Motor braking.

[BrianG]

Aragon, do you think this stiffness is enough to stop a vehicle as fast as it does? I figured the ESC would basically energize all phases at once effectively locking the rotor. Of course, this can be controlled via PWM.

If it's just the motor braking by shorting (more or less) the phases, then the motor is taking the majority of the abuse except whatever voltage is dropped across the FETs. This is the only way I can see any energy being fed back to the batteries, but the induced voltage has to be higher than the battery voltage for them to be charged. Maybe this is what Griffin explained above, but I seem to be too dense to understand it.

If the ESC locks the rotor by PWM energizing all phases at once, then the ESC seems like it would be taking the abuse.

[GriffinRU]

ESC only shorts motor phases.

[zeropointbug]

Simple and short, my opinion on braking.

BL motor braking: (most likely mechanics)

first: a shorted motor has incredible resistance from turning, and, the faster it spins, the more resistance it has.

- Using full motor shorting for braking would undoubtedly flip an R/C car over it's lid with ease.

- So, you need to control this shorting in order to control the motor shaft resistance, to create an average resistance or torque.

-Along came PWM... the controller sets up the FET's to unify all motor phases to create a short. So the controller will change PWM duty cycle depending on throttle position... so anywhere between say 1% to 95% (for ABS, guessing) duty cycle for proportional braking.

-These latest controllers (like the Quark) most likely have some good algorithms for braking.

This is just my simple theory on motor braking, not going into detail what happens in FET's.

:025:

[GriffinRU]

Originally Posted by zeropointbug

Simple and short, my opinion on braking.

BL motor braking: (most likely mechanics)

first: a shorted motor has incredible resistance from turning, and, the faster it spins, the more resistance it has.

- Using full motor shorting for braking would undoubtedly flip an R/C car over it's lid with ease.

- So, you need to control this shorting in order to control the motor shaft resistance, to create an average resistance or torque.

-Along came PWM... the controller sets up the FET's to unify all motor phases to create a short. So the controller will change PWM duty cycle depending on throttle position... so anywhere between say 1% to 95% (for ABS, guessing) duty cycle for proportional braking.

-These latest controllers (like the Quark) most likely have some good algorithms for braking.

This is just my simple theory on motor braking, not going into detail what happens in FET's.

:025:

I like it :)

[zeropointbug]

thanks, i tried :)

[BrianG]

Originally Posted by zeropointbug

Simple and short, my opinion on braking.

BL motor braking: (most likely mechanics)

first: a shorted motor has incredible resistance from turning, and, the faster it spins, the more resistance it has.

- Using full motor shorting for braking would undoubtedly flip an R/C car over it's lid with ease.

- So, you need to control this shorting in order to control the motor shaft resistance, to create an average resistance or torque.

-Along came PWM... the controller sets up the FET's to unify all motor phases to create a short. So the controller will change PWM duty cycle depending on throttle position... so anywhere between say 1% to 95% (for ABS, guessing) duty cycle for proportional braking.

-These latest controllers (like the Quark) most likely have some good algorithms for braking.

This is just my simple theory on motor braking, not going into detail what happens in FET's.

:025:

Thanks ZP. I was questioning if the motor phases, when shorted, would have enough braking force. I took a 10XL hooked to an emaxx tranny and attached a drill to the output shaft. I turned the drill on to spin the motor. The drill I used is rated for 3600rpm (assuming unloaded). Working backwards from the tranny (1.722:1) and spur/pinion (51T/14T), the motor was spinning around 23,000 rpm, which was pretty good to emulate the real-world speed of a vehicle.

Anyway, when I shorted the motor phases, the rpms of the drill dropped considerably, but didn't seem enough to support the idea that this braking force would flip a truck or provide adequate braking force for a heavier vehicle (~10lbs), let along using PWM to control the braking force, which would reduce it. That's why I was thinking that maybe the ESC was locking the rotor in a controlled fashion.

Of course, locking the rotor would probably be extremely bad for the ESC and batteries, but it was just an idea.

Also, I fully understand the theory on PWM. ;)

[BrianG]

OK, thanks! That makes sense. I guess I just didn't understand the statement:

Originally Posted by GriffinRU

If controller doesn't open FET's then BackEMF from motor will be rectified by built-in diodes and will be charging battery, if battery voltage will be less then backEMF+voltage drop. You can optimize this process if you open FETs in sync with diodes, then you will cut off diodes voltage drop and losses.
If you noticed braking can be done with only one side, while recuperating involved both.

My confusion stems from the diode aspect. I know the diode is built into the FETs. So, if the FET simply shorts the phases, how can you eliminate the v drop of the diodes?

[GriffinRU]

Originally Posted by BrianG

OK, thanks! That makes sense. I guess I just didn't understand the statement:



My confusion stems from the diode aspect. I know the diode is built into the FETs. So, if the FET simply shorts the phases, how can you eliminate the v drop of the diodes?

Diode conducts in one way only, if you close corresponding FET then you eliminate voltage drop associated with diode. ESC knows exact rotor position and then polarity of the BackEMF, this allows to use FET's instead of diodes to do the rectification.

I agree with zpb. I'd also like to raise another question on this talk of ABS...

ABS is an abbreviation for Antilock Braking System. By saying antilock one assumes that there is a mechanism in place to detect when locking occurs so that action can be taken to avoid it.

Do ESCs even do this detection?

I have a suspicion they do not, and I also think that they don't really need to.

As ZPB already pointed out, a motor's braking force increases with RPM assuming a constant shorting resistance across its poles. This leads me to think that ABS would be unnecessary in a brushless braking system as the motor itself naturally applies less braking force as it slows down.

The only time I can see ABS being of value is if there were devices monitoring the rotation of the wheels themself so that braking force could be released if only one wheel locked (while the diff action kept the motor spinning).

What do you all think?

[GriffinRU]

Originally Posted by Aragon

I agree with zpb. I'd also like to raise another question on this talk of ABS...

ABS is an abbreviation for Antilock Braking System. By saying antilock one assumes that there is a mechanism in place to detect when locking occurs so that action can be taken to avoid it.

Do ESCs even do this detection?

I have a suspicion they do not, and I also think that they don't really need to.

As ZPB already pointed out, a motor's braking force increases with RPM assuming a constant shorting resistance across its poles. This leads me to think that ABS would be unnecessary in a brushless braking system as the motor itself naturally applies less braking force as it slows down.

The only time I can see ABS being of value is if there were devices monitoring the rotation of the wheels themself so that braking force could be released if only one wheel locked (while the diff action kept the motor spinning).

What do you all think?

The key element of ABS is to keep track on traction (maintain traction), have nothing to do with RPM

Originally Posted by GriffinRU

The key element of ABS is to keep track on traction (maintain traction), have nothing to do with RPM

Yes, but avoiding a wheel lock (or skid) is a key part of maintaining traction. Static frictional force is always greater than kinetic frictional force given the same two surfaces...

[GriffinRU]

Originally Posted by Aragon

Yes, but avoiding a wheel lock (or skid) is a key part of maintaining traction. Static frictional force is always greater than kinetic frictional force given the same two surfaces...

You lost me here, or smth else

[zeropointbug]

Yeah, it's not true ABS (like the Quark), it's just the controllers algorithm controlling it. It knows how fast the rotor is spinning, etc.... there is a minimum rpm the motor can spin when shorted anyways.

It is quite simple how it would 'detect' the rotor rpm, it's just a matter of making a good algorithm to do some braking magic. I imagine that controllers only use a fraction of the motor braking resistance.

[GriffinRU]

Originally Posted by zeropointbug

Yeah, it's not true ABS (like the Quark), it's just the controllers algorithm controlling it. It knows how fast the rotor is spinning, etc.... there is a minimum rpm the motor can spin when shorted anyways.

It is quite simple how it would 'detect' the rotor rpm, it's just a matter of making a good algorithm to do some braking magic. I imagine that controllers only use a fraction of the motor braking resistance.

Of course controllers do simple things, we do not have individual sensors and brakes per wheel, but idea is the same.
And check your last statement, because it is the motor which stops the vehicle not the ESC or Battery

[zeropointbug]

Originally Posted by GriffinRU

Of course controllers do simple things, we do not have individual sensors and brakes per wheel, but idea is the same.
And check your last statement, because it is the motor which stops the vehicle not the ESC or Battery

I know that, I was stating that it's up to the controller to do good braking, as many ppl here know that different controllers have much different braking characteristics.