How does electronic braking work?

[kitefighter]

Hello everyone.

As I was having fun running my Tamiya M-07 in the large open plan office after working hours (as you do), I tried practicing driving faster and faster and then triggering the brakes. It was great fun on the carpet and even more on the shiny floor with the car skidding to a halt.

The only thing is, I haven't really much idea of what electronic braking is and how it works. Anyone care to explain?

Is this how it works?

https://en.wikipedia.org/wiki/Dynamic_braking

Thanks.

[Uranium]

(this might be the worst explanation or i could've missed the entire point but here is my two cents.)

since the front diff is geared right to the motor, the motor shaft just stops spinning when you apply the brake, making the gears stop and making the diff stop rotating as well, and therefore the car will brake and come to a halt.

[nbTMM]

On brushed motors the brake function on the ESC usually just connects the two terminals of the motor together. A motor can function as generator, it depends on the EMF voltage versus the voltage applied at the terminals. If EMF > terminal voltage it's a generator, if EMF < terminal voltage it's a motor. By connecting the motor terminals together, the EMF voltage is shorted out, current flows in the armature which causes a magnetic field which opposes the force of the wheels/gearbox back-turning the motor. The motor heats up due the current flowing in the armature. The braking force is dependant on the speed since EMF voltage depends on motor speed - at high speed there is a lot of braking force and when stationary there is no braking force - you can imagine if the motor was just sitting on the bench and you shorted it out nothing would happen ;).
Stronger braking could be achieved on a brushed motor by applying voltage from the battery in the reverse direction, which would actually achieve regenerative braking and charge the battery. The problem with this however is that in brushed RC setups the ESC doesn't know which direction and speed the motor is rotating, so it is unable to perform that type of braking. If you needed to brake suddenly with a brushed motor you could manually apply reverse throttle. except that if the car then spun 180degrees you'd then need to manually apply forwards throttle to continue braking, otherwise you'd then be accelerating in reverse!

On sensored brushless motors, since the ESC monitors the direction and speed of the motor, it can actually battery voltage in which ever direction of rotation is necessary to slow the speed of the motor, achieving very strong regenerative braking at any speed and both in forwards and reverse. Since the ESC doesn't know how fast the car is travelling, only how fast the motor is turning, it is still possible to lockup the wheels under braking since the ESC is just doing whatever is necessary to make the motor completely stationary! To prevent lockups you can turn down the braking force in your ESC settings or transmitter settings, or just be more careful on the controls .

I'm not 100% sure about sensorless brushless setups although I suspect it depends on the ESC as to which braking technique it uses.

[speedy_w_beans]

In sensorless ESC/motor setups the ESC typically monitors the back EMF of the motor to figure out direction and speed.  The older Castle systems would drive the phases of the motor in sequence initially with no regard to feedback; once the motor reached a high enough speed it would generate enough back EMF that the ESC could figure out the rotor position and then control it much like a sensored system.  It was that initial startup at low speeds that was a little rough and not as good as full sensored setups.

Article on sensorless brushless motors here.

[GooneyBird]

On 3/9/2019 at 5:16 AM, nbTMM said:

Stronger braking could be achieved on a brushed motor by applying voltage from the battery in the reverse direction, which would actually achieve regenerative braking and charge the battery. The problem with this however is that in brushed RC setups the ESC doesn't know which direction and speed the motor is rotating, so it is unable to perform that type of braking.

Akshually!

More expensive ESCs can do regenerative braking with brushed motors. They still don't know in which direction the motor is spinning (since it's not sensored) but it does know what + and - is, and it can 'softly' reverse the polarity of the motor (as if you'd be putting it in reverse) and measure the current flowing from the motor to the ESCs. Once that current approaches 0 it will stop braking to prevent rolling the car backwards.

You can tell what the ESC is doing by throwing the brakes on and pushing the car by hand. If the ESC 'beeps' (like it does when you're slowly rolling forward) it's actually applying reverse to the motor. If you don't hear anything it's shorting out the motor.