The clever engineering of James Webb's mirror actuators

there is a lot of really cool technology packed into the james webb space telescope everything from the reflector the size of a tennis court but thinner than a human hair the cryo cooler keeping the instrumentation super cold the ultra lightweight brilliant mirrors everything about it is just a technological marvel but i think the most impressive piece of technology on the entire telescope is this this is a working replica of the jwst mirror actuators it's possibly the coolest mechanism that i've seen in quite a while so i decided to build one these actuators are required because jwst's primary mirror is composed of 18 hexagonal segments and these segments eventually will form a concave ellipsoid which will reflect the light into the instruments but this is a really challenging task because each of the segments needs to be aligned in seven degrees of freedom to nanometer precision which is difficult on earth let alone on a satellite out in space the actuator that we're looking at today handles the kind of kinematic adjustments so there's tip tilt roll y translation x translation and then piston or z translation the seventh degree of freedom is actually the curvature of the mirror itself and that's handled by a different actuator as if this wasn't difficult enough already it's on a satellite up in space which means it needs to survive the rocket launch to get it up there and even if it survives the launch it needs to survive hard vacuum and cryogenic temperatures once it's out to its final destination and because it's a satellite it needs to be as lightweight as possible but still stiff enough to hold the mirrors in their nanometer perfect precision position and because everything on the jwst behind the sunshield can indirectly affect the thermal performance of these mirrors it needs to be as cold as possible so no active positioning that might emit heat holding its position this is like an impossible set of engineering problems to solve and yet it has been solved which is just truly remarkable to me these actuators were built by ball airspace and frankly i think they're just geniuses for coming up with it let me read the specs for you it's got a course range of 21 millimeters two centimeters of actual travel distance which is huge the step size on the course is 58 nanometers the fine range is 10 microns and the fine step size is 2 nanometers that's ridiculous the thing only weighs 600 grams about a pound and a half and there are 108 of these total across the back of the jwst six of them are placed on each mirror segment and they form a hexapod or a stewart platform to control the six degrees of freedom this is of course a 3d printed replica of the actuators that are on jwst there is a paper that ball aerospace published in 2006 or so which has some technical drawings and a few critical details about the gear train and dimensions of the actuator and from that i managed to essentially work backwards and cat up a replica as close as i could get it using the components i have this is a true to scale replica the actuators are about 150 millimeters mine's actually a little bit bigger because the bearings that i have are too big and so it kind of forced everything to be a bit larger than it should but otherwise it's pretty close to what's on the jwst the gear reductions are very similar if not identical depending on the stage the flexure mechanism is essentially verbatim so this is as close as we're going to get without ball aerospace releasing the technical drawings themselves each actuator has two stages a core stage which is you to roughly position the mirror where it needs to be and then a fine stage to really dial in the alignment there's a single stepper motor which is tucked inside of the mechanism which couples directly to a 1 to 60 planetary gear reduction drive train the stepper that i used is one of these 28 byj super common steppers which actually has a 1 to 64 gear reduction so it's very similar although amusingly it's quite a bit bigger than the ones that they used and it also has you know a lot more backlash and stuff the motor then drives a set of spur gears which gives you a three to one reduction and those spur gears in turn drive a bevel gear and a camshaft and it's the camshaft which gives you the fine control mechanism the camshaft has a slightly offset cam in the middle which couples to a bearing and drives the center part of this flexure as the cam rotates it places an upwards force on the center section and as it moves upwards it pulls the arms of the flexure in with it and as the arms pull inwards the very top portion of the flexor is forced to move as well because everything is kind of coupled but it gives you a essentially a gear reduction between the cam motion and the top of the flexure if we simulate this you can see how the center section moves a fairly large distance say a millimeter whereas the top only moves by a couple microns what's cool about this flexure is there's a lot of design latitude to come up with the exact requirements you want and the gear reduction needed you can adjust the distance between the arms the stiffness of the material you're using the cross sections of the different flexure joints the angle of the inner arm how much the cam displaces the center section like there's a lots of different things you can do to adjust how much the top of the flexor moves an interesting thing about this mechanism is it gives you a ton of precision essentially for free because you've got the resolution of the stepper itself you've got the gear reduction of the planetary gear you've got the gear reduction of the spur drive and then you've got the reduction of the flexure itself so one step on the stepper motor gives you in this case two nanometers of distance at the top of the flexure without having to do any super precise you know servo controlled and feedback mechanism it's just based on mechanics and the gearing and the flexure now they do have sensors to check how much things have moved but it's not required based on the mechanism itself which i think is just super cool the bevel gear that i mentioned earlier is used to drive the coarse stage of the actuator the bevel gear couples to another bevel gear just a one-to-one snow reduction air goes through a shaft through some bearings down to a coupling disc and these coupling discs are super ingenious in my opinion they're essentially flat discs that have two protruding pins one on either side and it's set up in such a way that there's essentially 320 degrees of backlash so the fine stage is allowed to rotate for 320 or 30 degrees whatever until it hits the protruding pin of the core stage and at that point it starts to drive the core stage because the two pins are in contact and as long as it keeps rotating in the same direction it will keep driving the core stage in that direction but if you stop rotating and go back the other way it disengages from the core stage and now you're back to just fine control which is super cool and so the way they've done this is that you get a single stepper motor that can control both coarse and fine positioning using this mechanism so if you want to go somewhere quickly do a big course adjustment you just rotate until you engage the pins and drive it to your desired location disengage and kind of go backwards a little bit and then you can adjust the fine within that 10 or so micron period if you need to backtrack and go the other way just throw it in reverse engage the pin on the couplers in the other direction and drive it back down it's just so cool the core stage has an additional one to eight gear reduction and the big gear is directly coupled to a ball screw and that's what moves it up and down finally the last important thing is this brace at the back of the actuator this is a torsional stabilizer and this is needed because everything is just sitting on this ball screw and the motor is on top of the actuator so if you were to just run it without any kind of stabilizer it would just spin in place you wouldn't actually get any lateral motion so the torsional stabilizer is just a flexure joint on the back that allows the whole thing to stretch kind of in a linear direction but not twist as i mentioned earlier there are six actuators per mirror segment and they're arranged in a hexapod or a stewart platform configuration and it's actually the combination of all these actuators working together which is what gives you the six degrees of freedom there's videos online about how hexapods work they're really cool mechanisms i thought about building one but it was already enough work getting this up and running let alone building six and all the kinematic control so maybe someone else will do it it'd be really cool to see one of these segments kind of working in real life if you're interested in printing this yourself i've uploaded it to you know all this 3d sites fair warning it's not very printer friendly nothing's tolerance to work right off a printer i did a lot of sanding and filing and gluing to get everything assembled so keep that in mind if you decide to do it yourself i'm using a coarse m8 screw as the ball screw because you can't find a 21 millimeter ball screw it was obviously custom made but it is a little loose just because it's not made for this kind of application so keep that in mind or add some teflon tape or maybe find a fine threaded instead of coarse threaded screw shout out to the orbital mechanics podcast that's where i heard about this originally it was on one of their recent episodes on jwst if you like space flight rocketry satellites missions upcoming missions history of missions all the technology behind any of it interviews with people in the industry highly recommend orbital mechanics i've been listening for a couple years now and it's great i love it so check that out if that sounds interesting to you link is down in the description below if you enjoyed the video today consider one of these fine videos that google thinks you will enjoy as well do the things people tell you to do at the ends of videos and otherwise i think that's it thanks for watching i'll see you next time

Channel: Breaking Taps

Views: 567,447

Keywords:

Id: 5MxH1sfJLBQ

Length: 11min 6sec (666 seconds)

Published: Mon Feb 07 2022

Reddit Comments

[u/bitse]

The author of the original paper describing the design of this device responded to the video in the YT comments! It's so cool that I wanted to share the response here so people don't miss it.

Robert Warden here. I wrote the paper back in 2006. I just wanted to say how impressed I am with your reverse engineering! Your graphics and description are very well done. Back then, we didn't have easy access to 3D printers, so I built the first model out of Legos, which is still on my desk! Wishing you all the best - Bob

And he shares this video later showing the lego model he built:

https://www.youtube.com/watch?v=3WBrqUa_1yk

[u/Scoopdoopdoop]

That is rad dude

[u/AgrippaDaYounger]

Well done, would have liked to see the full assembly though. The OP had already overcome design hurdles, why not keep going and show the full assembly.

[u/alexismolena]

What a great mechanism !

[u/LukesFather]

The pretty smart engineering and its cool he was able to replicate it. Its awesome that he also posted the .stl files

[u/work_alt_1]

How do they deal with like so many of the different issues that I think would impact measurements at such a small scale?

• Changes in temperature, impacting changes in material properties like stiffness, which would impact how much the "flexor" (or whatever the top part is called) would bend?
• Deterioration in the material, which would also change stiffness?
• Any slight "cocked" part in the assembly, which would result in the "flexor" moving more at a certain degree of rotation of any of the "cocked" parts, and less at other degrees of rotation
• Slight changes in curvature of each mirror?

Like essentially I'm assuming they figure out how much a certain amount of rotation moves the upper "flexor" part in nanometers, and then just assume that it's constant? I don't know why I'm assuming that this incredible piece of engineering is that simple, I guess I'm actually assuming it's more complicated, just interested specifically how? Like do they baseline/zero it out often? Re-check their measurements/calculations often? Validate the results?

• Is the whole piece actually made out of 3D printed parts?
• Does the actual part require lubrication like I saw in this video?
• If so, how are they re-applying it when it's up in space?

I just honestly feel like when we get down to anything even close to as small as a nanometer, there's so many things that could change it, like literally just breathing near it, I don't understand how you could eve create something that would be this precise.

[u/Philias2]

Wow, those disks for switching between coarse/fine adjustment are super clever.

[u/ArmadilloNo1122]

. Reminder to watch later