I have noticed in the forum a number of people advising the connection of ground (negative power) of all their ESCs to the APM. In some cases some people are also connecting multiple 5 volt lines to the APM in the hope of providing redundancy of the 5 volt line. In my opinion both these practices can lead to intermittant issues that could cause your copter to occassionly behave erratically, and at worse damage your APM (as connecting multiple ESC grounds to the APM can create ground loops). Ground loops will create a noisy and unstable ground for the APM, which is the last thing you want with sensitive gyros and accels. The random issues that ground loops/bad power can lead people to incorrectly assume that there is something buggy with the software, causing unnecessary work for the developers and testers.
Ideally a seperate, single BEC (5 volt regulator) should be used to power the APM as the 5 volt line from an ESC that is also powering a motor will get very hot, causing the 5 volt line to become noisy and possibly brown-out/shut-down in flight. The BEC should not get so hot that it is uncomfortable for you to keep your finger on it for more than a couple of minutes with APM+GPS+Xbee etc connected. If so, a BEC with a larger heatsink or a switch-mode BEC should be used. In addition using a seperate BEC also ensures that the APM does not share a noisy ground return of an ESC.
I think a lot of the intermittant and unexplainable issues people have had can be resolved by ensuring the power supply and powering wiring topology is sound to begin with.
Does anyone know who contact about having documentation updated to reflect the above?
Replies
I measured a lot of noise at altitude sonar/baro, I disconnected the wire for measuring at apm (volt divider), thus all the noise was gone. I put on a feritt core and plugged the cable into apm again, the problem is solved.
Very Important!!! I've been building power supplies for years and noise is always an issue- the more sensitive the circuit you're driving, the harder you have to work to clean it up to avoid glitches. Circuitry as sensitive as the APM can only benefit from clean power. So what about some of the standard DC filter systems? Toroidal chokes/ Caps/ Tuned low/high pass filters? Another solution is a small 2s Li-po battery to power all the electronics directly through the APM with a separate big battery for the esc/motors. A separate small battery has the advantage of acting as a broadband filter by just absorbing and overpowering most noise. Also- any wire can act as an unwanted antenna for noise so long as it is the correct (wrong) length. Lots of different length wires around lots of noise sources means more noise in your circuitry.
Redundant 5v supplies should definitely be avoided- they usually don't play nice operated in parallel. Crosstalk between input stages in separate regulators can cause all sorts of ugly noise. ALL grounds do need to be tied together to make sure all devices share the same ground voltage, but any additional grounds will only add noise. I would suggest removing the +5 pin from the esc/bec connectors and just connect the ground and sig-out to the esc's which would provide common ground to the drive system without creating any loops or introducing additional noise.
On a large camera mount, do you think it would be better to get the power for the mount servos from a seperate BEC and just get the signal from the APM. Instead of getting power and signal from the APM???
Thanks :)
I have tried using copteret without ground(-) from esc's to the apm completely unaware of this, because I switched to separate bec right into the pdb. That led to a crash for me. The copter got a strange behavior. When it was below 5 degrees outside, I do not get the copter armed, the engines just beep rapidly.
Good discussion. I have access to modify the Wiki and I can help you get it "published" there if we can come up with a good document.
I'm fairly well off in my case as I have a trad.heli. with a single ESC, and that ESC is opto-isolated, and the motor battery is completely separate from the radio system battery. But, my radio power system is somewhat confusing... I have an L7805 5V regulator powering my receiver and APM. Then I have a 6V switching BEC powering the servo rail on the APM for my three swash servos. And then my tail servo is direct 11.1V to the 3S battery. But all of those share a common ground back to the battery. In particular, the ground for the 3S servo actually goes THROUGH the APM and the APM ground. I had to do that, because when I had the ground for that servo going directly to the battery, I had a noise problem.
It's worth discussing in this article that you want to try to keep grounds seperate, but they must be connected in some way. A signal source operating at 5V, and a signal receiver operating at 5V will not communicate if their grounds aren't connected.
Even on Earth it is important to have a singular ground because ground has resistance and lightning strikes occur.
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