Phantom 2 - Auto Retracting Legs

Phantom 2 - Auto Retracting Legs

Earlier this year (April 2016) I purchased my first 'real' drone (second hand), a DJI Phantom 2 Vision. I had been given a Syma X5SW at christmas, and that little quad got me hooked.

I attended the Drone & Tech expo held in the RDS in Dublin this year. After which I went for some food....By the time I had finished my meal I had organised to buy and collect my 'new' Phantom 2.

One of the things I noticed about most of the larger expensive multi-rotors at the expo was they all had retracting legs. I hadn't even collected my Phantom when I ordered the components I needed to make retracting legs for it.

I am a Mechanical Engineer, but since I have bought this drone I am a full-time researcher, part-time Engineer.

I quickly learned that the Phantom 2 has only one spare channel on the Tx (Transmitter), this 7th channel is commonly used for gimbal pitch control. Since I had planned to mount a gimbal + GoPro as opposed to the normal vision camera (the egg!), using this channel to control the retracting legs was not an option.

There are two types of Phantom 2 transmitter. The old PVT581, and the upgraded version NPVT581. The most notable differences between the two are:

• The upgraded version has the gimbal tilt wheel added to the top left of the Tx.
• There is now battery power LED indicators beside the main power switch.

You should note that, on the older Tx, the 7th channel is hidden just to the right of the battery door. Use the upper hole, as shown in the next image. You will need to buy a gimbal tilt lever to use this channel. (Ebay, Amazon or I got mine 3D printed from Shapeways.)

I digress...

The point is (finally!)..You cannot EASILY manually control retracting legs using the Phantom 2's standard hardware. (Unless you buy this)

So, I made mine auto-retract using this super handy little HML350 board. As far as I know this little board uses a barometer to measure changes in air pressure. Once your Phantom is 2 meters off the ground this board will send a signal to retract the legs. Then on landing, around 1 - 1.5 meters from the ground the legs will move back into position so you can land safely without damaging your gimbal etc. (There are actually four height settings to choose from on this board).

Auto-retract is all well and good, but it's a hindrance if you are working at your drone and you have to raise it up in the air to get the legs to retract, then turn off the drone so they don't return to normal position. So, i also bought this small Dr Mad Thrust Smart Retract Controller. This connects between the HML350 and the retracts servos themselves. It has a small button on it which allows for manual cycling of the gear. It also has the option of soldering a small switch onto it, which you can then position in a convenient part of your Phantom.

Now, before we go any further..I have a 3D printer at my disposal, so I designed my own hinges for the legs. These hinges straddle the two mounting points of the legs and the retracts are mounted between them (see picture below).

You're thinking..why didn't he just buy that kit he mentioned!? Well, I didn't know about this kit when I ordered my retracts..Also, in reading reviews on those kits, most people say the landing gear becomes weak and can collapse on a rough landing. I wanted something strong, durable and I love designing!

I will make the design of these available on Shapeways. Shapeways uses powder printers as far as I'm aware. I printed my own from ABS plastic which is somewhat stronger than the powder version (In saying that there are a lot of different materials available from Shapeways, depending on how much you want to spend.)

You will have to make two metal part yourself for each hinge (or you could print it but it may be weak). If you look in the picture above, I have a 'M' shaped 3mm bar which transfers the torque from the retract to the hinges. This is fairly easily made (You could use a coat-hanger), but I will also put this as an option on Shapeways. Be warned, if printed this may not take the forces exerted on it and may fail. See below.

(Central pin not cut to size)

The wiring for this setup is fairly simple. I wouldn't be the best with a soldering iron, but you only need to solder two wires to the motherboard for power. I have drawn up a schematic for the wiring (below).

Always check on each board which pin is the signal output. This can be a squiggly symbol or a 'P' (pwm); this is where the orange wire should be connected. The red is your positive (+) and the brown is your ground.

I tested my assemblies on the bench before I opened my phantom.

And again when I fitted the assemblies to the Phantom.

After this  I split the Phantom and I soldered in the HML 350, I then found a place to fit the Dr Mad Retract Controller. I took a fin out from the side of the Phantom and placed the board here so that I could access the button to manually cycle the gear. You could purchase a small click button and solder it to this board, this would eradicate the need to remove a fin and would probably result in a neater discrete button.

The final product works well the majority of the time. After this video I changed the height setting on the HML 350 as Phantom needed to be very low to the ground in order to drop the legs. Since adjusting this, the legs drop about 2 Metres or so above ground level.

I have carried out this work for my Phantom 2, but this setup, aside from the 3D printed hinges can be used for any drone once you give the HML 350 a power supply.

One thing to remember with these auto-retract legs is that the HML 350 will gauge pressure where/when you turn on the drone. If you plan on flying and landing this drone on, for example a hill, 2M above your take off point these legs will not deploy. You may be better to buy the kit mentioned at the start of this post. As for this setup, this suits me and I have never had any problems with it.

*This blog post is a record of work I carried out on my drone for my own benefit. I, the author, accept no responsibility for any issues or damage to property as a result of following the  suggestions in this post.*