1 week ago

MeArm Pi review: open-source robot arm for Raspberry Pi

Long-time readers of The MagPi may recall that we reviewed the original MeArm Pi back in issue 33. While we loved the concept of an affordable, open-source robot arm, we did note that a missing piece of the puzzle was the need for users find their own way to drive the servos from a Raspberry Pi. The new MeArm Pi solves the problem by including a HAT with twin on-board joysticks, so now you have everything you need in one kit.

This review first appeared in The MagPi #62 and was written by Phil King.

Not only that, but the arm itself has been completely redesigned, eliminating two-thirds of the screws and using new fixings to make it easier to build. It certainly is a lot of fun to put together, like a cool motorised Meccano set.

Step-by-step illustrated instructions are supplied online – two versions, for a standard-size Pi or a Pi Zero. Most of the acrylic pieces simply slot together, secured by a few rubber bands, while three lengths of screws are used for the moving joints. A hex key is supplied for this purpose, but you need to avoid overtightening the screws so that the joints can move freely. The design is well thought out, with strategically placed holes for the servo cables to pass through to keep them tidy. We found the trickiest part was sliding the arm into the base, which required prising the two layers apart; we also needed to loosen the base servo horn slightly to enable it to move freely.

The case that houses the joysticks also features cut-outs for the Pi’s ports, although there’s no easy access to the SD card, so you’ll need to download and write the special OS image to it before you enclose the Pi. Based on Raspbian, the OS includes all the required software and creates a wireless access point so that you can connect to and program the MeArm Pi directly from a remote computer or tablet. This means it can be used headless, without the need for a monitor, keyboard, and mouse. There’s also the option of connecting it to your WiFi network if you prefer.


MeArm Pi review: Twist your arm

Once the system has booted up, which takes about 75 seconds on a Pi 3, the arm jerks into life and you can then control it using the twin joysticks. This is a lot of fun – like controlling a mini digger, albeit with a claw. The left stick handles base rotation and lower arm movement, and the right stick controls the upper arm and claw grip. While the servos are pre-calibrated to be used out of the box, we did encounter an issue with the grip not closing fully at first, but this was easily rectified by unscrewing and repositioning the servo horn. There is quite a lot of buzzing from the servos, but we found the control was accurate enough for us to pick up small objects, such as cherry tomatoes and socks.

A more interesting and educational way to control the arm is by programming it. While you can do this directly on the Pi, an easier way to get started is by connecting a PC or tablet to the MeArm Pi’s local web server, by pointing a browser to local.headlesspi.org. This gives you access to four apps, each for a different programming language: Blockly, Snap!, Python, and JavaScript. These include functions (and sample code) for moving the arm’s servos by degrees for accurate control. Naturally, you can set up loops and conditionals to get the arm to behave in sophisticated ways. There’s even the potential to add a Camera Module to the end of the arm (using an extra-long ribbon cable) and use OpenCV for image or face recognition to control it – as demonstrated by Mime.


MeArm Pi Review: Last Word

Despite a few teething troubles, we found the MeArm Pi a joy to build and use. Manual control using the joysticks is great fun, but programming it is ultimately more rewarding. The headless setup and local web server make it very easy to get started, offering a choice of four programming languages, although you could still opt to code it directly on the Pi instead.