When a drone slows down its prop motors, both in flight and on the ground, does it actively brake them?

[Deleted member 103366]

This question just popped into my mind.
When stopping them after landing they do seem to stop rather quickly. It made me wonder whether they actively brake them or rely on the interaction of the magnetic fields and or air resistance or all three.

 

[Yaros]

No, it just applies more power to the opposite side motors to brake faster if needed.
If you brake slowly then just reduce the power to the motors that are pushing it in the direction it was going.

 

[Deleted member 103366]

Thanks, but what happens when one commands a vertical descent or a motor stop?
I suppose the answer, for the former, is to look at a very magnified view of the motor speeds in the DAT vs throttle position.

 

[sar104]

No braking is required - the aerodynamic drag is large, and you can also feel the zero-current motor resistance if you turn the motors by hand.

 

[MARK (LI)]

It's part of what is so interesting about these things...with the 4 props working with and against each other to produce the different directional movements ...controlled by the little sticks in your hand, the command going through the air to the aircraft ...and virtually immediate response...the 6 prop ones must be really complex

 

[db4476]

I'll chime in on this, as I have experience writing code for brushless motors. The magic works by magnetic fields and pulses. There are two components to a brushless motor, on the left is the stator with the coils that when energized by a pulse from the driver (ESC) creates a magnetic field which pulls the rotors permanent magnets in one direction or the other depending on polarity.

To increase speed, the pulses from the driver increase in frequency, conversely to decrease speed the frequency decreases. It all happens very fast, up to thousands of time per second.

To do a quick motor emergency stop, there are two methods that can be used. Either hold the coils energized continuously (no pulses) or reverse the polarity of coils for a set period of time.

Hope this helps...

Cheers!

 

[sar104]

I'll chime in on this, as I have experience writing code for brushless motors. The magic works by magnetic fields and pulses. There are two components to a brushless motor, on the left is the stator with the coils that when energized by a pulse from the driver (ESC) creates a magnetic field which pulls the rotors permanent magnets in one direction or the other depending on polarity.

To increase speed, the pulses from the driver increase in frequency, conversely to decrease speed the frequency decreases. It all happens very fast, up to thousands of time per second.

To do a quick motor emergency stop, there are two methods that can be used. Either hold the coils energized continuously (no pulses) or reverse the polarity of coils for a set period of time.

Hope this helps...

Cheers!

View attachment 138551

Agreed, but are you saying that the DJI aircraft do use active braking? Or just that PWM inherently produces a retarding torque when reduced?

 

[db4476]

Agreed, but are you saying that the DJI aircraft do use active braking? Or just that PWM inherently produces a retarding torque when reduced?

There is no actual brake on the motors, it's all done by holding the coils high (coils energized) for a short period of time, say 1/1000th of a second creating a magnetic field which moves the motor. When the pulse ends, the magnetic field collapses leaving the rotor in "free state", but the rotor is moving and to keep it moving a series of pulses are needed. To brake you could just hold the coils high continuously locking the rotor's magnets to the coils position.

 

[sar104]

There is no actual brake on the motors, it's all done by holding the coils high (coils energized) for a short period of time, say 1/1000th of a second creating a magnetic field which moves the motor. When the pulse ends, the magnetic field collapses leaving the rotor in "free state", but the rotor is moving and to keep it moving a series of pulses are needed. To brake you could just hold the coils high continuously locking the rotor's magnets to the coils position.

No - I understand that there isn't a physical brake - I was asking if you think that the motors are actively slowed (by holding the stator coils high) or not.

 

[Deleted member 103366]

I have just hand spun and watched the motors on a switched off M2Z, then I did the same thing with the drone switched on. I would struggle to see any difference in how quickly the motors stopped rotating in those two situations,
But, when I watched the motors stop on landing, it seems to me that they stopped much more quickly than they did in the preceding situations.

 

[db4476]

No - I understand that there isn't a physical brake - I was asking if you think that the motors are actively slowed (by holding the stator coils high) or not.

To slow a motor down, such as taking the drone from hover to descend, it's just a matter of lowering the frequency of the PWM. Watching my drones descend on a breezy day I can hear the prop pitch change as the drone compensates for the breeze it is flying in.

Cheers!

 

[sar104]

To slow a motor down, such as taking the drone from hover to descend, it's just a matter of lowering the frequency of the PWM. Watching my drones descend on a breezy day I can hear the prop pitch change as the drone compensates for the breeze it is flying in.

Cheers!

Right, but that still doesn't address whether the coils are held high on shut down.

 

[db4476]

Right, but that still doesn't address whether the coils are held high on shut down.

The coils will be low, you can spin them

 

[DoomMeister]

The coils will be low, you can spin them

I believe you are addressing methods used to control brushless DC motors in general. Not specific methods that are used in DJI aircraft. I assume even if you wrote code for DJI you could not reveal any specifics due to a NDA.

 

[db4476]

I believe you are addressing methods used to control brushless DC motors in general. Not specific methods that are used in DJI aircraft. I assume even if you wrote code for DJI you could not reveal any specifics due to a NDA.

Controlling a motor is basically the same whether it's a drone, washing machine or anything else with a motor. You change the inputs to achieve the desired results. What is challenging is taking the sensor inputs and building a decision tree that works well under varying conditions.

Cheers!

 

[Capt KO]

It's Magic! or magnets...it works, as long as you have gps on. In atti you get to see the power changes required to slow and stop the drone as you become the brakes instead of the app. GPS is magic too.

 

[Brojon]

No braking is required - the aerodynamic drag is large, and you can also feel the zero-current motor resistance if you turn the motors by hand.

Exactly - the magnetic poles have a substantial effect on braking.

 

[AtmosphericArts]

As far as I know only racing/FPV style drones have active braking in ESCs. It makes a massive improvement to agility at the expense of huge voltage spikes and possibly decreased efficiency (could be wrong in that last part). Active braking with KISS and BLHeli ESCs was the single biggest performance leap for acro drones to date.