MakerBlock

Drawing Robot Pen Holders, Calligraphy Pens, and Thought Experiments

In discussing Sandy Noble’s Polargraph pen holder I mentioned how his design is optimized so that the point where the two cords meet is always the same as where the pen tip meets the paper.1 In the comments, he explained his rational, “So the pen tip is always at the tip of the hanging triangle, and there’s no distortion that way.” My response was that “…if the pen holder has a single ‘hanging triangle’ point in it with the pen tip a constant distance from that ‘hanging triangle point,’ the drawings should appear identical to those created at the ‘hanging triangle point’ – just offset by the constant distance.”

This morning Sandy updated his site with a post explaining, using several diagrams, the basis for his prior theory and how he came to agree with my point. (Not that he’s changing his pen holders… ;) )

Without as much fancy-schmancy maths and geometry, I figured I would explain the thought experiment I used to conclude that a pen tip that is always a constant distance and position from the “hanging triangle point” will always produce an accurate distortion-free drawing. To help illustrate these thought experiments, I’ve enlisted the help of Yoda. “Hi Yoda!”

Fig 1: Yoda, being drawn by a drawing robot

In the picture above, Yoda is being drawn by a drawing robot.

Above, I’ve labeled the important parts of the drawing. On the top left “Motor A,” on the top right “Motor B,” which are attached by cords to the pen holder indicated by the dark blue line. Here, I’ve shown Yoda as he would be drawn by a drawing robot, where the robot then draws two more points.

Let’s say, because we’re feeling whimsical today, we want to add a second pen to our pen holder. We’ll use a red pen and affix it below the blue pen in such a way that the red pen will always be directly below the blue pen by the same distance.

For the moment let’s pretend the red pen is capped so it won’t leave a mark. Now we’ll try to predict the position of the red pen at different points along the original drawing.

Fig 4: Where's the red pen? — Fig 4: Where’s the red pen?

It turns out this task is pretty easy. The red pen, at any given point during the Yoda drawing, will always be directly below the blue pen by the same exact distance between the two pens. Okay, now let’s draw Yoda again – this time with the red cap off.

We get two Yodas! How awesome is that! The reason I mentioned calligraphy pens in the title of this post is because it shows another way to think about this process. When we write with a calligraphy pen we don’t have one end of the pen wildly distorted – in theory the two points on the calligraphy pen are always a constant distance from one another and moving together (as long as we don’t rotate the pen when we write). You could imagine instead of a blue and red pen above, we’ve put a single calligraphy pen that’s as wide as the black line representing the distance between the two pens above. The resulting drawing would look like a Yoda – that had been smudged downwards by the same distance.

Let’s now draw Yoda again, but capping the blue pen and still tracking where the blue pen would be.

We should end up with a result very similar to Fig 4. It’s the same Yoda, only red and shifted down from the original blue Yoda by the distance between the two pens.

Let’s draw Yoda again – this time we’ve still got a pen holder which has the cord from Motor A meet Motor B at exactly one point. As Sandy points out, this is really easy to do when you aren’t worrying about making that exact point be the same precise point as the pen tip. Directly below point where the two cords meet on the pen holder, we’ll put the red pen. From a functional standpoint, this setup is identical scenario to Fig 6.

Now we have a red Yoda, shifted down on the paper by the distance between the point where the two cords meet and where the red pen touches the paper. It’s important to note that there’s no special magic to having the red pen directly below the point where the cords converge. This pen tip just needs to be a constant distance and position from the cord convergence point at any given time. While it might be more difficult to build a pen holder that holds the pen far off to one side, there’s no reason this wouldn’t work.

The lessons I take from this thought experiment are:

As long as the pen is a constant distance and constant position from the point where the two cords meet, your drawing will not appear distorted – just shifted by the same constant distance and position.
When calibrating the robot, the operator would need to calibrate the pen holder position by the cords convergence point – not the pen point. This means that the preview in your software won’t match exactly the position of your drawing on the paper.
While not part of the thought experiment per se, I think we can all agree that the more weight that is not centered on the cord convergence point, the more likely the pen holder is to sway.
I’m willing to defer to Sandy’s experience that pen holders that do not have the cord convergence point the same as the pen tip are, “Easier to design, easier to build, and cheaper, far, far cheaper.”

Thanks Yoda!

P.S. Just in case you’re wondering – the reason that SVG of Yoda above is so large is because it includes the full TSP version of Yoda I’m getting ready to draw. :)

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Photo courtesy of Kristina Alexanderson [↩]

I swear I’m not jealous!

Before Maker Faire announced the official call for Makers, I had already downloaded and printed out the “call for Makers” information from the most recent Maker Faire and handwritten all of my responses in the form – just so I would be ready to submit my application to be a Maker this year. ((Photo courtesy of @Doug88888)) I ended up submitting my application within just a few hours of the call going out.

After I had submitted my application, my daughter wanted to submit her own application. Of course I was happy to help her, so I helped her prepare an application and submitted it on her behalf a little after 11pm on March 14, 2013, just under the wire.

Today Maker Faire sent her an acceptance! My daughter is going to be exhibiting at Maker Faire Bay Area 2013!

…

I just hope my acceptance letter comes soon. :/ Senior year all over again…

A Study of Drawing Robot Pen Holders and Design Considerations

MakerBlock's Cardboard Gondola, Annotated — MakerBlock’s Cardboard Gondola, Annotated

The pen holder for a drawing robot is one of the most deceptively simple aspects of the entire machine. Stripped down to the most basic elements, the pen holder is nothing more than a small device used to connect to both cords from each motor to the pen. However, there are a number of extremely important, and subtle, design considerations that are not immediately evident.

Since Hektor’s debut in 2002, and arguably as far back as SIGGRAPH in 1988, people have been working on vertical pen plotters. In that time

I guess I should start this post with a discussion of the different gondolas out there.

In no particular order they are:

Binder clip. One version of the AS220 DrawBot used a simple binder clip holding a pen as the gondola/pen holder. It doesn’t get a whole lot simpler than that. It appears from the video associated with this post that the pen is held on a somewhat rigid rail. Similarly, Dustyn Roberts’ SADBot also used a big huge clip as a pen holder/weight and James Provost’s InternBot used a few binder clips. However, the most hacked together system is easily Josh Myer’s Muralizer which consisted of a lump of Play-Doh enveloping the pen.
AS220 Labs Pen Holder

SADBot Pen Holder

Muralizer Pen Holder – Powered by Play-Doh
Clip Stabilizer plus Binder Clip. The “production version” of the AS220 DrawBot included lasercut spool parts, motor mounts, and a “clip stabilizer.” The setup described in the assembly instructions appears to indicate that the pen can be held reasonably steady using this design. However, having spoken to Shawn Wallace about these designs, he advised that this is really a non-optimal setup that has a lot of wiggle to it.
AS220 Labs Clip Stabilizer Design

AS220 Labs Clip Stabilizer plus Binder Clip
Der Kritzler by Alex Weber. This “gondola” made use of several lasercut parts creating a long wooden “cage” which held the pen in place along with a servo activated pen lift. I can’t quite tell how the “pen lift” operates, whether it retracts the pen itself or whether it pushes something in front of the pen’s tip preventing the pen from leaving a mark. Either way, this drawing robot pen holder has a feature that I never really appreciated until now – the wooden cage is suspended by two wooden “wings” which keep the point of attachment to the toothed belt at it’s midpoint. I’ll discuss this feature more later.
Der Kritzler Pen Holder
My first gondola was one of my own design and it was a miserable failure. The central ring was too large to accommodate the marker pens I was using. Also, it wasn’t heavy enough to make the monofilament hang in a straight line.
Crappy Gondola
Sandy Noble’s Polargraph. Sandy has probably logged more hours with his drawing robots than just about anyone else. So, when he uses a particular setup for his pen holder, there’s got to be something to it. The interesting features of his gondola are that the weight is concentrated around the pen tube by use of several large bearings and, as with several other designs here, the cords to the pulleys are centered over the holder’s center of gravity. After I published this post, I found a printable Polargraph-style pen holder by Lanthan on Thingiverse.
Sandy Noble’s Polargraph Gondola
John Abela’s Gondola. I used John’s designs with my first drawbot, but without the blank CD. For the first time today I noticed that all of John’s pictures show the printed gondola glued to a blank CD for stability.1 When I used his design I just tied the top of the printed gondola to the monofilament line and added a ziplock baggie with dead batteries for weight. The result was a reasonably decent gondola that was pretty finicky. If the robot started drawing too close to one side or the other, the holder tended to twist and the pen made little to no contact with the paper. I can see why the blank CD was such a good idea.
John Abella’s Polargraph Gondola
Dealywhopper’s Dr. Scratchy Polargraph Gondola. Similar to John Abella’s Polargraph derivative is Dealywhopper’s Mr. Scratchy setup. It’s an amusing mixture of high tech 3D printed parts and hot glue hackery. There’s just something about its simplicity that really tickles me. Print the part, add some glue, slide the binder clip into the groove, glue the holder, some pennies for weight, and a micro servo to an old CD and you’re done. The interesting thing about this one is that the majority of the weight is off-center towards the drawing surface.
Dealywhopper’s Dr. Scratchy Polargraph Gondola
Dan Royer’s Makelangelo. In the spirit of open source Dan Royer has been working on and blogging about his Makeangelo and Makelangelo 2 robots. Dan’s Makeangelo is, like my first Polargraph derivative ‘bot, based on an Arduino Due and Adafruit Motor Shield. If you check out his Youtube channel, there’s about two dozen uploads documenting Dan’s experiments with different pen holder configurations. The version he’s shipping with his latest kit, which you can see below (the image is also a link to the video), uses three lasercut pieces to hold a pen and allow for an interesting pen lift. Although you can’t see it in the image below, there is a third lasercut piece which appears to slide forward and backwards with the micro servo. In the forward position it would push the pen holder top off the wall. The holder includes two rows of holes along the top for attaching the motor strings above the holder’s center of gravity.
Makeangelo 2 New Pen Holder
Drawing Machine by Ragnar. This drawing robot by Ragnar, a co-founder of Havtek, is exceptional for its incredibly high quality drawings and bespoke pen holder. Ragnar provides a detailed description of his setup in two posts. This may be the single most beautiful pen holder of the bunch. With heavy brass pieces, there appears to be no further need for any additional weights. As you’ll notice from his other photographs, the two brass arms are in the centered along the body of the pen holder. This pen holder looks like each of the parts came off of an assembly line just destined to be part of an awesome drawing robot.
Ragnar’s Drawing Machine Pen Holder
GarabatoBot (aka DoodleBot) by Made by Miguel Ángel de Frutos. This is one of the most interesting drawing robot pen holders ever made – if for no other reason than it integrates almost all of the critical components into the pen holder itself. As I recall, several other projects had tried to use integrated motors but found that the resulting pen holder/robot combination was too heavy to lift itself. Miguel’s design is well documented on his site and the printable parts are shared on Thingiverse.
GarabatoBOT robot by Miguel Ángel de Frutos
Harvey Moon’s Drawing Machine. What makes Harvey Moon’s drawing robot pen holder particularly interesting is his choice to have the pen actually move up and down. The pen holder incorporates a second non-drawing point and a rack-and-pinion system to advance and retract the pen. I have to admit, I really like the aesthetic quality of having a no-foolin’ pen lift.
Harvey Moon’s Drawing Machine Pen Holder
Darcy Whyte’s Mr. Drew. Darcy Whyte aka Mambohead has been blogging about his own Polargraph variant2 for quite some time and worked through several different pen holders, including one designed specifically for a smaller drawing robot. His designs use layered clear lasercut acrylic to build up a gondola which incorporates a springy clamp for holding the pen in place. This video gives a really good tour of Darcy’s pen holder and explains several of the more important features.
1. There are two pieces of threaded rod on either side of the pen holder. By stacking pieces of acrylic you can adjust the position where the wire connects to the gondola.
2. There is a pen clamp using a rubber band, as indicated above. The best part about this clamp is that he uses varying pieces just below the pen to adjust the tilt on the pen, in case it requires a slight angle to draw on a more vertical surface.
3. There is a third piece of threaded rod at the bottom of the gondola where additional weights can be attached.
4. The clear acrylic and strategically placed holes in the top of the pen clamp allow the operator to see where the pen contacts the paper.
  Darcy Whyte’s Mr. Drew Pen Holder
Stuart Childs DRBO. Stuart Childs sells a lasercut Polargraph-compatible robot kit. Once assembled it is a stand-alone drawing robot. The most interesting difference between Stuart’s robot and Sandy’s setup is the construction of the pen holder. I’ve included a picture of the front and back of the pen holder below to give you an idea of what it looks like. Per Stuart’s comment below, his own gondola was inspired by Darcy’s Mr. Drew.3 For a better idea of how it is assembled and how it operates, you should definitely check out Stuart’s excellent step-by-step assembly photographs. This pen holder has a small circular lasercut piece which attaches to the business end of a micro servo, to push ahead of the pen’s tip – allowing for “pen lifts.” There are two features in particular that I really like.
1. First, I like how the “arms” which connect to the motor cords can swivel. This is a very clever way around several potential problems. When tying the two cords to points on the pen holder, there are issues with placing them too far or to close together. Too far apart makes the pen holder extra stable, but the image drawn are distorted. Too close together minimizes distortion, but the pen holder can start to swing like a pendulum, causing wibbly wobbly lines. Additionally, if the cords from the motors have too much “twist” in them, the entire pen holder can actually be turned sideways and will stop drawing entirely. (I suspect just about any Polargraph-style pen holder which uses a large wide flat surface would be sufficient to combat the cord twisting/torquing problem.) Looking back to the AS220 Labs pen holder, you can see that it appears to use two rigid rails instead of string. The arms in Stuart’s robot essentially allow the cords from the motors to act as if they’re very close together – but probably wouldn’t allow much in the way of pendulum action.
2. Second, I appreciate his spring-loaded pen holder. This feature would allow his robot to accommodate a variety of pens or drawing implements. While a rubber band would obviously work as a quick hack, a true metal string would stand up to repeated use.
Stuart Childs’ DRBO Pen Holder
DrawBot Quick Change Pen Holder by UechiMike. Thingiverse user UechiMike designed his own pen holder which he identified as a derivative of Dan Royer’s Makeangelo. You’ll notice that UechiMike’s pen holder, like the DRBO immediately above, uses a rubber band in place of a spring as a way to accommodate a variety of pen sizes. UechiMike’s pen holder has holes on either side for routing the monofilament wire which, it looks like, are tied around. I have to wonder if the holder has any problem with torquing. You’ve got to love the recycling of dead AA batteries here. The only “gripe” with the design is that there isn’t any room for a micro servo to for pen lifts.
DrawBot Quick Change Pen Holder by UechiMike
Screwless Sharpie Holding Gondola by Bluemetal. Simple and sweet, this design doesn’t seem to have any weights or moving parts. Just a bit of printed plastic and a push-fit hole designed for Sharpies.
Screwless Sharpie Holding Gondola by Bluemetal
MakerBlock’s Cardboard Gondola. Okay, now the most intricate, well designed, and durable feat of engineering ever to meet a marker. My very own cardboard gondola. As you can see, I slapped four AA batteries and a pen to a jagged piece of cardboard. While it worked for several drawings4 it’s clearly nonoptimal. The cord attachment points are wide enough to cause distortion and not well balanced enough to prevent swinging. The only reason I slapped this together was that I was anxious to put my drawing robot to work.
MakerBlock’s Cardboard Gondola, Annotated

Frankly, my crappy gondola is a testament to the how forgiving DrawBot setups are. Even though I’ve been admiring drawing robot setups for probably a year and a half now, I’ve really only started to understand some of the design decisions. I’ll go ahead and publish this post5 and get to work on the next one laying out what I’ve learned from the different pen holders featured above.

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Face palm [↩]
How awesome is it that Darcy worked with Sandy on pen holder designs?! [↩]
TeamTeamUSA created his own variation on Stuart’s pen holder and made the plans freely available on Ponoko [↩]
Four Deathstars and a Firefly class starship [↩]
I’ve been sitting on it now for a few days [↩]

Dogs and cats living together!

I’m basically through with CD’s. I don’t even want the ones I have. I would much rather just have the MP3’s since they’re just so much easier to organize and play. This has been the case for a while now – the last time I bought a physical CD was four years ago. The CD before that was eight years ago.

I was looking at buying an album off of Amazon the other day only to discover it was $3 cheaper for me to buy the physical CD, which comes with an MP3 “autorip” copy in my Amazon account, than to just buy the MP3 album in the first place. Three dollars isn’t enough to make a tremendous difference in my lifestyle, but at the same time, that’s 1/3 the cost of the CD + MP3 album. I can’t imagine why I would want to pay $3 more for the MP3 album when I could save $3, get the CD and MP3 album, and give or throw away the CD.

Interestingly, now that the United States Supreme Court has upheld the “First Sale Doctrine,” at least in relation to international textbook sales, I wonder if I’m perfectly justified in buying the CD+MP3 for $9 and selling off the CD for a modest amount. (Are there still used CD buying places?!?) After all, it’s not as if I’m making unauthorized digital copies, I’m just selling the physical CD and keeping the digital copies I purchased… right?

Random Gripes of The Day

I was annoyed by two things today and I have decided to share them with you. ((Photo courtesy of Göran Arvidson)) It’s more about the sharing than actually imparting information, but these are two things I feel kinda strongly about. The day is young yet, so I reserve the right to add more things to this list. :)

I was going through my PayPal transactions for the last year and I discovered that providing a full and immediate refund costs me $0.30. That’s pretty lame. When they perform the refund, they get their money back. All they’re out is a few zeros and ones. And, frankly, it’s enough that I think my next few projects will be Stripe based rather than PayPal based. PayPal’s API is a pain, I’m convinced requiring PayPal over a credit card (via Stripe) for transaction probably costs me conversions ((Oh, I could go on and on about PayPal versus Stripe and the posts about the relative benefits and conversions)) , and their policies are worse.
Online forms that require multiple steps across multiple submission pages. The first thing this kind of UI tells me is that the people who wrote the page wanted something done quickly and easily (for them) and didn’t spend a lot of time thinking about usability. Because I have a tendency to overthink things, I sometimes go back and revise an earlier step depending upon what I see in a later step. When I see an online form that requires multiple steps I will typically feed it bogus information first so I can see the rest of the steps.

Um. That’s all I’ve got to grumble about today.

Actually, in hindsight, if those are my biggest problems of the day, I’m doing pretty well!

How to add a custom button to the WordPress Visual TinyMCE Editor

The other day I found a great post that explained how to add a custom button to the WordPress Visual TinyMCE editor.1 It was fairly simple and concise and even includes a zip file of all the parts you would need to install a button into WordPress’s visual TinyMCE editor. I’m not at all sure I could do a better job than that guy, so I’ll just direct you to the link above. :)

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Photo courtesy of Marco Bernardini [↩]

OCD Plugin Stats and the WordPress.org Statistics API

What you get when you don't use an API — What you get when you don’t use an API

The current version of my WordPress OCD Plugin Stats plugin, is somewhat… clunky.1 When I wrote it I didn’t know that WordPress.org had an API for it’s plugin statistics.2 Thus, the current version essentially scrapes particular WordPress.org plugin pages for information and drops it into the WordPress dashboard.

After poking into the stats on one of my plugin pages the other day I discovered that the source code for the page included several calls to a WordPress.org API. Why should I bother scraping and interpreting a page of HTML when I can just ask WordPress.org to deliver the exact data I need in a nifty JSON format?! I’m very much looking forward to adjusting this plugin to use a more direct and simple call to the WordPress API rather than scraping plugin pages.

In any case, if you’re looking for documentation on this, I found a page in the WordPress Codex and a site with a page devoted to documenting the WordPress.org API.

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Photo courtesy of peasap [↩]
Apparently they have more statistics than just on plugins [↩]

How to add a custom button to the WordPress editor

While posting about my DrawBot adventures, I’ve become increasingly annoyed with my own post series WordPress plugin. If only it had a simple button in the WordPress editor to drop the shortcode for an existing series, it would instantly become so much better. Thus inspired, I delved into WordPress.org and sought out the answers. What I discovered is that while it’s not particularly difficult to add a button to the WordPress text editor, it didn’t seem to be very well documented. I’m still working on how to add a graphical button to the WordPress TinyMCE visual editor, so stay tuned for that. I was ultimately able to glean the necessary information from the WordPress “Quicktags API. However, for your sake1 I’m distilling what I’ve learned right here.

First, I’ll give you an overview of what we’re going to do. If you’re looking to add a button to the WordPress editor, it’s probably because you’re working on a plugin. Thus, I’ll assume you have a working knowledge of plugin development. Your plugin will need to include, at a bare minimum, one action hook and one function which performs the double duty of creating the button and giving the button some kind of ability.

// This action adds a script to the script footer on administrative pages
add_action( 'admin_print_footer_scripts', 'MBSS_quicktags', 100 );

// This is the function that is run at the end of the administrative pages in the script footer
function MBSS_quicktags()
     {
     // Optional database interactions
     // I used this section in my plugin to run a query on the database and return information that would be used in the button
     ?>

     <script type="text/javascript">
     // Creates button in the text editor
     QTags.addButton( 'MBSS_tag_id', 'Add Series', MBSS_add_series_js );
     // Function to append the series shortcode to the end of the post
     function MBSS_add_series_js()
          {
          editor = document.getElementById('content');
          editor.value = editor.value + "\n" + "EXAMPLE TEXT TO APPEND TO END OF POST";
          }
     </script>
     <?php
     }

And there you have it! One action hook and one function which adds some javascript magic to your WordPress editor in the form of a simple text editor button!

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And, frankly, for my own future use [↩]

Apparently WordPress.org serves up even more delicious download stats

Lovin' that source code — Lovin’ that source code

While looking at the source code on my Simple Series plugin page, I began to wonder how WordPress.org was serving up all those nifty little stats into the downloads graph. ((Photo courtesy of Windell Oskay)) What I discovered is that there’s a little bit of jQuery magic going on that pulls download data from a WordPress.org API, formats it for niftiness, and inserts it into the dynamically draw SVG graphic of the downloads.

In case you’re interested, the API call looks like this:

http://api.wordpress.org/stats/plugin/1.0/downloads.php?slug=simple-series&limit=700&callback=?

I’m now wondering how I can use that data to show something interesting in my OCD Plugin Stats plugin.1

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I should probably rename the meta plugin. [↩]

Simple Series WordPress Plugin Update

This weekend, almost an entire year from the last update, I made some revisions to my Simple Series plugin for WordPress.1 This is one of the plugins that I use the most often for WordPress. It’s right up there, in terms of frequency, with the WP-Footnotes plugin that lets me insert footnotes.2

As much as I loved this plugin, it was always a little bit of pain to use. In order for two posts to be associated together in the same series, the title must be listed exactly the same in each post. So, to make sure that I was getting it right, I would open an old post in the relevant series, copy the exact plugin shortcode with the exact title, and paste it into the new post.

With this update you can click the “Add Series” button in the TinyMCE text editor for a post and insert any series title you’ve previously used. It’s not yet as elegant as I would like, but it makes a few button clicks out of a what used to be an annoying process.3 I’ve also added the ability to have a series between pages, posts, or any combination of the two kinds of content types.

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Photo courtesy of Tavo [↩]
Like this! [↩]
Described in the prior paragraph [↩]