Cruise Control Description and Operation (Adaptive Cruise Control) (Part 1)

CRUISE CONTROL DESCRIPTION AND OPERATION (ADAPTIVE CRUISE CONTROL)

ADAPTIVE CRUISE CONTROL (ACC)
The Adaptive Cruise Control System (ACC) is an enhanced Cruise Control System with the ability to sense and react to forward traffic. Compared to the common Cruise Control System found on many vehicles, the main functional enhancement of the ACC System is the ability to detect the presence of a vehicle in the path of the ACC vehicle. The ACC retains the existing cruise control feature that controls the vehicle speed to the driver selected speed. However, ACC allows a driver to set and maintain a following distance to the preceding nearest vehicle in the ACC vehicle path. The distance sensing cruise control (DSCC) module calculates a follow speed limit to ensure an acceptable distance is maintained to the preceding vehicle in front, should one be present. The ACC System automatically adjusts the speed of an ACC vehicle when the vehicle comes up behind a slower travelling vehicle in front. The ACC System applies limited automatic braking and throttle control without driver input when necessary to maintain the set following distance of the ACC vehicle. The preceding vehicle's speed and acceleration along with the ACC vehicle speed and acceleration and the distance between the two vehicles are factors used by the DSCC module to determine the ACC follow speed limit. The ACC vehicle speeds up to the original driver selected set speed when the pathway becomes clear without driver input.

ADAPTIVE CRUISE CONTROL (ACC) SYSTEM DESCRIPTION
The Adaptive Cruise Control (ACC) System depends on various modules on the vehicle to function and each module performs a function that is critical to the proper operation of the ACC System. ACC will not operate if any components fail. Communication between modules is via the High-Speed GMLAN serial data circuit. The instrument panel module (IPM) provides a translating gateway function for the GMLAN serial data links. The GMLAN utilizes the controller area network (CAN) communications protocol. The following is a functional description of the distance sensing cruise control (DSCC) module and the other associated components:

DISTANCE SENSING CRUISE CONTROL (DSCC) MODULE
The DSCC module contains the forward looking sensor (FLS) radar and the DSCC controller. The DSCC module utilizes the FLS radar to identify and classify objects in the road environment. The FLS radar scans the road environment to detect targets within its specified field of view. The DSCC controller then makes throttle and/or brake commands to the engine control module (ECM) and electronic brake control module (EBCM) via the GMLAN serial data circuit in order to control the vehicle acceleration/deceleration based on the data from the FLS radar. The ECM and EBCM provide throttle control and automatic braking needed for proper cruise speed adjustment. An internal yaw rate sensor is used by the DSCC module to determine the predicted path of the ACC vehicle. The main power feed for the DSCC module is the battery and this signal is always at battery potential. However, the ACC System only activates when the ignition 1 input is active and in the RUN position. The ignition 1 voltage signal to the DSCC module has to be 9.2 volts or more to be considered active by the DSCC module. When the ignition 1 signal is not active, all ACC functions are disabled. The following is a list of the DSCC module functions:
- The FLS radar processes the road environment to get data concerning any vehicle ahead of the ACC vehicle. Detection, parameter estimation, tracking, object classification and diagnostics are the primary FLS radar functions. When an object is detected, the DSCC controller calculates the object range, range rate, acceleration and azimuth angle parameters.
- The DSCC module performs ACC state processing automatically-distance control or speed control. The ACC System operates in 2 possible states-Cruise or Follow. The normal operating state is Cruise, whereby the vehicle speed is controlled to match the driver selected set speed. When a preceding forward target is identified by the FLS radar, the ACC System will automatically transition into the follow speed state to provide proper lane spacing behind the target vehicle in front. The preceding vehicle's speed and acceleration, with the ACC vehicle speed, acceleration and distance between the two vehicles will be used to determine the ACC follow speed limit. The ACC follow speed limit will ensure that an acceptable distance is maintained to the preceding vehicle.
- The DSCC module determines the follow speed limit for throttle control by the ECM.
- The DSCC module arbitrates the ACC System brake and throttle control between the EBCM and the ECM.
- The DSCC module requests brake light activation during automatic braking.
- The DSCC module provides operational feedback to the vehicle driver. The DSCC module sends signals for telltales and messages to be displayed on the head-up display (HUD) or driver information center (DIC) via the IPM. The DSCC module will provide audible feedback to the driver via the IPM and the radio amplifier as well.

INSTRUMENT PANEL MODULE (IPM)
The following are the ACC System functions provided by the IPM:
- The IPM provides a translating gateway for the high speed GMLAN serial data circuit.
- The IPM reads all cruise control switches and the gap switch. The DSCC module monitors a variety of user operated switches from the IPM switch status information sent via the GMLAN serial data circuit.
- The IPM illuminates the brake light based on a GMLAN message from the DSCC module during automatic braking.
- The IPM measures the brake pedal position and sends brake pedal travel status via GMLAN serial data circuit to the ECM. The ECM disengages the ACC System when the brake is applied by the driver pressing the brake pedal.
- The IPM serves as a gateway to the display systems, head-up display (HUD), instrument panel cluster (IPC), driver information center (DIC) and amplifier, visual and audible warnings. The IPM will generate visual and audible warnings based on high speed GMLAN serial data messages from the DSCC module.

ENGINE CONTROL MODULE (ECM)
The following are the ACC System functions performed by the ECM:
- The ECM provides the electronic throttle control to the ACC System. The ECM is also responsible for determining when a driver is overriding the adaptive cruise control throttle position by pressing the accelerator pedal.
- The ECM processes the cruise control switch requests received via the GMLAN serial data circuit from the IPM. Engaging and disengaging the ACC System are functions performed by the ECM.
- The ECM determines the driver selected vehicle speed. Unless the ACC module requests a lower vehicle speed, the ECM will control the vehicle speed to the driver selected set speed.
- The ECM allows automatic braking without disengaging the ACC.

ANTILOCK BRAKE SYSTEM (ABS)
The following are the ACC System functions performed by the EBCM:
- The EBCM controls the operation of the Antilock Brake System.
- The EBCM provides automatic braking for the ACC System. The DSCC module will request vehicle deceleration via high speed GMLAN serial data circuit.
- The EBCM determines when the driver-applied brake pressure is active. The EBCM will communicate this status via the GMLAN serial data circuit to the IPM.
- The EBCM releases vehicle automatic braking when there is a throttle override by the driver pressing the accelerator.
- The EBCM predicts brake temperatures. Due to automatic braking, the brake system may overheat. When the brakes are overheated, the EBCM requests the ACC System to be temporarily shut down by the ECM until the brakes cool to a normal operating temperature.

Refer to ABS Description and Operation.

TRANSMISSION CONTROL MODULE (TCM)
The transmission control module (TCM) provides a down shift to protect the brakes. When the TCM detects some driver braking activity or ACC automatic braking activity, the TCM down shifts the vehicle transmission to increase engine braking. This helps reduce braking activity, extend brake pad life and reduce brake system overheating.

DRIVER INFORMATION CENTER (DIC)
The DIC is a part of the IPC and displays the ACC System warning messages. The DSCC module requests messages to be displayed on the DIC by sending a GMLAN request to the IPM. The IPM sends a GMLAN serial data request to the DIC demanding the display of the warning message. When the message is acknowledged by the driver and the cause of the message resolved, the DIC turns the message off. These displays are accompanied by a chime warning internal to the DIC. The ACC System will not operate if the DIC fails. The following are the 3 ACC messages that can be displayed on the DIC:
- Service Radar Cruise
- Cruise Not Ready
- Clean Radar

HEAD-UP DISPLAY (HUD)
The HUD displays ACC System telltales and status information. The DSCC module sends a message to the IPM via the GMLAN serial data circuit requesting the display of a HUD telltale. The HUD displays the telltale when it receives a GMLAN serial data message from the IPM demanding illumination of the telltale. Feedback to a driver action takes precedence over other status information sent to the HUD by the IPM. The HUD is enabled in power mode RUN. The driver may have to adjust the position of the HUD or the intensity of the display to see the information being displayed. The ACC System will not operate if the HUD is not turned ON or if the HUD fails. The HUD displays the following telltales and ACC status indicators:
- Cruise Engaged Telltale
- Vehicle Ahead Telltale
- Follow Distance Display
- Set Speed Display
- Driver Alert Telltale
- Vehicle Speed Display
- Driver Throttle Override

RADIO AMPLIFIER
The amplifier receives a GMLAN serial data message from the IPM in order to request an audible warning. The chime alerts the vehicle driver to certain ACC vehicle conditions. The audible chime accompanies a visual indicator as a warning to the driver. The ACC System will not operate if the radio amplifier fails.

BRAKE PEDAL POSITION (BPP) SENSOR
The IPM monitors the brake pedal position (BPP) sensor in order to determine when pressure is being applied to the brake by the driver stepping on the brake pedal. The BPP sensor receives a low reference signal and a 5-volt reference from the IPM. When the brake is applied, a signal is applied from the BPP sensor through the BPP sensor signal circuit to the IPM. The IPM utilizes this signal in deciding when to command the activation of the stop lamps. The IPM continuously sends signals via the GMLAN serial data circuit to the EBCM and the ECM reflecting the brake pedal position status. This will include the initial brake travel signal required for the activation of the ACC System. The ACC System will not engage unless the initial brake pedal travel signal is valid and received by the IPM. When the IPM brake pedal position signal indicates that brake pedal pressure is being applied by the driver when the ACC System is engaged, the ECM immediately disengages the ACC System. Re-engaging the ACC System will require the driver pressing the set or resume switch. Refer to Brake Pedal Position Sensor Calibration and to Exterior Lighting Systems Description and Operation for more information on the brake pedal pressure sensor.

ACCELERATOR PEDAL
Deceleration of the ACC vehicle to maintain a certain distance and speed while a slower moving vehicle is in its path is achieved through throttle control by the ECM and the application of light automatic braking by the EBCM. During automatic braking the ECM is commanded to release the throttle by the DSCC module. The DSCC module via the GMLAN serial data circuit requests the ECM to release the throttle from the currently controlled cruise position. At the end of automatic braking, the ECM will control the vehicle speed to the current set vehicle speed. Driver throttle override occurs when the driver of the ACC vehicle depresses the accelerator pedal with the intention of requesting greater speed than the current throttle position. An example is an ACC vehicle driver stepping on the accelerator for a passing maneuver. The ECM continuously monitors the throttle and reports a throttle override condition. When no throttle override is present, automatic braking is allowed. When throttle override is detected by the ECM, automatic braking in not allowed to affect the vehicle acceleration.