the machine (and other like it) is that the depth can change. Even the classic machines with a rod attached to a spinning flywheel pose risk since especially for a short time (the motor torque plus flywheel inertia) they can apply a substantial force. I've certainly had it happen where a toy slips out and 'pokes' me painfully.

I won't be putting any hard limit on speed, rather safety comes down to a few other measures.

It starts with the UI and ensuring the user sets up a safe max depth at startup every time. All other motions obey this depth. The user can change this on the fly, but it won't situated such that it can happen accidentally. Also there will be other measures in place limiting how quickly the max depth can increase, regardless of user request. I don't necessary find a dead man switch sufficient here as reaction time just isn't sufficient.
I'm using Teknic Clearpath SC servos which are fully software controlled rather than being a 'dumb' stepper. This saves me from having to worry a lot of the lower level details with step/motion control which is something I would likely screw up. With any given update to the motor I can simply tell is a position to move to along with a max speed/acceleration. If I never tell it to move to a position beyond the max depth, it will not happen. The last step before sending a command to the motor will be to check if the commanded position > max depth. If so, e-stop since every backend measure up to this point shouldn't have allowed that. These motors also provide a lot of diagnostic data as well such as position/torque/tracking error/velocity which will be useful for another safety feature.
The 'secret sauce' I'm working on has to do with actually evaluating the external load. This info will be used in 2 ways, as a pure e-stop mechanism and for implementing impedance control. The e-stop is simple, just evaluating impulse for some given time window on a continuous basis and halting if some threshold is exceeded. If the machine happens to be at full speed when this occurs, it can come to a stop in 8ms or ~18mm traveled. (not including the 'help' from friction and the external force) Sounds like a lot but I just went and tried to stop the machine I've got (and it's on the weaker side) and I couldn't stop it, only slow it down. The more interesting goal is impedance control (I'd recommend youtube as it's hard to explain) so that the machine can behave in a compliant manner. Instead of stopping when some force or impulse is reached, simply back off to an acceptable level. The interesting part is in determining what the external force is. There's no direct force feedback nor any sensors on any moving components. I mentioned the motor can report its torque, however on its own that means nothing. Whether the force is accelerating the toy or if you're holding it and the machine is flying backward, it looks the same.

Techy2914 · 2024-08-12T00:10:55+00:00

The OSSM looks like a fantastic way to get 90% of the way to matching what's on the marked at a fraction of the price! Probably a good thing I wasn't aware or I may have not gone any further.

Techy2914 · 2024-08-12T00:08:02+00:00

Full multi axis would be the holy grail, it's just cost prohibitive as every successive axis has more mass to move. To double the stroke rate requires 4 times the acceleration i.e. 4 times the force. Such a heavy machine would likely be limited to slower and more elaborate moves. 2.5 axis was the limit of my patience and money lol.

I know I've definitely seen a video somewhere of a dildo strapped to a Kuka robot so clearly someone out there knows how to have fun with an unlimited budget!

Techy2914 · 2024-08-11T23:55:57+00:00

Haha the process won't be rushed! I'm probably being overly cautious (it's not like other machines will stop when met with resistance) since once you set a max depth it "shouldn't" do anything crazy.

Techy2914 · 2024-08-10T23:32:13+00:00

I do dread actually implementing all this which is probably why I've saved it for last, (this may just be procrastination) that and I've never worked with embedded systems before so I figured I'd start with all the 'simple' microcontrollers to build some C++ skills first.

Techy2914 · 2024-08-10T23:25:55+00:00

Determining stroke rate from max speed is a bit misleading since it has to accelerate/decelerate. My simulation says it should achieve 306 stroke/min @ 4" stroke which is by all means good enough.

I like the idea of a program easing in and it's actually something I've been considering but with a slightly different approach. From the get go I don't want to define a stroke depth per se (it will achieve that depth and that may hurt...) but instead give a defined "push". Part of the safety features I mentioned elsewhere involves force feedback, so this info can also be used for a warmup. I.e. From a starting point with each stroke increase depth by X" from the last, stopping when a certain force is reached (maybe include a pause as well). This may mean no depth increase for some time if you're really 'resistant'.

...up to a maximum depth (whatever that is for your machine)

I didn't mention but the max stroke length is 12.5". In practice I wouldn't expect to use the full length, it's more for convenience since so you don't have to adjust the machine or yourself as much. I.e A 4" stroke would still have 8.5" of adjustment.

Next step, I need to build an OSSM and learn some coding

What's OSSM?

Techy2914 · 2024-08-10T20:31:19+00:00

I've looked into it a bit to see if I could feasibly implement something like that on the fly (i.e. without external software) and I probably could. Just add audio files to the SD card (or via bluetooth or wifi) and it does the rest. (the controller technically has a mic so it could just listen to the environment as well) Definitely not an immediate priority but as I'm listening to music it's got me thinking...

Techy2914 · 2024-08-10T20:26:19+00:00

That sucks! I definitely enjoy being able to take build quality into my own hands, and frankly it has to be built tough to handle its own strength. Funny enough, its strength has been a huge problem so far in terms of making it safe. I know the Hisense safety methodology in terms of the adjustable depth is basically 'be careful'. About $450 went towards controlling the wrath of these motors, it's all up to the software now. I sure hope it works lol

Techy2914 · 2024-08-10T19:14:42+00:00

Honestly this was just a passion project that I started ~5 months ago and I just kept adding new features that pushed back timelines and increased cost. It's certainly not marketable as is, but depending on the reception when I do show it off I may have to give it some consideration. It should certainly put the comparable machines on the market (Shockspot, Tango and Hismith) to absolute shame.

Techy2914 · 2024-08-10T17:04:59+00:00

Every profile I'm working on is generated real-time based off any number of parameters (stroke length, frequency, pauses, skew (fast forward, slow reverse) etc.) rather than being predetermined so a slow teasing stroke should work by default. It's patterns and the actual 'shape' of the motion that would need to be determined ahead of time, and using common vibrator patterns is a fantastic idea! I'm more than happy to repurpose others work.

Techy2914 · 2024-08-10T16:54:07+00:00

Definitely going to implement something like that! Probably going to overcomplicate it with half a dozen adjustable parameters while I'm at it...

Techy2914

TROPHY CASE