Principles of Operation
Anti-Lock Control - Hybrid
Principles of Operation
Regenerative Braking
The regenerative brake system utilizes the electric motor portion of the hybrid electric powertrain as a generator to create electrical current. This recharges the High Voltage Traction Battery (HVTB) and delivers brake torque to be used in place of or in combination with the vehicles conventional friction brakes to slow the vehicle. The amount of brake torque provided by the electric motor is dependent upon the state of charge of the HVTB. When the HVTB is almost fully charged, the amount of regenerative braking is limited to avoid overcharging, and the requested deceleration is produced by conventional friction braking alone.
Conventional Friction Braking
If the HVTB is almost fully charged, or the requested amount of braking cannot be achieved by regenerative braking alone, the ABS module will apply the vehicles conventional friction brakes. The friction brakes utilize components similar to those found in a conventional vacuum assisted brake system with anti-lock control. For a list of components, refer to Brakes and Traction Control.
Brake-By-Wire
During a normal brake application, the brake pedal feel simulator will allow the pressure applied to the brake pedal by the drivers foot to act against a spring instead of the brake booster push rod. The brake pedal angle sensor is mounted to the brake pedal assembly and will provide the ABS module with an analog signal indicating the amount of deceleration the driver is demanding. If it is necessary for the conventional friction brakes to be applied, the ABS module will apply a Pulse-Width Modulation (PWM) voltage to a vacuum solenoid inside the active brake booster. The solenoid will allow vacuum from either the combustion engine (if the engine is running) or the electric vacuum pump to move the booster push rod in turn pushing the piston in the master cylinder creating hydraulic brake pressure. If the brake pedal is applied quickly and forcefully, the driver can overcome the brake pedal feel simulator spring pressure, allowing a direct connection between the brake pedal, booster push rod and master cylinder.
Brake Pedal Feel Simulator and Cut-Off Device
When the ignition is turned on, the ABS module runs a series of checks to make sure the system is running properly. Once these checks are complete, the ABS module closes the valve in the simulator cut-off device which puts the brake system into brake-by-wire mode. Brake-by-wire mode is when the pedal feedback to the driver is provided by the pedal feel simulator and not by the pressure in the hydraulic brake system. The simulator uses spring pressure to provide a familiar brake pedal sensation. When the driver presses the brake pedal, the ABS module interprets the pedal movement through the brake pedal angle sensor. Based on the pedal angle and the amount of brake torque that can be provided through regenerative braking, the ABS module determines how much brake torque will be provided by the powertrain and how much will be provided by the hydraulic brakes. The brake pedal is uncoupled from the brake booster push rod allowing the system to hold off hydraulic pressure when regenerative braking is providing the necessary brake torque. When hydraulic braking is required, the ABS module activates the booster solenoid which allows the vacuum booster to push on the master cylinder creating pressure in the hydraulic system. In the event a fault occurs which inhibits the brake-by-wire mode, the ABS module will open the cut-off device valve. This puts the system into conventional mode with vacuum boosted hydraulic brakes and no regenerative braking. In this mode, the pedal feedback to the driver is provided by the booster. When the driver pushes the pedal, the uncoupling between the pedal and the booster push rod is overcome, and the pedal arm will push on the booster push rod as in a conventional brake system.
Active Brake Booster
The active brake booster is similar to a conventional vacuum assisted brake booster with the addition of a brake booster solenoid, a brake booster travel sensor, and a vacuum pressure sensor. If it is necessary to apply the friction brakes due to inadequate deceleration from regenerative braking, the ABS module will send a PWM voltage to the brake booster solenoid that is proportional to the amount of brake torque requested. The booster solenoid will open, which allows the vacuum booster push rod to push on the master cylinder creating pressure in the hydraulic system.
The ABS module uses the signals produced by the vacuum pressure sensor to verify that vacuum maintained in the brake booster. The ABS module uses the signals produced by the brake booster travel sensor to monitor brake booster performance. The brake booster travel sensor and the vacuum pressure sensor can be serviced separately from the brake booster.
Electric Vacuum Pump
When the combustion engine is not running, an electric vacuum pump is used to maintain vacuum inside the brake booster. The ABS module turns the electric vacuum pump on and off through the use of a mechanical relay and a solid state relay. If the ABS module detects a vacuum pump motor circuit concern, a vacuum system leak or a vacuum pump performance concern, a DTC will set in the ABS module and the combustion engine will run to provide vacuum for the brake booster.
Anti-Lock Control
The ABS module continuously monitors and compares the rotational speed of each wheel. This is accomplished through the use of 4 active wheel speed sensors, one for each wheel. The wheel speed sensors are connected to the ABS module by 2 circuits. One circuit provides voltage for sensor operation and the other circuit provides sensor input to the ABS module. As the wheels spin, the wheel speed sensor tone rings pass through the magnetic field generated by the active wheel speed sensor. This creates a square-wave signal that is sent to the ABS module to indicate individual wheel speed.
The ABS module uses a Hydraulic Control Unit (HCU) to prevent the wheels from locking up during a braking event. The HCU contains several solenoid-controlled valves, a pump motor, an accumulator and several sensors. The fluid flows through 4 normally open inlet valves inside the HCU and through the outlet ports of the HCU to the brake calipers. If the ABS module senses that a wheel is about to lock up, based on wheel speed sensor data, the module will close the normally open inlet valve for that wheel. This prevents more fluid from getting to the brake caliper. The ABS module then reads the wheel speed sensor signal from the affected wheel again. If the wheel is still decelerating (locking up), the module opens the normally closed outlet valve. This allows a controlled amount of hydraulic pressure in the brake caliper to be relieved into the HCU accumulator. The ABS pump motor runs during an ABS braking event so that sufficient brake fluid pressure is consistently available to the HCU valves. This procedure of closing and opening valves is repeated several times per second until the speed of the affected wheel matches the speeds of the other wheels. Once this has been accomplished, the ABS module returns the solenoid valves to their normal positions.
The ABS module is self-monitoring. When the ignition switch is turned to the RUN position, the ABS module will carry out a preliminary electrical check of the wheel speed sensor circuitry by sending voltage through the sensor and checking for the voltage to return. At speeds above 20 km/h (12 mph), without the brake pedal being pressed, the pump motor will be commanded ON for less than 2 seconds to check pump motor operation. Also, during all phases of operation while the vehicle is in motion, the ABS module checks for correct operation of the wheel speed sensors. In the event that the ABS module detects a concern, the module will set a DTC and send a message to the Instrument Panel Cluster (IPC) over the High Speed Controller Area Network (HS-CAN) bus to illuminate the yellow ABS warning indicator, the stability/traction control indicator (also known as the sliding-car icon) and/or the red brake warning indicator. If the concern is severe enough, ABS may be disabled. Normal power-assisted braking, however, remains.
Electronic Brake Distribution (EBD)
The ABS incorporates a strategy called Electronic Brake Distribution (EBD). EBD uses the HCU as an electronic proportioning valve. On initial application of the brake pedal, full pressure is applied to the rear brakes. The ABS module uses wheel speed input to calculate an estimated rate of deceleration. Once vehicle deceleration exceeds a certain threshold, the ABS module closes the appropriate isolation valves in the HCU to hold the rear brake pressure constant while allowing the front brake pressure to build. This creates a balanced braking condition between the front and rear wheels and minimizes the chance of rear wheel lockup during hard braking. As the vehicle decelerates the valves are opened to increase the rear brake pressure in proportion to the front brake pressure.
If there is a DTC in the ABS module specifically for the HCU or there are 2 or more wheel speed sensor DTCs, EBD will be disabled. When EBD is disabled, the ABS warning indicator, the red brake warning indicator, and the sliding-car icon will be illuminated.
AdvanceTrac(R) with Roll Stability Control (RSC(R))
The AdvanceTrac(R) with RSC(R) system provides the following stability enhancement features for certain driving situations:
- Traction Control System - helps to avoid excessive drive-wheel spin and loss of traction.
- Electronic Stability Control (ESC) System - helps to avoid skids and lateral slides.
- RSC(R) - helps to avoid vehicle roll-over.
Traction Control System
The traction control system uses the ABS and the PCM to limit wheel spin. When the ABS module detects a wheel spinning excessively, brake pressure to the appropriate brake caliper is modulated. This is accomplished by opening and closing the appropriate solenoid valves inside the HCU while the hydraulic pump motor is activated. At the same time, the ABS module sends a message over the HS-CAN bus that a traction control event is taking place. When the IPC receives this message, it will flash the sliding-car icon (twice per second). If the event is severe enough, the ABS module will also send a message to the PCM over the HS-CAN bus to assist with traction control. When the PCM receives this message, it will adjust engine timing and decrease fuel injector pulses. Once the affected wheel returns to normal speed, the ABS module returns the solenoid valves in the HCU to their normal position, deactivates the hydraulic pump motor and sends another message over the HS-CAN bus indicating that the traction event has ended. The PCM returns engine timing and fuel injectors to normal operation and the Instrument Panel Cluster (IPC) extinguishes the sliding-car icon.
Once vehicle speed reaches or exceeds 100 km/h (62 mph), traction control is accomplished only through PCM intervention.
Certain DTCs in the ABS module will disable the traction control system. Depending on the DTCs present, the yellow ABS warning indicator and/or the sliding-car icon will be illuminated.
Electronic Stability Control (ESC) and Roll Stability Control (RSC(R)) Systems
The ESC and RSC(R) systems are controlled by the ABS module and use the same wheel speed sensors and tone rings that are used for anti-lock braking. The ESC and RSC(R) systems also use input from the steering wheel rotation sensor, the stability control sensors (yaw rate, roll rate, longitudinal acceleration and lateral acceleration) which are internal to the Restraints Control Module (RCM) and information from other modules sent over the HS-CAN bus to help maintain vehicle stability.
The ABS module uses the various sensors to continuously monitor vehicle roll motion and direction of travel relative to the driver's intended course. If the ABS module determines from all these inputs that the vehicle is unable to travel in the intended direction or that a condition exists for a potential roll-over event, it modulates brake pressure to the appropriate brake caliper(s) by opening and closing the appropriate solenoid valves inside the HCU while the hydraulic pump motor is activated. At the same time, the ABS module sends a message over the HS-CAN bus that a stability event is taking place. When the IPC receives this message, it will flash the sliding-car icon. During a stability event, conditions may require the involvement of the PCM to limit engine torque like it does for traction control. Once the stability condition has been corrected, the ABS module returns the solenoid valves in the HCU to their normal position, deactivates the hydraulic pump motor and sends another message over the HS-CAN bus indicating that the traction event has ended. The PCM returns engine timing and fuel injectors to normal operation and the Instrument Panel Cluster (IPC) extinguishes the sliding-car icon.
Certain DTCs being present in the ABS module may disable the ESC and RSC(R) systems. Depending on the DTCs present, the yellow ABS warning indicator and/or the sliding-car icon will be illuminated.
Stability Control Sensors
The stability control sensors for the AdvanceTrac(R) with RSC(R) system consist of the yaw rate sensor, lateral accelerometer and longitudinal accelerometer. The sensors are housed in the RCM which sends sensor information to the ABS module over a dedicated Controller Area Network (CAN) bus. If any of the sensors are defective, a new RCM must be installed. Refer to Restraint Systems for additional information on the RCM.
- The yaw rate sensor measures the relative vehicle motion about the vertical axis through the vehicle center of gravity.
- The longitudinal accelerometer measures the acceleration corresponding to the force involved when the vehicle moves forward and rearward in the horizontal plane, along the centerline of the vehicle's front and rear wheels.
- The lateral accelerometer measures the acceleration that corresponds to the force involved when the vehicle moves sideways.
Lateral acceleration has 2 forms. The first is the centrifugal acceleration that is generated when the vehicle travels around in a circle. The second is the acceleration due to gravity. On level ground there is no contribution from this acceleration. However, if the vehicle is parked sideways on a bank or incline, the sensor measures some lateral acceleration due to gravity, even though the vehicle is not moving.
Steering Wheel Rotation Sensor
The steering wheel rotation sensor measures the rate of rotation of the steering wheel by monitoring the steering wheel rotation ring as it passes through the sensor gap. The steering wheel rotation sensor uses the HS-CAN bus to transmit information to the ABS module about whether the steering wheel is being turned left or right and how far it is being turned. The steering wheel rotation sensor is integrated into the Electronic Power Assist Steering (EPAS) column assembly and is not serviced separately. For information and diagnostics on the EPAS system, refer to Steering.
Stability/Traction Control Switch
The stability/traction control switch provides the driver with the ability to disable only the traction control feature. Anti-lock control, ESC and RSC(R) systems will remain functional. The switch is hardwired to the ABS module. When the stability/traction control switch is pressed, a ground signal is sent to the ABS module. The ABS module then sends a message over the HS-CAN bus to the IC indicating that the driver has requested that the system be deactivated and the IC illuminates the stability/traction control indicator (sliding-car icon). The ABS module and the PCM will then ignore any traction events until the driver presses the switch again or until the ignition key is cycled.
The AdvanceTrac(R) system cannot be disabled when a MyKey(TM)-restricted key is in use that has the AdvanceTrac(R) always-on feature configured to "on". If an attempt is made to disable the AdvanceTrac(R) system while a MyKey(TM)-restricted key is in use, ADVTRAC ON MYKEY SETTING will display in the IC message center. For additional information about MyKey(TM), refer to Antitheft and Alarm Systems.
Stability/Traction Control Indicator (Sliding-Car Icon)
Status of the traction control, ESC, and RSC(R) systems is indicated by a stability/traction control indicator (sliding-car icon) located in the IC. When the driver disables the system through the use of the stability/traction control switch, this indicator will illuminate solidly. When one or more of the systems is currently active, the indicator will flash, and when the system is disabled due to DTCs being present in the ABS module, the indicator will illuminate in conjunction with the yellow ABS warning indicator.
ABS Module Calibration (Multi-Calibration Routine)
When an ABS fault has been corrected or an ABS component has been replaced it will be necessary to calibrate the ABS module. The calibration procedure is required for the brake-by-wire sensors to learn the "zero-position" of the vehicle, which means that the vehicle must not be moving and that the brake pedal must not be applied.
To carry out the calibration procedure, connect the scan tool, proceed to the Multi-Calibration routine and follow the scan tool instructions.
ABS Module Configuration
When a new ABS module is installed, it must be configured. To configure the ABS module, refer to Programmable Module Installation (PMI) in Information Bus. Programming and Relearning