The MeArm is a small and inexpensive robotic arm with 4 degrees of freedom. Originally developed as an educational tool for schoolchildren the MeArm has since become a popular device for robotics experimenters of all ages. Its simple open source design makes it an ideal tool for anyone developing software and electronics. Projects created with the MeArm can then be ported for use on a more expensive and capable full-sized robotics arm.
The MeArm is a great introduction to robotics, as well as a useful tool for seasoned experimenters
As the MeArm is an open source project the plans for building it are available freely on the Internet. To build a MeArm from scratch requires just four small servo motors and a material such as acrylic or wood to construct the arm itself. It is also possible to assemble a MeArm using parts produced with a 3D Printer.
For those of us who don’t have the facilities to cut or produce all of the MeArm components there is another option. The MeArm kit, available from Mime in the UK, contains all of the parts required to build a MeArm. This is the route that I have chosen to create my own MeArm and this is what I have documented in this article and its associated video.
There are over 50 steps that you need to take to assemble a MeArm and I’m going to go over all of them in this article, so as a result this is a very long document (and a very long video). For the most part the assembly went smoothly, however I did encounter a few errors in the MeArm manual that I needed to work around. This included one major omission that actually resulted in my needing to take apart and reassemble my MeArm! If you follow this guide along with the MeArm manual you’ll be assured of successfully completing the build without having to resort to such drastic measures.
Despite these small setbacks I found the MeArm to be a quality product that will no doubt be useful for several future robotics experiments. Now that it’s built I have moved on to designing and constructing an advanced controller for the arm which I will share with you in the next article in the MeArm series.
So grab yourself a MeArm kit and let’s get building!
The MeArm Kit is available in different colors with two “flavors” – with or without a controller (which Mime calls a “brains board”). I chose to purchase the kit without a controller as I intended to build my own controller. This is a popular option and the Mime website has instructions for using both an Arduino and a Raspberry Pi with the MeArm. For my initial build I chose to use an Arduino. Naturally I chose a blue MeArm to match my workshop!
When I first received the MeArm Kit I was surprised as to how small the box was, it didn’t look like you could actually fit all of the parts into such a small container. However this is just a testament to the efficiency of the MeArm design and upon opening the tiny box I was relieved to discover that everything was indeed included.
No instruction manual is packaged with the MeArm Kit, instead you can download a PDF document from the Mime website. I downloaded and printed the MeArm Assembly Manual Arduino Edition, my copy was Revision 1.0 September 2015. As I purchased my kit in December 2016 there are a few changes to the design that were not reflected in the manual, this is to be expected and with the exception of one change did not cause a lot of confusion.
The MeArm is constructed of acrylic plastic and within the kit you’ll find four sheets, each precut with all of the MeArm parts. This is actually where I made my major error and rather than make you read through to the end of the article I’ll tell you what that was right now.
The four sheets each come with a white plastic backing on one side. What I didn’t realize was that you NEED TO REMOVE THE PLASTIC before beginning construction! Unfortunately I didn’t do this and as a result my initial build didn’t work very well. Once I rebuilt the MeArm without the plastic backing on the components everything worked a lot better! In retrospect I should have realized this when I started, in my own meager defense the manual could of perhaps mentioned this step but it didn’t. So don’t make the same error I did – take the plastic off before you begin construction.
The photos (and video) of the MeArm assembly I’ve included here show the parts with the plastic still attached. While this is incorrect the rest of the assembly procedure is still valid. As an added bonus the plastic does make it easier to see the pieces in the photos and the video, so it wasn’t a complete failure!
The “Fixings Package” contains all of the nuts and screws that you’ll need to assemble the MeArm. The screws are all M3 metric fasteners and in many cases they self-tap into the acrylic. The parts list is as follows:
In addition the Fixings Package also contains four stick-on feet for the MeArm base (which ironically I neglected to install in Step 4, I corrected that near the end of the build) and a 2.5 mm hex Allen key. The Allen key is required to tighten the screws, in my case I used a 2.5 mm hex screwdriver from my toolbox as I found this a bit easier to work with.
After confirming that I was provided with the correct quantities of hardware I separated the nuts and screws and placed them into a small parts box with individual compartments for each one. If you don’t have a box like this you could simply use a egg carton to organize the parts.
The kit comes with four MG90S servo motors. These popular servo motors are the metal gear equivalent of the even more popular SG90S motors which I’m sure you’ve probably used before. Each servo is in its own package and each one is packed with three styles of servo arm, as well as with a the screws required to use the arms and attach them to the servo shaft.
In addition to the servo motors there is also a short 3-pin extension cable provided. This is used to extend the reach of the claw servo motor cable.
Prior to beginning construction of the MeArm you will need to calibrate the servo motors. Follow along and I’ll show you how that’s done. You’ll need an Arduino, some jumper wires, an external 5 volt power supply capable of enough current for the four servos and a permanent marker pen.
The MeArm manual has a diagram that shows how to hook the four servo motors up to an Arduino Uno. While the hookup diagram is indeed correct two of the colors illustrated for the servo motor wires didn’t match the ones on the motors I was supplied. You’ll probably run into this problem as well, so here is the list of the actual servo motor wire colors and how they “translate” to the ones shown in the MeArm manual:
The PWM wires for the motors are connected to four of the digital outputs on the Arduino. If you’re using a different type of Arduino you can move these to other outputs, providing you select outputs that are capable of PWM. If you do that you’ll need to modify the supplied Arduino sketch accordingly.
