Part 2
The anisotropic magneto resistive integrated circuit (AMRIC) (1) comprises of eight AMR elements (2) which are configured to form two Wheatstone bridges (3).
One of the Wheatstone bridges (4) is 45 degrees offset from the other to enable the AMRIC to produce sine (5) and cosine (6) output signal.
The AMR elements change resistance that corresponds to the changes in the angle of magnetic field projected to the AMRIC, regardless of the polarity of the magnetic field.
The evaluation circuit of the steering angle sensor evaluated and combines the output signal of the AMRIC to produce two identical linear output (A) that represents the 360 degrees angular rotation of the magnetic field.
The polarity of the AMR elements limits the signal output range of a single AMRIC from 0-180 degrees.
Steering Angle Sensor Layout
To increase the output signal range of the steering angle sensor, and enable it to produce an output signal that can represent the ±760 degrees of steering wheel rotation, two AMRICs are fitted to the steering angle sensor.
The magnets (1) that project magnetic fields to the AMRICs are mounted on the primary measuring gear (2) and secondary measuring gear (3).
The secondary gear has two less teeth when compared against the primary measuring gear. This causes the measuring gears to rotate at a different ratio.
Steering Angle Sensor Operation
The angular orientation of the magnetic fields produced by the measuring gear magnets (1) corresponds to the degree of steering wheel angular position.
This magnetic field changes the resistance value of the AMR elements, which enable the AMRICs (2) to produce a pair of signal outputs (3).
The difference in the rotational ratio between the measuring gears causes the signal output of the second AMRIC (A) to be shorter in range when compared against the signal output of the first AMRIC (B).
The evaluation circuit (4) compares and evaluates this difference, and the rate of change between the output signals of the AMRICs, at parallel points (5) to calculate that precise steering wheel angular position.
Antilock Brake System Operation
The antilock braking system (ABS) prevents wheel lock-up during hard or emergency braking by modulating the brake fluid pressure applied to the appropriate wheels. The ABS cycles through the following phases when the electronic brake control module (EBCM) detects the beginning of the wheel lock-up:
* Maintaining pressure
* Reducing pressure
* Increasing pressure
Note:
The following are conditions that may be experienced when the ABS is active and are considered normal:
* During ABS controlled braking, the braking pressure of the affected wheel is automatically adjusted to prevent wheel lock-up, regardless of pressure applied to the brake pedal.
* A series of rapid pulsations are felt through the brake pedal. These pulsations occur as solenoid valves within the hydraulic modulator changes position to modulate the brake hydraulic pressure.
* A ticking or popping noise in the hydraulic modulator occurs as the hydraulic modulator solenoid valves cycle rapidly to modulate the hydraulic brake pressure.
* Intermittent chirping noises may be heard as the tires approach slipping on dry pavement.
* Electric motor and pump noise and rapid brake pedal pulsation caused by the operation of the hydraulic modulator pump during the ABS reducing or increasing pressure phase or the EBCM self-test.
ABS Phase - Maintaining Pressure
The EBCM closes the inlet valve and keeps the outlet valve closed in order to isolate the system when wheel slip occurs. This maintains the pressure steady on the brake so that the hydraulic pressure does not increase or decrease.
ABS Phase - Reducing Pressure
The EBCM decreases the pressure to individual wheels during a deceleration when wheel slip occurs. The inlet valve is closed and the outlet valve is opened. The excess fluid is stored in the accumulator until the return pump can return the fluid to the master cylinder.
ABS Phase - Increasing Pressure
The EBCM increases the pressure to individual wheels during a deceleration in order to reduce the speed of the wheel. The inlet valve is opened and the outlet valve is closed. The increased pressure is delivered from the master cylinder.
Electronic Brake-Force Distribution System
The electronic brake-force distribution (EBD) system is part of the ABS software programmed into the electronic brake control module (EBCM). It is designed to replace the rear brake proportioning valve in reducing rear wheel slip during moderate braking.
The EBD system utilizes the existing ABS active controls to regulate the vehicle's rear brake fluid pressure. This enables the EBD system to provide dynamic front to rear brake proportioning under various vehicle loads, driving maneuvers, or road conditions.
In some situations, when the EBD system is activated, a brake pedal height drop of approximately 10 mm will be experienced when the driver varies the brake pedal pressure while performing brake stops. This is caused by the hydraulic modulator performing an adjustment on the rear brake fluid pressure and is considered normal.
EBD System Keep Alive Function
The EBD system plays an important role in vehicle stability during braking. For this reason, the EBD system has a Keep Alive Function integrated in its software. When the EBCM detects a fault in the ABS, depending on the type of fault, certain parts of the system are kept alive. This allows the EBD system to apply some rear wheel brake proportioning even under certain ABS faults.
Traction Control System
The traction control system (TCS) utilizes the active braking controls in the ABS and the engine torque reduction function of the engine management system to prevent longitudinal wheel spin during vehicle acceleration.
When drive wheel slip is noted while the brake is not applied, the EBCM will enter traction control mode.
First, the EBCM requests the ECM to reduce the amount of torque to the drive wheels via the requested torque signal circuit. The ECM reduces torque to the drive wheels by retarding spark timing and turning off fuel injectors. The ECM reports the amount torque delivered to the drive wheels via the delivered torque signal circuit.
If the engine torque reduction does not abolish drive wheel slip, the EBCM will actively apply the drive wheel brakes. During traction control braking, hydraulic pressure in each drive wheel circuit is controlled to prevent the drive wheels from slipping. The master cylinder isolation valve closes in order to isolate the master cylinder from the rest of the hydraulic system. The prime valve then opens in order to allow the pump to accumulate brake fluid in order to build hydraulic pressure for braking. The drive wheel inlet and outlet solenoid valves then open and close in order to perform the following functions:
* Pressure increase
* Pressure hold
* Pressure decrease
If at any time during TCS mode the brakes are manually applied, the EBCM will exit the TCS brake intervention mode and allow for manual braking.
Electronic Stability Program
The electronic stability program (ESP) is designed to provide optimum vehicle stability and steering control during high speed cornering or when driving on a slippery road surface.
Yaw rate is the rate of rotation about the vehicle's vertical axis. The VSES is activated when the EBCM determines that the desired yaw rate does not match the actual yaw rate as measured by the yaw rate sensor.
The desired yaw rate is calculated from the following parameters:
* The position of the steering wheel
* The speed of the vehicle
* The lateral, or sideways acceleration of the vehicle
The difference between the desired yaw rate and the actual yaw rate is the yaw rate error, which is a measurement of oversteer or understeer. If the yaw rate error becomes too large, the EBCM attempts to correct the vehicle's yaw motion by applying differential braking to the appropriate wheel. The amount of differential braking applied to the left or right front wheel is based on both the yaw rate error and side slip rate error.
The VSES activations generally occur during aggressive driving, in turns or on bumpy roads without much use of the accelerator pedal. When braking during VSES activation, the pedal pulsations feel different than the ABS pedal pulsations. The brake pedal pulsates at a higher frequency during VSES activation.
The ESP incorporates the following components:
* Steering angle sensor to determine driver steering input, which the EBCM uses to support the calculation of intended vehicle direction.
* Yaw rate sensor to measure vehicle rotation around its vertical axis, which the EBCM uses to support the calculation of actual vehicle behavior.
* Active braking and engine torque reduction functionality of the TCS to prevent wheel spin.
When the ESP detects a lateral wheel slip or when the calculated vehicle direction deviates from the actual vehicle direction, the ESP utilizes the engine torque reduction functionality and the active braking controls in the ABS-TCS to stabilize and steer the vehicle to the correct direction.
ESP Engine Torque Reduction
The ESP monitors the wheel speed sensor, yaw-rate sensor and the steering angle sensor to determine lateral wheel slip. When the ESP detects a lateral wheel slip, the ESP initially utilizes the torque reduction functionality of the TCS and sends a serial data communication signal to the engine control module (ECM) requesting engine torque reduction. In addition, if the ESP still detects a lateral wheel slip, the ESP applies active brake intervention.
ESP Brake Intervention
The ESP applies active brake intervention when the EBCM receives the following input signals and determines the vehicle is beginning to understeer.
* The steering angle sensor sends a signal to the EBCM the driver intends to steer in direction (A).
* The yaw rate sensor detects the vehicle begins to spin (B) with the front of the vehicle beginning to slide in direction (C).
The ESP uses the existing active braking control in the ABS-TCS to apply a calculated braking force to one or both inner wheels (1) of the vehicle to stabilize and steer the vehicle to the intended direction.
Note:
Applying brake-force to the inner wheel (1) slows down the inner side of the vehicle, which induces the vehicle to rotate on its vertical axis (A).
The EBCM monitors and compares signals from the yaw-rate sensor, steering angle sensor and each wheel speed sensor to determine wheel slip. If the EBCM detects the beginning of a vehicle understeer condition, the EBCM sends the following signal to the hydraulic modulator to:
* Close the front and rear isolating valves.
* Open the front and rear prime valves.
* Close the front right and rear right inlet valves.
* Operate the hydraulic modulator pump.
This results in the following actions:
* The rear isolation valve is closed to isolate the rear brake fluid circuits from the master cylinder and prevent the brake fluid returning to the brake master cylinder when the hydraulic pump builds-up the brake fluid pressure.
* The front right and rear right inlet valves are closed to isolate the right wheel hydraulic circuits, allowing the hydraulic modulator to supply brake fluid pressure only to the left wheels.
* The rear priming valve is open to allow fluid to be drawn from the master cylinder into the hydraulic pump.
* The hydraulic pump applies appropriate brake fluid pressure to the left brake callipers to steer the vehicle to the intended direction.
* The hydraulic modulator modulates the front left and rear left inlet valves and outlet valves to assist in obtaining maximum road surface traction in the same manner as in the TCS Mode.
Note:
If at any time during the ESP mode the brakes are manually applied, the EBCM will exit the ESP Brake Intervention Mode and allow normal braking.
The ESP applies active brake intervention when the EBCM receives the following input signals and determines the vehicle is beginning to oversteer.
* The steering angle sensor sends a signal to the EBCM the driver intends to steer in direction (A).
* The yaw-rate sensor detects the vehicle begins to spin (B) with the rear of the vehicle beginning to slide in direction (C).
The ESP uses the existing active braking control in the ABS-TCS to apply a calculated braking force to one or both outer wheels (1) of the vehicle to stabilize and steer the vehicle to the intended direction.
Applying brake-force to the outer wheel (1) slows down the outer side of the vehicle, which induces the inner wheel to rotate on its vertical axis (A).
The EBCM monitors and compares signals from the yaw-rate sensor, steering angle sensor and each wheel speed sensor to determine wheel slip. If the EBCM detects the beginning of a vehicle oversteer condition, the EBCM sends the following signal to the hydraulic modulator to:
* Close the front and rear isolating valves
* Open the front and rear prime valves
* Close the front right and rear right inlet valves
* Operate the hydraulic modulator pump
This results in the following actions:
* The rear isolation valve is closed to isolate the rear brake fluid circuits from the master cylinder and prevent the brake fluid returning to the brake master cylinder when the hydraulic pump builds-up the brake fluid pressure.
* The front left and rear left inlet valves are closed to isolate the right wheel hydraulic circuits, allowing the hydraulic modulator to supply brake fluid pressure only to the left wheels.
* The rear priming valves are open to allow fluid to be drawn from the master cylinder to the hydraulic pump.
* The hydraulic pump applies appropriate brake fluid pressure to the left brake callipers to stern the vehicle to the intended direction.
* The hydraulic modulator modulates the front right and rear right inlet valves and outlet valves to assist in obtaining maximum road surface traction in the same manner as in the TCS Mode.