Exactly how do the friction brakes work?

In answering some recent questions, it occurred to me that I have a hole in my knowledge about the GenII Prius braking system as it relates to the hydraulic friction brakes. In a nutshell, here is what I know, or at least think I know:

1) The brake pedal is attached to a potentiometer, two electrical switches, and a hydraulic piston.

2) The potentiometer is use as an input to a rate sensor, which measures the speed of pedal application to determine if a panic stop situation exists.

3) The two electrical switches are connected respectively to the brake lights, and brake interlock which prevents powering up without pressing the brake pedal.

4) The hydraulic piston is connected to a closed hydraulic system which also contains a pressure sensor and a spring piston device.

5) The spring piston device allows the brake pedal to move with the standard squishy feeling found in normal automotive braking systems.

6) The pressure sensor sends brake pedal position information to the brake system ECU.

7) When the brake ECU determines that braking is required, it negotiates with the other ECUs over regenerative braking verses hydraulic friction brakes. A panic stop situation calls for immediate use of the friction brakes and engages the braking assist system which speeds the application of the brakes (I can't remember Toyota's name for this assist system; what is it?)

8) If hydraulic friction brakes are required (low speed, insufficient regenerative capacity, or panic stop), the braking ECU applies hydraulic pressure to the friction brakes. This hydraulic pressure comes from a reservoir which is pressurized by an electrically driven pump (I usually call this the brake accumulator pump; what is the proper name for this pump?)

9) The electrically driven brake pump is supported by a bank of super capacitors. These super capacitors store enough electricity to briefly operate the pump in the event of a total electrical failure.

10) The parking brake cable operates the rear drum brakes (U.S. version; I'm not sure about this with rear disk systems?)

Here is the part I know I don't know: Is there a direct hydraulic connection between the brake pedal hydraulic piston and the actual hydraulic friction brakes? In other words, if all of the automatic electronic parts fail, when you step on the brake pedal will it manually supply hydraulic pressure to actuate the friction brakes?

If this is true, what keeps the friction brakes from dragging when regenerative braking is used? Does the spring piston in the brake system supply enough extra travel that manual actuation only occurs at the bottom of the stroke?

I'm really curious about this, and want to get it straight so I don't pass along bad information. Any corrections or additional information will be greatly appreciated.

Tom

 

Thanks Jayman, that's it exactly. Now we know.

The brake master cylinder is normally connected only to the stroke simulator, which is the spring piston device. If the power brakes fail for any reason, the skid control ECU throws some valves which disconnect the master cylinder from the stroke simulator and connects it directly to the front brake cylinders. In this emergency mode you have manual hydraulic control of the front brakes only.

This is a fairly robust system. Look at the layers of protection:

1) Regenerative and engine braking though the front wheels (antilock).

2) Power hydraulic braking through all four wheels (antilock) with emergency power backup through the super capacitor bank.

3) Manual hydraulic braking through the front wheels.

4) Cable operated braking of the back wheels (parking brake).

This is a triple-redundant system. How cool is that.

Tom

 

Here is another cool thing: Looking at the diagram jayman posted, you can see that the master cylinder is a dual master cylinder. Instead of being two cylinders in parallel, the two cylinders are tandem. The first cylinder is connected to the right front brake, the second cylinder is connected to the left front brake and the stroke simulator. The two master cylinder pistons are connected only by hydraulic fluid. In normal operation, when you depress the brake pedal, the pedal lever pushes on the first piston. The fluid behind this piston has nowhere to go, since the emergency valve is closed. The pressure behind the first piston presses on the second piston, which in turn presses on the fluid behind it. This fluid is coupled to the stroke simulator, so the piston in the stroke simulator moves, allowing the brake pedal to depress.

In the event of a power brake failure, the emergency valves drop into position. Now when you depress the brake pedal, the fluid pressure behind the first piston is transferred to the right front brake and the second master cylinder piston. The second master cylinder piston transfers the pressure to the fluid behind it. In the emergency configuration, the valve to the stroke simulator is closed, so the pressure goes to the left front brake.

The emergency system still works in the event of a hydraulic leak. If the fluid leaks from the right side, the master cylinder pistons mechanically touch, transferring pressure to the left side piston. If the fluid leaks out of the left side, the left piston collapses to the far end of the cylinder, allowing the right piston to build pressure. In either case, the brake peddle position will drop slightly and braking will be reduced to one front wheel.

It looks like a well though out system.

Tom

 

Damn! I did it again. That one gives me fits.

Thanks for the correction; I'll fix it.

Tom

 

That's when you drag your foot out the door, or aim for a tree.

Tom

 

I get it! So heat is involved. Gee, thanks. :yawn:

Tom

 

Hobbit, the key thing for me was seeing the fail-over solenoid valves that are used to reconfigure the hydraulic system in the event of a major failure. Without those, there would either be no manual hydraulic backup, or manual hydraulic braking action would have to be on all of the time, which would cause the brakes to drag during regen.

Your post explains the details in how the pads and shoes are retracted. I like the return valves that are used to relieve pressure at the actuators. It's a well thought out and very clever system.

As for cruise control, most systems use the brake light switch for cancellation. I don't know if this is true on the Prius. It could be either of the two brake pedal switches, either of the brake pressure switches, or more likely, just a signal from the brake ECU. Maybe hobbit knows.

Tom