I had initially raised this question in Arducopter 2.x group yesterday but since no one replied and I really need help, I am posting this question in the Arducopter User Group.

Here is the situation:

I am doing a project to augment Search and Rescue operations as we know it with Semi-Autonomous UAVs. About a month ago we tried to join modules together but we hit a major problem with making the Arducopter fly in AUTO mode. We need to get the Quadcopter following the waypoints before Friday, the project presentation date. 

Below is a screenshot of a log we downloaded yesterday. I have also attached the log to this post with our params file. The waypoint file for this flight only contained two waypoints and an RTL mode. Before this flight we tested Loiter mode and it worked well. In this flight however, we had multiple fly-aways with the Auto mode. 


We believe that our project has potential. We have presented at the NCUR conference last week and won an award yesterday for our project, but for this project to come together we need a video of the waypoint navigation working with system by Friday. 


Thank you Tsoi and Boland. 

Yesterday (4/21) I did three things before flying:

1. Downgraded the FW to 3.1.5

2. Twisted the GPS wires for them to stay together and used a toroid.

3. Waited until I had connection to more than 10 sats before flying auto. 

I ran two auto modes both worked well (the second ended abruptly at the second waypoint but that might be related to the problem I am about to explain).

The main issue that I am having now is the same the one discussed in the discussion forum called "AC 3.2.1 APM 2.5 Throttle pulsates in Loiter". As you can see from the image below and the log (named: 2015-04-21 20-18-29.log) attached, although the APM knows that the altitude is increasing and the error between desired alt and barAlt is increasing, it doesn't compensate.


red (barAlt) green (Desired Alt)


I flew again today (4/22) first with an Alt Hold P gain of 1 and then I increased it to 3 and tried it again. 

Stabilization when Alt Hold P gain = 1:


Stabilization when Alt Hold P gain = 3:


Both flight logs are attached in the response I left today at the bottom. 

Alt hold controller seems like it is broken. Not following the desired value at all. 

What now? Tomorrow is my last day to test and fly...


Frame: DJI F450 kit with motors and escs (original)

Autopilot: APM 2.5+

GPS & Compass: ublox RCtimer version (Ublox LEA_6H-0-002  (19) it also says v1.2) I also have a 3DR MTK v2.0 that I can use

I have done autotune to tune the craft. 


2015-04-19 12-40-12.log


2015-04-21 20-18-29.log

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    • I think the only thing that I had not calibrated yesterday after downgrading was the accelerometer. 

      I will do it now and give it a spin this afternoon. 

    • Have you checked your vibration levels?

      Errant vertical behaviour was what I encountered way back when they changed algorithms.

      Do a flight and log your IMU data.

      There is a good Wiki on vibration management.

    • X and Y vibration levels seem fine (+/- 3):


      the Z vibration level, however, is a bit abnormal:


      Would this much vibration effect the alt hold?

    • The Log for the graphs above is attached here

      2015-04-22 22-34-48.log

    • Yes.


      Generally it's assumed that you have carefully balanced your propellers, ensured that your motors are of a reasonable quality and therefore don't need any balancing. You should also ensure your APM mounting has some level of vibration damping too.

    • This.

    • Have you tried tuning the altitude hold parameter? http://copter.ardupilot.com/wiki/altholdmode/#tuning

    • My Alt hold P gain was set to 1.5 so I decreased it to 1 (which I assume is the default value). I will give it a spin today and see the results. 

      Wouldn't more P gain give me a faster controller.

  • Your HDop goes to 99.99, which looks like you are is simply loosing GPS signal totally. In another thread he has already been recommended to introduce some RF shielding between GPS and Raspberry Pi...  I'd also recommend to cleanup and check your wiring and plugs on both ends, especially the loose GPS wires can be twisted a few times to keep them together and wound through a toroid to help reduce EMI. Then watch carefully in GCS to ensure your HDop remains good throughout all test flights.. don't even think of switching to Auto until you have a reliable HDop less than 3, ideally 2 or lower.

    You also need to ensure that the GPS with onboard Magnetometer are mounted securely so they cannot move in flight, it appears you are using soft velcro which may twist and move in flight even a few degrees which is undesirable. 

    Where is your telemetry downlink located? I've seen others place their telemetry antenna right next to their GPS, which is a big NO-NO... poor little GPS will never hear that quiet quiet whisper of GPS satellites over that loud scream in it's ear :p  there shouldn't be any real need to separate your RC Receiver from your GPS, unless the RC Receiver is a two way telemetry type like many of the FrSky ones. You need to also be aware that processors are often clocked at RF frequencies, so your Raspberry Pi may be drowning the GPS in RF output around GPS 1.5GHz range...  so definitely do the RF shielding between the Pi and GPS.

    As others have mentioned, you really don't want to skimp on the GPS unit used, or the flight controller either really... sorry, not what you want to hear, but I agree, not all GPS units are created equal and your upgrade path for a APM 2.5 is absolutely zero.

    good luck!

    • Go to your local Guitar Center and buy some copper tape.  Put copper tape on the top board under the GPS to shield the RF EMI from below possibly generated by the R Pi.  Also, keep in mind the data received from the GPS unit are coming down to the APM via those wires you have hanging down the side and they can also collect EMI.  You may need to get some ferrite rings and wrap the section of wires (both the magnetometer and GPS) closer to the APM in ferrite rings to filter out the noise.

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