final project: fab-scaleFor my final project, I designed and fabricated an open-source analytical balance I'm calling Fabalance. As discussed in Week 1, analytical balances are a crucial scientific instrument that as of yet has not been reproduced in the open-source science community. Most chemistry and biology labs have at least one decent analytical balance, easily identified by the sliding draft shield and prominent spot on the bench. Unlike common scales, analytical balances don't use load cells; instead, they use an electromagnetic balance mechanism with an optical feedback loop, inferring mass by the power required to keep the weighing dish from moving. So, did it work? Short answer: I didn't get up to weighing anything, and crucially I never closed the control loop between the electromagnetic actuator and the optical sensor. But all of the subsystems worked, and the overall integration and packaging of the instrument can be considered a qualified success. Importantly, the learnings of the last Fabalance iteration paint a clear path to the next steps in the project, which I hope to take after I catch up on sleep. Many people helped support this project along the way. In particular, I'd like to thank Alfonso, for helping fabricate a number of aluminum parts on the CBA water jet cutter; Jiri, for suggesting the AFM-like mirror-based optical feedback mechanism; Jake, for helping steer me back from a few deep rabbit holes; and Danica, for putting up with week after week of late nights at the lab.
overviewFabalance consists of several distinct subsystems: the flexure, the electronics, the firmware, and the enclosure. As you'll see if you read through the project history on this page and others, the vast majority of development effort went in to the flexure; I tried a number of fabrication strategies, none of which were perfect but all of which taught me a great deal. The eventual design used a milled fiberglass frame reversibly coupled to a number of stainless steel foil flexural elements, which was functional but still problematically delicate.
final assemblyDuring final assembly, one of the 13 um stainless flexures broke: I hastily rebuilt the assembly fixture and cut a few fresh strips of foil; unfortunately, during reassembly the repeated strain on one of the 'fish' caused a more serious failure: I threw a hail-mary pass and got out the fancy quick-set epoxy, eschewing repairability for a last attempt at functionality: It worked! After a suitable cure time, I popped the flexure out of the fixture and quickly dropped it into the now-mostly-finished enclosure.
electronicsNeil has gotten excited about the new ATtiny 416 and 1614; they're priced like other ATtinys but have ATmega-like capabilities, along with a novel programming protocol called UPDI that eliminates the need for a dedicated ISP dongle. He gave me a bit of cut tape with a few of each processor, so I decided to give the platform a go for this project. Electronically, the microcontroller reads the differential signal between two phototransistors and adjusts the PWM duty cycle of the signal feeding the voice coil actuator, via an N-channel MOSFET. The scale also has a simple 8x2 LCD display and a big tare switch. I designed the PCB to be somewhat modular, breaking out the various offboard components into 2x2 and 2x3 headers or (in the case of the diode laser and voice coil) solder pads. I had previously left a generous 40 mm x 100 mm bolt pattern on the aluminum base plate, so PCB layout was simple and uncrowded: I set my design rules to 0.5 mm / 0.5 mm and milled the PCB on the CBA's SRM-20: I then used the megaTinyCore ATtiny1614 Arduino core to program the board, and Pololu's LCD library to get PCB woken up: I also twiddled the TCA registers on the t1614 until I got a 40 kHz PWM signal driving the coil, which stayed above the audio range and gave me ~10 bits of output resolution. Assembled with the scale, this allowed me to fully test the mechanism with the coil for the first time: Unfortunately, getting the LCD and the coil working at the same time proved evasive. It seems that the LCD library is hard-coded to use the same timer I'd selected for the PWM output, either directly or via Arduino defaults. Bah, this always seems to happen with third-party microcontroller libraries: they work in isolation, but the inherently resource-limited nature of embedded development makes intercompatibility challenging. For the class demo, I kept the LCD physically installed but decided to demonstrate the coil functionality.
thoughtsThis project has been a delightful if somewhat underresearched exploration into sensitive flexure design and digital fabrication. I say underreachered because I never got my hands on a commercial analytical balance to take apart, and I never undertook a comprehensive literature review to see how other folks have solved the problems I faced. That was kind of the point, to be honest; fab class gave me a chance to try to recreate the last few decades of analytical balance design entirely from the 80s-era diagram I found earlier this semester: It was a fun exercise to say the least, and Fabscale helped me get up to speed on a number of machines that will be important during my time at CBA: the Roland MDX-540 4-axis desktop mill; the Sodick SL400G wire-EDM; the humble Roland SRM-20 PCB mill; the Adams Maxwell coil winder; and, since I decided to be fancy with my stainless flexures and scale baseplate, the Oxford laser micromachining system and the OMAX water jet cutter. With the possible exception of the voice coil actuator metal piece (which is wire-EDMed), everything else can be easily manufactured in a FabLab; I think the electromagnet could be redesigned around easier methods too, likely with a significant loss of efficiency. Or maybe this is good motivation to push the Desktop Wire-EDM project further (hint hint, Will...). The next step for the Fabalance is another redesign of the flexure mechanism. After a good deal of pondering, I've decided that a stainless steel frame with spot-welded stainless steel foil flexures is worthy of exploration; both parts are easy enough to fabricate, welded joints are strong, and stainless steel foil seems like an ideal flexure material. To do this, I'd like to explore fabricating a simple micro-spot welder designed specifically for making precision flexures; ideally, it would make more of a uniform line than a spot, and would be integrated (or at least compatible with) a flexure assembly fixture. I'd also like to take a closer look at the electronics portion of the project. I think it's feasible to integrate all of the functionality a scale needs into the ATtiny1614, and I'm a big fan of using simple and cheap microcontrollers whenever possible. But I want to give the sensing side another look; AFMs use quad photodiodes to compensate for off-axis motion, which seemed to be one source of drift for my sensor. As an alternative, I'd like to try a neat sensing method we used in my micro/nano fab class this semester (2.675). The lab used home-built atomic force microscopes based on Scott Manalis and Maxim Shusteff's design from 20.309, which are based on a MEMS interdigitated (ID) interferometric finger apparatus as the target for the laser rather than a simple mirror. As the ID assembly displaces, the fingers produce a constructive and destructive interference pattern that repeats every 1/4 wavelength (roughly 160 nm). A huge advantage of this setup is its relative insensitivity to misalignment; while adding a custom MEMS device to the scale may seem to go against the fabricability requirement, it is a good excuse to test Prashant's laser-based direct-write MEMS fabrication techniques on a real application.
~december update: flexure foiblesor, "how to not fabricate highly sensitive flexural mechanisms" In the last few minutes of the final How to Make lecture before the final project showcase, Neil talked about demand- versus supply-side scheduling and spiral development. The former concept is about proactively planning and allocating time so that all facets of a complex problem can be addressed by a deadline; the latter is about iterating full prototypes of a concept rather than spending all of one's time on a particular aspect. Reader, I did not follow this advice. To be fair, I gave out this advice many times during the semester, and even felt at times that I needed to focus on other parts of the final project beyond the core mechanism; but here we are, a few days before the project presentation, and the majority of my time has been spent on sorting out the scale's magneto-optical feedback structure. Between documenting the optical feedback mechanism prototype in week nine and showing up for class an hour later (document as you go!), the powerful magnets attached to the acetal flexure unexpectedly snagged a metal plate and ripped one of the flexural elements apart. As I tried to recover parts of the mechanism for a class demo, more hair-thin acetal flexures snapped, leaving me with a sad pile of white plastic on my desk. I'm glad I got the pictures and videos I did when the mechanism was working, but the fragility of the system left me concerned about the durability of the finished scale. Time to shift gears.
wire-EDM adventureI decided to try to recreate the monolithic flexure in metal. Specifically, I wanted to use wire-EDM cut aluminum plate; while it's a bit stiffer than the acetal, my hope was that I could achieve tighter tolerances with the EDM process and end up with a more precise (and hopefully, more durable) flexure. I spent a good bit of time chasing down wire-EDM problems; when these were mostly resolved, I finalized an updated design for the component: This design required three separate wire-EDM cuts; two internal operations and one external cut to free the mechanism. I pre-drilled holes in the appropriate spots in a 2 mm aluminum plate, fixtured it in the machine, and spent the next few hours watching the Sodick break wires. I figured out that the high pressure water jet was vibrating the plate, snapping the wire; this didn't happen with 2.4 mm 17-4 because the stainless stock is much stiffer than aluminum. Following Will Langford's lead, I bolted a few aluminum plates together and got the first internal cut to work. Unfortunately, the waste slug of metal got wedged in the cut and contacted the wire-EDM's plastic nozzle, stalling the machine and putting a small nick in the [consumable, cheap] part. Since stalls require a machine restart, I lost the zero point of the cut and wasn't able to get the second operation aligned: Keen observers will notice the offset flexure in the lower-right corner of the image above, as compared to the CAD model. I started to be concerned about putting all of my eggs in the wire-EDM basket; even if the aluminum design had cut properly, it would have taken all day to finish. I then attempted a hybrid approach; wire-EDMed flexural inserts in a machined fiberglass frame. I started with a simple CAD sketch: I manifolded the parts together and tried cutting them with a number of different flexure thicknesses: 50 microns, 100 microns, 150 microns. In all cases, the aluminum flexure disintegrated in the machine, leaving me with a pile of dogbone halfs. 17-4 and 304 stainless fared better:
These parts fit nicely in a fiberglass support I machined; however, they proved to be substantially more stiff than the acetal flexure, and even the 304 stainless ones broke quite easily. The image below shows a close-up of a part getting ready to snap:Oof. Surface finish issues? Dial in the machine offsets? Or is it time for another major course-correction? I took a break and had a chat with Jake. Back to the drawing board with less than a week to go.