Electromechanical Brake Question 2

[DJShortyG]

Hello All,

We are working on a vehicle specifically designed to utilize some of the current Drive by Wire systems out there. Our end goal is to test the Human Factors that contribute to the desire to or not to include by-wire systems in vehicles today.

Currently we have identified sources for a steering system, as wells as suspension and shifting (the second two were pretty easy). Braking however has proven to be difficult to find out side of the Brembo System announced in the following link.

http://www.brembo.com/en/News/Pages/BREMBO-@-IAA-LESS-WEIGHT,-MORE-EFFICIENCY-.aspx

My question really relates to understanding why if there really are cost savings, weight savings, and performance benefits to this type of braking system, why are there not more companies out there for us to look to purchase a system from? Also if, we were under pressure to design our own system, where would the best place to get operational specs of a braking system be?

Thank you for your help, I really appreciate it.

Greg

 

[NormPeterson]

What sort of modulation can you realize from such short activation and release times?


Kind of separate, but there's a current thread elsewhere on this site that discusses "whiskers" in accelerator position sensors and the potential for them to badly confuse the electronics they serve under certain situations/operational sequences. Here's a report that was linked to there.

http://nepp.nasa.gov/WHISKER/reference/tech_papers/2011-NASA-GSFC-whisker-failure-app-sensor.pdf

I can't think of any single default brake activation scheme or level that would fit every possible driving scenario (or even just the likely ones). It's simply too dependent on specific conditions that the ECU won't know about.

From an operator viewpoint, what sort of feedback would be available? I'm not sure that synthesizing it would work all that well, particularly in a split hydraulic/electric system. Perhaps the average driver isn't consciously aware of how he's using that feedback to modulate his stopping, but he's got to be doing it to the extent necessary to generally stop reasonably close to the painted lines at traffic signal controlled intersections on the first try, and with at least a semblance of smoothness.


Norm

 

[DJShortyG]

You are absolutely correct Norm, there is no single activation scheme that would work, and therefore we are having to work very closely with an AUTOSar software provider in order to help us integrate the EMB calipers into the control scheme.

As for the feedback on the pedal, we are currently using a hydraulic system from Hitachi that provides boost to the front brakes called the eACT. This unit also comes with a pedal emulator that pushes directly on hydraulic fluid giving the feel that we believe is necessary for driver comfort/consumer perception. There is also a complex twin spring system that is semi-programmable in terms of how the sensor reads the input pressure as to how much braking force is applied.

The modulation times for the brake were taken directly from SAE Papers and a phone call with one of SAE's braking experts. According to him those numbers not only match but, actually beat the activation time for a hydraulic system.

Thanks for the link Norm. Very interesting presentation especially given the fact that Toyota is one of the reasons there are major consumer perception issues with by-Wire technology.

 

[MacGyverS2000]

UNLESS they are designed in a normally closed type configuration like train brakes such that if an electrical connector fails the brakes will be applied until it is fixed.

This goes back to my earlier example... the last thing I want to happen at speed is for my brakes to suddenly lock up.

As I said, there is no good failure mode for brakes, and that in and of itself is a major hurdle.

Dan - Owner

http://www.Hi-TecDesigns.com

 

[DJShortyG]

Just playing Devil's advocate here, but a couple of weeks ago our research team had the failure modes discussion with a leading ABS/ESC Tier 1 and the conversation was very interesting.

Currently the only failure mode for hydraulic brakes is the eBrake or parking brake. In the event of a catastrophic failure (i.e. burst or cut hydraulic line) the regular foundation brakes are useless. The ABS controller is not programmed to detect a leak and shut the valve that goes to that caliper independently.

In a car with Full EMB so no hydraulics, the failure mode is still the eBrake or parking brake. In the event of a catastrophic power failure (which I will admit is an easier scenario to imagine than a loss of hydraulic pressure) the foundation brakes are again useless.

The difference the Tier 1 representative noted was that in a full electronic system there are 4 redundant brakes because there is nothing tying them all together accept for the ABS controller. Also a small capacitor that stores enough power to be used as a back up to stop you a couple of times can also be added to the ABS controller as further back up. So failure modes would be:

EMB
1. Four Separate Brakes
2. Capacitor Back Up Power
3. Parking Brake/eBrake

Hydraulic Brakes
1. Boost Failure - Direct Connection to Hydraulic Fluid
2. Parking Brake

Not sure but, it seems to me it is easier to create back ups for the electronic system than the hydraulic one, according to our interviewee.

 

[MacGyverS2000]

Also a small capacitor that stores enough power to be used as a back up to stop you a couple of times can also be added to the ABS controller as further back up.

The power required for this is non-trivial, which means the storage requirement is also non-trivial, both in size and cost. You're robbing Peter to pay Paul. You have distributed the system to all four corners and therefore made it smaller/lighter, but you have to add back in four independent backups, adding to the size/weight. Tack on the extra cost involved and things go sideways real quick.

Dan - Owner

http://www.Hi-TecDesigns.com

 

[DJShortyG]

You are right MacGyverS, while I said small capacitor it would be a bank of them somewhere in the car with individual connections running to each wheel from an individual capacitor. Power consumption for a single caliper should be in the 14.4v - 30 amp area. At least that is what we are targeting. Panasonic has some "Super-Caps" that are currently used in the Toyota Hybrids as back ups for the electronic systems, that we are looking at using.

None of the things we are talking about are cheap and honestly the improved performance and features could be provided with hydraulics if enough money was thrown at the issue. I guess it really becomes a question of when does the investment it takes to advance hydraulic brakes further outweigh the added cost of the redundancy systems.

Thanks for the post MacGyverS.

 

[MiketheEngineer]

Use two wires - independently - please.

Dean Martin's son probably died in an F16 fly-by-wire crash that may have occurred because of faulty electronics...

 

[btrueblood]

"The ABS controller is not programmed to detect a leak and shut the valve that goes to that caliper independently."

For good reason. Not even when you know about it, can I, an admittedly "average" driver compensate for having only one brake or one side of a vehicle's brakes being applied, and not both fronts or both backs simultaneously. Backhoes have rear wheel brakes that can (in some models) be independently actuated by a split brake pedal, allowing "skid steering" on soft ground. You're supposed to engage a pin to lock the pedals together before going on-road. Failing to do that leads to interesting results (like suddenly pivoting a 10-ton machine thru a fence into a farmer's field at 15 mph).

 

[pwildfire]

With regards to hydraulic brake failures: On any vehicle that I have ever owned, there are separate hydraulic circuits for the front and rear axles. In other words, if a line blows, you will still have brakes at the other end of the car until you have leaked out all the fluid. Driver feedback in combination with low fluid indication lets the driver know there is a problem, and braking power is reduced, but not eliminated, so the driver can (usually) get safely to a safe place to park and investigate.

Macguyver: The reason that I specified a dedicated system on my 'train brakes' comment is due to the necessity for this system to apply only a percentage of braking power and/or apply braking gradually to avoid the lockup scenario.

Aircraft examples seem only mildly relevant to me. Aircraft maintenance is infinitely more controlled than automobile, not to mention that pilots are well trained, wheres the average driver I pass on my way to work is much more interested in figuring out how to drive, text, and eat a burrito simultaneously than they are in noticing that their brakes aren't working well. I used to work as mechanic and have had people bring in cars in which the brakes have gone so long without inspection that the pads are gone, and the calipers have actually worn the rotors into two concentric discs.

 

[MacGyverS2000]

it would be a bank of them somewhere in the car with individual connections running to each wheel from an individual capacitor. Power consumption for a single caliper should be in the 14.4v - 30 amp area.

That's a hefty amount of power, even for a supercap... you'll have to string a number of them together to get the required power, which increases cost/weight significantly. Putting them in a central location and running wire brings back the whole point of distributed systems... reducing construction cost from running wire all over the vehicle. You haven't solved the original problem, only changed the wire color.

due to the necessity for this system to apply only a percentage of braking power and/or apply braking gradually to avoid the lockup scenario.

That's still a bad scenario, IMO... by the time the typical driver realizes their brakes have just gone belly up and that is what is gradually slowing the car down, they're now doing 40mph in the middle lane of a super-highway surrounded by 30-ton semis doing 70mph, and still slowing. You can't stay in the lane, and you don't want to chance jerking the car over in front of a fast-moving vehicle. Pancake city.

Dan - Owner

http://www.Hi-TecDesigns.com

 

[DJShortyG]

Clearly the industry has a long way to go and consumer acceptance of an electromechanical brake system is only one small piece of the puzzle.

I guess that while the technology may be there, "Just because we can, doesn't mean we should." - OEM engineer

If EMB was going to be launched on a car what would the most likely scenario be for the launch? What kind of vehicle, which manufacturer, what geographic market? I mean just throwing it out there, I have limited understanding of the BRIC Markets but they all seem to be advancing at a rate faster than we ever did in terms of automotive. I know they are not caught up yet but they are moving quickly.

Greg

 

[DJShortyG]

By the way, btrueblood, a vehicle's safety systems can more than compensate for the kind of situation where only one caliper is actuating. EPAS and ESC have been able to right many of the situations that used to cause very dangerous and sometimes deadly accidents. Take it one step further and add Active Front Steering to and Electronically Assisted Power Steering System and you have a vehicle that can literally turn the wheels in the completely opposite direction of the steering wheel.

IMO the technology exists to help bring an automobile to a complete and safe stop even with one caliper. It may just take a mile to do.

Greg

 

[BrianPetersen]

The current electric and hybrid vehicles are using regenerative braking, which is invariably under electronic control these days. BUT ... they retain the hydraulic brakes also, and the regenerative braking obviously isn't done by friction braking, but rather by using the drive motor as a generator. Most of them are not capable of regenerating 100% of the kinetic energy, particularly at anything beyond a moderately-fast stop.

 

[DJShortyG]

The teams understanding is that the newer Hybrids/EVs are capable of braking all the way down to around 7-10 mph using just the regenerative brakes and then blends in the friction brakes. This blending is where the big issue is. Due to a very slight lag in the Hydraulic system there is a moment, and I mean a short moment, when you feel as though your brakes gave out when transitioning between the two systems.

HEV/EVs are a good starting point though as the higher voltage and need for any efficiency gains to combat range anxiety could make one a great target for at least rear EMB,

Greg

 

[NormPeterson]

Hybrids, maybe. A pure EV will ultimately end up trading range for braking since the energy to make the friction braking happen has to come out of the same total energy reservoir. A separate energy source for the BBW would presumably siphon off some of the regen braking energy some of the time and simply add a constant amount of weight and space demands all of the time

I would expect this to be a bigger factor where slow speeds and frequent braking are the rule rather than the exception.


Norm

 

[patprimmer]

Individual wheel brakes are common for tractors, more as a slip control for the diff when one wheel spins, but it can be used for steering.

It was also a fairly common practice for off road racing to have the hand brake rigged so one rear wheel could be braked to increase turn in on a dirt surface with little weight on the front wheels. It was effective at high speeds WITH PRACTICE.

Without practice it was very easy to induce a spin

Regards
Pat
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[MacGyverS2000]

Just so we're clear... pound for pound, a friction brake will outperform any brake that relies on magnetic fields. To what degree I do not have numbers for, but it's a good point to keep in mind. If my stopping distance is increased by 20% after getting rid of pads/rotors, I'm not a happy camper, efficiency be damned. I won't be declaring to everyone how efficient my new car is when the hood is crumpled up under the bumper of the car in front of me.

Dan - Owner

http://www.Hi-TecDesigns.com

 

[DJShortyG]

I do have to admit that the Automotive Industry in general is making some great and then some not so great decisions about the cars they manufacture.

Honestly, they charge $2000 extra for a Chevy Cruze ECO and the most prominent change is low rolling resistance tires. They are adding what consumers will pay more for which right now is MPG's and a little bit of safety. This means they are willing to take risks on certain things that they can get a premium for.

As gas gets more expensive consumer penchant for paying more for a slightly more efficient car will continue to go up. The G-Wiz board on this site is a great example of what consumers will do to save a little money at the pump. "Buy a deathtrap because it doesn't cost me as much and the risk is worth the savings." - Me thinking what purchasers of the G-Wiz would say.

Greg

 

[hydtools]

There is a large population of RV trailer owners who would not be eager to have electromechanical brakes on their tow vehicles based on their experience with the electromechanical brakes on their trailers.

Ted