The Official Raspberry Pi Projects Book - Projects_Book_v4

Review

raspberrypi.org/magpi The Official Raspberry Pi Projects Book 191

MEARM PI

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.

magpi.cc/2waqp7b

£70 / $92

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.

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 (magpi.cc/2ffXB2J).

Most of the acrylic pieces

simply slot together, secured

by a few rubber bands