MOT_THST_EXPO for 30" props on FOC ESC

I’m trying to configure an x8 multicopter that uses 30" props on a Hobbywing ESC/motor combo that uses FOC (field oriented control). Details here: https://www.foxtechfpv.com/foxtech-8120-100kv-foc-esc-hv-combo-2.html and https://www.hobbywing.com/article.php?id=205 (I note the latter states “have smoother throttle linearity”.)

The page at https://ardupilot.org/copter/docs/tuning-process-instructions.html suggests that for 30" props, a value of around 0.85 is a good value. However, I am concerned that the FOC ESCs might have totally different linear/exponential relationship between PWM and thrust, as they have a totally differnt method of controlling the motor.

Does anyone have any expierence of this with FOC ESCs?

Also, what will happen if I set the MOT_THST_EXPO to 0.85 and it proves too high? What about if I set it too low? Which runs the risk of uncontrollable climbing on first take off?

I am looking at using a thrust stand but for something so big this will cost more than the multicopter!

Many thanks,

David Ebling

.85 is fine/ I fly a hexa with the same motor/esc/prop combo and it just flies well. make sure you follows the tuning instructions and do an autotune.

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Many thanks Andras, that’s a great help.

This just in from Leonard too:

“MOT_THST_EXPO is the linearization of thrust vs PWM. If this is set too high we see an increase in gain at the lower end of the thrust range and a decrease in gain at the upper end. Therefore when set to high you can see instability at low throttle and if set too low you can see instability at high throttle”

…so you could experiment somewhat and see what MOT_THST_EXPO works best.

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Thanks Shawn. Am I correct to think that an incorrect curve could cause hover throttle to be way too high or too leading to the copter climbing or descending at mid throttle?

If you use ALTHOLD (for example) and have MOT_HOVER_LEARN set to 2 it should be OK. Arducopter doesnt take very long to learn the hover value.
What is more likely to cause uncontrolled climbing is Z vibrations. Have STABILIZE mode ready.

Try changing MOT_THST_EXPO a bit at a time and look for the effects that Leonard mentioned.

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I am having trouble getting these ESCs to work. They are all beeping once per second even when the PWM value is set to 1100 which is meant to be the minimum value. Same result when plugged into a servo tester.

I think they aren’t supposed to be calibrated as the max/min pwm values are apparently fixed.

The specs say the ESCs have no BEC but they seem to be outputting voltage to the pixhawk output rails and will power the servo tester. Perhaps I still need to add a UBEC?

Does anyone know what I might be doing wrong?

You very likely plugged the wrong cable, all you need is the ground and the signal wires, the long black and white wires. The short wires are for programming and firmware update, the yellow/white is for telemetry. (Dont’ ask, it looks like a proprietary protocol, no info is avail)

Thanks, yes you were right about the cables although they’re all the same length on mine. I eventually found a hobbywing manual after a lot of searching!

Hi,
Which esc protocol did you use for these foc esc ? Do they support Dshot and Multishot ? Do they require esc calibration?

Thanks.

Regards,
Raj Patel

PWM only, and no need (actually cannot) calibrate. PWM max min values ar in the leaflet the you got wit them.

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Thanks a lot . Did the default hexacopter pid values work for the initial takeoff? After the takeoff we can auto tune it.

Regards,
Raj Patel

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Depends on the configuration, please follow the Wiki on checks before autotune.

Hello. You can try thise parameters. Hexacopter with hobbywing x8. 10 kg payload. Including battery.