Wednesday, September 11, 2013

Crazyflie 10-DOF nano quadcopter repair

Our crazyflie's inaugural flight was quite exciting! She quickly soared to the daring height of roughly twelve feet. In fear of imminent collision with the the ceiling we let go of the thrust control. Crazyflie responded with equal daring, quickly succumbing to gravity's pull. She headed downward at great speed. She immediately sacrificed one wing to break her fall.

Now what? We were certainly prepared to do some tinkering: we expected to lose a motor or one of its mounts here and there. We had great plans for her. However, we did not expect to be presented with such a significant repair so early in her life.

Trough some trial and error I think we came up with a fairly good design:

  • a part that is easily manufactured in any quantity
  • trivial to install with a hot glue gun
  • maintain wing alignment
  • is light weight, shock absorbent, ...
  • but not too strong, such as to stress the main board or its components during impact

Our first repair attempt involved printing thingiverse 95216, but this added a lot of weight and quickly broke as well:

After some experimenting, we ended up with a pretty good toothpick design:
  • toothpicks are easy to acquire
  • toothpicks are very light
  • toothpicks slide snugly into the motor mount
  • toothpicks are easily mounted to the main board with hot glue 

Each toothpick would average about ten crashes before it had to be replaced. The design also had a couple of other minor issues:
  • As the toothpick had to extend to the other side of the main board, the broken wing's motor and its mount ended up lower than the remaining three motors, making initial leveling trickier.
  • Because the toothpicks broke regularly, we ended up spending a lot of time with the hot glue gun: removing old toothpick and installing new ones.

Inspired, we took our toothpick design to the next level, with a 3d part of our own design:

After spending some time flying, crashing and repairing our crazyflie, I thought I'd share some thoughts on the wing design. No matter what wing design you choose, you're making an engineering trade off between strength and weight. Strength is actually a complicated subject with its own trade offs. A stiffer part will be able to withstand greater impact forcing, but will transfer those forces to the rest of the system. A weaker part with absorb more of the impact, protecting the rest of the unit, but with increased risk of itself breaking.

The current crazyflie wing design has relatively thin (light) stiff wings which become wider at the point where they are attached to the main board. This means that when the wings break, they almost certainly will break at the point where their width goes from narrow to wide. When it comes to repairs, this is obviously the most inconvenient point for a break.

Why not just make the wings wider (stronger), so they don't break as easily? While a stiffer design would certainly save some wings from bigger crashes, at some point we're going to start having main boards go bad because of the higher g forces during ever bigger crashes.

What we really need is a sacrificial wing part. Just like the motor mounts, which were designed to sacrifice themselves in order to spare motors which would otherwise be damaged, I think we want sacrificial wings.

The thing with sacrificial parts is that they, well, sacrifice themselves. They break. So, these parts need to be cheap and easily replaced, ideally without any tools. The thing I don't like about the motor mount design is that replacing a motor mount requires a soldering iron to detach and reattach the motor wires after stringing them through the mount.

Here's my proposal: redesign crazyflie so that instead of four wings it has four wing mounts. Design a very simple wing which...
  • is the most likely part to break
  • is really cheap, such that you get a whole bag of them with your crazyflie
  • is easy to make on a 3d printer
  • ideally resembles toothpick dimensions
This last point is important, because it means you can use a toothpick in a pinch.

Friday, July 12, 2013

Standing up a phpBB instance on Google App Engine and Cloud SQL

Before we get started

I've been playing around with phpBB on App Engine's new experimental PHP runtime environment, using Cloud SQL for the backend database. While it wasn't too much effort to get it a site up and running I thought it would be useful to document the steps I went through here so others can follow along. Let me know in the comments if these steps don't work for you, or if there's additional changes you ended up making to the phpBB source to address more advanced features.


This guide follows roughly these steps:

  1. Get a basic phpBB instance up and running on the App Engine SDK's dev_appserver, connected to a locally available MySQL instance
  2. Walk through phpBB's initial setup process using the local the dev_appserver
  3. Make a couple of adjustments to the generated config.php
  4. Export the local MySQL database and import it into Google Cloud SQL
  5. Deploy our application to the App Engine production environment

TODO: Look into caching template files in memcache.

Here are the step by step instructions:

Setup your Cloud Console Project

  • From the Google Cloud Console create a new project. Pick a suitable project id (I used fredsa-phpbb). This cloud console project id will also serve as your App Engine app id.
  • Enable billing on your Cloud Console project (click the gear icon in the top right and select 'Billing'). Don't forget to click the verification link the email you'll receive. Confirming your contact email will ensure you receive timely billing notifications.
  • Also enable billing for your App Engine project. Click the 'Enable Billing' button from the 'Billing Status' page from the App Engine admin console. There's a link to the App Engine console from the Cloud Console.
  • Within your cloud console project  create a new Cloud Storage bucket. For convenience I chose my bucket name to be the same as my project id
  • Within your cloud console project create a new Cloud SQL instance. Again for convenience I chose and instance id which matches my project id. Instance id fredsa-phpbb in project fredsa-phpbb can be known as fredsa-phpbb:fredsa-phpbb

Verify Cloud SQL connectivity your-project-id:your-cloud-sql-instance-id

Download phpBB 3.0.11

cd phpBB3
  • To easily track your changes throughout the installation process you may find it convenient to create a local git repository. This step is entirely optional:
git init
git add -A
git commit -m 'initial contents of'

(Optional) Add a favicon.ico

Copy your favorite favicon.ico into the main phpBB directory (i.e the same directory where common.php can be found)

Modify the app to be App Engine compatible

  • Add a 404.php, which we'll use to explicitly disallow access to certain resources, with the following contents:
  • Add php.ini with the following contents
output_buffering = "On"
  • (Optional) add cron.yaml to run cleanup tasks at a desired interval
# See
- description: phpBB cleanup tasks
 url: /cron.php
 schedule: every 24 hours
  • Add the App Engine app.yaml configuration file, with the following contents:
application: your-app-id
version: 1
runtime: php
api_version: 1

# see

# Prevent users from accessing protected resources, which would otherwise be
# accessible because of wildcard handlers later in this files. This replaces:
# - .htaccess
# - cache/.htaccess
# - files/.htaccess
# - images/avatars/upload/.htaccess
# - includes/.htaccess
# - store/.htaccess
- url: /(common.php|config.php|(cache|files|images/avatars/upload|includes|store)/.*)
 script: 404.php

- url: /
 script: index.php

- url: /favicon\.ico
 static_files: favicon.ico
 upload: favicon\.ico

- url: /(.*\.php)
 script: \1

- url: /adm/images
 static_dir: adm/images

- url: /(adm/style/.*\.(css|js))
 static_files: \1
 upload: adm/style/.*\.(css|js)

- url: /images
 static_dir: images
 application_readable: true

- url: /(styles/.*\.(html))
 static_files: \1
 upload: styles/.*\.(html)
 application_readable: true

- url: /(styles/.*\.(css|gif|jpg|js|png))
 static_files: \1
 upload: styles/.*\.(css|gif|jpg|js|png)

# see
# default entries
- ^(.*/)?#.*#
- ^(.*/)?.*~
- ^(.*/)?.*\.py[co]
- ^(.*/)?.*/RCS/.*
- ^(.*/)?\..*
# custom entries, for resources we don't want to deploy to production
- ^cache/.*$
- ^install/.*$
- ^scripts/.*$
  • Review  the contents of  any .htaccess files (which are ignored by App Engine). Ensure that the access restrictions indicated by each file are implemented using one of the following strategies:
    • Outright removal of the affected files or directories. Users won't be able to access files which do not exist.
    • Use of a login: admin protected URL handler in app.yaml. This allows App Engine admins such as yourself to access administrative functions while denying access to all other users.
    • Use of a script: 404.php URL handler in app.yaml. This explicitly denies access to specific resources, including those resources which would otherwise be made accessible by later app.yaml URL handlers.
    • Use of a skip_files entry in app.yaml. This prevents access to files by making sure the specified files are not deployed to the production environment.

The above app.yaml should already take care of these phpBB 3.0.11 files, although it wouldn't hurt for you to independently verify this.

git ls-files | grep .htaccess

Run and install phpBB locally

  • Download and install MySQL, PHP and the App Engine PHP SDK
  • Create a local MySQL database named phpbb, which we'll later export and then import into a Cloud SQL database.
mysql -u root
Query OK, 1 row affected (0.00 sec)
  • Launch the dev_appserver from the phpBB directory containing the app.yaml. Note the final argument to this command is a '.' indicating the current directory. \
 --php_executable_path=/usr/local/bin/php-cgi \
 --mysql_user root \
  • Click the install tab

  • Click 'Proceed to next step' and verify the installation compatibility
    • Note: In the App Engine production environment, the local file system is not writable. In the dev_appserver we initially allow phpBB to assume that local directories are writable. We'll later modify config.php
  • Specify the details for the local MySQL instance

  • Configure the admin account

  • Configure your email settings:

  • Configure your server URL settings:

  • Create database tables

Apply App Engine specific config.php changes

  • Modify phpBB/config.php as indicated in bold:
// phpBB 3.1.x auto-generated configuration file
// Do not change anything in this file!
$dbms = 'phpbb_db_driver_mysql';
$dbhost = '';
$dbport = '';
$dbname = 'phpbb';
$dbuser = 'root';
$dbpasswd = '';
$table_prefix = 'phpbb_';
$adm_relative_path = 'adm/';
$acm_type = 'phpbb_cache_driver_file';

@define('PHPBB_INSTALLED', true);
// @define('DEBUG', true);

// ************************************************************************
// ************************************************************************
$DEVAPPSERVER = preg_match('/^Development/', $_ENV['SERVER_SOFTWARE']);

 $acm_type = 'null';
} else {
 $dbhost = ':/cloudsql/fredsa-phpbb:fredsa-phpbb';
 $acm_type = 'memcache';
  • Optionally review the files that have been created during the installation process
git status --ignored

Copy your local MySQL instance to Cloud SQL

  • Export the local MySQL phpbb database
mysqldump --databases phpbb -u root --hex-blob > phpbb.sql
  • Upload the resulting phpbb.sql file to your Cloud Storage bucket:
gsutil cp phpbb.sql gs://your-cloud-storage-bucket-name/
  • From the Cloud SQL console import the MySQL database dump (gs://your-cloud-storage-bucket-name/phpbb.sql)
    • Confirm in the Operations Log that the import Status indicates Done

Initial phpBB test in the App Engine production environment

  • Deploy the app (again from the directory containing app.yaml) --oauth2 update .
  • Verify that you can login as the admin using the password you specified during the phpBB installation process. Note not all functionality may be available yet (see next step)

Replace all uses of preg_replace() with /e (PREG_REPLACE_EVAL)

  • Use array_map instead of preg_replace in includes/acp/acp_*.php (see commit 67b243cfc53e3f5f8bf6cd2a5eb80df475a6dd4c)
  • Your homework: find and replace remaining uses of preg_replace() with /e, and of course share those changes with the community, preferably by submitting pull requests with your patch to the phpBB committers.

# 3.0.11 release - Your list of TODOs
git grep -l 'preg_replace(\(.\)\(.\).*\2[a-z]*e[a-z]*\1,'


That's it!