Cruise Control: Description and Operation
Speed Control
COMPONENT LOCATION
INTRODUCTION
There are two variants of speed control available, a standard system and an adaptive system. The standard system maintains a set speed selected by the driver until operation is suspended or cancelled by a further input from the driver and is controlled by the ECM (engine control module). The adaptive system includes the same functionality as the standard system, but also has the ability to:
- Reduce vehicle speed, to less than the set speed, in order to maintain a selected distance behind a slower moving vehicle
- Accelerate the vehicle back to the set speed, once the way ahead is clear, after reducing the set speed because of a slower moving vehicle
- Alert the driver when the vehicle comes within a given distance of a slower moving vehicle.
The standard system and the adaptive system both have the following components:
- A set +/- switch
- A RESUME switch
- A CANCEL switch.
Both systems also use:
- The ECM (engine control module)
- The stoplamp switch
- The APP (accelerator pedal position) sensor.
The adaptive system incorporates the following additional components:
- An adaptive speed control module
- An adaptive speed control sensor
- A forward alert switch
- A time gap switch.
STANDARD SPEED CONTROL
The standard speed control system is integrated with the EEC (electronic engine control) system and uses fueling intervention to automatically maintain a set vehicle speed. Once engaged, the system can also be used to accelerate the vehicle without using the accelerator pedal.
The standard speed control system also uses inputs from the stoplamp switch, the APP (accelerator pedal position) sensor, the ECM (engine control module) and the ABS (anti-lock brake system) module.
The standard speed control is operated by the driver using only the steering wheel switches. When standard speed control is active, the ECM (engine control module) controls the electronic throttle to adjust the fuel supply as required to maintain the set speed.
The minimum set speed for standard speed control is 18 mph (30 (km/h). Speed control is automatically suspended if the following conditions apply:
- Vehicle speed falls below 18 mph (30 km/h)
- The brake pedal is pressed
- The cancel button is pressed
- Neutral, park or reverse gear is selected
- The difference between actual speed and the set speed is too great
- When the vehicle speed reaches a maximum speed of 120 mph (192 km/h)
- If the accelerator pedal is used to accelerate beyond the set speed for too long.
Speed Control Switches
There are three standard speed control switches:
- Set speed adjustment switch
- Cancel
- Resume.
The standard speed control switches are located on the RH (right-hand) side of the steering wheel. The switches are connected via fly leads to the audio control switches. The standard speed control switches are resistive ladder type switches which vary the resistance of a 5 volt signal sent to them. The signal is returned along a LIN (local interconnect network) bus from the audio control switches to the instrument cluster. The instrument cluster routes the control signals to the ECM (engine control module) on the medium speed CAN (controller area network) bus.
Speed control is engaged by rotating the speed adjustment switch to the + or - positions. Once engaged the speed can be varied by the speed adjustment switch. Each press of the speed adjustment switch will increase or decrease the set speed in steps of 1 mph (2 km/h).
ADAPTIVE SPEED CONTROL
The adaptive speed control system uses a forward looking radar sensor to scan the road ahead, looking for objects that are moving at a different rate to itself. When a target is identified the adaptive speed control system will monitor the time gap between it and the target vehicle. When that gap falls below a set driver selected level the adaptive speed control system will intervene slowing the vehicle by backing off the throttle and/or applying the brakes, until the correct gap is attained. The driver can chose between four gap settings, 1, 1.4, 1.8 and 2.2 seconds.
The system will detect but not react to the following:
- Vehicles in the oncoming lane
- Stationary vehicles
- Pedestrians
- Vehicles not in the same lane.
Adaptive speed control is active when the vehicle is moving. Adaptive speed control only functions when a set speed is entered in normal speed control mode. The adaptive speed control system only intervenes with the set speed when it detects a target vehicle, and then only if the minimum time gap is breached.
It is important to note that the system is intended for use in limited driving situations, does not remove control and responsibility from the driver, and at all times can be quickly overridden. The adaptive speed control system is not a collision warning system and will not react to stationary objects. The system does not operate below a minimum speed of approximately 20 mph (32 km/h) since it is unsuitable for use in cities or congested traffic. The system is best suited to main roads/ highways with gradual bends.
The ECM (engine control module), throttle body and throttle control are unchanged from those used for the standard speed control variants.
Radar Sensing
The adaptive speed control system is based on the use of a front mounted radar sensor. The sensor transmits a 1.5° wide beam forward of the vehicle and detects the returning signals reflected off other vehicles and objects ahead.
The 1.5° wide radar beam is mechanically scanned at a rate of 10 sweeps/second across a total arc of 15° centered on the longitudinal axis of the vehicle. The radar operates at millimetric wavelengths (76 - 77 GHz) and transmits a frequency modulated continuous wave signal at a relatively low power level (no high power pulses).
With the ignition switched ON, the adaptive speed control module is powered up but no radar transmissions are emitted until the vehicle is in motion.
The radar sensor detects three primary parameters of objects within the scanned arc. These are:
- Range
- Relative velocity
- Angle.
Range
The radar sensor detects the presence and ranges of different vehicles and objects within the scanned arc up to a distance of approximately 130 meters (426.5 feet). The transmitted signal frequency changes continuously in a cyclic pattern (modulation). This means that, in the time taken for the signal wave front, to travel to and from a target vehicle (or other object), the transmission frequency will have changed. The difference between the received signal frequency and the new transmission frequency is proportional to the distance between the transmitting vehicle and the target vehicle.
Relative Velocity
When the signal is reflected off a vehicle moving at a different speed (opening or closing gap) an effect known as the Doppler shift causes an extra frequency modulation to be imposed on the signal. This Doppler frequency varies with the relative speed of the vehicle being followed, enabling the system to differentiate between vehicles traveling at different speeds and also between moving vehicles and stationary objects.
Angle
Using a narrow angle beam to scan horizontally enables the system to distinguish between vehicles in different lanes and between vehicles and roadside objects.
Follow Mode
A set speed is selected in the normal speed control manner and this speed is maintained until a slower vehicle is encountered in the lane ahead. When the vehicle ahead comes within the effective range of the radar sensor, the system identifies it as a target vehicle and an icon is illuminated on the instrument cluster to indicate that the system is in "follow mode". When the distance between the two vehicles closes to a set time gap, the adaptive speed control system closes the throttle and if necessary applies the brakes to maintain the set time gap. Follow mode is effectively a closed loop system. If several vehicles are ahead, the closest vehicle is chosen as the target to follow. If the target vehicle moves out of radar range, or if either vehicle changes lane or drops below the minimum operating speed, the system exits follow mode and the follow mode icon is extinguished. The adaptive speed control system will only raise its speed to the originally set speed, it
will not accelerate past this speed to maintain a time gap.
Driver operation of the foot brake or control switches will immediately cancel adaptive speed control.
When the vehicle is in follow mode the follow mode warning indicator is illuminated in the instrument cluster and the current gap setting will be displayed in the message center.
System Restrictions
The adaptive speed control system is only intended to provide enhanced speed control as described above in certain restricted conditions.
The following points should be noted:
- Automatic braking is limited to approximately 30% of full pressure (0.3 G deceleration) and is intended to provide a smooth, gradual deceleration in follow mode conditions. Harsh braking by the target vehicle or following the target vehicle down to very low speeds or to a halt will require driver override of the brakes
- While the radar sensor detects moving and stationary targets for assessment of the environment ahead, the system does not react to or provide any control in situations other than follow mode conditions. Stationary or slow moving vehicles (below 6 mph (10 km/h)), pedestrians, objects on the road and oncoming vehicles in the same lane are not recognized.
WARNING: It must be emphasized that the adaptive speed control system is not a collision warning or avoidance system and that, other than the limited conditions of follow mode, driver intervention will be necessary to control the vehicle speed.
In follow mode, some situations may cause target ambiguities for the detection system. These situations include:
- The nearby presence of a third vehicle when driving on a line slightly offset to the target vehicle
- Vehicles edging into the lane ahead which are not detected by the system until they have moved into the radar beam.
On the approach to, or exit from a bend, a target vehicle may be lost or a new target acquired as vehicles ahead change their angular position with respect to the radar sensor. On a straight road, if the sensing vehicle is in follow mode below its selected set speed, losing the target vehicle will cause the sensing vehicle to accelerate to this set speed. This acceleration is undesirable either on, or entering a bend when the target is suddenly lost, and in this situation the system inhibits the resumption of the set speed.
The speed control system compares vehicle speed data from the ABS (anti-lock brake system) system with the relative speed of an external object as detected by the radar sensor to ascertain whether the object is stationary or not.
NOTE:
If tires are fitted which are different in diameter from those specified for the vehicle, the vehicle speed calculated by the ABS (anti-lock brake system) will not be the true road speed. This situation may cause stationary objects to be falsely identified as moving vehicles and result in automatic deceleration on a clear road.
Adaptive Speed Control Switches
The adaptive speed control system is controlled from the steering wheel switches. The switches are the same as used for normal speed control with the addition of an extra time gap setting switch. The time gap setting switch allows the driver to adjust the time gap to one of four pre-set time gaps. The selected time gap is displayed in the vehicle message center.
The time gap will return to the default gap (the third setting) each time the ignition is cycled.
Adaptive Speed Control Module
The adaptive speed control module is located on the bottom of the RH (right-hand) A pillar. The control module is connected to the other vehicle systems via the high speed CAN (controller area network) bus. Signals from the adaptive speed control forward looking radar sensor are received on a dedicated CAN (controller area network) bus between the two modules.
Adaptive Speed Control Radar Sensor
The adaptive speed control radar sensor is located centrally behind the front bumper cover. The sensor is connected to the adaptive speed control module via a private CAN (controller area network) bus. If the unit is replaced in service the unit must be re-aligned vertically.
Horizontal alignment is achieved by putting the sensor in service mode with a Jaguar approved diagnostic system. The vehicle is then driven for a short period while the sensor calibrates itself. Calibration is complete when the 'follow' icon in the instrument cluster stops flashing.
Forward Alert
The forward alert system utilizes the adaptive speed control system components. Forward alert is turned on and off independently of adaptive speed control, via a switch mounted in the lower outboard knee bolster switchpack. Forward alert will notify the driver by means of a chime, and warning indicator in the instrument cluster and indication in the message center, when a target vehicle comes into range. The system will NOT use throttle or brake intervention to slow the vehicle.
Forward Alert Switch
The forward alert switch is connected to the instrument cluster in a resistive ladder with the luggage compartment opening switch and the fuel filler flap switch. The instrument cluster sends the forward alert information to the adaptive speed control module on the CAN (controller area network) bus.
WARNING: This system is intended as a driver aid and should be used as such. The system is NOT a collision warning or avoidance device.
The system sensitivity can be adjusted in the same manner as the adaptive speed control, via the steering wheel mounted switches. Each adjustment is accompanied by a message in the message center.
CONTROL DIAGRAM
NOTE:
A = Hardwired connection; D = High speed CAN (controller area network) bus; O = LIN (local interconnect network) bus; U = Private CAN (controller area network)
