what the.... the MOTOR takes the abuse, batteries are not involved with braking. Did you miss everything that was said? :032:

Serum, grab a brushless motor and give the shaft a spin. Now connect all three motor leads together and spin it again. Notice how much stiffer it is?

Now in that scenario tell us in what battery or ESC the energy is "being stuffed"?

Most of it will be absorbed by motor, some by batteries...

It would be really hard to measure with eagletree what's going on with braking with normal approach, but if you add current sensor on one of the motor leads then you can measure what we are looking for. But to make it clear what has more or less harm ABS or proportional braking would be still difficult. And even with the last one it would be impossible with eagletree's resolution to see anything useful...

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.

ESC only shorts motor phases.

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.

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 :)

thanks, i tried :)

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

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

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...

What resolution do you think would be necessary? My DPR can sample at upto 20 times per second (I think), and it is capable of measuring current flow in both directions...

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.

The software engineers of the controllers are putting 'assumptions' into the algorithms, if you catch what I mean? It's hard to explain this. But also, maybe it's monitoring current levels, and there is a max?

A simple braking alg. would prob do some pretty decent braking effect. But who knows how sophisticated they are, or maybe they are simpler than one might think?

Much faster, at least 2 times faster then number of PWM steps times switching frequency.

You lost me here, or smth else

I don't think so, but it is Ok to assume that :)

what do you mean?

NVM then

Yeah, if the motor stops dead immediately, but that's not the case. since you are trying to slow down the vehicle and not locking its tires.

Go figure boy..

Use the motor as a generator to slow down the vehicle, that's the principle of braking.

And this is correct.

BTW, the ABS can 'easely' be programmed; if a motor turns, it's a generator, and by the amount of energy it produces the speed of the turning motor can be determined. Slowing a vehicle down usually take a certain amount of time and the controller can measure the energy that the motor is still producing.

Who ever ran a quark on a rather direct drive vehicle (short gearing ratio) and a low KV motor knows that the brakes work too hard. It's just a matter of software, this software assumes a certain brake power is needed.

Again; that guy at schulze (who developped his brushless controllers) told me the energy was stored back into the batteries. No need to think he was wrong. He seemed very interested in my runtime of my savage, when i told him i was going to use the mechanical brake instead of the motorbrake. So something must be happening with the batteries, agree?

And zero; if the energy is stored in the motor like you suggested, why does a controller get hotter if you use the motorbrake? (the motor would get hotter too, no doubt, but with your theory ONLY the motor would get hotter.)

@ artur; why does the controller need to know the possition of the rotor? all three phases can be shorted with PWM instead of going the complex route of following the stator.

Did I say the motor is only takes energy? that's true, most of it, esc I would assume takes some, obviously a little from internal resistance (not much), and switching, and losses from whatever else goes on with the FET, like Artur said there is a voltage drop in FET?

I have said it before that braking IS hard on a ESC, but when you look at total energy , the esc probably dissipates small percentage of energy.

I can see regenerative braking, but let the question be asked whether the efficiency when motor is acting like generator is great enough to produce a voltage high enough to charge batts. It might depend of motor? Do you think?



"Yeah, if the motor stops dead immediately, but that's not the case. since you are trying to slow down the vehicle and not locking its tires.

Go figure boy..

Use the motor as a generator to slow down the vehicle, that's the principle of braking."

-What are you saying?

That would be determined by the internal resistance (inherent with KV) of the motor.

The motor generates energy once the vehicle is running; some motors KV value are determined by spinning the rotor at X rpm to make 1V. Don't know if this AC is measured over 2 wires or if it's the sum of all three phases.

Basically every electric motor that uses magnets can be used as a generator. It produces energy once it spins. This is the reason why shortening the wires affect the turning resistance of the motor.

You said it yourself; if it spins faster, it has more resistance with the wires shortened.

If you have no batteries attached and you spin the wheels at 60-70mph and the truck is geared for 20mph i am pretty sure the controller will burn.

Perhaps even sooner, but the idea is obvious i guess.

schulze tells the world about the loose contact; he told me that a loose contact is worse on braking than on accelerating; this combined with the fact of his 'store the energy in the batteries' made sense to me.

To be in sync with rectification provided by diodes, otherwise there will be less energy put back to battery.

To charge the battery you need current. Higher current means higher energy density, in battery system means more energy being put back in battery in short time.
Now when you discharge battery under normal driving (acceleration...) battery voltage goes down based on voltage drop associated with internal resistance. Internal resistance related to battery chemistry and driven how fast this chemistry goes. This mean that for a short period of time their would be a nice current going to the battery via diodes (or FET's if controlled, very efficient) due to the earlier voltage drop on battery and it's time period will be driven by battery recovery time and vehicle dynamics (it can slow down faster).

As simple as charging battery with 100C for a 0.1 sec...
That's also explains why nice cap is always helps, but that is different story on parallel thread :)

Schulze's comment about loose contact business very well explained in App. Note I mentioned earlier, but as for braking I haven't check their soft yet if their controller does that or its given for free with built-in diodes.

Good, I thought you and zero both shared the opinion on the fact that the motor took the punishement during braking, that's why i asked.

I never lost the 'store energy in the battery' theory out of sight; i thought you and zero both did. But i just read your earlier posts again, and saw you agreed on another thing with zero;

I checked the spec sheet and it can go upto 5000 samples per second in high resolution mode. Surely it has to capture _something_ if it is there, even if it is recorded intermittently...

So, then, can 'potentially' all the energy from momentum be put back into batteries IF voltage is high enough through rectification, or can only half be utilized, with diodes?

This is the part I am still stuck on.

Maybe we should get an esc engineer representative on here to explain how there controller braking works. Maybe from CC, or S&T?

I'm sure some of these theories are correct in way, but whether engineers use them or something diff, who knows? just a thought

Is it not a good idea for ppl agree with me?

Use impedance instead, some ppl might be confused when you use internal resistance.

Yes, they are correct. The main problem I have with the regenerative braking theory in the context of our setups is that not a single controller that I know of makes any provision for setting the exact battery chemistry type and cell count or even warns about the use of lithium based packs if motor braking is used.

If ESCs are performing regenerative braking they need to warn users about how to configure their product correctly to accomodate their battery chemistry under braking conditions. Anything less is a serious danger!

There is a guy over at rcuniverse under the screen name Novak2 that is pretty good about relaying technical questions to the Novak engineers. He's about the only one I know of off hand.
The Quark 33A allows the user to select NiMH/NiCD or Li-Poly, but I believe it is only for purposes of LVC.

I highly doubt that. To get the truck moving it takes X energy. All that energy taken from the batteries does not make it to the wheels because of losses in the batteries, ESC, and motor. The same is true for taking motion and converting it to electricity; there will always be losses. So, you will never be able to take full advantage of kinetic energy.

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. ;)

If you read my post better you would not post this question.
Selective reading abilities?

That's a nice experiment Brian; i would say the drill would stop; here is my experience with shortening the wires;

shorted the wires, put the shaft in the drill, and i was able to deanodize the motorcan with schotch brite; it was allmost impossible to hold the motor by hand, while my drill was doing a good 3000rpm.

It just sounds like your saying it's not wise to agree with me, i read it.