Brake by WIRE? Questions about the REGEN/Braking System

[Shaka]

Having this 2012 Leaf SL that my wife picked up a few weeks ago has been a wonderful learning experience. And these forums have been the basis for much of my knowledge. Granted even the data here must be sifted through, but it seems most of the folks here are well versed. So, thank you!

Having said that, one chat that I had with a friend recently brought up a few questions. Since the car is smart enough to limit regenerative braking when the battery is full, but then has the ability to use regen braking when the battery is low, am I correct to assume the braking system in this car is "brake by wire" ? (same as drive by wire)

It seems like this would have to be the case, right? Driving down the road in ECO mode, you lift off the gas and the car starts to brake on own using regen. Using the dashboard display, I'll see 2-3 dots to the left of center, showing it is regenerating some amount of power. I'll apply the brakes "LIGHTLY" and see more dots appear, am I correct to assume that means that although I am using the brake pedal, I'm actually applying more regen braking too, right?

At some point I assume the car manages to say "based on pedal input, you need more stopping power, so we must use the brake pads to assist" -- is that indeed how this all works? Any input would be great. I searched for 10minutes but found no thread that seemed to tackle this specific subject... thanks in advance!

 

[Deleted member 3147]

The actual brake hydraulics are physically controlled by the brake pedal, but not through its full range of travel. When you start pressing, first the electronics activates the regen breaking and then as you press further, the physical brakes activate. The only thing that changes as the available regen changes is that the electronics adjust when in the pedal travel the physical breaks engage. I'd assume that if you have a full computer or electrical failure, the worst thing that would happen is that you'd have to press the brake pedal a bit further to engage the brakes. You would never lose control of the physical brakes.

AFAIK, hybrids like the Prius work the same way. In this respect, the Leaf is nothing new.

 

[thankyouOB]

yes, this is something I have long puzzled over.

When braking, i never see any reduction of the number of regen circles when i apply the brake more firmly.
i owuld think that as i engage the physical -- rather than the regen -- brake, there would be a signal in the dash by fewer regen circles being lit.
but that is not the case until i hit full stop.

is there anyway to observe through the dash or center console on "energy use" the moment when regen braking becomes physical braking?

 

[Ingineer]

The braking system on the Leaf has an electric motor based servo to control hydraulic pressure. The Leaf's computers calculate how much regen/friction braking is needed for any given instant and applies the appropriate mix.

When you press the brake pedal, (in normal conditions) you are simply compressing a spring. The electric servo motor is doing almost all the actual work of creating hydraulic pressure. In the event of partial failure the system would act like a "normal" power brake system and provide boost. In the event of a total failure, it would take exceptional pressure on the pedal to stop the car. In this mode you would first compress the spring, then a mechanical/hydraulic failsafe would deliver hydraulic pressure to the wheel cylinders to stop the car.

Using LEAFSCAN™, I can see the amount of friction vs. regen for any given instant. It varies according to a complex alorgithm, but there is almost always some amount of friction braking occurring except when using the simulated "engine braking" mode (AKA Pedal-off regen).

Regen is also effectively sidelined anytime the computers sense the threat of a loss of traction.

Cutaway view of the brake servo assembly:

#1 is the electric motor, #2 is the hydraulic master cylinder

-Phil

 

[abasile]

Thanks, Phil! I'll be interested in more info on your 'B' mode retrofit (or something like that) when the time comes.

 

[thankyouOB]

thanks, phil.
that is perfectly clear.
cant wait for ur leafscan.

 

[garygid]

There is a modification of the SOC-Meter that reads the total braking force,
Regen braking force, and Mechanical braking force, and can display the values.

It will operate a "beeper" to let you know what percentage of the total
braking force is mechanical. It helps you learn to minimize
the mechanical braking, and better use the Regen braking.

I believe "ticktock" developed it, and knows more of the details.

 

[aqn]

The braking system on the Leaf has an electric motor based servo to control hydraulic pressure. The Leaf's computers calculate how much regen/friction braking is needed for any given instant and applies the appropriate mix. [...]
In the event of a total failure, it would take exceptional pressure on the pedal to stop the car.
[...]
In this mode you would first compress the spring, then a mechanical/hydraulic failsafe would deliver hydraulic pressure to the wheel cylinders to stop the car.

The brake pedal is attached directly to the master cylinder's input rod with a clevis and clevis pin. [Nissan LEAF Service manual, 2011 LEAF, pages BR-212 and BR-221.] In a total failure, hydraulic pressure in the master cylinder is then generated only by one's foot, no longer assisted/boosted with hydraulic pressure from the electric motor, hence the extreme effort.

So, to answer the OP's question, no, the LEAF does not have "brake by wire", only normal "hydraulically assisted brakes", except it uses the servo motor to generate assist hydraulic pressure, not engine manifold vacuum.

 

[Ingineer]

So, to answer the OP's question, no, the LEAF does not have "brake by wire", only normal "hydraulically assisted brakes", except it uses the servo motor to generate assist hydraulic pressure, not engine manifold vacuum.

There is no actual direct mechanical connection during normal operation, so I do consider this "by wire". While it's true there is a rod connected by a clevis, it's directly connected to the servo assembly and NOT directly to the master cylinder. The master cylinder is behind the servo, and if you look at the cutaway, you are only compressing springs in normal use.

Only during failure would your foot actually generate all the hydraulic pressure.

-Phil

 

[Ingineer]

Nissan also just released details of their new steer-by-wire system they will be releasing on certain Infiniti models this year. It too is "by-wire" during normal use. Only during a total failure would a clutch engage to allow mechanical backup.

http://www.extremetech.com/electronics/138136-nissaninfiniti-steer-by-wire-one-step-closer-to-accident-avoiding-and-self-driving-cars" onclick="window.open(this.href);return false;

-Phil

 

[aqn]

So, to answer the OP's question, no, the LEAF does not have "brake by wire", only normal "hydraulically assisted brakes", except it uses the servo motor to generate assist hydraulic pressure, not engine manifold vacuum.

There is no actual direct mechanical connection during normal operation, so I do consider this "by wire". While it's true there is a rod connected by a clevis, it's directly connected to the servo assembly and NOT directly to the master cylinder. The master cylinder is behind the servo, and if you look at the cutaway, you are only compressing springs in normal use.

Only during failure would your foot actually generate all the hydraulic pressure.

-Phil

You are right. Normally, the brake pedal's input rod does not provide any hydraulic pressure at all. There is a gap between it and the master cylinder's piston. The input rod just happens to be in a position to push on the master cylinder's piston in event of total electrical failure.

Normally, the brake pedal's stroke sensor dictates the required braking force. The "electrically-driven intelligent brake unit" then meters out the required hydraulic pressure (sensed by a pressure sensor in the master cylinder) depending on the requested/received amount of regen braking from the traction motor. [LEAF Service Manual, page BR-14]

When both the "electrically-driven intelligent brake unit" and the "brake power supply backup unit" fail, then "normal" mechanical/hydraulic braking occurs, one's foot prodiding all the hydraulic pressure:
"When brake control is stopped (immediately before vehicle stop or while vehicle is stopped), cooperative
regenerative brake control is not performed.
[...]
When Control Is Stopped
The input rod crosses the cooperative regenerative brake control gap and contacts the master cylinder piston,
generating fluid pressure. There is no boost force (assist force), and the braking force is determined by the
force pressing on the brake pedal."
Nissan LEAF Service Manual, page BR-17.

 

[Shaka]

Wow! As the OP of this thread I want to say that this has been a really cool thread. And basically it has answered my question with what I expected... brake, by wire, when all systems are working. With obvious smart, fail-safe backup plans.

PS: That steer-by-wire concept is wild... and seems to be the way cars may go in the future, especially as autonomous cars become available!

 

[Ingineer]

Steer-by-wire opens up a whole bunch of new possibilities, such as dynamically variable ratios, all kinds of variable feedback. You are basically turning an electric motor, and it can provide almost any kind of feel desired, it can resist your efforts to create a stiff feel, and it can provide "fake" road feel and centering forces. You can then implement sport modes, highway, etc.

Most cars now, including the Leaf, use EPS (Electric Power Steering) instead of the older hydraulic technology with an engine-driven hydraulic pump. The hydraulic system was complex, wasted a lot of energy, and was often a source of leaks as the car aged, requiring yet another fluid. EPS is more advanced, as software can now control the boost dynamics, and can have a much better feel if the software is done right. Personally I don't like most of the Japanese-tuned systems, as they are too soft and tend to damp road feel a bit much. One of the reasons they tune the systems like this is to help reduce torque-steer on front wheel drive cars, which can be especially pronounced on a high-torque front wheel drive EV, such as the Leaf.

All cars with EPS also have the inherent capability to have the computer control the steering without driver input. One of the reasons that Google uses the Toyota Prius for it's self-driving platform is because the car is 100% drive by wire, or at least 100% controllable by wire. (The steering is EPS, not full DBW)

One additional advantage (or disadvantage if a malfunction somehow occurred) is that the car can be made to steer without needing to move the wheel. For instance, in an automatic collision avoidance situation, the car would need to suddenly swerve, and with EPS it would have to fight the driver. In steer-by-wire, the wheel would not have to turn, so the avoidance maneuver would be unaffected if the driver has an iron-grip on the wheel.

This video highlights one of the possible uses for full drive-by-wire computer control:
[youtube]http://www.youtube.com/watch?v=LC2wiBQLHRw[/youtube]

-Phil

 

[TickTock]

I posted some initial observations in this thread. Short of the long is friction brakes are applied immediately and regen displaces it shortly later except in heavy braking. Once you cross a certain threshold that friction is applied, regen is backed off even if it was already being applied.

I love my chirp mod on my SOC meter. I doesn't chirp much anymore now that I am trained but occasionally I start to slide and it's there to remind me.