Power Brake Assist: Description and Operation
Service Tips
Description
Regeneration Brake System
During deceleration or braking of an electric vehicle or HEV, the drive motor acts as an alternator and charges the battery by converting the vehicle's kinetic energy generated during braking into electrical energy.
Regenerative braking amount depends on the vehicle speed, battery SOC, etc.
Achieves significant improvement in fuel efficiency in city driving with repeated acceleration and deceleration.
Regenerative Braking Cooperation Control (RBC)
The brake force apportion is distributed by controlling hydraulic braking and the total brake force (hydraulic + regenerative brake) output that the driver requires is generated.
In the case of regenerative brake failure, the total brake force that the driver requires is supplied by the hydraulic brake system.
The following are the details of regenerative braking cooperative control.
- The red wire in the diagram indicates hydraulic pressure change.
- "I": Braking is conducted by the regenerative braking (Initial stage of braking).
- "II": Braking is conducted by the hydraulic brake.
- "III": Regenerative braking range extends as hydraulic brake decreases.
- "IV": Rapidly increasing of hydraulic pressure to meet the required brake amount because the regenerative braking is dropping due to very low kinetic energy when the vehicle almost comes to a stop.
System Features and Components
The AHB system is composed of the hydraulic power unit, the brake actuation unit, and electric stability control.
The Hydraulic Power Unit generates the hydraulic pressure required for braking.
Similar to the boosting effect when the driver steps on the brake pedal in a system equipped with a vacuum booster, the hydraulic pressure stored in the cylinder is supplied to provide pressure throughout the entire brake line.
The brake actuation unit delivers pressure that is generated by the Hydraulic Power Unit to a caliper on each wheel via electric stability control. It is connected to the brake pedal to detect the brake force that is required by the driver, and to generate a brake feeling.
Electric stability control carries out the ABS, TCS, and ESC functions as in conventional vehicles.
AHB System Operation Principals
1. Initial Status
The internal components of the HPU include 3 Normally Open (NO) valves, 2 Normally Close (NC) valves and a motor pump. The NO VALVE 3 is connected to the Simulator Chamber of the Brake Actuation Unit and it is opened when the brake is not engaged and closed when the brake is engaged.
The NO VALVE 1 and 2 are mounted between the boosting chamber and the reservoir tank of the brake actuation unit. When they are opened, the pressure in the boosting chamber is reduced.
The NC VALVE 1 and 2 are mounted between the high pressure generating motor pump and the boosting chamber. The pressure generated by the motor pump is cut off from the boosting chamber when closed and is fed to the boosting chamber when opened.
2. Base Brake Function
During normal braking, NO VALVE 1, 2, and 3 are in the ON status and closed. The pump either turns on or off according to the chamber status.
At this point, the NC VALVE 1 and 2 become ON and opened. The pressure generated by the pump is supplied to the boosting chamber and pressure is formed in the cylinder.
NOTE:
- Motor : when charging Accumulator
3. RBCS Function
When the regenerative brake cooperation system (RBCS) is engaged, the NO VALVE 1, 2 and NC VALVE 1, 2 control the amperage to reduce or increase the pressure in the boosting chamber that operates regenerative braking.
When the hydraulic braking pressure increases, regenerative braking decreases from the total brake force (driver required brake force). When the hydraulic braking pressure decreases, regenerative braking force increases. As a result, the generated electricity also increases.
NOTE:
- Motor : when charging Accumulator
4. Back-up Brake Function: 4 Wheel Back-up / ESC Hydraulic Brake Boost
Describes brake circuit in case of Hydraulic Power Unit (HPU) failure.
As shown in the diagram, all valves are not active in the HPU. Because NO VALVE3 is not operating, the simulator chamber pressure is not generated when the brake pedal is engaged as the brake field of the simulator chamber is linked to the reservoir tank. However, when the simulator rod begins to move as the brake pedal is engaged, the boosting chamber rod is mechanically pushed. This generates pressure in the master cylinder. This pressure is transferred to the ESC, which then controls the wheels in pressurized mode to achieve braking. The ESC therefore controls braking if the HPU fails.
Circuit Diagram - HPU
HPU connector input/output
AHB HPU Bleeding & Filling Process
Some pressurization device is required to apply the pressure.
HPU is supplied with accumulator is not filled. (But HPU inner circuit is filled completely.)
AHB System Component
System Bleeding & Filling Process
Air bleeding must be conducted when changing the HPU (Hydraulic Power Unit) or the BAU (Brake Actuation Unit).
Also, air bleeding must be conducted when changing the hose or tube between HPU and BAU.
Above and beyond these changing, operate below step2 work.
CAUTION:
Use the air bleeding tool (SST : 09589-3D100), when operating air bleeding to caliper, ESC and connected brake tube/hose.
It might not complete exactly or takes a long time if you operate air bleeding work with brake pedal.
1. Disconnect the negative (-) battery terminal in trunk. Set the air bleeding tool (SST: 09580-3D100) to reservoir tank and operate HPU & BAU line bleeding.
2. Connect the negative (-) battery terminal to turn the HPU ECU ON. And then operate the HPU and BAU line bleeding again.
3. Operate the ESC and Wheel line bleeding under the same conditions.
Bleeding & Filling Process
1. HPU & BAU Line Filling
(1) Disconnect the negative (-) battery terminal to turn HPU ECU off.
(2) Pressurize the pressure (0.3 - 0.5 MPa (43.5 - 72.5 psi)) to reservoir using air bleeding tool (SST: 09580-3D100).
(3) Remove all of the air by opening the bleed screw (A) and then close again.
(4) Open the bleeder screw (B) and let fluid flow until all the air is removed.
2. HPU & BAU Line Filling
(1) Connect the negative (-) battery terminal to turn HPU ECU on.
(2) Pressurize the pressure (0.3 - 0.5 MPa (43.5 - 72.5 psi)) to reservoir using air bleeding tool (SST: 09580-3D100).
(3) Press the brake pedal 20 - 40mm (0.79 - 1.57 in) while opening the bleed screw (A) and hold for 10 seconds.
Try these operations 2 - 3 times to each wheel until the air does not come out from brake fluid.
And then close the bleed screw.
CAUTION:
- Be careful that bleed screw hose does not break away.
- Fill the brake fluid if it is necessary.
(4) Press the brake pedal fully while removing all of the brake fluid by opening the bleed screw (B) of wheel side, and then close the bleed screw and release the brake pedal. Try these operations 3 - 5 times to each wheels until the air does not come out from brake fluid.
(5) Open the bleeder screw (A). Then slowly press the brake pedal. Then close the bleed screw and release the brake pedal. Repeat this procedure 3 - 5 times until no air bubbles are observed.
(6) Remove the air bleeding tool (SST: 09580-3D100) from reservoir. With the reservoir cap off, slowly press the brake pedal 5 - 10 times.
CAUTION:
- Be careful not to overflow the Brake Fluid.
(7) If accumulator does not charge and motor operate continuously, start the bleeding process over again.
(8) If air bubbles are not be observed rising in the reservoir, the bleed process is complete cap the reservoir.
3. ESC & Wheel & Master Cylinder Line Filling
(1) Pressurize the pressure (0.3 - 0.5 MPa (43.5 - 72.5 psi)) to reservoir using air bleeding tool (SST: 09580-3D100).
(2) Remove all of the brake fluid by opening the bleed screw of each wheel side.
(3) Press the brake pedal fully while removing all of the brake fluid by opening the bleed screw of wheel side, and then close the bleed screw and release the brake pedal. Try these operations 3 - 5 times to each wheels until the air does not come out from brake fluid.
CAUTION:
- In this time, be careful not to get the air into the pipe because bleed screw is opened overly.
