The drone parachute

Let's talk about parachutes.

Multicopters can become deadly weapons if the motors stop spinning as they should. There are so many reasons for a failure to happen (bad motors, bad battery, radio problems,etc...)  that it is absolutely necessary for the pilot to anticipate it, especially if you intend to flight over people's head!  We made the choice to never fly directly above people, but if you're willing take that risk,  you might be interested into getting a parachute on your drone . That's why we designed last year a DIY parachute for our quadcopter  🙂

The principle is very simple : the parachute is carefully folded  in a tube on top of which lies a lid that a servomotor keeps closed. At the bottom of the tube, a big spring pushes strongly the parachute towards its top end. So if the servo lets the lid go, the spring will push the parachute in the air.

It may be hard to believe, but the parachute will fit in the tube !
It may be hard to believe, but the parachute will fit in the tube !
The spring is separated from the parachute by a CNC milled ring.
The spring is separated from the parachute by a CNC milled ring.

I recommend you use a strong spring, so the parachute is entirely pushed out when the servo releases the lid. The spring we used is 10cm long, has 8 turns and is 3.5cm wide. It can be fully compressed under pressure, which is what we wanted here, in order to get the smaller parachute we could.

A hook holds the string to the bottom plate
A hook holds the string to the bottom plate

One of the major problems with parachutes is that the air needs to get "under" the parachute envelope in order to make it act as it should. If it doesn't , the parachute will become useless. We followed the method described in the following video :

 

This is how the parachute looks before we insert the actual parachute. The spring will eventually be pressured until its minimum lenth
This is how the parachute looks before we insert the actual parachute. The spring will eventually be pressured until its minimum lenth
We used a glass fiber bottom plate as it needs to resist to a lot of pressure
We used a glass fiber bottom plate as it needs to resist to a lot of pressure
Bottom plate. It can attached to your frame by screws or zip-ties (we love them :) )
Bottom plate. It can attached to your frame by screws or zip-ties (we love them 🙂 )

Parachute surface calculation

 

Obviously, your parachute's surface needs to match the weight of your copter, and the desired landing speed you would like to reach. I think a commonly accepted falling speeds sits around 5m/s (18km/h).

We found in the past a pretty useful formula that gives the diameter of tissue to use depending on the weight you want to slow down and its expected falling speed:

 Diameter = \frac{70*\sqrt{m}}{V}

, with m in grams and V in km/h.

 

For our 1kg drone, it gives us a diameter of 122cm. As you can see, the parachute can quickly become huge compared to the size of your multicopter, and you should maybe start thinking about where to place it on your drone from the very beginning of its conception.

For our quadcopter, we used a slightly smaller parachute (taken from a distress rocket) than the 120cm recommended size given by the formula, which led to higher falling speeds than the expected 5m/s, but it stayed slow enough to keep the drone intact after many, many test flights ! 🙂

Parachute ready for insertion
Parachute ready for insertion
See, it fits ! :)
See, it fits ! 🙂
Ready to go!
Ready to go!
The servo arm is under a lot of pressure here.
The servo arm is under a lot of pressure here. We used the TGY-53317M servo for this parachute.

 

Here's the final video 🙂


Comment if you have any question ! 🙂

PiWeather part 4 : first PCBs

Hello again, dear readers!

I don't have so much time to give to this project, but PiWeather is still moving forward!

The first prototype only had a DS18B20 temperature sensor, and was working on a breadboard, so the next logical step was to design and realize a first PCB for the project. I also moved from the DS18B20 to the DHT22 sensor because it gives both temperature and humidity, and is pretty accurate. I added to that a pressure sensor : the BMP085, sometimes referred to as GY 65 on eBay! This would give my sensor unit the ability to read pressure, temperature and humidity.  That's a good start !

I used EAGLE to design the schematics and the PCB :


PiWeather 0.1 schematics under EAGLE.
PiWeather 0.1 schematics under EAGLE.
The first PCB design.
The first PCB design.

We made the PCB ourselves like we did for our old quadcopter flight controller shield. It is pretty hard to get good results with homemade PCBs, and the tracks have to be very big if you want to be sure there won't be any problems. We got a working PCB on which we soldered the components, and it succesfully worked, powered by two AA cells!

Yep. It is definitely homemade.
Yep. It is definitely homemade.
Isn't it beautiful ? ;)
Isn't it beautiful ? 😉
On this top view, you can see we accidently took a 2 layer plate for our PCB. So we had to remove the copper with a sander :D
On this top view, you can see we accidently took a 2 layer plate for our PCB. So we had to remove the copper with a sander 😀

 

I am pretty happy with this PCB as it works great, but it obviously has drawbacks:

  • PCB making is the worst. It never works as you want, there always is a problem, some tracks are too thin, some are too thick, you have to carefully check for possible shorts...
  • There is no protection against oxydation. Of course you can by some sprays for that, but I don't know if that would work great for an outdoor sensor.
  • The minimum track width is too big to make something small and reliable
  • It's very, very ugly 😀

So I started to look for inexpensive solutions for my PCB to get produced in a "professional" way. And I found exactly what I wanted on Seeedstudio.com .They offer a very cheap PCB service starting at 9.99$ for 5 PCBs of max 5cm x 5cm. Perfect!

Seeedstudio provides you design rule files for EAGLE, so you can see directly if your design will respect their process. I designed a new 2 layer PCB that Seedstudio produced and sent to me in a few days.

The 2 layer PCB for PiWeather v0.2
The 2 layer PCB for PiWeather v0.2

 

I was blown away by the quality of the boards 😀

Here are the two sides of the v0.2
Here are the two sides of the v0.2
It just looks perfect. Nothing more to say.
It just looks perfect. Nothing more to say.
A board ready to be soldered :)
A board ready to be soldered 🙂

 

The v0.2 finally took life and is ready for duty !

In red: the NCP1402 regulator In blue : GY65 barometer White: DHT22 Black: nrf24L01+ The Atmega328p proudly stands in the middle :)
In red: the NCP1402 regulator
In blue : GY65 barometer
White: DHT22
Black: nrf24L01+
The Atmega328p proudly stands in the middle 🙂

v0.2

 

In the next weeks we'll try to build the first outdoor sensor, which will be solar powered. Then we'll probably deploy the website hosting the data gathered by the numerous PiWeather stations 😀

Cheers 🙂

 

Update 25/10/14 : The schematics are on github : https://github.com/psykhi/PiWeatherEAGLE

I also created a Raspberry Pi shield in order to plug the nRF24L01+ 🙂

I will write an article about it!