Installing a Zener Diode and Capacitor to help bullet-proof your Pixhawk is easy. Both components may just help save your plane or multicopter someday.
The Pixhawk design allows for the servo power rail to be used as a secondary or backup power supply for the AP. However, the design has very low tolerances such that if power is to come from servo rail (via the ESC or an independent 5V UBEC – the latter is recommended) the servos must not pull more than 5.7v. Note that most micro servos operate in the 4-6v range (with digital servos going > 10v) and when the voltage pull goes beyond 5.7v, the Pixhawk will reboot, resulting in disaster!
To mitigate voltage spikes beyond 5.7v on this servo rail, the recommended mod is to add a Zener diode. When a zener diode is installed in reverse (ie the anode coupled to the ground and the cathode to the current) it acts as some kind of breaker, activating and capping the voltage spikes once its rated voltage is reached.. this means if the correct zener diode is attached in reverse polarity in the servo rail, it will prevent over voltage and therefore not result in the AP resetting midflight. Installing a capacitor in parallel* to the zener is also recommended as it helps smooth the voltage “ripples” further.
To make a Zener Diode and Capacitor for Pixhawk or HKPilot you need the following:
1) Two (2) servo leads 2) One 1N5339 Zener Diode 3) One 6.3v 220uF capacitor
1) For the Zener – make sure that the polarity is reversed, ie the side with the silver band must be soldered to the current (ie middle) servo lead and the other to the ground.
2) For the Capacitor – install as normal.. ie the positive to the current servo lead and the negative to the ground lead.
Here’s how it looks like installed:
and installed in the HKPilot32:
* Coming from an non-electronics background, understanding what parallel connections should be, is confusing. Thanks to “turdsurfer” over in diydrones.com for pointing out what this meant! =)
CTTools: CT Tool for MinimOSD Extra 22.214.171.124 Pre Release r727.zip
2) Setup EzUHF Receiver to send RSSI or LINK quality values. Since you are using a Pixhawk or HKPilot, the setup should be that all the channels goes through channel 1 which is ‘muxed’. Connect to your EzUHF receiver using a USB cable and open the ImmersionRC Configuration Tool. Once connected, just set channel 8 to either RSSI or LINK (only channel 8 will with MinimOSD for now so its pointless experimenting with other channels). You can use either RSSI or LINK, but I suggest you use LINK as RSSI is all about the signal strength and not the actual quality of the link. Upload the settings to the receiver. Here’s how it looks like:
4) Do not edit any parameters in APM or MP. You do not need to change or update any settings in the AP configuration so leave it as is.
5) Get the maximum and minimum RSSI or LINK PWM values in MinimOSD. For this next step, both segments of the MinimOSD must be powered or any updates in the configuration you make with the MinimOSD will not be saved. The OSD output end must be powered with 12v and the other end is plugged to your laptop with an FTDI cable. Like so:
Use the configuration tool to set up the RSSI values (in Panel 1). Make sure you write the configuration to MinimOSD. On the main ‘Config’ panel make sure that you’ve set the ‘RSSI Channel’ to Channel 8… AND make sure the RSSI Enable RAW is checked.
Save the configuration, remove the MinimOSD, plug it into the Pixhawk cable and Vtx/Camera cables and power up your system. On the OSD screen where you’ve set the RSSI to be, you should see a number. Take note of this as this is your ‘maximum’ link quality value. Unplug your transmitter and take note of the value. This time you should see a smaller number. This is the minimum link quality value. For my setup, the highest was 1793 and the lowest was 1056. I just set it to 1800 and 1050 since the OSD is limited to steps of 10. Now power down the system, disconnect all cables, connect 12v power source to the MinimOSD and plug back the MinimOSD to your laptop.
6) Update the MinimOSD PWM Min and Max RSSI Values. Finally, fire up the MinimOSD CTools again and change the Minimum and Maximum RSSI values to the one you got in step 5. Also set uncheck the ‘RSSI Enable Raw’ so you can see the values in percentage format. Save the configuration and test.
The HKPilot32 (and other PixHawk variants) use a single “muxed” connection from the receiver to the AP. What I was not ready for was that the channel assignments were all totally mixed up. The throttle channel is now registering as roll (ie aileron), the roll as pitch (elevator), and the pitch as the throttle! only the yaw channel remained.
Here’s the stock channels using the latest ArduPilot:Plane firmware (3.1.1):
The main issue here is that the channels are fixed on my transmitter (a Spektrum DX6i). A quick check online shows that the consensus seems to be to change the assignment on the transmitter (oh boy) and not on the AP. At any rate, this is not viable for me right now.. so I have to implement it on the plane.
Note that this is only for the hardware combination above and may be different in other systems so a little trial and error may be required. Also, while available, changing these settings are not recommended for APM2.5/2.6.
I decided to bite the bullet and go with the HobbyKing HKPilot32 instead of the Pixhawk for the reincarnation of my Skywalker FPV rig. By most accounts the HKPilot32 works as advertised and why shouldnt it? it has the same design and uses the same chips as the open source Pixhawk. Thank you open source and market forces!
Here’s the content of the packet (L to R: USB cord, Servo wire, Power Module, the HKPilo32, cables for the gps, buzzer, arming button, stickers and vibration pads, free memory card/usb adapter and extension board) and a 4MB sd card pre-installed in the device. The only major downside is it does not come with the 6-pin cable needed to connect the HKPilot to the Quanum Lea-6H GPS.
What surprised me is the size of the HKPilot. it has the same dimensions as the APM 2.6 which is great.
I’ve heard about the poor quality of the soldering and board layout of the HKPilot.. personally, for the price, I couldnt care less as long as it works reliably. Anyway, out of curiosity I checked the board.
In my opinion, the board design and layout is not bad, a bit dirty to be honest but not bad. The soldering while not topnotch seems to look solid enough at any rate.. of course I’m comparing this to the usual hardware I see (computing and networking appliance’s boards). You can judge for yourself above.
So does it work? so far yes, I was able to load the latest ArduPlane firmware and didn’t really have an issue connecting MissionPlanner (except I had to set the connection speed to 38400)
To complete the modifications (see part 1), I felt that not only should the attachments be secure but it should also not require too much effort to remove the elevator. The plastic plate that will be glued to the elevator has a plastic tab that gets ‘locked’ to the plastic plate on top of the rudder. To remove the elevator one needs to ‘lift then pull’ that piece of tab. If you ask me, its too small and requires some unnecessary effort which can damage the damned thing. Here’s what I mean:
It would have been great if they added more plastic to this tab. At any rate, the solution is straight forward, get a paperclip or metal cable tie (my preferred choice) and make a small hole near the tip of the tab like so:
And here’s the finished mod.. which can be easily accessed using whatever tool is at hand.
The updated Skywalker RC Plane 2013 1880cm version I got from FPVModel.com came with a T-Tail configuration. That is, the elevator sits on top of the rudder as opposed to being wedged at the base of the rudder. One of the things I noticed was that the locking mechanism used to connect the elevator to the top of the rudder requires some work as there is simply no way we can make the 2 plastic plate parts to lock. For this a dremel and some sanding paper is your best friend.
Here’s what I mean by the locking issue.
there is simply no way those notches will get in without breaking the plastic:
The solution to this is to dremel or sand the level section (left of the circle above) to provide space for the raised notch to slide on.
Strengthening the Tail Section
I also decided to strengthen the connection by adding the a screw which goes on top of the elevator to a washer and bolt under the plate that will be glued to the rudder.
For this you’d need a screw with a bolt and 2 washers. What worked perfectly for me is a 3.5cm screw. It just has enough length to cover the thickness of the elevator and the two connector plastic plates between the rudder and elevator.
To create the perfect fit do the following (note: this also assumes you have not glued the plates yet to the elevator and rudder):
1) Create a small hole on the lower plate. The lower plate is the plastic plate which attaches to the rudder. You’d want to make a hole just in front of the X studs of the plastic to prevent touching the wire channels (see picture below). Create a hole just enough to fit your screw. A reaming tool (or the exact sized drill bit if you’re using a dremel) is perfect for this type of job as you want to have the exact ‘fit’. Again you dont want the hole to be too big.
2) Attach the two plates together. I assume you have already dremeled or sand papered off some of the plastic for a good snug fit. After attaching, get a marking pen and mark the upper plate through the hole made in step 1. Remove the plate and make the same sized hole which is marked by the pen. I did not do this in one step as I’m using a reaming tool which has a gradually increasing diameter which is not a good idea.
3) Glue the washer and bolt. This one requires some steady hands, since we will be inserting the screw from the top, the washer and the screw must be located under the plate of the rudder. To do this, join the two plates together and insert the screw from the top (ie elevator plate) and insert the washer and bolt on the other end. Dont screw it all the way, instead just screw a bit so the washer and bolt wont fall off. Glue the washer first by using a medium fast acting glue (the 5 Minute Epoxy is perfect for this).. just put a small amount of glue on the plastic and away from the hole, just enough for the washer to be securely locked to the plastic. Slightly add pressure to the glueing process by pulling on the screw so the bolt will be pressed against the washer. Turn the screw every now and then to make sure it and the bolt are not glued in as well.
Allow the glue to set (about 30minutes), do the same to the bolt.. again just enough glue around the edges to set the bolt on top of the washer. After that has set add some a piece of plastic or plywood (in my case both) to add additional strength and secure the washer and bolt to the lower plate. Here’s how it looks like, take note of the location of the bolt relative to the studs of the lower plate. I had to dremel a bit of the plastic so the washer and bolt can fit.
4) Create ingress hole on the elevator. This requires a bit of good guesstimating.. first remove the screw and attach (without gluing) the elevator to the plates. Screw the screw from the the other side (ie from the bottom of the bottom plate where you attached the washer and bolt) and really slowly turn the screw make small adjustments to your angle to make sure it comes out centered and on top of the elevator. Once you’ve done this, remove the screw and screw it from the top and you should be able to have a good straight path right through the washer and bolt at the bottom. Here’s how it looks like:
5) Add a Base Plate on top. Finally look for some small plastic to glue on top of the rudder as the base of the screw like so:
Add a washer before inserting the screw. This mod will add a good solid connection between the rudder and elevator.
And in part 2, I added a some leverage to easily pull off the horizontal stabilizer from the rudder (see here).
This is the manual of the the Boscam TS582000 is a powerful 5.8Ghz VTx. The best thing about this transmitter is its listed output obviously.. but that’s the only advantage, in my opinion and one could get a lower powered transmitter which can probably get almost the same range. As effective range is affected by several factors afterall.
Here are my comments about this transmitter:
1) Replace the Stock Antenna. Using this with the stock yagi antenna, is pointless because of the donut hole effect which results faded signals when the plane is above you. It works best using a circular polarized antenna like the Immersion RC Spironet. In fairness, this holds true to all 5.8 transmitters available in the market today. All I’m saying the cost difference of putting in a circular polarized antenna (instead of the yagi) for such a powerful transmitter would probably have made little price difference.
2) Limited Mounting Options. You’d think by now that the VTx’s which are being sold as for ‘FPV’ would have better mounting options, but they dont. Again, this holds true for almost all of the antennas out there. In my case, I had to reverse the antenna so I can use the mounting screws of the fan to screw it to a custom mounting using a phone card.
3) Clunky Channel Panel. Unlike other transmitters, this particular one is a pain when changing channels. Not only are the switches really small, there’s no visual feedback to show which channel is active. Other transmitters have a small digital-like screen to show the active channel which is really handy specially in the field. In a couple of instances I had to embarrassingly request other FPV flyers if they can change their channel as the 2W is overwhelming everybody and as I said changing the channel on this transmitter is a real pain in the @ss.