Welcome to the second article in the Multirotor Beginner's Guide.
In theory drones aren't complicated; the only moving parts are the motors. In reality, there is far more to them. To build a drone from scratch requires at least a basic understanding of physics, maths and technology; and practical skills such as engineering, wiring and even coding. To then fly the things requires further skill and experience. Obviously the degree of knowledge and skill required increases with the sophistication of the UAV. Some multicopters come in plug 'n' play kit form, which means you side step a lot of the technical challenges.
My aim here to is to help reduce the learning curve for you, by providing some useful tips that will keep you on track or at least save a headache.
- When you first set out to build a multicopter, keeping the weight low is probably one of the considerations that reach the top of the list. While it is true there are many efficiency related benefits for keeping a low AUW (all up weight), don't take this decision instinctively; a heavier multirotor does have some advantages. Heavier multirotors tend to be more stable. A heavier craft typically requires a higher propeller speed which translates into greater stability. Additionally the extra mass makes a heavier multicopter less susceptible to being blown around in the wind. Finally and probably most interestingly, a heavier machine needs less pitch to generate forward flight. This is important when it comes to shooting aerial video and FPV, as a light UAV will naturally point the camera down when moving quickly - a problem if you don't have a good brushless gimbal.
- Don't over-tighten screws and bolts! You’ll likely to end up with a big problem on your hands if you do. Over-tightening can stripped out threads or heads, which will prevent you from taking things apart in the future. Instead, tighten fixings sympathetically and use some thread lock to prevent them from coming loose.
- Most drones use cylindrical booms as arms, to which the motor mounts attach. To prevent the mounts from rotating, sometimes it’s a good idea to wrap the tube ends in insulation tape. It can help better seat the motor mounts and provide a little bit of extra friction - carbon fibre arms in particular tend to be very smooth.
- Carbon Frames are conductive! If an exposed cable or connector touches the frame you risk an electrical short. Be particularly careful if you’re using a folding multicopter frame. If your wiring is tight and routed around edges, the insulation can wear over time as you assembled and disassemble your frame. Therefore it is a good idea to use heat shrink on vulnerable areas to provide an extra layer of insulation.
- Ideally your multirotor will operate without vibrations, however eliminating all vibrations is not always possible. Therefore it is good practice to use a thread locking compound on fixing screws to prevent them from coming undone over time. In particular, focus on the critical screws that relate to the motor mounts and prop collets.
- Many electrical components use small jumpers to allow the user to select the correct circuitry for their application. For instance, many UBECs provide a jumper to select between 5v and 6v. Once set, if you know you are never going to need to change it again, it is a good idea to apply a small drop of superglue to fix it in position and prevent it from vibrating loose.
- A good solder joint is crucial for reliability and good electrical conductivity; high resistance in an electrical circuit will hinder the current from flowing at its full potential. After you've completed a solder joint, you should visually inspect it to make sure the joint is good. A good joint will be smooth and shinny. A rough or lumpy surface is symptomatic of a cold joint - one where the solder did not completely melt. At the other extreme, a sign of burnt flux is evident of an overheating. You can also conduct a series of tests to detect a bad joint. The best way to see if you are loosing power from your connectors or wiring is to use two voltmeters (or use one but twice); put one across the battery terminals as close to the battery as you can get them and the other across the load as close to the load as you can get. If you get a voltage differential (known as a voltage drop) you have resistance and a bad connection. Furthermore if you divide the difference in the voltage reading by the current load, you get the impedance. For example, if you were drawing 10 amperes and had a 1 volt difference, the line-connector impedance would be 0.1 ohm.
- Printed circuit boards (PCBs) often have wires which are soldered directly onto them. Most of the time, this isn't a problem if the soldering is good. However, very thin wires (22 AWG) can be vulnerable to breaking off. The 3DR power distribution boards are particularly bad in this respect. It's therefore a good idea to a apply a little glue from a low temperature glue gun to provide a bit of extra strength. You can also use this technique to help keep cable connections apart when they are in close proximity.
- Brushless gimbals controllers (BGCs) and flight controllers (FCs) generally have USB ports for firmware updates, unfortunately they tend to be fragile. Plugging into them can be a bit fiddly; use too much force, or plug into them too often and they can break - scrapping your expensive PCB in the process! A USB extension cable solves the problem. Simply leave it plugged in and position the output in an accessible place.
- A “brown out” is caused insufficient power to the receiver or FC. It is not an issue with the receiver or FC itself, but rather with the power supply. At best you may suffer from momentary loss of control as the systems reboot, but it’s not uncommon to end in a crash. If you are suffering from control problems, a capacitor may be a solution. A capacitor works as a voltage protector and helps to prevent your receiver / flight controller (FC) from dropping below its required operating voltage (but only for a limited period). This helps to eliminate "brown outs" and prevents your receiver / FC from rebooting. While a capacitor will decrease the probability of a brown out, it’s not a complete solution. The only real way to ensure you won't have a brownout is to have an adequate power supply. It’s good practice to have to independent systems, one for the RX/FC and one for everything else, servos, lighting etc.
- Most anti-vibration gimbal mounts use small vibration damping rubber grommets. If these fail or come loose then the gimbal and payload will most likely be lost! As a last line of dence is it a good idea to use some zip ties. Tie them so that they pass through the center of the rubbers and around the edge of the mount, but not to tightly so that they can't interfere.
- Connectors should fit together tightly for a good connection. With use, sprung male bullets usually weaken. To ensure they fit together tightly, carefully prise the prongs apart a little with a small flat bladed screwdriver before conection. Not too much otherwise they will break. If the connector is going to be left semi-permanently together, for instance the ESC to Motor 3.5mm connection, it’s worth appealing some heat shrink over the connectors to help keep them together.
- Arguably this is the most important top tip. If something doesn't work at first, always assume you are at fault and recheck everything again, preferably after a break from it. Never overlook the simplest thing just because you think that it 'can't be wrong' - or 'I've already checked it'... check it again. It's very easy, almost instinctive to assume a component is faulty, often it turns out to be something simple, something you've done wrong yourself, or simply can't see... incorrect polarities, plugs in the wrong socket on an RX / FC or even a broken connection inside heatshrink.
Testing & Tunning:
- Use a servo / ESC tester when wiring up your motors, it’s a lot easier than plugging in your radio control equipment. If you are wiring up an octocopter you can soon end up with a nest of wires, even a quadcopter can be pretty bad. If you haven’t labeled the wires individually, you can easily forget which each wire relates to. With an inexpensive servo / ESC tester you can quickly test which wire connects to which motor and then test whether the spinning direction is correct. Remember if the motor spins in the wrong direction, all you need to do is swap the positions of 2 out of the 3 ESC output wires.
- Make sure your multirotor is properly balanced. Having a well balanced multicopter can greatly help efficiency and stability, as the motors aren’t fighting against each other. The center of gravity (Cog) should be at the point where all arms would theoretically cross if they were long enough. You can test this by suspending the drone from this point and observing if it falls to one side. If you have a more advanced flight controller (FC) you can also check by performing a GPS hold in calm conditions and then analyse the motors output logs. If the plots for each motor have similar averages then the craft is well balanced. Note that the position of the battery can greatly affect the CoG.
- This is a warning that you've probably already heard - If you're doing any configuration / testing work with a battery connected but don't intend to actually fly, it's a good idea to remove your propellers, and thus remove the danger. This is a simple safety precaution that can save you from serious injury. A large propeller can easily open a large wound if you are unlucky or foolish enough to get in the way. A quadcopter has 4 propellers, so there's a large "danger zone" and a very real risk of injury. This is especially true during the setup stage when you're constantly adjusting settings on the control board, messing around with wiring and so on. Quick release prop adapters make it possible to remove your props in seconds, so there's no excuse for not bothering.
- You should always conduct a post-construction ground stress test before the first flight. A non-contact infrared thermometer is an essential tool for post-build checks and testing; how else can you safely check the working temperature of your motors, ESCs and batteries etc? All electrical components have a normal and maximum operating temperature. HP-LEDS are expected to get warm (+100°C) when lit, but get too hot and they will dim or even fail. Note that +50°C is roughly the threshold of heat pain to your flesh… so testing if a part is too hot by merely touching it (I don’t recommend touching any powered circuit) is not a quantitative analysis to be relied on. It may be that by using an IR thermometer you spot a problem early and avert permanent damage to an electrical component (more experienced builders maybe familiar with the dreaded 'grey smoke' and the putrid smell of electrical burning - something to be avoided).
- If you are going to tweak your PID values, update new FC firmware or play with other related software, make sure you make a note of the previous good flying settings. There is nothing more annoying than having a lovely flying multicopter to make a small change and get left with something that won't even take off.
Finally, remember if you are stuck forums are a fantastic resource, 99 times out of 100 the answer is already there. Here are some useful resources: www.multi-rotor.co.uk, www.multirotorforums.com, www.diydrones.com, http://www.rcgroups.com.
More to come...