GreenHybrid - Hybrid Cars - View Single Post

sdctcher · #14 (**permalink**) 12-21-2004, 08:03 PM

Thanks Stevo -
I offer the following technical info on both the Accelerator and Brake Control.

Acceleration Control
The engine management system electronically operates the throttle of the engine in response to throttle pedal movements initiated by the driver. In the event of a system failure, the engine management system provides a "limp home" mode which allows the car to be driven with limited performance.

Brake System — Escape Hybrid
The brake system has the following components:

fixed position foot pedal
front-to-rear split hydraulic system
front disc brakes utilizing a single piston floating brake caliper
rear disc brakes utilizing a single piston floating brake caliper
mechanical parking brake system utilizing a drum-in-hat within the rear disc brakes
4-wheel anti-lock brake system (4WABS).

Regenerative Braking System
Instead of just using the brakes to stop the vehicle, the electric motor that drives the hybrid electric vehicle can also slow the vehicle. In this mode, the electric motor acts as a generator and charges the batteries while the vehicle is slowing down. To achieve this, the anti-lock brake system (ABS) module calculates the amount of speed reduction requested by the driver through the accelerator pedal position and brake pedal travel sensor. The ABS module measures the inertia of the vehicle through the longitudinal accelerometer and determines if the required deceleration has been achieved. During deceleration/braking, the powertrain control module (PCM) partially engages the electronically controlled continuously variable transmission (eCVT) to be turned by the vehicle's wheels and slow it down. The ABS module requests the powertrain more or less dependant upon the driver request through the pedals. If the desired deceleration is not achieved through regenerative braking, the ABS module applies the friction brake pads to accommodate the driver request. The driver does not, under normal circumstances, have direct control over the amount of brake pressure that is applied to the rotors. In the event that the ABS module detects a fault which requires the deactivation of this system, the ABS module relinquishes control of braking and the driver will still have the use of manual hydraulic brakes.

Since the regenerative braking uses the electric motor to slow the vehicle's front wheels, front brake pad wear is reduced. The rear brake pads, because of the regenerative braking, wear at approximately twice the rate of the front brake pads.

Principles of Operation

The hydraulic control unit (HCU) controls the brake assist function as well as the anti-lock functions. The HCU contains an accumulator, driven by an electric motor, to supply pressurized brake fluid to the disc brake calipers during brake application. The electric motor begins to build pressure in the accumulator whenever the vehicle doors are opened or when the dome light is activated. The electric motor also charges the accumulator and initiates the brake system self-test whenever the ignition switch is turned to the ON position or the brake pedal is applied. During this time, 12 mm (0.47 in) of brake fluid is drawn from the master cylinder reservoir into the accumulator. The accumulator discharges 4 minutes after the ignition switch is turned to the OFF position and the vehicle dome light turns off, returning the brake fluid used during the charging of the accumulator to the master cylinder reservoir.

The regenerative braking system recovers the vehicle kinetic energy during deceleration to replace the high voltage traction battery energy. A certain level of regenerative braking is commanded by releasing the accelerator pedal. Further regenerative braking is commanded by applying the brake pedal. Whenever the ABS is activated, the affects of regenerative braking are reduced until the ABS event is over and the accelerator is applied.

The series of regenerative braking feature provides isolation of the driver brake demand from the friction brakes in order to provide a portion of the braking through negative torque request to the electric drive motor. In addition, this torque request results in a change in the flow of energy. The energy flows into, rather than out of, the battery. The motor's change from drive motor to generator is referred to as regeneration.

Regenerative braking logic is as follows:

NOTE: The ABS module is part of the HCU. Do not separate the ABS module from the HCU for any reason.

The ABS module determines the total torque request from accelerator pedal and brake pedal input.
The ABS module compares the drivers total request for a reduction of speed to the powertrain control module's (PCM) ability to slow the vehicle or negative "torque limit."
The ABS module requests the reduction in speed up to the torque limit from the PCM, via a torque modification request message sent to the PCM.
The ABS module applies the friction brakes beyond the torque limit.