One note, T-Motor high-end ESC’s apparently have built-in linearization, so you have to lower MOT_THST_EXPO to .1 or even lower.
That is VERY good to know! I found a page on ardupilots documents archive that mentions this exact information: Motor Thrust Scaling — Copter documentation
@Eosbandi Thank you for bringing the possibility that T-Motor 80a 6s ESC’s have built in linearization to my attention. I will have to get in touch with T-Motor to see if they have any info regarding this. However after reading this thread:
(AutoTune in 32inch quad result in strange PIDs, crash in next flight - #6 by AngelAlbeniz)
it seems T-Motors official stance is linearization is not built into any of their ESC’s according to dkemxr in the above linked thread.
AngelAlbeniz from the above thread also discuses issues with MOT_THST_EXPO values regarding his T-Motor ESC’s and motors. The difference is AngelAlbeniz is rocking 32" props and what I can only assume are T-Motor U13 motors. What has me apprehensive about moving my MOT_THST_EXPO from my current .74 to .1 is the data that was obtained from AngelAlbeniz measuring the motor thrust expo in a thrust stand and obtaining the correct value to be .4 for his setup, in addition with the testimony from dkemxr that most T-Motor ESC’s do not have built in linearization (although I will have to confirm).
Because I do not have access to a thrust stand I believe the best course of action is to gradually reduce the value of MOT_THST_EXP of my build (18" props U7 V1 motors) by increments of .1 down to either .4 or .5 observing the results as I travel down the values.
On a side note does anyone have a suggestion on a thrust stand that is capable of collecting MOT_THST_EXPO data in a price range under $600? Ardupilot forum links to these 2 testers
and I’m not sure the $100 Turnigy is capable of collecting MOT_THST_EXPO data (correct me if I’m wrong)
Hopefully correcting MOT_THST_EXPO combined with setting up harmonic notch filtering (thank you @xfacta )Will eliminate the strange de-syncing behavior observed in my initial log post and eliminate the aircraft behavior of rocking diagonally during pitch and roll autotuning.
I will post about the results after implementing these changes.
Thank you all for taking the time to impart your knowledge!
People have had success with the T-Motor ESCs using MOT_THST_EXPO of 0.2 or less, as Andras says, and the combined T-Motor power units (motor with ESC in the bottom) have been measured at 0.4, but it could depend a little on the prop size or something else…
Use this rule of thumb for MOT_THST_EXPO
- set too high you can see instability at low throttle
- set too low you can see instability at high throttle
I’ve heard of other conversations with T-Motor don’t produce expected outcomes either so I’m not surprised that someone from T-Motor replied with contradictory information.
Hi,
In my tests I used a custom made thrust stand, which cost me only about 50 EUR in terms of materials.
I use a 10Kg load cell from Amazon mounted on the end of a large aluminum profile, where I can mount motors with propellers up to 32inch without any problem. Then I use a MAUCH battery monitor with hall effect sensor to simultaneously measure current and voltage. In my experience I do not recommend using battery monitors that have a shunt resistor to measure current as they do not have the necessary accuracy for these tests. Finally, the battery monitor and the load cell are connected to an arduino board that reads these signals and also controls the motor. For safety I also have a FrSky receiver connected to the arduino, with which I can start and emergency stop tests from a safe distance (you know, throttling up motors with 32-inch propellers to 100% indoors can be a little scary hahah). The arduino runs a simple custom code that automatically performs the tests and sends the information via serial to the laptop where I store it. The major drawback of the system is that the battery monitor must be calibrated very well to be able to obtain good results. It usually takes me longer to calibrate the battery monitor than to perform the test itself.
On the other hand, I can confirm you that in my case I noticed a great improvement when I set the MOT_THST_EXPO parameter to its correct value of 0.4.
Any questions you may have about the thrust stand or my previous tests, feel free to ask. 
Just to confirm again, can you say what motor and ESC gave you MOT_THST_EXPO 0.4 with the 32inch prop please.
The power system is an ESC/Motor combo witch came with the M1200 pre made chassis from T-Motor. The motor is labeled as a U8XL KV100 (which don’t even exist in their normal catalog). And the ESC sits below the motor in the motor mount, that is all I know.
Little to no information is available about the combo, even after asking.
The propeller used is a T-Motor Polymer Folding 30inch MF3016. This propeller is different than the original 32 inch fixed carbon propellers that the MF1200 came with, but we changed them because we needed them to be able to be folded.
We have had all kind of problems when tried to use T-Motor gear on large machines. Switched to Hobbywing X8 and never a problem since (excpet a prop failing in flight after a lot of flights).
M1200 has an unmodified 60A HV Flame ESC glued in the housing under the motor.
Good to know! 
When I opened the motor mount the ESC board was bare with no markings or name to identify it.
We opened a Flame 60HV alongside and the boards were identical 
That says it all hahahahaha. Good test!
I took the octoquad out for a flight today with the intention of testing the effects of lowering the MOT_THST_EXPO with respect to correcting erratic behavior on the roll axis, as well as the strange autotune rocking behavior and general instability caused by strange de-syncing of the ESC’s I mentioned in the initial post.
(All tests conducted in alt-hold)
To start out I lowered the MOT_THST_EXPO from .74 to .54 then tested flight characteristics at low altitude by slowly rolling and pitching the aircraft at shallow angles at first, then progressively pitching and rolling faster and at steeper angles or “twitching” the aircraft. (note the aircrafts max lean angle for pitch and role is 35 degrees, 10 less than the default at 45 degrees)
Rinse and repeat the process until MOT_THST_EXPO is .3 ( I went from .54 to .3)
After completing the low altitude twitching tests described above with the MOT_THST_EXPO at .3 and observing NONE of the undesirable instability or strange roll behavior I decided it would be a good time to test out the autotune to see if the rocking would be gone as well.
So I took the aircraft about 20m high and proceeded to engage autotune. Instead of rocking back and forth after the first autotune twitch the aircraft went into a full blown spin midair and proceeded to crash in spectacular fashion:
https://www.youtube.com/watch?v=4GLSU_YF4E0&ab_channel=FinnJohnson
Here’s the link to the LOG from this flight:https://drive.google.com/file/d/1alzGNYQ0ld2dQd2SqcIWxpzUU9QOZs5e/view
I believe a prime suspect in the crash was the fact that I changed the autotune aggressiveness to the maximum value (.1) from .05. I did this because I had been running autotune aggressiveness at .1 while the MOT_EXPO_THST value was .74 with no catastrophic issues, and stupidly my brain said “hey it worked before and its gonna get a really accurate tune, run it at max aggressiveness what could go wrong?”
If anyone has any other ideas on what could have caused this I would appreciate your input. Otherwise the next logical step once I rebuild the aircraft is to lower the autotune aggressiveness to .05 and try again, and if the aggressiveness is not the culprit I would prefer to avoid another incident such as this.
Took a quick look and still looking at it but i see the log is truncated. It looks like pixhawk turned off in flight. From when it start rolling to hitting the ground it takes about 2 seconds but only 0.3 are on logs.
Maybe the fast spinning somehow pulled off the power from the flight controller and that is not the cause but just an effect of the crash dynamic.
It really reminds me of the days i was trying to tune a big quad and it just kept killing itself in flight.
As said before we solved by going to Hobbywing X8. About 1000 flights later they still perform great and never seen a desync since.
I am not trying to bash on T-Motor products, as a matter of fact we have a smaller drone with a T-Motor power system (500 series) wich flies 4-5 missions a day since 2 years ago and never ever a single prob. We just didn’t have same luck on large systems from T-Motor.
Have had exactly the same reaction with a large onto when I failed to set the INS_ACCEL_FILTER to the recommended value as described in the Tuning Guide.
At 20, the default, which you have set in your parameters, On the first twitch the copter over reacted and flipped.
I only had it 5m up, not 20m.
Sorry to see it go down like that but you have to go over all parameters listed in the Tuning Guide.
As @Corrado_Steri pointed out you might also have some other issue as the log stopped very abruptly before impact.
When we did all the testing we used INS_ACCEL_FILTER down to 5 but it didn’t make any difference. I am not saying it isn’t his problem, i am just reporting what happened to us with those motor/esc combination.
Thank you for pointing out that the INS_ACCEL_FILTER was set to 20 instead of of 10 as is recommended in arducopter initial setup documentation.
I had previously gone over the arducopter setup several times during the very first setup of this octoquad and thought that I had everything setup correctly. Before the crash I had done around 10 - 15 flights, most of which used autotune. So either I changed the INS_ACCEL_FILTER setting for god knows what reason before the catastrophic flight, and or the culprit for the crash as soon as autotune began was the MOT_EXPO_THST value being moved from .74 to .3.
Because while I gradually moved the MOT_EXPO_THST value down to .3 and tested the flight characteristics of the aircraft manually I did not such gradual testing for the autotune. For my next autotune tests I will be starting with the MOT_EXPO_THST value at .74 & the INS_ACCEL_FILTER at 10. Which has been those perameters settings for the 10 - 15 previous successful flights, then slowly reduce the MOT_EXPO_THST value, testing the autotune at VERY low altitudes to start until a stable and effective autotune can be achieved.
Thank you all so much for lending your experience and knowledge. I will post an update when I have the aircraft fully fixed and can make time to send her up.
Cheers!
Tmotor tells this to me:
Hello ,
Yes, the ESC have a built in linearizing thrust, but it’s not a straight line, please kindly check attached picture.
As for the parameter: Our default setting is 0.65, but you may need to adjust according to pilot habit.
Please let me know if you have any other questions.
this is tmotor official service
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Can you please attach the image you are talking about and indicate the exact model of T-Motor ESC you are referring to? Thanks
I talk about Tdrones M1200 drone with U8XL KV100 motors
32" fixed carbon prop and integrated esc (i dont know the exact model of the esc, i think that its a standard alpha 60a.
we have well done setup and good flying performances with mot_expo parameter at 0.2
we have make some autotunes and the flight dynamics are very good.
we use 4x 22Ah Li-Ion solid state batterys with about 80min endurance without payload.
at the moment, we are very happy with this drone and this setup
ardu version 4.2.0
only we need to improve and setup, the dynamic notch filter, at the moment, its disabled.
regards from Spain
Saludos desde España tambien!