Hello,

Here are key points in a few words:

  1. The "BAD ACCELL HEALTH" error may be induced by two different causes:
    • IMU1 error - MPU6000 soldering issue while production in some early batches of PixHawk. Currently is fixed by 3DR.
    • IMU2 error - LSM303D improper discharge process during power off.
  2. This solution is only for the LSM303D case.
  3. Needed hardware parts always were there. They can be controlled by FMU firmware.
  4. For some reason this feature seems has never been used in FMU initialisation. But it was always accessible via nsh CLI as "fmu sensor_reset" command.
  5. It seems the reason why it isn't used in FMU initialisation is a bug in sensor_reset function. It breaks SPI functionality in firmware.
  6. The fix for broken SPI after "fmu sensor_reset" is here.
  7. It permits to use sensor_reset function in FMU initialisation to properly discharge IMU2 to avoid IMU2-related BAD ACCELL HEALTH.
  8. It has been implemented as firmware fix and available here.
  9. Pre-built 3.2.1 image with mentioned fixes are available here for testing purposes.
  10. Currently I testing v3.2.1 fixed firmware. You can do the same on your own risk.
  11. There is a chance it will not resolve the issue. Then hardware fix will be needed. It is proposed below also.
    • It could be implemented with a minor design change (Q601 replacement).
    • Also it may be implemented as post-production hardware workaround (SMD resistor soldered on top of C506).

UPDATE 1: 20ms is not enough

Tests were performed with some number of flights. This fix don't breaks anything. But it doesn't work with 20ms discharge time. Going to increase it up to 1s, but I have not too much hope it will do. It seems pure software solution will not fix it w/o hardware fixes.

UPDATE 2: It depends on temperature

I have two items of HKPilot32. I flashed item #2 with original ArduCopter-3.2.1 Item #1 already flashed with 1s power reset fix. I put item #1 in refrigerator and started with item #2.

Powering attempts numbered as they were performed in common sequence. Powering cycles were performed by battery disconnection and connection back. To reproduce issue a disconnection time should be less than 1 second. My results are below:

Item #2, original 3.2.1, room temp about 22C
--------------------------------------------
bad: 2 4 5 6 7 8 9 10 11 12 14 15 16 17 18 19
good: 1 3 13 20

Item #2, 1s reset fix for 3.2.1, room temp about 22C
------------------------------------------------------
bad: -
good: all from 21 to 40


WOW! Doesn't it work?! Let's check it with cooled item #1.
I took item #1 out of the refrigerator and put there an item #2.


Item #1, 1s reset fix for 3.2.1, from refrigerator
--------------------------------------------------
bad: 43 44 45 46 47 48 49 50 51 52 
good: 41 42 53 54 55 56 57 58 59 60

No luck. What about to warm it a bit?


Item #1, 1s reset fix for 3.2.1, after 30 minutes in room temperature
---------------------------------------------------------------------
bad: -
good: all from 61 to 80

It looks like it works for room temperature! But what about cooled item #2?


Item #2, 1s reset fix for 3.2.1, from refrigerator
--------------------------------------------------
bad: 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100
good: 81

Being cooled it doesn't work again.

All logs are here. Next step I going to play with 5sec power reset on cooled FMUs..

UPDATE 3: It mostly works for 5 sec power reset time

Item #1, 5s reset fix for 3.2.1, from refrigerator
--------------------------------------------------
bad: 107
good: 101 102 103 104 105 106 108 109 110 111 112 113 114 115 116 117 118 119 120

With 5s reset time it looks like it mostly works for the same item and same temperature.
Also it seems for the attempt #107 I've had really small (about 0.2s) disconnection time. It is an unlikely scenario of actual copter use.


Link to test image is updated, now it have 5sec reset time fix. It would be nice to have test results from people suffering from this problem.

UPDATE 4: It seems temperature dependency is not about capacitors

I measured discharge process on C506 of the same FMU item for two cases: a) FMU have room temperature and b) FMU has been cooled 30 minutes in refrigerator. It appears that measured values difference is really subtle. It is about 20mV difference for discharge level on 5 sec period. So it seems the temperature-capacity dependence is not a factor for this issue.


UPDATE 5: MOSFETs test results

Performed tests with SPICE models and in hardware (a number of MOSFETs were used) demonstrated that issue cannot be resolved reliable w/o hardware fix due to the fact Rds goes high when Vds and Vgs going down. VDD_3V3_SENSORS rail needs to be connected to GND via 220 Ohm resistor and it still need a software fix. As result the EN1 input of mic5332 doing its job perfectly w/o any MOSFETs. But it will not work reliable for the case with resistor connected between any other power line and GND, this way the issue still possible to happen for cases of short power disconnection.

UPDATE 6: hardware fix is enough for most cases

I must admit that it seems working ok even without a software fix. Just performed a number of tests with original ArduCopter-3.2.1 firmware on cooled (all night in refridgerator) board with 220 Ohm resistor which is installed in parallel to C506. As result: by disconnecting and connecting battery back with my hands I was unable do it in enough short time to reproduce issue. All logs from attempt 121 through attempt 140 are showing IMU2 works just fine. Software fix still needed for non-fixed boards only. In case you are able to (de)solder SMD parts the better place for resistor is instead of Q601, you need to desolder Q601 and solder resistor on emitter and collector pads.

Owners of non-modified boards are have to wait for a software fix. It have a chance to be included in ArduCopter-3.3.3.

Here is a long story:

Some time ago I decided to switch from APM to PixHawk, so I bought two items of HKPilot32 FMU from HobbyKing. So far I can see they are clones of PX4 FMU v2.4.5. Boards are labeled as "px4fmu-2.4".

Those days I had not too much experience with PX4 platform, so all intermittent issues and misbehaving were considered as a lack of my skills. When I first stuck with BAD ACCEL HEALTH issue I googled for the issue and found some mentions of power filtering and so on. So I decided to put some ferrite rings on power lines to FMU and FPV, to decouple possible noise and spikes on power lines. It seemed as it helped me for some time and I've seen no more BAD ACCEL HEALTH for a long time.

But later it happened again. So I decided to investigate it in a more details and googled this thread.

While checking my DataFlash logs I realised it wasn't MPU6K issue. It was definitely LSM303D issue. I read through all in the mentioned thread and I collected all important points together in a single document. Also I collected there some important points drom other forums like HobbyKing and Plololu. (All 'hello' and 'thanks' were skipped.)

I discovered that possible key to the problem is in the message by Artem on February 19, 2015 at 11:48am: "first hit is pololu community forum thread. Apparently lsm303d is very sensitive to how it needs to be powered down". So I googled once more and found original message on Pololu forum: "When we were testing the accelerometer on the LSM303, we noticed we could get a similar behavior where the accelerometer constantly reported a single value for all axes. It seems that if you interrupt power to the accelerometer in a certain way (like disconnecting or turning off/on power) so that the voltage falls below a certain amount but not all the way to 0, then it can brown out and get stuck in a bad state".

So I disassembled my HKPilot32 and downloaded all pix4fmu-2.4.5 schemas and pcb layouts. As you can see there are capacitors (C506, C507) on VDD_3V3_SENSORS net close to LSM303D, also there are some capacitors on the same net close to LDO and some capacitors should be close to other sensors:

Having a hope that I would be able to solder SMD resistor on top of C506 I found exact location of C506 and started to measure LSM303D power voltage on C506 pins:

Here is how LSM303D power drops down once battery get disconnected. There are no any other modules connected to FMU or PM (Power Module). As you can see it takes about 50ms to drop on level 0.8V and it takes about 0.1 sec to drop on level 0.6V. And it doesn't fall below 0.344V even after 1 minute since battery get disconnected from PM. It's definitely not good. And it can be even worse in case other modules are connected to FMU or PM.

I was ready to solder SMD 1k resistor on top of C506 when I suddenly discovered Q601. Then I traced VDD_3V3_SENSORS_EN net and..


WOW! It is controlled by PE3 pin of STM32F4 and it is there definitely to discharge sensors power line and to shutdown LDO output. By design it looks like it should do sensors power line shutdown in a proper way. It was designed to do exactly that we need to avoid BAD ACCEL HEALTH.
So why it doesn't work, what I missed? Broken Q601 transistor? PCB issue? Anything else? Well, let's measure it on R620.

To shutdown sensors power line the base of Q601 should go down with logical zero on PE3. And I believe it should happen on FMU initialisation. Ok, FMU is powered, pushing reset button..

During FMU initialisation it goes up for some (really long 5.4sec) time, until PE3 will be configured as an output. Ok, no problem. Then I setup trigger for my oscilloscope on 3V level to catch voltage drop and no luck. It means Q601 doesn't receive logical zero from PE3 on FMU initialisation. At all.

NOTE: Hereafter I will refer to source code of version 3.2.1.

Ok, it's to time to dig the source code. What about to grep PX4Firmware sources for "SENSORS_EN" word? Here it is. It is used in "sensor_reset" method to do exactly what we needed. Let's see where "sensor_reset" methods is used. Nowhere else, for the moment of v3.2.1 released it is used just in the same file "PX4Firmware/src/drivers/px4fmu/fmu.cpp". What about v3.3.1? Checked, the same.

After browsing fmu.cpp file I can state definitely: for versions 3.2.1 and 3.3.1 it is used just to provide ability to run nsh command "fmu sensor_reset" with an optional parameter. Nothing more. And it definitely doesn't used by FMU initialisation process. Ok, let's try to execute "fmu sensor_reset 20" to shutdown power line for 20ms.

It works! As you can see it falls down to 0.8V in 1.8ms and to 0.6V in 10ms.

The bad side is that it doesn't falls down to zero due to Vce(sat) parameter value for Q601. Q601 is BJT transistor of type MMBT3906 and it have Vce(sat) from 0.25V to 0.4V. So VDD_3V3_SENSORS net cannot be discharged below Vce(sat). In case a pure software solution with PE3 and Q601 will not do for BAD ACCELL HEALTH, then design for Q601 should be changed to use some MOSFET instead. It permits to discharge sensors power line much closer to zero level. Also there are low Vce(sat) BJT switching transistors on the market.

The good side is that level of power line discharge isn't an only important factor of issue. The rate of discharge may also affect LSM303D behaviour and we able to fix it right now. Anyway, here is chance it will do so let's try it.

I implemented it this way and performed FMU reset by pressing reset button. Next moment my board is turn on multicolour LED as red and it means a error while initialisation. I connected to PX4 terminal and found following startup error messages:

Starting APM sensors [MS5611_SPI] on SPI bus 1 at 3ms5611: interface init failed
Error in startup

Then I turned on debug messages and realised that SPI is affected:

Starting APM sensors [MS5611_SPI] on SPI bus 1 at 3ms5611: prom all zero
ms5611: prom readout failed
ms5611: interface init failed
Error in startup

After some reading of "sensor_reset" source code I discovered the cause of SPI issue. It doesn't enable back SCK/MOSI/MISO pins after reset. So I fixed it and then FMU loads ok.

As a bottom line:

  • I unable to see why mentioned hardware feature (Q601) being known for a long time is never been used in FMU initialisation.
  • It may fix IMU2-related BAD ACCEL HEALTH issue. Needs to be tested.
  • In case it will not, then really minor design changes are needed to fix it in reliable way.

Comments and corrections are appreciated!

Regards, Dmitry Prokhorov

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thank you!

Pull request to hardware repository has been submitted: https://github.com/PX4/Hardware/pull/33

Next are rc.sensors and rc.APM scripts

To make it clear what is going on for this moment with fixes..

There are 3 fixes needed:

1) Hardware fix for next minor version of board, submitted https://github.com/PX4/Hardware/pull/33

2) Software fix for PX4FMU::sensor_reset method to fix SPI error after reset performed
2a) Has been accepted by Lorenz Meier to master branch of PX4/Firmware repository. But this repository is for PX4 flight stack, not for Ardupilot. Ardupilot PX4Firmware just based on original PX4Firmware, it differs a bit and accept all fixes not immediately.
2b) It has been accepted by Andrew Tridgell to master branch of diydrones/PX4Firmware (it is Ardupilot version of PX4Firmware). But it hasn't been accepted to ArduCopter-3.3 branch, so I unsure it will be accessible in 3.3.x. To fix it in 3.3.x I just submitted my pull request to ArduCopter-3.3 branch https://github.com/diydrones/PX4Firmware/pull/44

3) It's still to do a fix for rc.APM startup script to use 'fmu sensor_reset' command in case lsm303d stuck in a wrong state. It has been proposed to do this way by Lorenz Meier https://groups.google.com/forum/#!msg/px4users/Ek8D5hRZeIs/dWvDyZs-BAAJ

Few points more are in UPDATE 5.

Hi Dmitry 

I had many BAD ACC HEALTH issues on PixHawk board V2.

In your last post on November the 28th I understood this :

- HW modification, I'm able to do it, please could you confirm me that this solution is accepted & tested ? Justo to avoid misunderstanding and folowing roll back I'm impressed by your analysis. 

- Firmware 3.3.1 for Quadcopter (Arducopter - line) , I read thet your mod proposal isn't yet accepted, isn't so clear if the HW mod requires the software update to be effective or not or more simply is for robusteness.

- I'm sure that with your answers I'll do your proposed mod

Thanks

Very Good

Paolo

Hi.

So does LSM303D issue affects only Pixhawk clones or 3DR boards too?

My own experience shows that original ones aren't affected.

I'm sorry in case you pointed it somewhere earlier.

Hi, I'm using a clone one.

I'm doing some check with careful acc & mag calibration,I'll post results.

Thanks for yor reply

Paolo

Hi Paolo,

What company is manufacturer of your Pixhawk? Is it original Pixhawk from 3DR?

I can confirm that hardware fix is tested by me, results you can see in my update 6 in this topic. It hasn't been tested by other persons so far I can see. But Lorenz Meier explained me such kind of fix has been adopted two years ago by some manufacturers. Guess 3DR may have the same fix adopted. So far I can see my board change hasn't been accepted yet https://github.com/PX4/Hardware/pull/33 but Lorenz asked me to provide it. HW fix is enough for most of users. Being used alone, w/o SW fix, I believe HW fix able to provide robustness, say, about 95-95%.

Currently changes for Arducopter are in process of communicating with developers. They are not yet accepted and still in progress.  SW fix is just for 100% robustness. It needs to be used for boards w/o HW fix and with really little probability for boards with HW fix.

Hope it helps,

Dmitry

So far I have no my personal evidence that 3DR original Pixhawks are affected by this issue. But some persons in this discussion are reported they does affected.

Will test original Pixhawk as soon as I will have it available.

Yes, but they wrote about generic "BAD ACCEL HEALTH" one. It can be IMU1 fail.

Good point, thanks. It means they need to contact 3DR to replace bad items. Anyway we cannot state anything definitely so far. Neither "original pixhawks are good" nor "they are bad".

Hello, my PixHawk (clone bougth directly in Cina) is the version version2.

I found both the capacitor, the one to be paralleled with resistor, and the transistor to be exchanged by the resistor.

I adopted the first solution, the bad accel health disappears at low temperatures, in Turin, Italy, in the evening is cold roughly 4°Celsius, so I did the test directly on my balcony.

I need to do more trials to confirm / measure the effectiveness.

I saw in GIT HUB related PixHawk hardware, there are many version, the Q601 seems not present in earlier versions, dos is it correct ? 

Many thanks 

Hi Dmitry!

I have a 2.4.5 based board with the same issue. The pixhawk works fine always, except when changing battery (ie not used in a few hours), but now i have issues when it is cold outside, i am unable to arm at all, even if i wait a long time.

After all the testing you have done, what should i solder onto the board to fix it? replace R621 10 Ohm resistor to 220 Ohm and to connect D and S pins of Q601?

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