Apr 18, 2017

[2.70] Seek and Geek #9: U8 Outrunner BLDC Motor

This post is about bearing placement, shaft alignment, and hi-speed cantilever moment stiffness in outrunner brushless motors!

A disassembled U8 brushless motor

The point of a motor is to transmit torque, so motor bearings play an integral role in taking on load and minimizing friction losses. This particular motor supports a max thrust of 2.6kg using two bearings with load ratings of 2070N.


Guesstimating at moment resisted by the bearings...
Max stall torque from the motor is 0.912 Nm, and the rotor shaft is pressed on with probably a 5µm tolerance. Assuming no axial loading on the bearings, each bearing experiences half this torque as a radial force = approx. 50N.

We call the distance between bearings $a = 18mm$. F_bearing is the load on each bearing, which includes both applied loads and misalignment loads. By treating this shaft like a cantilever beam, we can calculate forces on the bearings due to misalignment:

$F = kx$, 
and
 $F_{bearing}\cdot a = K_{moment}\cdot\alpha = \frac{2EI}{L}\cdot \frac{\delta_{tol}}{L}$.
$F_{bearing} = \frac{2EI\delta_{tol}}{aL^2} + F_{applied}$

For this motor, L = 25mm, E = 69 GPa (material assumed to be 6061 AL), and I = 4019mm^4
$F_{misalignment}$ = 247N worst case.

So each bearing experiences 300N at max. torque of the motor, or nominally 15% of load rating.

Apr 17, 2017

[2.70] Cardboard Concept Desk



Modeling a concept desk out of cardboard, just as a quick stupid-check before designing components in detail.



The goal here was to see what this desk idea would look and feel like, without worrying about geometrical tolerances. I also played around with adding trusses to the interior of the desk (and added a drawer!)

Construction was all hot-glued corrugated cardboard, with the exception of the simple linear axis which makes a final reappearance here. So, the vertical column was assumed to have substantially higher stiffness than the rest of the cardboard desk.

Adding more thickness (box drawer) to the desk significantly helped improve moment of inertia and reduce deflections, but I don't think the interior trusses contributed much. I think future desk might scrap the truss idea in favor of having just clean box walls.


Derp-cardboard desk's only attachment to the carriage was via hot-glued foamcore, and clearly future desk will need a more load-bearing attachment system. Surprisingly, hotglue held the desk and 200g of motor components just fine.


Real desk will also likely be a wall-mounted desk, for simplicity.

Apr 9, 2017

[2.70] Seek and Geek #8: Stiletto Heels

National Geographic has a 2014 article about Hugh Turvey, a British photographer who uses a fusion of x-rays and visible light to create art. In particular, a photo of his wife's foot in a stiletto heel caught my eye.

"Femme Fatale", Hugh Turvey
What's with that heel?

After some internet searching, it's not a fancy flexure that allows the heel shank to snap off before the ankle twists...
It's actually the five nails used to attach the shank to the shoe!



Pressure under one of these stiletto heels can be more than 3000 kPa!

[2.70] Seek and Geek #7: Truss Geometries

2.70 coursework is pivoting towards specific components of the desk, so this seek&geek is about possible truss geometries for the desktop. I expect a constant distributed load (from desk weight and stuff placed on top) as well as a worst case point-load on the end (elbows and bodyweight). So how do other real-life objects achieve lightweight rigid planar structures?

Trusses are statically determinate structures solely consisting of two-force members, so basically assemblies of pinned beams. Trusses are most commonly seen in bridges, where the top beams "top chords" are typically under compression and the bottom chords are under tension.

Warren truss bridge

In my case, a desktop with a truss frame would experience tension on top and compression on the bottom. This design problem is similar to overhead cantilever road signs seen on the highway! Therefore, my desktop design can draw from a wealth of experience and calculations.

Highway signs also have to deal with significantly higher lateral loads than my desk does
http://s-steel.com/overhead-sign-structures/
Highway signs typically use a Pratt truss geometry to minimize sensitivity to buckling

Drawings and calculations for a highway overhead sign in I-85 Atlanta, GA

[2.70] Seek and Geek #6: Disposable Lighter

I signed up for a bunch of grill shifts this weekend, so now I have a lighter on my desk.

A very standard disposable lighter

Butane lighters like this one work by releasing compressed liquid butane as a narrow stream of gas. When this stream of gas meets a spark, it ignites and produces a flame. Holding open the valve continues to draw fuel out of the reservoir to support the flame and the outrush of gas prevents the flame from traveling inside the canister irrespective of orientation.

Lighters are dirt cheap, yet have a seemingly long working lifetime (it runs out of fuel long before it breaks) They do well on the cost/performance curve because they have very few moving parts - just the thumb-lever fork and the rotating spark wheel. They are also fairly idiot-proof - one swipe of the thumb rotates the sparkwheel with enough force to strike the flint and produce sparks, and that same motion lands the thumb on the fork lever to open the gas valve.

Parts of a lighter
 www.mybiclighter.com
An interesting note: the "flint" used in lighters and other modern firestarters is not the true sedimentary rock. "Flint" is actually the synthetic alloy "Ferrocerium" invented in 1903, which has an ignition temperature below 180 °C (easily generated by thumb friction) and produces sparks reaching temperatures up to 3,000 °C. 

Mar 30, 2017

[2.70] Linear Axis v2

Finally hooked everything up and ran the tests! 

TL;DR: This linear axis system has position error of 0.44mm, with only 3.5μm error resulting from backlash. It falls within the desired error budget of 0.5mm, at least in the no-load condition.


So this is what the setup looks like - I have my linear stage on a desk, and the laser shines on a piece of paper far (4.5m) away. I took three tests:

  1. Command the stepper motor to move 5000 counts forwards and backwards, and find out what distance this is (turning off the power supply between each jump)
    • testing ability to reliably move a set distance
    • measure open-loop error when turning on the system
  2. Command the stepper to jump (each 5000 counts) forwards 4 times, then to jump backwards 4 times (leaving power supply on)
    • testing ability to return to a position
  3. Command the stepper to bounce between two positions (bouncing 
    • measure cumulative backlash error

  • Distance from "zero" point of rail to wall "L"= 449cm
  • Travel distance of the linear axis "x" is defined differently every test
  • "Radius" of the most sensitive part of the carriage "r" = 20mm
  • α  and δ are resulting angular and distance errors projected onto the wall, from which we can measure machine error "err" of this linear axis
$\alpha = \frac{\delta}{L} $
and
$err = \frac{\delta x}{L+ x} $

Where for the purposes of my experiments, I assume all projected error comes from distance errors, even though some constant portion of it comes from angular error instead. (My analyses farther down ignore angular errors)

((For the results of these experiments, skip to the end))

Linear Axis v2 accomplishes two things. First, it has an actuator (nema17 stepper motor + 1/4-20 threaded rod), which allows me to send distance commands. Second, it has an anti-backlash device so switching directions accumulates less error.

Me planning out components for this actuator (ended up buying a flexible coupling instead of making one)
Some machining notes
This version2 reuses most of the parts from v1, with some modifications. I replaced one of the steel rails with a 1/4"-20 threaded rod, and added some bushings to the bearing blocks to accommodate the new diameter. I also made a plywood stand to put all the things on so I could use the linear axis without needing an empty optical table.

The big new thing here is the carriage for this threaded rod. It uses the same anti-backlash system as Austin's granite mill (Seek&Geek#1), which uses two adjustable-offset nuts for its preload. The threads of one of the nuts will always* be contacting the threads of the rod when traveling in either direction.

(* not actually always; since these are hand-tightened there will be some amount of user error here) 


Carriage and modified bearing block
The fixed part of the carriage consists of a milled aluminum block with a tapped hole on one side and an oversized-slot on the other. The HDPE wedge and its brass threaded-insert floats within the slot and gets pushed outwards by one of the adjustment screws (the other one keeps the wedge centered)

Carriage!

I chose to replace one of the rails with this actuator, which has some advantages and disadvantages compared to having two rails with the actuator in the center.

Two Rails, with Actuator acting as Rail

Advantages:
  • Much easier to build with the preexisting parts
  • Don't have to worry about overconstraint from the two other rails
Disadvantages:
  • Sensitive to threaded-rod imperfections, especially in roll
  • Actuator will always apply a moment, which magnifies error

Two Rails + Actuator in Center

Advantages:
Disadvantages:
  • Have to take care to add compliance to avoid binding from overconstraint
  • Not as easy to modify v1 rail to accomplish vs. the other method
Now for experiments! I used a standard nema17 stepper and an Arduino microcontroller, so nothing fancy. 

(A stepper motor is a brushless DC motor that divides a full rotation into an equal number of steps, so they will precisely rotate a fixed rotor angle without needing feedback. They usually do this by having tooth-shaped electromagnets and a gear-shaped rotor.)
Did I bring my 2.70 work and my calipers on a plane to Sweden? Yes, yes I did.
Results!
Experiment 1 - Turn on power to the system, command a set distance, turn power off
The carriage started at position 2.2cm. I programmed it to move 5000 stepper counts, then turned the power on and let the program run. Once the carriage stopped (5.3cm), I turned the power off. 
I flipped the direction and did this again, for a total of three trials (2 forwards, 1 backwards)

Looking at a ruler next to the moving carriage, it seems like the machine consistently moves 3.1cm per 5000-count jump. Looking at the laser deviations, we can get a better error resolution. The standard deviation for the three jumps was 1.54mm and the magnification for this experiment was 120.7. So, for this experiment the machine moves 3.1cm with an error of 12.8μm - pretty consistent commutation by the motor.

The Arduino is a generalist microcontroller, and when first powered on it briefly supplies 5V to all its logic pins. This slightly energizes the stepper motor on startup and causes additional error between what should be identical (or within 1.5mm on paper) landing points. This error was an average of 15.7μm, of which approx. 3μm should be start-up error.

Experiment 2 - Motor moves forwards, with pauses to measure distance traveled. Turn off power and restart. Then motor moves back to the starting point, again with pauses.


The carriage started at position 13.5cm. It moved four times, each time 5000-counts (nom. 3.1cm) and ended at position 2.2cm. This distance seemed a bit short, being an average 2.825cm instead of the expected distance. The system was turned off and reprogrammed to move backwards four times, again 5000-counts but ended at position 14.7cm - giving an average jump-distance of 3.125cm.

This is exciting. From Experiment1, we determined that turning the system on gives ~3μm error, and movement will have an average error of 13μm per jump. This machine error projected on the wall should give us an expected average jump error of 1.65mm. Experiment2's average jump-distance is off from our expected 3.1cm by 0.25mm and falls within the expected amount of error.

Experiment2 also allows me to measure average error of returning to a position. This ended up being 0.44mm error for my travel range of ~12cm, which is within my original desired error budget (500μm) for this machine. Woo!

While the carriage was moving, the laser dot moved back and forth on the page. This is partially due to vibrations transmitted by the motor, partially from contact vibrations between the carriage nut and the threaded rod, and partially from the laser pointer only being taped onto the carriage bed. The tip of the laser pointer displaces 0.115mm from these vibrations. 

Experiment 3 - Motor travels back and forth between two points, with pauses for recording position

The carriage started at position 6.6cm. Moving forwards and backwards 5000-counts, it consistently landed at positions 6.6cm and 3.4cm (5 trials) so traveled 3.2cm distances, not 3.1cm. So that's odd given the results of the other two experiments, but at least it's repeatable here.

Position error for this experiment was an average of 3.4μm, which means my machine is pretty good at rejecting backlash. While the carriage was moving, the tip of the laser wobbles 38μm (I did a better job clamping down the base platform for this experiment)

Bonus Analysis - Angle of linear axis machine relative to the wall
From Experiment3, we see that the the entire linear axis system is not quite square* with the laser paper (if it were square, there would be no systematic difference between the front measurements and the rear measurements). If we assume this discrepancy is entirely due to angular** error relative to the wall, we can get an estimate of what that angle is.



$\theta = tan^{-1}(\frac{\Delta }{x}) = 0.28^\circ$

$\Delta$ is the distance between the average front and back points (3.4cm and 6.6cm, resp.) multiplied by Experiment3's magnification factor. This angular error seems around right for lining things up using the floor tiles.

*I know that the laser pointer itself is not colinear with the axis-direction-of-travel... but I'm just combining this angular error with the main one and calling the whole thing "machine error"
**I'm also assuming my system didn't move between/during experiments, which probably isn't true.


Mar 21, 2017

[2.70] Linear Axis v2 (replacement threaded rod)

Still not ready for a proper update yet (need to run laser repeatability tests with set stepper counts), but I did replace the threaded rod.

Everything is so much better now.