Ball Aerospace JWST Actuator Uses Flexure Train for 2-Nm Steps

Breaking Taps

A single stepper motor achieves both 21-millimeter travel and 2-nanometer resolution open-loop by combining an intentional 320-degree backlash coupling with a monolithic flexure motion divider.

AI-assisted summary of How to achieve nanometer precision in space? — original source. Check the original for full context.

Source published: 2022-02-07

Key ideas

The James Webb Space Telescope uses 108 cryogenic actuators built by Ball Aerospace to align its 18 hexagonal primary mirror segments across six kinematic degrees of freedom with nanometer precision. (0:00)
Each actuator integrates both a coarse stage providing 21 mm of travel with 58 nm steps and a fine stage offering 10 µm of travel with 2 nm step resolution, all within a 600 g package. (2:11)
A single stepper motor operates both stages without active closed-loop power, using an intentional 320° deadband pin coupling to toggle between coarse ball screw drive and fine planar flexure displacement. (6:47)
To prevent the moving assembly from rotating around the ball screw, the design utilizes an external flexure linkage as a torsional stabilizer that allows linear movement while resisting rotation. (8:37)

Detailed notes

## Background Published on YouTube by the engineering channel Breaking Taps on February 7, 2022, this video explores the cryogenic mirror actuators developed for the James Webb Space Telescope. According to the description, the piece investigates how these devices achieve long travel and nanometer precision while surviving launch vibrations, extreme cold, and hard vacuum using intricate flexure mechanisms.

## TL;DR To properly focus light onto its scientific instruments, the James Webb Space Telescope relies on 108 cryogenic actuators across its 18 hexagonal primary mirror segments, operating in hexapod configurations to provide six kinematic degrees of freedom. Built by Ball Aerospace, each actuator achieves both a 21 mm coarse travel stage with 58 nm resolution and a 10 µm fine adjustment stage capable of 2 nm per step using a single internal stepper motor. The system avoids active heat-generating feedback by chaining planetary and spur gear reductions into an offset camshaft that drives a monolithic planar flexure motion divider. Furthermore, a clever pin-coupling lost-motion mechanism with roughly 320° of backlash allows the single motor to alternate between fine flexure adjustments and coarse ball screw actuation without adding weight or thermal penalties.

## Atmosphere & style The presentation carries the enthusiastic, analytical vibe of an inquisitive maker reverse-engineering elite aerospace hardware at a home workbench. The host's delivery blends genuine reverence for Ball Aerospace's engineering solutions with casual, dry humor about the practical headaches of 3D printing flight-spec tolerances on consumer machines. It feels less like an academic lecture and more like a hands-on mechanical teardown, guiding the viewer from awe at the brutal environmental constraints of deep space into pure appreciation for the clever simplicity of gears and flexures.

## Bulletin - [00:00](https://www.youtube.com/watch?v=5MxH1sfJLBQ&t=0s) The James Webb Space Telescope uses 108 cryogenic actuators built by Ball Aerospace to align its 18 hexagonal primary mirror segments across six kinematic degrees of freedom with nanometer precision. - [02:11](https://www.youtube.com/watch?v=5MxH1sfJLBQ&t=131s) Each actuator integrates both a coarse stage providing 21 mm of travel with 58 nm steps and a fine stage offering 10 µm of travel with 2 nm step resolution, all within a 600 g package. - [06:47](https://www.youtube.com/watch?v=5MxH1sfJLBQ&t=407s) A single stepper motor operates both stages without active closed-loop power, using an intentional 320° deadband pin coupling to toggle between coarse ball screw drive and fine planar flexure displacement. - [08:37](https://www.youtube.com/watch?v=5MxH1sfJLBQ&t=517s) To prevent the moving assembly from rotating around the ball screw, the design utilizes an external flexure linkage as a torsional stabilizer that allows linear movement while resisting rotation.

## Key takeaways - [00:00](https://www.youtube.com/watch?v=5MxH1sfJLBQ&t=0s) The primary mirror assembly requires aligning 18 hexagonal segments into a single parabolic contour across seven degrees of freedom. - [00:35](https://www.youtube.com/watch?v=5MxH1sfJLBQ&t=35s) Six spatial degrees of freedom (tip, tilt, roll, and X, Y, Z translation) are driven by a hexapod arrangement of six actuators per mirror, while radius of curvature is adjusted separately. - [01:12](https://www.youtube.com/watch?v=5MxH1sfJLBQ&t=72s) Actuators must survive launch vibrations, operate in hard vacuum and cryogenic temperatures, and hold positions indefinitely without active power emitting heat near the cold optics. - [02:11](https://www.youtube.com/watch?v=5MxH1sfJLBQ&t=131s) The Ball Aerospace flight unit provides 21 mm of coarse travel (58 nm resolution) and 10 µm of fine travel (2 nm resolution) at roughly 600 grams mass and 150 mm in length. - [04:04](https://www.youtube.com/watch?v=5MxH1sfJLBQ&t=244s) The fine positioning stage achieves 2 nm step resolution open-loop using a compound reduction chain of a 1:60 planetary gear, a 3:1 spur gear, and a planar flexure joint. - [04:58](https://www.youtube.com/watch?v=5MxH1sfJLBQ&t=298s) The monolithic flexure functions as a mechanical motion divider, converting roughly 1 mm of camshaft displacement at the center into just a few microns of travel at the mounting interface. - [06:47](https://www.youtube.com/watch?v=5MxH1sfJLBQ&t=407s) A lost-motion pin coupler with roughly 320° of rotational backlash allows the same motor to adjust the fine stage without engaging the coarse drive mechanism. - [07:36](https://www.youtube.com/watch?v=5MxH1sfJLBQ&t=456s) Driving past the pin coupler's deadband engages a 1:8 reduction gear and a precision ball screw to translate the coarse positioning stage over its full 21 mm travel range. - [08:05](https://www.youtube.com/watch?v=5MxH1sfJLBQ&t=485s) Reversing motor direction immediately disengages coarse translation, re-establishing fine adjustment within the new coarse position. - [08:37](https://www.youtube.com/watch?v=5MxH1sfJLBQ&t=517s) An external flexure-based torsional stabilizer prevents the actuator body from spinning about the ball screw while accommodating axial travel. - [09:08](https://www.youtube.com/watch?v=5MxH1sfJLBQ&t=548s) Coordinated extension and retraction across the six Stewart platform actuators fully defines each mirror segment's kinematic position.

## Detailed walkthrough ### [00:00](https://www.youtube.com/watch?v=5MxH1sfJLBQ&t=0s) The Engineering Marvel of JWST Mirror Actuators

The James Webb Space Telescope is packed with cutting-edge technology, from a tennis-court-sized sunshield thinner than a human hair to specialized cryocoolers and ultra-lightweight mirrors. Yet, the single most impressive piece of engineering on the entire observatory is its mirror actuator system. The primary mirror consists of 18 hexagonal segments that must unite into a seamless, concave ellipsoid to properly focus light into scientific instruments.

Aligning these mirrors requires positioning each segment across seven degrees of freedom to nanometer precision—a feat that is extraordinarily difficult on Earth, let alone in deep space. Six kinematic degrees of freedom are controlled directly by these actuators: tip, tilt, roll, X translation, Y translation, and Z translation (piston). The seventh degree of freedom—the radius of curvature of each mirror segment—is handled by a separate actuator.

Beyond sheer mechanical precision, the operational constraints are brutal. The actuators must survive the violent vibrations of a rocket launch, followed by the hard vacuum and cryogenic temperatures of deep space. They must be as lightweight as possible while retaining sufficient stiffness to hold nanometer positions indefinitely. Furthermore, because components behind the sunshield can thermally radiate and disrupt mirror performance, the actuators must remain completely cold—meaning they cannot rely on continuous, heat-emitting active power to hold their position. Regarding this staggering list of constraints, the host remarks: “This is like an impossible set of engineering problems to solve and yet it has been solved which is just truly remarkable to me.”

- **Developer:** Ball Aerospace designed and manufactured the actuators. - **Total Deployed Actuators:** 108 units across the primary mirror assembly (6 actuators per segment). - **Architecture:** 6 actuators per hexagonal segment arranged in a hexapod (Stewart platform).

---

[02:11](https://www.youtube.com/watch?v=5MxH1sfJLBQ&t=131s) Actuator Specifications and 3D Printed Replica

The operational specifications achieved by Ball Aerospace are staggering considering the size and mass constraints of space hardware. The unit provides both massive macro-level travel and extreme micro-level refinement:

| Specification | Coarse Stage | Fine Stage | |:--- |:--- |:--- | | **Travel Range** | 21 mm (~2 cm) | 10 µm | | **Step Resolution** | 58 nm | 2 nm | | **Mass** | ~600 g (~1.5 lbs) total unit mass | — |

To demonstrate how such precision is physically achieved, the host reverse-engineered and 3D printed a true-to-scale functional replica. Using a 2006 technical paper published by Ball Aerospace that contained partial mechanical drawings and gear train details, he CAD-modeled the mechanism around off-the-shelf components. The actual flight units measure roughly 150 millimeters in length. While the 3D-printed replica is marginally larger due to the physical size of standard hobbyist bearings, it maintains the core architecture, reproducing the gear reductions and flexure geometries almost verbatim.

---

[04:04](https://www.youtube.com/watch?v=5MxH1sfJLBQ&t=244s) The Fine Positioning Stage and Flexure Mechanism

The core mechanical challenge is achieving 2-nanometer steps without relying on complex, active closed-loop servo feedback that generates heat. The actuator accomplishes this through pure mechanical advantage across a multi-stage gear and flexure reduction train powered by a single internal stepper motor.

```mermaid flowchart LR accTitle: Actuator Drive Train Architecture accDescr: Mechanical power flow from the stepper motor through gear reductions to the fine flexure and coarse ball screw stages.

subgraph Drive["Input & Primary Reduction"] M["Stepper Motor"] --> P["1:60 Planetary Gear"] P --> S["3:1 Spur Gears"] end

subgraph FineStage["Fine Stage"] S --> C["Offset Camshaft"] C -->|"displaces center ~1 mm"| F["Planar Flexure Joint"] F -->|"deflects top ~2 µm"| OUT1["Fine Output (2 nm / step)"] end

subgraph CoarseStage["Coarse Stage"] S --> B1["1:1 Bevel Gears"] B1 --> CP["Pin Coupler (~320° Backlash)"] CP --> G["1:8 Gear Reduction"] G --> BS["Lead / Ball Screw"] BS --> OUT2["Coarse Output (21 mm travel)"] end ```

The flight unit uses an internal stepper coupled to a 1:60 planetary gearhead (substituted in the replica with a common 28BYJ-48 stepper motor, which has an internal 1:64 reduction). The output turns a spur gear set providing a 3:1 reduction, which directly rotates an offset camshaft.

This camshaft drives the center section of a monolithic planar flexure. As the offset cam rotates, it displaces the center section of the flexure upward by a macroscopic amount (around 1 mm in simulation). This central displacement pulls the lateral flexure arms inward, transferring an attenuated motion to the very top mounting interface:

- The flexure behaves as a mechanical motion divider: a 1 mm vertical displacement at the center results in only a few microns of travel at the top. - The flexure geometry offers vast design latitude: engineers can fine-tune arm spacing, material stiffness, joint cross-sections, and arm angles to achieve the exact displacement ratio desired. - Compounding the stepper angle, the planetary gearhead (1:60), the spur gear set (3:1), and the flexure attenuation produces an astonishing 2 nanometers of linear displacement per motor step.

As the host notes regarding the elegant simplicity of the system: “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.” While sensor suites exist on the telescope to measure position, the mechanism itself achieves its fidelity open-loop purely through geometry and mechanics.

---

[06:47](https://www.youtube.com/watch?v=5MxH1sfJLBQ&t=407s) Coarse Positioning Stage and Lost-Motion Pin Coupling

Rather than adding the weight and thermal penalty of a second motor, Ball Aerospace engineered a way to drive both the fine stage and the 21 mm coarse stage using the exact same stepper motor. The primary spur gear meshes with a 1:1 bevel gear set, routing drive torque down a central shaft to a specialized coupling disc.

This coupling mechanism exploits an intentional "lost motion" design using protruding pins on mating disc faces, creating roughly 320° to 330° of rotational backlash:

1. **Fine Control Window:** Within that ~320° window of rotation, the drive pin moves freely through open space without contacting the coarse stage pin. In this zone, only the camshaft and flexure operate, giving precise control over the 10 µm fine adjustment range. 2. **Coarse Engagement:** When the motor continues rotating in one direction past the deadband, the drive pin strikes the mating pin, physically locking the coarse stage drive. Continuous rotation in that direction drives a secondary 1:8 gear reduction connected directly to a precision ball screw, translating the entire mechanism along its 21 mm coarse path. 3. **Disengagement & Direction Reversal:** To return to fine positioning, the motor simply reverses direction. The pins immediately separate, freeing the system to make fine sub-micron adjustments within the new coarse position. If coarse adjustment in the opposite direction is needed, the motor reverses through the full 320° deadband until the pins make contact on their opposing faces.

---

[08:37](https://www.youtube.com/watch?v=5MxH1sfJLBQ&t=517s) Torsional Stabilizer, Hexapod Kinematics, and DIY Notes

Because the actuator rides atop a rotating ball screw with its drive motor mounted on top of the moving assembly, running the screw would naturally cause the actuator body to spin freely in place instead of translating vertically. To prevent this, Ball Aerospace integrated a torsional stabilizer along the spine of the unit. This stabilizer is an external flexure linkage that permits free linear extension along the primary axis of travel while remaining extremely stiff against torsional rotation.

Six of these linear actuators are connected to each of the 18 primary mirror segments to form a Stewart platform (hexapod). By coordinating the relative extension and retraction of all six actuators simultaneously, the system achieves complete kinematic control across all six spatial degrees of freedom.

For makers looking to replicate the project using the files posted to platforms like Thingiverse and Printables, the host offers practical advice: - The CAD files are faithful to the flight design and are not optimized for loose 3D-printer tolerances; extensive filing, sanding, and gluing are required to get parts fitting smoothly. - A standard M8 screw was substituted in place of the custom 21 mm space-flight ball screw. Because standard hardware introduces play, makers should anticipate loose fits, wrap threads in Teflon tape, or source fine-pitch threaded rods to reduce slop. - Credit for bringing the technical details of the actuator to light goes to *The Orbital Mechanics Podcast*, whose JWST coverage inspired the build.

[10:33](https://www.youtube.com/watch?v=5MxH1sfJLBQ&t=633s) Podcast Recommendation and Wrap-Up

Wrapping up the discussion on the James Webb Space Telescope's mirror actuators, the host gives an enthusiastic plug for *The Orbital Mechanics* podcast, recommending it to anyone interested in the space industry. He mentions having followed the show for quite some time, noting, “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.” A link to the podcast is provided in the video description for curious viewers who want deeper dives into aerospace engineering and spaceflight news.

Turning to the standard YouTube sign-off, the host steers the audience toward the algorithmically suggested content on screen, inviting them to check out whichever video Google thinks they might like next. Rather than running through a rote checklist of calls to action, he wryly urges viewers to “do the things people tell you to do at the ends of videos,” before offering a final thanks and signing off until the next project.

## Mentioned Ball Aerospace, Breaking Taps, Destin Sandlin, Google, James Webb Space Telescope, NASA Goddard Space Flight Center, Printables, Smarter Every Day, Stewart platform, The Orbital Mechanics Podcast, Thingiverse

## Video description The cryogenic mirror actuators on JWST are amazing feats of engineering. Capable of long travel and nanometer precision, able to survive rocket launch, cryogenic temperatures and hard vacuum... they are really impressive devices. And they use flexures!

CONSIDER SUBSCRIBING 🥰 ☕Buy me a coffee? https://www.buymeacoffee.com/Breakingtaps 🔬Or Patreon if that's your jam: https://www.patreon.com/breakingtaps 📢Twitter: https://twitter.com/BreakingTaps 💻Discord: https://discord.gg/R45uCXcEv4

==== Links ==== - "Cryogenic Nano-Actuator for JWST": https://www.esmats.eu/amspapers/pastpapers/pdfs/2006/warden.pdf - Orbital Mechanics Podcast: https://theorbitalmechanics.com/ - NASA Goddard Spaceflight Center's video on mirror alignment: https://www.youtube.com/watch?v=2dh41uouutU - Ball & Ball Aerospace history, because lol what? 😂 https://www.ball.com/our-company/our-story/history-timeline

- Thingiverse: https://www.thingiverse.com/thing:5232214 - PrusaPrinters: https://www.prusaprinters.org/prints/129373-jwst-mirror-actuators

- Shirt is from Destin over at Smarter Every Day: https://www.bonfire.com/store/smarter-every-day/

==== Timeline ==== 0:00 Mirror actuator 2:11 Actuator specs 3:06 3D printed replica 4:04 Fine positioning stage 6:47 Coarse positioning stage 8:37 Torsional stabilizer 9:08 Hexapod arrangement 9:38 Assembling and printing notes

[Source](https://www.youtube.com/watch?v=5MxH1sfJLBQ)

中文简报

鲍尔航天JWST作动器采用柔节机构实现2纳米步进单台步进电机结合特意设计的320度侧隙联轴器与整体式柔性运动分度机构，以开环方式同时实现了21毫米行程与2纳米分辨率。为了将光线精准聚焦到科学仪器上，詹姆斯·韦伯空间望远镜在18块六边形主镜镜片上依赖108台低温作动器，以六足结构排列提供六个运动学自由度。这些作动器由鲍尔航天制造，每台均仅凭单一内部步进电机，便同时实现了具有58纳米分辨率的21毫米粗调行程，以及单步可达2纳米的10微米精调行程。该系统避免使用产生热量的有源反馈，而是将行星齿轮和正齿轮减速机构级联至偏置凸轮轴，以此驱动整体平面柔性运动分度机构。此外，采用具有约320°侧隙的巧妙销轴联轴空回机构，使这台单电机能够在精细柔性调节和粗调滚珠丝杠驱动之间切换，而不会增加重量或带来热损耗。詹姆斯·韦伯空间望远镜采用鲍尔航天制造的108台低温作动器，在六个运动学自由度上以纳米级精度校准其18块六边形主镜镜片。 (0:00) 每台作动器均在600克的封装内，集成了行程为21毫米且步距为58纳米的粗调级，以及行程为10微米且步距分辨率达2纳米的精调级。 (2:11) 单台步进电机无需闭环主动供电即可驱动两个阶段，利用特意设计的320°死区销轴联轴器，在粗调滚珠丝杠驱动与精调平面柔性位移之间进行切换。 (6:47) 为防止移动组件绕滚珠丝杠旋转，该设计采用外部柔性连杆作为扭转稳定器，在允许线性运动的同时阻抗旋转。 (8:37)

Reading summary

To align the James Webb Space Telescope's primary mirror segments to nanometer precision without emitting disruptive heat, Ball Aerospace engineered dual-stage mechanical actuators driven by a single motor. Across 108 hexapod-mounted units, this design achieves both macro-scale travel and 2-nanometer step resolution through open-loop mechanical reductions and flexures. Operational Constraints and Dual-Stage Capabilities (0:00) The James Webb Space Telescope (JWST) primary mirror relies on 18 hexagonal segments that must operate together as a seamless concave ellipsoid. Aligning these segments requires nanometer-scale control across six kinematic degrees of freedom: tip, tilt, roll, and X, Y, and Z translations (piston), while a separate mechanism manages the radius of curvature. Six actuators per segment—totaling 108 flight units arranged as Stewart platforms—handle these kinematic adjustments under extreme physical constraints, including launch vibration, hard vacuum, and cryogenic operating temperatures. Because thermal emissions behind the sunshield would degrade mirror performance, the actuators must hold positions cold and rigid without relying on continuous, heat-generating active power. Nanometer Precision via Mechanical Flexure Attenuation (4:04) To bridge the gap between gross deployment and nanoscale tuning, Ball Aerospace designed a dual-stage actuator weighing approximately 600 grams and measuring roughly 150 millimeters long. The coarse stage provides 21 millimeters of travel with 58-nanometer step resolution, while the fine stage provides 10 micrometers of travel at 2 nanometers per step. Instead of using complex closed-loop servo systems that generate unwanted heat, the fine stage relies on open-loop mechanical reduction powered by an internal stepper motor. The motor drives a 1:60 planetary gearhead, followed by a 3:1 spur gear reduction connected to an offset camshaft. This camshaft displaces the center of a monolithic planar flexure vertically by roughly 1 millimeter. The flexure functions as a mechanical motion divider, attenuating that displacement to only a few microns at the top mounting interface. By compounding motor step increments through the planetary gearhead, spur gears, and flexure geometry, the assembly reliably produces 2 nanometers of linear displacement per motor step purely through mechanical kinematics. Single-Motor Actuation Through Lost-Motion Coupling (6:47) To avoid the mass and thermal penalties of installing separate drive motors for the coarse and fine stages, the mechanism drives both stages using a single stepper motor via an intentional lost-motion coupling. Primary spur gear torque is routed down a central shaft through a 1:1 bevel gear set to mating discs equipped with protruding drive pins, creating roughly 320° to 330° of rotational backlash. Within this deadband window, the drive pin rotates freely without contacting the coarse stage, allowing the camshaft to manipulate the flexure across its 10-micrometer fine range. When the motor rotates continuously past the deadband in either direction, the drive pin contacts the mating pin to lock the coarse drive train. This drives a secondary 1:8 gear reduction attached to a precision ball screw, translating the mechanism along its 21-millimeter coarse range. Reversing direction instantly disengages the coarse stage, allowing immediate fine sub-micron adjustments from the newly established coarse baseline. Torsional Stabilization and Hexapod Geometry (8:37) Because the actuator body and drive motor ride directly on the rotating ball screw, driving the screw would cause the entire unit to spin in place rather than translate linearly. Ball Aerospace resolved this by mounting a torsional stabilizer along the spine of the unit. This external flexure linkage provides high torsional rigidity to prevent axial spinning while permitting unconstrained linear extension along the travel axis. Coordinating the linear extension and retraction of all six actuators within each segment's Stewart platform yields precise spatial control over the primary mirror. In bench testing and hobbyist 3D-printed replications based on Ball Aerospace's 2006 technical disclosures, achieving functional motion requires tight mechanical tolerances, as standard off-the-shelf threaded hardware introduces thread play that the flight-grade ball screws avoid. Ball Aerospace’s mirror actuator design shows that severe thermal and mass constraints in spaceflight can be overcome through pure kinematic geometry. By utilizing flexure-based motion division and lost-motion pin couplings, the mechanism achieves nanometer-level alignment open-loop, bypassing the thermal footprint and complexity of active electronic servos.

精炼阅读摘要

詹姆斯·韦伯空间望远镜（JWST）的主镜执行器由 Ball Aerospace 设计制造，在严苛深空环境下无需依赖产生热量的主动闭环伺服系统，仅凭单电机驱动与纯机械结构便实现了 21 毫米大行程与 2 纳米微调精度的兼容控制。深空环境限制与多自由度调节要求 (0:00) JWST 主镜由 18 块六边形镜片拼接而成，必须精确对齐为一体化的凹面椭球体，才能将光线准确汇聚至科学仪器。每个镜片由 6 个执行器组成的 Stewart 平台（六足机构）控制俯仰、倾斜、滚转以及 X、Y、Z 轴平移等 6 个运动自由度，曲率半径则由独立的第 7 个执行器调节，主镜阵列总计布置了 108 个主执行器。这套机构必须耐受火箭发射过程中的剧烈震动，并在深空的硬真空与极低温环境下维持稳定的刚度与纳米级精度。由于执行器位于遮阳板后方，任何组件向外辐射的热量都会干扰红外望远镜的观测性能。因此，执行器在保持定位时必须保持完全冷却，无法依靠持续通电发热的传统主动伺服系统来维持位置。节目主持人指出，在同时满足超轻量化、极端刚度、无热辐射维持以及纳米级精度的前提下，该系统成功攻克了在地面上都极其罕见的一组工程矛盾。双级调节指标与柔性微调机构减速原理 (4:04) 该执行器单体质量约为 600 克，飞行件总长约 150 毫米，兼具宏观行程与极高分辨率：其粗调级具有 21 毫米的行程范围与 58 纳米的步进分辨率；微调级行程为 10 微米，步进分辨率达 2 纳米。为了在不产生热量的开环条件下达到 2 纳米控制，机构采用了由单个内部步进电机驱动的机械减速链。步进电机输出首先经过 1:60 行星齿轮减速器，再通过 3:1 的正齿轮组减速，进而驱动一根偏心凸轮轴。凸轮轴直接推动一块整体式平面柔性铰链的中心部位。凸轮旋转带动柔性铰链中心产生约 1 毫米的垂直位移，该变形拉动两侧柔性臂向内收紧，通过机械几何比例将其衰减为顶部安装面仅数微米的微动。通过结合步进角、齿轮系与柔性铰链的逐级衰减，机构以纯机械几何方式实现了单步 2 纳米的位移输出。工程师可以通过调整臂间距、材料刚度及铰链截面精确设定运动缩放比，完全摆脱了对发热电子伺服机构的依赖。基于空行程销轴离合器的单电机复用方案 (6:47) 为避免配置两台电机所带来的额外重量与热辐射惩罚，Ball Aerospace 设计了同时驱动微调级与 21 毫米粗调级的单电机分动方案。动力由主正齿轮分出至 1:1 伞齿轮副，通过传动轴引入带有凸出销轴的特制耦合盘。该机构利用端面销轴之间约 320° 至 330° 的旋转游隙，构建出有意设计的“空行程”（lost motion）死区。在 320° 的游隙窗口内，驱动销在开阔区域空转而不接触从动销，动力仅传递至偏心凸轮与柔性铰链，实现 10 微米范围内的微米级调节。当电机向单一方向持续旋转超出该死区时，两销撞击咬合，锁死粗调驱动轴，通过 1:8 的二级减速齿轮驱动精密滚珠丝杠，推动整个执行器沿 21 毫米粗调导向移动。若需切回微调，电机仅需反向旋转即可使销轴瞬间脱离接触，机构随即在新粗调位置上恢复微调控制。抗扭稳定器设计与桌面级制造验证 (8:37) 由于微调执行机构与电机整体搭载在旋转滚珠丝杠之上，若无约束，丝杠转动会导致执行器本体自转而无法实现轴向直线位移。设计团队在装置外脊部集成了一个柔性抗扭稳定器，该连杆结构在允许主轴向自由线性伸缩的同时，对扭转方向具备极高刚度，从而保证了推进精度。六台执行器协同工作，便能完全控制镜片空间位置。主持人根据 Ball Aerospace 在 2006 年发表的技术论文中的图纸与齿轮传动数据，利用 3D 打印与通用轴承对该机构进行了功能复刻。制作经验表明，航天原始 CAD 模型未针对桌面 3D 打印公差进行优化，装配需要大量打磨与粘接修正；此外，复刻版使用标准 M8 螺杆代替了专用的航天滚珠丝杠，必须通过使用生料带缠绕螺纹等手段来消除间隙，以缓解民用标准紧固件引入的机械侧隙问题。 JWST 镜面执行器展示了以机械几何优势替代主动闭环伺服的设计范式，在无热功耗和质量受限的环境下解决了纳米级控制难题；该系统的地面复刻不仅验证了纯机械减速与销轴空行程传动的精妙逻辑，也指出了民用加工公差与航天专用精密机构之间的工艺差距。

Detailed notes — zh

## Background 该视频由 YouTube 频道 Breaking Taps 于 2022 年 2 月 7 日发布。视频简介指出，詹姆斯·韦伯太空望远镜（JWST）上的低温反射镜执行器是一项令人惊叹的工程壮举，不仅具备大行程与纳米级精度，更能经受火箭发射、深冷环境与硬真空的考验，其核心采用了精巧的柔性铰链机构。 ## TL;DR 视频深入解析了 Ball Aerospace 为詹姆斯·韦伯太空望远镜研发的低温纳米执行器（Cryogenic Nano-Actuator）。JWST 的 18 块六边形子镜背面共安装了 108 个该类执行器，每 6 个构成斯图尔特六足平台以精确控制镜片的 6 个空间自由度。作者通过逆向工程 3D 打印了一个可实际工作的复刻模型，生动拆解了其如何仅凭单台步进电机，借助柔性铰链、偏心凸轮实现 2 纳米精度的微调，并通过巧妙的 320 度回差联轴销与滚珠丝杠实现 21 毫米的大范围粗调，同时依靠背部抗扭稳定器消除自旋干扰。 ## Atmosphere & style 整期视频洋溢着创客与精密工程爱好者的极客热情与由衷赞叹。主讲人的讲解思路极为清晰、节奏明快，借助 3D 打印物理实物模型与机械运动动画，将深奥晦涩的空间机构学化繁为简。整场演示兼具严谨的学术剖析与接地气的 DIY 动手经验分享，让观众在直观领略航天顶尖机构精妙构思的同时，深切体会到机械设计所蕴含的纯粹之美。 ## Bulletin - [00:00](https://www.youtube.com/watch?v=5MxH1sfJLBQ&t=0s) 詹姆斯·韦伯太空望远镜（JWST）主镜由18块六边形子镜构成，每块子镜需在7个自由度上实现纳米级对齐，背面共布置108个超精密执行器。 - [02:11](https://www.youtube.com/watch?v=5MxH1sfJLBQ&t=131s) 由Ball Aerospace研发的低温执行器单机仅重约600克，兼具21毫米粗调行程（58纳米/步）与10微米精调行程（2纳米/步）。 - [04:04](https://www.youtube.com/watch?v=5MxH1sfJLBQ&t=244s) 系统通过单台步进电机配合多级齿轮减速与柔性铰链机构，无需复杂闭环伺服反馈即可实现2纳米的纯机械级微步位移。 - [06:47](https://www.youtube.com/watch?v=5MxH1sfJLBQ&t=407s) 执行器巧妙引入约320度空行程回差的联轴盘设计，使单一电机能够无冲突地自由切换粗调丝杠传动与精调柔性微动。 ## Key takeaways - [00:00](https://www.youtube.com/watch?v=5MxH1sfJLBQ&t=0s) JWST主镜需拼合成凹面椭球体，各子镜需精准调整俯仰、倾斜、滚动、三轴位移等6个运动学自由度及第7个曲率自由度。 - [00:00](https://www.youtube.com/watch?v=5MxH1sfJLBQ&t=0s) 执行器必须在硬真空、低温深冷及火箭发射剧烈振动等极端环境下工作，且不可依靠持续耗电发热的主动控制来维持位置。 - [02:11](https://www.youtube.com/watch?v=5MxH1sfJLBQ&t=131s) 执行器具备粗精双阶段调节：粗调行程达21毫米且步距58纳米，精调行程为10微米且单步步距仅2纳米。 - [02:11](https://www.youtube.com/watch?v=5MxH1sfJLBQ&t=131s) 每块子镜背面由6个执行器构成斯图尔特六足位移平台（Hexapod），全镜共计108个执行器协同控制刚体姿态。 - [02:11](https://www.youtube.com/watch?v=5MxH1sfJLBQ&t=131s) 主讲人根据2006年Ball Aerospace公开论文中的工程参数，成功3D打印复刻出等比例的功能性工作模型。 - [04:04](https://www.youtube.com/watch?v=5MxH1sfJLBQ&t=244s) 精调机构依靠1:60行星齿轮、3:1正齿轮与偏心凸轮驱动柔性铰链，将毫米级凸轮位移缩减至微米乃至纳米级输出。 - [04:04](https://www.youtube.com/watch?v=5MxH1sfJLBQ&t=244s) 柔性铰链的设计参数（如臂间距、截面厚度及内臂夹角）高度可调，为系统提供了巨大的纯机械级减速比与刚度保障。 - [06:47](https://www.youtube.com/watch?v=5MxH1sfJLBQ&t=407s) 粗精调切换通过带有双向驱动销的联轴盘实现，利用约320度的空行程回差将两套传动机制彻底解耦。 - [06:47](https://www.youtube.com/watch?v=5MxH1sfJLBQ&t=407s) 粗调模式下，电机旋转越过空行程后直接啮合1:8二级齿轮及滚珠丝杠，反转微小角度即可脱开并重新回到精调区间。 - [08:37](https://www.youtube.com/watch?v=5MxH1sfJLBQ&t=517s) 执行器背部配置了刚性极强的扭转稳定器，允许构件轴向伸缩的同时锁死周向旋转，防止电机自转打滑失稳。 - [08:37](https://www.youtube.com/watch?v=5MxH1sfJLBQ&t=517s) 复刻打印该执行器需要大量的后处理打磨与公差修整，若以标准M8螺杆替代精密滚珠丝杠需缠绕生料带消隙。 ## Detailed walkthrough ### [00:00](https://www.youtube.com/watch?v=5MxH1sfJLBQ&t=0s) 詹姆斯·韦伯太空望远镜的工程挑战与执行器詹姆斯·韦伯太空望远镜（JWST）融合了令人叹为观止的尖端技术：网球场大小但比发丝还薄的遮阳板、使仪器保持极度低温的低温制冷机，以及超轻质的铍反射镜。然而，主讲人 Breaking Taps 认为，整台望远镜上最令人惊叹的技术结晶，其实是安装在镜片后方的执行器（Mirror Actuator）。JWST 的主镜由 18 个六边形子镜拼合而成，这些镜片最终必须形成一个凹面椭球体，从而将光线精准反射到仪器中。要完成这一任务，每个子镜都必须在 7 个自由度（Degrees of Freedom）上实现纳米级对齐精度。在地面上做到这一点已属不易，在遥远的深空轨道中更是一场严峻考验。执行器负责控制其中的 6 个运动学自由度调整：俯仰（Tip）、倾斜（Tilt）、滚动（Roll）、Y 轴平移、X 轴平移以及活塞运动（Piston，即 Z 轴平移）；第 7 个自由度则是子镜本身的曲率半径调整，由另外专门的执行机构负责。除了超高精度的挑战之外，整个机构还面临着严酷的环境约束：必须承受火箭发射时的剧烈振动；到达目标轨道后要耐受硬真空和深冷环境；必须极其轻量化，同时具备足够的刚度以将镜片锁定在纳米精度的固定位置；此外，由于遮阳板后方的一切都会影响光学性能，执行机构必须尽可能保持低温，绝对不能依靠持续耗电发热的“主动定位控制”来维持位置。谈及 Ball Aerospace 攻克这一难题时，主讲人赞叹道：“This is like an impossible set of engineering problems to solve and yet it has been solved which is just truly remarkable to me.” - 主镜由 18 块六边形子镜组合形成凹面椭球体。 - 运动自由度划分：单镜片通过执行器调整 6 个位移与转动自由度，第 7 个自由度为子镜自身曲率调节。 - 空间环境挑战：发射振动、高真空、极低温度（Cryogenic）、不可散发多余热量的主动位置维持。 - 研发单位：Ball Aerospace。 --- ### [02:11](https://www.youtube.com/watch?v=5MxH1sfJLBQ&t=131s) 执行器技术指标与 3D 打印复刻过程 Ball Aerospace 研发的这款低温纳米执行器在工程指标上堪称极端。它具备粗调（Coarse）和精调（Fine）两个行程阶段：粗调行程达 21 毫米（超过 2 厘米的位移行程对于纳米级机构而言极其庞大），粗调单步步距仅为 58 纳米；精调行程为 10 微米，精调步距则压缩到了惊人的 2 纳米。整台执行器重量仅约 600 克（约合 1.5 磅）。在整个 JWST 主镜背面，一共布置了 108 个这样的执行器；每块子镜背面配置 6 个，构成六足位移台（Hexapod）或斯图尔特平台（Stewart Platform），借此统筹控制 6 个自由度的姿态。主讲人根据 Ball Aerospace 于 2006 年前后公开发表的一篇论文（包含工程图样、齿轮传动比与关键尺寸数据）进行了逆向建模，制作了 1:1 的 3D 打印工作复刻模型。真实执行器全长约 150 毫米，而复刻模型由于采用了现成且尺寸偏大的轴承，整体轮廓略有放大。但内部的齿轮减速比与原始设计高度接近，核心的柔性铰链机构（Flexure Mechanism）更是几乎原样再现，代表了目前公开资料下能够达到的最高还原则。 | 机构参数 | 粗调阶段（Coarse Stage） | 精调阶段（Fine Stage） | |:--- |:--- |:--- | | **有效行程** | 21 毫米（mm） | 10 微米（μm） | | **单步步距** | 58 纳米（nm） | 2 纳米（nm） | | **系统规模** | 全镜共 108 个执行器；每子镜 6 个执行器构成六足平台 | | **单机重量** | 约 600 克（约 1.5 磅） | --- ### [04:04](https://www.youtube.com/watch?v=5MxH1sfJLBQ&t=244s) 单步 2 纳米：精调机构与柔性铰链原理执行器内部巧妙地仅使用了一台步进电机，通过多级传动和柔性放大机构来实现精调。电机隐藏在壳体内部，首先直连一套 1:60 行星齿轮减速箱；在复刻模型中，主讲人使用了极为常见的 28BYJ-48 步进电机（自带 1:64 减速），虽然体积偏大且侧隙（Backlash）较重，但功能逻辑完全一致。电机动力随后传入一套 3:1 减速的正齿轮组，正齿轮再进一步驱动一套伞齿轮以及一根关键的凸轮轴（Camshaft）。精调阶段的核心就在于这根凸轮轴：轴中央包含一个微量偏心凸轮，凸轮外圈嵌套轴承，顶推柔性铰链的中心连接部。当凸轮旋转并向上顶推铰链中心部分时，几何结构迫使柔性臂向内收拢，从而连带拉动机构最顶端产生微量位移。这种柔性铰链在偏心轮输入与顶部输出之间构成了巨大的运动减速比——在仿真中可以看到，中心部分移动整整 1 毫米的大位移，顶端仅仅产生几微米的形变位移。主讲人盛赞这种机构设计自由度极大，工程师可通过调整臂间距、材料刚度、铰链截面厚度、内臂夹角或凸轮偏心距等，精准定制所需的传动比。得益于电机自身步距、行星齿轮、正齿轮与柔性铰链的层层缩放，整个系统无需依赖复杂昂贵的高精度闭环伺服反馈，“基本上免费获得了海量的机械精度”——单步步进即可在顶端稳定输出 2 纳米的位移。关于纯机械减速实现的极端精度，主讲人明确指出：“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.” ```mermaid flowchart LR accTitle: Fine Adjustment Drive Train accDescr: 展现从步进电机到柔性铰链顶部输出两纳米级位移的动力传递流程 subgraph Power["动力输入与齿轮级"] M["步进电机"] --> G1["1:60 行星减速"] G1 --> G2["3:1 正齿轮减速"] end subgraph MotionConvert["机构变换级"] G2 --> B["伞齿轮与凸轮轴"] B --> C["偏心凸轮顶推 (毫米级)"] end subgraph FlexureStage["柔性铰链位移缩放"] C --> F["柔性臂向内收拢"] F --> OUT["顶端输出 (2 纳米/步)"] end ``` --- ### [06:47](https://www.youtube.com/watch?v=5MxH1sfJLBQ&t=407s) 粗调机构与双向 320 度回差联轴器为了在不增加第二台电机的情况下实现 21 毫米的大范围粗调，设计者引入了一个构思绝妙的粗精调切换机构。驱动凸轮的正齿轮同轴带动一套 1:1 伞齿轮副，将动力通过垂直传动轴和轴承传递给位于底部的联轴盘（Coupling Discs）。这个联轴盘两面各设有一个突出的驱动销，并被人为设计出约 320 度的自由空行程回差（Backlash）。在这一大段回差旋转范围内，精调机构可以自由转动而不触碰粗调级；而一旦连续转动约 320 度到 330 度，驱动销便与粗调机构的撞针贴合，开始强行带着粗调级同向运转。粗调传动链随后经过一套 1:8 的齿轮减速，驱动一颗大齿轮及其直接相连的滚珠丝杠，驱动整个执行器沿轴向进行长距离升降。当粗调到位后，只需命令电机轻微反向旋转，销轴立刻脱离接触，机构重新落入 320 度的空行程保护区，在此区间内（约 10 微米行程）继续进行纳米级的精细微调。如果要反向粗调，只需向相反方向持续旋转越过死区、重新顶上反向销钉即可。主讲人惊叹于这种用单一电机实现粗精两用且互不干涉的离合设计：“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.” - 联轴盘机械逻辑：利用约 320 度的回差空行程隔离精调与粗调，使单电机能够兼顾全局移动与局部位姿。 - 啮合与脱开流程：连续单向转动越过空行程后销钉硬接触，驱动粗调；反转几度脱开接触，立刻重回精调模式。 - 粗调丝杠传动：包含 1:8 的二级齿轮减速，最终驱动滚珠丝杠实现 21 毫米总行程。 --- ### [08:37](https://www.youtube.com/watch?v=5MxH1sfJLBQ&t=517s) 扭转稳定器、六足平台与复刻打印建议整个执行器的主体安装在滚珠丝杠上，电机又位于机构顶部。为了避免电机运转时产生反作用力矩导致执行器在丝杠上原地自转打转、无法产生有效轴向推力，Ball Aerospace 在执行器背部设计了一个刚性极强的扭转稳定器（Torsional Stabilizer）。这是一个特殊的平行柔性铰链机构，其物理特性表现为：允许整个部件沿直线轴向自由伸缩拉长，但在扭转方向上具备极高的抗扭刚度，从根本上杜绝了旋转晃动。在应用架构上，主镜的每块六边形子镜均由 6 个执行器以斯图尔特六足平台（Hexapod）的形式联合支撑。6 根连杆执行机构协同伸缩，共同解算并输出空间 6 个自由度的任意刚体位姿变换。主讲人坦言六足联调的运动学逆解与打印装配工作量巨大，因此仅完整制作了单根执行器样机供社区研究。对于想要自行 3D 打印复刻的观众，主讲人发出警示：该文件并非“即打即用”的平民模型，零件没有预留宽松公差，需要配合大量的砂纸打磨、锉削修整和粘接装配；此外，由于市面上买不到行程 21 毫米的微型精密滚珠丝杠，复刻件使用普通粗牙 M8 螺杆代替，存在一定晃动间隙，建议装配时缠绕生料带（Teflon Tape）或改用细牙螺纹以提升紧固度。最后，主讲人感谢了播客节目 The Orbital Mechanics，正是该节目对 JWST 技术的深入探讨激发了他制作该复刻实物的灵感。 - 扭转稳定器作用：作为背部抗扭柔性铰链，放行轴向单向直线位移，彻底锁死沿丝杠轴线的周向扭转自旋。 - 姿态解算构型：6 组执行器并联排布为 Stewart 平台，通过协调伸缩精确映射 6 个空间自由度。 - DIY 打印制造难点：缺乏预留装配公差，要求手工打磨锉削；替代标准件 M8 螺杆间隙偏大，需采用生料带或细牙螺杆优化。 ### [10:33](https://www.youtube.com/watch?v=5MxH1sfJLBQ&t=633s) 播客推荐与结尾致谢在视频的尾声，作者向对航天与太空工程感兴趣的观众强烈推荐了播客节目《The Orbital Mechanics》（轨道力学）。他分享道，自己已经连续收听这个播客好几年了，里面的内容非常精彩、扎实，对于想要了解航天产业内幕的人来说十分有价值。他指出：“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.”（强烈推荐《Orbital Mechanics》，我已经听了好几年了，它棒极了，我非常喜欢，如果你对此感兴趣的话一定要去听听看。）相关的播客链接也已经附在视频下方的简介区供大家查阅。随后，作者以轻松幽默的方式向观众道别。他打趣地提醒大家可以看看旁边由 Google 算法推荐的精彩视频，并戏谑地表示：“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.”（去做那些大家在视频结尾总叫你们做的事情吧；除此之外我想就这些了，感谢观看，我们下次再见。）以此号召大家完成点赞、订阅等常规支持操作，正式结束了整期视频。 - **推荐资源**：《The Orbital Mechanics》播客（作者已长期收听数年，专注航天工程与太空行业深度内容，链接附于视频简介栏中）。 - **互动结语**：以调侃平台常规口播的轻松口吻，向观众致谢并告别。 ## Mentioned 詹姆斯·韦伯太空望远镜, JWST, Breaking Taps, Ball Aerospace, The Orbital Mechanics, 28BYJ-48, NASA Goddard Spaceflight Center, Destin, Smarter Every Day, Google ## Video description The cryogenic mirror actuators on JWST are amazing feats of engineering. Capable of long travel and nanometer precision, able to survive rocket launch, cryogenic temperatures and hard vacuum... they are really impressive devices. And they use flexures! CONSIDER SUBSCRIBING 🥰 ☕Buy me a coffee? https://www.buymeacoffee.com/Breakingtaps 🔬Or Patreon if that's your jam: https://www.patreon.com/breakingtaps 📢Twitter: https://twitter.com/BreakingTaps 💻Discord: https://discord.gg/R45uCXcEv4 ==== Links ==== - "Cryogenic Nano-Actuator for JWST": https://www.esmats.eu/amspapers/pastpapers/pdfs/2006/warden.pdf - Orbital Mechanics Podcast: https://theorbitalmechanics.com/ - NASA Goddard Spaceflight Center's video on mirror alignment: https://www.youtube.com/watch?v=2dh41uouutU - Ball & Ball Aerospace history, because lol what? 😂 https://www.ball.com/our-company/our-story/history-timeline - Thingiverse: https://www.thingiverse.com/thing:5232214 - PrusaPrinters: https://www.prusaprinters.org/prints/129373-jwst-mirror-actuators - Shirt is from Destin over at Smarter Every Day: https://www.bonfire.com/store/smarter-every-day/ ==== Timeline ==== 0:00 Mirror actuator 2:11 Actuator specs 3:06 3D printed replica 4:04 Fine positioning stage 6:47 Coarse positioning stage 8:37 Torsional stabilizer 9:08 Hexapod arrangement 9:38 Assembling and printing notes [Source](https://www.youtube.com/watch?v=5MxH1sfJLBQ)

Original transcript

[0:00] there is a lot of really cool technology [0:02] packed into the james webb space [0:03] telescope everything from the reflector [0:06] the size of a tennis court but thinner [0:08] than a human hair the cryo cooler [0:10] keeping the instrumentation super cold [0:12] the ultra lightweight brilliant mirrors [0:15] everything about it is just a [0:16] technological marvel but i think the [0:19] most impressive piece of technology on [0:21] the entire telescope is this [0:24] this is a working replica of the jwst [0:27] mirror actuators [0:29] it's possibly the coolest mechanism that [0:31] i've seen in quite a while so i decided [0:34] to build one these actuators are [0:35] required because jwst's primary mirror [0:38] is composed of 18 hexagonal segments [0:41] and these segments eventually will form [0:43] a concave ellipsoid which will reflect [0:46] the light into the instruments [0:48] but this is a really challenging task [0:49] because each of the segments needs to be [0:51] aligned in seven degrees of freedom to [0:54] nanometer precision which is difficult [0:56] on earth let alone on a satellite out in [0:58] space the actuator that we're looking at [1:00] today handles the kind of kinematic [1:02] adjustments so there's tip [1:05] tilt [1:07] roll [1:08] y translation [1:11] x translation and then piston or z [1:14] translation [1:15] the seventh degree of freedom is [1:17] actually the curvature of the mirror [1:18] itself and that's handled by a different [1:20] actuator as if this wasn't difficult [1:22] enough already it's on a satellite up in [1:25] space [1:26] which means it needs to survive the [1:28] rocket launch to get it up there and [1:29] even if it survives the launch it needs [1:32] to survive hard vacuum and cryogenic [1:34] temperatures once it's out to its final [1:36] destination [1:37] and because it's a satellite it needs to [1:39] be as lightweight as possible but still [1:41] stiff enough to hold the mirrors in [1:43] their nanometer perfect precision [1:45] position [1:47] and because [1:48] everything on the jwst behind the [1:51] sunshield can indirectly affect the [1:54] thermal performance of these mirrors it [1:56] needs to be as cold as possible so [1:59] no active positioning that might emit [2:01] heat holding its position [2:03] this is like an impossible set of [2:05] engineering problems to solve and yet it [2:08] has been solved which is just truly [2:10] remarkable to me these actuators were [2:12] built by ball airspace and frankly i [2:15] think they're just geniuses for coming [2:17] up with it [2:24] let me read the specs for you it's got a [2:26] course range of 21 millimeters [2:29] two centimeters of actual travel [2:31] distance which is huge [2:33] the step size on the course is 58 [2:37] nanometers [2:38] the fine range [2:40] is 10 microns and the fine step size is [2:44] 2 nanometers [2:46] that's ridiculous the thing only weighs [2:48] 600 grams about a pound and a half and [2:51] there are 108 of these total across the [2:54] back of the jwst [2:56] six of them are placed on each mirror [2:58] segment and they form a hexapod or a [3:00] stewart platform to control the six [3:03] degrees of freedom this is of course a [3:06] 3d printed replica of the actuators that [3:08] are on jwst [3:10] there is a paper that ball aerospace [3:12] published in 2006 or so which has some [3:15] technical drawings and a few critical [3:17] details about the gear train and [3:20] dimensions of the actuator and from that [3:23] i managed to [3:24] essentially work backwards and cat up a [3:27] replica as close as i could get it using [3:29] the components i have [3:31] this is a true to scale replica the [3:34] actuators are about 150 millimeters [3:36] mine's actually a little bit bigger [3:38] because the bearings that i have [3:40] are too big and so it kind of forced [3:42] everything to be a bit larger than it [3:43] should but otherwise it's pretty close [3:46] to what's on the jwst [3:48] the gear reductions are very similar if [3:51] not identical depending on the stage the [3:54] flexure mechanism is essentially [3:56] verbatim so this is as close as we're [3:59] going to get without ball aerospace [4:01] releasing the technical drawings [4:02] themselves each actuator has two stages [4:05] a core stage which is you to roughly [4:07] position the mirror where it needs to be [4:09] and then a fine stage to really dial in [4:12] the alignment there's a single stepper [4:14] motor which is tucked inside of the [4:16] mechanism which couples directly to a 1 [4:19] to 60 planetary gear reduction drive [4:22] train [4:23] the stepper that i used is one of these [4:25] 28 byj super common steppers which [4:29] actually has a 1 to 64 gear reduction so [4:31] it's very similar although amusingly [4:33] it's quite a bit bigger than the ones [4:35] that they used and it also has you know [4:37] a lot more backlash and stuff the motor [4:39] then drives a set of spur gears which [4:41] gives you a three to one reduction and [4:43] those spur gears in turn drive a bevel [4:46] gear and a camshaft and it's the [4:48] camshaft which gives you the fine [4:50] control mechanism the camshaft has a [4:53] slightly offset cam in the middle which [4:56] couples to a bearing and drives the [4:58] center part of this flexure [5:00] as the cam rotates it places an upwards [5:03] force on the center section and as it [5:06] moves upwards it pulls the arms of the [5:10] flexure in with it and as the arms pull [5:12] inwards the very top portion of the [5:14] flexor is forced to move as well because [5:16] everything is kind of coupled but it [5:18] gives you a [5:20] essentially a gear reduction between the [5:22] cam motion and the top of the flexure if [5:25] we simulate this you can see how the [5:28] center section moves a fairly large [5:31] distance say a millimeter whereas the [5:33] top only moves by a couple microns [5:36] what's cool about this flexure is [5:38] there's a lot of design latitude to come [5:39] up with the exact requirements you want [5:42] and the gear reduction needed you can [5:44] adjust the distance between the arms the [5:46] stiffness of the material you're using [5:48] the cross sections of the different [5:50] flexure joints the angle of the inner [5:53] arm how much the cam displaces the [5:55] center section like there's a lots of [5:57] different things you can do to adjust [5:59] how much [6:00] the top of the flexor moves an [6:03] interesting thing about this mechanism [6:04] is it gives you a ton of precision [6:06] essentially for free because you've got [6:09] the resolution of the stepper itself [6:11] you've got the gear reduction of the [6:12] planetary gear you've got the gear [6:14] reduction of the spur drive and then [6:16] you've got the reduction of the flexure [6:18] itself so one step on the stepper motor [6:22] gives you in this case two nanometers of [6:25] distance at the top of the flexure [6:27] without having to do any super precise [6:30] you know servo controlled and feedback [6:32] mechanism it's just based on mechanics [6:35] and the gearing and the flexure now they [6:37] do have sensors to check how much things [6:40] have moved but it's not required based [6:42] on the mechanism itself which i think is [6:44] just super cool [6:45] the bevel gear that i mentioned earlier [6:47] is used to drive the coarse stage of the [6:50] actuator the bevel gear couples to [6:53] another bevel gear just a one-to-one [6:54] snow reduction air goes through a shaft [6:57] through some bearings down to a coupling [6:59] disc and these coupling discs are super [7:02] ingenious in my opinion they're [7:04] essentially flat discs that have two [7:06] protruding pins one on either side [7:10] and it's set up in such a way that [7:11] there's essentially [7:14] 320 degrees of backlash so the fine [7:18] stage is allowed to rotate for [7:21] 320 or 30 degrees whatever until it hits [7:24] the protruding pin of the core stage and [7:27] at that point it starts to drive the [7:29] core stage because the two pins are in [7:32] contact [7:33] and as long as it keeps rotating in the [7:35] same direction it will keep driving the [7:37] core stage in that direction [7:39] but if you stop rotating and go back the [7:42] other way it disengages from the core [7:44] stage and now you're back to just fine [7:47] control which is [7:49] super cool and so the way they've done [7:51] this is that you get a single stepper [7:53] motor that can control both coarse and [7:56] fine positioning using this mechanism so [7:59] if you want to go somewhere quickly do a [8:01] big course adjustment you just rotate [8:03] until you engage the pins and drive it [8:05] to your desired location [8:07] disengage [8:08] and kind of go backwards a little bit [8:11] and then you can adjust the fine within [8:14] that 10 or so micron period [8:16] if you need to backtrack and go the [8:18] other way just throw it in reverse [8:20] engage the pin on the couplers in the [8:22] other direction and drive it back down [8:24] it's just so cool the core stage has an [8:27] additional one to eight gear reduction [8:30] and the big gear is directly coupled to [8:33] a ball screw and that's what moves it up [8:34] and down finally the last important [8:37] thing is this brace at the back of the [8:39] actuator this is a torsional stabilizer [8:42] and this is needed because everything is [8:44] just sitting on this ball screw and the [8:47] motor is on top of the actuator so if [8:49] you were to just run it without any kind [8:52] of stabilizer it would just spin in [8:53] place you wouldn't actually get any [8:55] lateral motion [8:56] so the torsional stabilizer is just a [8:58] flexure joint on the back that allows [9:01] the whole thing to stretch kind of in a [9:04] linear direction but not twist [9:06] as i mentioned earlier there are six [9:08] actuators per mirror segment and they're [9:11] arranged in a hexapod or a stewart [9:13] platform configuration and it's actually [9:15] the combination of all these actuators [9:17] working together which is what gives you [9:19] the six degrees of freedom [9:21] there's videos online about how hexapods [9:23] work they're really cool mechanisms i [9:26] thought about building one but it was [9:28] already enough work getting this up and [9:30] running let alone building six and all [9:32] the kinematic control so maybe someone [9:34] else will do it it'd be really cool to [9:35] see one of these segments kind of [9:37] working in real life if you're [9:39] interested in printing this yourself [9:40] i've uploaded it to you know all this 3d [9:43] sites fair warning it's not very printer [9:45] friendly nothing's tolerance to work [9:48] right off a printer i did a lot of [9:50] sanding and filing and gluing to get [9:52] everything assembled so keep that in [9:54] mind if you decide to do it yourself i'm [9:56] using a coarse m8 screw as the ball [9:59] screw because you can't [10:01] find a 21 millimeter ball screw it was [10:03] obviously custom made but it is a little [10:06] loose just because it's not made for [10:09] this kind of application so keep that in [10:10] mind or add some teflon tape or maybe [10:12] find a fine threaded instead of coarse [10:15] threaded screw [10:16] shout out to the orbital mechanics [10:18] podcast that's where i heard about this [10:20] originally it was on one of their recent [10:22] episodes on jwst [10:24] if you like space flight rocketry [10:27] satellites missions upcoming missions [10:29] history of missions all the technology [10:31] behind any of it interviews with people [10:33] in the industry [10:35] highly recommend orbital mechanics i've [10:37] been listening for a couple years now [10:38] and it's great i love it so check that [10:40] out if that sounds interesting to you [10:42] link is down in the description below [10:44] if you enjoyed the video today consider [10:47] one of these fine videos that google [10:49] thinks you will enjoy as well [10:51] do the things people tell you to do at [10:53] the ends of videos and otherwise i think [10:55] that's it thanks for watching i'll see [10:57] you next time