Figure 1: general wiring diagram for an 8 motors
& APM drone
Practical build blog: powering your APM drone (or don’t brownout an APM like the US gov would):
1. Foreword & Introduction:
I would like to introduce a build blog on a critical aspect of a drone’s assembly: how to power your drone & APM. The objective of such a build blog is to be practical. Wherever possible in this blog links will be provided to get the described components. This blog is merely a synthesis of gathered advices I received from many helpful people within this drone community (particular thanks to Randy, Bill, Forrest and many others who taught me lots and are still teaching me lots of stuff)
As a topic I chose “Powering the APM” as it is in my opinion one of the most critical aspect you should care for in assembling your drone. I’d probably be not too far from the truth if I said a majority of drone incidents & crashes are related to power issues or bad wiring/testing (brownouts, wiring mistakes, ground loops, wrong voltage/amps dimensioning, etc.).
Experienced members in this community will find this blog obvious. The blog does target people at early stages of drone build’s learning, like the state I was in a few months ago. It was very hard to find all bits and pieces of information to get a global detailed picture. Let’s hope this blog can help others in their build.
This blog will be written in successive posts as it is a fairly long subject and my time is quite limited. It will also allow others to interact and add complementary input between my posts; to be enriched progressively.
2. Powering an APM Drone: Objectives and Constraints?
Cf. figure 1 above illustrates a general wiring diagram for powering a typical drone. It illustrates 8 motors and ESCs but would remain applicable for fewer motors (battery voltage would probably go down to 3S or 4S for quad motors).
First, what are the objectives ?
-As illustrated in figure 1, a first objective is to power ESCs and associated motors with a high power source in terms of amps/volts, able to sustain the maximum throttle consumption and voltage required by the motors and propellers configuration (with a margin is even better). A good way to dimension this for your own setup is to either measure real values on a test bench (not so practical because it implies you would already know what to do and what pieces to buy), or more practically to use an online tool which is not so bad to give rough estimates here : http://www.ecalc.ch/xcoptercalc.htm?ecalc&lang=en
Another good information source about motor testing is an excel sheet with test bench measurements done by Forrest. Forrest, if your read this, could you post your excel sheet please ? Thx.
The eCalc tool asks you to enter your number of motors, the all up weight of your model, the battery type, the ESC capacity in amps, the motor’s brand and model, propeller’s type and model. You then click on calculate and the tool displays at the bottom 5 results columns. The most interesting columns for our exercise are “Motor@maximum” and “Motor@Hover”, because it will give you the consumed amps at maximum and at hover, the voltage and throttle % to hover (ideally should be at 50%). If you are under dimensioned or over dimensioned (under optimized), the tool will display some warning messages. The tool allows you thus to check that all the parts are dimensioned correctly to function together. For example, assume the tool tells you that your hover power consumption is X. However your motor’s maximum power specification is Y. Verify that X < Y. If not change the motors or use smaller props or lower pitch props (same verifications to do with max amps, max volts, max RPM, etc).
I have used this tool to produce a table for a particular motor brand, RC tiger motors here: http://www.diydrones.com/group/arducopterusergroup/forum/topics/tiger-motors-propellers-combinations-for-an-heavy-lift-octocopter?xg_source=activity
-A second objective is to power all other components that are not part of the high power gear, namely APM and all other bits and pieces around it: a receiver, a telemetry unit, a GPS/compass module, etc. Optional servos that are require to power for gimbals for example ARE NOT low powered items; they CANNOT be powered through APM but must be powered directly from the high power source (via UBECs). As far as electronics is concerned, the objective is to feed a very stable and reliable power source at 5 volts (there may be other specific FPV/OSD components that require other voltages such as 12v or 7v but I exclude these components from this blog for now). This second objective seems apparently easy to reach but it is actually a very sensitive and tricky one as we will see in more details later.
Second, what are the constraints ?
We want to reach above described objectives within constraints:
-The power chains must be reliable and as light as possible (typically if we can use the main battery for everything, it will be lighter than if you need a separate battery for every component).
-There is better one main battery as the power source for everything (which avoids different ground references, avoids potential ground loops, makes your build lighter). So we have a constraint to split the power chains to different voltages/amps (main voltage for ESCs and motors, and 5V for APM and associated electronics). These LIPOs are power monsters people should be aware of: they are capable of delivering very high amperage (e.g.: more than 100 amps in my X8 drone application) at a set voltage between 3S (11.1V) and 6S (22.2V) for most drones. If you make the math, we speak here of order of magnitudes of between 1 to a few kilowatts of power!
There is a link you may consult about batteries common brands discussion here: http://www.diydrones.com/group/arducopterusergroup/forum/topics/6s-battery-comparison-on-some-most-common-brands-turnigy-zippy?xg_source=activity
-The main power source must feed high amps and high voltage to the ESCs+motors. We have a constraint to measure both amps and voltage fed to the whole drone as it is a critical piece of information you want to check continuously on your GCS (or goggles) while flying. It would be insane to fly without such real time information, as you do not want to let your copter fall out of the sky once your battery becomes empty.
An assembly constraint is thus to wire every bits and pieces in such a manner that your current and voltage sensors measure the TOTAL (SUM) of all amps consumption. If you use two batteries , do obviously not connect your sensors after one of the two batteries. Another classical mistake is to connect items pumping current and voltage from the sensors wires!
Avoid connecting items in such a way you create ground loops. Connect everything in a STAR topology, with the center of the star connected right after the current/voltage sensor on the high power leads/wires. As it is difficult to visualize your wiring in the practice, make a wiring diagram of your connections and check it out (like I did on figure 1 for example)!
As you will notice on figure 1, I kept a ground loop with the ground wire of the Attopilot module, but it is impossible to connect otherwise. It is one of the weak point of Attopilot sensor usage, unfortunately. Some posts have been done about this here : http://www.diydrones.com/forum/topics/attopilot-volt-current-sensor-significant-parasitic-ground
-At the same time the main power source must be derived to feed a set of sensitive electronics at 5 volts. APM has a constraint to be fed at a voltage between 4.6V minimum and 5.25V maximum. If you are below the minimum voltage, your APM will brownout. If you are above you will fry APM. However it is not so easy to get a stable 5V source as the load varies. Take a look at the picture below that illustrates how unstable an APM 5V power source could be (and there is even worse):
Figure 2 : Bad Vcc illustration
Figure 2 shows a 5V power source from a switching BEC regulator. The measured voltage on this BEC was 5.14V unconnected. We observe from this log picture that once connected the voltage immediately drops down to 4.9V which is not good. Further a strong variability of the voltage between 4.6V and 4.9V is observed. This brings the voltage close to the power limit of APM’s power requirements with a risk of brownout. We will see later in this blog how this problem was solved.
3. Tested solutions to fulfill powering objectives and constraints:
For the main battery, I chose a dual battery setup to reduce total cost of ownership (and increase max amp capacity). In an objective to increase flight duration to the maximum I looked initially at high capacity batteries such as maxamp 12.000 mah or equivalent from other brands. In a 6S voltage these batteries are awfully expensive (sometimes more expensive than a complete 3DR RTF kit!). Therefore, I found best for me to use two smaller 6000mah batteries. It weighs the same or even less than a 12.000 mah battery, it costs a third or a fourth of the price if you get a turnigy nanotech or zippy brand. There comes a physical challenge to install multiple batteries on a drone without ruining your possibilities to attach a camera gimbal underneath, and without ruining your possibility to use a protective dome for electronics on top. So for my octoquad, I’ve custom cut a dual battery holder CF plate that screws on the center plate, providing two side platforms left/right to fix the two batteries (thx VulcanUAV for their help!):
It looks like this once assembled on the drone’s frame:
After choosing the battery , we need to think about how to wirre in a STAR TOPOLOGY.
I found the easiest way to do this was to use a power distribution board, like this:
I personally chose to use a 250 amps capacity power distribution board from VulcanUAV here : http://www.vulcanuav.com/accessories.html
It is much better and safer than using cable solutions, for example like this:
3.2-Power distribution board
I will now start to detail this power distribution board, based on figure3:
Figure 3 : Power distribution board placement assembly
More to follow (digest this first:)...
UPDATE Oct 28th : Parts 2 and 3 were added on page 1 and 2 of this blog.