stopcrazypp

It looks like the Toyota Prius is undergoing investigation for braking problems.
Breaking: NHTSA opens investigation into Prius brake issue — Autoblog Green
If Electronics are to Blame for Toyota’s Vehicle Failures, What Does this Mean for the Volt?
2010 brake system - PriusChat Forums

This sounds very similar to what is discussed in this thread:
NHTSA Complaint wrt Regen and TC

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doug

True in that in involves how the software deals with the interaction of the regen and friction brakes with the ABS.

I believe, however, that the Roadster is performing as designed. That doesn't mean it's necessarily the optimal solution, though.

stopcrazypp

Yes, it seems like the system is supposed to perform this way. The only complication that the Prius and the Tesla adds is regen braking, but it should still be managable.

Also I found a similar complaint for the WRX: it was found not to be a design/safety fault by the NHTSA, and Subaru release a similar ECU recalibration to mask the sensation of braking loss.
http://www.scoobymods.com/02_03_mt_wrx-t5701.html

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Tdave

I'm really glad the Roadster associates the regen with the accelerator, not the brake. Why aren't these other manufacturers doing it that way? It's easier to design, and easier to drive.

efusco

I don't understand what you're saying here...could you elaborate?

Since this is a more advanced audience I thought I'd explain, a little, why this braking transition issue is more noticeable on the 2010.

With the Prius 2G the regen maxed out at about -100 amps, that, apparently, has been boosted up to around -125 amps (maybe more?) in the Prius 3G (at least the ones sold before January). Thus, the stronger regen gave the benefit of more pronounced braking before the friction brakes kick in. That's great, except that when you hit a bump and the skid control ECU senses a wheel spin differential or possible loss of traction it transitions to friction...that transition takes about 1/2 second...since you were braking pretty hard with regen, then the regen suddenly stops before friction kicks in that decrease in deceleration rate becomes very noticeable...even some experienced 2G Prius drivers have described the sensation as startling or unsettling.

I'm not convinced that this transition, unsettling though it might be, causes any significant increase in braking distance or that it's a safety hazard (and thus shouldn't be a "recall"), but I think it's unsettling enough and in light of the current accelerator recall hysteria that Toyota will be forced to have a recall to reduce maximum regen amperage and thereby reduce the transition impact.

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EVan E. Fusco, MD
Nixa, MO
Prius owner Eagerly awaiting my Model S- R 77 Sequence number

Tdave

You elaborated well on the Prius issue. The Ford Fusion has a similar issue with the regen/friction braking transition, according to reports I've read yesterday/today.

My point was that those vehicles have an additional technical design hurdle to overcome because they tie both the regen and friction braking to the brake pedal, and need to control both with that one control pedal, including the transition between the two.

The Roadster in contrast ties the regen to accelerator lift, and the brake pedal is solely used for friction brakes. This is a more elegant design, easier to engineer, and has operator advantages most of us here appreciate.

TEG

This situation was discussed at length in the past on this forum and the TM blogs. Rather than dredge up those old posts, here is a brief summary from my perspective:

It boils down to expected accelerator pedal behavior.

In a high end sportcar like the Roadster (e.g.: a Ferrari), many people are used to a high revving performance engine with a manual transmission where letting up on the go pedal produces strong engine braking and associated slow down. So the Roadster can get away with fairly strong regen just by lifting the "torque pedal" which is familiar behavior to manual transmission sportscar drivers.

A car like the Prius, on the other hand, is trying to attract family car drivers who might otherwise be used to something like an automatic transmission Camry or Taurus. Drivers used to automatic transmissions don't expect there to be engine braking when they ease up on the accelerator pedal.

I am sure Toyota could have engineered the Prius to do regen on accelerator pedal lift up, but too many customers would have found that unexpected/unusual/unfamilliair.

Tdave

I say tough. Get used to it working the right way, like the Roadster.

efusco

Interesting...
There is some mild regen with accelerator pedal lift in the Prius (around -15amps give or take based upon speed and battery SOC). But you can vary the amount of regen when braking up to a max of around -100 to -125 amps under ideal conditions, though I tend to stay in the -40 to -80 amp range for most of my common braking situations.

So, if I'm understanding this right, with the Roadster maximum regen is "coasting" with the foot off of both brake and accelerator--correct? Any idea what kind of regen power that is? It would seem like it would be fairly limited compared to the Prius-style system. Does regen cut out as soon as the brake pedal is depressed?

Finally, in the Prius 1G friction braking and regen were simultaneous...with mild braking there was very little friction braking, but it was there, with harder braking the friction and regen increased, but eventually the regen would stop and you'd be on friction only. Thus, with the 1G Prius there was no 'transition' like there is with the 2G and 3G. In an effort to increase the effectiveness of regen succeeding generations of the Prius increased regen, stopped friction until the max regen threshold was exceeded or for "emergency" conditions (which includes bumps, skids, uneven braking surfaces, uneven wheel spin rates, etc.)

So, I don't really consider the Tesla system better...it may be smoother and avoid the transition, but it limits driver control of regen and probably reduces the overall effectiveness possible out of the regen system. It's obviously simpler to implement an smoother...maybe even "safer"...so I'm not bashing, just saying I wouldn't call it better.

Indeed, if Toyota can find a way to smooth that transition form high regen to friction I'd have to proclaim that system better b/c of the ability to maximize regen and minimize friction brake use/wear.

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EVan E. Fusco, MD
Nixa, MO
Prius owner Eagerly awaiting my Model S- R 77 Sequence number

doug

I wouldn't necessarily call it more elegant, but it certainly is more simple and is my preference. Regardless, the issue at hand is similar: how traction control deals with regen.

Perhaps it's even more dramatic in the Roadster where the regen is likely stronger and you're less likely to have your foot ready over the brake pedal. If TC engages there's a sudden loss of regen which makes the driver experience positive jerk (decrease in deceleration), which could be unsettling.

Do you have these numbers in terms of power? With that and the mass of the car one can make an estimate of braking force and acceleration.