Here’s a little teaser of a current project that I’ve been working on.
If you take a delta robot and replace the parallel linkages with the a pair of parallel wires wound over motor pulleys then you get a robot that can scale up to giant size. Like the size of a giant room. Yeah, it gets big. I’ve got it as big as a triangle 20m×19m×12m and 5m high so far, but it could go bigger!
I’ve been describing Arduinos as a “gateway microcontroller” a lot while introducing them to new users. They’re great to get started with, you build your first project include your first Arduino board in it, and then you have to go out and buy another one. And then you’re hooked on Arduinos!
When you’re just starting with Arduino, before you’ve had time to build up your ever increasing collection of microcontrollers, this can seen uncommonly expensive: you have to go back and buy another £25 board, just to include it in your next project and have to spend another £25. But it doesn’t it doesn’t have to be like that…
As well as having other suppliers who produce Arduino compatible boards that are cheaper, the design is open so you can take the components and reuse them in your own circuits. You can ignore the physical layout part of the Arduino and layout the components on a breadboard or include them on a complete board of your whole project as a standalone device.
Stripping back the Arduino to the bare minimum of components is known as a “barebones arduino compatible”. The minimum possible circuit to run is the bare ATMega328 chip with a Lillypad bootloader, although to be practical, most circuits will need a few external components. One project that strips the Arduino down to a minimum and is made to be suitable for beginners is Shrimping.it.
I’ve taught Shrimp kit building to beginners from 8 year olds to adults and it’s surprisingly easy to get on with. Push the components into a breadboard, connect to a USB port on your computer for power and it will start flashing its LED. If you open the Arduino IDE, open one of the example sketches and press upload, you can start building the Arduino examples just the same as if you had the official blue board. Continue reading Shrimp-on-a-shield
Reading the contents of a coffee machine has precedent on the internet; the inspiration for the very first webcam was the Trojan Room Coffee Monitor that was installed in Cambridge University Computer Laboratory from 1991 to 2001. So now in DoES Liverpool, we have an online record of the amount of coffee in the coffee machine from the weight measurement of the machine, pots and water.
A set of bathroom scales lives underneath the coffee machine in the kitchen, connected to an Arduino Ethernet that measures the weight, calculates the number of cups of coffee in the machine and sends the data out to Xively. All the measuring electronics of the scales has been bypassed and the Arduino measures the mass of the water in the coffee machine by reading resistance change from the strain gauges at each corner of the scales, via an INA125 instrumentation amplifer. Detail on connecting the instrumentation amplifier to the Arduino is shown in the ArduinoInstAmp repository in GitHub and I’ve written about it previously.
As part of my role as DoES Liverpool’s technician, I get to give introductions to new users to get them started with using DoES’s laser cutter. However, because it’s only an introduction I only get to spend an hour or so going over the basics when I’d like to spend longer and help them on their project in more detail.
So, to get around this, I’ll be running an introductory laser cutting course in Liverpool to help more people get started with using this technique for cutting their own designs. It’s a course for everyone; laser cutting is a very accessible process, and if you can use a computer, draw out a design and press the start button on the laser, then you can use a laser cutter.
21st September 2013
Would you like to get started with using a laser cutter?
Do you want to be able to produce your own designs and cut them on a laser cutter?
Then this course can help. You can book online now at defproc.eventbrite.co.uk for just £69.55 including materials. The minimum number of tickets sold for this course to go ahead is 1. If there’s an attendee, this course will run.
Starting from the basics of laser cutting and operating a hobby laser cutter, this course will take you through preparing designs for cutting or engraving to making your own items.
How a laser cutter works.
Cutting, line engraving and fill engraving,
Using Inkscape to produce cut-files
2D to 3D, joining and jointing techniques.
Concentrated help and support for protyping and refining your own designs
This course is much more in depth than the normal introduction I give to new users of DoES Liverpool’s laser cutter, so at the end of the day you should be proficient enough to come in and use a hobby laser cutter by your self.
One of the smaller projects that I took to Maker Faire last month was a strain gauge reading circuit for Arduino. While the Arduino in the demo setup was actually redundant (the voltage input from the scales was output to a 5V dial gauge directly) there was also a circuit diagram setup to show how to use an instrumentation amplifier to boost the voltage output from the strain gauges in a set of supermarket weighing scales to a value great enough that the Arduino could read it as an analogue input.
One of the reasons I took this to Maker Faire UK was because I couldn’t find a guide to connecting strain gauges to Arduino that said “here’s the amplifier that you need, here’s how to connect it and here’s how to take readings”.
I’m sure you’ve seen the write up from Mudlark and the blog post from BBC R&D about their new Perceptive Radio that localises content and adjusts playback characteristics depending on listener interaction. I was approached by MCQN Ltd as the electronics part of the radio was nearing completion and asked if I could help with creating an enclosure that would portray the project well as a radio-type device.
While I am expecting to get the clearance to open-source the case design the same way that the electronics and code has been, in the meantime I can show you some of detail that went into the case build and some of the laser cut parts that place the components.
If you want to see how the perceptive radio works, the radio play is available online at futurebroadcasts.com
This coming weekend, Maker Faire UK is at the Centre for Life in Newcastle. I’ll be exhibiting as part of the DoES Liverpool stall this year.
While I initially started going to DoES Liverpool, it was to get some hands-on experience with running a laser cutter, which would improve the work I was doing with the Atomic Duck. Since the start of this year, I’ve been working as the technician for the co-working, workshop and makerspace.
Along with helping users get started with the laser cutter and the 3D printer in the workshop, I’ve also been able to do some really interesting smaller projects too and I’m looking forward to showing some of those off at Maker Faire this year.
Included on the stand from me will be:
The clip-together “Clip-R-Pi” >Raspberry Pi boxes showing the integrated that I developed.
A £6 Arduino-shield-compatible board based on the shrimping.it project (a £3 barebones arduino clone-on-a-breadboard).
The Doodlebot-Pro, a more permanent version of the pens+cup+tape+vibrating motor robot. This version has a laser cut chassis with 120 degree finger joints and integrated elastic clips so it can be assembled and disassembled in seconds.
An animated, 90 LED wordclock that is driven from 10 digital pins on the Arduino using Charlieplexing. Also there will be a demo circuit showing the charlieplexing schematic and the persistence of vision effect.
Arduino scales built using an instrumentation amplifier to get readings directly from the strain gauges in a commercial kitchen scales.
I’ve previously shown for laser-cut elastic-clips for comb joints what the equations are for calculating maximum bending stress and operating force, using the distance that the clip has to deflect in use as the starting point. I also explained how a tapering clip profile was preferable to a straight one. This time I’m going to share an example of a practical clip geometry that I’ve been using regularly to hold small parts together in 3mm acrylic.
While the clip is useful as a component of other parts, it’s helpful to see how it is used to make something practical, so I’ve also made a clip-together box that can be completely made from laser cut acrylic, with no other parts or tools needed. Both the clip geometry and the box are available on thingiverse for you to download, use and remix.
As a measure of how robust the clips that hold the box together are, we’re now also producing some Raspberry Pi cases using a derivative of the box design above. Manufactured in Liverpool, in the UK, we have some of the first batch available to buy in the shop now. (More on the Clip-R-Pi cases next time)
During a makerday, I saw a someone struggling to put together a Raspberry Pi box that they’d laser cut, and were trying to hold together:
two pieces of acrylic to make the joint,
and the Raspberry Pi board,
and a nut into a slot,
while also attempting to screw in a bolt,
and not drop everything.
So I started thinking about if it would be possible to extend the work I have been doing on making flexible areas in acrylic to make a clip mechanism that could be laser cut to make self-fixing comb-jointed parts.
A comb joint (also called a finger joint) is a carpentry joint that has come to the attention of makers with the increasing accessibility of laser cutting. The finger joint can be used to make boxes from laser cut sheet materials (there are a couple of automated tools available including BoxMaker and Box-o-Tron). It is relatively easy to implement and the fitting of the crenelations gives much better alignment of the joining parts than butting together the edges of the adjoining sheets (this is a butt joint). Because “complexity is included” with laser cutting, the cutting effort needed for a comb-jointed box is not much more than for a straight-edged butt-jointed box.
However, to stay as one piece, the comb joints must either be friction fitted together or glued. For both these methods, the mechanical strength of the joint is limited by the relatively small area of the mating surfaces. To give the joint greater strength, one set of combs can be closed, so it is mechanically supported in 2 directions (which then makes it a mortice-and-tenon joint), and if a captive T-slotted nut and bolt is also included (this effectively “closes” the other set of combs — making a bolted mortice-and-tenon joint) the joint is well supported against movement in all directions planar to the component plates. Variations on the t-slot and nut exist, such as the using delrin clips, and while they are very robust, they all require hardware in addition to the laser cut acrylic.
Flexible plastic clips are a staple of contemporary product design, just look at your phone; there’s almost certainly a version of a moulded plastic clip that hold the parts of the case together. If you’ve got an older/non-smart phone, then it’s quite possible that you also have moulded button for the back panel with a living (elactically deforming) hinge in. Having already demonstrated that it is possible to make acrylic flexible with a lattice cut living hinge, I investigated how to cut acrylic into an elastic clip that could be used to secure a comb joint.
Elastic Clip Geometry
I’ve been calling this an elastic clip because of it’s structural properties. To operate successfully, the material must only be operating in the elastic-region of it’s stress/strain capabilities. Under elastic deformation, once any force is removed, the material will return to it’s original shape. If the yield stress of the material is exceeded, it enters plastic deformation where there will be a permanent change in the shape of the structure after all force has been removed; because the applied force was great enough to start permanently re-aligning the molecules that make up the material. For a brittle material, such as acrylic, the difference between the yield stress and ultimate stress (the absolute maximum it can sustain before it breaks) is very small, so for a clip to stand repeated use, the maximum stress in operation needs to stay well away from the ultimate stress of the material.
Any kind of integrated clip is going to take the form of a cantilevered beam, where the operation of the clip bends the beam along its length, until the clip is “open”. Having a back stop will limit the size of the maximum deformation of the clip, and therefore the maximum stress it will experience, so the limit of motion gives a starting point for calculating maximum operating stress.
36MPa — While this is a low enough stress for normal, gentle handling and can bend to 90 degrees; this is likely to break if mistreated, and does not bend much beyond 90 degrees before breaking.
20MPa — Better than the 36MPa sample, this one can easily bend to 90 degrees, but may break if the sample is bent as far as 180 degrees, especially if the sample is cool.
10MPa — More robust again, this can bend comfortably to touch both ends of the sample together, but is noticably less stiff than the 20MPa sample.
The minimum internal bend radius for 3mm panels with square cross-section links so the inner links do not bind was shown to be 44mm, so the test samples includes a 44mm radius corner.
The samples have 3 different sized hinges, where the torsional link length varies to affect the maximum stress that those links experience in a 90 degree bend (the design specification). The different lengths also affect the stiffness of the hinge too, so the longest (28mm) sample is much more flexible, and allows the hinge to twist slightly when handled as well as bend, though the lower stress give a much more robust hinge that can deal with rougher handling without breaking. By comparison, the stiffest hinge (8mm links) may break if moved too quickly or at too low a temperature; though it may be suitable for permanent or pre-assembled structures and the reduced length may be an advantage for use in shallow structures.