This is a method of color screenprinting shirts called CMYK printing, which uses Cyan, Magenta, Yellow, and blacK to product a full-color image, just like a color 2D printer would.
In order to ensure proper alignment of each layer for each Tee shirt, precise coupling of some kind must be used. If you look at the bottom right, it seems some kind of endstop is used to ensure each screen is located properly in the horizontal direction, and the rood itself butts up against the metal frame to align the vertial direction and prevent rotations. While this may not exactly be micron repeatability, this certainly seems good enough for this application! Perhaps I should make a Kinematic Screen Printing Coupling? Maybe in the future!
Here's a video of a photographic image recreated by this process. This time, a machine is used with four stations and four tee shirt holders that move from station to station, where each station only puts down one color. I imagine some kind of final alignment features are used in order to get each station to be repeatably applied to each tee shirt.
The Ultimaker is a great 3D printer that is known in the desktop 3D printing community for printing quickly and reliably. It can use both ABS and PLA filament, and the original is made from a lasercut plywood construction and assembled using slotted "t-nuts" (The community calls these bolted joints t-nuts even though these aren't real T-Nuts, which you can use to clamp parts to a mill bed, among other things).
This sounds similar to my FASBot! Here I will look into the similarities and differences between the two.
To start, the material, while also wood, is plywood, as opposed to MDF. According to some online sources (http://www.diffen.com/difference/MDF_vs_Plywood), plywood is stiffer than MDF an is less sensitive to moisture.
How much stiffer is plywood than MDF? According to Matweb, the Young's Modulus of plywood is 9.8 GPa. According to MakeItFrom, the Young's Modulus of MDF is only 4GPa. Plywood is over twice as stiff!
This is probably bcause MDF is compressed bits of wood fibers while plywood has long grains. It's the difference between baking a cake of carbon fiber shavings and epoxy versus making a tube from longer strands of carbon fiber epoxied together. The latter will be stronger.
The main structure forms a cube that is held together with slots and tabs for alignment, and "T-nuts" to keep everything together.
The printer uses belt drive for the X-Y axes, and a hefty leadscrew for the Z axis. Here are the main guides that transmit force to the printer head. A single, long bushing is pressed through each, that looks about 3X as long as the diameter of the shaft. Saint Venant strikes again! The belts are tensioned using those little clamps sticking out the sides.
Each axis has a pair of these guides, spaced far apart. Each slides along a shaft that also transmits motor torque. The axis shafts are held at each end with a radial bearing that are held in place with a pair of plates (shown above disassembled).
Like my FASBot clamp axes, the Ultimaker transmits motor force to one direct leader slide and one follower slide through a belt transmission. Using this construction, force can be applied to both sides at once, preventing jamming. Each pair of slides pushes another shaft sideways, causing the printer head to move. The printer can still slide along this shaft, but is constrained by an orthogonal shaft coupled to another pair of slides.
The build platform is the carriage of the Z axis. It has two beefy sides to allow it to be cantilevered without much deflection, and a pair linear bearings allow it to move only up and down.
The bearings are held in place with stacked parts. Note that the length of this bearing is about 3X the diameter of the guide shaft. Saint Venant!
The leadscrew is constrained at the bottom using a rigid shaft coupling that attaches a stepper motor located underneath the printer bottom plate. A bearing supports the shaft coupling, and thus the leadscrew, axially. The leadsrew is not constrained at all from the top, which would allow it to. This is acceptable, because the printer process is only sensitive while the bed is moving downward, and this occurs in tiny steps of 0.001" (~ 20 microns) at a time. The only forces the leadscrew will see are downward gravitational forces.
So for my FASBot, I decided to use this electric screwdriver with a nut driver attachment to performing my fastening. This is my machine's "spindle", though I've modified it in order to better fit into my machine. First, to reverse-engineer it!
The whole thing comes apart in four pieces. One piece, the black one to the left, contains the tool holder and a planetary gearbox. In the middle, an orange housing separates into two halves, which houses the drive motor, a triggering mechanism, and the detachable battery pack. Because my machine will have its own power, I can chuck the batteries, which eliminates the need for about half of the orange housing.
These pins press into the orange housing sides and force the two halves to mate.
A flexure provides the electrical contacts between the battery pack and the trigger mechanism on the rear side of the motor.
Flexures also keep the battery pack attached to the orange housing. Press both ends in, and the battery pack detaches. Push the battery pack back into the orange housing and it'll click into place.
The sun gear of the first stage in the planetary geartrain is attached to the motor shaft. You can see plenty of lubrication inside where the plastic gears live.
You can see here the U-shaped pin that passes through two holes that go through the black gear housing and the orange housing, to couple the two together.
This is the trigger mechanism in back of the motor. Push the trigger down, the motor spins one way. Push the trigger up and the motor spins the other. I looked into the mechanism and saw that there are flexural electrical contacts, one power, one ground, that the trigger interchangeably applies to the motor terminals to change the motor spin direction.
Very cool! I will design a custom piece that holds the motor and mates with the black gearbox housing.
So my passenger-side mirror got busted on my car. Upon replacing it ( for only $25.00 on Amazon Prime and 10 minutes to repair), I decided to take it apart for my Seek and Geek! Making the most out of a busted mirror!
Here it is in its temporary-fix Gaff-taped glory.
I used a chunk of a sheet or mirrored acrylic as a temporary fix so I could legally still drive (and practically still see behind me!).
Putting the "Why?!" back in DIY (DIWhy?!?!?!)
The whole mirror assembly comes off with three self-tapping screws for plastic. There is a connector with three conductors, presumably for Motor1/Motor2-, Motor1+ and Motor2+. Using these three, you can spin both motors forward and reverse, but only one at a time. Foam gaskets are glued to the inside to dampen noise and catch any water/dust/roadstuff that may make its way in there.
The glass is (was) connected to this inner face. There is a single snap-fit ball joint in the middle, about which the mirror pitches and rolls. Two additional smaller snap-fit ball joints, one on the bottom, and another on the right, are connected internally to the outputs of the motor assembly.
A better view of the middle ball joint, and the two ball-joints that connect to the motor outputs. The balls are at the ends of linear actuators. The motor assembly pushes or pulls the bottom ball of the mirror plate to aim it up or down, and pushed/pulls on the right ball to aim it left-right.
A closer view of the motor assembly, with the big middle ball joint which acts as a pivot point for the mirror plate, and the two linear actuators, each with a ball end. Grease is applied liberally as lubricant.
Upon taking the motor apart, you can see two small motors with identical transmissions that do a few things.
First, a worm drive right out of the motor enables it to apply plenty of torque onto a long idler gear. This idler gear then attaches to the actuator output gear, which is threaded onto a screw. As the long idler gear spins, the big output gear also spins, and the ball output moves up or down along the screw. The idler gear is long enough to continually engage throughout the length of travel.
(Idler gear removed). Here is the output threaded down to its lowest point.
And here is the output stage threaded up to its highest point, about 0.75" of travel.
Here is the plastic screw on which the output shaft spun.
Taking a look underneath the Five Elastically Averaged Flexures used as the leadscrew "nut"! Smart! There are no limit switches or feedback control on the mechanism, so it isn't a servo, just an open loop motor. With these flexures, when the shaft output (the mirror angle) reaches its limit, the flexures move out of the way and it can click down or up along the plastic screw without causing damage to any device. BRILLIANT!
The most recent revival of the Battlebots TV show on ABC was pretty exciting to watch, and this year there are a few teams affiliated with MIT that will be competing in the 2016 season. I'm happy to say I'm helping out a bit with one of those teams, a bot called The Dentist. While most of the design has been done already by the core team members, I've been helping out with some machining and general integration.
I'd like to take a look at some of the most successful and worst robots from last year's season, analyze what worked, and what didn't. While it's too late for me to make use of these lessons in designing The Dentist (I was roped in after most of the design was done), I think this will be useful for engineering in general, if not for a future battlebot.
Successful Bot: Tombstone (or Last Rites)
Tombstone's Strategy: Have the best weapon, maximize its kinetic energy.
Defending against Tombstone: be fast to avoid impact, have armor designed to take a direct impact, have enough traction to stay put when hit
Tombstone (and its previous iteration, Last Rites) is ridiculously powerful. It chews through anything by having as much of the total 250lbs allowed in the bot as possible in the weapon. The more mass the weapon, and the faster it spins, the more kinetic energy it has. The more energy you have, the harder you can hit, and you can brute force your way through any competitor.
The issue is, however, Tombstone needs to be able to handle its own impacts. If it hits another robot and sends it flying, it may be able to handle the reaction impulse, because the rotor will not stop spinning. In the slow motion video above, you can see it chew through furniture with minimal reaction force (the furniture just melts away).
If Tombstone hits a wall, however, the rotor has nowhere to go because it cannot push the wall away, and so the robot takes the brunt of the impulse and sends itself flying. The bearings and structure need to be able to deal with this, as well as the electronics within. When Tombstone is successful, however, it is terrifying to watch, like when it went up against Counter-Revolution.
Speaking of which...
Unsuccessful Bot: Counter-Revolution
The gist: weak weapons, armor cannot handle impacts, weak drivetrain.
This sad robot looked pretty cool at first. It had two vertical spinning weapons, but they never even got to do any damage before Tombstone came in and made quick work of the thin aluminum armor. The drivetrain was also not well-designed, as it is unable to provide any traction if the bit is lifted up at all, and is too weak to be able to maneuver and dodge Tombstone.
Successful Bot: Bite Force
The gist: RELIABLE. Strong armor, strong drivetrain, re-configurable weapons.
Bite Force ended up winning the overall competition! It has a beefy frame of either steel or aluminum, which takes a beating and never lets up. I don't think it ever took any actual damage throughout the entire competition. In this video it's up against MIT's Overhaul, and while they have similar designs, reliability is KEY. Bite Force can lift other bots without ever losing traction due to reliable design. Bite Force can take a monstrous beating without ever having a part break because the armor is thick as hell and is well-designed. Bite Force's lifter can be strategically reconfigured depending on the immediate competition to either be a lifter primarily, or a wall of steel with very few appendages.
Here is the championship fight, and Bite Force was a match for Tombstone because it was the worst thing Tombstone could possibly come up against: a wall of steel. Tombstone is normally relentless because it can chew through most armor it comes up against, but Bite Force has been designed to take a worst-case direct impact from Tombstone without taking any permanent (yield) damage.
Neutral Bot: Witch Doctor
The good: Vertical spinner with high kinetic energy: can throw around Tombstone. Nice front armor.
The bad: Unprotected wheels, no means of self-righting when flipped upside-down.
Witch Doctor is pretty cool, because it gave Tombstone a run for its money. It actually broke Tombstone's main weapon! Its compact vertical spinner was effective in throwing around Tombstone like a toy, and the surrounding armor held up quite well against Tombstone. Its cantilevered wheels were right out in the open and unprotected, however, and it was unable to self-right when flipped over, leading to its loss against Tombstone.
The other day I saw a Hydaulic Cherry Picker like the one above parked between Kresge and McCormick which had been used to put the "MIT 100 Years In Cambridge Celebration" posters up on Building 7.
The whole thing has a telescopic arm made of square extrusion. I couldn't get a good peek inside of the mechanism it uses to actuate the telescoping mechanism, but considering everything else on this machine is hydraulic (which is good because there is only one power source that is distributed across the whole machine) I bet this is probably actuated hydraulically, with the piston motion amplified in some way through gearing or a linkage.
Speaking of linkages, the whole thing has a "wrist" that's on a parallelogram 4-bar linkage, so it keeps the basket holding any operators parallel to the ground.
A workspace analysis found online shows off just how much reach this thing has. The wrist allows fine positioning once the big arm has been extended.
The big arm is actuated by a massive hydraulic piston in a 3-bar linkage. The forces it has to exert to hold the arm in place change given the angle of the arm, and the horizontal configuration requires it to bear the highest load. Luckily, static force holding is something inherently free in hydraulic systems!
You can see the cable/hose carrier, bringing both electronic cables and hydraulic hoses up the arm to the the pistons located near the basket.
The wheels are hydraulic! Both drive and steering are achieved through hydraulic actuators. This thing is really slow when driving, no more than 15MPH, but at least no additional forms of power are needed. The same hydraulic power unit is used to move the arm, the basket, steer, and drive the wheels.
There are two pistons that orient the basket! One is coupled to the 4-bar parallelogram linkage, and the other will simply change the angle of the entire basket with respect to the arm. This is probably coupled to the motion of the arm's beefy main piston near its base, so the basket is always oriented horizontally.
There also is a left-right rotational joint at the basket's wrist, also hydraulically actuated.
