How does the brake pedal work? Brake by wire?

Rhino, the braking system is probably most complex system in the car. It includes main braking ECU (BA, ABS, VSC+, EBD, TRC sub-systems), hydraulic pressure pump, valves controlling braking pressure for each wheel individually as well as wheel rotation feedback sensors and lot of other sensors.
The pedal has electronic sensors (simulator of braking pressure) and hydraulic linkage. In standard conditions the hydraulic part of the pedal works as a sensor for the ECU controlling the elecric braking fluid pressure pump + valves and regenerative braking through main MG2. In case of ECU malfunction there is a direct linkage to the hydraulic braking, so car is able to stop in any case (hopefully, not tested ).
Regenerative braking appears only on front wheels as the MG2 is directly connected with them. Rear brakes are always friction one.
Parking brake - foot operated is mechanical. The joystick parking position operates via PSD mechanical block.

 

the braking system works well in worse case scenario braking... like when your 12v line shorts out on a windy road... a cliff on the outside of a long right hand turn.. with a fire truck in the oncoming lane two seconds before you 12v shorts (completely my fault)

the entire car blinks and turns off. it's rather stiff steering that's really not even close to bad ( i was doing 40mph ) .. the brakes work as if the car is on (like a normal car.. no floating feeling from the regenerative braking)

in case all fails, the car has a 12v capacitor bank... i think that's the 3rd and final system.... before you have to pull the parking brake. i don't remember the exact breakdown of the system anymore.. just key items.

 

Ouch

 

My understanding is that the master cylinder is disconnected during normal operation (brake by wire, completely computer controlled). If there's a problem (e.g. complete power loss), the master cylinder reconnects and it works like a normal car. I haven't checked, but my guess is that there's no brake booster so it'll probably be pretty stiff.

 

Actually the brakes are fully mechanical, with the first part of the pedal travel electronically monitored and that actuates braking (including regen), with feedback control present. If that fails you will still press the mechanical pedal/master "piston" further and actuate brakes as per any other car. It's not "brake by wire". It's "brake control by wire" until that fails to work. Then it's just hydraulic.

As stated above the parking brake is fully mechanical with no electronic control (other than a switch to warn you it's on). I say parking brake because it's not an "emergency brake", as it's almost impossible to modulate how much it's on in use. It's an on/off system.

 

PaJa, you said "The joystick parking position operates via PSD mechanical block."
What exactly happens when I push the Park button? Is it anything like a normal auto tranny in this regard?

 

I don't think so as I accidently hit my Park button while driving all it did then was put trans in neutral. A little shocking but I'm ok with it, as is much better than any other auto trans put into park while moving.

 

When you press the brake pedal, hydraulic fluid is moved in a piston just as in a regular car. However, in normal use, that piston doesn't then move fluid in the brake cylinders. Instead the pressure change is monitored by a pressure sensor connected to the Skid Control ECU. (The fluid enters a device called the Stroke Simulator, which provides the same sort of feel you expect from the brake cylinders.)

From there, the Skid Control ECU works out how much braking force you want, and asks the Hybrid Vehicle ECU to provide up to that amount through regeneration. The HV ECU replies saying how much it can provide. The Skid Control ECU then makes up the difference, if any, by controlling valves from a high-pressure reserve (the Brake Accumulator) to the brake cylinders, which increase the pressure in the brake cylinders, and other valves from the cylinders to the fluid reserve tank, which reduce the pressure.

The rear wheels are always braked with friction brakes as regeneration only operates on the front wheels (the motors are only connected to the front wheels).

If the wheels lose traction, the Skid Control ECU asks the HV ECU to stop providing regeneration and it modulates the pressure on the wheel cylinders to try to keep the wheels just on the verge of locking up. This is ABS.

There is a variable resistor - like a classic volume control on an amplifier - which monitors how fast you press the brake pedal. If you press the brake pedal quickly, it skips asking for regeneration (the HV ECU can't react quickly) and just applies the brakes. It also applies more force than you asked for - this is called Brake Assist - as it's known that drivers frequently don't brake hard enough in emergencies.

If there's a problem with regeneration, the HV ECU will report no braking effort and the Skid Control ECU does it all with the friction brakes. If the Skid Control ECU doesn't see a response from the HV ECU it assumes no regen is available and does all braking with friction brakes.

If there's a problem with the electronically-controlled braking system, the Gen 2 opened and closed valves to isolate the Stroke Simulator and send the fluid moved by the driver's foot directly to the front brakes only, with no power assistance. There are actually two pistons in the Master Cylinder, both of which are pressed by the rod that the brake pedal connects to, which gives two independent brake lines. One connects to the front left wheel and the other to the front right.

Gen 3 (2010 Prius) improves on this by using the accumulator as a booster and sending fluid from one Master Cylinder piston to the front wheels, the other to the rear. I believe it can keep electronic control of one pair and use driver's effort for the other.

Hopefully this fail-safe mode is never needed, but some people here have experienced it.

 

Park drives a "gear" (looks like a "cycloid" gear from an oil pump) into a mate which locks the drive wheels but not the engine. It won't do this unless you are moving -very- slowly or you are at a stop. The computer is smarter than us drivers. You can even hit it on the highway and you'll only succeed in going to "N".

BTW, this is the only sliding gear in the car. Amazing eh? (see, I'm Canadian).

 

as it's been said, the parking "gear" is actually just a pin.

if you're under 8mph, it will try to drop the pin.. (maybe it was a tad slower for P)... anyway.. you'll hear clicking and feel a violent stop. if you hear a loud "ping" and you still roll, you've destroyed your parking pin/claw. (hard to do)


as the rest of the gears (since we are off subject)... they only switch when you're under 8mph... D to R.. and R to D.... when doing 3 point turns, i don't stop, ever.. i simply go from D to R and back to D as the car rolls... it makes the prius the fastest car to turn on and do quick turn arounds in. (imo)

 

No, the parking "gear" is a gear. It is driven into a mating gear that is locked (bolted) to the housing. The whole assembly is removable without full disassembly of the HSD, presumably because the engineers thought some owners might damage it easily. It looks like a hypoid gear because it's designed to not lock in position in case the car rolls against it and to ensure it can slide into position even when you'd think the teeth would interfere. Unfortunately there is no good drawing of it in the GII shop manual. Not sure what's in the GIII manual.

 

Not a pin, as pointed out above. The concept is the same, but the gear system is much more robust.

Tom

 

Parking lock mechanism.

 

That's right -- though the manual does not mention it being retaining some control... only full or none? Anyway, as long as the accumulator pump can run (there is power) you will get assist. If you have a total power failure you are on your own. I would assume you get a couple pushes of the brake pedal before it becomes very stiff.

The powered fail-safe is engaged when the ECU detects problems with itself or with any of the hydraulic sensors. There is another failure mode where you have electronic braking, but ABS,VSC,Brake Assist and TRAC don't work.

Also, as far as I can tell, the brake system is, by default, in the non electronic mode. Once the system comes online "READY" then the "Switching solenoid valve" switches to electronic mode.

From the service manual:

2. FAIL-SAFE FUNCTION OF HYDRAULIC SYSTEM
(a) If a malfunction is detected in the brake booster with master cylinder (skid control ECU or brake actuator) or an individual sensor, control will be stopped and brake effort will be generated by the brake booster with master cylinder (hydraulic brake booster).

(b) If brake control is stopped due to a malfunction in the hydraulic pressure source, the pressure generated in the master cylinder by the driver is applied to the wheel cylinders to ensure braking force.