PlotBot: Building the Machine

With the research all done, I started thinking about how I wanted to build my PlotBot.
Having looked at the other designs, I found they were either mounted on a wooden frame and then a piece of paper is taped onto the wooden panel, or they draw directly onto a surface like glass or a wall. Given that the aim is just to make something that catches peoples eye, rather than making posters or drawings for people, I think the best course of action would be to use a whiteboard. I can get one reasonably cheaply, and the mounting is pretty much already sorted.

The whiteboard and mountings.

Once I had bought a whiteboard (600mm x 450mm) I started lining up the parts I had as to how I would mount them.
I had also bought 2 Pololu 1204 Stepper Motors and an Adafruit Motorshield v2 (AFMSv2). I did have a few concerns with these parts combined together, in that the motors only draw 600mA and the motorshield provides 1.2A per channel, therefore the motors might get a little hot if they start drawing more than they should – but we’ll see how it goes!

Roughly lining up the parts on a sheet of acrylic.

To mount the acrylic sheet to the whiteboard I used two of the mounts supplied with the whiteboard secured on the top of the sheet. These then hook onto the edge of the whiteboard, and the mounts on the side are adjustable to “lock in” the sheet to the sides of the board. Finally I decided to neatly mount the arduino and AFMSv2 in the center of the acrylic sheet.

     

     

I picked up two remote control car wheels at a local hobby store, along with 50m of fishing line, which would form the basis for my reels.

I found some nuts in the garage that fitted the inside of the wheel, and used Araldite (metal glue) to fill the gap around the stepper motor shaft hoping that this wouldn’t go wrong.

Araldite’s in, I was a little bit messy dripping it everywhere!

 With the luck of the gods, after leaving it 24 hours to cure I was able to punch the stepper motor shaft out of the nut, leaving a nice shaped hole. The advantage of this method being that I can very easily remove the reels and use the steppers in other projects.  

  

Now that I have the reels mounted on the steppers, I was able to complete the main build; mounting the steppers onto the acrylic sheet, and winding the fishing line onto the wheels – happy days!

  

 

 

 

PlotBot: Brief

So as time draws on we are getting closer to the start of the new academic year, and of course that means Fresher’s Fair!
At Kent we have several creative societies including SpaceSoc and their “Build a Rocket” sessions, Engineering Soc  with their focus on robotics, and TinkerSoc who want to help people build without limits.
With TinkerSoc it has become somewhat of a tradition to build and showoff a project at the fresher’s fair. In previous years we have had a laser engraver making custom name tags and furbies singing bohemian rhapsody, basically something to grab peoples attention and imagination.
Having seen a number of vertical plotters online I have decided now is the time to build one.

The standard vertical plotter is made up of 2 stepper motors, a servo, a motor controller and a microcontroller. By providing a stream of polar coordinates to the robot, the two motors can be wound in and out to move a pen across the whiteboard. This produces drawings where the pen never leaves the surface however that does not limit the styles of drawings that can be produced. Drawings can be developed further by adding a server or linear actuator to the pen carriage in order to push the pen off the drawing surface, thus allowing mush more freedom to implement different drawing styles.

Obviously we cannot draw above, or on either side of the motors, however the effectiveness of the plotter changes depending on the position of the pen carriage.
As such the most effective drawing area is a rectangle in the centre of the drawing surface with the tension on a cord being too low on either side, and the resolution is too low at the top due to the large angles. (http://2e5.com/plotter/V/design/

 

There have been a great many vertical plotters in the past, a great list can be found at plotterbot.com.
Overall there seem to be two different styles of drawing with vertical plotters.

Single line, where the pen never leaves the surface, is technically less challenging and can provide great results however you can be left with the odd scrawl across the surface that you didn’t want.

Multi line, where the pen can be lifted/pushed away from the surface, allows much more flexibility with regards to what can be drawn as the robot won’t scrawl connecting lines across the surface however does add the extra complexity of having a servo or linear actuator to push the pen carriage away from the surface.

Bearing in mind the saying, the more complex it is, the more likely it is to break.

 

Tinkerlog’s “Der Krizler” is definitely one of the more popular V-plotters out there, drawing on glass to amuse passers-by.

https://www.flickr.com/photos/8123185@N02/8181763825/

 

Dan Royer’s Makelangelo is a very impressive V-plotter. Commercialised as a kit, it’s reliability has been tested extensively!

Makelangelo 2.5

 

And probably the oldest V-plotter around from 1988 developed at MIT using lego!

 

 

Having a go at Book Binding

For some time I have wanted to have a go at book binding. It seems like a great thing to do as a gift for a loved one, but maybe that’s just me. Well I decided to make my girlfriend a photo album and sketch book as a Christmas present. Hence why this is being published after Christmas to avoid her surprise being ruined. On the next occasion that I decide to make a book I would choose paper with a lower gsm that I have done for these. I selected 300 gsm, which is great for the photo album, however for the book it is a little too thick to easily flick the pages.

Anyway, to the book binding process. After having folded 20 sheets of paper I clamped them together in-between two other books (sheets of wood are better) and then coated the folded edges with Copydex, a latex based contact adhesive, and a cut of cloth placed over it. By doing this the Copydex soaks into the cloth and the paper to make a reasonably strong joint.

Now that the pages have been bound, its time to make the cover. Using 5mm rigid foam and sticky back cloth, I cut out the front, back and the spine and laid the parts out on the cloth. When doing this make sure to leave a large border of cloth which is used in the next step to cover the edges.

Alas I forgot to take enough photos but the next step is to fold the edges of the cloth up and over to cover the edges of the board. this can be a little tricky because of the sticky-ness but it looks good if you persevere.

In order to securely join the pages and the cover, we have to use a joining page which gets glued on the front and back cover, and the front and back of the stack of pages. Finally we are starting to look a bit more professional and are ready to glue in the pages.

Make sure to use a nice big weight when gluing in the pages just to make sure everything is nice and flat!

No photo’s of the finished book. Sad times.

So You’ve got yourself a Lilypad?

So as part of my involvement with TinkerSoc, we now have our new hoodies. These hoodies are designed so that a Lilypad Arduino can be sewn onto it and then components added to it. We will be wearing hoodies that we can literally tinker with.

For those that don’t know, the Arduino Lilypad is an Arduino development platform intended for clothing and e-textiles. Using conductive thread you can sew tracks and components onto any fabric and then programmed.
The Lilypad doesn’t have a USB plug like an ordinary Arduino Uno. This is because the Lilypad does not have a FTDI Chip (Future Technology Devices International Ltd) unlike the Uno and many other Arduino’s. The FTDI chip converts the USB to serial communication.

So because the Lilypad doesn’t have a USB connection we can use an Arduino Uno instead. To do this we need to remove the ATMega 328p from the Arduino Uno, then break out the header on the Lilypad. As per the diagram above from left to right, the pins connect to Gnd, Gnd, 5v, Rx, Tx and Reset. With these connections made, we can proceed to connect the Uno to a computer, and then start up the Arduino IDE. While in the IDE, make sure you change the board to the correct version of the Lilypad you are using. (If in doubt try to read the number on the Microcontroller on the centre of the board)

And with that done you are ready to start programming, I suggest loading up the example blink program first. Enjoy.

Adjustable Voltage Regulator Problems

So yesterday I had no lectures, so instead of spending my time working on assignments that I don’t have, I decided to go into the engineering lab and prototype the MIC2941 adjustable voltage regulator circuit.
The datasheet for the 2941 contains a example circuit with maths to go with it. This circuit should be capable of outputting between 1.2v and 26v, perfect for what I want to do.

So in the lab I found the components I needed, and set to work.

In order to calculate the values on R1 and R2 I used the equation provided, Vout=Vref(1+(R1/R2)) re-arranging so that, R1/R2=(Vout/Vref)-1. With this I then had a ratio that I could scale to any values I wanted.

Due to the output voltage being limited to between 1.2v and Vin – 1v I chose to aim for a output of 15v initially, because the power supply unit’s (PSU’s) my university have on workbenches only go up to 20v.
So with that value in mind I calculated R1/R2 = 11.195 to 3dp, therefore I selected a pre-set 2kΩ for R2 and a variable 22kΩ for R1. In theory I should then be getting approximate output voltages of:

14.76v at full rotation
7.995v at centre point
1.5375v at low rotation

As seen in the video below, with the reference voltage at 1.2 – 1.3v (poor resolution on the power supply’s behalf) and 19.9v on the input, the output voltage seems to fluctuate between 6.6v and then back up to 15.1v and back down.

My plan in the next couple of days is to hock it up to the oscilloscope and investigate the input current, which was limited to 0.5A by the PSU.

Uni Update

Life at university is going well, however lab sessions are insultingly easy. They comprise of simply measuring the resistance, voltage and current with different combinations of resistors. Having said that I do understand that they are partially there as a method of learning how to use the test equipment that each workstation is equipped with.
I have been nominated as the Foundation Year course rep which means that people on my course are supposed to go to me with their problems regarding the School of Engineering and Digital Arts.
As well as that, I have joined the Caving Club, the Shooting Society, and become the Treasurer of the TinkerSoc, the University’s Engineers Society.
With the TinkerSoc we have plans for several big projects that I look forward to updating everyone on!