Fighting with computers

One of the things I did not like about a new printer was that the belt that moved the carriage was not laying parallel to the axis of motion (left) but at an angle, as shown in the picture below (right): Such a belt path is not right as for each inch of motion in the axis a slightly different amount of belt is moved (the hypotenuse of a triangle instead of its long side). Most of the time, the angle is so small the difference is tiny. That is why the arrangement works and printers are printing even though they have this ugly hack. The problem is worse when the carriage is close to any of the two ends, as then one of the angles is not so small and that creates a bigger error. But the question is: how big the error can be? Well, it took me a good part of a weekend to figure that out, among other things because wxMaxima does not like to calculate the intersection of two circles easily . So the first thing I needed was to model the anomalous belt path in a geometric model I co...

Some of my ongoing projects required a larger 3D printer and I saw a good deal on a printer that had good reviews (except for its weird bed leveling system) so I bought one off Anycubic's Aliexpress shop shipped from Germany. I got in in less than a week and putting it together took me a bit more than one hour. It is basically two parts, shipped flat, that you place at a square angle to make a Prusa i3 like configuration. The printer comes with all the tools you need, including the wire cutter to use to remove the zip ties that keep the bed from moving while in transit. The 400x400 mm bed features glass on top with a special texture that does what it promises: good adhesion while hot and easy release of parts when cold. These two brackets will reinforce the vertical axis beams fixed to the horizontal extrusions. Once built, the manual called for a manual or automatic bed leveling. That I did not like as it seems the user has to decide which one is best. Someth...

Some time ago I developed 4xiDraw , a 2D drawing machine using a single belt, a 3D printable clone of AxiDraw(tm) pen plotter. Instead of the EBB board they used, I opted for an Arduino-based solution, with a CNC shield and regular Pololu-like stepper motor drivers. Initially, I started using various Inkscape plug-ins that will convert the drawing into a G-code file. Once the G-code was created, I would use the UniversalGcodeSender program to stream it to the drawing machine. That worked ok, both wired and wirelessly (using Bluetooth). But later, a kind user named Torsten Martinsen migrated the existing AxiDraw plug-in to the 4xiDraw hardware controller. This way, plotting happened right inside Inkscape program without the need to messing with external files or other additional software. Very cool indeed! I left the project living its own life, with a lot of users creating their own version of it and doing all kinds of interesting hacks, till I received an awkward request fro...

I recently attended a one-day tutorial about Pynq by Xilinx. While it was on planet Earth I was not familiar with some of these names that sounded alien to me. But I was decided to get some update about the new FPGA offerings by Xilinx company. I am glad I did as I discovered a wide range of technologies I know nothing about that I reckon can be pretty useful. Unfortunately, becoming an expert on any of them will most likely require many hours of training. But I can share with you some of what I learned: Pynq is an open-source project by Xilinx to make it easier to use the programmable logic included in some of their SoCs. And that brings us to the second name I have never heard about: Zynq-7000 is a system-on-a-chip (SoC) by Xilinx that bundles an ARM microcontroller with a large FPGA.  In a nutshell, Pynq is a small Linux distribution that runs on a Zynq SoC with access to all the power of the embedded FPGA. But I guess the question here is what this can be useful for. Ri...

If you build a piece of interactive Art with an Arduino, there is not much harm you can cause it by just powering it down anytime. But when you use a computer, either a PC or a Raspberry Pi, bad things may happen if you just switch off the mains power unannounced. A common occurrence is that you are asked by the museum or exhibit managers for instructions on how to properly switch off your interactive Art installation. Experience shows us that the more difficult it is, the more likely one day will be done improperly. And I really hate when a piece that took a lot of effort to create goes to waste because of a problem triggered by a "rough" shutdown.  So one idea that has worked for us is to set up our system as a wireless access point. The person in charge of shutting the system down will use her phone to connect to our wireless network using a password we provided. Once connected just visiting a specific address and port number will cause the shutdown (first time th...

I have been working a few days improving an existing project based on a Raspberry Pi. It is a art piece with a vertical plotter that draws lines on with a pen. Software and data were already developed and I was just adding some finishing touches. I was given a new RPi3+ board to upgrade the system but when I connected the new board with the existing SD card the system did not boot up. As our system did not include an HDMI display but an embedded one, we did not have display unless the system could successfully boot. My first thought that then new board could be more power hungry than the existing one and therefore the existing power supply was not enough to keep up with the demand. I had to put back the older board and left the one for further testing. Once I managed to get an HDMI display connected to the board, I could see the image above. Not sure whether it was some Harry Potter add on the upper left corner or, maybe, someone trying to tell me there was something wrong with ...

A few weeks ago I mentioned a new Android app was available to assist during the assembly of projects made of blocks of stacked layers of material. That will not be complete with a similar app for those of you own Apple cellphones and iPad tablets.  For the moment the app is available for iOS 12 devices . Supporting older versions has not been a priority as we did not want development to lag behind but we might venture there if we get enough requests.  As with the Android app, this version is free too. A sample model is embedded so you can see how it works (go to the gear menu).

Collaboration with a local company paved the way to the development of an Android app to represent 3D models that are built by joining a set of 3D blocks. Each block is made of a set of layers of parts machined by a 3-axis CNC machine out of foam sheets.  The manufacturing process is more or less outlined in this video: The problem here was for work workers to have an easy way to know the assembly of the parts without the need of having a computer screen. Given each one of them happens to have a smartphone that seemed the obvious choice. And that is the result (portrait video warning): As you can see in the video each block has a red number hovering it. And the low polygon count of the 3D mesh is just to ease on the amount of data transmitted and not a limitation of the program itself. I wanted to make this tool  useful for other users, so the file format is a regular ZIP file with .mim extension that contains three different elements: one binary STL file for each...

Algorithms for free-form nesting has a cost that depends on the complexity of the polygons that represent the parts to be placed. The more complex (the more points) these polygons are, the longer the computing time. One might accept a slight waste of material if that can speed up the nesting significantly. The obvious idea is to simplify the polygons and for that task, there is a considerable amount of algorithms available. However, there is a bit of a catch in our case: If the simplification process renders a new polygon that is slightly smaller than the original one, this could be a problem as the actual part will not fit in the space allocated for it by the nesting software. The simplification performed by the red arrow creates a new polygon that, while it has fewer points, it cannot contain the original polygon inside and so it would be a bad idea to use the new for nesting purposes. On the other hand, the green arrow simplification does create a new polygon with fewer p...

I was looking for a simple mock-up to be used to explain the operation of a stepper motor to my students. This is what I came up with: Just four coils on the edges of a plus-sign shape, where the two horizontal coils are connected in serial in a way that they create opposite polarity fields. Same arrangement for the two vertical coils. So we end up with a vertical and a horizontal coil. Once this is set, we use a pill-shaped magnet that will align with the vertical or horizontal magnetic field created by the coils. What is even better, current is ramped-up on one axis while it is ramped-down on the other, a smoother movement can be achieved (aka micro-stepping). Of course, that pill-shaped magnet will look familiar to you if you ever have been to a chemistry lab as they are used for magnetic stirrers. So for powering this "motor", I have used a stepper motor driver A4988 and as you can see, if field changes are fast enough, the rotor can spin and even stir a liq...

To determine the projection of a given 3D part, I am slicing the parts in a sequence of cuts at different heights. Each cut creates one or more perimeters or rings. The approximated desired projection is obtained by performing the union of each cut.   The image above represents the output perimeter in red color, and the number tells me there is just one. Each perimeter start is marked by a circle and its end by a square. The image below, however, does have two different output perimeters, that is, the vertical projection can be decomposed in two different disconnected areas.  I am coding in Java and using the Area class from java.awt. The main reason is that the class does include a method to do the union of areas.  However, after some weird results and some debugging, I realized the output of such a union method is not always clean. Sometimes the output is polluted with some additional paths that are not really useful or valid. In my case, I was getting some ...

In the previous entry, I discussed how the silhouette of a cut, which would be a closed polygonal line, could be turned into a triangulated surface so these triangles could form a closed lid for the sliced triangular mesh. But there are many cases when that surface is not that simple as a closed polygonal line. So the general case is a little bit more challenging. Let us see some sample cases in the image below: So the simple case I talked about would be the circle label with the number 1. But the rest of the cases 2,3, and 4 are a bit trickier. Case 2: We have a hole inside the exterior cut. That would be the result of cutting a tube-like shape. Here we do not want to cover the whole circle as in case number 1, but we want to keep the hole uncovered. Luckily, the Earcut library handles the cases where the external polygonal line encloses an area with one or more holes. But we still need to tell the library which points delimit the external perimeter and which ones are the ...

Once a triangular mesh is cut by an intersecting plane, the resulting halves are no longer a water-tight mesh, because some holes are created by the cutting action of the plane. In the image here, the volume of a [solid] hat was cut by a vertical plane, splitting it in two. In order for this half to be back a 2-manifold triangular mesh, all the triangles with black edges have been added to create a flat surface that covers what the cut plane would otherwise reveal (the inside of the hat). The problem is how given a certain polygon, like the red polygonal line here, a collection of triangles can be created to cover entirely all the area delimited by such a perimeter (aka polygon triangulation). The problem gets even more interesting once we consider that some meshes can present holes inside these perimeters obtained after cutting the mesh with a plane. Fortunately, there are different algorithms to solve that problem, and I will settle with the so-called Ear clipping . Fortuna...

When we use a plane to cut a triangular mesh, several cases can be considered. If there is a non-null intersection between the plane and a given triangle, there are mostly five different cases: The plane contains one vertex of the triangle: not very useful for cutting purposes if the rest of the triangle lays above the cutting plane and we want to keep all that is below. The plane contains two vertices of the triangle, then the intersection is a line segment but we will only keep that triangle if the other vertex is below the cutting plane. The plane contains three vertices of the triangle: that is a keeper. The plane does not contain any vertices of the triangle, but there is an intersection with two of the edges: this is what we will talk about today here. A fifth case can be considered when the plane contains one vertex but the two other are on opposite sides of the plane, and it can easily be incorporated to the previous case. The following picture shows us a couple of t...

I recently gave Adafruit's IO service a try. You can get a few data sensor feeds recorded and graphed on the web. They have a somehow limited version for free and a more complete offering for paying customers. A current project required to record a few sensor samples per minute and Adafruit's free service seemed to fit the bill. I was using ESP32 with BME280 sensor to gather humidity, temperature, and atmospheric pressure and to store it somewhere in the cloud. I was testing this on campus, where we have a corporate wifi network that uses a Radius server for PEAP authentication. I assumed ESP32 could only use WPA with a pre-shared key but I was wrong. I googled around and found that some people were using it with Eduroam . Eduroam is a European-wide Academic wifi network and it is present in our campus too, so what harm was in trying? I found this code that might work so I tried it out, and it did work at the first try! Brilliant! But there were a couple of detai...

Not being an expert in motor control, I created dcservo  project to make a basic platform to create cheap servomotors that could replace stepper motors on certain systems, like 3D printers. I chose to implement a full PID controller but that might have been a bit overkill. I was talking today to a more knowledgeable colleague who asked me why I needed a PID. My answer was that without the integral action I was getting a significant position error in the steady state, and the derivative action, though a really small value, was needed to keep the system from overshooting. However, he suggested me to focus only on a simpler, proportional controller with a twist: most of my position error is coming from the fact that there is a dead zone where motor does not respond to control action. It is true, though the motor will move faster the higher the PWM value it is applied to the motor drive, it is also true that low values (between 1 and 50 in my test system) do not cause the moto...

Bugfix-1.1 branch of Marlin firmware does include a fifth-degree (quintic) Bézier polynomial for the speed profile. If you want to use it enable S_CURVE_ACCELERATION define. You can have a look at the source code , luckily well commented so we can understand where it is coming from. I was curious to check how different that curve was from the sinusoidal speed profile I mentioned in a  previous entry . Although the source code comments are very thorough on how they reach their expression, in layman's terms is something the speed along a move is s(t)= (6*t 5 -15*t 4 +10*t 3 )*|v 1 -v 0 |+v 0 where v 1 is the final speed, v 0 is the initial speed and t varies from 0 to 1 to trace the motion. I was surprised on how it looked quite similar to the sinusoidal case, so I went on and did a comparison with the sinusoidal s-curve: And the result is that there is not much of a difference so I don't expect much different behavior when using one or the other. ...

A local company built a ram extruder for us. Not very different from a syringe extruder but massive in terms of size and pressure. Our assumption was that parts had been sized properly, so we did not question the 8.7Nm NEMA34 stepper, nor the 5mm-thick acrylic tube with a 60mm internal diameter. After all, it was not the first time they have built a similar contraption to be used with clay. Unfortunately, though we try to be very gentle on the extrusion test, we manage to break the tube in a few minutes. Luckily for us, it cracked without sending debris around. We did not expect that at all and when we contacted the seller all sorts of questions were asked, questions we did not have an answer for, like "what was the operating pressure?". I did not have a clue as I was just trying to get the stepper rotating very slowly. Their diagnose was we did it wrong as we set the nominal current (6A) for the stepper driver. From torque to pressure So I was determined to figure ...

In previous entries, I have delved into the idea of using different shapes for the speed pattern for a motion using steppers and have presented how the timing can be based on a look-up table. Such a table would be pre-calculated and flashed with the firmware. But a single table provides a pattern for a given max speed, acceleration, number of steps and speed shape. You can use that for timing the steps of the motor and it will accelerate properly as many steps as entries on your table. But it won't be practical if we want to change any of these parameters for other movements. The idea I am not yet sure this works as expected, but first tests look ok, finally, as it took me a while to figure this out. I have created a lookup table as the graph above, where each entry contains the delay needed for each step, for a movement of 1000 steps total, where speed will increase to 2Π while time goes till 2Π too. Maximum speed for each different motion pattern will be different th...

Though I have been experimenting with Arduino, I realize that as soon as the computing requirements grow a bit the results quickly show the processor is actually slowing down my results. So now I am using a DOIT Espduino featuring an ESP32 processor that makes that sensitivity go away. It was a bit of a challenge to get this board to work with CNCshield v3. The culprit was resistor R1 (enable pin) that would affect the booting capability of the ESP32. Once R1 was removed (brutally destroyed with a wire cutter in my case) the problem was gone and I could upload and run code with the Arduino IDE. I created a simple program that uses some pre-calculated delays to perform the motion using different expressions for the acceleration. Four have been used and shown in the video below: Uniform acceleration a=k Sinusoidal acceleration a=1-cos(t) Exponential a=(e^(cos(t/2)-1)*sin(t/2))/2 Exponential2 a=(e^(-(t-sin(t))/pi)*(1-cos(t)))/pi The expectation from a former entry was t...