Thursday, September 26, 2013

QUADframe/3DR uBlox GPS Mount

elevated GPS mount

The hexacopter I am building is based on the APM 2.6 flight control system.  That system uses a uBlox GPS module for autopilot capabilities.  Most professional multirotor helicopters have the GPS unit mounted on a rod above the airframe.  For ultimate accuracy the GPS unit must have an unobstructed view of the GPS satellites and be far from any electrical interference from the flight control system and motors; hence the elevated mounting.

Unfortunately, neither QUADframe nor 3D Robotics makes such a mount.  There are some kits on the market but I didn't feel like spending the money or waiting for shipping.  So I decided to make one.  I found some fiberglass rod that I salvaged from a broken umbrella; just the right diameter and stiffness.  I decided to use some rivet nuts to affix the rod to the frame plate and GPS housing.

GPS dissasembled and drilled for rivet nut

I disassembled the plastic GPS housing and used a 5/16" brad point bit to drill a hole in the base of the housing.  I had to drill the hole off center to avoid hitting any of the circuitry.

8-32 rivet nut installed in GPS case

I crimped the 8-32 rivet nut into place.

upper frame plate drilled for rivet nut

8-32 rivet nut installed

I did the same to the frame plate.  The location of the hole in the frame plate was dictated by the length of the GPS wires.  It needed to be relatively close the the APM unit.

fiberglass rod with thread

I settled on a 5" length for the rod.  I cut it to length with an abrasive wheel on my Dremel and threaded the ends of the rod to 8-32.  As expected, the threads didn't hold too well so I secured the rod in the inserts with epoxy.  Once it set, I reassembled the GPS module.


Now the GPS unit is mounted well above any obstructions or interference.  I'll have to wait until everything is assembled until we know just how clean and accurate the GPS signal is.  Fingers crossed!

Sunday, September 15, 2013

Saw Blade Airgun Target

Saw blade target

The past few days I've been suffering from maker's block.  A project I've been working on has been getting the best of me.  I needed a quick and easy project to break out of my funk.  A few months ago I lent my chop saw to a friend who was installing a laminate floor.  When I got it back, the 12" blade was as sharp as a wet noodle.  Rather than throw it away, I decided to save it for a future project.  Good saw blades use high quality steel.  I figured this blade would make a great airgun target.

dull chop saw blade

I grabbed some of my favorite curb-side steel; old bed frames.  I used a cut-off wheel on my grinder to cut the pieces.  I cut the top piece 19" long and the vertical pieces 25" long; 20" for the height and 5" for the ground spike.  I screwed up and cut the spike on the right side on the wrong end of the piece, making the face frame uneven on that side.  I thought about scrapping that piece and making a new one, but it is only an aesthetic problem.  It will function just fine as is.

pieces cut

I welded the pieces together and dressed the welds and sharp edges with the grinder.  I drilled two holes spaced 10" apart on the top of the frame to support the blade.  I cut some old chain to length and bolted it to the blade and frame with 10-24 screws.  Luckily the blade already had expansion slots in it that fit the hardware perfectly.  Drilling through good saw blades is an exercise in futility without carbide bits.  I finished off the frame with a little spray paint over the bare spots.

welded together

weld detail

The blade has a tantalizing hole right in the center; perfect for a bullseye.  In the future, I think I will have to mount some sort of bell behind it.  As I don't have one right now, some Daisy ShatterBlast targets will have to suffice.  I was going to add some sort of mount for the Daisy targets on the back of the blade, but duct tape will work just as well.

targets for bullseye
The blade makes a nice sound when you hit it and the ShatterBlast discs make for a nice challenge at longer distances.  I think I may add a horizontal piece under the blade to serve as a stepping point to drive the spikes into the ground and prevent the legs from twisting.  But for now, it's time to shoot and have some fun.

Sunday, September 8, 2013

Mounting DJI 4114 Motors to QUADframe Mounts

DJI 4114 motor

We decided to use DJI 4114 motors from the S800 for our hexacopter.  They are low KV motors which means they will hopefully draw fewer amps and give us longer flight times.  Unfortunately they have an odd bolt hole pattern and do not fit on the QUADframe motor mounts.

bolt hole patterns do not match up

The DJI wires come out of the bottom of the motor and hit the mounting plate.  So first I had to make a clearance cut in the mounts for the wires.  I put two mounts into my milling vise at a time and located the center line.  I then used a 0.50" end mill to drill through the two mounts leaving a 0.25" deep semicircle on the end of each mount.  The mounts are made of G10 which machines easily.

cutting clearance for motor wires

The next step was measuring the hole pattern on the motor.  The holes are in a triangular pattern.  After much careful measuring, I came up with the pattern below.

DJI bolt hole pattern

I used the DJI motor mounts that came with the motor to locate the bottom hole.  I centered the DJI mount over the QUADframe mount and marked the bottom hole.  I then drilled the bottom hole in each of the QUADframe motor mounts with a #30 drill bit.  With that done, I centered the chuck over the bottom hole and used the X and Y dials to position the bit to drill the other two holes according to the above pattern.

compare original (L) to modified mount (R)

holy cow, it fits!

Luckily all of my measurements were correct and the QUADframe mounts fit the DJI motors on the first try. However, the captive nuts just barely hit the bottom of the DJI motors preventing them from sitting flush on the mount.

captive nuts hit base of motor

So I needed to remove some material from the base of the motors.  I used a grinding wheel in my Dremel to carefully grind small notches in the base where they hit the captive nuts.  I taped up the motors with painters tape first to keep the metal dust out of the motors.  When finished, I blew the motors out with some canned air just to be sure.

small notch ground into base of motor

With that problem solved it was time to mount the motors back on the frame, right?  Wrong.  One motor mount screw was directly under one of the boom blocks.  I had to cut a clearance notch in each boom block so it would sit flush over the screw.  And the other boom block pinched the wires so I had to file a notch in those as well.

clearance cuts on boom blocks

Finally everything fit.  I used a little thread lock on the motor screws and screwed the motors to the mounting plates.  The motors mounted nice and solid; almost a factory fit.  A more elegant solution would have been to machine new motor mounting plates, but I'm under a deadline and don't have the time.  However I expect I will have some carbon fiber mounting plates custom made at some point.

finally mounted

Thursday, September 5, 2013

LiPo Charging Station

The new hexacopter I am building will feature three 8000 mAh 6S Thunder Power batteries.  That's over $1,700 per set, and I plan on having two sets!  Now with that much capacity, my Accucel 6 would take forever to charge them.  And there is no way I am trusting $1,700 of batteries to a $23 charger.  Time to step up my charging game.  I figure Thunder Power must know how to charge their own batteries, so I selected their top of the line charger the 1430C.  It is capable of charging at a whopping 1000 watts!

Thunder Power 1430C

But to charge at that rate, you need a big power supply.  I chose the 1200 watt EFuel power supply.  It's good for 50 amps at 24 volts.  With that combination I should be able to charge a set of three batteries in parallel in about 30 minutes.

EFuel 1200 watt power supply

Now the charger and power supply are not delicate pieces of equipment.  But I would like to mount them inside a case for easy transport.  Also I would like to wire everything up once to reduce wear and tear on the connectors and minimize the chance of wiring something incorrectly.  I have this old equipment case that will fit everything nicely.  However am worried about the air circulation.  This stuff is likely to get very warm and I don't want to recirculate hot air into the equipment.  The best option is to mount everything onto a removable platform that will fit inside of the case for storage.

case for the charging system

I found an old street sign in my pile of materials.  I decided to bend it to fit the case.  The bent ends will serve as legs to allow for storage underneath the platform.  I tried to bend the sheet with my Harbor Freight bending brake.  I knew the brake wasn't rated for stock that thick but I figured I'd give it a try anyway.  Not a chance; the sheet wouldn't even budge.  So I clamped the sign to my steel bench and resorted to a 5 lbs mallet to "gently ease" it into shape.  Once bent to shape I trimmed the legs to bring the top to a height of 2 inches.

bad day to be a street sign

Next I disassembled the charger and power supply so I could punch mounting holes in them.  I guess I could have just zip tied them to the platform but I wanted a cleaner look than that.  I carefully chose the locations so that the screws would fit without touching any electrical contacts.  I then laid out the units on the platform, marked the mounting holes, and drilled the platform.

holes punched in bases

sign bent and ready to mount parts

Both units were held in place with 8-32 screws.  The power supply mounted easily.  I bolted down the base and then reassembled the unit.  The charger was a little more difficult.  Because the sides of the unit also act as the feet, I couldn't tighten down the charger until the unit was first reassembled.  So I put toothed lock washers under the screw heads so that I could tighten the nuts without access to the screw heads.

PSU and charger mounted to base
I decided to add a handle to make it easier to remove from the case.  I found a piece of aluminum bar and hand-bent it into shape in a vise.  The middle was sagging a little from the weight so screwed the handle into wooden blocks underneath the base.


wooden blocks for support

Next I needed to wire the charger to the power supply.  I could have just inserted the banana plugs into the binding posts.  But I wanted something a bit more permanent.  So I soldered some gold plated ring terminals to the charger wires and clamped them down in the binding posts.  Then I secured the wires to the base with cable clamps.  No need to worry about the wires coming out or someone hooking things up incorrectly.

ring terminals for binding posts

The charging system fits perfectly in the case.  Now it is nice and easy to transport which helps considering the system is pretty heavy.  A premium storage solution for a premium charging system.  I feel much better now knowing this $430 system is properly protected.

completed charging system

QUADframe Landing Gear Assembly

QUADframe 215mm landing legs

Lately I have been working a lot with multi-rotor helicopters.  I have built the Turnigy Micro Quad v2 and the JDrones ArduCopter Quad v1.1.  Well now it's time to step it up a bit.  I am going to be building a hexacopter based on the ArduCopter APM 2.6 flight control system and the QUADframe PRO SIX frame.  I will be posting build logs for the complete assembly.  In this first part I assemble the landing gear.  QUADframe has five different sets of landing legs available; from 50 to 310 mm clearance.  I chose the 215 mm landing legs thinking they were a good trade-off between clearance and weight.  I ordered mine with the optional accessory plates for extra mounting options for sensors, batteries, camera, etc.  Assembly is not complicated but since QUADframe does not supply any manual or build videos, I thought I would make my own.

Parts included in the kit.

Included hardware.

Assembly tools: medium thread locker, 2.5mm allen key, 5.5mm wrench, cutters

The first step is to screw three of the thin aluminum tubes to one landing leg.  Use medium strength thread locker to make sure the screws do not vibrate loose.  Once that is done screw a second leg onto the opposite side with three more screws.  Repeat the above steps for the other side.

Attach small tubes to bottom 3 holes of legs.

Next we attach the face plates.  The plates go outside of the legs and are screwed into the small aluminum tubes.  Notice which holes are used in the picture.  Repeat for the other leg assembly.

Attach face plates to legs with small tube.

Face plates assembled.
Then the aluminum angle brackets are attached to the top holes of the face plates using the washers and nylock nuts.

Attach angle brackets to top of face plates with small bolts, nylock nuts and washers.
The two large aluminum tubes are then screwed in with the M4 panhead screws to the lowest holes in the faceplate.
Attach large tubes to bottom holes in face plate with large pan head bolts.
The two accessory plates are attached to the middle aluminum tubes with zip ties.  Be sure to pull the zip ties gradually and evenly to ensure that the plates are centered between the tubes.  If you pull one side too tight, the plate will not be centered.

Attach accessory plates to legs with zip ties.
Next we attach the battery plates to the uppermost tubes with zip ties.  I did not add the velcro yet as I am not sure where I am going to mount my batteries.

Zip tie battery plates to supports.
The four rubber grommets are placed in the large holes on the bottom of each leg.

Insert four rubber grommets into legs.
I marked the center of each of the aluminum skids before inserting them through the grommets.  That made it easier to center the skids between the legs.  Finally the four rubber caps are placed on the ends of the skids.

All in all, not too difficult.  The legs are nice and sturdy.  I can't wait to build the rest of the frame.  Total build time was about 43 minutes.  Final weight is 320 grams.  I was left with some leftover hardware; 2 washers, 2 small bolts, and one nut.  If the above instructions aren't clear enough, you can watch my video of the assembly below.