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.

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.

Fig 3: Identifying Slide Direction
Courtesy of GENERAL MOTORS CORP.
• 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.