Assembly

Time to Build a Drone

So, last two weeks were pretty heavy on the content- hypercomplex closed manifolds, SPI busses, I2C busses and all that.

 

Let's get back to working with our hands- time to actually build the drone.

 

Now, we'll be doing this a little differently to last year- you've build carbon fibre main frames, but it'll be up to you to design the 3D printed components that enable flight. A list of components which will be  mandatory is as follows: 

 

  • Battery carrier- this will hold your battery above the drone, away from the propellers
  • Camera mount- provides somewhere to mount the camera. Best to use something like TPU, to absorb vibrations
  • Crash guard- extends past the propellers to prevent propeller strikes

 

However, nobody wants to do all that before actually flying. As such, I'm giving all of you a fully integrated traing frame. There's just one catch: you'll need to each buy a second UBRobotics membership. Just kidding, but the catch is that the training frame only supports 50mm propellers as opposed to the 75mm the carbon frame will fit. With only 2500mm^2 of sweep vs the 5625mm^2 of the 75mm fans, you'll be taking a huge hit to thrust, agility and efficiency. It's also built like a brick, so while it should take crashes like a brick it'll also fly like... a brick. A redeeming feature of the frame is that it should be possible to swap the electronics from it to the 75mm frame with no soldering or complex operations- so you should be able to drop in your carbon frame upgrade easily.

 

We'll be using these frames to learn about PID control- one of the most versatile ways to control robotic systems. You'll also need to demonstrate competence with the 50mm frame before you can fly with the 75mm frame.

 

As per usual, supporting materials can be found on the Github:

You should have been given all of the parts shown below:

Step 1: The Fun Part: Fitting the Grommets

Last year, everyone loved this part. If you don't love it, you're the outlier here. You should have four yelllow grommets with your ESC, and we will provide you with a further 4 grommets for your flight controller.

 

You'll need to cram these through the four 3mm mounting holes on each of the PCBs. If you're struggling with this, ask Tayyab for help. He's really good at this. When he comes over, make sure to move the tutorial off screen so he can't see I told you this.

 

I find pinching the grommet into a point and pushing enough of this through to grab from the other side is a good approach. You can also use a tool such as an allen key to push it through from the back, but be careful not to poke holes in them. 

Step 2: Soldering 2: Solder Harder

Hopefully you got some soldering experience during Sumobots. Good news, you're about to get some more.

 

Soldering ESCs (Electronic Speed Controllers) is a bit different to soldering DC motors and simple motor drivers- an ESC is designed to distribute and get rid of heat as much as possible. This is great to stop it from overheating while you fly, but very annoying when you're soldering it. Today, you'll be using a "tap soldering" method. It's not the best way to solder, but it works very well for drones and it's easy to pick up.

 

To create a successful joint, the most important factor is heat. Solder flows towards heat, and will only properly bond if the pad is hot enough. Normally I'd suggest to turn up the heat on your iron, but in the CTL the irons have fixed heat. The important thing is that we want to apply a very sharp, fast heat to the joint so that the overall PCB doesn't have time to heat up- copper pads might be fine at 250C for quite a while, but the motor driver's controllers are probably only rated for 80C or so. If these spend much time above 100C, you're gonna have a bad time.

 

To ensure proper heat transfer, follow these steps:

  1. Heat up the iron. Solder should melt instantly in contact with it
  2. Apply a little solder to the tip
  3. Wipe hard on the brass cleaner provided
  4. Apply a little more solder on the tip. This solder isn't to go into the joint as much as it is to improve thermal conductivity
  5. Put the iron on the pad you wish to solder. Allow it to heat for a second, and then start to add more solder. There should be enough to cover the pad, maybe 1mm high or so
  6. "Tin" the motor wires. This means to coat them with solder, which should provide an easy surface for the solder on the pad to stick to
  7. Use the iron to melt the solder on the pad, and then push the tinned wire into it. When you see the solder on the pad flow onto the wire, withdraw the iron. Hold the wire as steady as you can until the solder is solid

 

Solder the connections as follows:

Pay careful attention to the polarity of the motor leads, with the blue wires towards the inside and the red wires towards the outside. This means two motors will spin in reverse, which is important to keep the drone stable (more on that next week).

 

There should be small polarity indicators on the large battery pads near the top of the ESC- make sure red is positive and black is negative, or you'll cook the ESC. You can push the capacitor's legs through the tiny holes in the battery pad, but this can make it very hard to remove- so I'd advise against this. The capacitor MUST be connected the right way around also, or it may explode. There should be a stripe with a bunch of negative symbols down one side of the capacitor to indicate this.

 

Last off, make sure to add some heat shrink tubes to the capacitor's legs. You really don't want these touching each other when connected to a LiPo.

Step 3: Insert the Electronics

With the ESC soldered to the motors, we can now put it into the frame:

Step 4: Install the First Bolts

To mount the motors, we'll screw a bolt all the way through and then align the mounting hole with it. You can use 2 or 4 bolts per motor, but start with just one. Ensure the bolts can spin freely in the hole in the frame- some of the holes may be a little undersized. If they do catch, just spin the bolt until is can freely rotate.

 

You can also route the wires through the slits in the ducts. The wires take an odd path, but for good reason- the wires don't need to be trimmed, making assembly easier.

Step 5: Secure the Motors

With everything in place, we can now secure the motors. Make sure the bolts are fully though the frame, and then align the motor's mounting holes with the bolt. Tighten it down, and then move on to the next motor. Once all motors are in place, install the second bolt on each motor.

Step 6: Wire up the Air Unit

Put all the electronics into the frame.

 

It's good design practice to ensure that all connectors only fit if they're supposed to. However, I don't get paid for this so you can only expect so much from me. As such, the digital camera port and the motor port both use JST 1.0 6p connectors. There is a slight difference in how they use them however. The upshot of this is that if you plug the ESC into the CAM port, the board will explode. Not in a cool way either, just a small pop and a lingering feeling of regret. Don't do this.

 

The cable shouldn't be long enough for this to even happen, but please don't prove me wrong here :(

 

(The worst part is your ESC comes with a 8p plug so I could've just used this. Come back next year for your Air Unit MK3)

Step 7: Bolt Down the Air Unit and Add Fans

To prevent overheating, you'll need to add 4 fans to the drone. Seems excessive, but go with it.

 

Anyway, bolt down the air unit and install the propellers as follows. Lastly, make sure your motors are fully secured before installing the propellers- this takes quite a bit of force, so a partially secured motor may be damaged here.

Finished Drone

You're now the proud owner of a bird, or something close. An aviculturalist, if you will. Next week, we'll be doing static testing where you'll see how we can keep the drone steady in the air by adjusting control parameters.