Change max throttle in different modes?

Is it possible have mot_thr_max set to different levels for different modes? I want to keep it at 100 for manual mode but not more than 50 for auto/guided.

Another thing is that changing mot_thr_max doesn’t affect turning for some reason. So even if you have mot_thr_max set to 40 in manual mode, turning will still be with 100% motor power

Hi @uchiha,

We don’t support having different maximum throttle levels for different modes but you could use a Lua script to change the parameter value and I think it would take immediate effect. TBH, I’m not sure this is a great idea though and I’d recommend instead changing the desired speed (e.g. WP_SPEED) and then leave it to the speed controller to decide how much throttle it needs. If you find the speed controller is too aggressive then there are parameters like ATC_ACCEL_MAX which can make it less aggressive.

I’m surprised to hear that MOT_THR_MAX is not being respected. Do you have an onboard log of this?

Thanks for the tip. I will see what I am able to do.

On QGC. There is a dashboard that shows throttle percentage. When is set the throttle max to 50. I can see in the dashboard that the percentage doesn’t go above 50 as expected. When I turn left/right. The dashboard shows 0. When I put the boat on a table and look at the thrusters. I can see forward reverse rotates slower according to the throttle max setting. But it turns with full power. So there is no doubt it ignores the throttle max. What logs do you want to see? I have never downloaded logs from pixhawk before.

Throttle max is for maximum forward thrust, not for limiting the steering output on a skid steer boat.

I believe you’re running into this issue where there is no way of limiting the steering thrust.

The only way I found around it is to manually set the pwm range to reduce the thrust from the motors but that will also limit your total forward thrust.

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We have a wiki page here describing how to download the onboard (aka “dataflash”) logs.

Ah, I understand what you’re saying now. This is a skid steering vehicle and you were expecting MOT_THR_MAX to affect the maximum output of each motor. We could change it to work that way but at the moment it only limits the “throttle” component that is sent into the “mixer”. It doesn’t limits the steering component.

So I guess what you’re trying to do is slow the turn rate of the vehicle during missions? There’s an ATC_STR_RAT_MAX parameter that can be used for this.

I can totally understand that when you see the motors go to full (in an autonomous mode) the natural reaction is, “let’s limit the motors” but I think a better response is to reduce the demanded turn rate or acceleration. The reason is that you’ll get better performance if the vehicle only attempts maneuvers that are well within its capabilities with some extra available thrust left over to also handle disturbances.

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The problem with that is it assumes symmetrical thrust output from both motors and propellers dont work like that. With almost any twin screw boat with regular asymetric propellers, you have to either add extra reverse thrust or limit forward thrust in order to get symmetrical forward and reverse thrust, so the boat will always move forwards with motors getting equal power in opposite directions resulting in the boat doing circles rather than turning on the spot.


Ok, thanks for that clarification. Maybe we can add a parameter to allow specifying the maximum thrust that the motors can produce in reverse (expressed as a percentage relative to the thrust they produce when rotating forwards).

… I’m not sure that this is what Uchiha is struggling with though. At least when I read the comments above I don’t immediately come to this conclusion… I just see the concern about the motors hitting 100% in some modes.

it’s just a different result of not being able to limit the steering thrust.

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Yes you are both right @geofrancis @rmackay9

I know about the ATC_STR_RAT_MAX, but I expected MOT_THR_MAX to also limit the steering thrust. Now I know that’s not the case and I will focus on turn rate instead. Thanks for clarifying

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