I'm using analog brake signal with my HAL sensor brake leaver.
I was wondering. Is brake always REGEN or is there also a active brake? (So always negative current or is there a positive current component too?)
I know there is a handbrake feature that actively holds the wheel. But that is something else of course. But it got me wondering.
Thanks
no it does not have to be regen . you can have regen or active or both , its all down to what currents you set, just to much active is just going to create a lot of heat in the mosfets/motor
C green
Could you point me to that setting? I can only see Motor->Genral->Currents Battery Current Max Regen and Motor Current Max Brake.
The Help text for both of them refer to them as Negative currents going into the battery. So Regen.
I find no setting for positive current/active braking.
Only Regen would make sense to me. I would just like to know this is what it happening.
When is regen braking supposed to cut in? Perhaps I just have my currents to low and I don't notice the effect.
Is it triggered only when the brake is engaged? Or also when I go off the gas? My other scooter does that, when I let go of the gas it goes into regen the more I let go off the gas. I quite like that effect. With VESC I assumed I have to run in Speed Control mode to get this effect.
Braking always involves removing energy from the motor (and load). The VESC can neither store this energy nor dissipate it - all it can do is transfer it on to the battery. As such, braking will always be regenerating.
Also, I'm not sure what you mean by "active" braking, as opposed to regenerative? The VESC is as active when it brakes as when it accelerates - it still needs to track the rotor and switch the FETs to regulate current.
no .. have you tried turning a shorted motor as far as i am aware vesc can do the same job on braking using the mosfets to create the short. why do you think there is two seperate currents listed to set.
set regen to zero and max brake to 50amps and give it a go ;)
C green
As far as I know there is indeed only regenerative braking for when the motor is turning. (mosfets can only be fully on/ fully off, no partial options)
There is a parking brake mode but that is only for when the motor is stationary (the vesc will short the wires).
@badger1666: That's a good experiment. Unfortunately I don't have an inertial load setup available to test it right now. Yes, the VESC could short the motor phases and act as a parking brake. I guess this could be called passive brake? But as soon as you start regulating/limiting the current, you should automatically get regeneration: each time you swith a coil off to limit the current, its voltage should spike due to inductance, and push current into the battery. The more you allow the current to rise, the more energy should be pushed back into the battery each cycle.
But yeah, I need to think more about this, it's past bedtime and I'm too tired right now. ;) I've actually been a bit confused about the different braking modes before. Time to look into it! :)
I also believe this active mode is parking brake only. (Servo like position hold mode)
i tried said experiment today while scootering along a coast path. its correct the vesc must regen for braking :(
kind of makes no sense in having a max braking amps and max regen amps then ?
C green
There is no way to brake in a controlled way without pushing the current back into the battery. At very low speeds when the short circuit current is less than the requested braking current nothing is pushed back, but when going faster than that it always is.
If all phases are shorted at high speed and not released, the current would rise to hundreds of amps until something breaks in a fraction of a second. Modulated shorting of all phases at high speed (switching between shorting them and not) is also regenerative as every time they are released the inductive current spikes will be "rectified" into the battery through the body diodes in the MOSFETs, and this method makes a terrible noise (I think this is what castle controllers do). If the was a way to partly turn on the MOSFETs and waste the energy in them they would melt in a fraction of a second. All of this means that I would be quite impressed if some motor controller supports non-regenerative braking at any speed as opposed to regenerative braking.
Regarding the current limits, all of them are meaningful and important. The first thing to understand is that the battery current is a fraction of the motor current, depending on the modulation (duty cycle), roughly duty cycle * motor current. The duty cycle is more or less proportional to the motor speed, meaning that at low speed a high motor current only gives a low battery current, and at high speed the motor current and battery current are almost the same. This means that if the battery current is set rather low because of e.g. how much current the cells can handle, the motor braking current can still be high at low to mid speed and gradually decrease as the speed increases. The same thing is true for acceleration; setting a higher motor current than battery current means that high acceleration is possible at low to mid speeds, and then the acceleration gradually decreases as the speed increases. Therefore four different current limits give the maximum flexibility. It should be noted that the current limits behave such that no current limit is exceeded at any point, meaning that if you set the battery currents to what the battery can handle the motor currents can be set freely, and the software will do the best it can to provide them based on what is possible at any moment.
Regarding the handbrake function, it is only useful when standing still. It creates an open loop current vector that forces the motor to stay in a position. It can be used on some application where the motor has to stay in a position without access to an encoder for proper position control at zero speed.
Thank you, i have been looking for this answer for sometime :)
I think that this concept of controlled braking but not using the battery to sink the current is called 'dynamic braking' instead of 'regenerative braking' and in industrial drives it is done with a resistive load bank. I assume that some EV's (cars) must do something similar otherwise when the battery is full their braking performance would be much less. But this is just assumption.
Dean
To be honest, indiferently pumping multiple Amps of power into a potentially fully charged cell freaks me the fuck out.
I agree, though full BMS _should_ maintain 'charging' / regen current and cell voltages within safe limits. Without integration to ESC then it could mean sudden cut off and no more regen brake (hopefully not causing accident). Ideal would be nice integration that slowly reduces regen as the limits of the battery are approached. OR a braking resistor and use that instead once battery gets too close to its limits.
A redundant mechanical brake seems like a reasonable solution? Even if you add e.g. a braking resistor the brakes will still fail if a power connector to the VESC comes loose, for example.
From that discussion, it comes out that the braking is inherently limited to what the battery can handle for charging, so with a typical 2A(maybe 4A), at 36V, the braking power will be in the order of 100W. Which is not that much when you need to slow down a 100kg moving vehicle. Short of having stupidly beefy batteries, couldn't it be possible to have a braking circuitry that just diverts any current over the regen threshold onto something that can properly dissipate it, like an halogen bulb, or a heater coil? Not too sure how that translates in terms of electronics, though...
In the meantime, got to practice those Coleman slides!
I had the idea to add a crowbar circuit that snuffs out everything that is above max VBatt. That would basically soak up all energy that the Battery can't take. (Kind of like a electonic Zener diode)
In theory easy. But I suspect in practice anything but. I'm not really a Elecchicken. My suspicion is it's going to be hard to have it sensitive enough and also be sure it won't snubb any energy flowing back from the battery.
Difficult subject. But I would love to see some progress.
What you have described seems to be pretty much how it works :)
https://www.motioncontroltips.com/faq-what-is-dynamic-braking-and-when-i...
The max limit of how much current a shorted motor generates is equal to the ratio of flux linkage [Weber] to inductance [Henry] of the motor (assuming brushless motor with surface mounted magnets). If you look up the base units and divide [Weber] by [Henry], then you get [Ampere]. This ratio is often called characteristic current (or I_char) of a motor. It is certainly possible, and sometimes desired, to have a motor with flux linkage / inductance ratio that will lead to safe currents when shorted -- one can also add phase inductors to reduce I_char.
Most skateboard motors have I_char higher than what is safe for VESC. For example my 6055 outrunner with 200 rpm/V has 3.4 mWb / 16.5 mH = 242 A. If that motor had a winding with more turns, e.g. 100 rpm/V (or maybe even 130), then it would be safe for VESC to short it for a few seconds. VESC measures both flux linkage and inductance during setup, and could therefore determine automatically if a motor is safe to short circuit.
Hi!
Very interesting discussion, I have a problem related to this (posted a thread on general discussion here on vesc forums), where I have been considering a circuit which removes everything above PSU voltage.
What you say here, is that dynamic braking - which is incorporated in vesc as a short between the three poles of the bldc, or "parking brake", is safe to use when braking low kV (rpm/V) motors. I have ordered a 70KV 5055 sensored Maytech motor. Would it be safe, and would it be viable using VESC, to set up such a braking method? I am building a thread/yarn winding machine, so the torque is extremely low compared to a skateboard with a person on. It would almost be comparable to breaking when the motor was spinning freely.