Radar based regenerative breaking

Not sure anyone should DIY venture into safety-critical systems like this without serious, lab-quality fault-tolerance vetting.

If the system threw a false panic stop on an interstate, who would be liable for the ensuing multi-car mayhem?

 

I would believe that a Toyota hydrid using any type of cruise control, radar assisted or not, will regenerate energy into the battery anytime it decides to slow down in response to excessive speed or another car, just as it does when you manually let up on the gas pedal or when you are going downhill. Toyota has offered some sort of adaptive or dynamic cruise control as a tech upgrade since the mid 2000s and made it standard on almost all as of 2017. So there is likely no benefit to trying to add something.

In a Toyota hybrid system, all braking down to 7 mph is handled by the regen system unless aggressive braking is needed. When a panic stop or very strong braking is required above 7 mph, the hydraulic brakes are also applied. These systems are almost 100% brake by wire except for a rarely used failsafe mode when a pair of solenoids allow pedal hydraulic pressure to directly activate the brake cylinders. A side effect of this level of regenerative braking are sets of brake pads that can last over 200k miles and still pass state inspections.

 

My 2019 Rav4 hybrid seems to have such a system. If a car on the highway pulls in front of mine and the spacing between the cars isn't the length I selected for the speed I am at, the car slows down to restore the gap.

 

That's what I would have pointed out too. The Prius already does this, maybe without making as much of a publicity event of it.

It's a bit more complicated than that; regen at the low end of speeds loses effectiveness, hence the brakes get involved, but also regen at highway speeds is like drinking from a firehose, hence brakes also get involved (if you are actually braking, not just backing off the go pedal), with a sweet spot for regen sort of in between.



But any way you slice it, the car makes effective use of its regen capacity pretty much whenever it can.

(That picture is Gen 1; details change over generations, but not the basic ideas.)

 

You overstate the amount of braking that can be regenerative. It is capped by the battery charge rate limit of 27kW (or less when hot or cold), which is reached with just moderate braking at lower speeds or very light braking at highway speed. All braking stronger than that must be non-regenerative. And many drivers, including one in my household, have natural braking rates stronger than that.

For a better idea of when one is hitting that threshold, watch the HSI display.

 

I have a ScanGauge on my Prius and I monitor battery current.

I have noticed that there is a current limit on charge/discharge of around 120 amps. As discussed by others, this is proportional. At freeway speeds, just a light brake pedal pressure can exceed 120 amps. At slow speeds, MG1 is not turning fast enough to generate 120 amps. I noticed if I maintain a constant pedal pressure while braking the amps decline as the vehicle speed is reduced. Regeneration seems to be most effective from about 15 to 50 mph which should be the bulk of city driving.

Also as other have mentioned below about 8 mph, the braking is all friction brakes. This is a front wheel drive car so regeneration is only available on the front wheels. The rear brakes are full time friction brakes. Rear brakes contribute more during a gentle stop but they don't contribute as much as front friction brakes/regeneration during heavier braking due to weight transfer to the front wheels..

I am in awe of the engineers that figured out how to proportion brake force between front and rear brakes when you add in varying levels of regeneration.

 

I'm even more in awe of the old-school engineers who figured out ways to make that kind of stuff happen using only, like, springs and plungers and stuff. (A conventional car has such a valve "figuring out" the right proportion of hydraulic pressure between the fronts and the rears.) Carburetors. Old automatic-transmission valvebodies. PCV valves, there's a weird one.

By contrast, the Prius brake ECU just flat-out calculates in numbers how much torque it can give you from regen, what fluid pressure is needed to get what brake torque at the fronts and rears, subtracts a from b, and applies the result. Where'd all the old-school cleverness go?

 

Into the chip and software design...

 

I agree, our 97 Odyssey had some sort of mechanical proportioning valve that was connected to the body. A lever was connected to the rear axle with a spring. With a load in the van, it sat lower in the rear. That gadget applied the appropriate amount of rear braking force based on the vehicle loading.

There was a mechanical calibration procedure for that valve. You had to have a certain amount of gas in the tank, van on level ground etc, take a measurement and make an adjustment.

Today, everything is done with electrons.

 

Sure, that stuff has its own cleverness.

But at the same time, the control design problem used to be "figure out the engineering formula for what the brake pressure p needs to be under braking demand d with front/rear ratio r" and now design some crazy redavination with springs and stuff that somehow ends up producing a pressure close to p.

And now it's more like "put that formula right in the code, hang pressure sensors off the brake lines, and have the code open the valve until the pressure is p."

 

There's a map for everything.